Table of contents

We present the results of a meta-analysis of Keck spectra of extragalactic globular clusters (GCs) in a sample of eight galaxies, ranging from dwarf galaxies to massive elliptical galaxies. We infer ages for the metal-poor and metal-rich GCs in these galaxies through comparisons to Galactic GCs. Both subpopulations appear to be no younger than their Galactic counterparts, with ages ≳10 Gyr. This is the largest sample of galaxies for which ages have been constrained spectroscopically. Our results support the formation of most GCs in massive galaxies at high redshift. We propose a scenario for the formation of GC subpopulations that synthesizes aspects of both accretion and in situ approaches in the context of galaxy formation through hierarchical merging.

We present the first results from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Grism Parallel Survey, a large program obtaining deep, slitless ACS grism spectroscopy of high-latitude HST parallel fields. We report on 11 high Galactic latitude fields here, each with grism integration times of more than 12 ks. We identify 601 compact emission-line galaxies at z ≤ 1.6, reaching emission lines to a flux limit of ≳5 × 10^-18 ergs cm^-2 s^-1 (3 σ). We determine redshifts by cross-correlation of the target spectra with template spectra, followed by visual inspection. We measure star formation rates from the observed [O II] λ3727, [O III] λ5007, and Hα line fluxes. Follow-up observations with the Keck telescope of one of the survey fields confirms our classification and redshifts with σ(z) ≃ 0.02. This is one of the deepest emission-line surveys to date, covering a total area of 121 arcmin². The rough estimate of the comoving number density of emission-line galaxies in our survey at 0.3 < z < 1.3 is ∼4.5 × 10^-3h Mpc^-3. We reach deeper into the emission-line luminosity function than either the STIS or NICMOS grism parallel surveys, finding an apparent space density of emission-line galaxies several times higher than those surveys. Because of the ACS high spatial resolution, our survey is very sensitive to faint, compact galaxies with strong emission lines and weak continua. The ACS grism survey provides the comoving star formation density at z ≤ 1.6 at a high level of completeness.

We measure the buildup of the stellar mass of galaxies from z = 6 down to z = 1. Using 15 band multicolor imaging data in the NICMOS Ultra Deep Field we derive photometric redshifts and masses for 796 galaxies down to H_AB = 26.5 mag. The derived evolution of the global stellar mass density of galaxies is consistent with previous star formation rate density measurements over the observed range of redshifts. Beyond the observed range, maintaining consistency between the global stellar mass and the observed star formation rate suggests the epoch of galaxy formation was z = 16.

The presence of dust in quasar absorbers, such as damped Lyα (DLA) systems, may cause the background QSO to appear reddened. We investigate the extent of this potential reddening by comparing the optical-to-infrared colors of QSOs with and without intervening absorbers. Our QSO sample is based on the Complete Optical and Radio Absorption Line System (CORALS) survey of Ellison and coworkers. The CORALS data set consists of 66 radio-selected QSOs at z_em ≥ 2.2 with complete optical identifications. We have obtained near-simultaneous B- and K-band magnitudes for a subset of the CORALS sample and supplemented our observations with further measurements published in the literature. In total, we have B - K colors for 42 of the 66 QSOs, of which 14 have intervening DLA systems. To account for redshift-related color changes, the B - K colors are normalized using the Sloan Digital Sky Survey QSO composite. The mean normalized B - K color of the DLA subsample is +0.12, whereas the mean for the no-DLA sample is -0.10; both distributions have rms scatters of ∼0.5. Neither a Student's t-test nor a Kolmogorov-Smirnov test indicate that there is any significant difference between the two color distributions. Based on simulations that redden the colors of QSOs with intervening DLA systems, we determine a reddening limit that corresponds to E(B - V) < 0.04 (SMC-like extinction) at 99% confidence (3 σ), assuming that E(B - V) is the same for all DLA systems. Finally, we do not find any general correlation between absorber properties (such as [Fe/Zn] or neutral hydrogen column density) and B - K color. The two reddest QSOs with DLA systems in our sample have H I column densities that differ from each other by an order of magnitude and moderate gas-to-dust ratios as inferred from chemical abundances. One of these two QSOs shows evidence for strong associated absorption from X-ray observations, an alternative explanation for its very red color. We conclude that the presence of intervening galaxies causes a minimal reddening of the background QSO.

This paper is the first of a series describing the results of the Australia Telescope Hubble Deep Field–South radio survey. The survey was conducted at four wavelengths, 20, 11, 6, and 3 cm, over a 4 year period and achieves an rms sensitivity of about 10 μJy at each wavelength. We describe the observations and data reduction processes and present data on radio sources close to the center of the Hubble Deep Field–South. We discuss in detail the properties of a subset of these sources. The sources include both starburst galaxies and galaxies powered by an active galactic nucleus and range in redshift from 0.1 to 2.2. Some of them are characterized by unusually high radio-to-optical luminosities, presumably caused by dust extinction.

This paper is part of a series describing the results from the Australia Telescope Hubble Deep Field–South (ATHDFS) survey obtained with the Australia Telescope Compact Array. This survey consists of observations at 1.4, 2.5, 5.2, and 8.7 GHz, all centered on the Hubble Deep Field–South. Here we present the first results from the extended observing campaign at 1.4 GHz. A total of 466 sources have been cataloged to a local sensitivity of 5 σ (11 μJy rms). A source extraction technique is developed that (1) successfully excludes spurious sources from the final source catalogs and (2) accounts for the nonuniform noise in our image. A source catalog is presented, and the general properties of the 1.4 GHz image are discussed. We also present source counts derived from our ATHDFS 1.4 GHz catalog. Particular attention is given to ensuring that the counts are corrected for survey incompleteness and systematic effects. Our counts are consistent with other surveys (e.g., the Australia Telescope ESO Slice Project, VIRMOS, and the Phoenix Deep Field), and we find, in common with these surveys, that the HDF-N counts are systematically lower.

We present first-epoch, milliarcsecond-scale linear polarization images at 15 GHz of 133 jets associated with active galactic nuclei (AGNs) from the MOJAVE (Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments) survey. MOJAVE is a long-term observational program to study the structure and evolution of relativistic outflows in AGNs. The sample consists of all known AGNs with Galactic latitude |b| > 25, J2000.0 declination greater than -20°, and correlated 15 GHz Very Long Baseline Array (VLBA) flux density exceeding 1.5 Jy (2 Jy for sources below the celestial equator) at any epoch during the period 1994–2003. Of the 133 AGNs that satisfy these criteria, 96 are also part of the VLBA 2 cm Survey. Because of strong selection effects, the sample is dominated by blazars with parsec-scale morphologies consisting of a bright core component at the extreme end of a one-sided jet. At least one-third of the cores are completely unresolved on the longest VLBA baselines, indicating brightness temperatures above 10¹¹ K. These cores tend to have electric vectors that are better aligned with the inner jet direction, possibly indicating the presence of a stationary shock near the base of the jet. The linear polarization levels of the cores are generally low (<5%), but many of the extended jet regions display exceedingly high fractional polarizations (>50%) and electric vectors aligned with the jet ridge line, consistent with optically thin emission from transverse shocks. The cores and jets of the radio galaxies show very little or no linear polarization. Both the weak- and strong-lined blazar classes (BL Lac objects and high-polarization radio quasars) show a general increase in fractional polarization with distance down the jet, but the BL Lac jets are generally more polarized and have electric vectors preferentially aligned with the local jet direction. We show that these differences are intrinsic to the jets and not the result of observational biases. We find that distinct features in the jets of gamma-ray-loud (EGRET) blazars are typically twice as luminous as those in non-EGRET blazars and are more highly linearly polarized. These differences can be adequately explained if gamma-ray blazars have higher Doppler-boosting factors, as the result of better alignment with the line of sight and/or higher bulk Lorentz factors.

We present total and polarized intensity images of 15 active galactic nuclei obtained with the Very Long Baseline Array at 7 mm wavelength at 17 epochs from 1998 March to 2001 April. At some epochs the images are accompanied by nearly simultaneous polarization measurements at 3 mm, 1.35/0.85 mm, and optical wavelengths. Here we analyze the 7 mm images to define the properties of the jets of two radio galaxies, five BL Lac objects, and eight quasars on angular scales ≳0.1 mas. We determine the apparent velocities of 106 features in the jets. For many of the features we derive Doppler factors using a new method based on a comparison of the timescale of decline in flux density with the light-travel time across the emitting region. This allows us to estimate the Lorentz factors (Γ), intrinsic brightness temperatures, and viewing angles of 73 superluminal knots, as well as the opening angle of the jet for each source. The Lorentz factors of the jet flows in the different blazars range from Γ ∼ 5 to 40 with the majority of the quasar components having Γ ∼ 16–18, while the values in the BL Lac objects are more uniformly distributed. The brightest knots in the quasars have the highest apparent speeds, while the more slowly moving components are pronounced in the BL Lac objects. The quasars in our sample have similar opening angles and marginally smaller viewing angles than the BL Lacs. The two radio galaxies have lower Lorentz factors and wider viewing angles than the blazars. Opening angle and Lorentz factor are inversely proportional, as predicted by gasdynamical models. The brightness temperature drops more abruptly with distance from the core in the BL Lac objects than in the quasars and radio galaxies, perhaps owing to stronger magnetic fields in the former resulting in more severe synchrotron losses of the highest energy electrons. In nine sources we detect statistically meaningful deviations from ballistic motion, with the majority of components accelerating with distance from the core. In six sources we identify jet features with characteristics of trailing shocks that form behind the primary strong perturbations in jet simulations. The apparent speeds of these components increase with distance from the core, suggestive of acceleration of the underlying jet.

We present the historical light curve of the BL Lac object S5 0716+714, spanning the time interval from 1953 to 2003, built using Asiago archive plates and our recent CCD observations, together with literature data. The source shows an evident long-term variability, over which well-known short-term variations are superposed. In particular, in the period from 1961 to 1983 the mean brightness of S5 0716+714 remained significantly fainter than that observed after 1994. Assuming a constant variation rate of the mean magnitude, we can estimate a value of about 0.11 mag yr^-1. The simultaneous occurrence of decreasing ejection velocities of superluminal moving components in the jet reported by Bach and coworkers suggests that both phenomena are related to the change of the direction of the jet to the line of sight from about 5° to 07 for an approximately constant bulk Lorentz factor of about 12. A simple explanation is a precessing relativistic jet, which should at present be close to the smallest orientation angle. One can therefore in the next 10 years expect a decrease of the mean brightness of about 1 magnitude.

Near-IR images of the prototype LINER/Seyfert type 1 galaxy NGC 1097 observed with the Very Large Telescope using adaptive optics disclose with unprecedented detail a complex central network of filamentary structure spiraling down to the center of the galaxy. The structure, consisting of several spiral arms, some almost completing a revolution about the center, is most prominent within a radius of about 300 pc. Gas and dust may be channelled to the center of NGC 1097 along this central spiral. Some filaments can be traced farther out, where they seem to connect with the nuclear star-forming ring at a 0.7 kpc radius. Straight principal shocks running along the primary large-scale bar of NGC 1097, seen in the optical images as prominent dust lanes, curve into this ring, but radio polarization vectors cross the nuclear ring at a rather large angle. Here we attempt to explain this morphology in terms of three-dimensional gas flow in a barred galaxy. In our scenario, parts of the principal shock that propagate in the off-plane gas can cross the nuclear star-forming ring and excite waves inward from it. If the dispersion relation of the excited waves allows for their propagation, they will naturally take the shape of the observed central spiral. The nuclear region of NGC 1097 remains unresolved at subarcsecond scales in the near-IR, with an upper size limit of <10 pc FWHM. Thus, any putative central dusty torus or gaseous disk envisaged by the active galactic nucleus (AGN) unified schemes has to be smaller than 10 pc in diameter at near-IR wavelengths. The extinction in the region between the nuclear star-forming ring and the nucleus increases very moderately, reaching A_v ∼ 1 at the immediate surroundings of the nucleus. Thus, if the nuclear filaments are tracing cold dust, they contribute to a very low extinction in the line of sight and are likely to be distributed in a rather thin disk.

We investigate the environmental dependence of star formation in cluster virial regions and infall regions as part of the Cluster and Infall Region Nearby Survey (CAIRNS), a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. We use complete, homogeneous spectroscopic surveys of K_s-limited samples in eight of the CAIRNS clusters. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to the general large-scale structure. Galaxies in infall regions probe whether processes affecting star formation are effective over scales larger than cluster virial regions. The fraction of galaxies with current star formation in their inner disks as traced by Hα emission increases with distance from the cluster and converges to the "field" value only at 2–3 virial radii, in agreement with other investigations. However, among galaxies with significant current star formation (EW[Hα] ≥ 2 Å), there is no difference in the distribution of EW[Hα] inside and outside the virial radius. This surprising result, first seen by Carter and coworkers, suggests that (1) star formation is truncated either on very short timescales or only at moderate and high redshifts or (2) that projection effects contaminate the measurement. We quantify the possible impact of mechanisms that only affect the outer parts of galaxies and thus might not be detected in this survey or any fiber-based survey. The number density profiles of star-forming and non-star-forming galaxies indicate that, among galaxies projected inside the virial radius, at least half the former and 20% of the latter are "infall interlopers," galaxies in the infall region but outside the virial region. We show that the kinematics of star-forming galaxies in the infall region closely match those of absorption-dominated galaxies. This result shows that the star-forming galaxies in the infall regions are not interlopers from the field and excludes one model of the backsplash scenario of galaxy transformation. Finally, we quantify systematic uncertainties in estimating the global star formation in galaxies from their inner disks.

We conduct a photometric and spectroscopic survey of a 10 deg² region surrounding the nearby NGC 5846 group of galaxies, using the Canada-France-Hawaii and Keck I telescopes to study the population of dwarf galaxies as faint as M_R = -10. Candidates are identified on the basis of quantitative surface brightness and qualitative morphological criteria. Spectroscopic follow up and a spatial correlation analysis provide the basis for affirming group memberships. Altogether, 324 candidates are identified, and 83 have spectroscopic membership confirmation. We argue on statistical grounds that a total of 251 ± 10 galaxies in our sample are group members. The observations, together with archival Sloan Digital Sky Survey, ROSAT, XMM-Newton, and ASCA data, suggest that the giant ellipticals NGC 5846 and NGC 5813 are the dominant components of subgroups separated by 600 kpc in projection and embedded in a 1.6 Mpc diameter dynamically evolved halo. The galaxy population is overwhelmingly early type. The group velocity dispersion is 322 km s^-1, its virial mass is 8.4 × 10¹³M_⊙, and M/L_R = 320 M_⊙L. The ratio of dwarfs to giants is large compared with other environments in the Local Supercluster studied, and, correspondingly, the luminosity function is relatively steep, with a faint-end Schechter function slope of α_d = -1.3 ± 0.1 (statistical) ±0.1 (systematic) at our completeness limit of M_R = -12.

We derive the number of dynamically close companions per galaxy (N_c) and their total luminosity (L_c) for galaxies in the Millennium Galaxy Catalogue. N_c is similar to the fraction of galaxies in close pairs and is directly related to the galaxy merger rate. We find N_c = 0.0174 ± 0.0015 and L_c = (252 ± 30) × 10⁶L_⊙ for galaxies with -22 < M_B - 5 log h < -19 and ⟨z⟩ = 0.123, and N_c = 0.0357 ± 0.0027 and L_c = (294 ± 31) × 10⁶L_⊙ for galaxies with -21 < M_B - 5 log h < -18 and ⟨z⟩ = 0.116. The integrated merger rate to z = 1 for both samples is about 20%, but this depends sensitively on the fraction of kinematic pairs that are truly undergoing a merger (assumed here to be 50%), the evolution of the merger rate [here as (1 + z)³], and the adopted timescale for mergers (0.2 and 0.5 Gyr for each sample, respectively). Galaxies involved in mergers tend to be marginally bluer than noninteracting galaxies and show an excess of both early-type and very late type objects and a deficiency of intermediate-type spirals. This suggests that interactions and mergers partly drive the star formation and morphological evolution of galaxies.

We present near-infrared observations in the J, H, and K' passbands of the interacting pair of galaxies NGC 5394/95 (KPG 404). The total magnitudes, colors, surface brightnesses, and color profiles are calculated. In addition, aperture magnitudes are compared against previous determinations. We also perform a structural (disk + bulge) analysis, as well as a two-dimensional Fourier analysis, to gain insight into the morphology of the pair. The disk + bulge fit shows that NGC 5394 (KPG 404A) is more compact than normal galaxies, while NGC 5395 (KPG 404B) is less concentrated. The two-dimensional Fourier analysis shows that NGC 5394 is an H2β galaxy in the dust-penetrated (DP) classification by Block & Puerari. NGC 5395, in contrast, displays a very complex structure that needs a number of Fourier coefficients to be explained. A tightly wound m = 1 coefficient (DP class H1α) is the main structure, but other m = 1 and m = 2 coefficients (suggesting modulation) are also present in the Fourier spectra. The m = 1 coefficients represent a pseudo-ring-type structure, indicative of a collision rather than a passage. Based on our results we are able to assert that the scenario of the interaction between the galaxy members of KPG 404 should take into account a crossing of NGC 5394 through the disk of NGC 5395 in a Cartwheel-like encounter rather than a passage as in M51-type pairs. Numerical simulations could help to unravel the structural and morphological evolution of this interacting pair.

Petrosian magnitudes were designed to help with the difficult task of determining a galaxy's total light. Although these magnitudes [taken here as the flux within 2R_P, with the inverted Petrosian index 1/η(R_P) = 0.2] can represent most of an object's flux, they do of course miss the light outside the Petrosian aperture (2R_P). The size of this flux deficit varies monotonically with the shape of a galaxy's light profile, i.e., its concentration. In the case of a de Vaucouleurs R^1/4 profile, the deficit is 0.20 mag; for an R^1/8 profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total (Sérsic) magnitudes from Petrosian magnitudes using only the galaxy concentration (R₉₀/R₅₀ or R₈₀/R₂₀) within the Petrosian aperture. The corrections hold to the extent that Sérsic's model provides a good description of a galaxy's luminosity profile. We show how the concentration can also be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) Petrosian parameters, yielding good agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the New York University Value-Added Galaxy Catalog Sérsic R^1/n fits by Blanton and coworkers. Although the corrective procedure described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.

We devise improved photometric parameters for the morphological classification of galaxies using a bright sample from the First Data Release of the Sloan Digital Sky Survey. In addition to using an elliptical aperture concentration index for classification, we introduce a new texture parameter, coarseness, which quantifies deviations from smooth galaxy isophotes. The elliptical aperture concentration index produces morphological classifications that are in appreciably better agreement with visual classifications than those based on circular apertures. With the addition of the coarseness parameter, the success rate of classifying galaxies into early and late types increases to ≃88% with respect to the reference visual classification. A reasonably high success rate (≃68%) is also attained in classifying galaxies into three types, early-type galaxies (E+S0) and early-type (Sa+Sb) and late-type (Sc+Sdm+Im) spiral galaxies.

We present the results of high-quality long-slit spectroscopy of planetary nebulae (PNe) and H II regions in the two dwarf irregular (dIrr) galaxies Sextans A and B, which belong to a small group of galaxies just outside the Local Group. The observations were obtained with the New Technology Telescope ESO Multi-Mode Instrument. In Sextans A we obtained the element abundances in its only known PN and in three H II regions with the classical T_e method. The oxygen abundances in these three H II regions of Sextans A are all consistent within the individual rms uncertainties, with an average 12 + log(O/H) = 7.54 ± 0.06. The oxygen abundance of the PN in Sextans A is, however, significantly higher: 12 + log(O/H) = 8.02 ± 0.05. This PN is even more enriched in nitrogen and helium, suggesting a classification as a PN of type I. The PN abundances of S and Ar, which are presumably unaffected by nucleosynthesis in the progenitor star, are well below those in the H II regions, indicating lower metallicity at the epoch of the PN progenitor formation (∼1.5 Gyr ago, according to our estimates based on the PN parameters). In Sextans B we obtained spectra of one PN and six H II regions. Element abundances with the T_e method could be derived for the PN and three of the H II regions. For two of these H II regions, which have a separation of only ∼70 pc in projection, the oxygen abundances do not differ within the rms uncertainties, with a mean of 12 + log(O/H) = 7.53 ± 0.05. The third H II region, which is about 0.6 kpc northeast of the first two, is twice as metal-rich, with 12 + log(O/H) = 7.84 ± 0.05. This suggests considerable inhomogeneity in the present-day metallicity distribution in Sextans B. Whether this implies a general chemical inhomogeneity among populations of comparable age in Sextans B, and thus a metallicity spread at a given age, or whether we happen to see the short-lived effects of freshly ejected nucleosynthesis products prior to their dispersal and mixing with the ambient interstellar medium will require further study. For the PN we measured an O/H ratio of 12 + log(O/H) = 7.47 ± 0.16, consistent with that of the low-metallicity H II regions. We discuss the new metallicity data for the H II regions and PNe in the context of the published star formation histories and published abundances of the two dIrr galaxies. Both dIrrs show generally similar star formation histories in the sense of continuous star formation with amplitude variations but differ in their detailed enrichment timescales and star formation rates as a function of time. If we combine the photometrically derived estimates for the mean metallicity of the old red giant branch population in both dIrrs with the present-day metallicity of the H II regions, both dIrrs have experienced chemical enrichment by at least 0.8 dex (lower limit) throughout their history.

We analyze the vertical distribution of the resolved stellar populations in six low-mass (V_max = 67–131 km s^-1), edge-on, spiral galaxies observed with the Hubble Space Telescope Advanced Camera for Surveys. In each galaxy we find evidence for an extraplanar stellar component extending up to 15 scale heights (3.5 kpc) above the plane, with a scale height typically twice that of two-dimensional fits to K_s-band Two Micron All Sky Survey images. We analyze the vertical distribution as a function of stellar age by tracking changes in the color-magnitude diagram. The young stellar component (≲10⁸ yr) is found to have a scale height larger than the young component in the Milky Way, suggesting that stars in these low-mass galaxies form in a thicker disk. We also find that the scale height of a stellar population increases with age, with young main-sequence stars, intermediate-age asymptotic giant branch stars, and old red giant branch (RGB) stars having successively larger scale heights in each galaxy. This systematic trend indicates that disk heating must play some role in producing the extraplanar stars. We constrain the rate of disk heating using the observed trend between scale height and stellar age and find that the observed heating rates are dramatically smaller than in the Milky Way. The color distributions of the RGB stars well above the midplane indicate that the extended stellar components we see are moderately metal-poor, with peak metallicities around [Fe/H] = -1 and with little or no metallicity gradient with height. The lack of metallicity gradient can be explained if a majority of extraplanar RGB stars were formed at early times and are not dominated by a younger heated population. Our observations suggest that, like the Milky Way, low-mass disk galaxies also have multiple stellar components. In its structure, mean metallicity, and old age, the RGB component in these galaxies seems analogous to the Milky Way thick disk. However, without additional kinematic and abundance measurements, this association is only circumstantial, particularly in light of the clear existence of some disk heating at intermediate ages. Finally, we find that the vertical dust distribution has a scale height somewhat larger than that of the main-sequence stars.

A sample of 34 dwarf irregular galaxies (dIs) in the Local Volume, most nearer than 5 Mpc, has been imaged in the near-infrared (NIR) in J and K_s at the Canada-France-Hawaii Telescope (CFHT) in Hawaii and the Observatorio Astronómico Nacional in the Sierra San Pedro Mártir, in Mexico. Absolute magnitudes in K_s range from -14 to -18. In the CFHT images, stars brighter than M ∼ -7.5 were resolved. We show that the resolved component comprises more than 50% of the light from star formation bursts within the last 3 Gyr. In most cases, the resolved population down to M = -7.5 represents less than 5% of the total NIR flux in K_s, with fractions in J being 1.5–2 times larger. Thus, the NIR light of dIs can be considered to be predominantly contributed by stars older than about 4 Gyr. Although exponential at large radii, surface brightness profiles for the unresolved component flatten in the centers. They can be fitted across the whole range of radii with a hyperbolic secant (sech) defined as a function of two parameters: the central surface brightness and the scale length of the exponential. With respect to this model, only two galaxies (NGC 1569 and NGC 3738) show an excess of flux in the center, both of which are hosting starbursts. Isophotal, total, and fitted sech magnitudes have been calculated for all galaxies for which the unresolved component was detected, along with semimajor axes at μ_J = 23 mag arcsec^-2 and μ = 22 mag arcsec^-2. The scale length and the semimajor axes correlate linearly with absolute isophotal magnitude. The same is true for colors and the central brightness. More luminous dIs tend to be larger, redder, and brighter in the center. The fraction of light contributed by young stars is independent of both luminosity and central surface brightness. The Tully-Fisher relation shows considerable scatter, but residuals are tied to surface brightness. The galaxies appear to lie in a "fundamental plane" defined by the sech absolute magnitude, the sech central surface brightness, and the H I line width. The rms of residuals in M_K is only 0.4 mag, which implies that the plane can be used to evaluate the distances of star-forming dwarfs. Corrections for tilt do not reduce the residuals, so line widths must be governed predominantly by random motions. Color-magnitude diagrams (CMDs) are presented for 29 galaxies in which stars were resolved. Most show a finger centered around J - K_s = 1 mag. In some cases, there is a red tail extending to J - K_s = 2.5 mag. Most color profiles constructed for the unresolved component show a remarkably constant J - K_s = 0.8–1.0 mag, matching the color of the finger in the CMDs.

We have used Hubble Space Telescope Wide Field Planetary Camera 2 observations to construct a color-magnitude diagram (CMD) for the bulge of M31 at a location ∼1.6 kpc from the galaxy's center. Using scaled solar abundance theoretical red giant branches with a range of metallicities, we have translated the observed colors of the stars in the CMD to abundances and constructed a metallicity distribution function (MDF) for this region. The MDF shows a peak at [M/H] ∼ 0 with a steep decline at higher metallicities and a more gradual tail to lower metallicities. This is similar in shape to the MDF of the Milky Way bulge but shifted to higher metallicities by ∼0.1 dex. As is the case with the Milky Way bulge MDF, a pure closed-box model of chemical evolution, even with significant pre-enrichment, appears to be inconsistent with the M31 bulge MDF. However, a scenario in which an initial infall of gas enriched the bulge to an abundance of [M/H] ∼ -1.6 with subsequent evolution proceeding as a closed box provides a better fit to the observed MDF. The similarity between the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to the significant differences in the MDFs of their halo populations. This suggests that the bulk of the stars in the bulges of both galaxies were in place before the accretion events that occurred in the halos could influence them.

We present Antarctic Submillimeter Telescope and Remote Observatory observations of ¹²CO J = 4 → 3 and ¹²[C I] emission in the 30 Doradus complex in the Large Magellanic Cloud. We detected strong ¹²CO J = 4 → 3 emission toward R140, a multiple system of Wolf-Rayet stars located on the rim of the expanding H II shell surrounding the R136 cluster. We also detected a high-velocity gas component as a separate feature in the ¹²CO J = 4 → 3 spectrum. This component probably originates from molecular material accelerated as a result of the combined motion induced by the stellar winds and explosions of supernovae, including several fast-expanding H II shells in the complex. The lower limit on the total kinetic energy of the atomic and molecular gas component is ∼2 × 10⁵¹ ergs, suggesting that this comprises only 20% of the total kinetic energy contained in the H II complex structure.

We present unfiltered CCD photometry of a sample of 71 candidate RR Lyrae stars during 2003 and 2004, out of 148 candidate stars that were selected from Sloan Digital Sky Survey (SDSS) commissioning data by Ivezić and coworkers. We obtain light curves of 69 candidate stars and present a catalog of their positions, amplitudes, periods, types, and mean magnitudes. We confirm that the 69 stars are true RR Lyrae variables based on periods and light curves, for the first time for 41 of them. We identify 57 type ab, 11 type c, and 1 type d (double-mode) in our sample. The double-mode RR Lyrae star shows properties similar to globular cluster stars and are unlike field stars of the Galactic halo in the Petersen diagram. We also find one RR Lyrae star that is related to Pal 5 and recover another. In our sample, only two candidate stars do not have a reasonable light curve. We conclude that the ratio of contamination in our sample is less than ∼8%, which shows that the criteria used by Ivezić and coworkers to select RR Lyrae stars from the SDSS two-epoch data are reliable.

The birthrate of stars of masses ⩾10 M_⊙ is estimated from a sample of just over 400 O3–B2 dwarfs within 1.5 kpc of the Sun and the result extrapolated to estimate the Galactic supernova rate contributed by such stars. The solar-neighborhood Galactic-plane massive star birthrate is estimated at ∼176 stars kpc^-3 Myr^-1. On the basis of a model in which the Galactic stellar density distribution comprises a "disk+central hole" like that of the dust infrared emission (as proposed by Drimmel and Spergel), the Galactic supernova rate is estimated at probably not less than ∼1 nor more than ∼2 per century and the number of O3–B2 dwarfs within the solar circle at ∼200,000.

We report the discovery of 152 new high proper motion systems (μ ≥ 04 yr^-1) in the southern sky (δ = -47° to 00°) brighter than UKST plate R_59F = 16.5 via our SuperCOSMOS-RECONS (SCR) search. This paper complements Paper XII in the Solar Neighborhood series, which covered the region from δ = -90° to -47° and discussed all 147 new systems from the southernmost phase of the search. Among the total of 299 systems from both papers, there are 148 (71 in Paper XII, 77 in this paper) new systems moving faster than 05 yr^-1 that are additions to the classic Luyten Half Second sample. These constitute an 8% increase in the sample of all stellar systems with μ ≥ 05 yr^-1 in the southern sky. As in Paper XII, distance estimates are provided for the systems reported here based on a combination of photographic plate magnitudes and Two Micron All Sky Survey photometry, assuming that all stars are on the main sequence. Two SCR systems from the portion of the sky included in this paper are anticipated to be within 10 pc, and an additional 23 are within 25 pc. In total, the results presented in Paper XII and here for this SCR sweep of the entire southern sky include five new systems within 10 pc and 38 more between 10 and 25 pc. The largest number of nearby systems have been found in the slowest proper motion bin, 06 yr^-1 > μ ≥ 04 yr^-1, indicating that there may be a large population of low proper motion systems very near the Sun.

A list of 4131 dwarfs, subgiants, and giants located or suspected to be located within 33 pc of the Sun is presented. All the stars are drawn from the new Lépine Shara Proper Motion (LSPM)–North catalog of 61,976 stars with annual proper motions larger than 015 yr^-1. Trigonometric parallax measurements are found in the literature for 1676 of the stars in the sample; photometric and spectroscopic distance moduli are found for another 783 objects. The remaining 1672 objects are reported here as nearby star candidates for the first time. Photometric distance moduli are calculated for the new stars based on the (M_V, V - J) relationship, calibrated with the subsample of stars that have trigonometric parallaxes. The list of new candidates includes 539 stars that are suspected to be within 25 pc of the Sun, including 63 stars estimated to be within only 15 pc. The current completeness of the census of nearby stars in the northern sky is discussed in light of the new candidates presented here. It is estimated that ≈32% (≈18%) of nuclear-burning stars within 33 pc (25 pc) of the Sun remain to be located. The missing systems are expected to have proper motions below the 015 yr^-1 limit of the LSPM-North catalog.

We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1 ≲ [Fe/H] ≲ +0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster we provide a ridgeline tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses.

We present high spatial resolution spectroscopic observations of the proplyd 167-317 (LV2) near the Trapezium cluster in the Orion Nebula, obtained during the system verification run of the Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (IFU) at the Gemini South Observatory. We have detected 38 forbidden and permitted emission lines associated with the proplyd and its redshifted jet. We have been able to detect three velocity components in the profiles of some of these lines: a peak with a 28–33 km s^-1 systemic velocity that is associated with the photoevaporated proplyd flow, a highly redshifted component associated with a previously reported jet (which has receding velocities of about 80–120 km s^-1 with respect to the systemic velocity and which is spatially distributed to the southeast of the proplyd), and a less obvious, approaching structure that may possibly be associated with a faint counterjet with a systemic velocity of -75 ± 15 km s^-1. We find evidence that the redshifted jet has a variable velocity, with slow fluctuations as a function of the distance from the proplyd. We present several background-subtracted, spatially distributed emission-line maps, and we use this information to obtain the dynamical characteristics over the observed field. Using a simple model and extinction-corrected Hα fluxes, we estimate the mass-loss rate for both the proplyd photoevaporated flow and the redshifted microjet, obtaining _proplyd = (6.2 ± 0.6) × 10^-7M_⊙ yr^-1 and _jet = (2.0 ± 0.7) × 10^-8M_⊙ yr^-1, respectively.

We present near-IR (NIR) J, H, and K_s images and K-band spectroscopy of two newly discovered stellar clusters at different stages of evolution. Our spectra suggest the presence of massive young stellar objects in the heavily embedded cluster in the star-forming region near radio source G353.4-0.4 and an O5–O6 V star in the cluster near radio source G305+00.2. We determine a K-band luminosity function (KLF) for both clusters and an initial mass function (IMF) for the cluster near G305+00.2. The derived IMF slope is Γ = -1.5 if the KLF is used to derive the IMF and is Γ = -0.98 if the color-magnitude diagram (CMD) and spectra are used. The more reliable CMD-based slope is flatter than the Salpeter value usually found for stellar clusters. We find that using the KLF alone to derive an IMF is likely to produce an overly steep slope in stellar clusters subject to variable extinction.

We present the results of optical spectroscopy of 139 stars obtained with the Hydra multiobject spectrograph. The objects extend over a 1.3 deg² area surrounding the main cloud of the ρ Oph complex. The objects were selected from narrowband images to have Hα in emission. Using the presence of strong Hα emission, lithium absorption, location in the Hertzsprung-Russell diagram, or previously reported X-ray emission, we were able to identify 88 objects as young stars associated with the cloud. Strong Hα emission was confirmed in 39 objects with line widths consistent with their origin in magnetospheric accretion columns. Two of the strongest emission-line objects are young, X-ray-emitting brown dwarf candidates with M8 spectral types. Comparisons of the bolometric luminosities and effective temperatures with theoretical models suggest a median age for this population of 2.1 Myr, which is significantly older than the ages derived for objects in the cloud core. It appears that these stars formed contemporaneously with low-mass stars in the Upper Scorpius subgroup, likely triggered by massive stars in the Upper Centaurus subgroup.

Multiepoch imaging of the Orion equatorial region by the Sloan Digital Sky Survey has revealed that significant variability in the blue continuum persists into the late-M spectral types, indicating that magnetospheric accretion processes occur below the substellar boundary in the Orion OB1 association. We investigate the strength of the accretion-related continuum veiling by comparing the reddening-invariant colors of the most highly variable stars against those of main-sequence M dwarfs and evolutionary models. A gradual decrease in the g-band veiling is seen for the cooler and less massive members, as expected for a declining accretion rate with decreasing mass. We also see evidence that the temperature of the accretion shock decreases in the very low mass regime, reflecting a reduction in the energy flux carried by the accretion columns. We find that the near-IR excess attributed to circumstellar disk thermal emission drops rapidly for spectral types later than M4. This is likely due to the decrease in color contrast between the disk and the cooler stellar photosphere. Since accretion, which requires a substantial stellar magnetic field and the presence of a circumstellar disk, is inferred for masses down to 0.05 M_⊙, we surmise that brown dwarfs and low-mass stars share a common mode of formation.

We present a new 11.7 μm mosaic image of the inner Orion Nebula obtained with the T-ReCS instrument on Gemini South. The map covers 27 × 16, with a diffraction-limited spatial resolution of 035; it includes the BN/KL region, the Trapezium, and OMC-1 South. Excluding BN/KL, we detect 91 thermal-IR point sources, with 27 known proplyds and over 30 "naked" stars showing no extended structure in Hubble Space Telescope (HST) images. Within the region we surveyed, ∼80% of known proplyds show detectable thermal-IR emission, almost 40% of naked stars are detected at 11.7 μm, and the fraction of all visible sources with 11.7 μm excess emission (including both proplyds and stars with unresolved disks) is roughly 50%. These fractions exclude embedded sources. Thermal dust emission from stars exhibiting no extended structure in HST images is surprising and means that they have retained circumstellar dust disks comparable to the size of our solar system. Proplyds and stars with IR excess are not distributed randomly in the nebula; instead, they show a clear anticorrelation in their spatial distribution, with proplyds clustered close to θ¹ Ori C and other IR sources found preferentially farther away. We suspect that the clustered proplyds trace the youngest ∼0.5 Myr age group associated with the Trapezium, while the more uniformly distributed sources trace the older 1–2 Myr population of the Orion Nebula Cluster. This suggests that small protoplanetary disks persist for a few million years in irradiated environments and hints that hierarchical subclustering has been important on ∼30'' scales around the Trapezium. We detect 11.7 μm emission from the five brightest members of the Trapezium (θ¹ Ori A, B, C, D, and E), caused by free-free stellar wind emission and possibly emission from dusty disks around companion stars. Within 30'' of θ¹ Ori C, 100% of known proplyds are detected at 11.7 μm, and they exhibit remarkable limb-brightened dust arcs at the collision of the proplyd mass loss and the wind from θ¹ Ori C. The star θ¹ Ori D is associated with the most prominent mid-IR dust arc of the Ney-Allen Nebula. We propose that this arc is the consequence of θ¹ Ori D being the closest member of the Trapezium to the background cloud. Finally, we detect dust emission from Herbig-Haro jets in Orion, including HH 202, 529, 513, and 514. This is the first detection of mid-IR continuum emission from dust in the body of a collimated HH jet or bow shock. The presence of dust implies that some jet material must be lifted from large radii in the accretion disk (outside the dust sublimation radius) or entrained from the circumstellar environment.

We present narrowband images of η Carinae in the light of [O III] λ5007 obtained with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2, as well as a ground-based image in the same emission line with a larger field of view. These images show a thin veil of [O III] emission around η Car and its ejecta, confirming the existence of an oxygen-bearing "cocoon" inferred from spectra. This [O III] veil may be the remnant of the preoutburst wind of η Car, and its outer edge probably marks the interface where η Car's ejecta meet the stellar wind of the nearby O4 V((f)) star HD 303308 or other ambient material in the Carina Nebula; i.e., it marks the "astropause" in η Car's wind. This veil is part of a more extensive [O III] shell that appears to be shaped and ionized by HD 303308. A pair of HST images with a 10 yr baseline shows no significant proper motion, limiting the expansion speed away from η Car to 12 ± 13 km s^-1, or an expansion age of a few times 10⁴ yr. Thus, this is probably the decelerated preoutburst luminous blue variable wind of η Car. The [O III] morphology is very different from that seen in [N II], which traces younger dense knots of CNO-processed material; this represents a dramatic shift in the chemical makeup of material recently ejected by η Car. This change in the chemical abundances of the ejecta may have resulted from the sudden removal of the star's outer envelope during η Car's 19th century outburst or an earlier but similar event.

We present Hubble Space Telescope NICMOS imaging of the H₂ 2.12 μm emission in five fields in the Helix Nebula ranging in radial distance from 250'' to 450'' from the central star. The images reveal arcuate structures with their apexes pointing toward the central star. These molecular hydrogen knots are most highly structured in the fields closest to the central star and become increasingly less structured with increasing radius. Comparison of these images with ground-based images of comparable resolution reveals that the molecular gas is more highly clumped than the ionized gas line tracers. From our images, we determine an average number density of knots in the molecular gas ranging from 162 knots arcmin^-2 in the denser regions to 18 knots arcmin^-2 in the lower density outer regions. The decreasing number density of H₂ knots in the outer regions creates a lower filling factor of neutral and molecular gas emission in the radio observations of CO and H I and may explain why these outer regions, where we clearly detect H₂ 2.12 μm, fall below the detection limit of the radio observations. Using this new number density, we estimate the total number of knots in the Helix to be ∼23,000, which is a factor of 6.5 larger than previous estimates. The total neutral gas mass in the Helix is 0.35 M_⊙ assuming a mass of ∼1.5 × 10^-5M_⊙ for the individual knots. The H₂ emission structure of the entire Helix Nebula supports the recent interpretation of the Helix as a nearly pole-on polypolar planetary nebula (PN). The H₂ intensity, (5–9) × 10^-5 ergs s^-1 cm^-2 sr^-1, remains relatively constant with projected distance from the central star, suggesting a heating mechanism for the molecular gas that is distributed almost uniformly in the knots throughout the nebula. The temperature and H₂ 2.12 μm intensity of the knots can be approximately explained by photodissociation regions (PDRs) in the individual knots; however, theoretical PDR models of PNs underpredict the intensities of some knots by a factor of 10. The brightest H₂ emission (∼3 × 10^-4 ergs s^-1 cm^-2 sr^-1) may be enhanced by a larger than unity area filling factor of H₂ knots or may be an individual H₂ knot exposed to direct starlight, causing rapid photoevaporation compared with the more embedded knots of the disk.

Using optical (Hα and [S II]), near-IR (H₂ and K_s), and submillimeter (850 and 450 μm) data, we have examined the region surrounding the Barnard 1 (B1) core and found a multitude of new shocks from protostellar outflows. We trace several flows, some of which are large, parsec-scale outflows with dynamic ages of order 10⁴ yr, indicating that star formation has been taking place in B1 for at least that long. We can confidently identify eight protostars that are driving outflows. Of those eight protostars, one source, SMM 2 (SMM J033330+31095) is a new Class 0 source, giving B1 a total of three Class 0 protostars. Based on the number of shocks and protostars in this region, B1 appears to be a much more active region of star formation than previously thought. The number of shocks is comparable to or greater than those of other active star-forming regions in Perseus (e.g., IC 348, L1455, and L1448).

An Hα emission survey of the young cluster NGC 2362 resulted in the detection of 130 Hα emission stars in an 11' × 11' field approximately centered on the fourth magnitude O9 Ib multiple star τ CMa. The survey was carried out using the wide-field grism spectrograph on the University of Hawaii 2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. Deep optical VR_CI_C (to V ∼ 23.0) and near-infrared (NIR) photometry (JHK) to K ∼ 16 were obtained for several fields within the cluster. Spectra covering the 6000–8000 Å region at a resolution of R ∼ 3000 (adequate for the determination of Li I λ6708 line strengths) were also acquired for ∼200 pre-main-sequence (PMS) candidates with GMOS. Ages and masses for the Hα emitters in NGC 2362 were inferred from the isochrones and evolutionary tracks of D'Antona & Mazzitelli, as well as those of Baraffe et al. An estimated cluster age of ∼1.8 Myr follows from the models of D'Antona & Mazzitelli and 3.5–5.0 Myr from those of Baraffe et al. The fraction of the T Tauri star (TTS) population that is composed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 for the TTS population of NGC 2264. On the basis of W(Hα) alone, the fraction of TTSs still undergoing accretion is 5%–9%, comparable to the inner disk fraction determined from JHKL-band excesses by Haisch and coworkers (12%). Approximately 15% of the PMS sample in this study exhibits possible NIR excess, having E_H-K > 0.1 mag. Given the lack of NIR excess and strong Hα emission from the majority of cluster members, it is inferred that the inner disk regions of the TTS population have dissipated significantly. The mean level of chromospheric activity among the WTTS population of NGC 2362 is log(L_Hα/L_bol) = -3.65, significantly greater than that of the low-mass population of the 600 Myr old Hyades cluster, log(L_Hα/L_bol) = -3.90. The total mass of the Hα emitters and the OB stellar population of NGC 2362 defines a lower limit for the cluster mass of ∼300 M_⊙. Allowance for A- and F-type stars still on the radiative track, multiplicity, outlying members, and the low-mass population lying below the completeness limit of the Hα emission survey increases this lower limit to well over 500 M_⊙. The derived relaxation, disruption, and evaporation timescales for the cluster imply that NGC 2362 will likely survive beyond the age of the Pleiades, but statistics of galactic cluster lifetimes favor its disruption well before the age of the Hyades.

NGC 6397 is the closest globular cluster and hence the ideal place to search for faint stellar populations such as cataclysmic variables (CVs). Hubble Space Telescope (HST) and Chandra observers have identified nine certain and likely CVs in this nearby cluster, including several magnetic CV candidates. We have combined our recent UV imagery with archival HST images of NGC 6397 to search for new CV candidates and especially to look for dwarf nova–like eruptive events. We find remarkable and somewhat unexpected dwarf nova–like eruptions of the two well-known cataclysmic systems CV2 and CV3. These two objects have been claimed to be magnetic CVs, as indicated by their helium emission-line spectra. Magnetic fields in CVs are usually expected to prevent the disk instability that leads to dwarf nova eruptions. In fact, most field magnetic CVs are observed to not undergo eruptions. Our observations of the dwarf nova eruptions of CV2 and CV3 can be reconciled with these objects' He II emission lines if both objects are infrequently erupting intermediate polars, similar to EX Hya. If this is the case for most globular cluster CVs, then we can reconcile the many X-ray and UV-bright CV candidates seen by Chandra and HST with the very small numbers of erupting dwarf novae observed in cluster cores.

Fluxes and upper limits in the wavelength range from 3.6 to 70 μm from the Spitzer Space Telescope are provided for 20 solar-mass Pleiades members. One of these stars shows a probable mid-IR excess, and two others have possible excesses, presumably due to circumstellar debris disks. For the star with the largest, most secure excess flux at MIPS (Multiband Imaging Photometer for Spitzer) wavelengths, HII 1101, we derive log(L_dust/L_*) ∼ -3.8 and an estimated debris disk mass of 4.2 × 10^-5M for an assumed uniform dust grain size of 10 μm. If the stars with detected excesses are interpreted as stars with relatively recent, large collisional events producing a transient excess of small dust particles, the frequency of such disk transients is ∼10% for our ∼100 Myr, Pleiades G dwarf sample. For the stars without detected 24–70 μm excesses, the upper limits to their fluxes correspond to approximate 3 σ upper limits to their disk masses of 6 × 10^-6M using the MIPS 24 μm upper limit or 2 × 10^-4M using the MIPS 70 μm limit. These upper limit disk masses (for "warm" and "cold" dust, respectively) are roughly consistent with, but somewhat lower than, predictions of a heuristic model for the evolution of an "average" solar-mass star's debris disk based on extrapolation backward in time from current properties of the Sun's Kuiper Belt.

We have used the F160W filter (1.4–1.8 μm) and the coronagraph on the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope to survey 45 single stars with a median age of 0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7 mag. For the median age we were capable of detecting a 30M_J companion at separations between 15 and 200 AU. A 5M_J object could have been detected at 30 AU around 36% of our primaries. For several of our targets that were less than 30 Myr old, the lower mass limit was as low as 1M_J, well into the high mass planet region. Results of the entire survey include the proper-motion verification of five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).

In this paper a simple, semianalytical method is introduced for solving partial eclipsing binaries, using differential observations on two different colors. A matrix formulation of the problem is introduced, and the results are evaluated by comparing them against the synthetic light curves generated by Wilson and Devinney's light curve (LC) computer program. The surface temperatures of the components are also estimated using color curves. Although various proximity effects are neglected in this analytical approach, the procedure may prove useful for a rapid, first-order solution of many detached partial or total eclipsing binaries. These first-order solutions can be improved using detailed numerical calculations such as the LC and differential correction algorithms.

We have constructed a sample of M dwarfs out to a distance of ∼100 pc. This sample of 605 stars (574 M dwarfs and 31 K dwarfs) have been spectroscopically observed, yielding spectral types, radial velocities, Hα equivalent widths, and molecular band indices. Photometric observations have been compiled, with photographic plates providing RI magnitudes and JHK_s magnitudes from Two Micron All Sky Survey observations. Proper motions were determined by a match to USNO-B, and distances were computed employing a spectroscopic parallax, allowing the computation of UVW space motions. These data were used to investigate the distribution of magnetically active stars within the solar neighborhood. We find that there is no statistically significant variation in activity for equidistant locations above and below the Galactic plane. We also confirm prior kinematic analysis of large samples of M dwarfs, finding that active stars form a dynamically colder population.

We present the results of a detailed multiphase spectroscopic analysis of six classical Cepheids with pulsation periods between 3 and 6 days. For each star we have derived phased values of effective temperature, surface gravity, microturbulent velocity, and elemental abundances. We show that the elemental abundance results for these Cepheids are consistent for all pulsational phases.

We report here on the δ Scuti pulsation characteristics of low-amplitude oscillations in the α Persei cluster star V459 Per. The 105 hr of high-quality light curves over 10 nights confirm the low-amplitude oscillation characteristics of V459 Per. Amplitude spectra indicate multiperiodic pulsation behavior, and two pulsation frequencies, f₁ = 22.4822 and f₂ = 28.0172 cycles day^-1, have been confidently identified. Two other frequency candidates, f₃ = 11.3212 and f₄ = 19.9229 cycles day^-1, were suggested with a confidence level of more than 60%. Pulsation constant values and frequency ratios imply that f₁ and f₂ pulsations are in nonradial p₁ and p₂ modes with l = 1, f₃ is in agreement with a g₁ mode of l = 2, and f₄ corresponds to a radial fundamental.

We present results from the most recent (2002–2004) observing campaigns of the eclipsing system KH 15D, in addition to rereduced data obtained at Van Vleck Observatory (VVO) between 1995 and 2000. Phasing 9 yr of photometric data shows substantial evolution in the width and depth of the eclipses. The most recent data indicate that the eclipses are now approximately 24 days in length, or half the orbital period. These results are interpreted and discussed in the context of the recent models for this system put forward by Winn and coworkers and Chiang & Murray-Clay. A periodogram of the entire data set yields a highly significant peak at 48.37 ± 0.01 days, which is in accord with the spectroscopic period of 48.38 ± 0.01 days determined by Johnson and coworkers. Another significant peak, at 9.6 days, was found in the periodogram of the out-of-eclipse data at two different epochs. We interpret this as the rotation period of the visible star and argue that it may be tidally locked in pseudosynchronism with its orbital motion. If so, application of Hut's theory implies that the eccentricity of the orbit is e = 0.65 ± 0.01. Analysis of the Ultraviolet and Visual Echelle Spectrograph VLT spectra obtained by Hamilton and coworkers shows that the v sin i of the visible star in this system is 6.9 ± 0.3 km s^-1. Using this value of v sin i and the measured rotation period of the star, we calculate the lower limit on the radius to be R = (1.3 ± 0.1) R_⊙, which concurs with the value obtained by Hamilton and coworkers from its luminosity and effective temperature. Here we assume that i = 90°, since it is likely that the spin and orbital angular momenta vectors are nearly aligned. One unusually bright data point obtained in the 1995/1996 observing season at VVO is interpreted as the point in time when the currently hidden star B made its last appearance. Based on this datum, we show that star B is 0.46 ± 0.03 mag brighter than the currently visible star A, which is entirely consistent with the historical light curve. Finally, well-sampled V_J and I_J data obtained at the Cerro Tololo Inter-American Observatory Yale 1 m telescope during 2001/2002 show an entirely new feature: the system becomes bluer by a small but significant amount in very steady fashion as it enters eclipse and shows an analogous reddening as it emerges from eclipse. This suggests an extended zone of hot gas located close to but above the photosphere of the currently visible star. The persistence of the bluing of the light curve shows that its length scale is comparable to a stellar radius.

We present metallicity estimates for seven open clusters based on spectrophotometric indices from moderate-resolution spectroscopy. Observations of field giants of known metallicity provide a correlation between the spectroscopic indices and the metallicity of open cluster giants. We use χ² analysis to fit the relation of spectrophotometric indices to metallicity in field giants. The resulting function allows an estimate of the target-cluster giants' metallicities with an error in the method of ±0.08 dex. We derive the following metallicities for the seven open clusters: NGC 1245, [M/H] = -0.14 ± 0.04; NGC 2099, [M/H] = +0.05 ± 0.05; NGC 2324, [M/H] = -0.06 ± 0.04; NGC 2539, [M/H] = -0.04 ± 0.03; NGC 2682 (M67), [M/H] = -0.05 ± 0.02; NGC 6705, [M/H] = +0.14 ± 0.08; NGC 6819, [M/H] = -0.07 ± 0.12. These metallicity estimates will be useful in planning future extrasolar planet transit searches, since planets may form more readily in metal-rich environments.

We present spectral types for OGLE (Optical Gravitational Lensing Experiment) transiting planet candidates OGLE-TR-134 through 137 based on low-resolution spectra taken at Kitt Peak. Our main objective is to aid those planning radial velocity monitoring of transit candidates. We obtain spectral types with an accuracy of 2 spectral subtypes, along with tentative luminosity classifications. Combining the spectral types with light-curve fits to the OGLE transit photometry, and with Two Micron All Sky Survey counterparts in two cases, we conclude that OGLE-TR-135 and 137 are not planetary transits, while OGLE-TR-134 and 136 are good candidates and should be observed with precision radial velocity monitoring to determine whether the companions are of planetary mass. OGLE-TR-135 is ruled out chiefly because a discrepancy between the stellar parameters obtained from the transit fit and those inferred from the spectra indicates that the system is a blend. OGLE-TR-137 is ruled out because the depth of the transit combined with the spectral type of the star indicates that the transiting object is stellar. OGLE-TR-134 and 136, if unblended main-sequence stars, are each orbited by a transiting object with radius below 1.4 R_J. The caveats are that our luminosity classification suggests that OGLE-TR-134 could be a giant (and therefore a blend), while OGLE-TR-136 shows a (much smaller) discrepancy of the same form as OGLE-TR-135, which may indicate that the system is a blend. However, since our luminosity classifications are uncertain at best, and the OGLE-TR-136 discrepancy can be explained if the primary is a slightly anomalous main-sequence star, the stars remain good candidates.

Many astronomers have speculated that the solar system contains undiscovered massive planets or a distant stellar companion. The acceleration of the solar system barycenter can constrain the mass and position of the putative companion. In this paper we use the most recent timing data on accurate astronomical clocks (millisecond pulsars, pulsars in binary systems, and pulsating white dwarfs) to constrain this acceleration. No evidence for nonzero acceleration has been found; the typical sensitivity achieved by our method is a_⊙/c ∼ a few × 10^-19 s^-1, comparable to the acceleration due to a Jupiter-mass planet at 200 AU. The acceleration method is limited by the uncertainties in the distances and by the timing precision for pulsars in binary systems, and by the intrinsic distribution of the period derivatives for millisecond pulsars. Timing data provide stronger constraints than residuals in the motions of comets or planets if the distance to the companion exceeds a few hundred AU. The acceleration method is also more sensitive to the presence of a distant companion (≳300–400 AU) than existing optical and infrared surveys. We outline the differences between the effects of the peculiar acceleration of the solar system and the background of gravitational waves on high-precision timing.

Matched filter (MF) processing has been shown to provide significant performance gains when processing stellar imagery used for asteroid detection, recovery, and tracking. This includes extending detection ranges to fainter magnitudes at the noise limit of the imagery and operating in dense cluttered star fields as encountered at low Galactic latitudes. The MF software has been shown to detect 40% more asteroids in high-quality Spacewatch imagery relative to the currently implemented approaches, which are based on moving target indicator (MTI) algorithms. In addition, MF detections were made in dense star fields and in situations in which the asteroid was collocated with a star in an image frame, cases in which the MTI algorithms failed. Thus, using legacy sensors and optics, improved detection sensitivity is achievable by simply upgrading the image-processing stream. This in turn permits surveys of the near-Earth asteroid (NEA) population farther from opposition, for smaller sizes, and in directions previously inaccessible to current NEA search programs. A software package has been developed and made available on the NASA data services Web site that can be used for asteroid detection and recovery operations utilizing the enhanced performance capabilities of MF processing.