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We present the discovery of seven quasars at z > 5.7, selected from ∼2000 deg² of multicolor imaging data of the Sloan Digital Sky Survey. The new quasars have redshifts z from 5.79 to 6.13. Five are selected as part of a complete flux-limited sample in the SDSS northern Galactic cap; two have larger photometric errors and are not part of the complete sample. One of the new quasars, SDSS J1335+3533 (z = 5.93), exhibits no emission lines; the 3 σ limit on the rest-frame equivalent width of the Lyα+N V line is 5 Å. It is the highest redshift lineless quasar known and could be a gravitational lensed galaxy, a BL Lac object, or a new type of quasar. Two new z > 6 quasars, SDSS 1250+3130 (z = 6.13) and SDSS J1137+3549 (z = 6.01), show deep Gunn-Peterson absorption gaps in Lyα. These gaps are narrower than the complete Gunn-Peterson absorption troughs observed among quasars at z > 6.2 and do not have complete Lyβ absorption.

We compiled a sample of 29 flat-spectrum radio quasars (FSRQs) and 48 BL Lac objects for which both B - V and U - B are available. The two-color (U - B, B - V) stellar evolution diagram of FSRQs and BL Lac objects in this sample shows that FSRQs and BL Lac objects can be considered to be in the same family, but they occupy different extreme regions in the diagram. The result of the two-color diagram of FSRQs and BL Lac objects in this paper is consistent with the intrinsic accretion rate-luminosity relation for FSRQs and BL Lac objects that was obtained in a previous paper. In addition, based on the quasi-simultaneous spectral properties of core-dominated radio sources observed by Punsly, we compiled another sample of FSRQs and BL Lac objects for which the quantities Δ = log[(F_υ)_cm/(F_υ)_mm] have been well obtained. We found that there is a significant correlation between the intrinsic bolometric luminosity log L and the quantity . We also found that FSRQs have a relatively higher bolometric luminosity and larger than BL Lac objects. This result is also consistent with the intrinsic accretion rate-luminosity relation because the physical parameters Δ are strongly correlated with the accretion rates () and nuclear gas densities.

We present optical and X-ray identifications for the 64 radio sources in the Great Observatories Origins Deep Survey Chandra Deep Field-South Advanced Camera for Surveys (ACS) field revealed in the Australia Telescope Compact Array 1.4 GHz survey of the Chandra Deep Field-South. Optical identifications are made using the ACS images and catalogs, while the X-ray view is provided by the Chandra X-Ray Observatory 1 Ms observations. Redshifts for the identified sources are drawn from publicly available catalogs of spectroscopic observations and multiband photometric-based estimates. Using this multiwavelength information we provide a first characterization of the faint radio source population in this region. The sample contains a mixture of star-forming galaxies and active galactic nuclei, as identified by their X-ray properties and optical spectroscopy. A large number of morphologically disturbed galaxies are found, possibly related to star formation. In spite of the very deep optical data available in this field, seven of the 64 radio sources have no optical identification to z₈₅₀ ∼ 28 mag. Only one of these is identified in the X-ray.

PKS 2149-306 and CXOCDFS J033225.3-274219 each exhibit several emission lines in their optical spectra from which their redshifts have been determined. However, an ASCA spectrum of the first object has detected an emission line at 5 keV observed wavelength, and the Chandra ACIS-I spectrum of the second object shows a very strong emission feature at the observed wavelength of 6.2 keV. The two lines cannot be identified with any known search line on the basis of their redshift values without invoking extremely large outflow bulk velocities of 0.6c-0.75c. Such high bulk velocities are practically unheard of in any other observations of extragalactic objects. We show here that all the lines in both the optical and X-ray spectra of the two objects can be identified with lines at longer wavelengths that are blueshifted. We propose a scenario in which the spectrum is blueshifted due to net apparent motion toward the observer, resulting from an ejection.

Central black hole masses for 117 spiral galaxies representing morphological stages S0/a through Sc and taken from the large spectroscopic survey of Ho et al. are derived using K_s-band data from the Two Micron All Sky Survey. Black hole masses are found using a calibrated black hole-K_s bulge luminosity relation, while bulge luminosities are measured by means of a two-dimensional bulge-disk decomposition routine. The black hole masses are correlated against a variety of parameters representing properties of the nucleus and host galaxy. Nuclear properties such as line width (FWHM [N II]), as well as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [N II]/Hα, and [S II]/Hα), show a very high degree of correlation with black hole mass. The excellent correlation with line width supports the view that the emission-line gas is in virial equilibrium with either the black hole or bulge potential. The very good emission-line ratio correlations may indicate a change in ionizing continuum shape with black hole mass in the sense that more massive black holes generate harder spectra. Apart from the inclination-corrected rotational velocity, no excellent correlations are found between black hole mass and host galaxy properties. Significant differences are found between the distributions of black hole masses in early-, mid-, and late-type spiral galaxies (subsamples A, B, and C) in the sense that early-type galaxies have preferentially larger central black holes, consistent with observations that Seyfert galaxies are found preferentially in early-type systems. The line width distributions show a marked difference among subsamples A, B, and C in the sense that earlier type galaxies have larger line widths. There are also clear differences in line ratios between subsamples A+B and C that likely are related to the level of ionization in the gas. Finally, a K_s-band Simien & de Vaucouleurs diagram shows excellent agreement with the original B-band relation, although there is a large dispersion at a given morphological stage.

In order to examine the relative importance of powerful starbursts and Compton-thick active galactic nuclei (AGNs) in NGC 6240, we have obtained mid-infrared images and low-resolution spectra of the galaxy with subarcsecond spatial resolution using the Keck telescopes. Despite the high spatial resolution (∼200 pc) of our data, no signature of the hidden AGNs has been detected in the mid-infrared. The southern nucleus, which we show provides 80%-90% of the total 8-25 μm luminosity of the system, has a mid-infrared spectrum and a mid-/far-infrared spectral energy distribution consistent with starbursts. At the same time, however, it is also possible to attribute up to 60% of the bolometric luminosity to an AGN, consistent with X-ray observations, if the AGN is heavily obscured and emits mostly in the far-infrared. This ambiguity arises because the intrinsic variation of properties among a given galaxy population (e.g., starbursts) introduces at least a factor of a few uncertainty even into the most robust AGN-starburst diagnostics. We conclude that with present observations it is not possible to determine the dominant power source in galaxies when AGN and starburst luminosities are within a factor of a few of each other.

We report first-epoch circular polarization results for 133 active galactic nuclei in the MOJAVE program to monitor the structure and polarization of a flux-limited sample of extragalactic radio jets with the VLBA at 15 GHz. We found strong circular polarization (≥0.3%) in approximately 15% of our sample. The circular polarization was usually associated with jet cores; however, we did find a few strong jet components to be circularly polarized. The levels of circular polarization were typically in the range of 0.3%-0.5% of the local Stokes I value. We found no strong correlations between fractional circular polarization of jet cores and source type, redshift, EGRET detections, linear polarization, or other observed parsec-scale jet properties. There were differences between the circular-to-linear polarization ratios of two nearby galaxies versus more distant quasars and BL Lac objects. We suggest this is because the more distant sources have either (1) less depolarization of their linear polarization, and/or (2) poorer effective linear resolution, and therefore, their VLBA cores apparently contain a larger amount of linearly polarized jet emission. The jet of 3C 84 shows a complex circular polarization structure, similar to observations by Homan & Wardle 5 years earlier; however, much of the circular polarization seems to have moved, consistent with a proper motion of 0.06c. The jet of 3C 273 also has several circularly polarized components, and we find that their fractional circular polarization decreases with distance from the core.

We present 118 new optical redshifts for galaxies in 12 clusters in the Horologium-Reticulum supercluster (HRS) of galaxies. For 76 galaxies, the data were obtained with the Dual Beam Spectrograph on the 2.3 m telescope of the Australian National University at Siding Spring Observatory. After combining 42 previously unpublished redshifts with our new sample, we determine mean redshifts and velocity dispersions for 13 clusters in which previous observational data were sparse. In 6 of the 13 clusters, the newly determined mean redshifts differ by more than 750 km s^-1 from the published values. In three clusters, A3047, A3109, and A3120, the redshift data indicate the presence of multiple components along the line of sight. The new cluster redshifts, when combined with other reliable mean redshifts for clusters in the HRS, are found to be distinctly bimodal. Furthermore, the two redshift components are consistent with the bimodal redshift distribution found for the intercluster galaxies in the HRS by Fleenor and coworkers.

Chemical abundance indicators are studied using composite spectra, which we provide in tabular form. Tables of line strengths measured from these spectra and parameters derived from these line strengths are also provided. From these we find that at fixed luminosity, early-type galaxies in low-density environments are slightly bluer, with stronger O II emission and stronger Hδ and Hγ Balmer absorption lines, indicative of star formation in the not very distant past. These galaxies also tend to have systematically weaker D4000 indices. The Lick indices and α-element abundance indicators correlate weakly but significantly with environment. For example, at fixed velocity dispersion, Mg is weaker in early-type galaxies in low-density environments by 30% of the rms scatter across the full sample, whereas most Fe indicators show no significant environmental dependence. The galaxies in our sample span a redshift range that corresponds to look-back times of ∼1 Gyr. We see clear evidence for evolution of line-index strengths over this time. Since the low-redshift population is almost certainly a passively aged version of the more distant population, age is likely the main driver for any observed evolution. We use the observed redshift evolution as a model-independent clock to identify indicators that are more sensitive to age than to other effects such as metallicity. In principle, for a passively evolving population, comparison of the trends with redshift and environment constrain how strongly the luminosity-weighted ages and metallicities depend on environment. We develop a method for doing this that does not depend on the details of stellar population synthesis models. Our analysis suggests that the galaxies that populate the densest regions in our sample are older by ∼1 Gyr than objects of the same luminosity in the least dense regions, and that metallicity differences are negligible. We also use single-burst stellar population synthesis models, which allow for nonsolar α-element abundance ratios, to interpret our data. The combination of Hβ, Mg b, and ⟨Fe⟩ lines suggests that age, metallicity, and α-enhancement all increase with velocity dispersion. The objects at lower redshifts are older but have the same metallicities and α-enhancements as their counterparts of the same σ at higher redshifts, as expected if the low-redshift sample is a passively aged version of the sample at higher redshifts. In addition, objects in dense environments are less than 1 Gyr older and α-enhanced by ∼0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that in dense regions, the stars in early-type galaxies formed at slightly earlier times and on a slightly shorter timescale than in less dense regions. Using Hγ_F instead of Hβ leads to slightly younger ages but the same qualitative differences between environments. In particular, we find no evidence that objects in low-density regions are more metal-rich.

A sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Cluster has been imaged in the near-infrared (NIR) in J and K_s on the 2.1 m telescope at the Observatorio Astronómico Nacional in the Sierra San Pedro Mártir in Mexico. Isophotes as faint as μ_J = 24 mag arcsec^-2 and μ = 23 mag arcsec^-2 have been reached in most of the targets. Surface brightness profiles can be fitted across the whole range of radii by the sum of two components: a hyperbolic secant (sech) function, which is known to fit the light profiles of dwarf irregular galaxies (dIs), and a Gaussian component, which quantifies the starburst near the center. Isophotal and total fitted NIR magnitudes have been calculated, along with semimajor axes at μ_J = 23 mag arcsec^-2 and μ = 22 mag arcsec^-2. The diffuse underlying component and the young starburst have been quantified using the profile fitting. Most color profiles show a constant color, between J - K_s = 0.7 and 0.9 mag. The diffuse component represents the overwhelming majority of the NIR light for most BCDs, with the starburst enhancing the flux by less than about 0.3 mag. Linear correlations were found between the sech scale length and the sech magnitude and between the sech semimajor axis and the sech magnitude. Overall, galaxies with more luminous diffuse components are larger and brighter in the center. The central burst correlates with the diffuse component, with brighter BCDs having stronger starbursts, suggesting that more massive objects are forming stars more efficiently. BCDs lie on the "fundamental plane" defined by dIs in Paper I, following the same relation between sech absolute magnitude, sech central surface brightness, and the hydrogen line width W₂₀, although the scatter is larger than for the dIs. On the other hand, correlations between the sech absolute magnitude and the sech central surface brightness in K_s for BCDs and dIs are equally good, indicating that BCD line widths may be enhanced by turbulence or winds.

We have studied an unbarred Sb galaxy with a nuclear star-forming ring, NGC 7742, by means of two-dimensional spectroscopy, long-slit spectroscopy, and imaging and have compared the results with the properties of another galaxy of this type, NGC 7217, which was studied by us earlier. Both galaxies have many peculiar features in common: each has two global exponential stellar disks with different scale lengths, each possesses a circumnuclear inclined gaseous disk with a radius of 300 pc, and each has a global counterrotating subsystem, a gaseous one in NGC 7742 and a stellar one in NGC 7217. We suggest that a past minor merger is the probable cause of all these peculiarities, including the appearance of nuclear star-forming rings without global bars; the rings might be produced as resonance features by tidally induced oval distortions of the global stellar disks.

Curves in a family derived from powers of the polar-coordinate formula for ellipses are found to provide good fits to bound orbits in a range of power-law potentials. This range includes the well-known 1/r (Keplerian) and logarithmic potentials. These approximate orbits, called p-ellipses, retain some of the basic geometric properties of ellipses. They satisfy and generalize Newton's apsidal precession formula, which is one of the reasons for their surprising accuracy. Because of their simplicity the p-ellipses make very useful tools for studying trends among power-law potentials, and especially the occurrence of closed orbits. The occurrence of closed or nearly closed orbits in different potentials highlights the possibility of period resonances between precessing, eccentric orbits and circular orbits, or between the precession period of multilobed closed orbits and satellite periods. These orbits and their resonances promise to help illuminate a number of problems in galaxy and accretion disk dynamics.

We present Hubble Space Telescope Advanced Camera for Surveys images and color-magnitude diagrams for 25 nearby galaxies with radial velocities V_LG < 500 km s^-1. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, are found to be the nearest known representatives of blue compact dwarf objects. Using high-quality data on distances and radial velocities of 110 nearby field galaxies, we derive their mean Hubble ratio to be 68 km s^-1 Mpc^-1 with a standard deviation of 15 km s^-1 Mpc^-1. Peculiar velocities of most of the galaxies, V_pec = V_LG - 68D, follow a Gaussian distribution with σ_v = 63 km s^-1 but with a tail toward high negative values. Our data display the known correlation between peculiar velocity and galaxy elevation above the Local Supercluster plane. The small observed fraction of galaxies with high peculiar velocities, V_pec < -500 km s^-1, may be understood as objects associated with nearby groups (Coma I, Eridanus) outside the local volume.

We complement our Hubble Space Telescope (HST) observations of the inner ring of the galaxy NGC 3081 using an analytical approach and n-body simulations. We find that a gas cloud inner (r) ring forms under a rotating bar perturbation with very strong azimuthal cloud crowding where the ring crosses the bar major axis. Thus, star formation results near to and "downstream" of the major axis. From the dust distribution and radial velocities, the disk rotates counterclockwise (CCW) on the sky like the bar pattern speed. We explain the observed CCW color asymmetry crossing the major axis as due to the increasing age of stellar associations inside the r ring major axis. These move faster than the pattern speed. The exterior point of the r ring at the bar major axis has an orbital rate equal to the pattern speed. We show how the perturbation strength can be estimated from the ring shapes and relative spacing over the inner through outer ring regions. The bar strength (maximum tangential/radial force) appears to be constant from 6 to 15 kpc. We derive how the perturbation, the fractional long wavelength m = 2 intensity, and the rotation curve can be used to calculate the disk surface mass density versus radius. The disk surface density at 7 kpc is 13 M_⊙ pc^-2 rising to 19 at 13 kpc. The latter is insufficient by a factor of seven to generate the observed rotation curve, implying halo domination. The surface density may have been reduced at 7 kpc due to inner ring gas cloud scattering. The surface density plus the observed surface brightness gives a disk mass-to-light (M/L) ratio which increases from 7 kpc through 13 kpc, contradicting the usual assumption in bar strength calculations. The simulation ring lifetime of several billion years is consistent with our ∼400 Myr HST estimates. With a sufficiently high gas cloud surface mass density, our simulations form gas cloud "associations" near the ends of the bar as observed. Too high a density destroys the ring.

M83 is one of the nearest galaxies with enhanced nuclear star formation, and it presents one of the best opportunities to study the kinematics and physical properties of a circumnuclear starburst. Our three-dimensional spectroscopy data in the R band confirm the presence of a secondary nucleus or mass concentration (previously suggested by Thatte and coworkers). We determine the position of this hidden nucleus, which would be more massive than the visible one and was not detected in the optical Hubble Space Telescope images due, probably, to the strong dust extinction. Using a Keplerian approximation, we estimated for the optical nucleus a mass of (5.0 ± 0.8) × 10⁶M_⊙/ sin i (r < 15), and for the hidden nucleus, located 4'' ± 1'' to the northwest (position angle of 271° ± 15°) of the optical nucleus, a mass of (1.00 ± 0.08) × 10⁷M_⊙/ sin i (r < 15). The emission-line ratio map also unveils the presence of a second circumnuclear ring structure, previously discovered by IR imaging (Elmegreen and coworkers). The data allow us to resolve the behavior of the interstellar medium inside the circumnuclear ring and around the binary mass concentration.

The highly anisotropic distribution and apparent alignment of the Galactic satellites in polar great planes begs the question of how common such distributions are. The satellite system of M31 is the only nearby system for which we currently have sufficiently accurate distances to study the three-dimensional satellite distribution. We present the spatial distribution of the 15 currently known M31 companions in a coordinate system centered on M31 and aligned with its disk. Through a detailed statistical analysis we show that the full satellite sample describes a plane that is inclined by -56° with respect to the poles of M31 and has an rms height of 100 kpc. At 88% the statistical significance of this plane is low, and it is unlikely to have a physical meaning. We note that the great stellar stream found near Andromeda is inclined to this plane by 7°. Most of the M31 satellites are found within <±40° of M31's disk; i.e., there is little evidence for a Holmberg effect. If we confine our analysis to early-type dwarfs, we find a best-fit polar plane within 5°-7° from the pole of M31. This polar great plane has a statistical significance of 99.7% and includes all dSphs (except for And II), M32, NGC 147, and PegDIG. The rms distance of these galaxies from the polar plane is 16 kpc. The nearby spiral M33 has a distance of only ∼3 kpc from this plane, which points toward the M81 group. We discuss the anisotropic distribution of M31's early-type companions in the framework of three scenarios, namely, as remnants of the breakup of a larger progenitor, as a tracer of a prolate dark matter halo, and as a tracer of collapse along large-scale filaments. The first scenario requires that the breakup must have occurred at very early times and that the dwarfs continued to form stars thereafter to account for their stellar population content and luminosity-metallicity relation. The third scenario seems to be plausible, especially when considering the apparent alignment of our potential satellite filament with several nearby groups. The current data do not permit us to rule out any of the scenarios. Orbit information is needed to test the physical reality of the polar plane and of the different scenarios in more detail.

We compare the near-infrared (JHK) globular cluster luminosity functions (GCLFs) of the Milky Way, M31, and the Sculptor Group spiral galaxies. We obtained near-infrared photometry with the Persson's Auxiliary Nasmyth Infrared Camera on the Baade Telescope for 38 objects (mostly globular cluster candidates) in the Sculptor Group. We also have near-infrared photometry from the Two Micron All Sky Survey (2MASS)-6X database for 360 M31 globular cluster candidates and aperture photometry for 96 Milky Way globular cluster candidates from the 2MASS All-Sky and Second Incremental Release databases. The M31 6X GCLFs peak at absolute reddening-corrected magnitudes of M = -9.18, M = -9.73, and M = -9.98. The mean brightness of the Milky Way objects is consistent with that of M31 after accounting for incompleteness. The average Sculptor absolute magnitudes (correcting for relative distance from the literature and foreground reddening) are M = -9.18, M = -9.70, and M = -9.80. NGC 300 alone has absolute foreground-dereddened magnitudes M = -8.87, M = -9.39, and M = -9.46 using the newest Gieren et al. distance. This implies either that the NGC 300 GCLF may be intrinsically fainter than that of the larger galaxy M31 or that NGC 300 may be slightly farther away than previously thought. Straightforward application of our M31 GCLF results as a calibrator gives NGC 300 distance moduli of 26.68 ± 0.14 using J, 26.71 ± 0.14 using H, and 26.89 ± 0.14 using K.

Globular clusters have long been known for presenting (at times) significant deviations from spherical symmetry. While rotation has been the main proposed explanation, other complicating factors such as their constant interaction with the strong gravitational potential of their host galaxy have made it difficult for a consensus to be reached. To address this question we have obtained high-resolution spectra of WLM-1, the lone, old globular cluster associated with the isolated, low-mass dwarf irregular galaxy WLM. Using archival HST WFPC2 data, we measure the radial ellipticity profile of WLM-1, finding it to be highly elliptical, with a mean value of 0.17 in the region 05-5'', which is comparable to what is found in our Galaxy for the most elliptical globular clusters. There is no evidence of isophote twisting, except for the innermost regions of the cluster (r < 05). To investigate whether the observed flattening can be ascribed to rotation, we have obtained long-slit high-resolution Very Large Telescope UVES spectra of this cluster along and perpendicular to the axis of flattening. Using cross-correlation we find that the velocity profile of the cluster is consistent with zero rotation along either axis. Thus, neither cluster rotation nor galactic tides can be responsible for the flattened morphology of WLM-1. We argue that the required velocity dispersion anisotropy between the semimajor and semiminor axes that would be required to account for the observed flattening is relatively small, of order 1 km s^-1. Even though our errors preclude us from conclusively establishing that such a difference indeed exists, velocity anisotropy remains at present the most plausible explanation for the shape of this cluster.

We examine the nature of the progenitor of the giant stellar stream in M31 using as constraints new radial velocity measurements of stream red giant stars (presented in the companion paper by Guhathakurta and coworkers) along with other M31 data sets available in the literature. We find that the observations are best fitted by orbits that are highly eccentric and close to edge-on, with apocenter to pericenter ratios on the order of 25-30 and with apocenters at or only slightly beyond the southern edge of the current data. Among these orbits we are able to find a few that plausibly connect the stream with the northern spur or with the low surface brightness feature of high metallicity similar to that of the stream (originally reported by Ferguson and coworkers) to the east of M31's center. In the latter case, if the connection is real, then the eastern debris should lie well in front of M31, near the apocenter of the orbit. Both the width of the debris and velocity dispersion measurements imply a rough lower limit on the mass of the progenitor of 10⁸M_⊙. We use this limit and our orbits to discuss which of M31's satellites could be plausibly associated with the stream. In addition, we predict that the width of the stream should increase beyond the southern edge of the current data around the apocenter of the orbit and that the line-of-sight velocity dispersion should exhibit significant variations along the stream.

This article presents a measurement of the proper motion of the Sculptor dwarf spheroidal galaxy determined from images taken with the Hubble Space Telescope using the Space Telescope Imaging Spectrograph in the imaging mode. Each of two distinct fields contains a quasi-stellar object that serves as the "reference point." The measured proper motion of Sculptor, expressed in the equatorial coordinate system, is (μ_α, μ_δ) = (9 ± 13, 2 ± 13) mas century^-1. Removing the contributions from the motion of the Sun and the motion of the local standard of rest produces the proper motion in the Galactic rest frame: (μ, μ) = (-23 ± 13, 45 ± 13) mas century^-1. The implied space velocity with respect to the Galactic center has a radial component of V_r = 79 ± 6 km s^-1 and a tangential component of V_t = 198 ± 50 km s^-1. Integrating the motion of Sculptor in a realistic potential for the Milky Way produces orbital elements. The perigalacticon and apogalacticon are 68 (31, 83) and 122 (97, 313) kpc, respectively, where the values in the parentheses represent the 95% confidence interval derived from Monte Carlo experiments. The eccentricity of the orbit is 0.29 (0.26, 0.60), and the orbital period is 2.2 (1.5, 4.9) Gyr. Sculptor is on a polar orbit around the Milky Way: the angle of inclination is 86° (83°, 90°).

We have determined the proper motion (PM) of the Large Magellanic Cloud (LMC) relative to four background quasi-stellar objects, combining data from two previous studies made by our group and new observations carried out in three epochs not included in the original investigations. The new observations provided a significant increase in the time base and the number of frames, relative to what was available in our previous studies. We have derived a total LMC PM of μ = (+2.0 ± 0.1) mas yr^-1, with a position angle of θ = 624 ± 31. Our new values agree well with most results obtained by other authors, and we believe we have clarified the large discrepancy between previous results from our group. Using published values of the radial velocity for the center of the LMC, in combination with the transverse velocity vector derived from our measured PM, we have calculated the absolute space velocity of the LMC. This value, along with some assumptions regarding the mass distribution of the Galaxy, has in turn been used to calculate the mass of the Milky Way. Our measured PM also indicates that the LMC is not a member of a proposed stream of galaxies with similar orbits around our Galaxy.

One of the main endeavors of fundamental astrometry is to establish a practical realization of a nonrotating, inertial reference frame anchored to celestial objects whose positions are defined in the barycentric coordinates of the solar system matching the current level of astrometric observational accuracy. The development of astrometric facilities operating from space at a microarcsecond level of precision makes the nonuniformity of the Galactic motion of the barycenter an observable and nonnegligible effect that violates the desired inertiality of the barycentric frame of the solar system. Most of the observable effect is caused by the nearly constant (secular) acceleration of the barycenter with respect to the center of the Galaxy. The acceleration results in a pattern of secular aberration that is observable astrometrically as a systematic vector field of the apparent proper motions of distant quasars.

We employ the classic approximations of planar epicycle and vertical harmonic oscillation for the Sun's Galactic motion to estimate the magnitude of secular acceleration components in the Galactic coordinates and to show that the peculiar accelerations are smaller than the main galactocentric component. We employ the vector spherical harmonic formalism to describe the predicted field of proper motions and evaluate the amplitude of this field at each point on the celestial sphere. We show that the pattern of secular aberration is fully represented by three low-order electric-type vector harmonics; hence, it is easily distinguishable from the residual rotations of the reference frame and other possible effects, such as the hypothetical long-period gravitational waves, which are described by other types of vector or tensor harmonics. Comprehensive numerical simulations of the grid astrometry with Space Interferometry Mission (SIM) PlanetQuest are conducted assuming that 110 optically bright quasars are included as grid objects and observed on the same schedule, but to lower precision due to their faintness, as regular grid stars. The full covariance matrix of the simulated grid solution is used to evaluate the covariances of the three electric harmonic coefficients, representing the secular aberration pattern of proper motions. This is the only reliable method to estimate such sample-based statistics in view of the considerable star-to-star correlations in SIM global astrometry. We conclude that the grid astrometry with SIM PlanetQuest will be sensitive to the main galactocentric component of secular acceleration, arising from the circular motion of the local standard of rest (LSR) around the Galactic center, while the peculiar acceleration of the Sun with respect to the LSR is expected to be too small to be detected with this astrometric space interferometer.

Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 μm from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within Galactic longitudes 65° > |l| > 10° and latitudes |b| < 1°. Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow the separation of bright H II regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.2-0.3, Kes 69, G22.7-0.2, 3C 391, W44, 3C 396, 3C 397, W49B, G54.4-0.3, Kes 17, Kes 20A, RCW 103, G344.7-0.1, G346.6-0.2, CTB 37A, G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (nine remnants), fine-structure lines (three remnants), polycyclic aromatic hydrocarbons (four remnants), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.

We present observations of intermediate-velocity clouds (IVCs) found at positive velocities in the Canadian Galactic Plane Survey Phase I region (74° < l < 148°, -35 < b < +56). A catalog of 138 distinct IVCs and 13 IVC complexes has been compiled. From the global characteristics of our sample we demonstrate that the majority of these IVCs are directly associated with energetic phenomena, such as supernova explosions and massive star-forming regions, within the disk of the Galaxy and thus form a different population than the high-velocity clouds and high-latitude IVCs. We report what we believe to be the first detection of H I 21 cm absorption of a background source by an IVC and attempt to determine its physical properties and distance. We discuss in some detail a few of the IVCs that illustrate the connection between the clouds and energetic phenomena in the Galaxy. We suggest that the velocity shift of these IVCs away from the bulk of the Galactic H I emission provides an opportunity to study the structure of the low-latitude H I emission in a relatively confusion-free environment and present two examples of large (degree-scale) complexes of H I emission particularly suitable for such studies.

We present observations of 16 planetary nebulae (PNs) in the 2 μm (K band) spectral region, obtained with a long-slit near-infrared spectrometer at McDonald Observatory. In general, the strongest features in our spectra are recombination lines of H I, He I, and (in some cases) He II. Half the sample shows emission from vibrationally excited H₂. Some of the observed PNs (e.g., M 1-13) display H₂ line ratios characteristic of shocked, thermalized gas, while others (e.g., BD +30 3639) have ratios intermediate between pure radiative (UV) and shock excitation, consistent with either a combination of the mechanisms or UV illumination of dense material. Our spectra of J900 and M 1-13 confirm that published narrowband images trace the H₂ emission, and we find that the H₂ emission in SwSt 1 has a larger spatial extent than previously reported. In IC 5117, SwSt 1, and NGC 40 we detect the [Kr III] 2.199 μm line identified by Dinerstein in 2001, with strengths indicating that krypton is enriched relative to the solar abundance, most markedly so in NGC 40. We also detect several lines from the ³G term of [Fe III] in Vy 2-2, SwSt 1, and marginally in Cn 3-1. The [Kr III] and [Fe III] lines fall near in wavelength to H₂ transitions, which are often used as diagnostics for UV excitation because they arise from higher vibrationally excited levels (v = 2, 3). For moderate spectral resolving power, R ≤ 600, these lines may be blended with, or even mistaken for, the corresponding H₂ lines, leading to misinterpretation of the H₂ emission. The strength of both the Kr and Fe nebular emission lines can be enhanced by special circumstances, Kr because of nucleosynthetic self-enrichment in the progenitor star and Fe due to inefficient initial dust condensation or partial destruction of the dust after formation, causing a larger fraction of the elemental iron to reside in the gas phase.

L988 is a large (∼05 × 07) dark cloud complex at about 600 pc that contains several bright pre-main-sequence objects (such as V1331 Cyg and LkHα 321), but this paper deals in detail only with a small region on its eastern edge, near the HAeBe star LkHα 324. That star and its distant companion LkHα 324SE lie at the apex of a V-shaped area apparently excavated from the edge of L988, and are the brightest members of a small cluster containing about 50 Hα-emission stars. A median age of about 0.6 Myr (with large dispersion) is inferred from its color-magnitude diagram, constructed from VRI photometry to V = 22. Keck HIRES spectra show that LkHα 324SE is probably also an HAeBe. Its image is nonstellar, and within 3'' to the northwest are three condensations having complex [S II] and [O I] profiles and radial velocities up to -200 km s^-1. They probably originate in an outflow from LkHα 324SE. A bright Ap star with strong Si II lines is embedded in the heavy obscuration 8' to the west. It illuminates a small reflection nebulosity, has several faint Hα emitters nearby, and shares the radial velocity of L988, so clearly it was formed in that cloud. It demonstrates again that such chemical peculiarities can be established very early in young stars of moderate mass.

We describe a long-term project aimed at deriving information on the chemical evolution of the Galactic disk from a large sample of open clusters. The main property of this project is that all clusters are analyzed in a homogeneous way to guarantee the robustness of the ranking in age, distance, and metallicity. Special emphasis is devoted to the evolution of the earliest phases of the Galactic disk evolution, for which clusters have superior reliability with respect to other types of evolution indicators. The project is twofold: on one hand we derive the age, distance, and reddening (and indicative metallicity) by interpreting deep and accurate photometric data with stellar evolution models, and on the other hand, we derive the chemical abundances from high-resolution spectroscopy. Here we describe our overall goals and approaches and report on the midterm project status of the photometric part, with 16 clusters already studied, covering an age interval from 0.1 to 6 Gyr and galactocentric distances from 6.6 to 21 kpc. The importance of quantifying the theoretical uncertainties by deriving the cluster parameters with various sets of stellar models is emphasized. Stellar evolution models assuming overshooting from convective regions appear to better reproduce the photometric properties of the cluster stars. The examined clusters show a clear metallicity dependence on the galactocentric distance and no dependence on age. The tight relation between cluster age and magnitude difference between the main-sequence turnoff and the red clump is confirmed.

The present paper presents a tabulation of data on all 600 Galactic open clusters for which it is currently possible to calculate linear diameters. As expected, the youngest "clusters," with ages <15 Myr, contain a significant (≥20%) admixture of associations. Among intermediate-age clusters, with ages in the range 15 Myr to 1.5 Gyr, the median cluster diameter is found to increase with age. Small, compact clusters are rare among objects with ages >1.5 Gyr. Open clusters with ages >1 Gyr appear to form what might be termed a "cluster thick disk," part of which consists of objects that were probably captured gravitationally by the main body of the Galaxy.

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color-magnitude diagrams (CMDs) are constructed and reach well past the main-sequence turnoff for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7.0 ± 0.5 Gyr for NGC 188 and 10.0 ± 1.0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal-rich inner halo/thick-disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791 are the oldest open clusters, with ages of 10 Gyr. They are 2 Gyr younger than the thick-disk globular clusters. These results confirm the status of Berkeley 17 as one of the oldest known open clusters in the Milky Way, and its age provides a lower limit to the age of the Galactic disk.

We present near- and mid-infrared photometry obtained with the Spitzer Space Telescope of ∼300 known members of the IC 348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to investigate both the frequency and nature of the circumstellar disk population in the cluster. The Spitzer observations span a wavelength range between 3.6 and 24 μm, corresponding to disk radii of ∼0.1-5 AU from the central star. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar initial mass function. Using measurements of infrared excess between 3.6 and 8.0 μm, we find the total frequency of disk-bearing stars in the cluster to be 50% ± 6%. However, only 30% ± 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin, anemic disks. Both these values are below previous estimates for this cluster. The disk fraction appears to be a function of spectral type and stellar mass. The fraction of stars with optically thick disks ranges from 11% ± 8% for stars earlier than K6 to 47% ± 12% for K6-M2 stars to 28% ± 5% for M2-M6 stars. The disk longevity and thus conditions for planet formation appear to be most favorable for the K6-M2 stars, which are objects of comparable mass to the Sun for the age of this cluster. The optically thick disks around later type (>M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally, we find that the presence of an optically thick dust disk is correlated with gaseous accretion, as measured by the strength of Hα emission. A large fraction of stars classified as classical T Tauri stars possess robust, optically thick disks, and very few such stars are found to be diskless. The majority (64%) of stars classified as weak-lined T Tauri stars are found to be diskless. However, a significant fraction (12%) of these stars are found to be surrounded by thick, primordial disks. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

We present the results of deep near-infrared observations searching for very low mass young stellar objects (YSOs) in the massive star-forming region of S106 taken with the Subaru Telescope. The survey, whose limiting magnitude exceeds 20 mag in the JHK' bands, is sensitive enough to provide unprecedented details in the two nebular lobes. In addition, it reveals a census of the stellar population down to objects below the deuterium-burning limit, a fiducial boundary between brown dwarfs and planetary-mass objects. Based on color-color diagrams, nearly 600 embedded YSO candidates with near-infrared excesses have been identified in an area of ∼5' × 5' that are not uniformly distributed but centrally concentrated. Combining the reddening-corrected luminosity of the YSO candidates with the theoretical evolutionary models, we suggest that there exists a substantial substellar population. If we divide the cluster into three subgroups based on stellar and cloud properties, both their luminosity functions and mass functions across the stellar-substellar boundary demonstrate local variation on a subparsec scale. Nevertheless, all the mass functions have distinct characteristics in common: they do not have any turnover and do not decline down to the completeness limits. Furthermore, the mass function for S106 appears to be more abundant in young substellar objects compared to those obtained for other young clusters, such as Trapezium and IC 348. This implies that the substellar mass function for young clusters may have a variation.

The Hubble Ultra Deep Field Survey has provided the deepest optical and near-infrared views of the universe yet, and has enabled a search for the most distant supernovae, to z ∼ 2.2. We have found four supernovae by searching spans of integrations of the Ultra Deep Field and the Ultra Deep Field Parallels taken with the Hubble Space Telescope paired with the Advanced Camera for Surveys and with NICMOS. Interestingly, none of these events were Type Ia supernovae (SNe Ia) above a redshift of 1.4, despite the substantially increased sensitivity per unit area to such objects over other surveys, including the Great Observatories Origins Deep Survey. However, we find that the low frequency of SNe Ia observed at 1.4 < z < 2.4 is statistically consistent with current estimates of the Type Ia supernova rate per unit volume, including the global star formation history combined with the nontrivial assembly time of SN Ia progenitors.

We present previously unpublished BVRI photometry of the Type Ia supernovae 1999cc and 2000cf along with revised photometry of SN 1999cl. We confirm that SN 1999cl is reddened by highly nonstandard dust, with R_V = 1.55 ± 0.08. Excepting two quasar-lensing galaxies whose low values of R_V are controversial, this is the only known object with a published value of R_V less than 2.0. SNe 1999cl and 2000cf have near-infrared absolute magnitudes at maximum in good agreement with other Type Ia supernovae of midrange decline rates.

Using archival data of low-redshift (z < 0.01; Center for Astrophysics and SUSPECT databases) Type Ia supernovae (SNe Ia) and recent observations of high-redshift (0.16 < z < 0.64) SNe Ia, we study the "uniformity" of the spectroscopic properties of nearby and distant SNe Ia. We find no difference in the measurements we describe here. In this paper we base our analysis solely on line-profile morphology, focusing on measurements of the velocity location of maximum absorption (v_abs) and peak emission (v_peak). Our measurement technique makes it easier to compare low and high signal-to-noise ratio observations. We also quantify the associated sources of error, assessing the effect of line blending with assistance from the parameterized code SYNOW. We find that the evolution of v_abs and v_peak for our sample lines (Ca II λ3945, Si II λ6355, and S II λλ5454, 5640) is similar for both the low- and high-redshift samples. We find that v_abs for the weak S II λλ5454, 5640 lines and v_peak for S II λ5454 can be used to identify fast-declining [Δm₁₅(B) > 1.7] SNe Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II λ3945, an event also observed, although infrequently, in low-redshift SN Ia spectra (6 out of 22 SNe Ia in our local sample). Moreover, echoing the recent studies of Dessart & Hillier in the context of Type II supernovae (SNe II), we see similar P Cygni line profiles in our large sample of SN Ia spectra. First, the magnitude of the velocity location at maximum profile absorption may underestimate that at the continuum photosphere, as observed, for example, in the optically thinner line S II λ5640. Second, we report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P Cygni line profiles in SN Ia spectra, by as much as 8000 km s^-1. All the high-z SNe Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration and are now publicly available.

We report photometric and spectroscopic observations of the SU UMa-type dwarf nova KS Ursae Majoris during its 2003 February superoutburst. Modulations with a period of 0.07017 ± 0.00021 days, which is 3.3% larger than the orbital period, have been found during the superoutburst and may be positive superhumps. A maximum trough-to-peak amplitude of around 0.3 mag is determined for this superhump. The spectra show broad absorption-line profiles. The lines display blue and red troughs that alternate in depth. The radial velocity curve of the absorption wings of Hβ has an amplitude of 40 ± 11 km s^-1 and a phase offset of 0.12 ± 0.03. The γ-velocity of the binary is 3 ± 9 km s^-1 and varies on the order of 50 km s^-1 from day to day. From other clear evidence for a precessing eccentric disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The inner part of the disk is also eccentric, as evidenced by asymmetric and symmetric wings in the lines. Therefore, the whole disk is eccentric, and the variation of the γ-velocity and the evolutionary asymmetric line profiles could be criteria for a precessing eccentric accretion disk.

We present a spectroscopic sample of 747 detached close binary systems from the Sloan Digital Sky Survey (SDSS) Fourth Data Release. The majority of these binaries consist of a white dwarf primary and a low-mass secondary (typically M dwarf) companion. We have determined the temperature and gravity for 496 of the white dwarf primaries and the spectral type and magnetic activity properties for 661 of the low-mass secondaries. We have estimated the distances for each of the white dwarf-main-sequence star binaries and use white dwarf evolutionary grids to establish the age of each binary system from the white dwarf cooling times. With respect to a spectroscopically identified sample of ∼8000 isolated M dwarf stars in the SDSS, the M dwarf secondaries show enhanced activity with a higher active fraction at a given spectral type. The white dwarf temperatures and gravities are similar to the distribution of ∼1900 DA white dwarfs from the SDSS. The ages of the binaries in this study range from ∼0.5 Myr to nearly 3 Gyr (average age ∼0.20 Gyr).

We present the results of 37 nights of CCD unfiltered photometry of nova V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 days (≈3.40 hr) and 0.14773 days (≈3.55 hr) in the 2005 data. The 2004 data show variability on a similar timescale, but no coherent periodicity was found. We suggest that the longer periodicity is the orbital period of the underlying binary system and that the shorter period represents a negative superhump. The 3.40 hr period is about 4% shorter than the orbital period and obeys the relation between superhump period deficit and binary period. The detection of superhumps in the light curve is evidence of the presence of a precessing accretion disk in this binary system shortly after the nova outburst. From the maximum magnitude-rate of decline relation, we estimate the decay rate t₂ = 17 ± 4 days and a maximum absolute visual magnitude of M = -7.7 ± 1.7 mag.

Using published observations with the Rossi X-Ray Timing Explorer satellite of the peculiar low-mass X-ray binary Circinus X-1, we investigate its spectral evolution during a long-term dip just before periastron. The spectra can be fitted well by a two-blackbody model, the Eastern model or the modified Western model, plus partial covering absorption. The derived partial covering fraction and equivalent hydrogen column of partial covering absorption are correlated with the position on the hardness-intensity diagram. The heavy equivalent hydrogen column of partial covering (N_H > 10²⁴ cm^-2) suggests that partial covering absorption is the crucial determinant for the long-term dip behavior observed in Cir X-1. A low-energy component with luminosity between 3% and 4% of the nondip spectrum is not affected by the partial covering matter. A prominent iron emission line around 6.4 keV with variable flux is detected in the spectrum. The covering matter is distributed not only in the inner system but also in the disk and extends as far as the iron-line-emitting region in the disk. Several possible candidates for covering matter are discussed.

We report spectroscopic observations of HD 27638 B, the secondary in a visual binary in which the physically associated primary (separation ∼19'') is a B9 V star. The secondary shows strong Li λ6708 absorption, suggesting youth, and has attracted attention in the past as a candidate post-T Tauri star, although this has subsequently been ruled out. It was previously known to be a double-lined spectroscopic binary (F8+G6) with a period of 17.6 days and to show velocity residuals indicating a more distant massive third companion with a period of at least 8 yr. Based on our radial velocity measurements covering more than two cycles of the outer orbit, along with other measurements, we derive an accurate triple orbital solution giving an outer period of 9.447 ± 0.017 yr. The third object is more massive than either of the other two components of HD 27638 B but is not apparent in the spectra. We derive absolute visual magnitudes and effective temperatures for the three visible stars in HD 27638. Isochrone fitting based on those properties gives an age of 200 ± 50 Myr for the system. We also infer an inclination angle of ∼533 for the inner orbit of HD 27638 B. We detect a near-infrared excess in HD 27638 B, which we attribute to the third star being a close binary composed of late-type stars. This explains its large mass and lack of a visible signature. Modeling of this excess allows us to infer not only the masses of the components of the unseen companion but also the inclination angle of the outer orbit (∼73°). The HD 27638 system is thus at least quintuple.

We present the details of LinBrod, a program that calculates synthetic spectra of cool Roche lobe-filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of nonrotating single stars to yield approximate synthetic spectra. We use the line-broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars.

It has been shown in earlier papers that o And is a quadruple star. Based on spectroscopic observations, Hill and coworkers calculated the orbital elements for a binary, assigning it to the B component. We have calculated the orbital elements for the subsystems Aa-B and Aa using speckle interferometry measurements. The individual masses of the o And components are determined from the orbital elements for these three subsystems and the trigonometric parallax given in the Hipparcos Catalogue. The results obtained indicate that the A component is a spectroscopic binary.

We have used new spectroscopic and photometric observations of HD 149420 to obtain an improved orbital element solution and determine some basic properties of the system. This double-lined binary has an orbital period of 3.39430 days, a circular orbit, and a mass ratio of 0.52. The primary is somewhat evolved, with an A9 IV spectral type, while the secondary is estimated to be an F5: dwarf. The secondary may be synchronously rotating, but the primary is rotating substantially slower than its synchronous velocity. Our photometric observations confirm the light variability and period found by Hipparcos. We determine a photometric period of 1.698 days, essentially half the value of the orbital period, with a peak-to-peak amplitude of 0.028 mag in V. In addition to this ellipsoidal light variation, we also detect more rapid, low-amplitude variability with a period of 0.076082 days and a peak-to-peak V amplitude of 0.008 mag, as well as a possible second short 0.059256 day period with a peak-to-peak amplitude of only 0.005 mag. This indicates that the A9 IV primary in HD 149420 is a pulsating δ Scuti variable. Our binary model that best fits both the radial velocity and photometric observations indicates the possibility of extremely shallow grazing eclipses, a circumstance not unexpected given the large minimum masses from the orbital solution, the evolved nature of the primary, and the short orbital period.

Following the suggestion of Zuckerman and coworkers, we consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historically classified as unrelated main-sequence stars in the solar neighborhood, are instead a wide physical binary system and members of the young β Pic moving group (BPMG). The BPMG is the nearest (d ≲ 50 pc) of several groups of young stars with ages around 10 Myr that are kinematically convergent with the Oph-Sco-Cen association (OSCA), the nearest OB star association. Combining South African Astronomical Observatory optical photometry, Hobby-Eberly Telescope high-resolution spectroscopy, Chandra X-Ray Observatory data, and Second US Naval Observatory CCD Astrograph Catalog kinematics, we confirm with high confidence that the system is indeed extremely young. GJ 3305 itself exhibits very strong magnetic activity but has rapidly depleted most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member of the OSCA, lying 110 pc from the main subgroups. The system is similar to the BPMG wide binary HD 172555/CD -64 1208 and the HD 104237 quintet, suggesting that dynamically fragile multiple systems can survive the turbulent environments of their natal giant molecular cloud complexes, while still having high dispersion velocities imparted. Nearby young systems such as these are excellent targets for evolved circumstellar disk and planetary studies, having stellar ages comparable to that of the late phases of planet formation.

Only seven radio-quiet isolated neutron stars (INSs) emitting thermal X-rays are known, a sample that has yet to definitively address such fundamental issues as the equation of state of degenerate neutron matter. We describe a selection algorithm based on a cross-correlation of the ROSAT All-Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS) that identifies X-ray error circles devoid of plausible optical counterparts to the SDSS g ∼ 22 mag limit. We quantitatively characterize these error circles as optically blank; they may host INSs or other similarly exotic X-ray sources such as radio-quiet BL Lac objects and obscured active galactic nuclei. Our search is an order of magnitude more selective than previous searches for optically blank RASS error circles and excludes the 99.9% of error circles that contain more common X-ray-emitting subclasses. We find 11 candidates, 9 of which are new. While our search is designed to find the best INS candidates and not to produce a complete list of INSs in the RASS, it is reassuring that our number of candidates is consistent with predictions from INS population models. Further X-ray observations will obtain pinpoint positions and determine whether these sources are entirely optically blank at g ∼ 22, supporting the presence of likely INSs and perhaps enabling detailed follow-up studies of neutron star physics.

The purpose of this paper is to predict the temperature at the fundamental blue edge (FBE) of the instability strip for RR Lyrae (RRL) variables from the pulsation equation that relates temperature to period, luminosity, and mass. Modern data for the correlations between period, luminosity, and metallicity at the FBE for field and cluster RRL stars are used for the temperature calculation. The predicted temperatures are changed to B - V colors using an adopted color transformation. The predicted temperatures at the FBE become hotter as [Fe/H] changes from 0 to -1.5, and thereafter cooler as the metallicity decreases to -2.5. The temperature range over this interval of metallicity is Δ log T_e = 0.04, or 640 K at 6900 K. The predicted color variation is at the level of 0.03 mag in B - V over most of this range. The predictions are compared with the observed RRL colors at the FBE for both the field and cluster variables, showing general agreement at the level of 0.02 mag in (B - V)₀, which, however, is the uncertainty of the reddening corrections. The focus of the problem is then reversed by fitting a better envelope to the observed FBE relation between color and metallicity for metallicities smaller than -1.8, which, when inserted in the pulsation equation, gives a nonlinear calibration of the absolute magnitude of the average evolved level of the horizontal branch (HB) of M_V = 1.109 + 0.600([Fe/H]) + 0.140([Fe/H])², where the zero point has been set by the observed RR Lyrae stars in the LMC at ⟨V⟩₀ = 19.064 by Clementini et al. for [Fe/H] = -1.5, and using an adopted LMC modulus of (m - M)₀ = 18.54 from Tammann et al. that is independent of the LMC Cepheids. This equation gives M_V = 0.52 at [Fe/H] = -1.5.

However, the calibration fails for the extreme second-parameter clusters NGC 6388 and NGC 6441. Proof that the M_V absolute magnitudes for their RR Lyrae variables are ∼0.4 mag brighter than the calibration equation predicts is from the unusually long periods for given amplitudes at their high metallicities of [Fe/H] ∼ -0.5. All second-parameter clusters are believed to violate the equation, but less severely than these two extreme examples. An additional complication in using RRL stars as distance indicators at the 0.1 mag level is shown by the difference of Δ log P = 0.029 ± 0.007 in the position of the envelope locus at the shortest periods for the observed period-metallicity correlation between the field and cluster variables. The field variables have shorter periods than cluster variables at the envelope. This requires the cluster RRL stars to be 0.09 mag brighter than the field variables at the same temperature and mass, or to have a temperature difference of Δ log T_e = 0.008 at fixed luminosity and mass. The field and cluster variables also differ in the near absence of cluster RR Lyrae stars in the -1.7 > [Fe/H] > -2.0 metallicity interval, whereas the field variables show no such gap. A test is proposed for different origins for the field and the cluster variables by comparing the morphology of the HBs in the local dwarf spheroidal galaxies with that in the Galactic globular clusters in the inner halo and by relating the differences with the relevant second-parameter indicators.

We present the abundance analysis of stars from the tip of the red giant branch (RGB) to below the RGB bump in the globular cluster NGC 2808 based on high-resolution echelle spectra. We derive abundances of Al, α-process elements (Si I, Ca I, Ti I, and Ti II), and Fe-group elements (Sc II, V I, Cr I, Cr II, Mn I, Co I, and Ni I). Apart from Mg being somewhat reduced, likely because it has been depleted at the expense of Al in the MgAl cycle, the other α-element ratios show the overabundance typical of halo stars of similar metallicity. Mn is underabundant, whereas Fe-group elements have typical abundance ratios near the solar value. We detect star-to-star differences in Al abundances from the RGB tip down to the faintest star below the RGB bump, correlated with Na abundances at all luminosities. The slope of the Na-Al correlation is similar to the one found in M13 by Sneden et al., but it is different from those in other globular clusters of similar metallicity. We find that the amount of chemical inhomogeneities along the Na-O and Mg-Al anticorrelations in globular cluster red giants is correlated with the present-day cluster mass and ellipticity. Moreover, we find for the first time a correlation between the spread in proton-capture elements and orbital parameters of clusters. The chemical anomalies are more extended in clusters having large-sized orbits and longer periods and those with larger inclination angles of the orbit with respect to the Galactic plane.

We present results from a campaign of multiple-epoch echelle spectroscopy of relatively faint (V = 9.5-13.5 mag) red giants observed as potential astrometric grid stars for the Space Interferometry Mission (SIM PlanetQuest). Data are analyzed for 775 stars selected from the Grid Giant Star Survey, spanning a wide range of effective temperatures (T_eff), gravities, and metallicities. The spectra are used to determine these stellar parameters and to monitor radial velocity (RV) variability at the 100 m s^-1 level. The degree of RV variation measured for 489 stars observed two or more times is explored as a function of the inferred stellar parameters. The percentage of RV-unstable stars is found to be very high—about two-thirds of our sample. It is found that the fraction of RV-stable red giants (at the 100 m s^-1 level) is higher among stars with T_eff ∼ 4500 K, corresponding to the calibration-independent range of infrared colors 0.59 < (J - K_s)₀ < 0.73. A higher percentage of RV-stable stars is found if additional constraints of surface gravity and metallicity ranges, 2.3 < log g < 3.2 and -0.5 < [Fe/H] < -0.1, respectively, are applied. Selection of stars based on only photometric values of effective temperature (4300 K < T_eff < 4700 K) is a simple and effective way to increase the fraction of RV-stable stars. The optimal selection of RV-stable stars, especially in the case in which the Washington photometry is unavailable, can rely effectively on the 2MASS colors constraint 0.59 < (J - K_s)₀ < 0.73. These results have important ramifications for the use of giant stars as astrometric references for the SIM PlanetQuest.

We present a moderate-resolution (R ≡ λ/Δλ ≈ 2000), 0.8-4.1 μm spectrum of LSR 1610-0040, a high proper motion star classified as an early-type L subdwarf by Lépine and collaborators based on its red-optical spectrum. The near-infrared spectrum of LSR 1610-0040 does not fit into the (tentative) M/L subdwarf sequence but rather exhibits a mix of characteristics found in the spectra of both M dwarfs and M subdwarfs. In particular, the near-infrared spectrum exhibits an Na I doublet and CO overtone band heads in the K band and Al I and K I lines and an FeH band head in the H band, all of which have strengths more typical of field M dwarfs. Furthermore, the spectrum of Gl 406 (M6 V) provides a reasonably good match to the 0.6-4.1 μm spectral energy distribution of LSR 1610-0040. Nevertheless, the near-infrared spectrum of LSR 1610-0040 also exhibits features common to the spectra of M subdwarfs, including a strong Ti I multiplet centered at ∼0.97 μm, a weak VO band at ∼1.06 μm, and possible collision-induced H₂ absorption in the H and K bands. We discuss a number of possible explanations for the appearance of the red-optical and near-infrared spectra of LSR 1610-0040. Although we are unable to definitively classify LSR 1610-0040, the preponderance of evidence suggests that it is a mildly metal-poor M dwarf. Finally, we tentatively identify a new band of TiO at ∼0.93 μm in the spectra of M dwarfs.

We present the first high-resolution (R ≈ 31,000) spectra of the cool L subdwarf (sdL) 2MASS 0532+8246 and what was originally identified as an early-type sdL, LSR 1610-0040. Our work, in combination with contemporaneous work by Cushing and Vacca, makes it clear that the latter object is more likely a mid-M dwarf with an unusual composition that gives it some subdwarf spectral features. We use the data to derive precise radial velocities for both objects and to estimate space motion; both are consistent with halo kinematics. We measure the projected rotational velocities, revealing a very slow rotation for the old sd?M6 object LSR 1610-0040. The object 2MASS 0532+8246 exhibits a rapid rotation of v sin i = 65 ± 15 km s^-1, consistent with the behavior of L dwarfs. This means that the braking time for L dwarfs is extremely long, or that perhaps they never slow down. A detailed comparison of the atomic Rb and Cs lines with spectra of field L dwarfs shows that the spectral type 2MASS 0532+8246 is consistent with being mid- to late-L. The Rb I and K I lines of LSR 1610-0040 are like those of an early-L dwarf, but the Cs I line is like that of a mid-M dwarf. The appearance of the Ca II triplet in absorption in this object is very hard to understand if it is not at least as warm as M6. We explain these effects in a consistent way using a mildly metal-poor mid-M model. M subdwarfs have weak metal oxides and enhanced metal hydrides relative to normal M dwarfs. LSR 1610-0040 exhibits metal hydrides like an M dwarf but metal oxides like a subdwarf. The same explanation that resolves the atomic-line discrepancy explains this as well. Our spectra cover the spectral region around a previously unidentified absorption feature at 9600 Å and the region around 9400 Å in which detection of TiH has been claimed. We identify the absorption around 9600 Å as being due to atomic lines of Ti and a small contribution of FeH, but we cannot confirm a detection of TiH in the spectra of cool sdLs. In 2MASS 0532+8246, both metal oxides and metal hydrides are extremely strong relative to normal L dwarfs. It may be possible to explain the strong oxide features in 2MASS 0532+8246 by invoking effects due to inhibited dust formation. High-resolution spectroscopy has aided in beginning to understand the complex molecular chemistry and spectral formation in metal-deficient and ultracool atmospheres and the properties of early ultra-low-mass objects.

We have investigated the abundance anomalies of lithium for stars with planets in the temperature range of 5600-5900 K reported by Israelian and coworkers, as compared to 20 normal stars in the same temperature and metallicity ranges. Our result indicates a higher probability of lithium depletion for stars with planets in the main-sequence stage. It seems that stellar photospheric abundances of lithium in stars with planets may be somewhat affected by the presence of planets. Two possible mechanisms are considered to account for the lower Li abundances of stars with planets. One is related to the rotation-induced mixing due to the conservation of angular momentum by the protoplanetary disk, and the other is a shear instability triggered by planet migration. These results provide new information on stellar evolution and the lithium evolution of the Galaxy.

Signal extraction is a vital link between science and systems-engineering requirements. In this paper we present a Terrestrial Planet Finder (TPF) interferometer planet-signal-extraction algorithm and demonstrate the performance of several nulling-interferometer designs on canonical TPF astronomical scenes. We create the output response of a linear phase-chopping dual Bracewell nulling interferometer and a matrix version of the correlation method employed to generate dirty images. We derive general and specific map parameters, such as signal-to-noise ratio, signal-to-artifact ratio, and detection confidence, used for individual maps or comparing array architectures. We implement a matrix form of CLEAN that removes map artifacts, produces reconstructed images, and retrieves planetary signals. Monte Carlo simulations show that some fixed-length structurally connected interferometer configurations can detect Earth-like planets for systems at 10 pc in the presence of stellar Poisson noise. Since angular resolution depends on baseline length, a design that can vary array configuration for each specific scene is superior to an interferometer with a fixed array length. Thus, a flexible free-flying architecture should satisfy the science requirements for more TPF candidates, compared to a fixed-length structurally connected architecture.

We use a hybrid, multiannulus, n-body-coagulation code to investigate the growth of kilometer-sized planetesimals at 0.4-2 AU around a solar-type star. After a short runaway growth phase, protoplanets with masses of ∼10²⁶ g and larger form throughout the grid. When (1) the mass in these "oligarchs" is roughly comparable to the mass in planetesimals and (2) the surface density in oligarchs exceeds 2-3 g cm^-2 at 1 AU, strong dynamical interactions among oligarchs produce a high merger rate, which leads to the formation of several terrestrial planets. In disks with lower surface density, milder interactions produce several lower-mass planets. In all disks, the planet formation timescale is ∼10-100 Myr, similar to estimates derived from the cratering record and radiometric data.

We present near-IR JHK broadband photometry for 17 Kuiper Belt objects (KBOs) and Centaurs. The observations were performed within the ESO Large Program on the "Physical Properties of Kuiper Belt Objects and Centaurs" from 2001 January to 2002 August. We used the ISAAC instrument at the ESO 8 m Very Large Telescope. We compiled visible-near-IR colors for a total of 51 published objects and performed a statistical analysis. Color-color correlations show that the same coloring process is probably acting on Centaur and KBO surfaces in the visible-near-IR range. Centaurs with H - K smaller than the Sun (0.06) systematically display the reddest B - V colors (at the 2.5 σ level). These Centaur surfaces are suspected of harboring material that has spectral signatures around 1.7-2.2 μm (water ice is a possibility; it was reported for the three objects that have published spectroscopy). We report no statistically significant evidence for a bimodal structure of the VJHK Centaur colors (Kolmogorov-Smirnov and dip tests on up to 17 objects). The Centaur H - K colors show some robust evidence (significance level >99.99%) for a continuous structure. We also report a statistically significant bimodal structure of the Centaur B - R distribution, which is compatible with the results published by Peixinho et al. in 2003 with different data. Classical KBOs show no trends at the 3 σ level. The V - J color is marginally correlated with perihelion distance q (which is consistent with results reported by Doressoundiram et al. in 2005 on B - R colors). Resonant and scattered disk objects are underrepresented (seven and nine objects, respectively) and show no statistically significant trend. Some of the marginal trends are mentioned as worthy of subsequent monitoring.

The evolution of the lunar spin axis is studied. Prior work has assumed that the inclination of the lunar orbit is constant and that the node regresses uniformly. This work takes into account the nonconstant inclination and nonuniform regression of the node as determined from averaged models of the motion of the lunar orbit. The resulting dynamics is considerably more rich, exhibiting additional resonances, period doubling and tripling, and chaos.

This paper presents the fourth extension to the VLBA Calibrator Survey, containing 258 new sources not previously observed with very long baseline interferometry (VLBI). This survey, based on three 24 hr Very Long Baseline Array observing sessions, fills remaining areas on the sky above declination -40° where the calibrator density is less than one source within a 4° radius disk in any given direction. The share of these areas was reduced from 4.6% to 1.9%. Source positions were derived from astrometric analysis of group delays determined at 2.3 and 8.6 GHz frequency bands using the Calc/Solve software package. The VCS4 catalog of source positions, plots of correlated flux density versus projected baseline length, contour plots, and fits files of naturally weighted CLEAN images, as well as calibrated visibility function files, are available online and can be found using the search keyword "VCS4."

This paper investigates the stability of equilibrium points under the influence of small perturbations in the Coriolis and the centrifugal forces, together with the effects of oblateness and radiation pressures of the primaries. It is found that collinear points remain unstable. It is also seen that triangular points are stable for 0 ≤ μ < μ_c and unstable for μ_c ≤ μ ≤ , where μ is the critical mass parameter and depends on the above parameters. It is further observed that the Coriolis force has a stabilizing tendency, while the centrifugal force, radiation, and oblateness of the primaries have destabilizing effects; the presence of any one or more of the latter makes weak the stabilizing ability of the former. Therefore, the overall effect is that the range of stability of the triangular points decreases.