Table of contents

We present a set of cluster models that link the present-day properties of clusters to the processes that govern galaxy formation. These models treat the entropy distribution of the intracluster medium as its most fundamental property. Because convection strives to establish an entropy gradient that rises with radius, the observable properties of a relaxed cluster depend entirely on its dark matter potential and the entropy distribution of its uncondensed gas. Guided by simulations, we compute the intracluster entropy distribution that arises in the absence of radiative cooling and supernova heating by assuming that the gas density distribution would be identical to that of the dark matter. The lowest entropy gas would then fall below a critical entropy threshold at which the cooling time equals a Hubble time. Radiative cooling and whatever feedback is associated with it must modify the entropy of that low-entropy gas, changing the overall entropy distribution function and thereby altering the observable properties of the cluster. Using some phenomenological prescriptions for entropy modification based on the existence of this cooling threshold, we construct a remarkably realistic set of cluster models. The surface brightness profiles, mass-temperature relation, and luminosity-temperature relation of observed clusters all naturally emerge from these models. By introducing a single adjustable parameter related to the amount of intracluster gas that can cool within a Hubble time, we can also reproduce the observed temperature gradients of clusters and the deviations of cooling flow clusters from the standard luminosity-temperature relation.

We describe the X-ray properties of a large sample of z ~ 3 Lyman break galaxies (LBGs) in the region of the Hubble Deep Field-North, derived from the 1 Ms public Chandra observation. Of our sample of 148 LBGs, four are detected individually. This immediately gives a measure of the bright AGN fraction in these galaxies of ~3%, which is in agreement with that derived from the UV spectra. The X-ray color of the detected sources indicates that they are probably moderately obscured. Stacking of the remainder shows a significant detection (6 σ) with an average luminosity of 3.4 × 10⁴¹ ergs s^-1 per galaxy in the rest-frame 2-10 keV band. We have also studied a comparison sample of 95 z ~ 1 "Balmer break" galaxies. Eight of these are detected directly, with at least two clear AGNs based on their high X-ray luminosity and very hard X-ray spectra. The remainder are of relatively low luminosity (<10⁴² ergs s^-1), and the X-rays could arise from either AGNs or rapid star formation. The X-ray colors and evidence from other wave bands favor the latter interpretation. Excluding the clear AGNs, we deduce a mean X-ray luminosity of 6.6 × 10⁴⁰ ergs s^-1, a factor of ~5 lower than the LBGs. The average ratio of the UV and X-ray luminosities of these star-forming galaxies L_UV/L_X, however, is approximately the same at z = 1 as it is at z = 3. This scaling implies that the X-ray emission follows the current star formation rate, as measured by the UV luminosity. We use our results to constrain the star formation rate at z ~ 3 from an X-ray perspective. Assuming the locally established correlation between X-ray and far-IR luminosity, the average inferred star formation rate in each LBG is found to be approximately 60 M_☉ yr^-1, in excellent agreement with the extinction-corrected UV estimates. This provides an external check on the UV estimates of the star formation rates and on the use of X-ray luminosities to infer these rates in rapidly star-forming galaxies at high redshift.

We discuss the effects of microlensing on the broad emission lines (BELs) of QSOs in the light of recent determinations of the size of the broad-line region (BLR) and its scaling with QSO luminosity. Microlensing by star-sized objects can produce significant amplifications in the BEL of some multiple-imaged QSOs, and could be very relevant for high-ionization lines. We have identified a group of 10 gravitational lens systems (~30% of the selected sample) in which microlensing could be observed. Using standard kinematic models for active galactic nuclei, we have studied the changes induced in the line profile by a microlens located at different positions with respect to the center of the BLR. We found that microlensing could produce important effects such as the relative enhancement of different parts of the line profile or the displacement of the peak of the line. The study of BEL profiles of different ionization in a microlensed QSO image could be an alternative method for probing the BLR structure and size.

We discuss a sample of 29 active galactic nuclei (AGNs) (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman break galaxies (LBGs) at z ≃ 3. Reaching apparent magnitudes of _AB = 25.5, the sample includes broad-lined AGNs approximately 100 times less UV luminous than in most surveys to date covering similar redshifts and in the first statistical sample of UV/optically selected narrow-lined AGNs at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ≃3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGNs are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGNs in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined M_BH versus M_bulge relation given estimates of the stellar masses of LBGs. The clustering properties of the AGNs are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star formation phase, is ~10⁷ yr.

We describe the results of an optical spectral monitoring campaign on the Seyfert galaxy 3C 390.3 carried out with a CCD spectrograph at the 2.6 m Shajn Telescope of the Crimean Astrophysical Observatory in 1992-2000. Light curves for the Hα and Hβ emission lines and for the continuum in both the Hα and Hβ spectral regions are given. The lag between the continuum and Hβ emission-line variations (specifically the cross-correlation centroid τ_cent) is found to be τ_cent = 89 days, more than twice the lag found in an earlier investigation by Dietrich et al. We attribute this discrepancy to the difference between continuum autocorrelation functions for these two time series, which have very different durations. We show that a single emission-line transfer function is able to reproduce both cross-correlation results. The recovered transfer function for the Hβ line has a peak at a lag of 20 days, a tail that extends up to 300 days, and little or no correlation near zero lag. We find that the Hα line, which varies with a markedly lower amplitude than Hβ, has a lag of τ_cent = 162 days. We find no significant difference in lag between the blue and red wings of either Hα or Hβ, effectively ruling out kinematics dominated by simple radial motion. However, emission-line profile variations were clearly detected over the duration of the monitoring campaign. The mean and rms profiles computed for three selected time intervals are significantly different and show no correlation between profile width and continuum flux. Over the duration of the campaign, the red central part of the line profiles strongly decreased with respect to the total line flux. Moreover, the red bump nearly completely disappeared, while the blue bump has become more prominent. We find that some discrete profile features appear to propagate across the line profile, and we see such features moving in both directions, blue to red and vice versa. Variations in two profile segments at line-of-sight velocity ±(7500-9500) km s^-1 are found to be poorly correlated with variations in the continuum or in other profile segments. Our conjecture is that the observed evolution of the Balmer line profiles in 3C 390.3 may be the result of the rotational redistribution of matter in a Keplerian disk and changes in the relative strength of an additional line component that dominates in the red central part of the profile. We conclude that the evolution of the broad-line profiles in 3C 390.3 in the context of the current models of the broad-line region remains unclear.

We report the discovery of a highly luminous optical transient (OT), SDSS J124602.54 + 011318.8, associated with a galaxy at a redshift of 0.385. In this paper, we consider the possibility that the OT may be a gamma-ray burst (GRB) afterglow. Three sets of images and two sets of spectra were obtained as part of the normal operations of the Sloan Digital Sky Survey (SDSS). In the first two image sets, observed two nights apart, the object appears as a point source at r* ≈ 17. The third image set, observed about 410 days later, shows an extended source which is more than 2.5 mags fainter. The spectra were observed about 400 and 670 days after the first two image sets, and both show an apparently normal galaxy at a redshift of 0.385. Associating the OT with the galaxy, the absolute magnitude was M = -24.8, which is over 4 mag brighter than the most luminous supernova ever measured. The spectral energy distributions of the galaxy-subtracted OT derived from the first two image sets are well-fitted by single power laws with indices of β_ν = -0.92 and -1.29, respectively, similar to most GRB afterglows. Based upon the luminosity of the OT, nondetections in contemporaneous ROTSE I images and the change in spectral slope, the OT, if an afterglow, was likely discovered early during a plateau or slowly fading phase. The discovery of a GRB afterglow at this stage of the SDSS is consistent with expectations, but only if the optical emission is much less strongly beamed than the gamma rays. We emphasize that other explanations for the OT cannot be ruled out; a recent follow-up study by Gal-Yam et al. (2002) provides strong evidence that this source is in fact an unusual active galactic nucleus (AGN).

We report evidence for the first time that merging clusters of galaxies are a promising candidate for the origin of high Galactic latitude, steady, unidentified EGRET gamma-ray sources. Instead of using past optical catalogs of eye-selected clusters, we made a matched filter survey of galaxy clusters over 4° × 4° areas around seven steady unidentified EGRET sources at |b| > 45°, together with a 100 deg² area near the south Galactic pole as a control field. In total, 154 Abell-like cluster candidates and 18 close pairs/groups of these clusters, expected to be possibly merging clusters, were identified within estimated redshift z_est ≤ 0.15. Five among the seven EGRET sources have one or two cluster pairs/groups (CPGs) within 1° from them. We assess the statistical significance of this result by several methods, and the confidence level of the real excess is maximally 99.8% and 97.8% in a conservative method. In contrast, we found no significant correlation with single clusters. In addition to the spatial correlation, we also found that the richness of CPGs associated with EGRET sources is considerably larger than those of CPGs in the control field. These results imply that a part of the steady unidentified EGRET sources at high latitude are physically associated with close CPGs, not with single clusters. We also discuss possible interpretations of these results. We argue that, if these associations are real, they are difficult to explain by hadronic processes but best explained by the inverse-Compton scattering by high-energy electrons accelerated in shocks of cluster formation, as recently proposed.

The study of soft X-ray emission of 38 X-ray-selected galaxy clusters observed by the ROSAT Position Sensitive Proportional Counter indicates that the "soft excess" phenomenon may be a common occurrence in galaxy clusters. Excess soft X-ray radiation, above the contribution from the hot intracluster medium, is evident in a large fraction of sources and is clearly detected with large statistical significance in the deepest observations. The investigation relies on new, high-resolution 21 cm H I observations. The sample selection also features analysis of infrared images to further ensure reliability of results with respect to the characteristics of Galactic absorption. The possibility of background or calibration effects as cause of the excess emission is likewise investigated; a detailed analysis of the distribution of the excess emission with respect to detector position and Galactic H I column density shows that the excess emission is a genuine celestial phenomenon. We find evidence for a preferential distribution of the soft excess emission at distances larger than ~150-200 kpc from the centers of clusters; this behavior may be naturally explained in the context of a nonthermal inverse Compton scenario. Alternatively, we propose that the phenomenon may be caused by thermal emission of very large scale "warm" filaments seen in recent hydrodynamic simulations. This new interpretation relieves the very demanding requirements of either the traditional intracluster "warm" gas and the nonthermal scenarios. We also investigate the possibility of the soft excess originating from unresolved, X-ray-faint cluster galaxies.

The 40 ks Chandra ACIS-S observation of A1367 provides new insights into small-scale structures and point sources in this dynamically young cluster. Here we concentrate on small-scale extended structures. A ridgelike structure around the center ("the ridge") is significant in the Chandra image. The ridge, with a projected length of ~8' (or 300 h kpc), is elongated from northwest to southeast, as is the X-ray surface brightness distribution on much larger scales (~2 h Mpc). The ridge is cooler than its western and southern surroundings, while the differences from its eastern and northern surroundings are small. We also searched for small-scale structures with sizes of about an arcminute. Nine extended features, with sizes from ~05 to 15, were detected at significance levels above 4 σ. Five of the nine features are located in the ridge and form local crests. The nine extended features can be divided into two types. Those associated with galaxies (NGC 3860B, NGC 3860, and UGC 6697) are significantly cooler than their surroundings (0.3-0.9 keV vs. 3-4.5 keV). The masses of their host galaxies are sufficient to bind the extended gas. These extended features are probably related to thermal halos or galactic superwinds of their host galaxies. The existence of these relatively cold halos implies that galaxy coronae can survive in a cluster environment (e.g., Vikhlinin and coworkers in 2001). Features of the second type are not apparently associated with galaxies. Their temperatures may not be significantly different from those of their surroundings. This class of extended features may be related to the ridge. We consider several possibilities for the ridge and the second type of extended features. The merging scenario is preferred.

We have obtained deep, long-slit spectroscopy along the major axis of NGC 6166, the cD galaxy in the cluster A2199, in order to measure the kinematics of intracluster stars at large radii. The velocity dispersion initially decreases from the central value of 300 to 200 km s^-1 within a few kiloparsecs and then steadily rises to 660 km s^-1 at a radius of 60 kpc (H₀ = 75 km s^-1 Mpc^-1, Ω_m = 0.3, Ω_Λ = 0.7), nearly reaching the velocity dispersion of the cluster (σ_A2199 = 775 ± 50 km s^-1). These data suggest that the stars in the halo of the cD galaxy trace the potential of the cluster and that the kinematics of these intracluster stars can be used to constrain the mass profile of the cluster. In addition, we find evidence for systematic rotation (V/σ ≈ 0.3) in the intracluster stars beyond 20 kpc. Such rotation is not seen in the kinematics of the cluster members. The surface brightness and velocity dispersion profiles can be fitted using a single-component mass model only by making unphysical assumptions about the level of anisotropy for both the stars in the cD galaxy and the kinematics of the galaxies in the cluster. Two-component mass models for the cD galaxy and its halo are subsequently explored using the kinematics of known cluster members as an additional constraint on the total enclosed mass beyond the extent of the stellar kinematics. Under the assumption of isotropy, the observed major-axis kinematics can be reproduced only if the halo, parameterized by a generalized Navarro-Frenk-White (NFW) profile, has a soft core, i.e., α < 1 (a generalized NFW halo with α = 1 is excluded because of low implied stellar mass-to-light ratios). This result is inconsistent with the predictions of current N-body simulations for dark matter halos. To test the consistency of our halo profiles with those derived from strong lensing measurements in intermediate-redshift clusters, we calculate the critical radii for tangential arcs, assuming that our best-fit mass models for A2199 were placed at cosmological redshifts between 0.2 ≤ z ≤ 0.5. The calculated critical radii for our best-fit two-component isotropic models range from 5'' to 40'', depending on the assumed source redshift, consistent with the radii for gravitational arcs observed in intermediate-redshift clusters. We also present the results of Monte Carlo simulations testing the general reliability of velocity dispersion measurements in the regime of low signal-to-noise ratio and large intrinsic Doppler broadening.

We report the optical identifications of two X-ray-luminous point sources in the spiral galaxy NGC 4565 based on archive data of Chandra and the Hubble Space Telescope. The central X-ray point source, RX J1236.3+2559, is found to be the nucleus of NGC 4565 with an X-ray luminosity of L_X ≈ 4.3 × 10³⁹ ergs s^-1. We show that its multiband properties are consistent with its being a low-luminosity active galactic nucleus. A faint optical counterpart with B ≈ 25.1 and I ≈ 24.0 was identified for the off-nucleus X-ray point source, RX J1236.2+2558. Its extinction-corrected B magnitude is estimated to be 24.5. The X-ray-to-optical flux ratio (f_X/f_B) is about 540. From the optical and X-ray properties, we argue that RX J1236.2+2558 is an ultraluminous X-ray compact source with L_X ≈ 6.5 × 10³⁹ ergs s^-1. The source is probably located in a faint globular cluster at the outer edge of NGC 4565's bulge.

Using Chandra and HST, we show that X-ray and Hα filaments that form the 1.3 kpc diameter superbubble of NGC 3079 have strikingly similar patterns at ~08 resolution. This tight optical line/X-ray match seems to arise from cool disk gas that has been driven by the wind, with X-rays being emitted from upstream, standoff bow shocks or by conductive cooling at the cloud/wind interfaces. We find that the soft X-ray plasma has thermal and kinetic energies E_TH ~ 2 × 10⁵⁶η ergs and E_KE ~ 5 × 10⁵⁴η ergs, where η_X is the filling factor of the X-ray gas and may be small; these are comparable to the energies of the optical line-emitting gas if η_X is large. Hydrodynamical simulations reproduce the observations well using a disk-mass loaded superwind. X-rays are also seen from the base of the radio counterbubble which is obscured optically by the galaxy disk and from the nucleus (whose spectrum shows the Fe Kα line at 6 keV as well as gas absorbed by a moderate neutral hydrogen column). The superbubble is surrounded by a fainter conical halo of X-ray emission that fills the area delineated by high-angle, Hα-emitting filaments, supporting our previous assertion that these filaments form the contact discontinuity/shock between galaxy gas and shocked wind. This X-ray emission is not significantly edge brightened, indicating a partially filled volume of warm gas within the shocked wind, not a shell of conductively heated gas. About 40'' (3 kpc) above the galaxy disk, an X-ray arc may partially close above the bubble, but the northeast quadrant remains open at the surface brightness attained by Chandra, consistent with the notion that the superwind reaches into at least the galaxy halo.

We model the 1990 giant X-ray flare of the quiescent galaxy NGC 5905 as the tidal disruption of a star by a supermassive black hole. From the observed rapid decline of the luminosity, over a timescale of a few years, we argue that the flare was powered by the fallback of debris rather than subsequent accretion via a thin disk. The fallback model allows constraints to be set on the black hole mass and the mass of debris. The latter must be very much less than a solar mass to explain the very low luminosity of the flare. The observations can be explained either as the partial stripping of the outer layers of a low-mass main-sequence star or as the disruption of a brown dwarf or a giant planet. We find that the X-ray emission in the flare must have originated within a small patch rather than over the entire torus of circularized material surrounding the black hole. We suggest that the patch corresponds to the "bright spot" where the stream of returning debris impacts the torus. Interestingly, although the peak luminosity of the flare was highly sub-Eddington, the peak flux from the bright spot was close to the Eddington limit. We speculate on the implications of this result for observations of other flare events.

The infrared emission from interstellar dust in the Small Magellanic Cloud is modeled using a mixture of amorphous silicate and carbonaceous grains, including a population of polycyclic aromatic hydrocarbon (PAH) molecules. We show the following: (1) This dust model is able to reproduce the spectral energy distribution from near-IR to far-IR for the entire SMC Bar region, provided that the PAH abundance in the SMC Bar region is very low. (2) The IR spectrum of the SMC B1 No. 1 molecular cloud can also be reproduced by our dust model, provided that the PAH abundance is increased relative to the overall SMC Bar. The PAHs in SMC B1 No. 1 incorporate ~3% of the SMC C abundance, compared to less than 0.4% in the SMC Bar. (3) The spectrum of SMC B1 No. 1 is best reproduced if the PAH mixture has intrinsic IR band strengths that differ from the band strengths that best fit Milky Way PAH mixtures. The variation in the PAH IR band strengths would imply different PAH mixtures, presumably a consequence of differing metallicity or environmental conditions. Other possibilities such as superhydrogenation of PAHs and softening of the starlight spectrum are also discussed.

We have mapped two regions at the northern tip of the Magellanic Stream in neutral hydrogen 21 cm emission, using the Arecibo telescope. The new data are used to study the morphology and properties of the Stream far away from the Magellanic Clouds, as well as to provide indirect constraints on the properties of the Galactic halo. We investigate confinement mechanisms for the Stream clouds and conclude that these clouds cannot be gravitationally confined or in free expansion. The most likely mechanism for cloud confinement is pressure support from the hot Galactic halo gas. This allows us to place an upper limit on the halo density: n_h(15 kpc) = 10^-3 cm^-3 and/or n_h (45 kpc) = 3 × 10^-4 cm^-3, depending on the distance. These values are significantly higher than predicted for an isothermal stratified halo.

We present K-band λ/Δλ ~ 2600 spectroscopy of five stars (K ~ 14-16 mag) within 05 of Sgr A*, the radio source associated with the compact massive object suspected to be a 2.6 × 10⁶M_☉ black hole at the center of our Galaxy. High spatial resolution of ~009 and good Strehl ratios of ~0.2 achieved with adaptive optics on the 10 m Keck telescope make it possible to measure moderate-resolution spectra of these stars individually for the first time. Two stars (S0-17 and S0-18) are identified as late-type stars by the detection of CO band head absorption in their spectra. Their absolute K magnitudes and CO band head absorption strengths are consistent with early K giants. Three stars (S0-1, S0-2, and S0-16) with r_proj < 0.0075 pc (~02) from Sgr A* lack CO band head absorption, confirming the results of earlier lower spectral and lower spatial resolution observations that the majority of the stars in the Sgr A* cluster are early-type stars. The absolute K magnitudes of the early-type stars suggest that they are late O, early B main-sequence stars of ages less than 20 Myr. The presence of young stars in the Sgr A* cluster so close to the central supermassive black hole poses the intriguing problem of how these stars could have formed or could have been brought within its strong tidal field.

We use the Chandra X-Ray Observatory to analyze interactions of the blast wave and the inhomogeneous interstellar medium on the western limb of the Cygnus Loop supernova remnant. This field of view includes an initial interaction between the blast wave and a large cloud as well as the encounter of the shock front and the shell that surrounds the cavity of the supernova progenitor. Uniquely, the X-rays directly trace the shock front in the dense cloud, where we measure temperature kT = 0.03 keV. We find kT ≈ 0.2 keV in regions where reflected shocks further heat previously shocked material. Applying one-dimensional models to these interactions, we determine the original blast-wave velocity v_bw ≈ 330 km s^-1 in the ambient medium. We do not detect strong evidence for instabilities or nonequilibrium conditions on the arcsecond scales we resolve. These sensitive, high-resolution data indicate no exceptional abundance variations in this region of the Cygnus Loop.

We study the intermittency and field-line structure of the MHD turbulence in plasmas with very large magnetic Prandtl numbers. In this regime, which is realized in the interstellar medium, some accretion disks, protogalaxies, galaxy-cluster gas, the early universe, etc., magnetic fluctuations can be excited at scales below the viscous cutoff. The salient feature of the resulting small-scale magnetic turbulence is the folded structure of the fields. It is characterized by very rapid transverse spatial oscillation of the field direction, while the field lines remain largely unbent up to the scale of the flow. Quantitatively, the fluctuation level and the field-line geometry can be studied in terms of the statistics of the field strength and of the field-line curvature. In the kinematic limit, the distribution of the field strength is an expanding lognormal, while that of the field-line curvature K is stationary and has a power tail ~K^-13/7. The field strength and curvature are anticorrelated, i.e., the growing fields are mostly flat, while the sharply curved fields remain relatively weak. The field, therefore, settles into a reduced-tension state. Numerical simulations demonstrate three essential features of the nonlinear regime. First, the total magnetic energy is equal to the total kinetic energy. Second, the intermittency is partially suppressed compared to the kinematic case, as the fields become more volume-filling and their distribution develops an exponential tail. Third, the folding structure of the field is unchanged from the kinematic case: the anticorrelation between the field strength and the curvature persists, and the distribution of the latter retains the same power tail. We propose a model of back-reaction based on the folding picture that reproduces all of the above numerical results.

We report the tentative detection of molecular oxygen in the interstellar medium. Deep integrations using the Submillimeter Wave Astronomy (SWAS) satellite of the ρ Oph A cloud have resulted in a detection of an emission feature consistent with the N_J = 3₃ → 1₂ transition of O₂ at a local standard of rest velocity of 6.0 km s^-1. The line width and velocity are suggestive of the redshifted wing emission seen in species that trace the molecular outflows seen in this region. The fractional abundance of O₂ relative to H₂ in this high-velocity gas is approximately 10^-5. The fractional abundance of ortho-H₂O in this material is 8 × 10^-8, somewhat higher than found in quiescent material by earlier SWAS observations but less than that found in younger outflows. We suggest a unified scenario that explains the low O₂ abundance in cloud cores previously reported [X(O₂) ≤ few × 10^-7], along with the much enhanced abundance in the outflow region. In this picture, quiescent clouds are characterized by significant depletion of gas-phase oxygen, which ends up largely as water ice on the dust grains. A shock associated with the outflow results in almost all oxygen being processed into gas-phase water. In the cool postshock gas, the standard gas-phase chemistry reasserts itself, and after an elapsed time of a few times 10⁵ yr, the O₂ abundance has increased to the point that this species is one of the main repositories of oxygen atoms. At the same time, the gas-phase water abundance drops drastically, in agreement with the SWAS observations that we report here. On a somewhat longer timescale, depletion again dominates and the gas-phase abundance of O₂ drops by several orders of magnitude, with a significant fraction of oxygen remaining in atomic form.

Many astrophysical flows occur in inhomogeneous (clumpy) media. We present results of a numerical study of steady, planar shocks interacting with a system of embedded cylindrical clouds. Our study uses a two-dimensional geometry. Our numerical code uses an adaptive mesh refinement, allowing us to achieve sufficiently high resolution both at the largest and the smallest scales. We neglect any radiative losses, heat conduction, and gravitational forces. Detailed analysis of the simulations shows that interaction of embedded inhomogeneities with the shock/postshock wind depends primarily on the thickness of the cloud layer and arrangement of the clouds in the layer. The total cloud mass and the total number of individual clouds is not a significant factor. We define two classes of cloud distributions: thin and thick layers. We define the critical cloud separation along the direction of the flow and perpendicular to it, distinguishing between the interacting and noninteracting regimes of cloud evolution. Finally, we discuss mass loading and mixing in such systems.

Using recent dust continuum data, we generate the intrinsic ellipticity distribution of dense, starless molecular cloud cores. Under the hypothesis that the cores are all either oblate or prolate randomly oriented spheroids, we show that a satisfactory fit to observations can be obtained with a Gaussian prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we show that correlations exist between the apparent axis ratio and both the peak intensity and total flux density of emission from the cores, the sign of which again favors the prolate hypothesis. The latter result shows that the mass of a given core depends on its intrinsic ellipticity. Monte Carlo simulations are performed to find the best-fit power law of this dependence. Finally, we show how these results are consistent with an evolutionary scenario leading from filamentary parent clouds to increasingly massive, condensed, and roughly spherical embedded cores.

We report high- and low-dispersion spectroscopy, optical imaging, and high-resolution Very Large Array-A λ3.6 cm continuum observations of the peculiar planetary nebula IC 2149. These observations show that "bipolar" is a suitable morphological classification for IC 2149. Most nebular material is concentrated in a knotty, bright ring seen edge-on, embedded in an apparently oblate ellipsoidal shell from which remnant or incipient bipolar lobes emerge. We confirm the previously reported depletion in heavy elements and deduce a very low ejected nebular mass ≲0.03 M_☉. All this information indicates that the formation of IC 2149 is the result of the evolution of a low-mass central star.

The morphology and kinematics of molecular clouds (MCs) are best explained as the consequence of supersonic turbulence. Supersonic turbulence fragments MCs into dense sheets, filaments, and cores and large low-density "voids," via the action of highly radiative shocks. We refer to this process as turbulent fragmentation.

In this work we derive the mass distribution of gravitationally unstable cores generated by the process of turbulent fragmentation. The mass distribution above 1 M_☉ depends primarily on the power spectrum of the turbulent flow and on the jump conditions for isothermal shocks in a magnetized gas. For a power spectrum index β = 1.74, consistent with Larson's velocity dispersion-size relation as well as with new numerical and analytic results on supersonic turbulence, we obtain a power-law mass distribution of dense cores with a slope equal to 3/(4 - β) = 1.33, consistent with the slope of the stellar initial mass function (IMF). Below 1 M_☉, the mass distribution flattens and turns around at a fraction of 1 M_☉, as observed for the stellar IMF in a number of stellar clusters, because only the densest cores are gravitationally unstable. The mass distribution at low masses is determined by the probability distribution of the gas density, which is known to be approximately lognormal for an isothermal turbulent gas. The intermittent nature of the turbulent density distribution is thus responsible for the existence of a significant number of small collapsing cores, even of substellar mass.

Since turbulent fragmentation is unavoidable in supersonically turbulent molecular clouds, and given the success of the present model in predicting the observed shape of the stellar IMF, we conclude that turbulent fragmentation is essential to the origin of the stellar IMF.

The h and χ Per "double cluster" is examined using wide-field (098 × 098) CCD UBV imaging supplemented by optical spectra of several hundred of the brightest stars. Restricting our analysis to near the cluster nuclei, we find identical reddenings [E(B-V) = 0.56 ± 0.01], distance moduli (11.85 ± 0.05), and ages (12.8 ± 1.0 Myr) for the two clusters. In addition, we find an initial mass function slope for each of the cluster nuclei that is quite normal for high-mass stars, Γ = -1.3 ± 0.2, indistinguishable from a Salpeter value. We derive masses of 3700 M_☉ (h) and 2800 M_☉ (χ) integrating the present-day mass function from 1 to 120 M_☉. There is evidence of mild mass segregation within the cluster cores. Our data are consistent with the stars having formed at a single epoch; claims to the contrary are very likely due to the inclusion of the substantial population of early-type stars located at similar distances in the Perseus spiral arm, in addition to contamination by G and K giants at various distances. We discuss the uniqueness of the double cluster, citing other examples of such structures in the literature but concluding that the nearly identical nature of the two cluster cores is unusual. We fail to settle the long-standing controversy regarding whether or not the double cluster is the core of the Per OB1 association and argue that this may be unanswerable with current techniques. We also emphasize the need for further work on the pre-main-sequence population of this nearby and highly interesting region.

In the high-density cores of globular clusters, multibody interactions are expected to be common, with the result that black holes in binaries are hardened by interactions. It was shown by Sigurdsson and Hernquist, Kulkarni, Hut, and McMillan, and others in 1993 that 10 M_☉ black holes interacting exclusively by three-body encounters do not merge in the clusters themselves, because recoil kicks the binaries out of the clusters before the binaries are tight enough to merge. Here we consider a new mechanism, involving four-body encounters. Numerical simulations by a number of authors suggest that roughly 20%-50% of binary-binary encounters will eject one star but leave behind a stable hierarchical triple. If the orbital plane of the inner binary is strongly tilted with respect to the orbital plane of the outer object, a secular Kozai resonance, first investigated in the context of asteroids in the solar system, can increase the eccentricity of the inner body significantly. We show that in a substantial fraction of cases, the eccentricity is driven to a high enough value that the inner binary will merge by gravitational radiation, without a strong accompanying kick. Thus, the merged object remains in the cluster; depending on the binary fraction of black holes and the inclination distribution of newly formed hierarchical triples, this mechanism may allow massive black holes to accumulate through successive mergers in the cores of globular clusters. It may also increase the likelihood that stellar-mass black holes in globular clusters will be detectable by their gravitational radiation.

We study the growth rate of stars via stellar collisions in dense star clusters, calibrating our analytic calculations with direct N-body simulations of up to 65,536 stars, performed on the GRAPE family of special-purpose computers. We find that star clusters with initial half-mass relaxation times ≲25 Myr are dominated by stellar collisions, the first collisions occurring at or near the point of core collapse, which is driven by the segregation of the most massive stars to the cluster center, where they end up in hard binaries. The majority of collisions occur with the same star, resulting in the runaway growth of a supermassive object. This object can grow up to ~0.1% of the mass of the entire star cluster and could manifest itself as an intermediate-mass black hole (IMBH). The phase of runaway growth lasts until mass loss by stellar evolution arrests core collapse. Star clusters older than about 5 Myr and with present-day half-mass relaxation times ≲100 Myr are expected to contain an IMBH.

The time-dependent evolution of the accretion disk around the black hole is computed. The classical description of the α-viscosity is adopted so the evolution is driven by the instability operating in the innermost radiation pressure-dominated part of the accretion disk. We assume that the optically thick disk always extends down to the marginally stable orbit, so it is never evacuated completely. We include the effect of the advection, coronal dissipation, and vertical outflow. We show that the presence of the corona and/or the outflow reduces the amplitude of the outburst. If only about half of the energy is dissipated in the disk (with the other half dissipated in the corona and carried away by the outflow), the outburst amplitude and duration are consistent with observations of the microquasar GRS 1915+105. Viscous evolution explains in a natural way the lack of direct transitions from the state C to the state B in the color-color diagram of this source. Further reduction of the fraction of energy dissipated in the optically thick disk switches off the outbursts, which may explain why they are not seen in all high accretion rate sources being in the very high state.

The number of orphan radio afterglows associated with gamma-ray bursts (GRBs) that should be detected by a flux-limited radio survey is calculated. It is shown that for jetted GRBs, this number is smaller for a smaller jet opening angle θ, contrary to naive expectation. For a beaming factor f ≡ (θ²/2)^-1 ≃ 500, roughly the value inferred by Frail et al. from analysis of afterglow light curves, we predict that between several hundred to several thousand orphan radio afterglows should be detectable (over all sky) above 1 mJy at GHz frequencies at any given time. This orphan population is dominated by sources lying at distances of a few hundred Mpc and having an age of ~1 yr. A search for pointlike radio transients with flux densities greater than 6 mJy was conducted using the FIRST and NVSS surveys, yielding a list of nine orphan candidates. We argue that most of the candidates are unlikely to be radio supernovae. However, the possibility that they are radio-loud active galactic nuclei cannot be ruled out without further observation. Our analysis sets a conservative 95% CL upper limit for the all-sky number of radio orphans, which corresponds to a lower limit f > 13 on the beaming factor. Rejection of all candidates found in our search would imply f > 80. This, and the possibility that some candidates may indeed be radio afterglows, strongly motivate further observations of these transients.

We analyze the time-tagged event (TTE) data from observations of gamma-ray bursts (GRBs) and soft gamma repeaters (SGRs) by the Burst and Transient Source Experiment (BATSE). These data provide the best available time resolution for GRBs and SGRs. We have performed an extensive search for weak periodic signals in the frequency range of 400-2500 Hz using the burst records for 2203 GRBs and 152 SGR flares. The study employs the Rayleigh power as a test statistic to evaluate the evidence for periodic emissions. We find no evidence of periodic emissions from these events at these frequencies. In all but a very few cases, the maximum power values obtained are consistent with what would be expected by chance from a nonperiodic signal. In those few instances in which there is marginal evidence for periodicity, there are problems with the data that cast doubt on the reality of the signal. For classical GRBs, the largest Rayleigh power occurs in bursts whose TTE data appear to be corrupted. For SGRs, our largest Rayleigh power, with a significance of ≈1%, occurs in one record for SGR 1900+14 (at ≈2497 Hz) and in no other outbursts associated with this source; we thus consider it unlikely to represent detection of a real periodicity. From simulations, we deduce that the Rayleigh test would have detected significant oscillations with a relative amplitude of ≈10% about half the time. Thus, we conclude that high-frequency oscillations, if present, must have small relative amplitudes.

We have carefully measured the pulse profile of the binary pulsar PSR B1913+16 at 21 cm wavelength for 20 years, in order to search for variations that result from general relativistic geodetic precession of the spin axis. The profile width is found to decrease with time in its inner regions, while staying essentially constant on its outer skirts. We fitted these data to a model of the beam shape and precession geometry. Four equivalent solutions are found, but evolutionary considerations and polarization data select a single preferred model. While the current data sample only a limited range of latitudes owing to the long precessional cycle, the preferred model shows a beam elongated in the latitudinal direction and hourglass-shaped.

We report new measures of radial velocities and rotation rates (v sin i) for 51 F and early G stars in the open cluster NGC 2516 and combine these with previously published data. From high signal-to-noise ratio spectra of two stars, we show that NGC 2516 has a relative iron abundance with respect to the Pleiades of Δ[Fe/H] = +0.04 ± 0.07 at the canonical reddening of E(B-V) = 0.12, in contrast to previous photometric studies that placed the cluster 0.2-0.4 dex below solar. We construct a color-magnitude diagram based on radial velocity members and explore the sensitivity of photometric determinations of the metallicity and distance to assumed values of the reddening. For a metal abundance near solar, the Hipparcos distance to NGC 2516 is probably underestimated. Finally, we show that the distribution of rotation rates and X-ray emission does not differ greatly from that of the Pleiades, when allowance is made for the somewhat older age of NGC 2516.

We present a theoretical study of the radially pulsating δ Scuti and SX Phoenicis variables, concentrating on the blue straggler SX Phoenicis variables found in globular clusters. We have evolved a grid of stellar models with the metal abundance of the globular cluster M55, including models with alpha-enhanced metal abundances, and tested these models for radial pulsations observed in the high-amplitude δ Scuti and SX Phoenicis stars. Our grid includes models with globally enriched helium content to simulate the effects of stellar collisions and global mixing possible in blue stragglers. We find that global enrichment of helium strongly affects the temperature and luminosity of a given star, but the location of the instability strip blue edge and the slope of the period-luminosity (PL) relation are unchanged. This suggests that the PL relation is not affected by blue straggler formation if blue stragglers are fully mixed stellar mergers. Our blue edges and PL relations are in agreement with other theoretical determinations and also with the observational PL relation of M55, but they are not in agreement with the PL relation previously derived for high-amplitude δ Scuti stars in the field. Analysis of the double-mode variable, V41, suggests either that the star may not be pulsating in the first and second overtones as claimed or that normal stellar models may not be accurate models of blue straggler stars.

In the present study, we analyze the effects of a flux of Alfvén waves acting together with radiation pressure on grains as an acceleration mechanism of the wind of late-type stars. In the wind model, we simulate the presence of grains through a strong damping of the waves, and we use a nonisothermal profile for the temperature that is coherent with grain formation theories. We examine the changes in the velocity profile of the wind, and we show that if the grains are created in the region 1.1 < r/r₀ < 2.0, their presence will affect the mass loss and terminal velocity. The model is applied to a K5 supergiant star and to Betelgeuse (α Ori).

We examine the late stages of terrestrial planet formation around each star in the α Centauri A and α Centauri B binary system. Each integration begins with a "bimodal" mass distribution of 14 large embryos embedded in a disk of smaller planetesimals orbiting one of the stars. These initial conditions were chosen because when they are used in simulations about a single star with giant planets, they lead to systems most closely resembling our solar system. However, it is far from certain that such a planetesimal distribution actually occurs either in single or binary star systems. We follow the evolution of the accreting bodies at various values of the inclination of the midplane of the disk relative to the binary orbit for 200 Myr to 1 Gyr. In simulations in which the midplane of the disk was inclined ≤30° relative to the binary orbital plane, three to five terrestrial planets were formed around α Cen A. When the embryos in the disk were moving retrograde relative to the binary plane, four or five terrestrial planets formed. From two to five planets formed in a disk centered around α Cen B, with α Cen A perturbing the system in the same plane. The aforementioned resulting terrestrial planetary systems are quite similar to those produced by calculations of terrestrial planet growth in the Sun-Jupiter or Sun-Jupiter-Saturn systems. In contrast, terrestrial planet growth around a star lacking both stellar and giant planet companions is slower and extends to larger semimajor axis for the same initial disk of planetary embryos. In systems with the accreting disk initially inclined at 45° relative to the binary star orbit, two to five planets formed despite the fact that more than half of the disk mass was perturbed into the central star. When the disk was inclined at 60° to the binary plane, the stability of the planetary embryos decreased dramatically because of larger variations in eccentricity caused by the Kozai resonance, and almost all of the planetary embryos and planetesimals were lost from these systems.

Recent radio wave interplanetary scintillation observations have improved our knowledge of the level of turbulent fluctuations of plasma density in the inner solar wind at heliocentric distances of 16-26 R_☉ and less accurately outside this range. This information relates to fluctuations in the inertial subrange of the turbulence. Of particular concern in this paper is the heliocentric distance dependence of the power-law normalization parameter C. This measurement of C, when combined with independent measurements of the mean plasma density in the inner solar wind, indicates that the dimensionless amplitude of the turbulence ≡ σ_n/ is low, of the order of 6%-15%, where σ_n is the standard deviation of the density fluctuations and is the mean value. The deduced value for depends on an assumed value for the outer scale of the turbulence, l_o. I adopt a fiducial value of l_o = 1 R_☉. A comparison is made with independent estimates of , from both radio remote sensing in the same part of space and in situ measurements at greater distances. Satisfactory agreement occurs. I discuss the possibility that this dimensionless density amplitude might be a proxy for the corresponding amplitude of the magnetic field fluctuations. This is a controversial assertion. However, if it is the case, the measurements suggest that the turbulence in the inner solar wind is weak, in the sense of being dynamically and thermodynamically important.

Hydromagnetic structural and stability properties of global magnetic fields with current sheets are discussed in this paper. These fields describe solar coronal magnetic structures that form when a fresh magnetic field of opposite polarity has emerged into the corona containing a preexisting magnetic field. Three classes of axisymmetric fields are treated. The first is characterized by a continuous normal field distribution at the boundary and an infinitesimally thin current sheet in the field. As ideal hydromagnetic equilibria, these fields are globally stable, but they are resistively unstable within the current sheet. Magnetic reconnection is unavoidable no matter how large the electrical conductivity is. The other two classes of fields may be stable without this kind of resistive nonequilibrium. Of particular interest among them are equilibrium fields with finite-thickness current sheets in force balance with pressure gradients and gravity. These fields are shown to have enough free magnetic energy to let some parts of its field to become open during magnetic reconnection, an effect important for the dynamics of coronal mass ejections.

Recent Transition Region and Coronal Explorer observations of active loops in the EUV showed that there is a class of EUV loops that consists of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the uniform loop heating model. These results support coronal heating mechanisms operating in or near the chromosphere and transition region. We propose a new local heating model of loop footpoints in the chromosphere in which there occurs nonuniform heating by slingshot magnetic reconnection during two loops' interaction, driven by the moving solitary magnetic kink, which was recently found by three-dimensional MHD simulation and is characterized as a moving solitary magnetic flux ring with a pair of counterrotating vortex rings. We also show that the loop interaction results in the formation of helical up- and downflows driven by the slingshot magnetic reconnection.

We examine the global MHD instability of solar tachocline latitudinal differential rotation and the coexisting broad toroidal magnetic field, using a "shallow-water" model that captures the simplest effects of subadiabatic stratification. We assume a single fluid shell that has a fixed bottom but variable thickness. This model is the MHD generalization of a hydrodynamic model that we have previously applied to the tachocline, although the solution method is somewhat different. Stratification in the model is characterized by an "effective gravity" G (G = 0 for adiabatic stratification). The radiative (lower) part of the tachocline thus has high G (~10²) and the overshoot part, low G (less than 1). We obtain growth rates, phase velocities, and spatial structures of unstable modes for a wide range of toroidal field strengths and effective gravities, as well as differential rotations that are consistent with helioseismic observations. We recover known two-dimensional MHD stability results in the limit of large G and hydrodynamic instability results in the limit of vanishing toroidal field. For strong magnetic fields, only longitudinal wavenumber m = 1 is unstable, but for weak fields m = 2 is also. For peak toroidal fields of 20 kG and above, the growth rates and disturbance structures are essentially independent of the effective gravity, until it becomes so small that the fluid shell shrinks to zero in low latitudes, whereupon the instability is cut off. In contrast, the instability evolves radically at low G when toroidal field is increased from zero. In both overshoot and radiative parts of the tachocline, unstable modes grow fastest for toroidal fields of the order of 10² kG. The structure of the unstable disturbances is always governed by the latitude location of singular or critical points at which the Doppler-shifted phase velocity of the disturbance equals the local (angular) Alfvén speed. All unstable disturbances possess kinetic helicity, narrowly concentrated in the neighborhood of the same critical points. Just as shown by Dikpati & Gilman for the hydrodynamic case, such disturbances could provide an "α-effect" for the solar dynamo. But unlike the hydrodynamic case, this α-effect would be a function of the toroidal field itself.

The asymmetry of Stokes V profiles of the spectral lines λ6301.5 and λ6302.5 was utilized to systematically study a sunspot observed close to the disk center. The Stokes spectra were taken with the National Solar Observatory (NSO)/High Altitude Observatory Advanced Stokes Polarimeter (ASP). The NSO low-order adaptive optics system was used to record a data set of consistently high resolution. We find the following results from this analysis: (1) a strong correlation between the center-of-gravity (COG) velocity derived from the intensity profiles with the V-profile asymmetry; (2) the amplitude asymmetry is much more sensitive to changes in the COG velocity than the area asymmetry; and (3) plotting area versus amplitude asymmetry for the entire active region results in a bimodal distribution. Different areas within the active region, such as penumbra, umbra, light bridge, and small-scale fields outside the sunspot, are clearly separated in this plot. The light bridges and the small-scale magnetic fields surrounding the observed sunspot show larger amplitude asymmetry compared to the area asymmetry, whereas the penumbra shows larger area asymmetry. In comparison, the Stokes V spectra measured in the umbra show little area and amplitude asymmetry. In this paper, we use bisector analysis of the V intensity profile as a new tool to determine the gradients in the physical parameters in a more direct way. The gradients derived from the bisector method provide further and more direct evidence for the physical picture derived from the study of the asymmetries. For light bridges we find that the data is consistent with a picture of small convective cells confined to lower layers of the atmosphere. Asymmetries in the penumbra are caused by steep vertical gradients in the Evershed velocities in combination with the gradient in the line-of-sight inclination angle, confirming the earlier observations. For small-scale fields, the picture is consistent with the canopy effect. We also compare the average velocity and the magnetic field strength derived from this bisector analysis with the velocity and magnetic field strength derived from the ASP inversion and find excellent agreement between these independent methods. Apart from these asymmetries, we also observe extremely asymmetric V profiles, such as one-lobed profiles and multiple reversals, mostly at the edges of the limb-side penumbra. In these regions, we also find differences in the V profiles of λ6301.5 and λ6302.5 that suggest steep gradients in the physical parameters. The asymmetries derived from a single scan match well with the ones derived from the time-averaged properties obtained by averaging 14 such scans separated by 7.5 minutes. This suggests that, in a statistical sense, the Stokes V asymmetries do not vary with time and describe a global/general property of magnetic features found in regions such as light bridge, umbra, penumbra, and small-scale fields.

We are in the process of updating and extending the OPAL equation-of-state (EOS) and opacity data to include low-mass stars. The EOS part of that effort now is complete, and the results are described herein. The new data cover main-sequence stars having mass ≥0.1 M_☉. As a result of the more extreme matter conditions encountered with low-mass stars, we have added new physics. The electrons are now treated as relativistic, and we have improved our treatment of molecules. We also consider the implications of the new results for helioseismology.

The Sun is believed to have been the recipient of a substantial amount of metal-rich material over the course of its evolution, particularly in the early stages of the solar system. With a long diffusion timescale, the majority of this accreted matter should still exist in the solar convection zone, enhancing its observed surface abundance, and implying a lower abundance core. While helioseismology rules out solar models with near-zero metallicity cores, some solar models with enhanced metallicity in the convection zone might be viable, as small perturbations to the standard model. Because of the reduced interior opacity and core temperature, the neutrino flux predicted for such models is lower than that predicted by the standard solar model. This paper examines how compatible inhomogeneous solar models of this kind are with the observed low and intermediate degree p-mode oscillation data, and with the solar neutrino data from the Sudbury Neutrino Observatory Collaboration. We set an upper limit on how much metal-rich accretion took place during the early evolution of the Sun at ~2 M_⊕ of iron (or ~40 M_⊕ of meteoric material).

The observed clustering of the Solar and Heliospheric Observatory (SOHO) sungrazing comets is a product of runaway fragmentation that occurs throughout their orbits about the Sun. Since the sungrazers in tight pairs occasionally appear simultaneously in the field of view of the SOHO coronagraphs, their offsets can be used to determine their separation parameters, including the time of their parent's breakup, by applying the author's standard model for split comets. The fragmentation mode of seven sungrazer pairs is shown to differ fundamentally from that of a SOHO non-Kreutz double comet. Further support for runaway fragmentation is provided by a statistically significant argument that employs an orbit-based search for pairs among the sungrazers.

We show that the long-time-averaged "effective" flux of extraterrestrial particles (ETPs) on the Earth over a wide range of particle sizes is significantly different from that of particles incident on the Earth's atmosphere, as a result of the ablation and fragmentation of larger incident objects in the atmosphere. We have analytically deduced the "modified" size spectrum of the ETPs using the equilibrium fragmentation model developed by Dohnanyi for meteoroid population in the interplanetary space and direct observations on fragmentation of a large number of meteorites in the atmosphere. Our model shows that most of the secondary particles are produced in the fragmentation of objects of radii from 1 cm to 1 m, the size range where the differential number spectrum is relatively much flatter, compared to the spectrum on either end. A significant fraction of impacting mass, about 50%, is volatilized during ablation and would be globally deposited, as will particles of size smaller than a few microns, in contrast to the remaining mass of particles of greater than 10-20 μm size, which we expect would be deposited close to the point of impact of the object. Although the long-time-averaged particle fluxes of smaller particles are greatly enhanced, these particles would not be sampled efficiently in stratospheric collections, since they are produced in fragmentation of larger size objects whose impact rates are small. Thus, stratospheric collections would be biased toward collection of the primary incident ETPs, whereas terrestrial accumulations representing large (area × time) products, as in the case of ocean sediments, would efficiently sample the fragmented particles.

We have measured resonance strengths and energies for dielectronic recombination (DR) of Fe XIX forming Fe XVIII via N = 2 → N' = 2 and N = 2 → N' = 3 core excitations. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also calculated these resonance strengths and energies using two independent, state-of-the-art techniques: the perturbative multiconfiguration Breit-Pauli (MCBP) and multiconfiguration Dirac-Fock (MCDF) methods. Overall, reasonable agreement is found between our experimental results and theoretical calculations. The most notable discrepancies are for the 3l3l' resonances. The calculated MCBP and MCDF resonance strengths for the n = 3 complex lie, respectively, ≈47% and ≈31% above the measured values. These discrepancies are larger than the estimated ≲20% total experimental uncertainty in our measurements. We have used our measured 2 → 2 and 2 → 3 results to produce a Maxwellian-averaged rate coefficient for DR of Fe XIX. Our experimentally derived rate coefficient is estimated to be good to better than ≈20% for k_BT_e ≥ 1 eV. Fe XIX is predicted to form in photoionized and collisionally ionized cosmic plasmas at k_BT_e ≫ 1 eV. Hence, our rate coefficient is suitable for use in ionization balance calculations of these plasmas. Previously published theoretical DR rate coefficients are in poor agreement with our experimental results. None of these published calculations reliably reproduce the magnitude or temperature dependence of the experimentally derived rate coefficient. Our MCBP and MCDF results agree with our experimental rate coefficient to within ≈20%.

The pure rotational spectra of CrO (X⁵Π_r) and CrN (X⁴Σ^-) have been recorded using millimeter/submillimeter-wave spectroscopy in the frequency range 248-636 GHz. These radicals were created by the reaction of chromium vapor, produced in a Broida-type oven, with N₂O or NH₃ under DC discharge conditions. For CrO, 12 rotational transitions were recorded, in which up to five spin-orbit components were observed—the first measurement of the pure rotational spectrum of a molecule in a ⁵Π electronic state. Nine rotational transitions were recorded for CrN; here all four spin components were measured in every transition. The data obtained for both radicals were analyzed using an appropriate effective Hamiltonian. The resulting spectroscopic parameters of these species were determined to high precision, including those related to fine structure and lambda doubling. This work will enable radioastronomical searches for CrN and CrO to be carried out. CrO has already been observed in the optical spectra of stellar atmospheres.

Ultraviolet photolysis of various coronene-ice mixtures at low temperature and pressure caused the addition of amino (–NH₂), methyl (–CH₃), methoxy (–OCH₃), cyano/isocyano (–CN, –NC), and acid (–COOH) functional groups to the polycyclic aromatic hydrocarbon (PAH) coronene (C₂₄H₁₂), in addition to previously reported alcohol (–OH) and ketone (>C=O) formation. This work represents the first experimental evidence that ice photochemistry may have contributed to the aromatics bearing carbon and nitrogen containing side groups that are detected in primitive meteorites and interplanetary dust particles. Furthermore, these results suggest that a wide range of modified PAHs should be expected in interstellar ices and materials that predated solar system formation. The implications of these results for interstellar and meteoritic chemistry are discussed.

For decades, the determination of the mean density of matter (Ω_M) has been tied to the distribution of light. This has led to a "bias," perhaps as large as a factor of 2, in determining a key cosmological parameter. Recent measurements of the physical properties of clusters, the cosmic microwave background anisotropy, and the power spectrum of mass inhomogeneity now allow a determination of Ω_M without "visual bias." The early data lead to a consistent picture of the matter and baryon densities, with Ω_B = 0.039 ± 0.0075 and Ω_M = 0.33 ± 0.035, respectively.

The halo approach to large-scale structure provides a physically motivated model to understand the clustering properties of galaxies. An important aspect of the halo model involves a description of how galaxies populate dark matter halos or what is now called the halo occupation distribution. We discuss a way in which clustering information, especially in the nonlinear regime, can be used to determine moments of this halo occupation number. We invert the nonlinear part of the real space power spectrum from the IRAS Point Source Catalog galaxy redshift survey to determine the second moment of the halo occupation distribution in a model-independent manner. The precise measurement of higher order correlations can eventually be used to determine successive higher order moments of this distribution.

As part of a program to study the evolution of active galactic nuclei (AGNs) in clusters of galaxies, we present our results for A2104. A deep Chandra observation of this massive, z = 0.154 cluster reveals a significant X-ray point-source excess over the expectations of blank fields, including eight X-ray counterparts with R < 20 mag. Our spectroscopy shows that all six X-ray sources associated with red counterparts are cluster members and their X-ray properties are consistent with all of them being AGNs. Only one of the six has the emission lines characteristic of optically selected AGNs; the remaining five would not have been classified as AGNs based on their optical spectra. This suggests the existence of a large population of obscured, or at least optically unremarkable, AGNs in clusters of galaxies. These six sources correspond to a lower limit of ~5% of the AGN fraction in cluster galaxies with R < 20 mag (rest-frame M_V = -19.5 mag) and are comparable to the blue galaxy fraction in the cluster. Such an obscured AGN population in clusters of galaxies has many implications for cluster galaxy evolution, the hidden growth of their central, supermassive black holes, estimates of the star formation rate at infrared and radio wavelengths, and the observed variance in the hard X-ray background.

Relative ages for the globular cluster (GC) subpopulations in the Virgo giant elliptical galaxy M87 (NGC 4486) have been determined from Strömgren photometry obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. Using a variety of population synthesis models, and assuming the GC mass at the turnover of the luminosity function is the same for both subpopulations, differential ages have been determined from the observed magnitudes at the turnover of the globular cluster luminosity function and from the mean color of each subpopulation. We measure an age difference between the two subpopulations of 0.2 ± 1.5 (systematic) ± 2 (random) Gyr, in the sense that the blue GCs are formally older. Thus, to within our measurement errors, the two subpopulations are found to be coeval. Combined with previous spectroscopic age determinations for M87 GCs, our results favor a picture in which the GCs associated with this galaxy are formed at high redshift and within a period of a few gigayears.

With the Ultraviolet Visual Echelle Spectrograph mounted at the Very Large Telescope, we have observed at unprecedented spectral resolution the absorption spectrum toward reddened stars in the Magellanic Clouds over the wavelength range of 3500-10500 Å. This range covers the strong transitions associated with neutral and charged large carbon molecules of varying sizes and structures. We report the first detection of diffuse interstellar bands (DIBs) at 5780 and 5797 Å in the Small Magellanic Cloud and the variation of the 6284 Å DIB toward several targets in the Large Magellanic Cloud. The variation of DIBs in the Magellanic Clouds compared with Galactic targets may be governed by a combination of the different chemical processes prevailing in low-metallicity regions and the local environmental conditions.

We report the discovery of a microlensing candidate projected 2'54'' from the center of M32, on the side closest to M31. The blue color (R-I = 0.00 ± 0.14) of the source argues strongly that it lies in the disk of M31, while the proximity of the line of sight to M32 implies that this galaxy is the most likely host of the lens. If this interpretation is correct, it would confirm previous arguments that M32 lies in front of M31. If more events are discovered in this direction in a dedicated experiment, they could be used to measure the mass function of M32 up to an unknown scale factor. By combining microlensing observations of a binary-lens event with a measurement of the M31-M32 relative proper motion using the astrometric satellites Space Interferometry Mission or Global Astrometric Interferometer for Astrophysics, it will be possible to measure the physical separation of M31 and M32, the last of the six phase-space coordinates needed to assign M32 an orbit.

We describe first results of a spectroscopic probe of selected fields from the Grid Giant Star Survey. Multifiber spectroscopy of several hundred stars in a strip of 11 fields along δ ≈ -17°, in the range 12 hr ≲ α ≲ 17 hr, reveals a group of eight giants that have kinematical characteristics differing from the main field population but that as a group maintain coherent, smoothly varying distances and radial velocities with position across the fields. Moreover, these stars have roughly the same abundance, according to their MgH+Mgb absorption line strengths. Photometric parallaxes place these stars in a semiloop structure, arcing in a contiguous distribution between 5.7 and 7.9 kpc from the Galactic center. The spatial, kinematical, and abundance coherence of these stars suggests that they are part of a diffuse stream of tidal debris, and one roughly consistent with a wrapped, leading tidal arm of the Sagittarius dwarf spheroidal galaxy.

The MACHO and OGLE collaborations have argued that the three longest duration bulge microlensing events are likely caused by nearby black holes, given the small velocities measured with microlensing parallax and nondetection of the lenses. However, these events may be due to lensing by more numerous lower mass stars at greater distances. We find a posteriori probabilities of 76%, 16%, and 4% that the three longest events are black holes, assuming a Salpeter initial mass function (IMF) and a 40 M_☉ cutoff for neutron star progenitors; the numbers depend strongly on the assumed mass function but favor a black hole for the longest event for most standard IMFs. The longest events (>600 days) have an a priori ~26% probability of being black holes for a standard mass function. We propose a new technique for measuring the lens mass function using the mass distribution of long events measured with the Advanced Camera for Surveys on the Hubble Space Telescope, the Very Large Telescope Interferometer, the Space Interferometry Mission, or the Global Astrometric Interferometer for Astrophysics.

We have identified the third known accretion-powered millisecond pulsar, XTE J0929-314, with the Rossi X-Ray Timing Explorer. The source is a faint, high Galactic latitude X-ray transient (d ≳ 5 kpc) that was in outburst during 2002 April-June. The 185 Hz (5.4 ms) pulsation had a fractional rms amplitude of 3%-7% and was generally broad and sinusoidal, although occasionally double-peaked. The hard X-ray pulses arrived up to 770 μs earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of = (-9.2 ± 0.4) × 10^-14 Hz s^-1; the spin-down torque may arise from magnetic coupling to the accretion disk, a magnetohydrodynamic wind, or gravitational radiation from the rapidly spinning pulsar. The pulsations were modulated by a 43.6 minute ultracompact binary orbit, yielding the smallest measured mass function (2.7 × 10^-7M_☉) of any stellar binary. The binary parameters imply an ≃0.01 M_☉ white dwarf donor and a moderately high inclination. We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates. This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries.

We analyze a high-resolution, high signal-to-noise ratio spectrum of the carbon-rich, extremely metal poor star CS 29498-043, obtained with the Subaru Telescope High Dispersion Spectrograph. We find its iron abundance is extremely low ([Fe/H] = -3.7), placing it among the few stars known with [Fe/H] ≤ -3.5, while Mg and Si are significantly overabundant ([Mg/Fe] = +1.8 and [Si/Fe] = +1.1) compared with stars of similar metallicity without carbon excess. Overabundances of N and Al were also found. These characteristics are similar to the carbon-rich, extremely metal poor star CS 22949-037. Although the sample is small, our discovery of CS 29498-043 suggests the existence of a class of extremely metal poor stars with large excesses of C, N, Mg, and Si.

New astrometric analysis of four Wide Field Planetary Camera 2 images of the isolated neutron star RX J185635-3754 shows that its distance is 117 ± 12 pc, nearly double the originally published distance. We argue that the star's birthplace was in the Upper Scorpius association but that a connection with ζ Ophiuchi is now unlikely. Assuming birth in Upper Sco, the revised distance yields an age of 5 × 10⁵ yr and a space velocity of about 185 km s^-1. The radiation radius inferred from fitting the full spectral energy distribution lies between 12 and 26 km, with a best fit R_∞ = 16.4 ± 0.3 km for a two-blackbody model. These radii are in the range of many equations of state, both with and without exotic matter, and remove the observational support for an extremely soft equation of state.

The recent detection of X-ray emission from HH 2 and HH 154 with the Chandra and XMM-Newton satellites (respectively) have opened up an interesting, new observational possibility in the field of Herbig-Haro objects. In order to be able to plan further X-ray observations of other HH objects, it is now of interest to be able to estimate their X-ray luminosities in order to choose which objects to observe. This Letter describes a simple, analytic model for predicting the X-ray luminosity of a bow shock from the parameters of the flow (i.e., the size of the bow shock, its velocity, and the preshock density). The accuracy of the analytic model is analyzed through a comparison with the predictions obtained from axisymmetric, gasdynamic simulations of the leading working surface of an HH jet. We find that our analytic model reproduces the observed X-ray luminosities of HH 2 and HH 154, and we propose that HH 80/81 is a good candidate for future observations with Chandra.

The damping mechanism of recently discovered coronal loop transverse oscillations provides clues to the mechanism of coronal heating. We determine the scaling of the damping time with the parameters of the loops observed in extreme ultraviolet by the Transition Region and Coronal Explorer. We find excellent agreement of the scaling power to the power predicted by phase mixing and poor agreement with the power predicted by the wave leakage or ideal decay of the cylindrical kink mode mechanisms. Phase mixing leads to rapid dissipation of the Alfvén waves due to the variation of the Alfvén speed across the wave front and formation of small scales. Our results suggest that the loop oscillations are dissipated by phase mixing with anomalously high viscosity.

We report the detection of the 2s²2p²³P_{1 → 0} line of Si IX using the McMath-Pierce telescope on Kitt Peak. Observations were made of the solar disk and at various heights above the limb between 2002 May 13 and 17, under nonideal sky conditions, using the 13.5 m vertical spectrograph and an InSb single-diode detector. We report a new rest wavelength for the line and discuss its potential use as a diagnostic of coronal magnetic fields using ground-based telescopes. Our observations give λ_rest = 3.93434 ± 0.00007 μm, consistent with earlier less accurate values, but it places the blue wing of the line under a strong telluric N₂O line. In the active regions observed, the line's intensity is comparable to or larger than predicted in earlier work for the quiet Sun.

It is shown that a generalized magneto-Bernoulli mechanism can effectively generate high-velocity flows in the solar subcoronal regions; sharp amplification of the flow speed is accompanied by a significant fall in density.

We report low- and high-resolution spectra of comet C/2002 C1 (Ikeya-Zhang) from the McDonald Observatory. The comet has a well-developed ion tail including CO⁺, CO, CH⁺, and H₂O⁺. We used our high-resolution spectra to search for N. None was detected, and we placed upper limits on N/CO⁺ of 5.4 × 10^-4. N was detected in the low-resolution spectra, but we show that this emission was probably telluric in origin (if cometary, we derive N/CO⁺ = 5.5 × 10^-3, still very low). We discuss the implications for the conditions in the early solar nebula of the nondetection of N. These depend on whether the H₂O ice was deposited in the amorphous or crystalline form. If H₂O was deposited in its crystalline form, the detection of CO⁺ but not N has implications for H₂O/H₂ in the early solar nebula.

The intensity ratios of the 3s → 2p and 3d → 2p lines in Fe XVII were measured on the Livermore electron beam ion trap employing a complementary set of spectrometers, including a high-resolution crystal spectrometer and the Goddard 32 pixel calorimeter. The resulting laboratory data are in agreement with satellite measurements of the Sun and astrophysical sources in collisional equilibrium such as Capella, Procyon, and NGC 4636. The results disagree with earlier laboratory measurements and assertions that processes not accounted for in laboratory measurements must play a role in the formation of the Fe XVII spectra in solar and astrophysical plasmas.