Table of contents

I point out that an effective upper limit of approximately 20(0.2/Ω_m)^1/2 Gyr (for a Hubble constant of 72 km s^-1 Mpc^-1), or alternatively, 1.49(0.2/Ω_m)^1/2 for the H₀-independent quantity H₀t₀, exists on the age of the universe, essentially independent of the unknown equation of state of the dominant dark energy component in the universe. If astrophysical constraints on the age of the universe can convincingly reduce the upper limit to well below this value, a useful lower limit on the equation of state parameter w for this component can be obtained. Direct dating by stars does not provide a useful constraint, but model-dependent cosmological limits from supernovae and the CMB observations can do so.

Motivated by the recent detection by the Wilkinson Microwave Anisotropy Probe of a large optical depth to Thomson scattering, implying a very early reionization epoch, we assess a scenario where the universe was reionized by "miniquasars" powered by intermediate-mass black holes (IMBHs), the remnants of the first generation of massive stars. Pregalactic IMBHs form within minihalos above the cosmological Jeans mass collapsing at z > 20, get incorporated through mergers into larger and larger systems, sink to the center as a result of dynamical friction, and accrete cold material. The merger history of dark halos and associated IMBHs is followed by Monte Carlo realizations of the merger hierarchy in a ΛCDM cosmology. Our model is based on the assumptions that quasar activity is driven by major mergers and nuclear IMBHs accrete at the Eddington rate a fraction of the gas in the merger remnant. The long dynamical frictional timescales leave many IMBHs "wandering" in galaxy halos after a minor merger. While seed IMBHs that are as rare as the 3.5 σ peaks of the primordial density field evolve largely in isolation, a significant number of BH binary systems will form if IMBHs populate the more numerous 3 σ peaks instead. In the case of rapid binary coalescence a fraction of IMBHs will be displaced from galaxy centers and ejected into the intergalactic medium (IGM) by the "gravitational rocket" effect, rather than accrete and shine as miniquasars. We show that, under a number of plausible assumptions for the amount of gas accreted onto IMBHs and their emission spectrum, miniquasars powered by IMBHs, and not their stellar progenitors, may be responsible for cosmological reionization at z ~ 15. Reionization by miniquasars with a hard spectrum may be more "economical" than stellar reionization, as soft X-rays escape more easily from the dense sites of star formation and travel farther than EUV radiation. Energetic photons will permeate the universe more uniformly, make the low-density diffuse IGM warm and weakly ionized prior to the epoch of reionization breakthrough, set an entropy floor, and reduce gas clumping. Future 21 cm observations may detect a preheated, weakly ionized IGM in emission against the cosmic microwave background.

We present the results of a near-infrared imaging study of the host galaxies of 17 quasars in the redshift range 1 < z < 2. The observations were carried out at the ESO VLT UT1 8 m telescope under excellent seeing conditions (~04). The sample includes radio-loud (RLQs) and radio-quiet (RQQs) quasars with similar distribution of redshift and optical luminosity. For all the observed objects but one we have been able to derive the global properties of the surrounding nebulosity. The host galaxies of both types of quasars appear to follow the expected trend in luminosity of massive ellipticals undergoing simple passive evolution. However, we find a systematic difference by a factor ~2 in the host luminosity between RLQs and RQQs [_RLQ (host) = -27.55 ± 0.12 and _RLQ (host) = -26.83 ± 0.25]. Comparison with other samples of quasar hosts at similar and lower redshift indicates that the difference in the host luminosity between RLQs and RQQs remains the same from z = 2 to the present epoch. No significant correlation is found between the nuclear and the host luminosities. Assuming that the host luminosity is proportional to the black hole mass, as observed in nearby massive spheroids, these quasars emit at very different levels (spread ~1.5 dex) with respect to their Eddington luminosity and with the same distribution for RLQs and RQQs. Apart from a factor of ~2 difference in luminosity, the hosts of RLQs and RQQs of comparable nuclear luminosity appear to follow the same cosmic evolution as massive inactive spheroids. Taken together, our results support a view where nuclear activity can occur in all luminous ellipticals without producing a significant change in their global properties and evolution. Quasar hosts appear to be already well formed at z ~ 2, in disagreement with the predictions of models for the joint formation and evolution of galaxies and active nuclei based on the hierarchical structure formation scenario.

Gamma-ray bursts (GRBs) are promising tools for tracing the formation of high-redshift stars, including the first generation. At very high redshifts the reverse shock emission lasts longer in the observer frame, and its importance for detection and analysis purposes relative to the forward shock increases. We consider two different models for the GRB environment, based on current ideas about the redshift dependence of gas properties in galaxies and primordial star formation. We calculate the observed flux as a function of the redshift and observer time for typical GRB afterglows, taking into account intergalactic photoionization and Lyα absorption opacity, as well as extinction by the Milky Way. The fluxes in the X-ray and near-IR bands are compared with the sensitivity of different detectors such as Chandra, XMM, Swift XRT, and the James Webb Space Telescope (JWST). Using standard assumptions, we find that Chandra, XMM, and Swift XRT can potentially detect GRBs in the X-ray band out to very high redshifts z ≳ 30. In the K and M bands, the JWST and ground-based telescopes are potentially able to detect GRBs even 1 day after the trigger out to z ~ 16 and 33, if present. While the X-ray band is insensitive to the external density and to reverse shocks, the near-IR bands provide a sensitive tool for diagnosing both the environment and the reverse shock component.

The luminosity-size and mass-size distributions of galaxies out to z ~ 3 are presented. We use very deep near-infrared images of the Hubble Deep Field-South in the J_s, H, and K_s bands, taken as part of FIRES at the VLT, to follow the evolution of the optical rest-frame sizes of galaxies. For a total of 168 galaxies with K_s,AB ≤ 23.5, we find that the rest-frame V-band sizes r_e,V of luminous galaxies ( ~ 2 × 10¹⁰h^-2L_☉) at 2 < z < 3 are 3 times smaller than for equally luminous galaxies today. In contrast, the mass-size relation has evolved relatively little: the size at mass ~ 2 × 10¹⁰h^-2M_☉ has changed by 20% (±20%) since z ~ 2.5. Both results can be reconciled by the fact that the stellar M/L ratio is lower in the luminous high-z galaxies than in nearby ones because they have young stellar populations. The lower incidence of large galaxies at z ~ 3 seems to reflect the rarity of galaxies with high stellar mass.

The redshift interval 1.4 ≲ z ≲ 2.5 has been described by some as the "redshift desert" because of historical difficulties in spectroscopically identifying galaxies in that range. In fact, galaxies can be found in large numbers with standard broadband color selection techniques coupled with follow-up spectroscopy with UV and blue-sensitive spectrographs. In this paper we present the first results of a large-scale survey of such objects, carried out with the blue channel of the LRIS spectrograph (LRIS-B) on the Keck I Telescope. We introduce two samples of star-forming galaxies, "BX" galaxies at ⟨z⟩ = 2.20 ± 0.32 and "BM" galaxies at ⟨z⟩ = 1.70 ± 0.34. In seven survey fields we have spectroscopically confirmed 749 of the former and 114 of the latter. Interlopers (defined as objects at z < 1) account for less than 10% of the photometric candidates, and the fraction of faint active galactic nuclei is ~3% in the combined BX/BM sample. Deep near-IR photometry of a subset of the BX sample indicates that, compared with a sample of similarly UV-selected galaxies at z ~ 3, the z ~ 2 galaxies are on average significantly redder in (-K_s), indicating longer star formation histories, increased reddening by dust, or both. Using near-IR Hα spectra of a subset of BX/BM galaxies to define the galaxies' systemic redshifts, we show that the galactic-scale winds that are a feature of star-forming galaxies at z ~ 3 are also common at later epochs and have similar bulk outflow speeds of 200-300 km s^-1. We illustrate with examples the information that can be deduced on the stellar populations, metallicities, and kinematics of redshift desert galaxies from easily accessible rest-frame far-UV and rest-frame optical spectra. Far from being hostile to observations, the universe at z ~ 2 is uniquely suited to providing information on the astrophysics of star-forming galaxies and the intergalactic medium, and the relationship between the two.

The unresolved transition array (UTA) of iron M-shell ions is a prominent absorption feature in the X-ray spectrum of many active galactic nuclei (AGNs). Modeling photoionized plasmas in an attempt to match the observed silicon and oxygen lines fails to predict the level of ionization of iron as inferred by this feature. It is suggested that the discrepancy is due to underestimation of the low-temperature dielectronic recombination rates for iron M-shell ions. Modified ionization balance calculations, based on new (guessed) atomic data, support this idea. The results are shown and compared to the global properties of several observed UTAs. Implications for AGN absorbing gas are discussed, including an analysis of the ionization parameter distribution in such sources. The need for real calculations of such atomic data is stressed.

We examine the effects of low-temperature, or Δn = 0, dielectronic recombination (DR) on the ionization balance of the Fe M shell (Fe IX-Fe XVI). Since Δn = 0 rates are not available for these ions, we have derived estimates based on the existing rates for the first four ionization states of the CNO sequence and newly calculated rates for L-shell ions of third-row elements and Fe. For a range of ionization parameter and column density applicable to the intrinsic absorbers detected in ASCA, Chandra, and XMM-Newton observations of Seyfert galaxies, we generated two grids of photoionization models, with and without DR. The results show that the ionization parameter at which the population of an Fe M-shell ion peaks can increase in some cases by a factor of more than 2 when these rates are included. More importantly, there are dramatic changes in the range in ionization parameter over which individual M-shell ions contain significant fractions of the total Fe (e.g., >10%) in the plasma. These results may explain the mismatch between the range of Fe ionization states detected in the X-ray spectra of Seyfert galaxies, identified by the energies of the M-shell unresolved transition array, and those predicted by photoionization models of the X-ray absorbers that reproduce lines of second- and third-row elements. The results suggest that care should be taken in using third- and fourth-row ions to constrain the physical conditions in photoionized X-ray plasmas until accurate DR rates are available. This underscores the importance of atomic physics in interpreting astronomical spectroscopy.

We present wavelength measurements of K-shell resonance lines of O V and O VI using the University of California Lawrence Livermore National Laboratory EBIT-I electron beam ion trap. The wavelength accuracy of better than 140 parts per million is sufficient to determine gas outflow velocities of warm absorbers associated with active galactic nuclei to within 40 km s^-1 and better. Our measurements confirm that the outflow velocities associated with NGC 5548 and derived from the O V and O VI lines are similar to those derived from the O VII lines. These kinematic measurements make for further evidence that the X-ray and UV absorbers in these systems are truly two manifestations of the same physical outflow.

The Infrared Space Observatory (ISO) was used to search for a tracer of the warm and dense neutral interstellar medium, the [O I] 63.18 μm line, in four ultraluminous IRAS sources lying at redshifts between 0.6 and 1.4. While these sources are quasars, their infrared continuum emission suggests a substantial interstellar medium. No [O I] flux was detected after probing down to a 3 σ sensitivity level sufficient for detecting line emission in starbursts with similar continuum emission. However, if the detection threshold is slightly relaxed, one target is detected with 2.7 σ significance. For this radio-quiet quasar, there is likely a substantial dense and warm interstellar medium; the upper limits for the three radio-loud sources do not preclude the same conclusion. Using a new, uniformly processed database of the ISO extragalactic far-infrared spectroscopy observations, we show that nearby Seyfert galaxies typically have higher [O I]-to-far-infrared ratios than do normal star-forming galaxies, so the lack of strong [O I] 63 μm emission from these high-redshift ultraluminous sources cannot be attributed to their active cores.

Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT All-Sky Survey in several galaxies with no evidence of Seyfert activity in their ground-based optical spectra. These flares had the properties predicted for a tidal disruption of a star by a central supermassive black hole. We report Chandra observations of three of these galaxies taken a decade after their flares that reveal weak nuclear X-ray sources that are from 240 to 6000 times fainter than their luminosities at peak, supporting the theory that these were special events and not ongoing active galactic nucleus (AGN) variability. The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the (t - t_D)^-5/3 decay predicted for the fallback phase of a tidal disruption event, but only if ROSAT was lucky enough to catch the event exactly at its peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also consistent with (t - t_D)^-5/3. In the H II galaxy NGC 5905 we find only resolved, soft X-ray emission that is undoubtedly associated with starburst activity. When accounting for the starburst component, the ROSAT observations of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are consistent with a (t - t_D)^-5/3 decay by at least a factor of 1000. Although we found weak Seyfert 2 emission lines in Hubble Space Telescope spectra of NGC 5905, indicating that a low-luminosity AGN was present prior to the X-ray flare, we favor a tidal disruption explanation for the flare itself.

It is frequently debated in literature whether a "standard" initial mass function (IMF)—meaning an IMF of the kind usually adopted to explain the chemical evolution in the local solar neighborhood—can account for the observed metal enrichment and iron mass-to-light ratio in clusters of galaxies. We address this problem by means of straightforward estimates that should hold independently of the details of chemical evolution models. It is crucial to compute self-consistently the amount of mass and metals locked-up in stars by accounting for the stellar mass-to-light ratio predicted by a given IMF. It then becomes clear that a "standard" solar neighborhood IMF cannot provide enough metals to account for the observed chemical properties in clusters: clusters of galaxies and the local environment must be characterized by different IMFs. Alternatively, if we require the IMF to be universal, in order to explain clusters such an IMF must be much more efficient in metal production than usually estimated for the solar vicinity. In this case, substantial loss of metals is required from the solar neighborhood and from disk galaxies in general. This "nonstandard" scenario of the local chemical evolution would challenge our present understanding of the Milky Way and of disk galaxy formation.

We present a weak-lensing mass reconstruction of the interacting cluster 1E 0657-558, in which we detect both the main cluster and a subcluster. The subcluster is identified as a smaller cluster that has just undergone initial infall and pass-through of the primary cluster and has been previously identified in both optical surveys and X-ray studies. The X-ray gas has been separated from the galaxies by ram pressure-stripping during the pass-through. The detected mass peak is located between the X-ray peak and galaxy concentration, although the position is consistent with the galaxy centroid within the errors of the mass reconstruction. We find that the mass peak for the main cluster is in good spatial agreement with the cluster galaxies and is offset from the X-ray halo at 3.4 σ significance, and we determine that the mass-to-light ratios of the two components are consistent with those of relaxed clusters. The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster. This proof of dark matter existence holds true even under the assumption of modified Newtonian dynamics (MOND); based on the observed gravitational shear-optical light ratios and the mass peak-X-ray gas offsets, the dark matter component in a MOND regime would have a total mass that is at least equal to the baryonic mass of the system.

We present an analysis of a Chandra observation of the massive, nearby galaxy cluster A2319. A sharp surface brightness discontinuity—suggested by previous, lower angular resolution X-ray imaging—is clearly visible in the ACIS image. This ~300 kpc feature suggests that a major merger is taking place with a significant velocity component perpendicular to the line of sight. The cluster emission-weighted mean temperature is 11.8 ± 0.6 keV, somewhat higher than previous temperature measurements. The Chandra temperature map of A2319 reveals substructure resembling that anticipated based on hydrodynamic simulations of cluster mergers and shows an associated cool core not previously known. The map shows a separation between the intracluster medium (ICM) and galaxies of one subcluster, indicating a transient state in which the ICM has been stripped from the subcluster galaxies (and presumably the dark matter). Detailed analysis of the merger feature shows a pressure change across the surface brightness discontinuity by a factor of ≲2.5. The higher density side of the front has a lower temperature, suggesting the presence of a cold front similar to those in many other merging clusters. The velocity of the front is roughly sonic. We compare bulk properties of the ICM and galaxies in A2319 to the same properties in a large sample of clusters as a way of gauging the effects of the major merger. Interestingly, by comparing A2319 to a sample of 44 clusters studied with the ROSAT PSPC, we find that the X-ray luminosity, isophotal size, and ICM mass are consistent with the expected values for a cluster of its temperature; in addition, the K-band galaxy light is consistent with the light-temperature scaling relation derived from a sample of ~100 clusters studied with the Two Micron All Sky Survey. Together, these results indicate either that the merger in A2319 has not been effective at altering the bulk properties of the cluster or that there are large but correlated displacements in luminosity, isophotal size, ICM mass, galaxy light, and emission-weighted mean temperature in this cluster.

More than two-thirds of disk galaxies are barred to some degree. Many today harbor massive concentrations of gas in their centers, and some are known to possess supermassive black holes (SMBHs) and their associated stellar cusps. Previous theoretical work has suggested that a bar in a galaxy could be dissolved by the formation of a mass concentration in the center, although the precise mass and degree of central concentration required is not well established. We report an extensive study of the effects of central masses on bars in high-quality N-body simulations of galaxies. We have varied the growth rate of the central mass, its final mass, and its degree of concentration to examine how these factors affect the evolution of the bar. Our main conclusions are the following: (1) Bars are more robust than previously thought. The central mass has to be as large as several percent of the disk mass to completely destroy the bar on a short timescale. (2) For a given mass, dense objects cause the greatest reduction in bar amplitude, while significantly more diffuse objects have a lesser effect. (3) The bar amplitude always decreases as the central mass is grown and continues to decay thereafter on a cosmological timescale. (4) The first phase of bar weakening is due to the destruction by the central mass concentration (CMC) of lower energy, bar-supporting orbits, while the second phase is a consequence of secular changes to the global potential that further diminish the number of bar-supporting orbits. We provide detailed phase-space and orbit analysis to support this suggestion. Thus, current masses of SMBHs are probably too small, even when dressed with a stellar cusp, to affect the bars in their host galaxies. The molecular gas concentrations found in some barred galaxies are also too diffuse to affect the amplitude of the bar significantly. These findings reconcile the apparent high percentage of barred galaxies with the presence of CMCs and have important implications for the formation and survival of bars in such galaxies.

We study the early dynamical evolution of young dense star clusters by using Monte Carlo simulations for systems with up to N = 10⁷ stars. Rapid mass segregation of massive main-sequence stars and the development of the Spitzer instability can drive these systems to core collapse in a small fraction of the initial half-mass relaxation time. If the core-collapse time is less than the lifetime of the massive stars, all stars in the collapsing core may then undergo a runaway collision process leading to the formation of a massive black hole. Here we study in detail the first step in this process, up to the occurrence of core collapse. We have performed about 100 simulations for clusters with a wide variety of initial conditions, varying systematically the cluster density profile, stellar initial mass function (IMF), and number of stars. We also considered the effects of initial mass segregation and stellar evolution mass loss. Our results show that, for clusters with a moderate initial central concentration and any realistic IMF, the ratio of core-collapse time to initial half-mass relaxation time is typically ~0.1, in agreement with the value previously found by direct N-body simulations for much smaller systems. Models with even higher central concentration initially, or with initial mass segregation (from star formation) have even shorter core collapse times. Remarkably, we find that, for all realistic initial conditions, the mass of the collapsing core is always close to ~10^-3 of the total cluster mass, very similar to the observed correlation between central black hole mass and total cluster mass in a variety of environments. We discuss the implications of our results for the formation of intermediate-mass black holes in globular clusters and super star clusters, ultraluminous X-ray sources, and seed black holes in proto-galactic nuclei.

We present the results from a ~53 ks XMM-Newton observation of NGC 2276. This galaxy has an unusual optical morphology with the disk of this spiral appearing to be truncated along the western edge. This XMM-Newton observation shows that the X-ray source at the western edge is a bright intermediate X-ray object (IXO). Its spectrum is well fitted by a multicolor disk blackbody model used to fit optically thick standard accretion disks around black holes. The luminosity derived for this IXO is 1.1 × 10⁴¹ ergs s^-1 in the 0.5-10 keV band, making it one of the most luminous discovered to date. The large source luminosity implies a large-mass black hole if the source is radiating at the Eddington rate. On the other hand, the inner-disk temperature determined here is too high for such a massive object given the standard accretion disk model. In addition to the IXO, we find that the nuclear source in this galaxy has dimmed by at least a factor of several thousand in the 8 years since the ROSAT HRI observations.

Recent observations with the Chandra X-Ray Observatory have provided us with the capability to discriminate point sources, such as the supermassive black hole Sgr A*, from the diffuse emission within the inner 10'' of the Galaxy. The hot plasma producing the diffuse X-radiation, estimated at ≈7.6 × 10³¹ ergs s^-1 arcsec^-2 in the 2-10 keV band, has an rms electron density ≈26 cm^-3 and a temperature kT ≈ 1.3 keV, with a total inferred mass of ≈0.1 M_☉. At least some of this gas must be injected into the interstellar medium via stellar winds. In the most recent census, about 25 bright, young stars have been identified as the dominant sources of the overall mass efflux from the Galactic center. In this paper we use detailed three-dimensional smoothed particle hydrodynamics (SPH) simulations to study the wind-wind interactions occurring in the inner 3 pc of the Galaxy, with a goal of understanding what fraction, if any, of the diffuse X-ray flux measured by Chandra results from the ensuing shock heating of the ambient medium. We conclude that this process alone can account for the entire X-ray flux observed by Chandra in the inner 10'' of the Galaxy. Understanding the X-ray morphology of the environment surrounding Sgr A* will ultimately provide us with a greater precision in modeling the accretion of gas onto this object, which appears to be relatively underluminous compared to its brethren in the nuclei of other galaxies.

Cosmic rays scatter off magnetic irregularities (Alfvén waves) with which they are resonant, that is, waves of wavelength comparable to their gyroradii. These waves may be generated either by the cosmic rays themselves, if they stream faster than the Alfvén speed, or by sources of MHD turbulence. Waves excited by streaming cosmic rays are ideally shaped for scattering, whereas the scattering efficiency of MHD turbulence is severely diminished by its anisotropy. We show that MHD turbulence has an indirect effect on cosmic-ray propagation by acting as a damping mechanism for cosmic-ray-generated waves. The hot ("coronal") phase of the interstellar medium is the best candidate location for cosmic-ray confinement by scattering from self-generated waves. We relate the streaming velocity of cosmic rays to the rate of turbulent dissipation in this medium for the case in which turbulent damping is the dominant damping mechanism. We conclude that cosmic rays with up to 10² GeV could not stream much faster than the Alfvén speed but 10⁶ GeV cosmic rays would stream unimpeded by self-generated waves, unless the coronal gas were remarkably turbulence-free.

Recently, a nonlinear theory for perpendicular diffusion of charged particles was presented. This theory is called the nonlinear guiding center theory and provides an integral equation for the perpendicular mean free path. In this paper we consider analytical solutions of this equation in the case of magnetostatic turbulence. The resulting formulas for the perpendicular mean free path are discussed. We also compare these new results with results of the quasi-linear theory for parallel diffusion and with observational results.

We present the results of an analysis of the large-scale anisotropy of cosmic rays in the PeV range. The Rayleigh formalism is applied to the right ascension distribution of extensive air showers measured by the KASCADE (Karlsruhe Shower Core and Array Detector) experiment. The data set contains about 10⁸ extensive air showers in the energy range 0.7-6 PeV. No hints of anisotropy are visible in the right ascension distributions in this energy range. This accounts for all showers, as well as for subsets containing showers induced by predominantly light or heavy primary particles, respectively. Upper flux limits for Rayleigh amplitudes are determined to be between 10^-3 at a primary energy of 0.7 PeV and 10^-2 at 6 PeV.

I report here on the analysis and interpretation of a Chandra observation of the supernova remnant Kes 32. Kes 32 is rather weak in X-rays because of a large interstellar absorption, which is found to be ~4 × 10²² cm^-2, larger than previously reported. Spectral analysis indicates that the ionization age of this object is very young, with n_et ~ 4 × 10⁹ cm^-3 s and a temperature of kT_e ~ 1 keV. The X-ray emission peaks at a smaller radius than in the radio. The low ionization age suggests that Kes 32 is a young remnant. However, a young age is in contradiction to the relatively large apparent size, which indicates an age of several thousand years instead of a few hundred years. This problem is discussed in connection with Kes 32's unknown distance and its possible association with the Norma Galactic arm.

We analyze the consequences of the local interstellar magnetic field being almost 3 times larger than the Galactic average (ordered) field on the structure of the heliospheric interface in the axisymmetric case when the field is parallel to the relative direction of motion between the local interstellar medium (LISM) and the Sun. A field of such strength is expected to exist in the Local Interstellar Cloud, if the latter condensed from material inside a magnetic flux tube rebounding from the wall of the Local Bubble cavity. The analysis is performed using a newly developed multifluid neutral MHD model. We show that the bow shock ahead of the heliopause still exists for supersonic and sub-Alfvénic LISM parameters. Our results agree well with the observations of the Lyα absorption spectra and yield positions of the termination shock and the heliopause similar to those obtained from the standard super-Alfvénic model.

We consider astrophysical maser radiation that is created in the presence of mildly supersonic, magnetohydrodynamic (MHD) turbulence. The focus is on the OH masers for which the magnetic field is strong enough that the separations of the Zeeman components are greater than the spectral line breadths. A long-standing puzzle has been the absence of the Zeeman π-components and the high circular polarization in the observed spectra of these masers. We first argue that the elongation of eddies along the field that has recently been recognized in MHD turbulence will enhance the optical depth parallel to the magnetic field in comparison with that perpendicular to the magnetic field. We then simulate maser emission with a numerical model of MHD turbulence to demonstrate quantitatively how the intensities of the linearly polarized π-components are suppressed and the intensities of the nearly circularly polarized σ-components are enhanced. This effect is also generic in the sense that most spectral lines in MHD turbulence with Mach number M ~ 1 should have larger optical depth parallel to the magnetic field than perpendicular. The effect is reduced considerably when M < 1. The simulations also demonstrate that the velocity and magnetic field variations due to the turbulence can (but do not necessarily) cause one of the σ-components to be much more intense than the other, as is often observed for mainline OH masers.

We use extensive 350 μm polarimetry and continuum maps obtained with the Hertz polarimeter and SHARC II (Submillimeter High Angular Resolution Camera II) along with HCN and HCO⁺ spectroscopic data to trace the orientation of the magnetic field in the Orion A star-forming region. Using the polarimetry data, we find that the direction of the projection of the magnetic field in the plane of the sky relative to the orientation of the integral-shaped filament varies considerably as one moves from north to south. While in IRAS 05327-0457 and OMC-3 MMS 1-6 the projection of the field is primarily perpendicular to the filament, it becomes better aligned with it at OMC-3 MMS 8-9, and well aligned with it at OMC-2 FIR 6. The OMC-2 FIR 4 cloud, located between the last two clouds, is a peculiar object in which we find almost no polarization. There is a relatively sharp boundary within its core where two adjacent regions exhibiting differing polarization angles merge. The projected angle of the field is more complicated in OMC-1, where it exhibits smooth variations in its orientation across the face of this massive complex. We also note that while the relative orientation of the projected angle of the magnetic field to the filament varies significantly in the OMC-3 and OMC-2 regions, its orientation relative to a fixed position on the sky shows far greater stability. This suggests that the orientation of the field is perhaps relatively unaffected by the mass condensation present in these parts of the molecular cloud. By combining the polarimetry and spectroscopic data, we were able to measure a set of average values for the inclination angle of the magnetic field relative to the line of sight. We find that the field is oriented quite close to the plane of the sky in most places. More precisely, the inclination of the magnetic field is ≈73° around OMC-3 MMS 6, ≈74° at OMC-3 MMS 8-9, ≈80° at OMC-2 FIR 4, ≈65° in the northeastern part of OMC-1, and ≈49° in the Orion bar. The small difference in the inclination of the field between OMC-3 and OMC-2 seems to strengthen the idea that the orientation of the magnetic field is relatively unaffected by the agglomeration of matter located in these regions. We also present polarimetry data for the OMC-4 region located some 13' south of OMC-1.

We have assembled a database of stars having both masses determined from measured orbital dynamics and sufficient spectral and photometric information for their placement on a theoretical H-R diagram. Our sample consists of 115 low-mass (M < 2.0 M_☉) stars, 27 pre-main-sequence and 88 main-sequence. We use a variety of available pre-main-sequence evolutionary calculations to test the consistency of predicted stellar masses with dynamically determined masses. Despite substantial improvements in model physics over the past decade, large systematic discrepancies still exist between empirical and theoretically derived masses. For main-sequence stars, all models considered predict masses consistent with dynamical values above 1.2 M_☉ and some models predict consistent masses at solar or slightly lower masses, but no models predict consistent masses below 0.5 M_☉, with all models systematically underpredicting such low masses by 5%-20%. The failure at low masses stems from the poor match of most models to the empirical main sequence below temperatures of 3800 K, at which molecules become the dominant source of opacity and convection is the dominant mode of energy transport. For the pre-main-sequence sample we find similar trends. There is generally good agreement between predicted and dynamical masses above 1.2 M_☉ for all models. Below 1.2 M_☉ and down to 0.3 M_☉ (the lowest mass testable), most evolutionary models systematically underpredict the dynamically determined masses by 10%-30%, on average, with the Lyon group models predicting marginally consistent masses in the mean, although with large scatter. Over all mass ranges, the usefulness of dynamical mass constraints for pre-main-sequence stars is in many cases limited by the random errors caused by poorly determined luminosities and especially temperatures of young stars. Adopting a warmer-than-dwarf temperature scale would help reconcile the systematic pre-main-sequence offset at the lowest masses, but the case for this is not compelling, given the similar warm offset at older ages between most sets of tracks and the empirical main sequence. Over all age ranges, the systematic discrepancies between track-predicted and dynamically determined masses appear to be dominated by inaccuracies in the treatment of convection and in the adopted opacities.

We report findings from the first set of data in a current survey to establish conclusively whether jets from young stars rotate. We observed the bipolar jets from the T Tauri stars TH 28 and RW Aur and the blueshifted jet from T Tauri star LkHα 321, using the Hubble Space Telescope Imaging Spectrograph. Forbidden emission lines show distinct and systematic velocity asymmetries of 10-25 (±5) km s^-1 at a distance of 03 from the source, representing a (projected) distance of ≈40 AU along the jet in the case of RW Aur, ≈50 AU for TH 28, and 165 AU in the case of LkHα 321. These velocity asymmetries are interpreted as rotation in the initial portion of the jet where it is accelerated and collimated. For the bipolar jets, both lobes appear to rotate in the same direction. Values obtained were in agreement with the predictions of MHD disk-wind models. Finally, we determine, from derived toroidal and poloidal velocities, values for the distance from the central axis of the footpoint for the jet's low-velocity component of ≈0.5-2 AU, consistent with the models of magnetocentrifugal launching.

The inner region of the accretion disk around a magnetized star (T Tauri star, white dwarf, or neutron star) is subjected to magnetic torques that induce warping and precession of the disk. These torques arise from the interaction between the stellar field and the induced electric currents in the disk. We carry out numerical simulations of the nonlinear evolution of warped, viscous accretion disks driven by the magnetic torques. We show that the disk can develop into a highly warped steady state in which the disk attains a fixed (warped) shape and precesses rigidly. The warp is most pronounced at the disk inner radius (near the magnetosphere boundary). As the system parameters (such as accretion rate) change, the disk can switch between a completely flat state (warping-stable) and a highly warped state. The precession of warped disks may be responsible for a variety of quasi-periodic oscillations or radiation flux variabilities observed in many different systems, including young stellar objects and X-ray binaries.

We present results of a pulsar population synthesis study that incorporates a number of recent developments and some significant improvements over our previous study. We have included the results of the Parkes multibeam pulsar survey in our select group of nine radio surveys, doubling our sample of radio pulsars. More realistic geometries for the radio and γ-ray beams are included in our Monte Carlo computer code, which simulates the characteristics of the Galactic population of radio and γ-ray pulsars. We adopted with some modifications the radio-beam geometry of Arzoumanian, Chernoff, and Cordes. For the γ-ray beam, we have assumed the slot gap geometry described in the work of Muslimov and Harding. To account for the shape of the distribution of radio pulsars in the -P diagram, we continue to find that decay of the magnetic field on a timescale of 2.8 Myr is needed. With all nine surveys, our model predicts that EGRET should have seen seven radio-quiet (below the sensitivity of these radio surveys) and 19 radio-loud γ-ray pulsars. AGILE (nominal sensitivity map) is expected to detect 13 radio-quiet and 37 radio-loud γ-ray pulsars, while GLAST, with greater sensitivity, is expected to detect 276 radio-quiet and 344 radio-loud γ-ray pulsars. When the Parkes multibeam pulsar survey is excluded, the ratio of radio-loud to radio-quiet γ-ray pulsars decreases, especially for GLAST. The decrease for EGRET is 45%, implying that some fraction of EGRET unidentified sources are radio-loud γ-ray pulsars. In the radio geometry adopted, short-period pulsars are core dominated. Unlike the EGRET γ-ray pulsars, our model predicts that when two γ-ray peaks appear in the pulse profile, a dominant radio core peak appears in between the γ-ray peaks. Our findings suggest that further improvements are required in describing both the radio and γ-ray geometries.

IRAS 16279-4757 belongs to a group of post-asymptotic giant branch (post-AGB) stars showing both polycyclic aromatic hydrocarbon (PAH) bands and crystalline silicates. We present mid-infrared images that resolve the object for the first time. The morphology is similar to that of the "Red Rectangle" (HD 44179), the prototype object with PAHs and crystalline silicates. A two-component model and images suggest a dense oxygen-rich torus, an inner, low-density, carbon-rich region, and a carbon-rich bipolar outflow. The PAH bands are enhanced at the outflow, while the continuum emission is concentrated toward the center. Our findings support the suggestion that mixed chemistry and morphology are closely related. We discuss the Infrared Space Observatory Short-Wavelength Spectrometer (ISO SWS) spectra of IRAS 16279-4757. Several bands in the ISO SWS spectrum show a match with anorthite: this would be the first detection of this mineral outside the solar system. Compared to HD 44179, the shapes of PAH bands are closer to those of planetary nebulae, possibly related to a population of small PAHs present in HD 44179 but absent around IRAS 16279-4757. Detailed examination of the spectra shows the individual character of these two objects. The comparison suggests that the torus found in IRAS 16279-4757 may have formed more recently than that in HD 44179.

Approximately 25%-30% of pulsating asymptotic giant branch (AGB) stars show long secondary periods (LSPs) of typical length ~400-1500 days, roughly 10 times longer than the primary period of pulsation. Here we seek an explanation for the LSPs. We describe the spectral variations over a 4 yr interval for three of these stars. The radial velocity is found to vary during the LSP with full amplitude of ~5 km s^-1, a result similar to that found by Hinkle and coworkers for six other variables of this type. Variations in the Hα and Na D line profiles throughout the LSP suggest that chromospheric activity and mass loss vary with the period of the LSP. Possible explanations for the photometric and radial velocity variations include eccentric motion of an orbiting companion of mass ~0.1 M_☉, radial and nonradial pulsation, rotation of an ellipsoidal-shaped red giant, episodic dust ejection, and star spot cycles. We discuss each of these models and show that they all have problems. The most likely explanation is that the LSPs result from a low degree g⁺ mode confined to the outer radiative layers of the red giant, combined with large-scale star spot activity to give the observed chromosphere and the irregularity of the light curve. We suggest that these stars may be the precursors of asymmetric planetary nebulae.

The present-day formation of cataclysmic variables (CVs) with brown dwarf (BD) secondaries (0.013 M_☉ ≤ M_s ≤ 0.075 M_☉) is investigated using a population synthesis technique. Results from the latest, detailed models for BDs have been incorporated into the population synthesis code. The present-day orbital period distribution of zero-age CVs (ZACVs) that form with BD secondaries is calculated. For our models, we find that ZACVs with BD secondaries have orbital periods in the range 46 minutes to 2.5 hr. We also find that ZACVs with BD secondaries comprise 18% of the total, present-day ZACV population. In addition, we find that 80% of ZACVs with BD secondaries have orbital periods shorter than 78 minutes. This implies that 15% of the present-day ZACV population should have orbital periods shorter than the observed orbital period minimum for CVs. We also investigate the dependence of the present-day formation rate of CVs with BD secondaries on the assumed value of the common envelope efficiency parameter, α_CE, for three different assumed mass ratio distributions in zero-age main-sequence (ZAMS) binaries. Surprisingly, we find that the common envelope process must be extremely inefficient (α_CE < 0.1) in order for CVs with BD secondaries not to be formed. Finally, we find that the progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations of smaller than 3 AU and ZAMS primary masses between ~1 and 10 M_☉, with ~75% of the primary masses of smaller than ~1.6 M_☉. Interestingly, these ranges in orbital separation and primary mass place the majority of the progenitor binaries within the so-called brown dwarf desert. The implications of these results are discussed in the context of V485 Cen and 1RXS J232953.9+062814, the only known CVs with orbital periods shorter than the period minimum, CVs above the period minimum that are strongly suspected of containing BD secondaries (such as WZ Sge), common envelope evolution involving very low mass secondaries, and the BD mass function in ZAMS binaries.

We examine recent claims that the T-type brown dwarf S Ori 053810.1-203626 (S Ori 70) is a spectroscopically verified low-mass (3M_Jup) member of the 1-8 Myr σ Orionis cluster. Comparative arguments by Martín & Zapatero Osorio asserting that S Ori 70 exhibits low surface gravity spectral features indicative of youth and low mass are invalidated by the fact that their comparison object was not the field T dwarf 2MASS 0559-1404, but rather a nearby background star. Instead, we find that the 1-2.5 μm spectra of S Ori 70 are well matched to older (age ~ few Gyr) field T6-T7 dwarfs. Moreover, we find that spectral model fits to late-type field T dwarf spectra tend to yield low surface gravities (log g = 3.0-3.5), and thus young ages (≲5 Myr) and low masses (≲3 M_Jup), inconsistent with expected and/or empirical values. Finally, we show that the identification of one T dwarf in the field imaged by Zapatero Osorio et al. is statistically consistent with the expected foreground contamination. Based on the reexamined evidence, we conclude that S Ori 70 may simply be an old, massive (30-60 M_Jup) field brown dwarf lying in the foreground of the σ Orionis cluster. This interpretation should be considered before presuming the existence of so-called "cluster planets."

We present a new system of narrowband filters in the near-infrared that can be used to classify stars and brown dwarfs. This set of four filters, spanning the H band, can be used to identify molecular features unique to brown dwarfs, such as H₂O and CH₄. The four filters are centered at 1.495 μm (H₂O), 1.595 μm (continuum), 1.66 μm (CH₄), and 1.75 μm (H₂O). Using two H₂O filters allows us to solve for the reddening of individual objects. This can be accomplished by constructing a color-color-color cube and rotating it until the reddening vector disappears. We created a model of predicted color-color-color values for different spectral types by integrating filter bandpass data with the spectra of known stars and brown dwarfs. We validated this model by making photometric measurements of seven known L and T dwarfs, ranging from L1 to T7.5. The photometric measurements agree with the model to within ±0.1 mag, allowing us to create spectral indexes for different spectral types. We can classify A through early M stars to within ±2 spectral types, late-type M and L dwarfs to within ±0.3 spectral types, and T dwarfs to within ±0.1 spectral types (1 σ). Thus, we can distinguish between a T1 and a T3 dwarf. The four physical bands can be converted into two reddening-free indexes μ₁ and μ₂ and an extinction A_V for the individual objects. This technique, which is equivalent to extremely low resolution spectroscopy, can be used to survey large areas to provide rough spectral classifications for all the stars in the area, ranging down to the coolest brown dwarfs. It should prove particularly useful in young clusters where reddening can be high.

We have developed fluid transport equations for fully ionized gases that improve the description of Coulomb collisions. The aim has been to develop simple and versatile equations that can easily be implemented in numerical models and thus be applied to a large variety of space plasmas, while they still accurately describe thermal forces and energy flows in collision-dominated plasmas. Based on exact solutions to the Boltzmann equation in the collision-dominated limit, the correction term to the velocity distribution function that account for particle flows is assumed to be proportional to the third power of the velocity, leading to a near isotropic core distribution. Applying the fluid equations derived from this new velocity distribution to a collision-dominated electron-proton plasma with a small temperature gradient, the resulting electron heat flux, as well as the thermal force between electrons and protons, deviate less than 25% from the exact results of classical transport theory. The new equations predict a factor of 4 reduction in the thermal force acting on heavy, minor ions caused by an imposed heat flux, compared with fluid equations that are in common use today. The improved description of thermal forces is expected to be important for modeling the composition of stellar atmospheres.

Improved energy levels and hyperfine structure constants for a selected set of Ho II levels were measured from spectra recorded using the 1 m Fourier transform spectrometer (FTS) at the US National Solar Observatory. Branching fractions for the strong blue-UV lines from these levels were also measured from the FTS data and combined with earlier radiative lifetimes from time-resolved laser-induced fluorescence measurements to determine accurate absolute transition probabilities for 22 lines of Ho II. These new laboratory measurements, along with a recently reported partition function for ionized Ho, were used to improve Ho abundance determinations in the Sun and three metal-poor Galactic halo giant stars. The derived solar photospheric holmium abundance, (Ho) = +0.51 ± 0.10, is consistent with its meteoritic value, (Ho) = +0.49 ± 0.02. In each of the metal-poor, neutron-capture-rich program stars, the holmium abundance relative to those of other rare earth elements agrees well with solar system rapid-neutron-capture abundance values.

The parallel mean free path of cosmic-ray particles in partially turbulent electromagnetic fields is calculated in the damping model of dynamical turbulence for pure two-dimensional turbulence geometry. Using the general results for the pitch-angle Fokker-Planck coefficient for pure slab geometry from Teufel & Schlickeiser, the parallel mean free path for composite slab/two-dimensional geometry is also calculated. The rigidity dependence and the absolute value of the mean free path for this general turbulence geometry are compared with observational results. We demonstrate that the composite model is able to explain observational results.

We explore a mechanism, entirely new to the fast solar wind, of electron heating by lower hybrid waves to explain the shift to higher charge states observed in various elements in the fast wind at 1 AU relative to the original coronal hole plasma. This process is a variation on that previously discussed for two-temperature accretion flows by Begelman & Chiueh. Lower hybrid waves are generated by gyrating minor ions (mainly α-particles) and become significant once strong ion cyclotron heating sets in beyond 1.5 R_☉. In this way the model avoids conflict with SUMER electron temperature diagnostic measurements between 1 and 1.5 R_☉. The principal requirement for such a process to work is the existence of density gradients in the fast solar wind, with scale length of similar order to the proton inertial length. Similar size structures have previously been inferred by other authors from radio scintillation observations and considerations of ion cyclotron wave generation by global resonant MHD waves.

The acceleration of electrons and protons caused by a super-Dreicer electric field directed along the longitudinal component B_y of the magnetic field is investigated. The three-component magnetic field in a nonneutral current sheet occurring at the top of the reconnecting flaring loops on the charged particle trajectories and energies is considered. Particle trajectories in the reconnecting current sheet (RCS) and their energy spectra at the point of ejection from the RCS are simulated from the motion equation for different sheet thicknesses. A super-Dreicer electric field of the current sheet is found to accelerate particles to coherent energy spectra in a range of 10-100 keV for electrons and 100-400 keV for protons with energy slightly increasing with the sheet thickness. A longitudinal B_y component was found to define the gyration directions of particles with opposite charges toward the RCS midplane, i.e., the trajectory symmetry. For the ratio B_y/B_z < 10^-6 the trajectories are fully symmetric, which results in particle ejection from an RCS as neutral beams. For the ratio B_y/B_z > 10^-2 the trajectories completely lose their symmetry toward the RCS midplane, leading to the separation of particles with opposite charges into the opposite halves from an RCS midplane and the following ejection into different legs of the reconnecting loops. For the intermediate values of B_y/B_z the trajectories are partially symmetric toward the midplane, leading to electrons prevailing in one leg and protons in the other.

We treat in detail positron production from the decay of radioactive nuclei produced in nuclear reactions of accelerated ³He. Because of their large cross sections and low threshold energies, these reactions can significantly contribute to positron production in solar flares with accelerated-particle compositions enriched in ³He. The addition of these ³He reactions extends earlier calculations of positron production by accelerated protons and α-particles. ³He reactions not only add significantly to the total positron yield in flares, but can also yield a positron depth distribution that peaks higher in the solar atmosphere. We discuss the impact these reactions have on the analysis of the annihilation line observed with RHESSI from the 2002 July 23 flare. A significant contribution from ³He reactions expands the utility of the annihilation line as a sensitive tool for investigating the structure of the flaring solar atmosphere.

An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well-observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field E_rec inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred E_rec is ~5.8 V cm^-1 and the peak mass acceleration is ~3 km s^-2, while for the M1.0 flare event E_rec is ~0.5 V cm^-1 and the peak mass acceleration is 0.2-0.4 km s^-2.

We present diffraction-limited observations of magnetic flux concentrations and penumbral and umbral fine structure within an active region observed at disk center. We recorded G-band images, magnetograms, Dopplergrams, and narrowband filtergrams, using the Universal Birefringent Filter (UBF) at the Dunn Solar Telescope (DST). The National Solar Observatory (NSO) adaptive optics system at the DST was used to achieve diffraction-limited long-exposure imaging with a high signal-to-noise ratio. The main results can be summarized as follows: Strong and spatially narrow downflows are observed at the edge of magnetic structures, such as small flux concentrations (sometimes also referred to as flux tubes), pores, a light bridge, and the sunspot umbrae. For the particular sunspot observed, we find strong evidence for what appear to be vigorous, small-scale convection patterns in a light bridge. We observe extremely narrow (<02) channels or sheets of downflowing plasma. Flux concentrations as seen in intensity expand from a height close to where the continuum is formed to the height of formation for the G band. These observations indicate that the G band forms in the mid-photosphere. We are able to identify individual penumbral fibrils in our data and find a bright (hot) upflow and a more vertical field structure at the filament footpoint near the umbral boundary. The observations are consistent with a filament geometry in which the field and flow turn to a nearly horizontal, dark structure over a distance of about 02. In the deep photosphere we observe strong upflows of the order of 1 km s^-1 in umbral dots. We compare our results with theoretical model predictions.

We use wavelet transforms to study the characteristic time scales of chromospheric oscillation "wave packets" that are observed in Transition Region and Coronal Explorer (TRACE) ultraviolet continuum image time series. Using several data sets, we investigate the statistical, spatial, and temporal intermittence of the number, duration, mean frequency, and delay ("wait time") between wave packets in the time series data. Further, we demonstrate that these characteristic values are consistent with newly developed pictures of the wave-mode suppression and conversion by the chromospheric magnetic "canopy." We propose that wavelet analysis may be fruitfully used in diagnosing the structure of the chromosphere and in identifying chromospheric oscillation wave packets temporally and spatially with their photospheric sources.

High-cadence multiwavelength optical observations were taken with the Dunn Solar Telescope at the National Solar Observatory, Sacramento Peak, accompanied by Advanced Stokes Polarimeter vector magnetograms. A total of 11 network bright points (NBPs) have been studied at different atmospheric heights using images taken in wave bands centered on Mg I b₁ - 0.4 Å, Hα, and Ca II K₃. Wavelet analysis was used to study wave packets and identify traveling magnetohydrodynamic waves. Wave speeds were estimated through the temporal cross-correlation of signals, in selected frequency bands of wavelet power, in each wavelength. Four mode-coupling cases were identified, one in each of four of the NBPs, and the variation of the associated Fourier power with height was studied. Three of the detected mode-coupling, transverse-mode frequencies were observed in the 1.2-1.6 mHz range (mean NBP apparent flux density magnitudes over 99-111 Mx cm^-2), with the final case showing 2.0-2.2 mHz (with 142 Mx cm^-2). Following this, longitudinal-mode frequencies were detected in the range 2.6-3.2 mHz for three of our cases, with 3.9-4.1 mHz for the remaining case. After mode coupling, two cases displayed a decrease in longitudinal-mode Fourier power in the higher chromosphere.

Observations show that solar activity is distributed nonaxisymmetrically, concentrating at "preferred longitudes." This indicates the important role of nonaxisymmetric magnetic fields in the origin of solar activity. We investigate the generation of the nonaxisymmetric fields and their coupling with the axisymmetric solar magnetic field. Our kinematic generation (dynamo) model operating in a sphere includes a model of solar differential rotation, as obtained by inversion of helioseismic data, modeled distributions of the turbulent resistivity, nonaxisymmetric mean helicity, and meridional circulation in the convection zone. We find that (1) the nonaxisymmetric modes are localized near the base of the convection zone, where the formation of active regions starts, and at latitudes around 30°; (2) the coupling of nonaxisymmetric and axisymmetric modes causes the nonaxisymmetric mode to follow the solar cycle; the phase relations between the modes are found; and (3) the rate of rotation of the first nonaxisymmetric mode is close to that determined in interplanetary space.

We investigate the possibility of measuring magnetic field strength in G-band bright points through the analysis of Zeeman polarization in molecular CH lines. To this end we solve the equations of polarized radiative transfer in the G band through a standard plane-parallel model of the solar atmosphere with an imposed magnetic field and through a more realistic snapshot from a simulation of solar magnetoconvection. This region of the spectrum is crowded with many atomic and molecular lines. Nevertheless, we find several instances of isolated groups of CH lines that are predicted to produce a measurable Stokes V signal in the presence of magnetic fields. In part this is possible because the effective Landé factors of lines in the stronger main branch of the CH A²Δ-X²Π transition tend to zero rather quickly for increasing total angular momentum J, resulting in a Stokes V spectrum of the G band that is less crowded than the corresponding Stokes I spectrum. We indicate that, by contrast, the effective Landé factors of the R and P satellite subbranches of this transition tend to ±1 for increasing J. However, these lines are in general considerably weaker and do not contribute significantly to the polarization signal. In one wavelength location near 430.4 nm, the overlap of several magnetically sensitive and nonsensitive CH lines is predicted to result in a single-lobed Stokes V profile, raising the possibility of high spatial resolution narrowband polarimetric imaging. In the magnetoconvection snapshot we find circular polarization signals of the order of 1%, prompting us to conclude that measuring magnetic field strength in small-scale elements through the Zeeman effect in CH lines is a realistic prospect.

We present a detailed, comparative study of low angular degree solar p-mode variations extracted by analyses of two sets of observational data. These were collected by the ground-based Birmingham Solar-Oscillations Network (BiSON) and the Global Oscillations at Low Frequency (GOLF) instrument on board the ESA/NASA SOHO satellite. The ~5.5 yr period analyzed covers the complete rising phase of solar activity cycle 23 (1996-2002). We find an excellent level of agreement in the uncovered variations, indicating that the two data sets are highly correlated and dominated by the same mode realization noise (the signature of the stochastic forcing of the resonances). The results lend further support to the surmise that changes in damping alone may account for observed variations in mode power and damping. While significant variations in peak asymmetry are uncovered in the near-continuous GOLF set, a similar analysis of the BiSON database yields a null result. This reflects the deleterious impact of its ground-based window function on the precision with which the asymmetry can be determined. As such we are unable to rule out the possibility that variations of magnitude similar to those in GOLF may be present in the BiSON observations.

We present a theoretical formalism by which the global and local mass functions of dark matter substructures (dark subhalos) can be analytically estimated. The global subhalo mass function is defined to give the total number density of dark subhalos in the universe as a function of mass, while the local subhalo mass function counts only those subhalos included in one individual host halo. We develop our formalism by modifying the Press-Schechter theory to incorporate the following: (1) the internal structure of dark halos; (2) the correlations between the halos and the subhalos; and (3) the subhalo mass-loss effect driven by the tidal forces. We find that the resulting (cumulative) subhalo mass function is close to a power law with a slope of ~-1, that the subhalos contribute approximately 10% of the total mass, and that the tidal-stripping effect changes the subhalo mass function self-similarly, all consistent with recent numerical detections.

Giant radio galaxies (GRGs) are prime and unique laboratories for constraining the plasma processes that accelerate relativistic electrons within large intergalactic volumes. The evidence for short radiative loss times rules out certain scenarios for energy transport within their very large dimensions. This, combined with their high energy content, large ordered magnetic field structures, the absence of strong large-scale shocks, and very low upper limits on their internal thermal plasma densities, points to a direct and efficient conversion of force-free magnetic field to particle energy. This is underlined by the evidence in GRGs that their internal Alfvén speeds are higher than the lobe expansion speeds. We discuss these constraints in the context of models in which the central black hole energy is initially extracted as electromagnetic Poynting flux that injects large amounts of magnetic flux into the lobes. Recent advances in the theory of collisionless magnetic reconnection make this a favored mechanism to explain the particle acceleration in these systems. The energy reservoir is likely to be force-free fields, which is independently consistent with recent models of initial electromagnetic energy transfer from the parent galaxy's supermassive black hole. Such a scenario has wide-ranging astrophysical consequences: it implies that space-distributed magnetic reconnection or some other highly efficient field-to-particle energy conversion process likely dominates in all extended extragalactic radio sources.

A puzzling feature of the Chandra-detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission, resulting in an outward-increasing radio-to-X-ray ratio. In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays. This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet's nonthermal electron distribution. In this Letter we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration. This notion is consistent with another recent finding, namely, that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors Γ ~ 2-3. The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from kiloparsec scales to the hot spots, where, in a final collision with the intergalactic medium, they slow down rapidly to the subrelativistic velocities of the hot spot advance speed.

GRB 941017, a gamma-ray burst of exceptional fluence, has recently been shown to have a high-energy component that is not consistent with the standard fireball phenomenology. If this component is the result of photomeson interactions in the burst fireball, it provides new and compelling support for substantial high-energy neutrino fluxes from this and similar sources. In this Letter, we consider what impact this new information has on the neutrino spectra of gamma-ray bursts and discuss how this new evidence impacts the prospects for detection of such events in next generation neutrino telescopes.

We have reexamined the relation between the mass of the central black holes in nearby galaxies, M_bh, and the stellar mass of the surrounding spheroid or bulge, M_bulge. For a total of 30 galaxies bulge masses were derived through Jeans equation modeling or adopted from dynamical models in the literature. In stellar mass-to-light ratios, the spheroids and bulges span a range of a factor of 8. The bulge masses were related to well-determined black hole masses taken from the literature. With these improved values for M_bh, compared to Magorrian et al., and our redetermination of M_bulge, we find that the M_bh-M_bulge relation becomes very tight. We find M_bh ~ M with an observed scatter of ≲0.30 dex, a fraction of which can be attributed to measurement errors. The scatter in this relation is therefore comparable to the scatter in the relations of M_bh with σ and the stellar concentration. These results confirm and refine the work of Marconi & Hunt. For M_bulge ~ 5 × 10¹⁰M_☉ the median black hole mass is 0.14% ± 0.04% of the bulge mass.

We use high-resolution collisionless N-body simulations to study the secular evolution of disk galaxies and, in particular, the final properties of disks that suffer a bar and perhaps a bar-buckling instability. Although we find that bars are not destroyed by the buckling instability, when we decompose the radial density profiles of the secularly evolved disks into inner Sérsic and outer exponential components, for favorable viewing angles, the resulting structural parameters, scaling relations, and global kinematics of the bar components are in good agreement with those obtained for bulges of late-type galaxies. Round bulges may require a different formation channel or dissipational processes.

We combine the Giant Metrewave Radio Telescope low-frequency radio observations of SN 1993J with the Very Large Array high-frequency radio data to get a near-simultaneous spectrum around day 3200 since explosion. The low-frequency measurements of the supernova determine the turnover frequency and flux scale of the composite spectrum and help reveal a steepening in the spectral index, Δα ~ 0.6, in the optically thin part of the spectrum. This is the first observational evidence of a break in the radio spectrum of a young supernova. We associate this break with the phenomenon of synchrotron aging of radiating electrons. From the break in the spectrum we calculate the magnetic field in the shocked region independent of the equipartition assumption between energy density of relativistic particles and magnetic energy density. We determine the ratio of these two energy densities and find that this ratio is in the range 8.5 × 10^-6 to 5 × 10^-4. We also predict the nature of the evolution of the synchrotron break frequency with time, with competing effects due to diffusive Fermi acceleration and adiabatic expansion of the radiative electron plasma.

The nature of ultraluminous X-ray (ULX) sources is presently unknown. A possible explanation is that they are accreting intermediate-mass black holes (IBHs) that are fed by Roche lobe overflow from a tidally captured stellar companion. We show that a star can circularize around an IBH without being destroyed by tidal heating (in contrast to the case of M_BH > 10⁶M_☉ massive black holes in galactic centers, where survival is unlikely). We find that the capture and circularization rate is ~5 × 10^-8 yr^-1, almost independently of the cluster's relaxation time. We follow the luminosity evolution of the binary system during the main-sequence Roche lobe overflow phase and show it can maintain ULX source-like luminosities for greater than 10⁷ yr. In particular, we show that the ULX source in the young cluster MGG-11 in starburst galaxy M82, which possibly harbors an IBH, is well explained by this mechanism, and we predict that ≳10% of similar clusters with IBHs have a tidally captured circularized star. The cluster can evaporate on a timescale shorter than the lifetime of the binary. This raises the possibility of a ULX source that outlives its host cluster, or even lights up only after the cluster has evaporated, in agreement with observations of hostless ULX sources.

We present high angular resolution (~01-04) Very Large Array observations at 2 and 6 cm made in 1983, 1986, and 1995 toward the ultracompact bipolar H II region NGC 7538 IRS 1. We find, at both wavelengths, clear evidence of a decrease in the emission from the lobes. This decrease, of the order of 20%-30%, has not been observed previously in any ultracompact H II region. Most likely, it is due to recombination of the ionized gas in the lobes as a result of a decrease in the available ionizing photon flux. It is unclear if this decrease in the ionizing photon flux is due to an intrinsic change in the exciting star or to increased absorption of ionizing photons in the optically thick core of the nebula.

We discuss the main properties of the Galactic globular cluster (GC) blue straggler stars (BSSs), as inferred from our new catalog containing nearly 3000 BSSs. The catalog has been extracted from the photometrically homogeneous V versus (B-V) color-magnitude diagrams (CMDs) of 56 GCs, based on Wide Field Planetary Camera 2 images of their central cores. In our analysis, we used consistent relative distances based on the same photometry and calibration. The number of BSSs has been normalized to obtain relative frequencies (F_BSS) and specific densities (N_S) using different stellar populations extracted from the CMD. The cluster F_BSS is significantly smaller than the relative frequency of field BSSs. We find a significant anticorrelation between the BSS relative frequency in a cluster and its total absolute luminosity (mass). There is no statistically significant trend between the BSS frequency and the expected collision rate. The value of F_BSS does not depend on other cluster parameters, apart from a mild dependence on the central density. Post-core-collapse clusters act like normal clusters as far as the BSS frequency is concerned. We also show that the BSS luminosity function for the most luminous clusters is significantly different, with a brighter peak and extending to brighter luminosities than in the less luminous clusters. These results imply that the efficiency of BSS production mechanisms and their relative importance vary with the cluster mass.

Recent interferometric observations of the brightest and angularly largest classical Cepheid, ℓ Carinae, with ESO's Very Large Telescope Interferometer have resolved with high precision the variation of its angular diameter with phase. We compare the measured angular diameter curve to the one that we derive by an application of the Baade-Wesselink-type infrared surface brightness technique and find a near-perfect agreement between the two curves. The mean angular diameters of ℓ Car from the two techniques agree very well within their total error bars (1.5%), as do the derived distances (4%). This result is an indication that the calibration of the surface brightness relations used in the distance determination of far-away Cepheids is not affected by large biases.

Extrasolar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wave-front errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5%-10% bandwidth and recorded by coronagraphic interferometers equipped with active control of wave-front phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wave-front components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wave front is removed by destructive interference made directly at the detector focal plane with an "antihalo," synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error-sensing data.

Ion tails of comets are known to extend radially away from the Sun over very large distances. Crossing these tails by spacecraft not specifically targeted to intercept them was believed to be extremely improbable, since that requires precise angular alignment of the spacecraft with a comet. We report here the fortuitous detection of cometary ions at large angular separation far from the comet. To explain this unexpected discovery, we conclude that these ions were ducted laterally along magnetic fields that were randomly distorted by a coronal mass ejection and that such transport increases the probability of an unplanned detection of comets.

Nonlinear inertial range hydrodynamic turbulence simulations, describing a high-β interstellar plasma fluid, demonstrate spectral anisotropy in the density spectrum. The anisotropic fluctuations, developed mainly in the high-frequency component of the fluid, are driven primarily by low-frequency incompressible fluctuations. The anisotropy occurs predominantly in large-scale structures; in short-scale structures, isotropy occurs in fully developed compressible turbulence.

We study the effects of high-energy pseudometastable levels in modeling Fe II spectra excited by Lyα fluorescence. We demonstrate that collisional couplings between metastable- and radiative-emitting levels are of fundamental importance in the formation of the spectrum under a wide range of physical conditions that encompass most astronomical fluorescent Fe II sources. Electron-impact-induced transitions from metastable levels strengthen the overall emission spectra and, in particular, strengthen the lines that results from secondary decay after Lyα pumping, like the so-called "1 μm lines."

We investigate the coalescence process of two parallel current loops with cohelicity by using the two-dimensional, electromagnetic, relativistic particle-in-cell code. We found that in a later stage of the two current loops' coalescence, fast magnetosonic waves are generated as a result of rebound of the coalescence, and they develop to shock waves propagating away from the coalesced loops. We also found that near the fast magnetosonic shock front the ions can be promptly accelerated by the surfatron acceleration mechanism. We apply the obtained simulation results to the proton acceleration during solar flares.