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We have produced a new conformal map of the universe illustrating recent discoveries, ranging from Kuiper Belt objects in the solar system to the galaxies and quasars from the Sloan Digital Sky Survey. This map projection, based on the logarithm map of the complex plane, preserves shapes locally and yet is able to display the entire range of astronomical scales from the Earth's neighborhood to the cosmic microwave background. The conformal nature of the projection, preserving shapes locally, may be of particular use for analyzing large-scale structure. Prominent in the map is a Sloan Great Wall of galaxies 1.37 billion light-years long, 80% longer than the Great Wall discovered by Geller and Huchra and therefore the largest observed structure in the universe.

We show that large, high-redshift (z > 10) galaxies with virial temperature in excess of 10⁴ K may be composed mostly of cold atomic clouds that were formerly minihalos. These clouds move at a speed of ~15-30 km s^-1 and may collide with one another on an average time interval of ~10⁷ yr. The supersonic collisions may result in spatially distributed and efficient star formation. Most of the subsequent star formation in cold atomic clouds may be triggered by shock waves launched from the first stars formed in colliding clouds. Those shock-wave-compressed clouds may be more widespread spatially, because of the large imparted velocities, and some may escape into the intergalactic medium. The resultant widespread star formation would allow the possibility of a much higher ionizing photon escape fraction. These favorable conditions may form a physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth τ_e = 0.11-0.14. Furthermore, a chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in intergalactic medium, if minihalos are strongly clustered. In this case, a still higher τ_e would be achievable.

The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale gas inhomogeneities due to structure formation. These inhomogeneities include both collapsed minihalos, which are generally self-shielding, and lower density structures, which are not. The minihalos are dense and sufficiently optically thick to trap intergalactic ionization fronts, blocking their path and robbing them of ionizing photons until the minihalo gas is expelled as an evaporative wind. The lower density structures do not trap these fronts, but they can slow them down by increasing the overall recombination rate in the intergalactic medium (IGM). In this paper we study the effects of both types of inhomogeneities, including nonlinear clustering effects, and we find that both IGM clumping and collapsed minihalos have significant yet qualitatively different impacts on reionization. While the number density of minihalos on average increases strongly with time, the density of minihalos inside H II regions around ionizing sources is largely constant. Thus the impact of minihalos is essentially to decrease the number of ionizing photons available to the IGM at all epochs, which is equivalent to a reduction in the luminosity of each source. On the other hand, the effect of IGM clumping increases strongly with time, slowing down reionization and extending it. Thus while the impact of minihalos is largely degenerate with the unknown source efficiency, IGM clumping can help significantly in reconciling the recent observations of cosmic microwave background polarization with quasar absorption spectra at z ~ 6, which together point to an early but extended reionization epoch.

We present a semianalytic model for cold dark matter halo substructure that can be used as a framework for studying the physics of galaxy formation and as an ingredient in halo models of galaxy clustering. The model has the following main ingredients: (1) extended Press-Schechter mass accretion histories, (2) host halo density profiles computed according to the trends observed in cosmological simulations, (3) distributions of initial orbital parameters of accreting subhalos measured in a high-resolution simulation of three Milky Way-size halos, and (4) integration of the orbital evolution of subhalos including the effects of dynamical friction and tidal mass loss. We perform a comprehensive comparison of the model calculations to the results of a suite of high-resolution cosmological simulations. The comparisons show that subhalo statistics such as the velocity and mass functions, the radial distributions, and the halo occupation distributions agree well over 3 orders of magnitude in host halo mass and at various redshifts. We find that both in the simulations and in our model the radial distributions of subhalos are significantly shallower than that of the dark matter density. The abundance of subhalos in a host is set by competition between tidal disruption and new accretion. Halos of high mass and halos at high redshift tend to host more subhalos because the subhalos have, on average, been accreted more recently. Similarly, at a fixed mass and epoch, halos that formed more recently host a larger number of subhalos. Observed "fossil groups" may represent an extreme tail of this correlation. We find a related correlation between host halo concentration and satellite abundance at fixed host mass, N_sat ∝ c, where a changes with redshift and host-to-subhalo mass ratio. Lastly, we use our substructure model to populate host halos in one of the high-resolution cosmological simulations, replacing the actual subhalos resolved in this simulation and using the host mass as the only input for the model calculation. We show that the resulting correlation function of such a hybrid halo ensemble is indistinguishable from that measured directly in the simulation. This supports one of the key tenets of the standard halo model, i.e., the assumption that the halo occupation distribution is statistically independent of the host halo environment.

We consider the feasibility of detecting Population III pair-instability supernovae (PISNe) at very high redshifts with the James Webb Space Telescope (JWST). Four published estimates for the PISN rate show a rather wide dispersion, between 50 and 2200 deg^-2 yr^-1. Correcting problems with several of these, we conclude that even a fairly optimistic estimate is probably a further order of magnitude lower than this range, at a rate of the order of 4 deg^-2 yr^-1 at z ~ 15 and 0.2 deg^-2 yr^-1 at z ~ 25, both with substantial uncertainty. Although such supernovae (SNe) would be bright enough to be readily detectable with the JWST at any relevant redshift, the lower number densities derived here would likely require either a dedicated wide-angle search strategy or a serendipitous search. We expect that typically about 1 deg² (or 500 JWST NIRCam images) at 4.5 μm must be imaged to detect one PISN at z ~ 15 and about 35 deg² to detect one at z ~ 25. If some Population III star formation persists to lower redshifts at z ~ 5, then PISNe may also be detectable in wide-angle ground-based Z-band imaging surveys at Z_AB ~ 23, at a density of the order of 1 deg^-2 of surveyed area. In the Appendix we consider the possible effects of intergalactic dust in obscuring high-redshift SNe or other high-redshift sources. We show that the obscuration at a given rest wavelength will peak at some maximum redshift and thereafter decline. While it may be a significant effect in observations of the very high redshift universe, it is unlikely, even under rather pessimistic assumptions, to completely obscure primordial objects.

Photometric (BVI) and redshift data corrected for streaming motions are compiled for 111 "Branch-normal," four 1991T-like, seven 1991bg-like, and two unusual supernovae of Type Ia (SNe Ia). Color excesses E(B - V)_host of normal SNe Ia, due to the absorption of the host galaxy, are derived by three independent methods, giving excellent agreement leading to the intrinsic colors at maximum of (B - V)⁰⁰ = -0.024 ± 0.010 and (V - I)⁰⁰ = -0.265 ± 0.016 if normalized to a common decline rate of Δm₁₅ = 1.1. The strong correlation between redshift absolute magnitudes (based on an arbitrary Hubble constant of H₀ = 60 km s^-1 Mpc^-1), corrected only for the extrinsic Galactic absorption, and the derived E(B - V)_host color excesses leads to the well-determined yet abnormal absorption-to-reddening ratios of _BVI = 3.65 ± 0.16, 2.65 ± 0.15, and 1.35 ± 0.21. Comparison with the canonical Galactic values of 4.1, 3.1, and 1.8 forces the conclusion that the law of interstellar absorption in the path length to the SN in the host galaxy is different from the local Galactic law, a result consistent with earlier conclusions by others. Improved correlations of the fully corrected absolute magnitudes (on the same arbitrary Hubble constant zero point) with host galaxy morphological type, decline rate, and intrinsic color are derived. We recover the result that SNe Ia in E/S0 galaxies are ~0.3 mag fainter than in spiral galaxies for possible reasons discussed in the text. The new decline rate corrections to absolute magnitudes are smaller than those by some authors for reasons explained in the text. The four spectroscopically peculiar 1991T-type SNe are significantly overluminous as compared to Branch-normal SNe Ia. The overluminosity of the seven 1999aa-like SNe is less pronounced. The seven 1991bg types in the sample constitute a separate class of SNe Ia, averaging in B 2 mag fainter than the normal Ia. New Hubble diagrams in B, V, and I are derived out to ~30,000 km s^-1 using the fully corrected magnitudes and velocities, corrected for streaming motions. Nine solutions for the intercept magnitudes in these diagrams show extreme stability at the 0.02 mag level using various subsamples of the data for both low and high extinctions in the sample, proving the validity of the corrections for host galaxy absorption. We shall use the same precepts for fully correcting SN magnitudes for the luminosity recalibration of SNe Ia in the forthcoming final review of our Hubble Space Telescope Cepheid-SN experiment for the Hubble constant.

Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to search 129 known Lyα absorption systems at z < 0.15 toward 31 active galactic nuclei (AGNs) for corresponding absorption from higher Lyman lines and the important metal ions O VI and C III. We detect O VI in 40 systems, over a smaller range of column density (log N = 13.0-14.35) than seen in H I (log N = 13.0-16.0). The coexistence of O VI and H I suggests a multiphase IGM with warm photoionized and hot ionized components. With improved O VI detection statistics, we find a steep distribution in O VI column density, d/dN ∝ N, suggesting that numerous weak O VI absorbers contain baryonic mass comparable to the rare strong absorbers. Down to 30 mÅ equivalent width (O VI λ1032) we find an absorber frequency d/dz ≈ 17 ± 3. The total cosmological mass fraction in this hot gas is at least Ω_WHIM = (0.0022 ± 0.0003)[h₇₀(Z_O/0.1 Z_☉)(f/0.2)]^-1, where we have scaled to fiducial values of oxygen metallicity, O VI ionization fraction, and the Hubble constant. Gas in the warm-hot intergalactic medium (WHIM) at 10⁵-10⁶ K contributes at least 4.8% ± 0.9% of the total baryonic mass at z < 0.15. We then combine empirical scaling relations for the observed "multiphase ratio," N/N ∝ N, and for hydrogen overdensity in cosmological simulations, N ∝ δ, with the H I photoionization correction to derive the mean oxygen metallicity, Z_O ≈ (0.09 Z_☉)(f/0.2)^-1 in the low-z multiphase gas. Given the spread in the empirical relations and in f, the baryon content in the O VI WHIM could be as large as 10%. Our survey is based on a large improvement in the number of O VI absorbers (40 vs. 10) and total redshift path length (Δz ≈ 2.2 vs. Δz ≈ 0.5) compared to earlier surveys.

On the basis of the disk galaxy formation theory within the framework of the standard ΛCDM hierarchical picture (Mo, Mao, & White), we selected modeled damped Lyα systems (DLAs), according to their observational criterion N 10^20.3 cm^-2 by Monte Carlo simulation using random inclinations to examine their observed properties. By best-fitting the predicted metallicity distribution to the observed ones, we get an effective yield of about 0.25 Z_☉ for DLAs, which is comparable to those for the SMC and LMC. And the predicted distribution is the same as that of the observations at a significance level of higher than 60%. The predicted column density distribution of modeled DLAs is compared with the observed ones with the corresponding number density, with a discussion of the gas content. We found that the predicted number density n(z) at redshift 3 agrees well with the observed value, but the gas content Ω_DLA is about 3 times larger than that observed, since our model predicts more DLAs with higher column density. It should be noted that the predicted star formation rate density contributed by DLAs is consistent with the most recent observations if the star formation timescale in DLAs is assumed to be 1-3 Gyr. Meanwhile, the connection between DLAs and Lyman break galaxies (LBGs) is discussed by comparing their UV luminosity functions, which shows that the DLAs' host galaxies are much fainter than LBGs. We also predict that only a few percent of DLAs can host LBGs, which is also consistent with current observations. However, there is a discrepancy between model prediction and observation in the correlation between metallicity and H I column density for DLAs. We suggest that this could result from either the inadequacy of a Schmidt-type star formation law at high redshift, the diversities of DLA populations, or the model limitations. Although our current simple model cannot fully reproduce the observed DLA velocity distribution, we argue that this kind of model can still provide valuable information about the nature of DLAs.

We study the spectroscopic properties of a sample of 10³ void galaxies identified in the Sloan Digital Sky Survey (SDSS) and compare these with the properties of galaxies in higher density regions (wall galaxies). This sample of void galaxies covers the range of absolute magnitude from M_r = -13.5 to M_r = -22.5 in regions with density contrast δρ/ρ < -0.6. We compare the equivalent widths of Hα, [O II], [N II], Hβ, and [O III] of void and wall galaxies with similar luminosities. We find that void galaxies have larger emission line equivalent widths, indicating that they are forming stars at a higher rate. A comparison of the Balmer break, as measured by the parameter D_n(4000), reveals that void galaxies have younger stellar populations than wall galaxies. Using standard techniques, we estimate Hα and [O II] star formation rates (SFRs) of the void and wall galaxies. Combining these measurements with estimates of the stellar masses, we find specific star formation rates (SFR per unit stellar mass) for void galaxies that are generally higher than for wall galaxies, consistent with the results from the equivalent widths.

In order to better understand the origin and evolution of relic radio bubbles in clusters of galaxies, we report on an extensive set of two-dimensional MHD simulations of hot buoyant bubbles evolving in a realistic intracluster medium (ICM). Our bubbles are inflated near the base of the ICM over a finite time interval from a region whose magnetic field is isolated from the ICM. We confirm both the early conjecture from linear analysis and the later results based on preformed MHD bubbles, namely, that very modest ICM magnetic fields can stabilize the rising bubbles against disruption by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. We find in addition that amplification of the ambient fields as they stretch around the bubbles can be sufficient to protect the bubbles or their initial fragments even if the fields are initially much too weak to play a significant role early in the evolution of the bubbles. Indeed, even with initial fields less than 1 μG and values of β = P_g/P_b approaching 10⁵, magnetic stresses in our simulations eventually became large enough to influence the bubble evolution. Magnetic field influence also depends significantly on the geometry of the ICM field and on the topology of the field at the bubble/ICM interface. For example, reconnection of antiparallel fields across the bubble top greatly reduced the ability of the magnetic field to inhibit disruptive instabilities. Our results confirm earlier estimates of 10⁸ yr for relic radio bubble lifetimes and show that magnetic fields can account for the long-term stability of these objects against disruption by surface instabilities. In addition, these calculations show that lifting and mixing of the ambient ICM may be a critical function of field geometries in both the ICM and the bubble interior.

Using Chandra archival data, we quantify the evolution of cluster morphology with redshift. Clusters form and grow through mergers with other clusters and groups, and the amount of substructure in clusters in the present epoch and how quickly it evolves with redshift depend on the underlying cosmology. Our sample includes 40 X-ray-selected, luminous clusters from the Chandra archive, and we quantify cluster morphology using the power ratio method of Buote & Tsai. The power ratios are constructed from the moments of the X-ray surface brightness and are related to a cluster's dynamical state. We find that, as expected qualitatively from hierarchical models of structure formation, high-redshift clusters have more substructure and are dynamically more active than low-redshift clusters. Specifically, the clusters with z > 0.5 have significantly higher average third- and fourth-order power ratios than the lower redshift clusters. Of the power ratios, P₃/P₀ is the most unambiguous indicator of an asymmetric cluster structure, and the difference in P₃/P₀ between the two samples remains significant even when the effects of noise and other systematics are considered. After correcting for noise, we apply a linear fit to P₃/P₀ versus redshift and find that the slope is greater than zero at better than 99% confidence. This observation of structure evolution indicates that dynamical state may be an important systematic effect in cluster studies seeking to constrain cosmology, and when calibrated against numerical simulations, structure evolution will itself provide interesting bounds on cosmological models.

For measurement of the active galactic nucleus (AGN) luminosity function and its evolution, X-ray selection samples all types of AGNs and provides reduced obscuration bias in comparison with UV excess or optical surveys. The apparent decline in optically selected quasars above z ~ 3 may be strongly affected by such a bias. The Chandra Multiwavelength Project (CHAMP) is characterizing serendipitously detected X-ray sources in a large number of fields with archival Chandra imaging. We present a preliminary measure of the comoving space density using a sample of 311 AGNs found in 23 CHAMP fields (~1.8 deg²) supplemented with 57 X-ray-bright AGNs from the Chandra Deep Field-North and Chandra Deep Field-South. Within our X-ray flux (f_{0.3-8.0 keV} > 4 × 10^-15 ergs cm^-2 s^-1) and optical magnitude (r' < 22.5) limits, our sample includes 14 broad emission-line AGNs at z > 3. Using this X-ray-selected sample, we detect a turnover in the comoving space density of luminous type 1 AGNs (log L_X > 44.5 ergs s^-1, measured in the 0.3-8.0 keV band and corrected for Galactic absorption) at z > 2.5. Our X-ray sample is the first to show a behavior similar to the well-established evolution of the optical quasar luminosity function. A larger sample of high-redshift AGNs and with a greater fraction of identified sources, either spectroscopic or photometric, at faint optical magnitudes (r' > 22.5) are required to remove the remaining uncertainty in our measure of the X-ray luminosity function, particularly given the possibility that AGNs might be more easily obscured optically at high redshift. We confirm that for z < 1, lower luminosity AGNs (log L_X < 44.5) are more prevalent by more than an order of magnitude than those with high luminosity. We have combined the Chandra sample with AGNs from the ROSAT surveys to present a measure of the space density of luminous type 1 AGNs in the soft X-ray band (0.5-2.0 keV) that confirms the broadband turnover described above.

The Whipple Observatory 10 m γ-ray telescope has been used to survey the error boxes of EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. Twenty-one unidentified sources detected by EGRET (more than 10% of the total number) have been included in this survey. In no case is a statistically significant signal found in the EGRET error box, which implies that, at least for this sample, the γ-ray spectra of these sources steepen between 100 MeV and 350 GeV. For each EGRET source location, we list candidate associations and derive upper limits on the integral γ-ray flux above 350 GeV.

We present new ultraviolet photometry of the jet in M87 obtained from Hubble Space Telescope (HST) WFPC2 imaging. We combine these ultraviolet data with previously published photometry for the knots of the jet in radio, optical, and X-ray and fit three theoretical synchrotron models to the full data set. The synchrotron models consistently overpredict the flux in the ultraviolet when fitted over the entire data set. We show that if the fit is restricted to the radio through ultraviolet data, the synchrotron models can provide a good match to the data. The break frequencies of these fits are much lower than previous estimates. The implied synchrotron lifetimes for the bulk of the emitting population are longer than in earlier work but still much shorter than the estimated kinematic lifetimes of the knots. The observed X-ray flux cannot be successfully explained by the simple synchrotron models that fit the ultraviolet and optical fluxes. We discuss the possible implications of these results for the physical properties of the M87 jet. We also observe increased flux for the HST-1 knot that is consistent with previous results for flaring. This observation fills in a significant gap in the time coverage early in the history of the flare and therefore sets constraints on the initial brightening of the flare.

We explore the galaxian luminosity-metallicity (L-Z) relationship in both the optical and the near-infrared (NIR) using a combination of optical photometric and spectroscopic observations from the Kitt Peak National Observatory (KPNO) International Spectroscopic Survey (KISS) and NIR photometry from the Two Micron All Sky Survey (2MASS). We supplement the 2MASS data with our own NIR photometry for a small number of lower luminosity emission-line galaxies (ELGs) that are underrepresented in the 2MASS database. Our B-band L-Z relationship includes 765 star-forming KISS galaxies with coarse abundance estimates from our follow-up spectra, while the correlation with KISS and 2MASS yields a total of 420 galaxies in our J-band L-Z relationship. We explore the effect that changing the correlation between the strong-line abundance diagnostic R₂₃ and metallicity has on the derived L-Z relation. We find that the slope of the L-Z relationship decreases as the wavelength of the luminosity bandpass increases. We interpret this as being, at least in part, an effect of internal absorption in the host galaxy. Furthermore, the dispersion in the L-Z relation decreases for the NIR bands, suggesting that variations in internal absorption contribute significantly to the observed scatter. We propose that our NIR L-Z relations are more fundamental than the B-band relation, since they are largely free of absorption effects and the NIR luminosities are more directly related to the stellar mass of the galaxy than are the optical luminosities.

We present BV CCD surface photometry, profiles, and images of the galaxy ESO 383-45, together with other galaxies in the same CCD field. We also present a B - V color map of the field and images of the galaxies enhanced by self-correlation of pixel values and by digital "unsharp masking." The extended halo and system of filaments of ESO 383-45 are seen clearly. We suggest that the evidence (radio jets and their curvature and areas of diffuse optical emission) of a dense intergalactic medium (IGM) in this field toward the center of the IC 4296 cluster may indicate that the galaxy ESO 383-45 is still undergoing ram pressure stripping of its gas, forming stars in the filaments, while the central galaxy has evolved to have a lenticular morphology. On the other hand, there are "knots" in the filaments that look like tidal dwarf galaxies in formation, and previous simulations of the tidal interaction of two disk galaxies have produced galaxies that can resemble ESO 383-45 from certain viewing angles. Other galaxies in the field appear to lie beyond the IC 4296 cluster and may be part of sheets of galaxies previously identified as connecting the Abell clusters of the Shapley supercluster. We identify many uncataloged faint extended objects that may represent background clusters of galaxies or knots (possibly of star formation) associated with the filaments and diffuse IGM of ESO 383-45. The present work represents the first multicolor surface photometric study for all of these galaxies, and only ESO 383-45 has previously been studied morphologically, using digitally co-added Schmidt plates obtained by some of the authors.

The central, or x₁, family of periodic orbits is the most important one in almost all two-dimensional numerical models of galactic bars in the literature. However, we present evidence that in two-dimensional models with sufficiently large bar axial ratios (a/c 6), stable orbits having propeller shapes play the dominant role. In our models this propeller family is in fact a distant relative of the x₁ family. There are also intermediate cases in which both families are important. The dominance of one family over the other may have direct consequences on the morphological properties of the bars that can be constructed from them, properties such as face-on bar thinness and strength as well as the boxiness of the outer isophotes.

We present kinematic measurements of the thick and thin disks in two edge-on galaxies. We have derived stellar rotation curves at and above the galaxies' midplanes using Ca II triplet features measured with the GMOS spectrograph on Gemini North. In one galaxy, FGC 1415, the kinematics above the plane shows clear rotation that lags that of the midplane by ~20%-50%, similar to the behavior seen in the Milky Way. However, the kinematics of the second galaxy, FGC 227, is quite different. The rotation above the plane is extremely slow, showing 25% of the rotation speed of the stars at the midplane. We decompose the observed rotation curves into a superposition of thick- and thin-disk kinematics, using two-dimensional fits to the galaxy images to determine the fraction of thick-disk stars at each position. We find that the thick disk of FGC 1415 rotates at 30%-40% of the rotation speed of the thin disk. In contrast, the thick disk of FGC 227 is very likely counterrotating if it is rotating at all. These observations are consistent with the velocity dispersion profiles that we measure for each galaxy. The detection of counterrotating thick disks conclusively rules out models in which the thick disk forms either during monolithic collapse or from vertical heating of an earlier thin disk. Instead, the data strongly support models in which the thick disk forms from direct accretion of stars from infalling satellites.

We present the results of a single-dish beam-matched survey of the three lowest rotational transitions of CO in a sample of 20 local (D 70 Mpc) luminous compact blue galaxies (LCBGs). These ~L*, blue, high surface brightness, starbursting galaxies were selected on the same criteria used to define LCBGs at higher redshifts. Our detection rate was 70%, with those galaxies having L_B < 7 × 10⁹ L_☉ not detected. We find that the H₂ masses of local LCBGs range from 6.6 × 10⁶ to 2.7 × 10⁹ M_☉, assuming a Galactic CO-to-H₂ conversion factor. Combining these results with our earlier H I survey of the same sample, we find that the ratio of molecular to atomic gas mass is low, typically 5%-10%. Using a large velocity gradient model, we find that the average gas conditions of the entire interstellar medium in local LCBGs are similar to those found in the centers of star-forming regions in our Galaxy and in the nuclear regions of other galaxies. Star formation rates, determined from IRAS fluxes, are a few M_☉ yr^-1, much higher per unit dynamical mass than normal spiral galaxies. If this rate remains constant, the molecular hydrogen depletion timescales are short, ~10-200 Myr.

We propose a simple model for the formation of dwarf spheroidal galaxies, in which stars are assumed to have formed from isothermal gas in hydrostatic equilibrium inside extended dark matter halos. After expelling the leftover gas, the stellar system undergoes a dynamical relaxation inside the dark matter halo. These models can adequately describe the observed properties of three (Draco, Sculptor, and Carina) out of four Galactic dwarf spheroidal satellites studied in this paper. We suggest that the fourth galaxy (Fornax), which cannot be fitted well with our model, is observed all the way to its tidal radius. Our best-fitting models have virial masses of ~10⁹ M_☉, halo formation redshifts consistent with the age of oldest stars in these dwarfs, and shallow inner dark matter density profiles (with slope γ ~ -0.5, ... ,0). The inferred temperature of gas is ~10⁴ K. In our model, the "extratidal" stars observed in the vicinity of some dwarf spheroidal galaxies are gravitationally bound to the galaxies and are a part of the extended stellar halos. The inferred virial masses make Galactic dwarf spheroidals massive enough to alleviate the "missing satellites" problem of Λ cold dark matter cosmologies.

We present new 2.2 μm diffraction-limited images from the W. M. Keck 10 m and Gemini 8 m telescopes of the cool Galactic center sources, IRS 1W, 5, 8, 10W, and 21, along with new proper motions for IRS 1W, 10W, and 21. These observations were carried out to test the bow shock hypothesis presented by Tanner et al. as an alternative to a very recent (10⁴ yr) epoch of star formation within the tidal stream of gas and dust known as the northern arm. Resolved asymmetric structure is detected in all the sources, with bow shock morphologies associated with IRS 1W, 5, 8, and 10W. For IRS 1W and 10W, there is an agreement between the position angle of the asymmetry and that of the inferred relative velocity vector of the near-infrared source with respect to the northern arm gas, strengthening the bow shock hypothesis. We therefore conclude that the observed morphology is indeed a bow shock generated by sources plowing through the northern arm. Furthermore, the large extent of the resolved structures (310-1340 AU), along with their luminosities (~10⁴-10⁵ L_☉), suggests that their central sources are Wolf-Rayet stars, comparable to the broad He emission-line stars, which have strong winds on the order of 1000 km s^-1. The bow shock geometry, along with the proper motion measurements, provide three-dimensional orbital solutions for this enigmatic class of objects. The orientations of the orbital planes of IRS 1W and 10W are consistent with that of the putative clockwise plane which has been proposed as a solution for the He I emission-line stars. While these observations eliminate the need to invoke star formation within the northern arm, they increase by 10% the total known population of massive, young stars with strong winds, whose origin remains unexplained in the context of the nearby supermassive black hole.

We present an X-ray absorption line spectroscopic study of the large-scale hot interstellar medium (HISM) in the Galaxy. We detect Ne IX Kα absorption lines in the Chandra grating spectra of seven Galactic low-mass X-ray binaries. Three of these sources also show absorption of O VII Kα, O VII Kβ, and/or O VIII Kα. Both the centroid and width of the lines are consistent with a Galactic HISM origin of the absorption. By jointly fitting the multiple lines, accounting for line saturation, and assuming the collisional ionization equilibrium, we estimate the average absorbing gas temperature as ~(2.4 ± 0.3) × 10⁶ K (90% confidence errors). We further characterize the spatial density distribution of the gas as 6.4 exp(-|z|/1.2 kpc) × 10^-3 cm^-3 (a disk morphology) or 6.2[1 + (R/2.3 kpc)²]^-1 × 10^-3 cm^-3 (a sphere morphology), where z and R are the distances from the Galactic plane and Galactic center (GC) respectively. Since nearly all the sight lines with significant absorption lines detected are somewhat toward GC and at low Galactic latitudes, these results could be severely biased. More observations toward off-GC sight lines and at high latitudes are urgently needed to further the study. Nevertheless, the results demonstrate the excellent potential of X-ray absorption line spectroscopy in the study of the HISM.

We use nondiffusive, nonrelativistic, test-particle numerical simulations to address the physics of particle acceleration by collisionless shocks. We focus on the importance of the shock normal angle, θ_Bn, in determining the energy spectrum of the accelerated particles. For reasonable parameters, we find that the injection velocity is weakly dependent on the mean shock normal angle and that low-energy particles are readily accelerated to high energies irrespective of θ_Bn. Our results are applicable for shocks that are nearly planar on scales larger than the coherence scale of the upstream magnetic turbulence and for particles whose gyroradii are smaller than this scale. We confirm previous results showing that the acceleration rate is larger for nearly perpendicular shocks compared to parallel shocks. However, we also find that the acceleration rate at parallel shocks moving through large-scale magnetic fluctuations is larger than that predicted by simple first-order Fermi acceleration. Our results can be understood in terms of the nature of the large-scale fluctuations and their effect on particle transport.

We discuss observations of the magnetic field, column density, and turbulence in the cold neutral medium (CNM). The observed quantities are only indirectly related to the intrinsic astronomical ones. We relate the observed and intrinsic quantities by relating their univariate and bivariate probability distribution functions (pdf's). We find that observations of the line-of-sight component of a magnetic field do not constrain the pdf of the total field B_tot very well but do constrain the median value of B_tot. In the CNM, we find a well-defined median magnetic field 6.0 ± 1.8 μG. The CNM magnetic field dominates thermal motions. Turbulence and magnetism are in approximate equipartition. We find that the probability distribution of column density N_⊥(H ) in the sheets closely follows N_⊥(H )^-1 over a range of 2 orders of magnitude, 0.026 N_⊥(H ) 2.6 × 10²⁰ cm^-2. The bivariate distributions are not well enough determined to constrain structural models of CNM sheets.

Much of the baryonic matter in the universe is in the form of H₂, which includes most of the gas in Galactic and extragalactic interstellar clouds. Molecular hydrogen plays a significant role in establishing the thermal balance in many astrophysical environments and can be important as a spectral diagnostic of the gas. Modeling and interpretation of observations of such environments requires a quantitatively complete and accurate treatment of H₂. Using this microphysical model of H₂, we present illustrative calculations of prototypical astrophysical environments. This work forms the foundation for future investigations of these and other environments in which H₂ is an important constituent.

We have developed an empirical and effective set of criteria, based on the Two Micron All Sky Survey (2MASS) colors, to select candidate classical T Tauri stars (CTTSs). This provides a useful tool to study the young stellar population in star-forming regions. Here we present our analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN 1616, LDN 1634, and Ori East to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiation-driven implosion model, in which the ionization fronts from OB stars compress a nearby cloud until the local density exceeds the critical value, thereby inducing the cloud to collapse to form stars. We find that only BRCs associated with strong IRAS 100 μm emission (a tracer of high density) and Hα emission (a tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding H II regions, and the ages of CTTSs, are consistent with sequential star formation. We also find that CTTSs are only seen between the OB stars and the BRCs, with those closer to the BRCs being progressively younger. There are no CTTSs leading the ionization fronts, i.e., within the molecular clouds. All of these findings provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the star-forming activities in the region.

We present new observations of circular polarization (CP) at 2.2 μm in the Orion (OMC-1) molecular cloud. Our results extend a previously published study of the region. We show that the degree of CP correlates spatially with the molecular cloud and appears to be generally very low in regions dominated by H II. We detect a feature with 3%-5% CP that extends approximately 60'' to the southwest of the BN/IRc2 region. Although the morphology of the observed CP is broadly consistent with a model in which radiation from a central source (probably IRc2) is scattered by aligned spheroidal grains, we conclude that dichroic extinction in the foreground molecular cloud also plays a major role in its production. Implications of our results for the hypothesis that CP radiation imposes chiral asymmetry upon prebiotic organic molecules in protoplanetary disks are discussed. Mechanisms invoked to explain the observed CP in the near infrared can also produce CP in the range of ultraviolet wavelengths capable of chiral selection by photolysis; however, the polarized flux is likely to be of limited spatial extent and to have lower percentage CP compared with the infrared.

Using the Owens Valley and Nobeyama Radio Observatory interferometers, we carried out an unbiased search for hot molecular cores and ultracompact (UC) H II regions toward the high-mass star-forming region G19.61-0.23. In addition, we performed 1.2 mm imaging with SIMBA and retrieved 3.5 and 2 cm images from the VLA archive database. The newly obtained 3 mm image brings information on a cluster of high-mass (proto)stars located in the innermost and densest part of the parsec-scale clump detected in the 1.2 mm continuum. We identify a total of 10 high-mass young stellar objects: one hot core (HC) and nine UC H II regions, whose physical parameters are obtained from model fits to their continuum spectra. The ratio between the current and expected final radii of the UC H II regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars formed simultaneously. Under the opposite assumption, namely, that star formation occurred randomly, we estimate that the HC lifetime is less than ~1/3 of that of UC H II regions on the basis of the source number ratio between them.

We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model possessing a central optically thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T_s ~1000-1500 K). Furthermore, we find a tight correlation of disk size with source luminosity R ∝ L^1/2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0-B3 stars) are undersized compared to predictions of the "optically thin cavity" model, likely because of optically thick gas within the inner AU.

We have mapped the CO, HCO⁺, N₂H⁺, and CS emission around a nearby T Tauri star, IRAS 16316-1540. A molecular outflow is seen in CO, HCO⁺, N₂H⁺, and CS emission originating from the IRAS source, while an envelope is seen in N₂H⁺ and HCO⁺ emission surrounding the IRAS source. The molecular outflow is bipolar with a southeastern (SE) lobe and a northwestern (NW) lobe. The structure and the kinematics of the SE lobe are well defined and can be explained with a simple kinematic model. In this model, the SE lobe is a U-shaped shell consisting of a wide-opening base mimicking the reflection nebula RNO 91 and a cylindrical shell with a constant radius mimicking the CO outflow shell. The wide-opening base is expanding toward the circumstellar envelope, while the cylindrical shell is expanding mainly laterally into the ambient medium. If the base of the SE lobe continues to expand at its current velocity, it will meet with the base of the NW lobe and disperse the circumstellar envelope in a few times 10⁴ yr. The circumstellar envelope is elongated perpendicular to the outflow axis, extending to the northeast and southwest of the IRAS source. The envelope has differential rotation with the velocity increasing toward the source. It may result from material infalling toward the source carrying its angular momentum. The HCO⁺ emission near the source may arise from an unresolved inner ring of the envelope, showing two peaks with one on each side of the IRAS source and a position-velocity structure consistent with the rotation law derived from the N₂H⁺ emission. The HCO⁺ emission in the outer part of the envelope likely traces the swept up envelope material, showing both the outflow motion and probably rotation.

We present the first high-resolution (R = 20,000-45,000, corresponding to 14 km s^-1 at 4200 Å to 6.6 km s^-1 at 9000 Å) observations of the optical afterglow of gamma-ray bursts. GRB 020813 and GRB 021004 were observed by UVES at the Very Large Telescope 22.19 and 13.52 hr after the trigger, respectively. These spectra show that the interstellar matter of the GRB host galaxies is complex, with many components contributing to each main absorption system, and spans a total velocity range of up to about 3000 km s^-1. Several narrow components are resolved down to a width of a few tens of km s^-1. In the case of GRB 021004 we detected both low- and high-ionization lines. Combined with photoionization results obtained with CLOUDY, the ionization parameters of the various systems are consistent with a remarkably narrow range with no clear trend with system velocity. This can be interpreted as due to density fluctuations on top of a regular R^-2 wind density profile.

We present here a comprehensive study of the optical/near-infrared (IR) upper limits for gamma-ray bursts that have an X-ray afterglow. We have extrapolated the X-ray afterglows to optical wavelengths based on the physics of the fireball blast wave model and compared these results with optical upper limits for a large sample of bursts. We find a small set of only three bursts out of a sample of 20 for which the upper limits are not compatible with their X-ray afterglow properties within the context of any blast wave model. This sparse sample does not allow us to conclusively determine the cause of this optical/near-IR deficit. Extinction in the host galaxy is a likely cause, but high redshifts and different afterglow mechanisms might also explain the deficit in some cases. We note that the three bursts appear to have higher than average gamma-ray peak fluxes. In a magnitude versus time diagram the bursts are separated from the majority of bursts with a detected optical/near-IR afterglow. However, two gamma-ray bursts with an optical afterglow (one of which is highly reddened) also fall in this region with dark bursts, making it likely that dark bursts are at the faint end of the set of optically detected bursts, and therefore the dark bursts likely form a continuum with the bursts with a detected optical afterglow. Our work provides a useful diagnostic tool for follow-up observations for potentially dark bursts; applied to the events detected with the Swift satellite, it will significantly increase our sample of truly dark bursts and shed light upon their nature.

We present the discovery and monitoring of the optical transient (OT) associated with GRB 020410. The fading OT was found by Hubble Space Telescope (HST) observations taken 28 and 65 days after burst at a position consistent with the X-ray afterglow, making this the first time that the optical afterglow of a gamma-ray burst (GRB) has been discovered by an orbiting observatory. Subsequent reexamination of early ground-based observations revealed that a faint OT was present 6 hr after burst, confirming the source association with GRB 020410. A deep nondetection after one week requires that the OT rebrightened between day 7 and day 28, and further late-time HST data taken approximately 100 days after burst imply that it is very red (F_ν ∝ ν^-2.7). We compare both the flux and color of the excess with supernova models and show that the data are best explained by the presence of a Type Ib/c supernova at a redshift z ≈ 0.5, which occurred roughly coincident with the day of the GRB.

The rapid acquisition of positions by the upcoming Swift satellite will allow monitoring for X-ray lines in gamma-ray burst (GRB) afterglows at much earlier epochs than was previously feasible. We calculate the possible significance levels of iron-line detections as a function of source redshift and observing time after the trigger for the Swift X-Ray Telescope (XRT), Chandra ACIS, and XMM-Newton EPIC detectors. For bursts with standard luminosities, decay rates, and equivalent widths of 1 keV assumed constant starting at early source-frame epochs, Swift may be able to detect lines up to z ~ 1.5 with a significance of 3 σ for times of t 10⁴ s. The same lines would be detectable with 4 σ significance at z 6 by Chandra and at z 8 by XMM-Newton for times of t 10⁵ s. For similar bursts with a variable equivalent width that peaks at 1 keV between 0.5 and 1 day in the source frame, Swift achieves the same significance level for z ~ 1 at t ~ 1 day, while Chandra reaches the previous detection significances around t ~ 1-2 days for z ~ 2-4; i.e., the line is detectable near the peak equivalent width times and undetectable at earlier or later times. For afterglows in the upper range of initial X-ray luminosity afterglows, which may also be typical of Population III bursts, similar significance levels are obtained out to substantially higher redshifts. A distinction between broad and narrow lines to better than 3 σ is possible with Chandra and XMM-Newton out to z ~ 2 and ~6.5, respectively, while Swift can do so up to z ~ 1 for standard burst parameters. A distinction between different energy centroid lines of 6.4 versus 6.7 keV (or 6.7 vs. Cobalt 7.2 keV) is possible up to z 0.6, 1.2, and 2 (z 1, 5, and 7.5) with Swift, Chandra, and XMM-Newton, respectively. For the higher luminosity bursts, Swift is able to distinguish at the 5 σ level between a broad and a narrow line out to z 5 and between a 6.7 versus a 7.2 keV line center out to z 5 for times of t 10⁴ s.

The net optical light curves and spectra of the supernova (SN) 2003dh are obtained from the published spectra of GRB 030329, covering about 6 days before SN maximum to about 60 days after. The bulk of the U-band flux is subtracted from the observed spectra using early-time afterglow templates, because strong line blanketing greatly depresses the UV and U-band SN flux in a metal-rich, fast-moving SN atmosphere. The blue-end spectra of the gamma-ray burst (GRB) connected hypernova SN 1998bw is used to determine the amount of subtraction. The subtraction of a host galaxy template affects the late-time results. The derived SN 2003dh light curves are narrower than those of SN 1998bw, rising as fast before maximum, reaching a possibly fainter maximum, and then declining ~ 1.2-1.4 times faster. We then build UVOIR bolometric SN light curve. Allowing for uncertainties, it can be reproduced with a spherical ejecta model of M_ej ~ 7 ± 3 M_☉, E_K ~ 3.5 ± 1.5 × 10⁵² ergs, with E_K/M_ej ~ 5 following previous spectrum modeling, and M(⁵⁶Ni) ~ 0.4 M_☉. This suggests a progenitor main-sequence mass of ~25-40 M_☉, lower than SN 1998bw but significantly higher than normal Type Ic SNe and the GRB-unrelated hypernova SN 2002ap.

We present observations that show dramatic evolution of the mean pulse profile of the relativistic binary pulsar J1141-6545 over a period of 5 yr. This is consistent with the precession of the pulsar spin axis due to relativistic spin-orbit coupling. Observations made between 1999 and 2004 with a number of instruments at the Parkes radio telescope demonstrate a steady, secular evolution of the mean total intensity profile, which increases in width by more than 50% during the 5 yr period. Analysis of the changing position angle of the linearly polarized component of the mean profile suggests that our line of sight is shifting closer to the core of the emission cone. We find that the slope of the position angle swing across the center of the pulse steepens with time and use a simplified version of the rotating vector model to constrain the magnitude and direction of the change in our line-of-sight angle relative to the pulsar magnetic axis. The fact that we appear to be moving deeper into the emission cone is consistent with the nondetection of this pulsar in previous surveys.

We present a new analysis of existing optical and ultraviolet spectra of the ONeMg nova V1974 Cygni 1992. Using these data and the photoionization code Cloudy, we have determined the physical parameters and elemental abundances for this nova. Many of the previous studies of this nova have made use of incorrect analyses, and hence a new study was required. Our results show that the ejecta are enhanced, relative to solar, in helium, nitrogen, oxygen, neon, magnesium, and iron. Carbon was found to be subsolar. We find an ejected mass of ~2 × 10^-4 M_☉. Our model results fit well with observations taken at IR, radio, submillimeter, and X-ray wavelengths.

Archival IUE spectra of the VY Sculptoris system MV Lyrae, taken during an intermediate state, can be best fit by an isothermal accretion disk extending half-way to the tidal cutoff radius. In contrast, a recent HST spectrum, while MV Lyr was in a high state, can be best fit with a standard T(R) profile for an accretion disk extending from an inner truncation radius to an intermediate radius with an isothermal accretion disk beyond. These fits use component-star parameters determined from a study of MV Lyr in a low state. Model systems containing accretion disks with standard T(R) profiles have continua that are too blue. The observed high-state absorption-line spectrum exhibits excitation higher than provided by the T(R) profile, indicating likely line formation in a high-temperature region extending vertically above the accretion disk. The absorption lines show a blueshift and line broadening corresponding to formation in a low-velocity wind apparently coextensive with the high-temperature region. Lines of N V, Si IV, C IV, and He II are anomalously strong relative to our synthetic spectra, indicating possible composition effects, but unmodeled excitation effects could also produce the anomalies. An analysis of a low state of MV Lyr, considered in an earlier study and extended in this paper, sets a limit of 2500 K for the T_eff of an accretion disk that may be present in the low state. This limit is in conflict with two recent models of the VY Sculptoris phenomenon.

We apply the Deloye & Bildsten isentropic models for donors in ultracompact low-mass X-ray binaries to the AM CVn population of ultracompact, interacting binaries. The mass-radius relations of these systems' donors in the mass range of interest (M₂ < 0.1 M_⊙) are not single-valued, but parameterized by the donor's specific entropy. This produces a range in the relationships between system observables, such as orbital period P_orb and mass transfer rate . For a reasonable range in donor specific entropy, can range over several orders of magnitude at fixed P_orb. We determine the unique relation between and M₂ in the AM CVn systems with known donor-to-accretor mass ratios q = M₂/M₁. We use structural arguments, as well as each system's photometric behavior, to place limits on and M₂ in each. Most systems allow a factor of about 3 variation in , although V803 Cen, if the current estimates of its q are accurate, is an exception and must have M₂ ≈ 0.02 M_⊙ and ≈ 10^-10 M_⊙ yr^-1. Our donor models also constrain each donor's core temperature (T_c) range and correlate T_c with M₂. We examine how variations in donor specific entropy across the white dwarf family of AM CVn systems affects this population's current Galactic distribution. Allowing for donors that are not fully degenerate produces a shift in systems toward longer P_orb and higher , increasing the parameter space in which these systems can be found. This shift increases the fraction of systems whose P_orb is long enough that their gravity wave (GW) signal is obscured by the background of detached double white dwarf binaries that dominate the GW spectrum below a frequency of ≈2 mHz.

A simultaneous light and radial velocity analysis of eight Large Magellanic Cloud (LMC) eclipsing binaries is presented. Combining spectroscopic observations obtained with UVES at the ESO Very Large Telescope and light curves available from the MACHO and OGLE databases, accurate masses and radii for the binary components, along with their spectral types and luminosities, are derived. These determinations allow us to construct the first mass-luminosity relation for late O and early B type stars in the LMC. This mass-luminosity relation looks very similar to that of the Milky Way, in spite of the different metallicities. The good agreement achieved in the comparison with recent theoretical isochrones is encouraging regarding the reliability of star models up to 20 M_☉.

We measured the spectrum of polarization for three proto-planetary nebulae (PPNs), IRAS 17411-2411, IRAS 08005-2356, and IRAS 04296+3429, and made model calculations with the dust-scattering Monte Carlo code DIRTY and the dust emission code 2Dust. We show that high levels of polarization in these PPNs correlate with extreme asymptotic giant branch (AGB) superwind mass-loss rates in excess of 10^-4 M_☉ yr^-1. All three objects show evidence for evacuated lobes cleared by collimated fast winds, and two indicate a significant equatorial mass enhancement. Our best-fit models require sharply peaked grain size distributions, suggesting that most of the light is being scattered by grains of a characteristic size in IRAS 17441-2411 and IRAS 08005-2356. IRAS 17441-2411 and IRAS 08005-2356 have lobes with wide opening angles, perhaps produced by deflection of a polar jet from an accreting companion by the AGB superwind. Modeling the spectropolarimetry of IRAS 04296+3429 indicates a point-symmetric, multipolar morphology in the PPN phase. The modeling of spectropolarimetry and other observations of PPNs provides a powerful way to constrain circumstellar morphology and dust parameters.

We focus on two Hubble Space Telescope Space Telescope Imaging Spectrograph (HST STIS) spectra of the Weigelt blobs B and D, extending from 1640 to 10400 Å, one recorded during the 1998 minimum (1998 March) and the other recorded in 1999 February, early in the following broad maximum. The spatially resolved spectra suggest two distinct ionization regions. One structure is the permanently low-ionization cores of the Weigelt blobs B and D, located several hundred AU from the ionizing source. Their spectra are dominated by emission from H I, [N II], Fe II, [Fe II], Ni II, [Ni II], Cr II, and Ti II. The second region, relatively diffuse in character and located between the ionizing source and the Weigelt blobs, is more highly ionized with emission from [Fe III], [Fe IV], N III], [Ne III], [Ar III], [Si III], [S III], and He I. Through photoionization modeling, we find that the radiation field from the more massive B-star companion supports the low-ionization structure throughout the 5.54 yr period. The radiation field of an evolved O star is required to produce the higher ionization emission seen across the broad maximum. This emission region is identified with slow-moving condensations photoionized by the O star and located in the extended mass flow emanating from the B-star primary. Comparison between the models and observations reveals that the high-ionization region is physically distinct (n_H ≈ 10⁷ cm^-3 and T_e ~ 10⁴ K) from the B and D blobs (n_H ≈ 10⁶ cm^-3 and T_e ~ 7000 K).

Medium- and high-resolution spectroscopy of U Equulei from 1 to 4 μm during 1997-2003 has revealed information about its unusual circumstellar envelope, observed previously at optical and radio wavelengths. Strong absorption bands of H₂O and of CO dominate the 1-4 μm spectrum. The gas has a mean temperature of 600 K, and ¹²C/¹³C ≤ 10. The CO 2-0 line profiles and velocities imply no net ejection or infall, and indicate either rapid radial gas motions being seen along a narrow continuum beam or absorption by orbiting gas that is nearly coincident with a highly extended continuum source. The gas could be located in a disklike structure. The observed high column densities of warm CO and H₂ normally would be associated with sufficient dust to completely obscure the star at optical wavelengths. The observations thus indicate either a highly abnormal gas-to-dust ratio, consistent with the earlier optical observation of abundant refractory metal oxides in the circumstellar gas, or peculiar geometry and/or illumination.

Using previous measurements and quantum chemical calculations to derive the molecular properties of the TiH molecule, we obtain new values for its rovibrational constants, thermochemical data, spectral line lists, line strengths, and absorption opacities. Furthermore, we calculate the abundance of TiH in M and L dwarf atmospheres and conclude that it is much higher than previously thought. We find that the TiH/TiO ratio increases strongly with decreasing metallicity, and at high temperatures can exceed unity. We suggest that, particularly for subdwarf L and M dwarfs, spectral features of TiH near ~0.52 and 0.94 μm and in the H band may be more easily measurable than heretofore thought. The recent possible identification in the L subdwarf 2MASS J0532 of the 0.94 μm feature of TiH is in keeping with this expectation. We speculate that looking for TiH in other dwarfs and subdwarfs will shed light on the distinctive titanium chemistry of the atmospheres of substellar-mass objects and the dimmest stars.

We present two-dimensional inviscid hydrodynamic simulations of a protoplanetary disk with an embedded planet, emphasizing the evolution of potential vorticity (the ratio of vorticity to density) and its dependence on numerical resolutions. By analyzing the structure of spiral shocks made by the planet, we show that progressive changes of the potential vorticity caused by spiral shocks ultimately lead to the excitation of a secondary instability. We also demonstrate that very high numerical resolution is required to both follow the potential vorticity changes and identify the location where the secondary instability is first excited. Low-resolution results are shown to give the wrong location. We establish the robustness of a secondary instability and its impact on the planet's torque. After the saturation of the instability, the disk shows large-scale nonaxisymmetry, causing the torque on the planet to oscillate with large amplitude. The impact of the oscillating torque on the protoplanet's migration remains to be investigated.

We report a method that uses "completeness" to estimate the number of extrasolar planets discovered by an observing program with a direct-imaging instrument. We develop a completeness function for Earth-like planets on "habitable" orbits for an instrument with a central field obscuration, uniform sensitivity in an annular detection zone, and limiting sensitivity that is expressed as a "delta magnitude" with respect to the star, determined by systematic effects (given adequate exposure time). We demonstrate our method of estimation by applying it to our understanding of the coronagraphic version of the Terrestrial Planet Finder (TPF-C) mission as of 2004 October. We establish an initial relationship between the size, quality, and stability of the instrument's optics and its ability to meet mission science requirements. We provide options for increasing the fidelity and versatility of the models on which our method is based, and we discuss how the method could be extended to model the TPF-C mission as a whole to verify that its design can meet the science requirements.

Within the framework of the quantum theory of polarized line formation, in the limit of complete frequency redistribution and of the collisionless regime, we derive explicit formulae describing the statistical equilibrium and the radiative emission of a multiterm atom with hyperfine structure, in the presence of an external magnetic field. The formulae we obtained for the radiative rates of the statistical equilibrium equations and for the radiative coefficients of the transfer equation for polarized radiation can be applied to investigate the formation of spectral lines for which both fine-structure and hyperfine-structure effects are important (e.g., the D1 and D2 lines of Na I in the solar atmosphere).

This paper provides a theoretical simulation of anisotropy measurements by the Low-Energy Charged Particle (LECP) experiment on Voyager. The model starts with an anisotropic pitch-angle distribution function in the solar wind plasma reference frame. It includes the effects of both Compton-Getting anisotropy and a perpendicular diffusion anisotropy that possibly exists in the upstream region of the termination shock. The calculation is directly applied to the measurements during the late 2002 particle event seen by Voyager 1. It is shown that the data cannot rule out either the model with zero solar wind speed or the one with a finite speed on a qualitative basis. The determination of solar wind speed using the Compton-Getting effect is complicated by the presence of a large pitch-angle distribution anisotropy and a possible diffusion anisotropy. In most high-energy channels of the LECP instrument, because the pitch-angle distribution anisotropy is so large, a small uncertainty in the magnetic field direction can produce very different solar wind speeds ranging from 0 to >400 km s^-1. In fact, if the magnetic field is chosen to be in the Parker spiral direction, which is consistent with the magnetometer measurement on Voyager 1, the derived solar wind speed is still close to the supersonic value. Given the uncertainty of the magnetic field direction, only the two lowest energy channels of the LECP instrument can give a definitive result for the solar wind speed. However, these channels contain very high levels of background from their response to isotropic cosmic rays. An uncertainty of just a few percent in the background level can entirely hamper the estimate of solar wind speed.

The observation by the Large Angle and Spectrometric Coronagraph (LASCO) instrument on the Solar and Heliospheric Observatory of blobs of plasma generated near the cusp region of the streamer belt has sparked the hope of gaining new insight into the genesis of the slow solar wind. The observations suggest that open and closed field lines reconnect near the cusp of the helmet streamer to form blobs of higher density plasmas that are ejected into the slow solar wind. Models of this process have been suggested, and simulation studies have been conducted. We take the recent simulation work of Einaudi et al. as our starting point to investigate the formation of the slow solar wind and of the blobs observed by LASCO. We report two main conclusions. First, our results confirm the overall scenario suggested in the above paper (Einaudi et al.), which is that reconnection indeed causes the formation of blobs and the acceleration of the slow solar wind. Second, by employing more realistic two-dimensional configurations including the cusp and bent field lines with fast solar wind flow, we find that reconnection giving rise to plasma blobs is driven by converging flows with reconnection rates largely insensitive to changes in resistivity.

Several ways have been proposed for heating the solar corona by magnetic reconnection in current sheets, depending on the nature of both the coronal magnetic field and the photospheric driving. Two ways that have recently been considered involve the formation of such current sheets either along separatrices (surfaces that separate topologically distinct regions) or along separators (intersections of separatrices linking one null point to another). The effect of slow photospheric motions on complex coronal magnetic configurations will in general be to generate three forms of electric current, namely, nonsingular distributed currents, singular currents on separatrices and singular currents on separators. These currents are not mutually exclusive but will in general coexist in the same configuration. The aim of this paper is to compare energy storage and heating that occurs at separatrices and separators. We use reduced MHD to model coronal loops that are much longer than they are wide, and we construct a series of examples for the formation of current sheets along separatrices and separators. We deduce that coronal heating is of comparable importance at separatrices and separators. Separatrices are twice as effective for observed small footpoint motions, while separators are twice as effective in the initial build-up of a new flux domain.

We studied the relationship between magnetic helicity injection and the formation of sigmoidal loops. We analyzed seven active regions: three regions showed coronal loops similar to the potential field, and four regions showed the sigmoidal loops. The magnetic helicity injection rate was evaluated using the method proposed by Kusano et al. In order to compare the helicity of regions of various sizes, we defined the normalized helicity injection rate as the magnetic helicity injection rate divided by the magnetic flux squared. We found that the sigmoidal regions and nonsigmoidal regions have comparable normalized helicity injection rates. Next, we calculated the magnetic helicity content of the sigmoidal loops by using the magnetic flux tube model (Longcope & Welsch) and compared it with the magnetic helicity injected from around the footpoints of three sigmoidal loops. For two sigmoidal loops, it is found that these values are comparable. Another loop showed significant disagreement between helicity injection rate and its magnetic helicity content. Excluding this region on the basis of its complexity (perhaps multiple loops forming a sigmoidal loop), we can conclude that geometric twist of the sigmoidal loops is consistent with the magnetic helicity injected from around the footpoints of the sigmoidal loops.

The physical mechanisms that cause the heating of the solar corona are still far from being completely understood. However, recent highly resolved observations with the current solar missions have shown clear evidence of frequent and very localized heating events near the chromosphere that may be responsible for the observable high temperatures of the coronal plasma. In this paper, we perform one-dimensional hydrodynamic simulations of the evolution of a hypothetical loop model undergoing impulsive heating through the release of localized Gaussian energy pulses near the loop's footpoints. We find that when a discrete number of randomly spaced pulses is released, loops of length L = 5 and 10 Mm heat up and stay at coronal temperatures for the whole duration of the impulsive heating stage, provided that the elapsed time between successive heat injections is 175 and 215 s, respectively. The precise value of the critical elapsed time is sensitive to the loop length. In particular, we find that for increased loop lengths of 20 and 30 Mm, the critical elapsed time rises to about 240 and 263 s, respectively. For elapsed times longer than these critical values, coronal temperatures can no longer be maintained at the loop apex in spite of continued impulsive heating. As a result, the loop apex undergoes runaway cooling well below the initial state, reaching chromospheric temperatures (~10⁴ K) and leading to the typical hot-cool temperature profile characteristic of a cool condensation. For a large number of pulses (up to ~1000) having a fully random spatiotemporal distribution, the variation of the temperature along the loop is highly sensitive to the spatial distribution of the heating. As long as the heating concentrates more and more at the loop's footpoints, the temperature variation is seen to make a transition from that of a uniformly heated loop to a flat, isothermal profile along the loop length. Concentration of the heating at the footpoints also results in a more frequent appearance of rapid and significant depressions of the apex temperature during the loop evolution, most of them ranging from ~1.5 × 10⁶ to ~10⁴ K and lasting from about 3 to 10 minutes. This behavior bears a tight relation with the strong variability of coronal loops inferred from SOHO observations in active regions of the solar atmosphere.

We present deep optical imaging of Geminid meteor stream parent 3200 Phaethon taken in search of low-level cometary activity (i.e., coma or dust trail). Although no unambiguous cometary behavior was observed, we find an upper limit on the object's cometary mass-loss rate of _lim ~ 0.01 kg s^-1. The corresponding active fraction (the fraction of the surface area that could consist of freely sublimating water ice) is f ≤ 7 × 10^-6, at least 2 orders of magnitude smaller than other known comets.

The synthetic routes to form acetaldehyde [CH₃CHO(X ¹A')] in extraterrestrial ices were investigated experimentally in a contamination-free ultrahigh vacuum scattering machine. Binary ice mixtures of carbon monoxide [CO(X ¹Σ⁺)] and methane [CH₄(X ¹A₁)] were condensed at 10 K onto a silver monocrystal and irradiated with 5 keV electrons to mimic the electronic energy transfer processes initiated by MeV cosmic-ray particle-induced δ-electrons in the "ultratrack" of MeV ion trajectories; the carbon monoxide-methane ices served as model compounds to simulate neighboring CO–CH₄ molecules in astrophysical ices, as present in cold molecular clouds and in cometary matter. Upon completion of the high-energy processing, the ice samples sublimed during the heating phase to 293 K, thus releasing the remaining reactants as well as the newly formed molecules into the gas phase. The experiment was monitored on line and in situ via a Fourier transform infrared (FTIR) spectrometer in absorption-reflection-absorption mode (solid state) and a quadrupole mass spectrometer (gas phase). Our investigations were combined with electronic structure calculations. At 10 K, the primary reaction step involved the cleavage of the carbon-hydrogen bond of the methane molecule via an electronic energy transfer process from the impinging electron to the methane molecule to form a methyl radical [CH₃(X ²A)] plus a hydrogen atom [H(²S_1/2)]. The H atom contains the excess energy in the form of translational motion; suprathermal hydrogen atoms can add to the carbon-oxygen triple bond of the carbon monoxide molecule, overcoming the entrance barrier, to yield the formyl radical [HCO(X ²A')]. Depending on the reactant geometry inside the matrix cage, the formyl radical recombined barrierlessly with the neighboring methyl radical inside the ices at 10 K. Upon warming of the ice sample, the acetaldehyde molecules sublime into the gas phase. This process mimics the sublimation of molecules from the grain mantles into the gas phase upon the transition of the molecular cloud to the hot molecular core phase. This mechanism to form acetaldehyde inside interstellar ices (cold molecular clouds; 10 K) upon high-energy processing, followed by a radical-radical recombination and sublimation in the hot core phase (molecular cores; few 100 K), presents a compelling route to account for high fractional abundances of acetaldehyde of a few times 10^-9 toward star-forming regions, as compared to abundances of only some 10^-10 in the cold cloud TMC-1, where solely gas-phase reactions are supposed to synthesize acetaldehyde.

The (000)-(000) band of the 4051.6 Å group ( ¹Π_u- ¹Σ) of C₃ was recorded in emission with a Bruker IFS 120HR Fourier transform (FT) spectrometer at the University of Waterloo. The band was excited by a microwave discharge in isopropanol (less than a few mtorr) diluted in helium (2 torr). Our new FT data provide more reliable and accurately calibrated transition wavenumbers than those from the grating spectra given by Gausset and coworkers. Analysis of our new spectrum combined with the data by McCall and coworkers confirmed that the lower J levels in the state were strongly perturbed, as reported by Gausset and coworkers. The unidentified lines observed by McCall and coworkers could be attributed to extra transitions to an unknown perturbing state.

Rate coefficients for radiative association of SO, SO⁺, and S₂ are estimated. For temperatures ranging from 300 to 14,000 K, the direct radiative association rate coefficients are found to vary with temperature from 1.73 × 10^-19 to 7.29 × 10^-19 cm³ s^-1 and from 1.49 × 10^-21 to 3.70 × 10^-19 cm³ s^-1 for S₂ and SO, respectively. The rate coefficients for formation through the inverse predissociation for S₂ are found to vary from 3.59 × 10^-18 to 1.44 × 10^-20 cm³ s^-1. For SO⁺, the direct rate coefficient varies rapidly with temperature from 3.62 × 10^-27 cm³ s^-1 at 2000 K to 2.34 × 10^-20 cm³ s^-1 at 14,000 K. The direct radiative association rate coefficients increase with the increase in temperature, but the inverse predissociation rate coefficients decrease with the increase in temperature.