Table of contents

In this paper we generate a new estimate of the high-energy neutrinos expected from GRBs associated with the first generation of stars in light of new models and constraints on the epoch of reionization and a more detailed evaluation of the neutrino emission yields. We also compare the diffuse high-energy neutrino background from Population III stars with the one from "ordinary stars" (Population II), as estimated consistently within the same cosmological and astrophysical assumptions. In disagreement with previous literature, we find that high-energy neutrinos from Population III stars will not be observable at current or near-future neutrino telescopes, falling below both the sensitivity of a km³ telescope and the atmospheric neutrino background, also under the most optimistic predictions for the GRB rate. This rules them out as a viable diagnostic tool for these still-elusive metal-free stars.

The isotropy of the Lyα forest in real-space uniquely provides a measurement of cosmic geometry at z > 2. The angular diameter distance for which the correlation function along the line of sight and in the transverse direction agree corresponds to the correct cosmological model. However, the Lyα forest is observed in redshift-space where distortions due to Hubble expansion, bulk flows, and thermal broadening introduce anisotropy. Similarly, a spectrograph's line-spread function affects the autocorrelation and cross-correlation differently. In this the second paper of a series on using the Lyα forest observed in pairs of quasi-stellar objects for a new application of the Alcock-Paczyński test, these anisotropies and related sources of potential systematic error are investigated with cosmological hydrodynamic simulations. Three prescriptions for galactic outflow were compared and found to have only a marginal effect on the Lyα flux correlation (which changed by at most 7% with use of the currently favored variable-momentum wind model vs. no winds at all). An approximate solution for obtaining the zero-lag cross-correlation corresponding to arbitrary spectral resolution directly from the zero-lag cross-correlation computed at full resolution (good to within 2% at the scales of interest) is presented. Uncertainty in the observationally determined mean flux decrement of the Lyα forest was found to be the dominant source of systematic error; however, this is reduced significantly when considering correlation ratios. We describe a simple scheme for implementing our results, while mitigating systematic errors, in the context of a future application of the Alcock-Paczyński test.

The SEDs and IR spectra of a remarkable sample of obscured AGNs selected in the MIR are modeled with recent clumpy torus models. The sample contains 21 AGNs at z = 1.3–3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, and FLS) by means of their extremely red IR to optical colors. All sources show the 9.7 μm silicate feature in absorption and have extreme MIR luminosities [L(6 μ m) ≃ 10⁴⁶ ergs s⁻¹]. The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining nine sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the NIR emission from hot dust. The best-fit torus models show a broad range of inclinations. Based on the unobscured QSO MIR luminosity function (Brown and coworkers) and on a color-selected sample of AGNs, we estimate the surface densities of obscured and unobscured QSOs with L(6 μ m) > 10¹²L_☉ and z = 1.3–3.0 to be about 17-22 and 11.7 deg⁻², respectively. Overall we find that ~35%-41% of luminous QSOs are unobscured, 37%-40% are obscured by the torus, and 23%-25% are obscured by a cold absorber detached from the torus. These fractions are consistent with a decrease of the torus covering fraction at large luminosities as predicted by receding torus models. An FIR component is observed in eight objects with luminosity greater than 3.3 × 10¹²L_☉, implying SFRs of 600–3000 M_☉ yr ⁻¹. In the whole sample, the average contribution from a starburst component to the bolometric luminosity, as estimated from the PAH 7.7 μm luminosity in the composite IR spectra, is ≤20% of the total bolometric luminosity.

We use quantitative metrics to characterize the variation of C IV λ1549 broad absorption lines (BALs) over 3-6 (rest-frame) years in a sample of 13 quasars at 1.7 ⩽ z⩽ 2.8 and compare the results to previous studies of BAL variability on shorter timescales. The strong BALs in our study change in complex ways over 3-6 yr. Variation occurs in discrete regions only a few thousand kilometers per second wide, and the distribution of the change in absorption equivalent width broadens over time. We constrain the typical C IV BAL lifetime to be at least a few decades. While we do not find evidence to support a scenario in which the variation is primarily driven by photoionization on multiyear timescales, there is some indication that the variation is produced by changes in outflow geometry. We do not observe significant changes in the BAL onset velocity, indicating that the absorber is either far from the source or is being continually replenished and is azimuthally symmetric. It is not possible in a human lifetime to expand the timescales in our study by more than a factor of a few using optical spectroscopy. However, the strong variation we have observed in some BALs indicates that future studies of large numbers of BAL QSOs will be valuable to constrain BAL lifetimes and the physics of variation.

Using the Sloan Digital Sky Survey Data Release 5 (SDSS-DR5), we survey proximate damped Lyα systems (PDLAs): absorption-line systems with H I column density N_HI ⩾ 2×10²⁰ cm⁻² at velocity separation δ v < 3000 km s⁻¹ from their background quasar. Many of these absorbers may be physically associated with their background quasars, and their statistics allow us to study quasar environments out to z ∼ 5. However, the large ionizing flux emitted by a quasar can ionize the neutral gas in a nearby galaxy, possibly giving rise to a "proximity effect," analogous to the similar effect observed in the Lyα forest. From a sample of 108 PDLAs, we measure the HI frequency distribution f(N_HI, X), incidence, and gas mass density of the PDLAs near luminous quasars over the redshift interval z = 2.2–5. The incidence and mass density of PDLAs at z ∼ 3 is approximately twice that of intervening DLAs, while at z < 2.5 and >3.5 the f(N_HI, X) distribution is enhanced but statistically consistent with the intervening population. We interpret the observed enhancement of PDLAs around quasars in terms of quasar-galaxy clustering and compare the strength of the clustering signal to the expectation from independent measures of the respective clustering strengths of DLAs and quasars, as well as a complementary analysis of the clustering of absorbers around quasars in the transverse direction. We find that there are a factor of 5-10 fewer PDLAs around quasars than expected and interpret this result as evidence for the hypothesis that the ionizing flux from the quasars photoevaporates H I in nearby DLA galaxies, thus reducing their cross section for DLA absorption. This constitutes the first detection of a "proximity effect" for DLAs.

A search for diffuse neutrinos with energies in excess of 10⁵ GeV is conducted with AMANDA-II data recorded between 2000 and 2002. Above 10⁷ GeV, the Earth is essentially opaque to neutrinos. This fact, combined with the limited overburden of the AMANDA-II detector (roughly 1.5 km), concentrates these ultra-high-energy neutrinos at the horizon. The primary background for this analysis is bundles of downgoing, high-energy muons from the interaction of cosmic rays in the atmosphere. No statistically significant excess above the expected background is seen in the data, and an upper limit is set on the diffuse all-flavor neutrino flux of E²Φ_{90% CL} < 2.7 × 10⁻⁷ GeV cm⁻² s⁻¹ sr⁻¹ valid over the energy range of 2 × 10⁵ to 10⁹ GeV. A number of models that predict neutrino fluxes from active galactic nuclei are excluded at the 90% confidence level.

We present an analysis of 109 moderate-luminosity (41.9 ⩽ log L_{0.5–8.0 keV} ⩽ 43.7) AGNs in the Extended Chandra Deep Field-South survey, which is drawn from 5549 galaxies from the COMBO-17 and GEMS surveys having 0.4 ⩽ z⩽ 1.1. These obscured or optically weak AGNs facilitate the study of their host galaxies since the AGNs provide an insubstantial amount of contamination to the galaxy light. We find that the color distribution of AGN host galaxies is highly dependent on (1) the strong color-evolution of luminous (M_V < − 20.7) galaxies, and (2) the influence of ~10 Mpc scale structures. When excluding galaxies within the redshift range 0.63 ⩽ z⩽ 0.76, a regime dominated by sources in large-scale structures at z = 0.67 and z = 0.73, we observe a bimodality in the host galaxy colors. Galaxies hosting AGNs at z≳ 0.8 preferentially have bluer (rest-frame U − V < 0.7) colors than their z≲ 0.6 counterparts (many of which fall along the red sequence). The fraction of galaxies hosting AGNs peaks in the "green valley" (0.5 < U − V < 1.0); this is primarily due to enhanced AGN activity in the redshift interval 0.63 ⩽ z⩽ 0.76. The AGN fraction in this redshift and color interval is 12.8% (compared to its "field" value of 7.8%) and reaches a maximum of 14.8% at U − V ∼ 0.8. We further find that blue, bulge-dominated (Sérsic index n > 2.5) galaxies have the highest fraction of AGN (21%) in our sample. We explore the scenario that the evolution of AGN hosts is driven by galaxy mergers and illustrate that an accurate assessment requires a larger area survey since only three hosts may be undergoing a merger with timescales ≲1 Gyr following a starburst phase.

The nearby elliptical galaxies NGC 4621 and NGC 4697 each host a supermassive black hole with M_• > 10⁸M_☉. Analysis of archival Chandra data and new NRAO Very Large Array data shows that each galaxy contains a low-luminosity active galactic nucleus (LLAGN), identified as a faint, hard X-ray source that is astrometrically coincident with a faint 8.5-GHz source. The latter has a diameter less that 0.3^'' (26 pc for NGC 4621, 17 pc for NGC 4697). The black holes energizing these LLAGNs have Eddington ratios L(2–10 keV)/L(Edd) ∼ 10⁻⁹, placing them in the so-called quiescent regime. The emission from these quiescent black holes is radio-loud, with log R_X = log ν L_ν(8.5 GHz)/L(2–10 keV) ∼ − 2, suggesting the presence of a radio outflow. Also, application of the radio-X-ray-mass relation from Yuan & Cui for quiescent black holes predicts the observed radio luminosities ν L_ν(8.5 GHz) to within a factor of a few. Significantly, that relation invokes X-ray emission from the outflow rather than from an accretion flow. The faint, but detectable, emission from these two massive black holes is therefore consistent with being outflow-dominated. Observational tests of this finding are suggested.

We address the ability of broad iron emission lines from black hole accretion disks to diagnose the spin of the black hole. Using a high-resolution three-dimensional MHD simulation of a geometrically thin accretion disk in a pseudo-Newtonian potential, we show that both the mid-plane density and the vertical column density of the accretion flow drop dramatically over a narrow range of radii close to the innermost stable circular orbit (ISCO). We argue that this drop of density is accompanied by a sharp increase in the ionization parameter of the X-ray photosphere, and that the resulting imprint of the ISCO on the X-ray reflection spectrum can be used to constrain spin. Motivated by this simulation, we construct a simplified toy model of the accretion flow within the ISCO of a Kerr black hole, and use this model to estimate the systematic error on inferred black hole spin that may result from slight bleeding of the iron line emission to the region inside of the ISCO. We find that these systematic errors can be significant for slowly spinning black holes but become appreciably smaller as one considers more rapidly rotating black holes.

We present a multiwavelength study of the nucleus, environment, jets, and hot spots of the nearby FR II radio galaxy 3C 321, using new and archival data from MERLIN, the VLA, Spitzer, HST, and Chandra. An initially collimated radio jet extends northwest from the nucleus of its host galaxy and produces a compact knot of radio emission adjacent (in projection) to a companion galaxy, after which it dramatically flares and bends, extending out in a diffuse structure 35 kpc northwest of the nucleus. We argue that the simplest explanation for the unusual morphology of the jet is that it is undergoing an interaction with the companion galaxy. Given that the northwest hot spot that lies ≳250 kpc from the core shows X-ray emission, which likely indicates in situ high-energy particle acceleration, we argue that the jet-companion interaction is not a steady state situation. Instead, we suggest that the jet has been disrupted on a timescale less than the light-travel time to the end of the lobe, ~10⁶ yr, and that the jet flow to this hot spot will only be disrupted for as long as the jet-companion interaction takes place. The host galaxy of 3C 321 and the companion galaxy are in the process of merging, and each hosts a luminous AGN. As this is an unusual situation, we investigate the hypothesis that the interacting jet has driven material on to the companion galaxy, triggering its AGN. Finally, we present detailed radio and X-ray observations of both hot spots, which show that there are multiple emission sites, with spatial offsets between the radio and X-ray emission.

We present Chandra and HST observations of the ultraluminous X-ray source (ULX) IC 342 X-1. The Chandra and HST images are aligned using two X-ray-emitting foreground stars. The astrometry-corrected position for X-1 is R .A . = 03^h45^m55.61^s, decl . = + 68°04'55.3'' (J2000.0), with an error circle of 0.2''. One extended optical source is found in the error circle, which could be the optical counterpart of X-1. The source shows an extended feature in HST images at long wavelengths, which is likely to be a superposition of two point sources, although it is possible that the dimmer one could be a jet. Both sources are much redder than typical for ULX optical counterparts. The brighter one has an absolute magnitude M_V = − 5.2 ± 0.2 and (B − V)₀ = 0.66 ± 0.13, and the dimmer star is not detected in B and has (B − V)₀ > 2.1. Their colors are consistent with an F8-G0 Ib supergiant or a carbon star, respectively. However, it is likely that part or most of the optical emission may be due to X-rays reprocessed by the companion star or the accretion disk. The stellar neighborhood of IC 342 X-1 lacks O stars and has a minimum age of ~10 Myr. This excludes the possibility that the surrounding nebula is powered by an energetic explosion of a single massive star that formed a black hole. We suggest that the nebula is most likely powered by an outflow from the X-ray source.

Spatially extended Lyα sources that are faint and/or compact in continuum are candidates for extremely young (≲10⁷ yr) galaxies at high redshifts. We present medium-resolution (R ∼ 2000) VLT/VIMOS spectroscopy of 18 such extended Lyα sources found in our previous study at z ∼ 3-5. The deep spectroscopy showed that all 18 objects have large equivalent widths (EWs), exceeding 100 Å in the rest frame. For about 30% of our sample (five objects), we identified conspicuous asymmetry in the profile of the Lyα line. They show broad wing emission components on the red side, and a sharp cutoff on the blue side of the Lyα line. Such asymmetry is often seen in superwind galaxies, and is also consistent with the theoretical prediction of superwinds. In fact, one of them shows systematic velocity structure in the two-dimensional spectrum, suggesting the existence of superwind activity. There are eight objects (8/18 ∼ 40% ) that have very large EWs, exceeding 200 Å (rest frame), and no clear signature of superwind activities. Such large EWs cannot be explained in terms of photoionization by a moderately old (>10⁷ yr) stellar population, even with a top-heavy IMF or an extremely low metallicity. These eight objects clearly show a positive correlation between the Lyα line luminosity and the velocity width. This suggests that these eight objects are good candidates for forming galaxies in a gas-cooling phase (cold accretion).

We study galaxy mergers using a high-resolution cosmological hydro/N-body simulation with star formation and compare the measured merger timescales with theoretical predictions based on the Chandrasekhar formula. In contrast to Navarro et al., our numerical results indicate that the commonly used equation for the merger timescale given by Lacey and Cole systematically underestimates the merger timescales for minor mergers and overestimates those for major mergers. This behavior is partly explained by the poor performance of their expression for the Coulomb logarithm, ln (m_pri/m_sat) . The two alternative forms ln (1 + m_pri/m_sat) and ½ln[1 + (m_pri/m_sat)²] for the Coulomb logarithm can account for the mass dependence of merger timescale successfully, but both of them underestimate the merger timescale by a factor 2. Since ln (1 + m_pri/m_sat) represents the mass dependence slightly better, we adopt this expression for the Coulomb logarithm. Furthermore, we find that the dependence of the merger timescale on the circularity parameter is much weaker than the widely adopted power law ^0.78, whereas 0.94^0.60 + 0.60 provides a good match to the data. Based on these findings, we present an accurate and convenient fitting formula for the merger timescale of galaxies in cold dark matter models.

Determining the scaling relations between galaxy cluster observables requires large samples of uniformly observed clusters. We measure the mean X-ray luminosity-optical richness (-₂₀₀) relation for an approximately volume-limited sample of more than 17,000 optically selected clusters from the maxBCG catalog spanning the redshift range 0.1 < z < 0.3. By stacking the X-ray emission from many clusters using ROSAT All-Sky Survey data, we are able to measure mean X-ray luminosities to ~10% (including systematic errors) for clusters in nine independent optical richness bins. In addition, we are able to crudely measure individual X-ray emission from ~800 of the richest clusters. Assuming a lognormal form for the scatter in the L_X-N₂₀₀ relation, we measure σ_{ln L} = 0.86 ± 0.03 at fixed N₂₀₀. This scatter is large enough to significantly bias the mean stacked relation. The corrected median relation can be parameterized by _X = e^α(₂₀₀/40)^β × 10⁴²h⁻² ergs s⁻¹, where α = 3.57 ± 0.08 and β = 1.82 ± 0.05. We find that X-ray-selected clusters are significantly brighter than optically selected clusters at a given optical richness. This selection bias explains the apparently X-ray-underluminous nature of optically selected cluster catalogs.

Flux-limited X-ray samples indicate that about half of rich galaxy clusters have cool cores. Why do only some clusters have cool cores while others do not? In this paper, cosmological N-body + Eulerian hydrodynamic simulations, including radiative cooling and heating, are used to address this question as we examine the formation and evolution of cool core (CC) and noncool core (NCC) clusters. These adaptive mesh refinement simulations produce both CC and NCC clusters in the same volume. They have a peak resolution of 15.6 h⁻¹ kpc within a (256 h⁻¹ Mpc)³ box. Our simulations suggest that there are important evolutionary differences between CC clusters and their NCC counterparts. Many of the numerical CC clusters accreted mass more slowly over time and grew enhanced CCs via hierarchical mergers; when late major mergers occurred, the CCs survived the collisions. By contrast, NCC clusters experienced major mergers early in their evolution that destroyed embryonic CCs and produced conditions that prevented CC reformation. As a result, our simulations predict observationally testable distinctions in the properties of CC and NCC beyond the core regions in clusters. In particular, we find differences between CC versus NCC clusters in the shapes of X-ray surface brightness profiles, between the temperatures and hardness ratios beyond the cores, between the distribution of masses, and between their supercluster environs. It also appears that CC clusters are no closer to hydrostatic equilibrium than NCC clusters, an issue important for precision cosmology measurements.

We have analyzed the redshift-dependent fraction of galactic bars over 0.2 < z < 0.84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg² field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ∼ 0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ∼ 0.84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z = 0.3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate.

We investigate the environment of infrared-luminous galaxies [L_IR(8–1000 μ m) > 10¹¹L_☉]. We focus on the redshift range 0.7 ⩽ z⩽ 1, where these galaxies dominate the star formation activity and play a significant role in galaxy evolution. We employ MIPS 24 μm data to identify infrared galaxies in the Extended Groth Strip (EGS). We use a local density indicator to probe the environment on a few Mpc scales and a group member catalog, both of which make use of the DEEP2 spectroscopic redshift catalog, to quantify the environment of these galaxies. We find that the local environment of LIRGs and ULIRGs is intermediate between that of blue and red galaxies. LIRGs and ULIRGs avoid underdense environments and inhabit local environments that are more dense on average than those of other DEEP2 galaxies at similar redshifts. However, when the comparison sample of the non-IR DEEP2 galaxies is restricted to have the same range of stellar mass, color, or luminosity as the IR galaxies, there is no longer any significant difference in environment: the IR galaxies follow the same trends in the color-environment and luminosity-environment relations observed at z ∼ 1. We also find that about 30% of the LIRGs and ULIRGs belong to groups, associated with a minimum dark matter halo of 6 × 10¹²M_☉h⁻¹. The group members constitute 20% of the sources responsible for the IR star formation rate density and comoving energy density at z ∼ 1.

We present deep mid-IR spectroscopy with Spitzer of 13 SMGs in the GOODS-N field. We find strong PAH emission in all of our targets, which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and AGN activity to the mid-IR emission. In the high-S/N composite spectrum, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. Individually, only 2/13 SMGs have continuum emission dominating the mid-IR luminosity; one of these SMGs, C1, remains undetected in the deep X-ray images but shows a steeply rising continuum in the mid-IR indicative of a Compton-thick AGN. We find that the mid-IR properties of SMGs are distinct from those of 24 μm-selected ULIRGs at z ∼ 2; the former are predominantly dominated by star formation, while the latter are a more heterogeneous sample with many showing significant AGN activity. We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template SEDs to determine the best estimate of the total IR luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multiwavelength analysis shows that the total IR luminosity, L_IR, in SMGs is dominated by star formation. We find that high-redshift SMGs lie on the relation between L_IR and L_{PAH ,6.2} (or L_{PAH ,7.7} or L_{PAH ,11.3}) that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the SFR in SMGs. SMGs are consistent with being a short-lived cool phase in a massive merger where the AGN does not appear to have become strong enough to heat the dust and dominate the mid- or far-IR emission.

We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18.5 ⩽ M_g < − 22.0 mag and redshift 0.01 ⩽ z < 0.03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Sérsic cuts yield a similar sample of ~2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined (60°) systems, we find the following results. (1) The optical r-band fraction (f_{opt − r}) of barred galaxies, when averaged over the whole sample, is ~48%-52%. (2) When galaxies are separated according to half light radius (r_e), or normalized r_e/R₂₄, which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f_{opt − r} rises sharply, from ~40% in galaxies that have small r_e/R₂₄ and visually appear to host prominent bulges, to ~70% for galaxies that have large r_e/R₂₄ and appear disk-dominated. (3) For galaxies with bluer colors, f_{opt − r} rises significantly (by ~30%). A weaker rise (by ~15%-20%) is seen for lower luminosities or lower masses. (4) While hierarchical ΛCDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that ~20% of disk galaxies appear to be ``quasi-bulgeless." (5) We outline how the effect of a decreasing resolution and a rising obscuration of bars by gas and dust over z = 0.2–1.0 can cause a significant artificial loss of bars, and an artificial reduction in the optical bar fraction over z = 0.2–1.0.

We present a new determination of the metallicity gradient in M33, based on Keck LRIS measurements of oxygen abundances using the temperature-sensitive emission line [O III] λ4363 in 61 H II regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12 + log (O/H) = 8.36 ± 0.04 and a slope of –0.027 ± 0.012 dex kpc⁻¹, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.

Using stellar evolution models in which atomic diffusion was included self-consistently throughout the evolution of a Population II 0.8 M_☉ star (Y = 0.2352, Z = 0.0001, α = 0.3) from the zero-age main sequence to the end of the core expansion phase on the horizonatal branch (HB), surface-abundance composition is calculated and compared to observations of HB stars in the globular cluster M15. The effect of turbulence is characterized by the outer mass it mixes. It is shown that the overabundances of Fe by a factor of 50-100 observed by Behr in the seven stars of that cluster with T_eff > 11,000 K require an outer mixed mass of about 10⁻⁷M_*. The abundance anomalies of Al, Si, S, Ca, Ti, Cr, and Ni, but not of Mg, are also reasonably well reproduced without any additional adjustment. This constitutes a convincing confirmation of the role of atomic diffusion driven by radiative accelerations in HB stars.

We have conducted a deep (15≲ r≲ 23), 20 night survey for transiting planets in the intermediate-age open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. In this paper we describe the observations and data reduction procedures for the survey and analyze the stellar content and dynamical state of the cluster. By combining high-resolution spectroscopy with existing BVI_CK_s and new gri color-magnitude diagrams, we determine the fundamental cluster parameters: t = 485 ± 28 Myr without overshooting (t = 550 ± 30 Myr with overshooting), E(B − V) = 0.227 ± 0.038, (m − M)_V = 11.57 ± 0.13, and [ M/H ] = + 0.045 ± 0.044, which are in good agreement with, although more precise than, previous measurements. We determine the mass function down to 0.3 M_☉ and use this to estimate the total cluster mass of 3640 ± 170 M_☉.

We have conducted a deep (15≲ r≲ 23), 20 night survey for transiting planets in the intermediate-age (~550 Myr) open cluster M37 (NGC 2099) using the Megacam wide-field mosaic CCD camera on the 6.5 m MMT. In this paper we present a catalog and light curves for 1445 variable stars; 1430 (99%) of these are new discoveries. We have discovered 20 new eclipsing binaries and 31 new short-period (P < 1 day ) pulsating stars. The bulk of the variables are most likely rapidly rotating young low-mass stars, including a substantial number (≳500) that are members of the cluster. We identify and analyze five particularly interesting individual variables, including a previously identified variable that we suggest is probably a hybrid γ Doradus/δ Scuti pulsator, two possible quiescent cataclysmic variables, a detached eclipsing binary (DEB) with at least one γ Doradus pulsating component (only the second such variable found in an eclipsing binary), and a low-mass (M_P ∼ M_S ∼ 0.6 M_☉) DEB that is a possible cluster member. A preliminary determination of the physical parameters for the DEB+γ Doradus system yields M_P = 1.58 ± 0.04 M_☉, M_S = 1.58 ± 0.04 M_☉, R_P = 1.39 ± 0.07 R_☉, and R_S = 1.38 ± 0.07 R_☉.

We present the first measurement of the proper motion of the young, compact Arches cluster near the Galactic center from near-infrared adaptive optics (AO) data taken with the recently commissioned laser-guide star (LGS) at the Keck 10 m telescope. The excellent astrometric accuracy achieved with LGS-AO provides the basis for a detailed comparison with VLT/NAOS-CONICA data taken 4.3 yr earlier. Over the 4.3 yr baseline, a spatial displacement of the Arches cluster with respect to the field population is measured to be 24.0 ± 2.2 mas, corresponding to a proper motion of 5.6 ± 0.5 mas yr⁻¹ or 212 ± 29 km s⁻¹ at a distance of 8 kpc. In combination with the known line-of-sight velocity of the cluster, we derive a three-dimensional (3D) space motion of 232 ± 30 km s⁻¹ of the Arches relative to the field. The large proper motion of the Arches cannot be explained with any of the closed orbital families observed in gas clouds in the bar potential of the inner Galaxy, but would be consistent with the Arches being on a transitional trajectory between x1 and x2 orbits. We investigate a cloud-cloud collision as the possible origin for the Arches cluster. The integration of the cluster orbit in the potential of the inner Galaxy suggests that the cluster passes within 10 pc of the supermassive black hole only if its true GC distance is very close to its projected distance. A contribution of young stars from the Arches cluster to the young stellar population in the inner few parsecs of the GC thus appears increasingly unlikely. The measurement of the 3D velocity and orbital analysis provides the first observational evidence that Arches-like clusters do not spiral into the GC. This confirms that no progenitor clusters to the nuclear cluster are observed at the present epoch.

In this paper we performed moderate-resolution spectroscopy on the data obtained with XMM-Newton of the northeastern limb of the Cygnus Loop. This observation was proposed as a higher resolution follow-up to that made by ASCA. We investigated the radial variation of the temperature, ionization timescale, elemental abundances, and hydrogen column density across our field of view. We confirmed that on average all the abundances are depleted. However, we could not confirm the jump structure found by Miyata et al. in the physical properties of the plasma at 0.9R_S. We also detected what looks like a new line in our spectra which probably comes from C VI. Comparing our results for heavy element abundances with those from Suzaku, we found that they agree to within a factor of 2-3. The heavy element ratios relative to O were relatively constant across the field of view and a comparison with models shows that the ISM in this part of the remnant was probably created by low- to intermediate-mass stars.

An imminent problem in mapping an astronomical object is that the true image is convolved with the sensitivity pattern of the observing telescope and, in addition, degraded by noise and other various sources of error. We approach the problem of deconvolving the observed image by introducing a totally scale-free prior map of intensity distributions which, using Bayes' equation, can be combined with observed data to form the posterior map of intensity distributions. Approximate equations for solving out the mean and the dispersion of the map posterior intensities are then derived.

We study the initial mass function (IMF) of one of the most massive Galactic star-forming regions NGC 3603 to answer a fundamental question in current astrophysics: is the IMF universal, or does it vary? Using our very deep, high angular resolution JHK_SL' images obtained with NAOS-CONICA at the VLT at ESO, we have successfully revealed the stellar population down to the subsolar mass range in the core of the starburst cluster. The derived IMF of NGC 3603 is reasonably fitted by a single power law with index Γ ∼ − 0.74 within a mass range of 0.4-20 M_☉, substantially flatter than the Salpeter-like IMF. A strong radial steepening of the IMF is observed mainly in the inner r≲ 30'' field, indicating mass segregation in the cluster center. We estimate the total mass of NGC 3603 to be about 1.0–1.6 × 10⁴M_☉. The derived core density is ≥6 × 10⁴M_☉ pc⁻³, an order of magnitude larger than, e.g., the Orion Nebula Cluster. The estimate of the half-mass relaxation time for solar-mass stars is about 10-40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster. The relaxation time for the high-mass stars can be comparable to the age of the cluster. We estimate that the stars residing outside the observed field cannot steepen the IMF significantly, indicating our IMF adequately describes the whole cluster. Analyzing thoroughly the systematic uncertainties in our IMF determination, we conclude that the power-law index of the IMF of NGC 3603 is Γ = − 0.74^{+ 0.62}_−0.47. Our result thus supports the hypothesis of a potential top-heavy IMF in massive star-forming clusters and starbursts.

We report interferometric observations of the high-mass star-forming object IRAS 23033+5951. Our observations reveal two massive molecular cloud cores, designated IRAS 23033+5951-MMS1 and IRAS 23033+5951-MMS2. MMS1 has already formed a massive protostar and MMS2 appears to be on the verge of doing so. The latter core may be an example of a massive analog to a Class 0 star-forming object. The more evolved core shows some evidence of N₂H⁺ destruction near the protostar, consistent with similar findings in low-mass star-forming objects. In addition to the already-known prominent HCO⁺ outflow, our SiO 2-1 and CH₃OH 2-1 maps show evidence for two more candidate outflows, both presumably less powerful than the main one. Both cores are embedded in an elongated feature whose major axis is oriented almost exactly perpendicular to the axis of the most prominent outflow in the region. Although it has many of the characteristics of a disk, the 87,000 AU (0.42 pc) diameter of this structure suggests that it is more likely to be the flattened, rotating remnant of the natal molecular cloud fragment from which the star-forming cores condensed. We conclude that IRAS 23033+5951 is an excellent example of massive star formation proceeding in relative isolation, perhaps by the method of monolithic collapse and disk accretion.

Bright-rimmed clouds (BRCs) are clouds that have been compressed by an external ionization shock front. We present the first high-resolution VLA observations of 20 of these BRCs in the northern hemisphere. We detected water maser emission from three objects: IRAS 21346+5714 (BRC 36), IRAS 21388+5622 (BRC 37), and IRAS 21445+5712 (BRC 39). The low detection rate supports the evidence that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and suggests that the embedded sources are in a more advanced evolutionary phase than Class 0 objects.

A large fraction of stars form within young embedded clusters, and these environments produce a substantial ultraviolet (UV) background radiation field, which can provide feedback on the star formation process. To assess the possible effects of young stellar clusters on the formation of their constituent stars and planets, this paper constructs the expected radiation fields produced by these clusters. We include both the observed distribution of cluster sizes N in the solar neighborhood and an extended distribution that includes clusters with larger N. The paper presents distributions of the FUV and EUV luminosities for clusters with given stellar membership N, distributions of FUV and EUV luminosity convolved over the expected distribution of cluster sizes N, and the corresponding distributions of FUV and EUV fluxes. These flux distributions are calculated both with and without the effects of extinction. Finally, we consider the effects of variations in the stellar initial mass function on these radiation fields. Taken together, these results specify the distributions of radiation environments that forming solar systems are expected to experience.

We present a census of circumstellar disks in the Chamaeleon I star-forming region. Using the Infrared Array Camera and the Multiband Imaging Photometer on board the Spitzer Space Telescope, we have obtained images of Chamaeleon I at 3.6, 4.5, 5.8, 8.0, and 24 μm. To search for new disk-bearing members of the cluster, we have performed spectroscopy on objects that have red colors in these data. Through this work, we have discovered four new members of Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three objects are highly embedded (A_J ∼ 5) and reside near known protostars, indicating that they may be among the youngest low-mass sources in the cluster (τ < 1 Myr ). The L0 source is the coolest known member of Chamaeleon I. Its luminosity implies a mass of 0.004-0.01 M_☉, making it the least massive brown dwarf for which a circumstellar disk has been reliably detected. To characterize the disk population in Chamaeleon I, we have classified the infrared spectral energy distributions of the 203 known members that are encompassed by the Spitzer images. Through these classifications, we find that the disk fraction in Chamaeleon I is roughly constant at ~50% from 0.01 to 0.3 M_☉. These data are similar to the disk fraction of IC 348, which is a denser cluster at the same age as Chamaeleon I. However, the disk fraction at M≳ 1 M_☉ is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%), indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus, low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters.

From a uniform analysis of a large (8.5 Ms) Rossi X-Ray Timing Explorer data set of low-mass X-ray binaries, we present a complete identification of all the variability components in the power spectra of black holes in their canonical states. It is based on gradual frequency shifts of the components observed between states and uses a previous identification in the black hole low-hard state as a starting point. It is supported by correlations between the frequencies in agreement with those previously found to hold for black hole and neutron stars. Similar variability components are observed in neutron stars and black holes (only the component observed at the highest frequencies is different), which therefore cannot depend on source-specific characteristics such as the magnetic field or surface of the neutron star or spin of the black hole. As the same variability components are also observed across the jet line, the X-ray variability cannot originate from the outer jet but is most likely produced in either the disk or the corona. We use the identification to directly compare the difference in strength of the black hole and neutron star variability and find that these can be attributed to differences in frequency and strength of high-frequency features and do not require the absence of any components. Black holes attain their highest frequencies (in the hard-intermediate and very high states) at a level a factor of ~6 below the highest frequencies attained by the corresponding neutron star components, which can be related to the mass difference between the compact objects in these systems.

We present an analysis of the X-ray variability of three symbiotic X-ray binaries, GX 1+4, 4U 1700+24, and 4U 1954+31, using observations made with the Swift Burst Alert Telescope (BAT) and the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM). Observations of 4U 1954+31 with the Swift BAT show modulation at a period near 5 hr. Models to explain this modulation are discussed, including the presence of an exceptionally slow X-ray pulsar in the system and accretion instabilities. We conclude that the most likely interpretation is that 4U 1954+31 contains one of the slowest known X-ray pulsars. Unlike 4U 1954+31, neither GX 1+4 nor 4U 1700+24 show any evidence for modulation on a timescale of hours. An analysis of the RXTE ASM light curves of GX 1+4, 4U 1700+24, and 4U 1954+31 does not show the presence of periodic modulation in any source, with the exception of a possible detection of the 5 hr period in 4U 1954+31, although there is considerable variability on long timescales for all three sources. There is no modulation in GX 1+4 on either the optical 1161 day orbital period or a previously reported 304 day X-ray period. For 4U 1700+24 we do not confirm the 404 day X-ray period previously proposed for this source from a shorter duration ASM light curve. We conclude that all three sources have substantial low-frequency noise in their power spectra that may give the appearance of periodic modulation if this noise is not properly accounted for, particularly if short-duration light curves are examined.

We report the results of simultaneous monitoring observations of the Galactic microquasar GRS 1915+105 with INTEGRAL and RXTE from 3 up to ~300 keV, and the Ryle Telescope at 15 GHz. We first identify the classes of variability in which GRS 1915+105 is found, and report some direct transitions between them. The accretion ejection connections are studied in a model-independent manner through the source light curves, hardness ratio, and color-color diagrams. During a period of steady "hard" X-ray state (class χ) we observe a steady radio flux interpreted as the signature of a compact jet. We then turn to three particular observations during which we observe several types of soft X-ray dip and spike cycles, followed by radio flares, corresponding to classes ν, λ, and β types of variability. This is the first time ejections are reported during a class λ observation. We generalize the fact that a (nonmajor) discrete ejection always occurs, in GRS 1915+105, as a response to an X-ray sequence composed of a spectrally hard X-ray dip terminated by an X-ray spike marking the disappearance of the emission above 18 keV. We identify the trigger of the ejection as the X-ray spike. A possible correlation between the amplitude of the radio flare and the duration of the X-ray dip is found. The X-ray dips prior to ejections could thus represent the time during which the source accumulates energy and material that is ejected later. The fact that these results do not rely on any spectral modelling enhances their robustness.

This is the second paper presenting the results of 2 yr of monitoring of GRS 1915+105 with INTEGRAL, RXTE, and the Ryle Telescope. We present the X-ray spectral and temporal analysis of four observations showing strong radio to X-ray correlations. During one observation GRS 1915+105 was in a steady state, while during the three others it showed cycles of X-ray dips and spikes (followed by radio flares). Through time-resolved spectroscopy of these cycles, we suggest that the soft X-ray spike is the trigger of the ejection. The ejected medium is then the coronal material responsible for the hard X-ray emission. In the steady state observation, the X-ray spectrum is indicative of the hard-intermediate state, with the presence of a relatively strong emission at 15 GHz. The X-ray spectrum is the sum of a Comptonized component and an extra power law extending to energies >200 keV without any evidence for a cutoff. We observe a possible correlation of the radio flux with that of the power law component, which may indicate that we see direct emission from the jet at hard X-ray energies. We study the energy dependence of a ~4 Hz QPO during the hard-intermediate state observation. The QPO ``spectrum'' is well modeled by a power law with a cutoff at an energy of about 11 keV, and clearly differs from the relative contribution of the Comptonized component to the overall flux. This may rule out models of global oscillations of the Compton corona.

An up-to-date catalog of nearby galaxies considered to be hosts of binary compact objects is provided, with complete information about sky position, distance, extinction-corrected blue luminosity, and error estimates. With our current understanding of binary evolution, rates of formation and coalescence for binary compact objects scale with massive-star formation, and hence the (extinction-corrected) blue luminosity of host galaxies. Coalescence events in binary compact objects are among the most promising gravitational-wave sources for ground-based gravitational-wave detectors such as LIGO. Our catalog and associated error estimates are important for the interpretation of analyses carried out for LIGO, in constraining the rates of compact binary coalescence, given an astrophysical population model for the sources considered. We discuss how the notion of effective distance, created to account for the antenna pattern of a gravitational-wave detector, must be used in conjunction with our catalog. We also note that the catalog provided can be used in other astronomical analysis of populations that scale with galaxy blue luminosity.

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4–3658, an X-ray transient with a recurrence time of ≈2 yr, using data from the Rossi X-Ray Timing Explorer covering four transient outbursts (1998-2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous measurements of this source into question), with typical upper limits of || ≲ 2.5 × 10⁻¹⁴ Hz s⁻¹ (2 σ). However, combining data from all the outbursts shows with high (6 σ) significance that the pulsar is undergoing long-term spin down at a rate = (−5.6 ± 2.0) × 10⁻¹⁶ Hz s⁻¹, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5 × 10⁸ G for the pulsar's surface dipole magnetic field and Q/I < 5 × 10⁻⁹ for the fractional mass quadrupole moment. We also measured an orbital period derivative of _orb = (3.5 ± 0.2) × 10⁻¹² s s⁻¹. This surprisingly large _orb is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called black widow millisecond radio pulsars, which further strengthens previous speculation that SAX J1808.4–3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0.15'') source position from optical data.

We present a detailed analysis of observations of the high-mass X-ray binary Cen X-3, spanning two consecutive binary orbits performed with the RXTE satellite in early March 1997. During this time Cen X-3 had a luminosity of L_{2 − 10 keV} ∼ (4–5) × 10³⁷ ergs s⁻¹ and a pulse period of 4.814 s. The PCA and HEXTE light curves both show a clear reduction in count rate after midorbit for both binary revolutions. We therefore analyze two broadband spectra for each orbit, before and after midorbit. Consistent with earlier observations, these four joint PCA and HEXTE spectra can be well described using a phenomenological pulsar continuum model, including an iron emission line and a cyclotron resonance-scattering feature. While no strong spectral variations were detected, the second half of orbit 2 shows a tendency toward being softer and more strongly absorbed. In order to follow the orbital phase-dependent evolution of the spectrum in greater detail, we model spectra for shorter exposures, confirming that most spectral parameters show either a gradual or sudden change for the second half of the second orbit. A comparison with a simple wind model indicates the existence of an accretion wake in this system. We also present and discuss high-resolution pulse profiles for several different energy bands, as well as their hardness ratios. PCA and HEXTE spectra were created for 24 phase bins and fitted using the same model as in the phase-averaged case. Systematic pulse phase-dependent variations of several continuum and cyclotron line parameters were detected, most notably a significant increase of the cyclotron line energy during the early rise of the main peak, followed by a gradual decrease. We show that applying a simple dipole model for the magnetic field is not sufficient to describe our data.

We analyze archival Chandra and XMM-Newton data of 4U 0142+61 within the context of the surface thermal emission and magnetospheric scattering model. We show that the 4U 0142+61 spectrum can be fit very well with this physical model that contains only four parameters. The system parameters can be tightly constrained from the fits, yielding a surface magnetic field strength of B = (4.75 ± 0.02) × 10¹⁴ G, a surface temperature of kT = 0.309 ± 0.001 keV, and a scattering optical depth of a few in the magnetosphere. These values do not vary between observations due to the stability of the source within the window of the observations. The detailed fits yield χ² values that are statistically much better than the traditionally employed blackbody+power-law and two blackbody fits. The spectroscopically measured surface magnetic field strength is higher than, but within, the theoretical uncertainties of the value inferred from the dipole spin-down formula.

We now have a good measurement of the cooling rate of G117-B15A. In the near future, we will have equally well determined cooling rates for other pulsating white dwarfs, including R548. The ability to measure their cooling rates offers us a unique way to study weakly interacting particles that would contribute to their cooling. Working toward that goal, we perform a careful asteroseismological analysis of G117-B15A and R548. We study them side by side because they have similar observed properties. We carry out a systematic, fine grid search for best-fit models to the observed period spectra of those stars. We freely vary four parameters: the effective temperature, the stellar mass, the helium layer mass, and the hydrogen layer mass. We identify and quantify a number of uncertainties associated with our models. Based on the results of that analysis and fits to the periods observed in R548 and G117-B15A, we clearly define the regions of the four-dimensional parameter space occupied by the best-fit models.

We compute rates of period change () for the 215 s mode in G117-B15A and the 213 s mode in R548, first for models without axions, and then for models with axions of increasing mass. We use the asteroseismological models for G117-B15A and R548 we derived in an earlier publication. For G117-B15A, we consider two families of solutions, one with relatively thick hydrogen layers and one with thin hydrogen layers. Given the region of parameter space occupied by our models, we estimate error bars on the calculated values using Monte Carlo simulations. Together with the observed for G117-B15A, our analysis yields strong limits on the DFSZ axion mass. Our thin hydrogen solutions place an upper limit of 13.5 meV on the axion, while our thick hydrogen solutions relaxes that limit to 26.5 meV.

We present FUSE observations of the hot white dwarfs in the post-common-envelope binaries Feige 24, EUVE J0720–317, BPM 6502, and EUVE J2013+400. The spectra show numerous photospheric absorption lines, which trace the white dwarf orbital motion. We report the detection of C III, O VI, P V, and Si IV in the spectra of Feige 24, EUVE J0720–317, and EUVE J2013+400 and the detection of C III, N II, Si III, Si IV, and Fe III in the spectra of BPM 6502. Abundance measurements support the possibility that white dwarfs in post-common-envelope binaries accrete material from the secondary star wind. The FUSE observations of BPM 6502 and EUVE J2013+400 cover a complete binary orbit. We used the FUSE spectra to measure the radial velocities traced by the white dwarf in the four binaries, where the zero-point velocities were fixed using the ISM velocities in the line of sight of the stellar systems. For BPM 6502 we determined a white dwarf velocity semiamplitude of K_WD = 18.6 ± 0.5 km s⁻¹, and with the velocity semiamplitude of the red dwarf companion (K_RD = 75.2 ± 3.1 km s⁻¹), we estimate the mass ratio to be q = 0.25 ± 0.01. Adopting a spectroscopic mass determination for the white dwarf, we infer a low secondary mass of M_RD = 0.14 ± 0.01 M_☉. For EUVE J2013+400 we determine a white dwarf velocity semiamplitude of K_WD = 36.7 ± 0.7 km s⁻¹. The FUSE observations of EUVE J0720–317 cover approximately 30% of the binary period and, combined with the HST GHRS measurements, we update the binary properties. FUSE observations of Feige 24 cover approximately 60% of the orbit, and we combine this data set with HST STIS data to update the binary properties.

We present new photometry of HD 149026 spanning five transits of its "super-Neptune" planet. In combination with previous data, we improve on the determination of the planet-to-star radius ratio: R_p/R_⋆ = 0.0491^{+ 0.0018}_−0.0005. We find the planetary radius to be 0.71 ± 0.05 R_Jup, in accordance with previous theoretical models invoking a high metal abundance for the planet. The limiting error is the uncertainty in the stellar radius. Although we find agreement among four different ways of estimating the stellar radius, the uncertainty remains at 7%. We also present a refined transit ephemeris and a constraint on the orbital eccentricity and argument of pericenter, ecos ω = − 0.0014 ± 0.0012, based on the measured interval between primary and secondary transits.

We show that interaction with a gas disk may produce young planetary systems with closely spaced orbits, stabilized by mean motion resonances between neighbors. On longer timescales, after the gas is gone, interaction with a remnant planetesimal disk tends to pull these configurations apart, eventually inducing dynamical instability. We find that this can lead to a variety of outcomes; some cases resemble the solar system, while others end up with high-eccentricity orbits reminiscent of the observed exoplanets. A similar mechanism has been previously suggested as the cause of the lunar late heavy bombardment. Thus, it may be that a large-scale dynamical instability, with more or less cataclysmic results, is an evolutionary step common to many planetary systems, including our own.

Gravitational instability of a vertically thin, dusty sheet near the midplane of a protoplanetary disk has long been proposed as a way of forming planetesimals. Before Roche densities can be achieved, however, the dust-rich layer, sandwiched from above and below by more slowly rotating dust-poor gas, threatens to overturn and mix by the Kelvin-Helmholtz instability (KHI). Whether such a threat is real has never been demonstrated: the Richardson criterion for the KHI is derived for two-dimensional Cartesian shear flow and does not account for rotational forces. Here we present three-dimensional numerical simulations of gas-dust mixtures in a shearing box, accounting for the full suite of disk-related forces: the Coriolis and centrifugal forces, and radial tidal gravity. Dust particles are assumed small enough to be perfectly entrained in gas; the two fluids share the same velocity field but obey separate continuity equations. We find that the Richardson number Ri does not alone determine stability. The critical value of Ri below which the dust layer overturns and mixes depends on the height-integrated metallicity Σ_d/Σ_g (surface density ratio of dust to gas). Nevertheless, for Σ_d/Σ_g between 1 and 5 times solar, the critical Ri maintains a nearly constant value of ~0.1. Keplerian radial shear stabilizes those modes that would otherwise disrupt the layer at large Ri. If the height-integrated metallicity is at least ~5 times greater than the solar value of 0.01, then midplane dust densities can approach Roche densities. Such a metal-rich environment might be expected to produce gas giant planets having similarly supersolar metallicities.

The growth of the gravitational instability in the dust layer of a protoplanetary disk is investigated. In order to see the effects of only the gravitational instability, we assume a laminar disk which has no radial pressure gradient as an unperturbed state so that the shear and the streaming instabilities do not grow. We neglect the relative velocity between the dust and gas parallel to the disk plane assuming that the dust and gas couple firmly by the mutual friction. However, we take account of the dust settling by using an analytic solution of dust density growth. We construct a two-dimensional thin disk model in which the radial and azimuthal directions in the midplane are taken as independent variables. In order to keep a certain amount of a disturbance, which is considered to exist not only at the beginning but all through the time evolution, we give perturbations repeatedly per Keplerian shear time in a local frame of reference. We find that the gravitational instability grows for the dust particle when the dimensionless gas friction time (the product of the gas friction time and the Keplerian angular velocity) is equal to 0.01. On the other hand, the gravitational instability does not grow sufficiently before the dust layer becomes infinitesimally thin if the dimensionless gas friction time is equal to 0.1. These results are consistent with the axisymmetric study by Yamoto and Sekiya. However, the gravitational instability grows nonaxisymmetrically, and trailing surface density patterns arise.

The physical mechanism responsible for the dissipation of the solar wind turbulence and the resulting plasma heating is not completely understood. To be a viable means of dissipation, any mechanism has to reproduce several observational features of the turbulence spectra. One important characteristic of the spectrum is its high-frequency break, where the spectral slope becomes considerably steeper than the Kolmogorov-like scaling law observed in the inertial range. The onset of the spectral steepening can be inferred from the observations fairly accurately, and it is a good benchmark to test various theories of the turbulence dissipation. In this paper, a large database of magnetic field spectra and plasma parameters at 1 AU measured by the ACE spacecraft is used to determine the spectral break. The statistical correlation of the data points calculated according to existing theoretical formulae for the break is analyzed, and the least-squares fits to the data are compared with the theoretically predicted scalings. It is concluded that the position of the spectral break is not determined just by a scale of the turbulent fluctuations, but by a combination of their scale and the amplitude at that scale. This suggests that the dissipation of the solar wind turbulence is an essentially nonlinear process.

Voyager 1's recent and long-anticipated passage into the heliosheath contradicted the prediction that we would observe the source of anomalous cosmic rays accelerated by the termination shock. The observed energetic protons reveal a power-law spectrum below several MeV, but above several MeV the spectrum falls more sharply, and we observe the familiar bump caused by modulation of anomalous cosmic rays. This paper develops the theoretical framework to include the motions and drift of particles during diffusive shock acceleration at a three-dimensional (3-D) termination shock, including cross-field diffusion, and curvature and gradient drifts. Our model supports the concept of McComas and Schwadron that because of the termination shock's blunt structure with a strong nose-to-tail asymmetry, there should be a strong deficit of locally accelerated anomalous cosmic rays (ACRs) near the nose. With reasonable parameters for the scattering mean free path and the ratio of perpendicular to parallel diffusion, the model produces an energy spectrum that agrees well with the Voyager 1 observations near the termination shock. These parameters also lead to an acceleration time to 10 MeV of about 1 yr, which is comparable to previous estimates derived from the observed charge states of ACRs. Thus, we provide a theory for diffusive acceleration at the blunt termination shock. The predictions of this theory are consistent with Voyager 1's observations, showing a lack of ACRs accelerated near the nose of the termination shock and the ACR acceleration timescale derived from ACR charge states.

We present a Monte Carlo method to model the transport of solar near-relativistic electrons in the interplanetary medium, including adiabatic focusing, pitch-angle dependent scattering, and solar wind effects. By taking into account the angular response of the LEFS60 telescope of the EPAM instrument on board the ACE spacecraft, we transform the simulated pitch-angle distributions into the sectored intensities measured by the telescope. The goal is to deconvolve the effects of the interplanetary transport in order to infer the underlying injection profile and the radial mean free path of the electrons. We apply the model to the near-relativistic electron event observed on 2000 May 1, associated with an impulsive X-ray flare, type III radio bursts, and a narrow fast CME. The deconvolved interplanetary transport conditions reveal a long radial mean free path of 0.9 AU and pitch-angle dependent scattering. The eight observed sectored intensities are fitted in detail for more than 90 minutes, except for a short period (~12 minutes) right after the time of peak intensities. This discrepancy may suggest that the assumed scattering model performs more efficiently than the actual scattering processes at work. The resulting injection profile consists of two main components, an initial component lasting 2-3 minutes and probably related to a type III radio burst observed by WIND WAVES at ~10:21 UT, and a delayed component starting at the Sun around 10:35 UT with a typical injection decay timescale of ~0.5 hr. The delayed component may be related to the CME-driven shock.

A simple model of the coronal magnetic field prior to the coronal mass ejection (CME) eruption on 1997 May 12 is developed. First, the magnetic field is constructed by superimposing a large-scale background field and a localized bipolar field to model the active region (AR) in the current-free approximation. Second, this potential configuration is quasi-statically sheared by photospheric vortex motions applied to two flux concentrations of the AR. Third, the resulting force-free field is then evolved by canceling the photospheric magnetic flux with the help of an appropriate tangential electric field applied to the central part of the AR. To understand the structure of the modeled configuration, we use the field line mapping technique by generalizing it to spherical geometry. We demonstrate that the initial potential configuration contains a hyperbolic flux tube (HFT) which is a union of two intersecting quasi-separatrix layers. This HFT provides a partition of the closed magnetic flux between the AR and the global solar magnetic field. Such a partition is approximate since the entire flux distribution is perfectly continuous. The vortex motions applied to the AR interlock the field lines in the coronal volume to form additionally two new HFTs pinched into thin current layers. Reconnection in these current layers helps to redistribute the magnetic flux and current within the AR in the flux-cancellation phase. In this phase, a magnetic flux rope is formed together with a bald patch separatrix surface wrapping around the rope. Other important implications of the identified structural features of the modeled configuration are also discussed.

In the present work we combine UV and radio observations of the quiet Sun to determine the differential emission measure (DEM) of the average quiet solar atmosphere from the photosphere (5600 K) to the corona. UV line intensities have been used to constrain the DEM above 30,000 K, and the radio spectrum from 1.5 to 345 GHz has been used to extend the DEM determination down to 5600 K. Radio observations are shown to provide a much more reliable diagnostic tool for DEM determination than UV and EUV lines at T < 30,000 K. The resulting average quiet-Sun DEM that we found is in excellent agreement with curves from the literature for temperatures larger than 60,000 K, but is lower than previous determinations by more than 1 order of magnitude in the 10,000-30,000 K temperature range. The present work determines the DEM below 10,000 K for the first time, in a temperature region where UV and EUV lines cannot be used.

Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and are powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of preexisting atmospheric magnetic flux subject to surface plasma flows or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA AR 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with four different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~10³² erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection.

The impulsive phase of a solar flare marks the epoch of rapid conversion of energy stored in the preflare coronal magnetic field. Hard X-ray observations imply that a substantial fraction of flare energy released during the impulsive phase is converted to the kinetic energy of mildly relativistic electrons (10-100 keV). The liberation of the magnetic free energy can occur as the coronal magnetic field reconfigures and relaxes following reconnection. We investigate a scenario in which products of the reconfiguration—large-scale Alfvén wave pulses—transport the energy and the magnetic field changes rapidly through the corona to the lower atmosphere. This offers two possibilities for electron acceleration. First, in a coronal plasma with β < m_e/m_p, the waves propagate as inertial Alfvén waves. In the presence of strong spatial gradients, these generate field-aligned electric fields that can accelerate electrons to energies on the order of 10 keV and above, including by repeated interactions between electrons and wave fronts. Second, when they reflect and mode-convert in the chromosphere, a cascade to high wavenumbers may develop. This will also accelerate electrons by turbulence, in a medium with a locally high electron number density. This concept, which bridges MHD-based and particle-based views of a flare, provides an interpretation of the recently observed rapid variations of the line-of-sight component of the photospheric magnetic field across the flare impulsive phase, and offers solutions to some perplexing flare problems, such as the flare "number problem" of finding and resupplying sufficient electrons to explain the impulsive-phase hard X-ray emission.

By considering a simple driven model involving the resistive 3D MHD interaction of magnetic sources, it is shown that it is essential to know the magnetic skeleton to determine (1) the locations of reconnection, (2) type of reconnection, (3) the rate of reconnection, and (4) how much reconnection is occurring. In the model, two opposite-polarity magnetic fragments interact in an overlying magnetic field with reconnection, first closing and then opening the magnetic field from the sources. There are two main reconnection phases: the first has one reconnection site at which the flux is closed, and the second has three sites. The latter is a hybrid case involving both closing and reopening reconnection processes. Each reconnection site coincides with its own separator, and hence all reconnection is via separator reconnection. All the separators connect the same two nulls and thus mark the intersection between the same four types of flux domain. In the hybrid state, the two competing reconnection processes (which open and close flux connecting the same two source pairs) run simultaneously, leading to recursive reconnection. That is, the same flux may be closed and then reopened not just once, but many times. This leads to two interesting consequences: (1) the global reconnection rate is enhanced and (2) heating occurs for a longer period and over a wider area than in the single-separator case.

Using 3 year WMAP data at the locations of ~700 X-ray-selected clusters, we have detected the amplitude of the thermal Sunyaev-Zeldovich (TSZ) effect at the 15 σ level, the highest statistical significance reported so far. Owing to the large size of our cluster sample, we are able to detect the corresponding cosmic microwave background distortions out to large cluster-centric radii. The region over which the TSZ signal is detected is, on average, 4 times larger in radius than the X-ray-emitting region, extending to ~3 h⁻¹₇₀ Mpc. We show that an isothermal β-model does not fit the electron pressure at large radii; instead, the baryon profile is consistent with the Navarro-Frenk-White profile, which is expected for dark matter in the concordance ΛCDM model. The X-ray temperature at the virial radius of the clusters falls by a factor ~3-4 from the central value, depending on the cluster concentration parameter. Our results suggest that cluster dynamics at large radii is dominated by dark matter and is well described by Newtonian gravity.

Hierarchical turbulent structure constituting a jet is considered to reproduce energy-dependent variability in blazars, particularly, the correlation between X- and gamma-ray light curves measured in the TeV blazar Markarian 421. The scale-invariant filaments are featured by the ordered magnetic fields that involve hydromagnetic fluctuations serving as electron scatterers for diffusive shock acceleration, and the spatial size scales are identified with the local maximum electron energies, which are reflected in the synchrotron spectral energy distribution (SED) above the near-infrared/optical break. The structural transition of filaments is found to be responsible for the observed change of spectral hysteresis.

We have performed a high mass and force resolution simulation of an idealized galaxy forming from dissipational collapse of gas embedded in a spherical dark matter halo. The simulation includes star formation and effects of stellar feedback. In our simulation a stellar disk forms with a surface density profile consisting of an inner exponential breaking to a steeper outer exponential. The break forms early on and persists throughout the evolution, moving outward as more gas is able to cool and add mass to the disk. The parameters of the break are in excellent agreement with observations. The break corresponds to a rapid drop in the star formation rate associated with a drop in the cooled gas surface density, but the outer exponential is populated by stars that were scattered outward on nearly circular orbits from the inner disk by spiral arms. The resulting profile and its associated break are therefore a consequence of the interplay between a radial star formation cutoff and redistribution of stellar mass by secular processes. A consequence of such evolution is a sharp change in the radial mean stellar age profile at the break radius.

New optical Hubble Space Telescope (HST), Spitzer Space Telescope, and XMM observations of the luminous infrared galaxy (LIRG) NGC 2623 are presented. This galaxy was observed as part of the Great Observatories All-sky LIRG Survey (GOALS). The prominent 3.2 kpc southern extension to the nucleus has been resolved by HST observations into ~100 star clusters, making it one of the richest off-nuclear concentrations of bright clusters observed in GOALS. The clusters have to –12.6 mag, which is within the magnitude range of Antennae galaxy clusters and in excess of 30 Doradus clusters at the high end. Their optical colors are primarily consistent with ages of ~1-100 Myr. Archival GALEX data show the off-nuclear region to be extremely bright in the far-ultraviolet, being equivalent in luminosity to the resolved nuclear region at 0.15 μm, but becoming less energetically significant at increasing wavelengths. In addition, [Ne V] 14.3 μm emission is detected with Spitzer IRS, confirming the inference from the X-ray and radio data that an active galactic nucleus (AGN) is present. Thus, the off-nuclear optical clusters are associated with a secondary burst of activity corresponding to a star formation rate ~0.1-0.2 M_☉ yr⁻¹; the bulk of infrared (and thus bolometric) luminosity is generated via star formation and an AGN embedded behind dust within the inner kiloparsec of the system. If the infrared luminosity is primarily reprocessed starlight, the off-nuclear starburst accounts for <1% of the present star formation in NGC 2623.

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, B − V color-magnitude diagram that reaches V ∼ 25.5 mag, showing the galaxy's main-sequence turnoff at V ∼ 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II, a fundamental-mode variable (RRab) with period P_ab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with P_c = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, ⟨ V⟩ = 21.48 ± 0.02 mag, is used to obtain a precision distance modulus of μ₀ = 21.02 ± 0.06 mag and a corresponding distance of 160^{+ 4}₋₅ kpc, for an adopted reddening E(B − V) = 0.015 mag.

We measure the metallicity of the unusual hypervelocity star HE 0437–5439 from high-resolution spectroscopy to be half-solar. We determine a spectral type of B2 IV-III for the star and derive an effective temperature T_eff = 21,500 ± 1000 K and a surface gravity log g = 3.7 ± 0.2 (cgs). We also present BV time series photometry and find the star to be nonvariable at the 0.02 mag level. We refine the magnitude of the hypervelocity star to V = 16.36 ± 0.04 mag, with a color B − V = − 0.23 ± 0.03 mag, confirming its early-type nature. Our metallicity result establishes the origin of HE 0437–5439 in the Large Magellanic Cloud and implies the existence of a massive black hole somewhere in this galaxy.

Recent laboratory measurements for the S⁺² + H₂ reaction find a total rate coefficient significantly larger than previously used in theoretical models of X-ray-dominated regions (XDRs). While the branching ratio of the products is unknown, one energetically possible route leads to the SH⁺ molecule, a known XDR diagnostic. In this work, we study the effects of S⁺² on the formation of SH⁺ and the destruction of S⁺² in XDRs. We find that the predicted SH⁺ column density for molecular gas surrounding an active galactic nucleus (AGN) increases by as much as 2 dex. As long as the branching ratio for S⁺² + H₂ → SH⁺ + H⁺ exceeds a few percent, doubly ionized chemistry will be the dominant pathway to SH⁺, which then initiates the formation of other sulfur-bearing molecules. We also find that the high rate of S⁺² + H₂ efficiently destroys S⁺² once H₂ forms, while the S⁺² abundance remains high in the H⁰ region. We discuss the possible consequences of S⁺² in the H⁰ region on mid-infrared diagnostics. The enhanced SH⁺ abundance has important implications in the study of XDRs, while our conclusions for S⁺² could potentially affect the interpretation of Spitzer and SOFIA observations.

Cyanoformaldehyde (CNCHO) has been detected toward the star-forming region Sagittarius B2(N) with the 100 m Green Bank Telescope (GBT) by means of four P-branch rotational transitions in emission, the 7(0, 7)-6(1, 6) at 8.6 GHz, the 8(0, 8)-7(1, 7) at 19.4 GHz, the 9(0, 9)-8(1, 8) at 30.3 GHz, and the 10(0, 10)-9(1, 9) at 41.3 GHz, and one P-branch transition in absorption, the 5(1, 5)-6(0, 6) at 2.1 GHz. The five b-type transitions have favorable transition line strengths (S_ijμ² > 10 D²) and occur in spectral regions that have little possibility of confusion with other molecular species. The transition line strengths and energy levels involved in the four cyanoformaldehyde transitions in emission are similar; however, transitions with larger beam sizes give systematically higher column densities, suggesting that CNCHO is spatially extended and not concentrated toward the Sgr B2(N-LMH) position. Moreover, with a GBT beamwidth of ~350^{' '}, the 5(1, 5)-6(0, 6) transition of CNCHO was detected in absorption, confirming the widespread spatial extent of this molecule. We suggest that cyanoformaldehyde is likely formed in a neutral-radical reaction of two other interstellar molecules known for widespread spatial distributions: formaldehyde (H₂CO) and the cyanide (CN) radical.

The millimeter-wave rotational emission lines (4₀₄-3₀₃ and 5₀₅-4₀₄) of protonated carbon dioxide, HCO₂⁺(HOCO⁺), have been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the IRAM 30 m telescope. This is the first detection of HCO₂⁺ except for the Galactic center clouds. The column density of HCO₂⁺ averaged over the beam size (29^'') is determined to be 7.6 × 10¹⁰ cm⁻², assuming a rotational temperature of 12.3 K. The fractional abundance of gaseous CO₂ relative to H₂ is estimated from the column density of HCO₂⁺ with the aid of a simplified chemical model. If the HCO₂⁺ emission only comes from the evaporation region of CO₂ near the protostar (T ≳ 50 K), the fractional abundance of CO₂ is estimated to be higher than 6.6 × 10⁻⁴. This is comparable to the elemental abundance of carbon in interstellar clouds, and hence, the direct evaporation of CO₂ from dust grain is unrealistic as a source of gaseous CO₂ in L1527. A narrow line width of HCO₂⁺ also supports this. On the other hand, the fractional abundance of CO₂ is estimated to be 2.9 × 10⁻⁷, if the source size is comparable to the beam size. These results indicate that gaseous CO₂ is abundant even in the low-mass star-forming region. Possible production mechanisms of gaseous CO₂ are discussed.

V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with XMM-Newton in 2007 March, 20 months after the optical outburst. The X-ray spectrum shows that the nova had evolved to a pure supersoft X-ray source, with no significant emission at energies above 1 keV. The X-ray light curve shows abrupt decreases and increases of the flux by a factor ~8. It is consistent with a periodicity of 2.97 hr, the orbital period suggested by Dobrotka and coworkers, although the observation lasted just a little more than a whole period. We estimate the distance to V5116 Sgr to be 11 ± 3 kpc. A simple blackbody model does not fit correctly the EPIC spectra, with χ²_ν > 4. In contrast, ONe-rich white dwarf atmosphere models provide a good fit, with N_H = (1.3 ± 0.1) × 10²¹ cm⁻², T = (6.1 ± 0.1) × 10⁵ K, and L = (3.9 ± 0.8) × 10³⁷(D/10 kpc)² ergs s⁻¹ (during the high-flux periods). This is consistent with residual hydrogen burning in the white dwarf envelope. The white dwarf atmosphere temperature is the same both in the low- and the high-flux periods, ruling out an intrinsic variation of the X-ray source as the origin of the flux changes. We speculate that the X-ray light curve may result from a partial coverage by an asymmetric accretion disk in a high-inclination system.

We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum of the single G-type giant HR 9024 during a large flare. In analogy to solar X-ray observations, we interpret the observed Fe Kα line as being produced by illumination of the photosphere by ionizing coronal X-rays, in which case, for a given Fe photospheric abundance, its intensity depends on the height of the X-ray source. The HETGS observations, together with three-dimensional Monte Carlo calculations to model the fluorescence emission, are used to obtain a direct geometric constraint on the scale height of the flaring coronal plasma. We compute the Fe fluorescent emission induced by the emission of a single flaring coronal loop that well reproduces the observed X-ray temporal and spectral properties according to a detailed hydrodynamic modeling. The predicted Fe fluorescent emission is in good agreement with the observed value within observational uncertainties, pointing to a scale height ≲ 0.3R_*. Comparison of the HR 9024 flare with that recently observed on II Peg by Swift indicates the latter is consistent with excitation by X-ray photoionization.

As stars evolve along the asymptotic giant branch (AGB), strong winds are driven from the outer envelope. These winds form a shell, which may ultimately become a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at the presence of a binary companion. Some post-AGB objects are surrounded by tori of crystalline dust, but there is no generally accepted mechanism for annealing the amorphous grains in the wind to crystals. Here, we show that the shaping of an AGB wind by a binary companion provides a possible mechanism for forming crystalline dust in the orbital plane.

We aim to understand cloud formation in substellar objects. We combined our nonequilibrium, stationary cloud model DRIFT (seed formation, growth, evaporation, gravitational settling, element conservation) with the general-purpose model atmosphere code PHOENIX (radiative transfer, hydrostatic equilibrium, mixing-length theory, chemical equilibrium) in order to consistently calculate cloud formation and radiative transfer with their feedback on convection and gas-phase depletion. We calculate the complete 1D model atmosphere structure and the chemical details of the cloud layers. The DRIFT-PHOENIX models enable the first stellar atmosphere simulation that is based on the actual cloud formation process. The resulting (T, p) -profiles differ considerably from the previous limiting PHOENIX cases DUSTY and COND. A tentative comparison with observations demonstrates that the determination of effective temperatures based on simple cloud models has to be applied with care. Based on our new models, we suggest a mean T_eff = 1800 K for the L dwarf twin-binary system DENIS J0205–1159, which is up to 500 K hotter than suggested in the literature. We show transition spectra for gas-giant planets which form dust clouds in their atmospheres and evaluate photometric fluxes for a WASP-1 type system.

Mid-infrared spectrophotometric observations have revealed a small subclass of circumstellar disks with spectral energy distributions (SEDs) suggestive of large inner gaps with low dust content. However, such data provide only an indirect and model-dependent method of finding central holes. We present here the direct characterization of a 40 AU radius inner gap in the disk around LkHα 330 through 340 GHz (880 μm) dust continuum imaging with the Submillimeter Array (SMA). This large gap is fully resolved by the SMA observations and mostly empty of dust with less than 1.3 × 10⁻⁶M_☉ of solid particles inside of 40 AU. Gas (as traced by accretion markers and CO M-band emission) is still present in the inner disk and the outer edge of the gap rises steeply—features in better agreement with the underlying cause being gravitational perturbation than a more gradual process such as grain growth. Importantly, the good agreement of the spatially resolved data and spectrophotometry-based model lends confidence to current interpretations of SEDs with significant dust emission deficits as arising from disks with inner gaps or holes. Further SED-based searches can therefore be expected to yield numerous additional candidates that can be examined at high spatial resolution.

We report the discovery of WASP-4b, a large transiting gas-giant planet with an orbital period of 1.34 days. This is the first planet to be discovered by the SuperWASP-South observatory and CORALIE collaboration and the first planet orbiting a star brighter than 16th magnitude to be discovered in the southern hemisphere. A simultaneous fit to high-quality light curves and precision radial velocity measurements leads to a planetary mass of 1.22^{+ 0.09}_−0.08M_Jup and a planetary radius of 1.42^{+ 0.07}_−0.04R_Jup. The host star is USNO-B1.0 0479–0948995, a G7 V star of visual magnitude 12.5. As a result of the short orbital period, the predicted surface temperature of the planet is 1761 K, making it an ideal candidate for detections of the secondary eclipse at infrared wavelengths.

We present the first-ever infrared image of Saturn's auroral emission taken from Earth, using the CSHELL instrument on NASA's InfraRed Telescope Facility. We adapt the emission-line imaging technique to use the spectrometer as an ultra-narrowband imager, measuring the H₃⁺ line at 3.953 μm with a spectral resolution of ~5000. Calibrating this technique with Jupiter shows a significant increase in sensitivity over the narrowest H₃⁺ filters available. The resulting image of Saturn's auroral region confirms that the infrared main auroral oval is broadly similar to that seen in ultraviolet observations with a partially filled-in polar cap. It also shows that significant variations in brightness occur within the oval itself.

Determining the origins of our solar system and, by proxy, other planetary systems, depends on knowing accurately and precisely the timing and tempo of the transformation of the disk of gas and dust to the solids that formed the planets. Relative ages based on the short-lived nuclide ²⁶Al indicate that high-temperature calcium-aluminum inclusions (CAIs) formed before lower temperature chondrules but these ages are heavily dependant on a model of homogeneous distribution of ²⁶Al within the protoplanetary disk. The competing X-wind model argues for heterogeneous distribution of ²⁶Al due to its formation by intra-solar system irradiation such that this system would have no chronological significance. We report a²⁰⁷Pb-²⁰⁶Pb isochron age of 4565.45 ± 0.45 Myr for chondrules from the CV chondrite Allende, an age that is 1.66 ± 0.48 Myr younger than the accepted Pb-Pb age for CAIs from this chondrite group. This age offset is in excellent agreement with the relative ages determined using the ²⁶Al-²⁶Mg system, an observation that supports a supernova origin for ²⁶Al and, importantly, the chronological significance of the ²⁶Al-²⁶Mg system in general. This is consistent with an early and brief CAI-forming event followed by recurrent chondrule formation throughout the life span of the protoplanetary disk. The paucity of old chondrules in chondrite meteorites may reflect their early incorporation into the parent bodies of differentiated meteorites after CAIs were effectively removed from the innermost regions of the protoplanetary disk. Lastly, the agreement between the absolute and relative chronology of CAIs and chondrules requires a solar system age younger than ~4567.5 Myr

We study the solar source of the ³He-rich solar energetic particle (SEP) event observed on 2006 November 18. The SEP event showed a clear velocity dispersion at energies below 1 MeV nucleon⁻¹, indicating its solar origin. We associate the SEP event with a coronal jet in an active region at heliographic longitude of W50°, as observed in soft X-rays. This jet was the only noticeable activity in full-disk X-ray images around the estimated release time of the ions. It was temporally correlated with a series of type III radio bursts detected in metric and longer wavelength ranges and was followed by a nonrelativistic electron event. The jet may be explained in terms of the model of an expanding loop reconnecting with a large-scale magnetic field, which is open to interplanetary space for the particles to be observed at 1 AU. The open field lines appear to be anchored at the boundary between the umbra and penumbra of the leading sunspot, where a brightening is observed in both soft and hard X-rays during the jet activity. Other flares in the same region possibly associated with ³He-rich SEP events were not accompanied by a jet, indicative of different origins of this type of SEP event.