Showing papers in "The Astronomical Journal in 1999"

A Hipparcos Census of the Nearby OB Associations

Abstract: A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions. OB associations are unbound moving groups, which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V 6 mag), with spectral types earlier than ~B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalog by combining de Bruijne's refurbished convergent point method with the Spaghetti method of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ~650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas–Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B- and A-type members in these groups and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre–main-sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere. Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ~500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk. The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt. Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be significantly larger than thought previously.

Long-Term Stability of Planets in Binary Systems

Abstract: A simple question of celestial mechanics is investigated: in what regions of phase space near a binary system can planets persist for long times? The planets are taken to be test particles moving in the field of an eccentric binary system. A range of values of the binary eccentricity and mass ratio is studied, and both the case of planets orbiting close to one of the stars, and that of planets outside the binary orbiting the systems center of mass, are examined. From the results, empirical expressions are developed for both (1) the largest orbit around each of the stars and (2) the smallest orbit around the binary system as a whole, in which test particles survive the length of the integration (10A4 binary periods). The empirical expressions developed, which are roughly linear in both the mass ratio mu and the binary eccentricity e, are determined for the range 0.0=e=0.7-0.8 and 0.1=mu=0.9 in both regions and can be used to guide searches for planets in binary systems. After considering the case of a single low-mass planet in binary systems, the stability of a mutually interacting system of planets orbiting one star of a binary system is examined, though in less detail.

The reddening-free decline rate versus luminosity relationship for type ia supernovae

Abstract: We develop a method for estimating the host galaxy dust extinction for type Ia supernovae based on an observational coincidence first noted by Lira, who found that the B-V evolution during the period from 30 to 90 days after V maximum is remarkably similar for all events, regardless of light-curve shape. This fact is used to calibrate the dependence of the Bmax-Vmax and Vmax-Imax colors on the light-curve decline rate parameter Δm15(B), which can, in turn, be used to separately estimate the host galaxy extinction. Using these methods to eliminate the effects of reddening, we reexamine the functional form of the decline rate versus luminosity relationship and provide an updated estimate of the Hubble constant of H0 = 63.3 ± 2.2(internal) ± 3.5(external) km s-1 Mpc-1.

BVRI Light Curves for 22 Type Ia Supernovae

Abstract: We present 1210 Johnson/Cousins B, V, R, and I photometric observations of 22 recent Type Ia supernovae (SNe Ia): SNe 1993ac, 1993ae, 1994M, 1994S, 1994T, 1994Q, 1994ae, 1995D, 1995E, 1995al, 1995ac, 1995ak, 1995bd, 1996C, 1996X, 1996Z, 1996ab, 1996ai, 1996bk, 1996bl, 1996bo, and 1996bv. Most of the photometry was obtained at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics in a cooperative observing plan aimed at improving the database for SNe Ia. The redshifts of the sample range from cz = 1200 to 37,000 km s-1 with a mean of cz = 7000 km s-1.

The Luminosity Function of Young Star Clusters in “the Antennae” Galaxies (NGC 4038/4039)*

Abstract: The Wide Field Planetary Camera 2 of the Hubble Space Telescope has been used to obtain high-resolution images of NGC 4038/4039 that go roughly 3 mag deeper in V than previous observations made during cycle 2. These new images allow us to measure the luminosity functions (LFs) of clusters and stars over a range of 8 mag (-14 < MV < -6). To first order, the LF is a power law, with exponent α = -2.12 ± 0.04. However, using a variety of different techniques to decouple the cluster and stellar LFs, which overlap in the range -9 MV -6, we find an apparent bend in the young cluster LF at approximately MV = -10.4. Brightward of this magnitude the LF has a power-law exponent α = -2.6 ± 0.2, while faintward the slope is α = -1.7 ± 0.2. The bend corresponds to a mass ≈ 1 × 105 M⊙, only slightly lower than the characteristic mass of globular clusters in the Milky Way (≈2 × 105 M⊙). It is currently not feasible to determine the cluster LF fainter than MV ≈ -8, where individual stars are likely to dominate. The stellar LF in the range -9 < MV < -6 is much steeper, with α = -2.9 ± 0.1, and is dominated by young red and blue supergiants. The star clusters of the Antennae appear slightly resolved, with median effective radii of 4 ± 1 pc, similar to or perhaps slightly larger than those of globular clusters in our Galaxy. However, the radial extents of some of the very young clusters (ages less than 10 Myr) are much larger than those of old globular clusters (e.g., the outer radius of knot S exceeds 450 pc). This may indicate that the tidal forces from the galaxies have not had time to remove some of the outer stars from the young clusters. A combination of the UBVI colors, Hα morphology, and Goddard High Resolution Spectrograph (GHRS) spectra enables us to age date the clusters in different regions of the Antennae. Star clusters around the edge of the dust overlap region appear to be the youngest, with ages 5 Myr, while clusters in the western loop appear to be 5–10 Myr old. Many star clusters in the northeastern star formation region appear to be ~100 Myr old, with an LF in V that has shifted faintward by ~1.0 mag relative to the younger (0–20 Myr) clusters that dominate over most of the rest of the galaxy. A third cluster population consists of intermediate-age clusters (~500 Myr) that probably formed during the initial encounter responsible for ejecting the tails. A handful of old globular clusters from the progenitor galaxies are also identified. Most of these lie around NGC 4039, where the lower background facilitates their detection. Age estimates derived from GHRS spectroscopy yield 3 ± 1 Myr for knot K (just south of the nucleus of NGC 4038) and 7 ± 1 Myr for knot S in the western loop, in good agreement with ages derived from the UBVI colors. Effective gas outflow velocities from knots S and K are estimated to be about 25–30 km s-1, based on the above cluster ages and the sizes of the surrounding Hα bubbles. However, the measured widths of the interstellar absorption lines suggest dispersion velocities of ~400 km s-1 along the lines of sight to knots S and K.

Spectral Irradiance Calibration in the Infrared. X. A Self-Consistent Radiometric All-Sky Network of Absolutely Calibrated Stellar Spectra

Abstract: We start from our six absolutely calibrated continuous stellar spectra from 1.2 to 35 ?m for K0, K1.5, K3, K5, and M0 giants. These were constructed as far as possible from actual observed spectral fragments taken from the ground, the Kuiper Airborne Observatory, and the IRAS Low Resolution Spectrometer, and all have a common calibration pedigree. From these we spawn 422 calibrated spectral templates for stars with spectral types in the ranges G9.5?K3.5 III and K4.5?M0.5 III. We normalize each template by photometry for the individual stars using published and/or newly secured near- and mid-infrared photometry obtained through fully characterized, absolutely calibrated, combinations of filter passband, detector radiance response, and mean terrestrial atmospheric transmission. These templates continue our ongoing effort to provide an all-sky network of absolutely calibrated, spectrally continuous, stellar standards for general infrared usage, all with a common, traceable calibration heritage. The wavelength coverage is ideal for calibration of many existing and proposed ground-based, airborne, and satellite sensors, particularly low- to moderate-resolution spectrometers. We analyze the statistics of probable uncertainties, in the normalization of these templates to actual photometry, that quantify the confidence with which we can assert that these templates truly represent the individual stars. Each calibrated template provides an angular diameter for that star. These radiometric angular diameters compare very favorably with those directly observed across the range from 1.6 to 21 mas.

Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure

Abstract: We have compiled a catalog of absolute proper motions of globular clusters from various sources. The sample consists of 38 clusters, from which most of the southern ones (15 clusters) were measured in our previous papers in this series. We have integrated orbits assuming two different Galactic potential models adopted from the literature and have calculated orbital parameters. The uncertainties associated with the orbital parameters were derived in a Monte Carlo approach, and we conclude that, overall, at the present level of measurement errors, orbital differences due to Galactic potential models are not significant. Three metal-poor clusters are found to have orbits similar to prototypical metal-rich disk clusters. These clusters are NGC 6254 (M10), NGC 6626 (M28), and NGC 6752. We interpret this as a potentially significant constraint on the formation of the disk. It is thus possible that part of the inner metal-poor halo is the low-metallicity tail of the thick disk. In this case, the ages of these clusters indicate that the formation of the disk partially overlapped with that of the halo. The clusters classified as young halo or red horizontal-branch by Zinn show a radially anisotropic velocity distribution, their orbits are of high total energy, with apocentric radii larger than 10 kpc and highly eccentric. In this sense they may represent an accreted component of our Galaxy. We also discuss ω Cen's orbit characteristics in the view of an accreted origin. We investigate the effect of the orbital motion on the internal dynamics of clusters. Adopting the formalism from Gnedin & Ostriker and their destruction rates due to two-body relaxation, we find that, in most cases, this internal process is more important than the destruction processes due to disk and bulge shocking. Hubble Space Telescope (HST) observations argue that NGC 6397's luminosity function is depleted at the faint end, and this is blamed on its high total destruction rate. We propose a list of clusters with similar destruction rates that may also have depleted luminosity functions. We also note the bias toward deriving higher destruction rates in studies that statistically assign tangential velocities based on a kinematic model of the globular cluster system, in contrast to the rates derived from the measured tangential velocities. Clusters prone to such biases are those that have circular orbits (kinematically thick-disk clusters) and some of those with orbits of high total energy.

Spectroscopic Classification of Late-M and L Field Dwarfs

Abstract: We present spectra for 12 new ultracool dwarfs found in the DENIS infrared survey. Seven of them have spectral types at the bottom of the M-class (M8–M9.5), and the other five belong to the cooler "L" class. We also present spectra for the two new L dwarfs found by the EROS 2 proper-motion survey. We introduce a scheme for L dwarf classification that is based on an extension to cooler spectra of a pseudocontinuum ratio previously defined for M dwarfs. For calibrating the spectral subclasses, we use a temperature scale for late-M and L dwarfs recently obtained by Basri et al. from synthetic spectrum fitting of high-resolution profiles of Cs I and Rb I resonance lines. We define that the subclass range from L0 to L6 corresponds to the temperature range from 2200 K to 1600 K. Our subclasses L0, L1, and L2 agree with recent findings by Kirkpatrick et al., but then they diverge such that our L6 is equivalent to their L8. We find that late-M and L dwarf subclasses can be assigned either in the optical with the PC3 index or in the near-infrared with the H2O H-band index. We discuss the main photospheric features present in L dwarf spectra, in particular in the region 400–650 nm, which has never been shown before. The TiO bands at 549.7, 559.7, 615.9, and 638.4 nm fade with decreasing temperature, but do not vanish until well inside the L domain (~L5). The Na I 589.0, 589.6 nm resonance doublet in our latest object (L6) becomes the broadest atomic feature ever seen in any cool dwarf. We do not detect Hα emission in our L dwarfs later than L3. We discuss the ages and masses of our objects using their temperatures and absence or presence of lithium. Finally, we compare two L1 dwarfs with different gravities (one with lithium and one without it) and discuss differences in spectral features.

A modified magnitude system that produces well-behaved magnitudes, colors, and errors even for low signal-to-noise ratio measurements

Abstract: We describe a modification of the usual definition of astronomical magnitudes, replacing the usual logarithm with an inverse hyperbolic sine function; we call these modified magnitudes asinh magnitudes. For objects detected at signal-to-noise ratios of greater than about 5, our modified definition is essentially identical to the traditional one; for fainter objects (including those with a formally negative flux), our definition is well behaved, tending to a definite value with finite errors as the flux goes to zero. This new definition is especially useful when considering the colors of faint objects, as the difference of two asinh magnitudes measures the usual flux ratio for bright objects, while avoiding the problems caused by dividing two very uncertain values for faint objects. The Sloan Digital Sky Survey data products will use this scheme to express all magnitudes in their catalogs.

SUMSS: A Wide-Field Radio Imaging Survey of the Southern Sky. I. Science Goals, Survey Design, and Instrumentation

Abstract: The Molonglo Observatory Synthesis Telescope, operating at 843 MHz with a 5 deg2 field of view, is carrying out a radio imaging survey of the sky south of declination -30°. This survey (the Sydney University Molonglo Sky Survey, or SUMSS) produces images with a resolution of 43'' × 43'' csc |δ| and an rms noise level of ~1 mJy beam-1. The SUMSS is therefore similar in sensitivity and resolution to the northern NRAO VLA Sky Survey. The survey is progressing at a rate of about 1000 deg2 per year, yielding individual and statistical data for many thousands of weak radio sources. This paper describes the main characteristics of the survey and presents sample images from the first year of observations.

The giant, horizontal, and asymptotic branches of galactic globular clusters. i. the catalog, photometric observables, and features

Abstract: A catalog including a set of the most recent color-magnitude diagrams (CMDs) is presented for a sample of 61 Galactic globular clusters (GGCs). We used this database to perform a homogeneous systematic analysis of the evolved sequences (namely, the red giant branch [RGB], horizontal branch [HB], and asymptotic giant branch [AGB]). Based on this analysis, we present (1) a new procedure to measure the level of the zero-age horizontal branch (VZAHB) and a homogeneous set of distance moduli obtained by adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H]CG97) and global metallicity ([M/H], including also the ?-element enhancement), such that the set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V, B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and the AGB bump (in nine GGCs). The dependence of these features on metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales, the earlier discrepancy between theory and observations (~0.4 mag) completely disappears.

Orbital Evolution of Planets Embedded in a Planetesimal Disk

Abstract: The existence of the Oort comet cloud, the Kuiper belt, and plausible inefficiencies in planetary core formation all suggest that there was once a residual planetesimal disk of mass ~10–100 M⊕ in the vicinity of the giant planets following their formation. Since removal of this disk requires an exchange of orbital energy and angular momentum with the planets, significant planetary migration can ensue. The planet migration phenomenon is examined numerically by evolving the orbits of the giant planets while they are embedded in a planetesimal disk having a mass of MD = 10–200 M⊕. We find that Saturn, Uranus, and Neptune evolve radially outward as they scatter the planetesimals, while Jupiter's orbit shrinks as it ejects mass. Higher mass disks result in more rapid and extensive planet migration. If orbital expansion and resonance trapping by Neptune are invoked to explain the eccentricities of Pluto and its cohort of Kuiper belt objects at Neptune's 3:2 mean motion resonance, then our simulations suggest that a disk mass of order MD ~ 50 M⊕ is required to expand Neptune's orbit by Δa ~ 7 AU, in order to pump up Plutino eccentricities to e ~ 0.3. Such planet migration implies that the solar system was more compact in the past, with the initial Jupiter-Neptune separation having been smaller by about 30%. We discuss the fate of the remnants of the primordial planetesimal disk. We point out that most of the planetesimal disk beyond Neptune's 2:1 resonance should reside in nearly circular, low-inclination orbits, unless there are (or were) additional, unseen, distant perturbers. The planetesimal disk is also the source of the Oort cloud of comets. Using the results of our simulations together with a simple treatment of Oort cloud dynamics, we estimate that ~12 M⊕ of disk material was initially deposited in the Oort cloud, of which ~4 M⊕ will persist over the age of the solar system. The majority of these comets originated from the Saturn-Neptune region of the solar nebula.

A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution* **

Abstract: We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) (LIR > 1012L⊙, z ≤ 0.3) using ISOPHOT-S on board the Infrared Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, LPAH/LIR, for starburst-dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we find that systems in the early stages of merging may well put out maximum luminosity. We also find that the total mass of interstellar gas, as estimated from the CO (1 → 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

The History of Low-Mass Star Formation in the Upper Scorpius OB Association

Abstract: We use a large sample of about 100 low-mass pre–main-sequence (PMS) stars in the Upper Scorpius OB association to explore the star formation history and the initial mass function of this association. Upper Scorpius is an ideal target for such a study, because the star formation process there is finished. The PMS stars have recently been found in a spatially unbiased wide-field survey of X-ray–selected stars in a 160 deg2 area, covering the Upper Scorpius association nearly completely. Following the optical characterization of these PMS stars, we present a new HR diagram for this association. We perform a detailed analysis of the HR diagram, taking proper account of the uncertainties and the effects of unresolved binaries, and derive ages and masses for the PMS stars. We find that the low-mass PMS stars have a mean age of about 5 Myr and show no evidence for a large age dispersion. This agrees very well with the age of 5–6 Myr previously found for the massive stars and shows that low-mass and high-mass stars are coeval and cospatial and thus have formed together. We conclude that the star formation process in Upper Scorpius was probably triggered by the shock wave of a supernova explosion in the nearby Upper Centaurus-Lupus association. After a short burst of very high star formation activity, which lasted only for a few Myr, star formation in Upper Scorpius was halted, probably by the strong winds and the ionizing radiation of the numerous massive stars that dispersed the molecular cloud.

The Nature and Evolution of Classical Double Radio Sources from Complete Samples

Abstract: We present a study of the trends in luminosity, linear size, spectral index, and redshift of classical double radio sources, from three complete samples selected at successively fainter low radio-frequency flux limits. We have been able to decouple the effects of the tight correlation between redshift and luminosity (inherent in any single flux-limited sample) which have hitherto hindered interpretation of the relationships between these four source properties. The major trends found are that (i) spectral indices increase with linear size, (ii) rest-frame spectral indices have a stronger dependence on luminosity than on redshift except at high (GHz) frequencies, and (iii) the linear sizes are smaller at higher redshifts. We reproduce the observed dependences in a model for radio sources (born throughout cosmic time according to a radio-source birth function) whose lobes are fed with a synchrotron-emitting population from compact hotspots, and which suffer inverse Compton, synchrotron, and adiabatic expansion losses. The magnetic energy density within each hotspot is proportional to the jet power, and synchrotron losses suffered in the hotspot mean that the energy spectrum of the emitting particles fed to the lobes is governed by the jet power. The axial ratios of radio sources in our model increase as the sources age, and axial ratios are higher in sources with higher jet power. In simulating the basic observed dependences, we find that there is no need to invoke any systematic change in the environments of these objects with redshift if the consequences of imposing a survey flux limit on our simulated data sets are properly included in the model. It is also necessary to include appropriate energy loss mechanisms (such as the effects of the cosmic microwave background and feeding the lobes from a compact hotspot), which cause decreasing luminosity through the life of a source. Although our study has broken the luminosity-redshift degeneracy, we present evidence that for such studies there is an unavoidable youth-redshift degeneracy, even though radio sources are short-lived relative to the age of the universe; it is imperative to take this into account in studies that seemingly reveal correlations of source properties with redshift such as the alignment effect.

HUBBLE SPACE TELESCOPE/NICMOS Imaging of Disks and Envelopes around Very Young Stars

Abstract: We present HST/NICMOS observations with ~01 ≈ 15 AU resolution of six young stellar objects in the Taurus star formation region. The targets of our survey are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS 04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu Tau/1) associated with Herbig-Haro jets. The broadband images show that the near-infrared radiation from these sources is dominated by light scattered from dusty circumstellar material distributed in a region 10–15 times the size of our solar system. Although the detailed morphologies of the individual objects are unique, the observed young stellar objects share common features. All of the circumstellar reflection nebulae are crossed by dark lanes from 500 to 900 AU in extent and from less than 50 to 350 AU in apparent thickness. The absorption lanes extend perpendicular to known optical and millimeter outflows in these sources. We interpret the dark lanes as optically thick circumstellar disks seen in silhouette against bright reflection nebulosity. The bipolar reflection nebulae extending perpendicular to the dust lanes appear to be produced by scattering from the upper and lower surfaces of the disks and from dusty material within or on the walls of the outflow cavities. Of five objects in which the central source is directly detected, two are found to be subarcsecond binaries. This minisurvey is the highest resolution near-infrared study to date of circumstellar environments around solar-type stars with age ≤ 1 Myr.

The Rotation Period Distribution of Pre-Main-Sequence Stars in and around the Orion Nebula

Abstract: We report rotation periods for 254 stars in an area 40' × 80' centered on the Orion Nebula. We show that these stars are likely members of the young (~106 yr) Orion OBIc/d association. The rotation period distribution we determine, which is sensitive to periods 0.1 < P < 8 days, shows a sharp cutoff for periods P < 0.5 days, corresponding to breakup velocity for these stars. Above 0.5 days the distribution is consistent with a uniform distribution; we do not find evidence for a gap of periods at 4–5 days. We find signatures of active accretion among stars at all periods; active accretion does not occur preferentially among slow rotators in our sample. We find no correlation between rotation period and near-IR signatures of circumstellar disks. In addition, we show that the distribution of v sin i among stars in our sample bears striking resemblance to that of low-mass Pleiades stars. We discuss the implications of our findings for the evolution of stellar angular momentum during the pre–main-sequence phase. We argue that all stars in our sample must still deplete angular momentum by factors of roughly 5–10, if they are to preserve their v sin i distribution over approximately the next 100 Myr. We consider in detail whether our findings are consistent with disk-regulated stellar rotation. We do not find observational evidence that magnetic disk-locking is the dominant mechanism in angular momentum evolution during the pre–main-sequence phase.

H I Shells in the Large Magellanic Cloud

Abstract: A recent high-resolution H I survey of the Large Magellanic Cloud (LMC) shows that the structure of the neutral atomic interstellar gas is dominated by numerous shells and holes, as well as complex filamentary and spiral-type structure. We present an up-to-date catalog of candidate H I supergiant and giant shells in the LMC. The candidates are visually selected from the H I data cube using selection and classification criteria that are described. Twenty-three supergiant shells, defined as those regions whose extent is much larger than the H I scale height, are cataloged; 103 giant shells (radii less than the scale height of the H I gas) are cataloged. We further classify the H I shells into five different types, based on the comparison of the H I with their associated Hα emission. For this purpose, we obtained new wide-field Hα images of the LMC with a CCD camera mounted on a 16 inch (0.41 m) telescope at Siding Spring Observatory. The pixel size of 20'' and the field of view of 12° are well matched to the H I survey. The size distribution of H I shells follows a crude power law, N(log R) ∝ R-1.5. For constant energy input to the H I shells and a constant shell creation rate, a shell luminosity spectrum of the form (L) ∝ L-β, where β = 1.75 ± 0.2, is obtained. This agrees well with the observed H II region luminosity spectrum for the LMC of Kennicut, Edgar, & Hodge, which has β = 1.75 ± 0.15. H I shells containing H II regions and OB associations seem to expand more rapidly than those without, providing direct evidence for substantial input of mechanical energy from regions of star formation.

Simulation of Stellar Objects in SDSS Color Space

Abstract: We present a simulation of the spatial, luminosity and spectral distributions of four types of stellar objects. We simulate (1) Galactic stars, based on a Galactic structure model, a stellar population synthesis model, stellar isochrones, and stellar spectral libraries; (2) white dwarfs, based on model atmospheres, the observed luminosity function, mass distribution, and Galactic distribution of white dwarfs; (3) quasars, based on their observed luminosity function and its evolution, and models of emission and absorption spectra of quasars; and (4) compact emission-line galaxies, based on the observed distribution of their spectral properties and sizes. The results are presented in the color system of the Sloan Digital Sky Survey (SDSS), with realistic photometric error and Galactic extinction. The simulated colors of stars and quasars are compared with observations in the SDSS system and show good agreement. The stellar simulation can be used as a tool to analyze star counts and constrain models of Galactic structure, as well as to identify stars with unusual colors. The simulation can also be used to establish the quasar target selection algorithm for the SDSS.

The Efficiency of Globular Cluster Formation

Abstract: The specific frequencies of globular cluster systems, SN ∝ tot/LV,gal ∝ Mgcs/Mstars, are discussed in terms of their connection to the efficiency of globular cluster formation in galaxy halos, which is claimed to reflect a generic aspect of the star formation process as it operates even at the current epoch. It is demonstrated that the total masses of GCSs are little affected by the dynamical destruction of low-mass clusters at small galactocentric radii. This permits direct, empirical estimates of the cluster formation efficiency by mass, cl ≡ M/M, even after 1010 yr of GCS evolution. However, the standard practice of using only the stellar luminosities of galaxies as indicators of their initial total gas masses (and thus relating SN to cl in one step) leads to serious conceptual problems, which are reviewed here. The first specific frequency problem, which is the well-known tendency for many brightest cluster galaxies to have higher than average SN, is a global one; the second specific frequency problem is a local one, in which the more extended spatial distribution of GCSs relative to halo stars in some (not all) bright ellipticals leads to SN-values that increase with radius inside the galaxies. Extending similar suggestions in the recent literature, it is argued that these trends in SN do not reflect any such behavior in the underlying cl; rather, both of these problems stem from neglecting the hot, X-ray emitting gas in and around many large ellipticals, and both may be alleviated by including this component in estimates of M. This claim is checked and confirmed in each of M87, M49, and NGC 1399, all of which have been thought to suffer from one or the other of these SN problems. Existing data are combined to construct GCS surface density profiles that extend over nearly the whole extents of these three galaxies, and a nonparametric, geometrical deprojection algorithm is developed to afford a direct comparison between the volume density profiles of their GCSs, stars, and gas. It is found, in each case, that ρcl ∝ (ρgas + ρstars) at radii beyond roughly a stellar effective radius, inside of which dynamical evolution may have depleted the initial GCSs. The constant of proportionality is the same in all three galaxies: cl = 0.0026 ± 0.0005. Taken together, these results suggest that GCSs generally should be more spatially extended than stellar halos only in gas-rich galaxies that also have a high global specific frequency. The implication that cl might have had a universal value is supported by global GCS data for a sample of 97 giant ellipticals, brightest cluster galaxies, and faint dwarfs. The total globular cluster populations in all of these early-type systems are in excellent agreement with the predictions of a constant cl at the level observed directly in M87, M49, and NGC 1399; all systematic variations in GCS specific frequency between galaxies are shown to result entirely from different relations, in different magnitude ranges, between M and the present-day LV,gal. An identical cl is also calculated for the Population II spheroid of the Milky Way and is indicated (although less conclusively) for the ongoing formation of open clusters. The inferred universal cluster formation efficiency, of 0.25% by mass, should serve as a strong constraint on general theories of star and cluster formation. The associated inference of a nonuniversal formation efficiency for unclustered stars is considered, particularly in terms of the suggestion that this might result, both in dwarf galaxies and at large galactocentric radii in the brightest ellipticals, from feedback and galactic winds. Implications for a merger-formation model of early-type GCSs, and for the proposed existence of intergalactic globulars in clusters of galaxies, are briefly discussed.

High-Resolution CO Observations of Luminous Infrared Galaxies

Abstract: We have performed a high-resolution imaging survey of the CO J = 1 → 0 emission in seven galaxies with infrared (IR) luminosities exceeding 3 × 1011 L☉—five of which are mergers. The resultant maps show that the molecular gas is very highly concentrated towards the cores of the mergers, with gas surface densities approaching or exceeding 104 M☉ pc-2 within 300–400 pc of the nuclei in three cases. This result supports earlier findings based on data from a smaller sample of luminous mergers. In the two mergers that show closely spaced double IR (stellar) nuclei, CO emission peaks between the nuclei and shows an extent roughly equal to the nuclear separation. The gas cores of the individual merging galaxies appear to be coalescing, while the stellar cores remain distinct. In the three single nucleus mergers, the CO peaks are coincident with the stellar nuclei, consistent with the hypothesis that these are relatively evolved merger remnants. In two of the three mergers with the most compact CO emission (Mrk 231 and NGC 6240), the empirical Galactic conversion factor from CO luminosity to molecular gas mass appears to overestimate the nuclear gas mass by a factor of more than 2 (3.6 in the case of Mrk 231). For Mrk 231, the high brightness temperature of the CO emission (Tb > 34 K) is the likeliest explanation for this overestimate. In the third such merger (NGC 2623), however, the geometry and kinematics suggest that the molecular gas mass is within a factor of 2 of the value given by using the Galactic conversion factor. Nonetheless, in all three of these objects, the molecular gas probably dominates the nuclear gravitational potential. We suggest that the molecular gas in objects with such high gas mass surface densities (~104 M☉ pc-2) is distributed in nuclear disks. These disks must be thin because of their self-gravity, with a full width of 30–40 pc (compared to radii of 300–400 pc) for a vertical velocity dispersion of 90 km s-1. The mean volume density of molecular hydrogen in such disks must be over 104 cm-3. The trend of increasing LFIR/LCO with increasing CO surface brightness is confirmed. The high concentrations of molecular gas thus appear intimately related to the high luminosities of these systems and probably serve as the fuel.

A Photometric Catalog of Herbig Ae/Be Stars and Discussion of the Nature and Cause of the Variations of UX Orionis Stars

Abstract: UBVR photometric monitoring of Herbig Ae/Be stars and some related objects has been carried out at Maidanak Observatory in Uzbekistan since 1983. More than 71,000 observations of about 230 stars have been obtained and are made available for anonymous ftp. Virtually all Herbig Ae/Be stars observed are irregular variables (called "UXors" after UX Ori), but there is a wide range of amplitudes from barely detectable to more than 4 mag in V. Our data confirm the results of previous studies, which indicate that large-amplitude variability is confined to stars with spectral types later than B8. The distribution of variability ranges is quite similar to what is seen in classical T Tauri stars. A careful search has failed to reveal any evidence for periodic variations up to 30 days, which can be interpreted as rotation periods. This is a clear distinction between the light variations of low-mass and high-mass pre–main-sequence stars. The Herbig Ae/Be stars evidently do not possess either the large, stable cool spots or persistent hot spots associated with strong surface magnetic fields and magnetically funneled accretion in classical T Tauri stars. A wide variety of shapes, timescales, and amplitudes exists, but the most common behavior is well illustrated by the light curve of LkHα 234. There are two principal components: (1) irregular variations on timescales of days around a mean brightness level that changes on a much longer timescale (typically years), sometimes in a quasi-cyclic fashion, and (2) occasional episodes of deep minima, occurring at irregular intervals but more frequently near the low points of the brightness cycles. Our data suggest that many T Tauri stars of K0 and earlier spectral type share the same variability characteristics as Herbig Ae/Be stars and should be regarded as UXors. Two FU Orionis stars ("FUors"), FU Ori and V1515 Cyg, also have recent light curves that are similar, in some respects, to UXors. The most developed model to account for the variations of some large-amplitude UXors involves variable obscuration by circumstellar dust clumps orbiting the star in a disk viewed nearly edge-on. However, there are problems in extending this model to the entire class, which lead us to propose an alternative mechanism, i.e., unsteady accretion. Evidence favoring the accretion model over the obscuration model is presented. It is suggested that the thermal instability mechanism responsible for outbursts in interacting binary system disks, and possibly FUors, may be the cause of the deep minima in UXors.

Galactic Globular Cluster Relative Ages

Abstract: We present accurate relative ages for a sample of 55 Galactic globular clusters. The ages have been obtained by measuring the difference between the horizontal branch and the turnoff in two internally photometrically homogeneous databases. The mutual consistency of the two data sets has been assessed by comparing the ages of 16 globular clusters in common between the two databases. We have also investigated the consistency of our relative age determination within the recent stellar model framework. All clusters with [Fe/H] -0.8 are ~1 Gyr younger than the most metal-poor ones, with a relatively small age dispersion, although the metal-rich sample is still too small to allow firmer conclusions. There is no correlation of the cluster age with the galactocentric distance. We briefly discuss the implication of these observational results for the formation history of the Galaxy.

The Rise Time of Nearby Type Ia Supernovae

Abstract: We present calibrated photometric measurements of the earliest detections of nearby Type Ia supernovae (SNe Ia). The set of ~30 new, unfiltered CCD observations delineate the early rise behavior of SNe Ia 18 to 10 days before maximum. Using simple empirical models, we demonstrate the strong correlation between the rise time (i.e., the time between explosion and maximum), the postrise light-curve shape, and the peak luminosity. Using a variety of light-curve shape methods, we find the rise time to B maximum for an SN Ia with ?m15(B) = 1.1 mag and peak MV = -19.45 mag to be 19.5 ? 0.2 days. We find that the peak brightness of SNe Ia is correlated with their rise time; SNe Ia that are 0.10 mag brighter at peak in the B band require 0.80 ? 0.05 days longer to reach maximum light. We determine the effects of several possible sources of systematic errors, but none of these significantly impacts the inferred rise time. We explore the degree to which comparisons between the observed and theoretically predicted rise times constrain SN Ia progenitor systems.

Globular Cluster Systems. I. V−I Color Distributions

Abstract: We have compiled data for the globular cluster systems of 50 galaxies from the Hubble Space Telescope Wide Field Planetary Camera 2 archive, of which 43 are type S0 or earlier. In this paper, we present the data set and derive the V-I color distributions. We derive the first four moments of the color distributions, as well as a measure for their nonunimodality. The number of globular clusters in each galaxy ranges from 18 (in NGC 2778) to 781 (NGC 5846). For those systems having more than 100 clusters, seven of 16 (44%) show significant bimodality. Overall, roughly half of all the systems in our sample show hints of a bimodal color distribution. In general, the distributions of the faint galaxies are consistent with unimodality, whereas those of the brighter galaxies are not. We also find a number of systems with narrow color distributions—with both mean red and blue colors—suggesting that systems exist with only metal-rich or only metal-poor globular clusters. We discuss their possible origins. In comparing the moments of the V-I distributions with various galaxy properties for the early-type galaxies, we find the following difference in the correlations between the field and cluster galaxy populations: the peak V-I color of the globular cluster distribution correlates well with the central velocity dispersion—and hence the Mg2 index and total luminosity—for galaxies in cluster environments; there exists no such correlation for field galaxies. This difference between cluster and field galaxies possibly reflects different formation scenarios for their globular cluster systems. Among the explanations for such a correlation, we consider either a larger age spread in the field populations or the possibility that cluster galaxies are always affected by significant accretion whereas some field galaxies could host pure "in situ" formed populations.

The Nature of Boxy/Peanut-Shaped Bulges in Spiral Galaxies

Abstract: We present a systematic observational study of the relationship between bars and boxy/peanut-shaped (B/PS) bulges. We first review and discuss proposed mechanisms for their formation, focusing on accretion and bar-buckling scenarios. Using new methods relying on the kinematics of edge-on disks, we then look for bars in a large sample of edge-on spiral galaxies with a B/PS bulge and in a smaller control sample of edge-on spirals with more spheroidal bulges. We present position-velocity diagrams of the ionized gas obtained from optical long-slit spectroscopy. We show that almost all B/PS bulges are due to a thick bar viewed edge-on, while only a few extreme cases may be due to the accretion of external material. This strongly supports the bar-buckling mechanism for the formation of B/PS bulges. None of the galaxies in the control sample show evidence for a bar, which suggests conversely that bars are generally B/PS.We consider the effects of dust in the disk of the galaxies but conclude that it does not significantly affect our results. Unusual emission-line ratios correlating with kinematical structures are observed in many objects, and we argue that this is consistent with the presence of strong bars in the disk of the galaxies. As expected from N-body simulations, the boxy-peanut transition appears to be related to the viewing angle, but more work is required to derive the precise orientation of the bars in the bulges. The reliable identification of bars in edge-on spiral galaxies opens up for the first time the possibility of studying observationally the vertical structure of bars.

Evidence for a Gradual Decline in the Universal Rest-Frame Ultraviolet Luminosity Density for Z < 1

Abstract: We have utilized various magnitude-limited samples drawn from an extremely deep and highly complete spectroscopic redshift survey of galaxies observed in seven colors in the Hawaii Survey Fields and the Hubble Deep Field to investigate the evolution of the universal rest-frame ultraviolet luminosity density from z = 1 to the present. The multicolor data (U', B, V, R, and I, J, HK') enable the sample selection to be made in the rest-frame ultraviolet for the entire redshift range. Because of the large sample size and depth (UAB = 24.75, BAB = 24.75, IAB = 23.5), we are able to accurately determine the luminosity density to z = 1. We do not confirm the very steep evolution reported by Lilly et al. but instead find a shallower slope, approximately (1 + z)1.5 for q0 = 0.5, which would imply that galaxy formation is continuing smoothly to the present time rather than peaking at z = 1. Much of the present formation is taking place in smaller galaxies. Detailed comparisons with other recent determinations of the evolution are presented.

Accretion in the Early Kuiper Belt II. Fragmentation

Abstract: We describe new planetesimal accretion calculations in the Kuiper Belt that include fragmentation and velocity evolution. All models produce two power law cumulative size distributions, NC ∝ r −2.5 for radii ∼ 0.3–3 km and NC ∝ r −3 for radii ∼ 1–3 km. The power law indices are nearly independent of the initial mass in the annulus, M0; the initial eccentricity of the planetesimal swarm, e0; and the initial size distribution of the planetesimal swarm. The transition between the two power laws moves to larger radii as e0 increases. The maximum size of objects depends on their intrinsic tensile strength, S0; Pluto formation requires S0 ∼ 300 erg g −1 . The timescale to produce Pluto-sized objects, τP, is roughly proportional to M −1 0 and e0, and is less sensitive to other input parameters. Our models yield τP ≈ 30–40 Myr for planetesimals with e0 = 10 −3 in a Minimum Mass Solar Nebula. The production of several ‘Plutos’ and ∼ 10 5 50 km radius Kuiper Belt objects leaves most of the initial mass in 0.1–10 km radius objects that can be collisionally depleted over the age of the solar system. These results resolve the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.

Signatures of the Giant Planets Imprinted on the Edgeworth-Kuiper Belt Dust Disk

Abstract: One method to detect extrasolar planetary systems is to deduce the perturbations of planets on the observed circumstellar dust disks. Our solar system, with its known configuration of planets, provides an excellent example to study how the distribution of dust in the Edgeworth-Kuiper belt (EKB) dust disk is affected by the existence of multiple and different planets. Numerical simulations of the orbital evolution of dust particles from EKB objects show that Neptune, by trapping dust particles in mean motion resonances, creates a ringlike structure along its orbit. Jupiter and Saturn, by ejecting dust particles from the solar system, create a radial brightness profile inside 10 AU that is quite different from that of a dust disk without their perturbations. On the other hand, Uranus and the terrestrial planets do not produce significant signatures on the EKB dust disk. Our solar system would be recognized as a system with at least two planets if observed from afar.

The Formation and Evolution of Candidate Young Globular Clusters in NGC 3256

Abstract: We present images of the recent galaxy merger NGC 3256 obtained with the Wide Field Planetary Camera 2 of the Hubble Space Telescope in B and I filters. We show that there is a large population of more than 1000 compact, bright, blue objects in this galaxy within the 7 kpc × 7 kpc region studied. These objects have sizes, colors, and luminosities like those expected for young Galactic globular clusters, with ages ranging from a few to several hundred megayears. On this basis, we identify at least some fraction of the compact, bright, blue objects in NGC 3256 as young globular clusters. The young cluster system makes up a significant fraction of the total luminosity of the galaxy within the region studied—15%–20% in B and half that in I, indicating a high efficiency of cluster formation on a galaxy-wide scale. In order to determine the properties of this young cluster system, the selection effects in size, color, and luminosity are carefully modeled. We find that the intrinsic color distribution is broad and there is no significant trend of color with magnitude. The combination of the broad range of observed colors and the lack of a trend of redder colors at fainter magnitudes cannot be fitted solely by a broad age distribution and/or differential reddening, although the latter is clearly present. The observations can be accounted for by either the preferential depletion/destruction of lower mass clusters as they age or a very young age (20 Myr) for the cluster population, comparable to or less than the dynamical time of the region in which the clusters are observed. We also find that the luminosity function of the young cluster system can be roughly fitted by a power law with an exponent of -1.8, with tentative evidence that it flattens at faint magnitudes. The clusters are compact in size, with typical estimated half-light radii of 5–10 pc, but there is no obvious cutoff for larger radii and only a shallow trend of size with luminosity. We discuss the implications of these results for models of the formation and dynamical evolution of globular clusters, as well as for interpretation of the properties of older globular cluster systems.