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Using a sample of 3814 quasars from the Early Data Release of the Sloan Digital Sky Survey, we confirm that high-ionization, broad emission lines, such as C IV, are significantly blueshifted with respect to low-ionization, broad emission lines, such as Mg II, which are thought to be close to the systemic redshift. We examine the velocity shifts of the Mg II and C IV emission lines with respect to [O III] and Mg II, respectively. C IV emission-line peaks have a range of shifts from a redshift of 500 km s^-1 to blueshifts well in excess of 2000 km s^-1 as compared with Mg II. We confirm previous results that suggest an anticorrelation between the shift of the C IV emission-line peak and the rest equivalent width of the C IV emission line. Furthermore, by creating composite quasar spectra as a function of C IV shift, we are able to study in detail the profiles of the line as a function of velocity shift. We find that the apparent shift of the C IV emission-line peak is not a shift so much as it is a lack of flux in the red wing for the composite with the largest apparent shift. This observation should strongly constrain models for the broad emission-line region in quasars. The emission-line blueshift and equivalent width of C IV are also discussed in light of the well-known anticorrelation between the equivalent width of C IV emission and continuum luminosity, otherwise known as the Baldwin effect. We further discuss the C IV emission-line shift as a function of other quasar properties, such as spectral index, radio and X-ray detection. We find a possible correlation between the C IV emission-line shifts and the radio properties of the quasars, which is suggestive of orientation as the cause of the C IV velocity shifts. Finally, we explore whether the C IV emission-line blueshifts correlate with the presence of broad absorption line absorption troughs or with narrow, "associated" absorption, and how these might be related to orientation.

We performed spectroscopic observations for a large infrared quasi-stellar object (QSO) sample with a total of 25 objects. The sample was compiled from the QDOT redshift survey, the 1 Jy ultraluminous IRAS galaxy survey, and a sample obtained by a cross-correlation study of the IRAS Point-Source Catalogue with the ROSAT All-Sky Survey Catalogue. Statistical analyses of the optical spectra show that the vast majority of infrared QSOs have narrow permitted emission lines (with FWHM of Hβ less than 4000 km s^-1) and more than 60% of them are luminous narrow-line Seyfert 1 galaxies. Two of the infrared QSOs are also classified as low-ionization broad absorption line (lo-BAL) QSOs. More than 70% of infrared QSOs are moderately or extremely strong Fe II emitters. This is the highest percentage of strong Fe II emitters in all subclasses of QSO Seyfert 1 samples. We found that the Fe II to Hβ line ratio is significantly correlated with the [O III] λ5007 peak and Hβ blueshift. Soft X-ray–weak infrared QSOs tend to have large blueshifts in permitted emission lines and significant Fe II 48, 49 (5100–5400 Å) residuals relative to the Boroson & Green Fe II template. If the blueshifts in permitted lines are caused by outflows, then they appear to be common in infrared QSOs. As the infrared-selected QSO sample includes both luminous narrow-line Seyfert 1 galaxies and lo-BAL QSOs, it could be a useful laboratory to investigate the evolutionary connection among these objects.

We present our discovery observations and analysis of RDCS 1317+2911, z = 0.805, and RDCS 1350+6007, z = 0.804, two clusters of galaxies identified through X-ray emission in the ROSAT Deep Cluster Survey (RDCS). RDCS 1317+2911 has an unusual morphology in our Chandra observations, with an asymmetric surface brightness profile and a bend in the distribution of X-ray emission. In contrast, RDCS 1350+6007 appears to be more like low-redshift clusters, with β = 0.49 ± 0.06 and r_core = 165 ± 5 kpc (Ω_m = 0.3, Ω_Λ = 0.7, H₀ = 65 km s^-1 Mpc^-1), though it also has an elliptical, slightly asymmetric surface brightness profile. We find a temperature of 3.7 keV and a bolometric luminosity of 8.2 × 10⁴³ ergs s^-1 for RDCS 1317+2911, and a temperature of 4.9 keV and a bolometric luminosity of 4.1 × 10⁴⁴ ergs s^-1 for RDCS 1350+6007. Our weak-lensing analysis of RDCS 1350+6007 confirms the general shape of the inner density profile but predicts twice the mass of the model based on the X-ray profile. There are two possibilities for this discrepancy: either there is a significant amount of mass near the redshift of the cluster that has not yet fallen into the potential well and shock-heated the gas, or, as we only see the X-ray emission from the core of the cluster, our β model fails to describe the true shape of the underlying potential. We combine the X-ray luminosities and temperatures for RDCS clusters of galaxies with such measurements of other clusters at high redshift (z > 0.7) and fit the luminosity-temperature relation. We find no statistically significant evolution in the slope or zero point of this relation at z_median = 0.83. This result is in agreement with models of intracluster medium evolution with significant preheating or high initial entropy values. Quantifying the bolometric luminosity-temperature relation as L = L₆(1 + z)^A(T/6 keV)^α, we find α = 2.9 ± 0.4, L₆ = 8.7 ± 0.9 × 10⁴⁴ ergs s^-1 and A = 0.3 ± 0.2, or A = 0.4 ± 0.2, depending on which low-redshift luminosity-temperature relation we compare with. With this result, we rule out at the 5 σ level the self-similar scaling model of intracluster medium evolution. We discuss how low-temperature, high-redshift clusters of galaxies will allow us to improve on this result, and we announce the discovery of two such objects, CXOU J0910.1+5419 and CXOU J1316.9+2914.

We derive the bolometric–to–X-ray correlation for a local sample of normal and starburst galaxies and use it, in combination with several UV reddening schemes, to predict the 2–8 keV X-ray luminosity for a sample of 24 Lyman break galaxies in the Hubble Deep Field and Chandra Deep Field North. We find that the mean X-ray luminosity, as predicted from the Meurer UV reddening relation for starburst galaxies, agrees extremely well with the Brandt stacking analysis. This provides additional evidence that Lyman break galaxies can be considered as scaled-up local starbursts, and that the locally derived starburst UV reddening relation may be a reasonable tool for estimating the UV extinction at high redshift. Our analysis shows that the Lyman break sample cannot have far-IR to far-UV flux ratios similar to nearby ultraluminous infrared galaxies, since this would predict a mean X-ray luminosity 100 times larger than observed, as well as far-IR luminosities large enough to be detected in the submillimeter. We calculate the UV reddening expected from the Calzetti effective starburst attenuation curve and the radiative transfer models of Witt & Gordon for low-metallicity dust in a shell geometry with homogeneous or clumpy dust distributions and find that all are consistent with the observed X-ray emission. Finally, we show that the mean X-ray luminosity of the sample would be underpredicted by a factor of 6 if the far-UV is unattenuated by dust.

The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time span of nearly 6 yr are presented and discussed. The optical light curve showed an overall variation greater than 3 mag, and the largest changes occurred in three strong flares. No periodicity was found in the light curve on timescales up to a year. The variability in the radio band is very different and shows moderate oscillations around an average constant flux density rather than relevant flares, with a maximum amplitude of ∼30%, without a simultaneous correspondence between optical and radio luminosity. The optical spectral energy distribution was always well fitted by a power law. The spectral index shows small variations, and there is indication of a positive correlation with the source luminosity. Possible explanations of the source behavior are discussed in the framework of current models.

We present results from a survey of an unbiased sample of 38 early-type (S0–Sa), low-inclination, optically barred galaxies in the field, using images both from the ground and from space. Our goal was to find and characterize central stellar and gaseous structures: secondary bars, inner disks, and nuclear rings. We find that bars inside bars are surprisingly common: at least one-quarter of the sample galaxies (possibly as many as 40%) are double barred, with no preference for Hubble type or the strength of the primary bar. A typical secondary bar is ∼12% of the size of its primary bar and extends to 240–750 pc in radius. Secondary bars are not systematically either parallel or perpendicular to the primary; we see cases where they lead the primary bar in rotation and others where they trail, which supports the hypothesis that the two bars of a double-bar system rotate independently. We see no significant effect of secondary bars on nuclear activity: our double-barred galaxies are no more likely to harbor a Seyfert or LINER nucleus than our single-barred galaxies. We find kiloparsec-scale inner disks in at least 20% of our sample; they occur almost exclusively in S0 galaxies. These disks are on average 20% the size of their host bar and show a wider range of relative sizes than do secondary bars. Nuclear rings are present in about a third of our sample. Most of these rings are dusty, sites of current or recent star formation, or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies (8% of the sample, but 15% of the S0's) appear to have purely stellar nuclear rings, with no evidence for dust or recent star formation. The fact that these central stellar structures are so common indicates that the inner regions of early-type barred galaxies typically contain dynamically cool and disklike structures. This is especially true for S0 galaxies, where secondary bars, inner disks, and/or stellar nuclear rings are present at least two-thirds of the time. If we interpret nuclear rings, secondary bars, and (possibly) inner disks and nuclear spirals as signs of inner Lindblad resonances (ILRs), then between one and two-thirds of barred S0–Sa galaxies show evidence for ILRs.

We have successfully constructed a catalog of H I–rich galaxies selected from the Minnesota Automated Plate Scanner catalog of the Palomar Observatory Sky Survey (POSS IThe POSS I field designations we use are the modified Luyten POSS I plate numbers, which are typically 1 greater than the plate reference numbers used in the Guide Star Catalog (Lasker et al. 1990)), based solely on optical criteria. We identify H I–rich candidates by selecting the bluest galaxies at a given apparent magnitude, those galaxies on the blue edge of POSS I color-magnitude parameter space. Subsequent 21 cm observations on the upgraded Arecibo 305 m dish detected over 50% of the observed candidates. The detected galaxies are H I–rich, with H I masses comparable to "normal" high surface brightness disk galaxies, and they have gas mass-to-light ratios ranging from 0.1 to 4.8 (in solar units). Comparison of our candidate galaxies with known low surface brightness galaxies (hereafter LSBs) shows that they exhibit similar optical and H I properties to that population. We also show that previously identified LSBs, including several LSBs with red B-V colors, preferentially occupy the blue edge of POSS I color-magnitude parameter space. Their presence on the blue edge appears to be a selection effect due to differing plate limits in the two POSS I bandpasses. This suggests the POSS I is a good filter for separating galaxies on the higher surface brightness end of the LSB population from the general population of galaxies in the night sky.

We present the final results from the Arecibo ObservatoryThe Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation. OH megamaser survey. We discuss in detail the properties of the remaining 18 OH megamasers detected in the survey, including three redetections. We place upper limits on the OH emission from 85 nondetections and examine the properties of 25 ambiguous cases for which the presence or absence of OH emission could not be determined. The complete survey has discovered 50 new OH megamasers (OHMs) in (ultra)luminous infrared galaxies ([U]LIRGs), which doubles the sample of known OHMs and increases the sample at z > 0.1 sevenfold. The Arecibo OH megamaser survey indicates that the OHM fraction in LIRGs is an increasing function of the far-IR luminosity (L_FIR) and far-IR color, reaching a fraction of roughly in the warmest ULIRGs. Significant relationships between OHMs and their hosts are few, primarily because of a mismatch in size scales of measured properties and an intrinsic scatter in OHM properties roughly equal to the span of the data set. We investigate relationships between OHMs and their hosts with a variety of statistical tools including survival analysis, partial correlation coefficients, and a principal component analysis. There is no apparent OH megamaser "fundamental plane." We compile data on all previously known OHMs and evaluate the possible mechanisms and relationships responsible for OHM production in merging systems. The OH-FIR relationship is reexamined by using the doubled OHM sample and found to be significantly flatter than previously thought: L_OH ∝ L. This nearly linear dependence suggests a mixture of saturated and unsaturated masers, either within individual galaxies or across the sample.

Recent observations of surface brightness distributions of both Milky Way and M31 satellite galaxies have revealed many instances of sudden changes or breaks in the slope of the surface brightness profiles (at some break radius r_break). These breaks are often accompanied by increasingly elliptical isophotes and sometimes by isophote twisting. We investigate the hypothesis of a tidal origin for these features by applying the same ellipse-fitting techniques that are used on observed galaxies to numerical simulations of the destruction of satellites, represented by spherical, single-component systems. We examine how observed quantities such as r_break, ellipticity e, and position angle ϕ of the fitted ellipses and amplitude of the extra-break population vary with the satellite's orbital eccentricity and phase, as well as our viewpoint relative to the orbit. We also look at orbit and viewpoint dependence of the rate of change of the latter three quantities with radius. We find that there are trends with orbital phase and eccentricity in all observed quantities, many of which are preserved through a wide variety of viewing angles. In particular, a generic feature of all simulations is a depletion zone just interior to an excess zone, regions in which the surface brightness is lower and higher, respectively, than the initial profile. A clear interpretation of any individual image, however, is likely to be hampered by the dependence of the observable features on these multiple parameters. For example, breaks can be excited by several physical processes and can occur well within the bound satellite population. Nevertheless, we do find we can place loose constraints on the tidal radius, mass-loss rate, orbital type and phase of the satellite, and nature of breaks, using photometric data alone.

The Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST) has been used to obtain high-resolution images of NGC 3610, a dynamically young elliptical galaxy in a group environment. These observations supersede shorter, undithered HST observations in which an intermediate-age population of globular clusters was discovered. The new observations show the bimodal color distribution of globular clusters more clearly, with peaks at V-I = 0.95 and 1.17. The luminosity function of the blue metal-poor population of clusters in NGC 3610 turns over, consistent with a Gaussian distribution with a peak M_V ≈ -7.0, similar to old globular cluster populations in elliptical galaxies. The red metal-rich population of clusters has a luminosity function that is more extended toward both the bright and faint ends, as expected for a cluster population of intermediate age. It is well fitted by a power law ϕ(L)dL ∝ L^αdL, with an exponent of α = -1.78 ± 0.05, or α = -1.90 ± 0.07 when corrected for observational scatter. A Kolmogorov-Smirnov test confirms the significant difference between the luminosity functions of the red and blue clusters, with a probability of less than 0.1% that they come from the same population. A comparison with the Fall & Zhang cluster disruption models shows marginal agreement with the observed data when comparing both the luminosity functions and the mean color distributions, although there are differences in detail. In particular, there is no clear evidence of the predicted turnover at the faint end, although deeper observations will be required to make a definitive test. A by-product of the analysis is the demonstration that at any given metallicity the peak of the luminosity function should remain nearly constant from 1.5 to 12 Gyr, since the effect of the disruption of faint clusters is almost perfectly balanced by the fading of the clusters. This may help explain the apparent universality of the peak of the globular cluster luminosity function.

We present a statistical system that can be used in the study of cluster populations. The basis of our approach is the construction of synthetic cluster color-magnitude-radius diagrams (CMRDs), which we compare with the observed data using a maximum likelihood calculation. This approach permits a relatively easy incorporation of incompleteness (a function of not only magnitude and color, but also radius), photometry errors and biases, and a variety of other complex effects into the calculation, instead of the more common procedure of attempting to correct for those effects. We then apply this procedure to our NGC 3627 data from Paper I. We find that we are able to successfully model the observed CMRD and constrain a number of parameters of the cluster population. We measure a power-law mass function slope of α = -1.50 ± 0.07 and a distribution of core radii centered at r_c = 1.53 ± 0.15 pc. Although the extinction distribution is less constrained, we measured a value for the mean extinction consistent with that determined in Paper I from the Cepheids.

We have used Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer broadband (at 1.6 μm) and narrowband Paα (λ_rest = 1.87 μm) images to identify, respectively, star clusters and H II regions in a sample of eight luminous infrared galaxies (LIRGs). These observations have revealed the presence of a large population of super–star clusters and bright H II regions. A significant fraction of the H II regions shows Hα luminosities above that of 30 Doradus, the prototypical giant H II region. The excess of extremely luminous H II regions in LIRGs has been confirmed by comparison with normal galaxies observed at similar spatial resolutions. Despite the large numbers of identified star clusters and H II regions in LIRGs, we find only a small fraction of coincidences, between 4% and 30% of the total number of detected sources. Using evolutionary synthesis models we have reproduced the relative fractions of young H II regions and intermediate and old star clusters observed in Arp 299 and the central region NGC 3256 by using a Salpeter initial mass function and instantaneous star formation. H II regions with no detected near-infrared cluster counterpart (25%–39% of the detected sources) represent the youngest sites of star formation, with ages up to approximately 5 Myr and mostly intermediate mass (≃10⁵M_⊙) ionizing clusters. For these two galaxies, within the present detection threshold we can detect only coincidences (4%–10% of the detected sources) between an H II region and a near-infrared star cluster for the most massive star clusters (≃10⁶M_⊙) during the first 7 Myr of their evolution. If there is significant extinction during the first million years, we may not detect the youngest star-forming regions, and hence the observed fractions of H II regions and coincidences will be lower limits. The identified near-infrared super–star clusters with no detectable Paα emission represent the "old" population (53%–66% of the detected sources), with ages between 7 and 20–40 Myr. Older clusters possibly created in this or previous episodes of star formation are likely to exist in these systems but cannot be identified with the present detection threshold. Our study demonstrates that Paα narrowband imaging of LIRGs and interacting galaxies identifies the youngest sites of star formation that could be otherwise missed by near-infrared broadband continuum surveys.

We present broadband 1.1, 1.6, and 2.2 μm images and a 2.37 μm narrowband image of the inner 19'' of the nearby radio galaxy M87, obtained with the Near Infrared Camera and Multi-Object Spectrometer of the Hubble Space Telescope (HST). The isophotes of the broadband images are almost perfectly circular to within approximately 05 (∼50 pc) of the active nucleus, and an r^1/4 law provides a good fit to the galaxy brightness profile at these wavelengths to within the same distance. This result agrees with predictions that the nuclear supermassive black hole will produce a nearly spherical distribution of the surrounding stars within a galaxy crossing time. A difference image formed from the 1.6 μm image and a V-band image obtained with the HST Wide Field Planetary Camera 2 does not show any clear evidence of a physically thick dusty torus around the nucleus, consistent with its lack of strong thermal mid-infrared emission. If such a torus is present, our data indicate its outer radius to be less than 50 pc. The infrared broadband colors and 2.37 μm image (which is sensitive to the strength of the stellar CO absorption) show no gradients to a distance of approximately 5'' (∼400 pc) from the nucleus and are consistent with a population dominated by late M giants, with no evidence of recent star formation. However, the globular clusters in this region are confirmed to consist of stars bluer than the underlying galaxy, indicating a different formation history. The images and associated colors also confirm that the regions beyond the nucleus do not contain strongly concentrated dust, in contrast to many other radio galaxies. In combination with other recent observations, these results indicate that M87 represents the dynamically evolved product of past galaxy mergers and suggest that its nucleus is in the final stages of activity.

We present the results of H I 21 cm observations of 139 actively star-forming dwarf galaxies obtained with the 305 m radio telescope at Arecibo Observatory. Our sample consists of all objects cataloged in objective-prism surveys for UV-excess or emission-line galaxies published prior to the start of the survey that have luminosities below M_B = -17.0 and that are located within the declination limits of the Arecibo telescope. Galaxies from the Markarian, Michigan, Case, Wasilewski, Haro, and Zwicky lists are included. The sample spans a wide range of both H I gas content and star formation levels. A total of 122 objects (88%) were detected; 82 galaxies have been observed for the first time in H I. The median velocity width for our sample is 88 km s^-1, and the median H I gas mass is 3.0 × 10⁸M_⊙. In general, the sample galaxies are gas-rich, with an average M_{H I}/L_B = 1.3 after correcting for the luminosity enhancement due to the starburst. The progenitors of many of the star-forming dwarfs have higher M_{H I}/L_B than typically seen in samples of nearby "normal" galaxies, emphasizing their distinct nature.

The properties of the velocity field in the local volume (cz < 550 km s^-1) have been difficult to constrain because of a lack of a consistent set of galaxy distances. The sparse observations available to date suggest a remarkably quiet flow, with little deviation from a pure Hubble law. However, velocity field models based on the distribution of galaxies in the 1.2 Jy IRAS redshift survey predict a quadrupolar flow pattern locally with strong infall at the poles of the local supergalactic plane. We probe this velocity field and begin to establish a consistent set of galactic distances. We have obtained images of nearby galaxies in the I and V bands from the W. M. Keck Observatory and in F814W and F555W filters from the Hubble Space Telescope. Where these galaxies are well resolved into stars, we can use the tip of the red giant branch (TRGB) as a distance indicator. Using a maximum likelihood analysis to quantitatively measure the I magnitude of the TRGB, we determine precise distances to seven nearby galaxies: Leo I, Sextans B, NGC 1313, NGC 3109, UGC 3755, UGC 6456, and UGC 7577.

We present first results from a spectroscopic survey designed to examine the metallicity and kinematics of individual red giant branch stars in the outer halo of the Andromeda spiral galaxy (M31). This study is based on multislit spectroscopy with the Keck II 10 m telescope and Low Resolution Imaging Spectrograph of the Ca II near-infrared triplet in 99 M31 halo candidates in a field at R = 19 kpc on the southeast minor axis with brightnesses from 20 < I < 22. The spectra are used to isolate M31 halo red giants from foreground Milky Way dwarf stars, faint compact background galaxies, and M31 disk giants. The observed distribution of radial velocities is well fitted by an equal mix of foreground Milky Way dwarf stars, drawn from a standard Galactic model and with velocities v ≲ 0 km s^-1, and M31 halo giants represented by a Gaussian of width σ ∼ 150 km s^-1 centered on its systemic velocity of v ≈ -300 km s^-1. A secure sample of 29 M31 red giant stars is identified on the basis of radial velocity (v < -220 km s^-1) and, in the case of four intermediate-velocity stars (-160 < v < -220 km s^-1), broadband B-I color. For this sample of objects, there is rough agreement between the metallicities derived in independent ways: two different calibrations of the Ca II absorption-line strength and a photometric estimate based on fitting model stellar isochrones to an object's location in a (B-I, I) color-magnitude diagram. The [Fe/H] distribution of M31 halo giants has an rms spread of at least 0.6 dex and spans the ≳2 dex range over which the abundance measurement methods are calibrated. The mean/median metallicity of the M31 halo is about ⟨[Fe/H]⟩ = -1.9 to -1.1 dex (depending on the details of metallicity calibration and sample selection) and possibly higher: the high-metallicity end of the distribution is poorly constrained by our data since the selection function for the secure M31 sample excludes over 80% of the giants in solar/supersolar metallicity range. Possible reasons are explored for the apparent discrepancy between the mean [Fe/H] found in our spectroscopic survey (corrected for metallicity selection bias) and the slightly higher mean values found in earlier photometric studies. Field halo red giants in M31 appear to be somewhat more metal-rich on average than their Milky Way counterparts. The M31 halo [Fe/H] distribution is comparable to that of M31 globular clusters, Galactic globular clusters, and Local Group dwarf satellite galaxies. The data in this 19 kpc outer halo field are broadly consistent with a scenario in which the halo is built from the accretion of small stellar subsystems. There are four stars in the secure M31 sample that have particularly strong Ca II lines, indicating solar metallicity, at a common velocity of ≈-340 km s^-1 close to the galaxy's systemic velocity, similar to what might be expected for M31 disk giants on the minor axis. An extrapolation of the inner disk brightness profile, however, falls far short of accounting for these four stars—the disk would instead have to be very large (R_disk ≳ 80 kpc) and/or warped. More likely, these four stars represent a metal-rich debris trail from a past accretion event in the halo.

We present a two-dimensional fitting algorithm (GALFIT) designed to extract structural components from galaxy images, with emphasis on closely modeling light profiles of spatially well-resolved, nearby galaxies observed with the Hubble Space Telescope. Our algorithm improves on previous techniques in two areas: by being able to simultaneously fit a galaxy with an arbitrary number of components and with optimization in computation speed, suited for working on large galaxy images. We use two-dimensional models such as the "Nuker" law, the Sérsic (de Vaucouleurs) profile, an exponential disk, and Gaussian or Moffat functions. The azimuthal shapes are generalized ellipses that can fit disky and boxy components. Some potential applications of our program include: standard modeling of global galaxy profiles; extracting bars, stellar disks, double nuclei, and compact nuclear sources; and measuring absolute dust extinction or surface brightness fluctuations after removing the galaxy model. When examined in detail, we find that even simple looking galaxies generally require at least three components to be modeled accurately, rather than the one or two components more often employed. Many galaxies with complex isophotes, ellipticity changes, and position angle twists can be modeled accurately in two dimensions. We illustrate this by way of 11 case studies, which include regular and barred spiral galaxies, highly disky lenticular galaxies, and elliptical galaxies displaying various levels of complexities. A useful extension of this algorithm is to accurately extract nuclear point sources in galaxies. We compare two-dimensional and one-dimensional extraction techniques on simulated images of galaxies having nuclear slopes with different degrees of cuspiness, and we then illustrate the application of the program to several examples of nearby galaxies with weak nuclei.

The double nucleus geometry of M31 is currently best explained by the eccentric disk hypothesis of Tremaine, but whether the eccentric disk resulted from the tidal disruption of an inbounding star cluster, from a nuclear black hole, or from an m = 1 perturbation of a native nuclear disk remains debatable. I perform detailed two-dimensional decomposition of the M31 double nucleus in the Hubble Space Telescope V-band to study the bulge structure and to address competing formation scenarios of the eccentric disk. I deblend the double nucleus (P1 and P2) and the bulge simultaneously using five Sérsic and one Nuker components. P1 and P2 appear to be embedded inside an intermediate component (r_e = 32) that is nearly spherical (q = 0.97 ± 0.02), while the main galaxy bulge is more elliptical (q = 0.81 ± 0.01). The spherical bulge mass (2.8 × 10⁷M_⊙), being coincident with the supermassive black hole mass (3 × 10⁷M_⊙), conjoined with a shallow bulge cusp, is consistent with the scenario that the bulge was scoured by spiraling binary supermassive black holes. In the two-dimensional decomposition the bulge is consistent with being centered near the UV peak of P2, but the exact position is difficult to pinpoint because of dust in the bulge. P1 and P2 are comparable in mass. Within a radius r = 1'' of P2, the relative mass fraction of the nuclear components is M_• : M_bulge : P1 : P2 = 4.3 : 1.2 : 1 : 0.7, assuming the luminous components have a common mass-to-light ratio of 5.7. The eccentric disk as a whole (P1 + P2) is massive, M ≈ 2.1 × 10⁷M_⊙, comparable to the black hole and the local bulge mass. As such, the eccentric disk could not have been formed entirely out of stars that were stripped from an inbounding star cluster. Hence, the more favored scenario is that of a disk formed in situ by an m = 1 perturbation caused possibly by the passing of a giant molecular cloud or the passing/accretion of a small globular cluster.

We investigate the interaction history of the M31 subgroup by comparing surface photometry of two of its satellites, M32 and NGC 205, with N-body simulations of satellite destruction. The recent discovery of a giant stream in the outer halo of M31, apparently pointed in the direction of M32 and NGC 205, makes such an investigation particularly relevant. The observational component of this study is based on 17 × 5° B- and I-band CCD mosaic images centered on M31 and covering both satellites. Standard ellipse-fitting techniques are used to model and remove M31 disk light and to perform surface photometry on the satellites to limiting brightness levels of (μ_B, μ_I) = (27, 25) mag arcsec^-2, corresponding to isophotal semimajor axis lengths of r = 420'' (1.6 kpc) and r = 720'' (2.7 kpc). A hint of excess light in the outer parts of M32 noted in earlier studies is confirmed; in particular, clear evidence is seen for a sharp (upward) break in the surface brightness profile at r = 150'' relative to a r^1/4 law that fits the inner region of M32. This break is accompanied by a steep increase in isophotal ellipticity , as well as position angle ϕ' twisting. In addition to this excess, evidence is seen for an inner downward break in the surface brightness profile at r = 50''. The robustness of the M32 isophotal features is demonstrated through their (1) insensitivity to the details of background subtraction, (2) symmetry about M32's center, and (3) narrow range of B-I color that is consistent with the interior regions of M32 but not with M31 residual spiral arm/dust lane features. The study of NGC 205 reveals pronounced isophote twisting at r ∼ 300'' that is coincident with a subtle downward break in the surface brightness profile, relative to an exponential law fitted to the inner region.

The simulation component of this project is based on the analysis of single-component, spherical satellites that are being tidally disrupted through interactions with their parent galaxy. Generic features of the simulations include an excess in the surface brightness profile at large radii, a depletion zone at intermediate radii, and isophotal elongation and twists that are coincident with breaks in the brightness profile. The two satellites, M32 and NGC 205, display most of these features consistently across the B and I bands, which is strongly suggestive of tidal interaction and probable stripping by M31. We discuss what these observed features can tell us about the satellites' orbital parameters and histories. Specifically, M32 is found to be on a highly eccentric orbit and away from pericenter. Investigating M32's unusual combination of high surface brightness and low luminosity (the hallmark of compact ellipticals), we make empirical estimates of the galaxy's intrinsic properties and conclude that it is not likely to be the residual core of a tidally stripped normal elliptical galaxy, as has been suggested, but rather that its precursor was intrinsically compact.

The Hubble Space Telescope Wide Field Planetary Camera 2 has been used to image Andromeda III, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude diagrams (CMDs) reveal for the first time the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda I and Andromeda II, and like most of the Galactic dSph companions, the HB morphology of And III is predominantly red, redder indeed than that of both And I and And II, despite And III having a lower mean metallicity. The And III HB morphology is also somewhat redder than that of the Galactic dSph Draco, which has a similar mean abundance to And III. We interpret this red HB morphology as indicating that the bulk of the And III population is ∼3 Gyr younger than the age of the majority of Galactic globular clusters. Nevertheless, the And III CMD does reveal the presence of a few blue HB stars, and a number of RR Lyrae variables are also evident in the data. This indicates that And III does contain an "old" population of age comparable to that of the Galactic globular clusters. There is no evidence, however, for any young stars in And III despite a claimed association between this dSph and an H I cloud. As was the case for And II, but not And I, no radial gradient was detected in the And III HB morphology. The mean V magnitude of the HB is 25.06 ± 0.04, leading to (m - M)₀ = 24.38 ± 0.06 for this dwarf. And III is then ∼75 kpc from the center of M31, comparable to the Galactocentric distances of Sculptor and Draco. Comparison with standard globular cluster red giant branches indicates a mean abundance for And III of ⟨[Fe/H]⟩ = -1.88 ± 0.11, the lowest mean abundance of any of M31's companions. This value, however, is consistent with the absolute magnitude–mean abundance relation followed by dSph galaxies. The same comparison yields an intrinsic abundance dispersion for And III of σ_int([Fe/H]) = 0.12, a low value compared to And I and And II and to the Galactic dSph's of comparable luminosity to And III. If confirmed by future spectroscopic studies, this low value would suggest that And III retained relatively little of the enrichment products generated during its evolutionary history. The list of candidate variables reveals one definite and one probable anomalous Cepheid variable star in And III. Such variables are common in Galactic dSph's, so their discovery in And III is not unexpected.

The tidal tails of the globular cluster Palomar 5 are analyzed over a 41 deg² area of the Sloan Digital Sky Survey photometric catalogs. The matched filter algorithm provides the maximum possible signal-to-noise detection of the cluster stars over the measured background, and the expected and actual effectiveness of the technique in the context of this data set is discussed. The stellar background is examined in some detail for systematic variation as a function of Galactic position in order to assess its effect on the detection efficiency. Of the total number of Pal 5 stars detected, 45% are out in the tails. The tails are found as the only additional 3 σ overdensity of cluster stars over the entire 41 deg² area studied. The annular-averaged density of stars along the tails is fitted to a power law in radius with best-fit index -1.58 ± 0.07, significantly steeper than that predicted from a constant orbit-averaged mass-loss rate.

Deep UBVI photometry for a large field covering the distant globular cluster M75 (NGC 6864) is presented. We confirm a previous suggestion that M75 possesses a bimodal horizontal branch (HB) bearing striking resemblance to the well-known case of NGC 1851. In addition, we detect a third, smaller grouping of stars on the M75 blue tail, separated from the bulk of the blue HB stars by a gap spanning about 0.5 mag in V. Such a group of stars may correspond to the upper part of a very extended, though thinly populated, blue tail. Thus M75 appears to have a trimodal HB. The presence of the "Grundahl jump" is verified using the broadband U filter. We explore the color-magnitude diagram of M75 with the purpose of deriving the cluster's fundamental parameters and find a metallicity of [Fe/H] = -1.03 ± 0.17 dex and -1.24 ± 0.21 in the Carretta & Gratton and Zinn & West scales, respectively. We discuss earlier suggestions that the cluster has an anomalously low ratio of bright red giants to HB stars. A differential age analysis with respect to NGC 1851 suggests that the two clusters are essentially coeval.

Copper abundances are presented for 40 red giant members of the massive Galactic globular cluster ω Centauri, as well as 15 red giant members of the globular clusters NGC 288, NGC 362, NGC 3201, NGC 6752, and M4 (NGC 6121). The spectra are of relatively high spectral resolution and signal-to-noise ratio. Using these abundances, plus published literature values for field stars, the abundance trends of [Cu/Fe] are defined as a function of [Fe/H]. The lowest metallicity stars in ω Cen have [Fe/H] ∼ -2.0, with the stars in this sample spanning a range from [Fe/H] ∼ -2.0 to -0.8. In the field star sample, [Cu/Fe] rises from about -0.8 at [Fe/H] = -2.5 to about -0.4 at [Fe/H] ∼ -1.4 and then rises rapidly to [Cu/Fe] ∼ 0.0 at [Fe/H] = -1.1. The globular clusters (other than ω Cen) tend to also follow the trend as displayed by the field stars. Unlike the field stars, however, ω Cen displays a constant ratio of [Cu/Fe] ∼ -0.5 all the way to [Fe/H] = -0.8. At the metallicity of [Fe/H] = -0.8, the values of [Cu/Fe] in ω Cen fall below the corresponding mean ratio in the field stars by roughly 0.5 dex. If copper is produced primarily in Type Ia supernovae (SNe Ia), as suggested in the literature, the lack of an increase in [Cu/Fe] in ω Cen would suggest very little contribution from SNe Ia to its chemical evolution within the metallicity range from [Fe/H] of -2.0 up to -0.8.

Intermediate-band photometry of the Hyades on the Caby system is presented for dwarf stars ranging from spectral type A through late K. A mean (hk, b-y) relation is constructed using only single stars without anomalous atmospheres and is compared with the field stars of the solar neighborhood. For the F dwarfs, the Hyades relation defines an approximate lower bound in the two-color diagram, consistent with an [Fe/H] between +0.10 and +0.15. These index-color diagrams follow the common convention of presenting stars with highest abundance at the bottom of the plot, although the index values for the metal-rich stars are numerically larger. For field F dwarfs in the range [Fe/H] between +0.4 and -1.0, [Fe/H] = -5.6δhk + 0.125, with no evidence for a color dependence in the slope. For the G and K dwarfs, the Hyades mean relation crosses the field star distribution in the two-color diagram, defining an approximate upper bound for the local disk stars. Stars found above the Hyades stars fall in at least one of three categories: [Fe/H] below -0.7, [Fe/H] above that of the Hyades, or chromospherically active. It is concluded that, contrary to the predictions of model atmospheres, the hk index for cool dwarfs at a given color hits a maximum value for stars below solar composition and, with increasing [Fe/H] above some critical value, declines. This trend is consistent, however, with the predictions from synthetic indices based upon much narrower Ca filters, where the crossover is caused by the metallicity sensitivity of b-y.

Hipparcos parallaxes and proper motions have made it possible to construct Hertzsprung-Russell (H-R) diagrams of nearby clusters with unprecedented accuracy. The standard deviation of high-fidelity, nonbinary, nonvariable stars about a model stellar evolution isochrone in the Hyades cluster is about 0.04 mag. We use this deviation to estimate an upper limit on the scatter in metallicities in stars in this cluster. From the gradient of the isochrone's evolution in the H-R diagram, we estimate an upper limit for the scatter of metallicities Δ[Fe/H] ≲ 0.03 dex, a smaller limit than has previously been measured spectroscopically. This suggests that stars in open clusters are formed from gas that is nearly homogeneous in its metallicity. We consider the hypothesis that processes associated with planet formation can pollute the convection zone of stars. The low observed scatter about the isochrone in the Hyades suggests that pollution effects are not common and strong. If the position on the H-R diagram is insensitive to the metallicity of the convection zone and atmosphere, then stars that have very polluted convection zones can be identified from a comparison between their metallicity and position on the H-R diagram. Alternatively, if the pollution of the star by metals results in a large change in the position of the star on the H-R diagram in a direction perpendicular to the isochrone, then the low scatter of stars in the Hyades can be used to place constraints on quantity of high-Z material that could have polluted the stars.

We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20 M_⊙. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001) for low-mass pre–main-sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5–18.0 in a 6 deg² area. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg² and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker) yields a sample of 250 PMS stars in the mass range ∼0.1 to ∼2 M_⊙. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low-mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 up to 20 M_⊙. A power-law fit to the mass function gives a slope of α ∼ -2.6 above ∼2 M_⊙ and a much flatter slope (α ∼ -0.9) below ∼0.6 M_⊙. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered and support previous conjectures that the triggering event was a supernova shock wave originating from the nearby Upper Centaurus–Lupus association.

We report optical and infrared spectroscopic observations of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both of which were hosted by the galaxy IC 5179. For SN 1999ee we obtained a continuous sequence with an unprecedented wavelength and temporal coverage beginning 9 days before maximum light and extending through day 42. Before maximum light SN 1999ee displayed a normal spectrum with a strong Si II λ6355 absorption, thus showing that not all slow-declining supernovae (SNe) are spectroscopically peculiar at these evolutionary phases. A comparative study of the infrared spectra of SN 1999ee and other Type Ia SNe shows that there is a remarkable homogeneity among the Branch-normal SNe Ia during their first 60 days of evolution. SN 1991bg–like objects, on the other hand, display spectroscopic peculiarities at infrared wavelengths. SN 1999ex was characterized by the lack of hydrogen lines, weak optical He I lines, and strong He I λλ10830, 20581, thus providing an example of an intermediate case between pure Ib and Ic supernovae. We conclude, therefore, that SN 1999ex provides the first clear evidence for a link between the Ib and Ic classes and that there is a continuous spectroscopic sequence ranging from the He-deficient SNe Ic to the SNe Ib, which are characterized by strong optical He I lines.

We present J, H, and K' images of 45 IRAS sources in the recent list by Campbell, Persson, & Matthews, which have been presumed to be luminous young stellar objects (YSOs) because of their infrared colors and their large IRAS fluxes. These objects have been classified into four groups according to two dust features in the 3 μm band—the H₂O ice absorption and the unidentified infrared band (UIB) emission. Our near-infrared images reveal that most of them (42/45) have nebulosity in at least one of the three bands. By examining the nebulosities around these YSOs, we find that (1) nebulae around the objects with the ice absorption can be explained by the scattered light of the central stars, (2) nebulae around the objects with the UIB emission cannot be explained by the scattered light alone—additional emission, most likely from very small grains, is necessary to explain the nebular colors and the surface brightness, and (3) the objects with neither the ice absorption nor the UIB emission tend to have faint or no nebulosity. We interpret the variation of nebular brightness and the color as evolutionary phases of circumstellar matter of intermediate- to high-mass YSOs.

The proper motions of Herbig-Haro (HH) objects in the Orion Nebula were measured with a ±10 km s^-1 accuracy using the Hubble Space Telescope Wide Field Planetary Camera 2 images in [S II], [N II], Hα, and [O III], taken 4–6 yr apart. Seven HH flows in the outer region as well as seven HH objects in the inner region of the BN-KL complex were identified. The H₂ finger system was confirmed to be created by an explosive event that took place approximately 1000 yr ago. We found a new HH flow toward the northwest originating from the vicinity of OMC-1S, which may be a part of the low-velocity bipolar flows centered at FIR 4. The proper motion of HH 202 was also measured with high accuracy for the first time. The proper-motion vectors of HH 202 and HH 203/204 are aligned well with their projected symmetric axis, which may indicate that they emanated from the same source. The proper-motion measurements in various emission lines provide generally the same results in the wide range of velocity from 20 to 400 km s^-1, as expected for shocks in a steady state.

New spectroscopic data of a sample of 10 southern [WC] stars are presented and described. Equivalent widths of carbon line transitions (C II λ4267, C III λ5696, and C IV λλ5801, 5812) are used to revise the classification of the seven late-type [WCL] stars of the sample. For them, our spectra allowed us to employ a quantitative system. We have confirmed the previous classification for three of the objects, but for three others, we have encountered different subtypes. Because of the weakness of the oxygen lines, it was not possible to suggest subtypes for the three [WCE] stars.

In this series of two papers, we present a high-dispersion spectroscopic analysis of eight candidate extremely metal-poor stars selected from the Hamburg/ESO Survey (HES) and of six additional very metal-poor stars. We demonstrate that with suitable vetting using moderate-resolution spectra, the yield of this survey for stars with [Fe/H] ≤ -3.0 dex is very high; three out of the eight stars observed thus far at high resolution from the HES are actually that metal-poor, three more have [Fe/H] ≤ -2.8 dex, and the remainder are only slightly more metal-rich. In preparation for a large-scale effort to mine the HES database for such stars about to get under way, we lay out in this paper the basic principles we intend to use to determine in a uniform way the stellar parameters T_eff, log g, and reddening.

We present a detailed abundance analysis of eight stars selected as extremely metal-poor candidates from the Hamburg/ESO Survey (HES). For comparison, we have also analyzed three extremely metal-poor candidates from the HK survey, and three additional bright metal-poor stars. With this work, we have doubled the number of extremely metal-poor stars ([Fe/H] ≤ 3.0 dex) with high-precision abundance analyses. Based on this analysis, our sample of extremely metal-poor candidates from the HES contains three stars with [Fe/H] ≤ -3.0 dex, three more with [Fe/H] ≤ -2.8 dex, and two stars that are only slightly more metal-rich. Thus, the chain of procedures that led to the selection of these stars from the HES successfully provides a high fraction of extremely metal-poor stars. We verify that our choices for stellar parameters, derived in Paper I and independently of the high-dispersion spectroscopic analysis, lead to acceptable ionization and excitation balances for Fe. Substantial non-LTE effects in Fe appear to be ruled out by the above agreement, even at these extremely low metallicities. For the α-elements Mg, Si, Ca, and Ti, the light element Al, the iron-peak elements Sc, Cr, and Mn, and the neutron-capture elements Sr and Ba, we find trends in abundance ratios [X/Fe] similar to those found by previous investigations. These trends appear to be identical for giants and for dwarfs. However, the scatter in most of these ratios, even at [Fe/H] ≤ -3.0 dex, is surprisingly small. Only Sr and Ba, among the elements we examined, show scatter larger than the expected errors. Future work (the "0Z Project") will provide much stronger constraints on the scatter (or lack thereof) in elemental abundances for a substantially greater number of stars. We discuss the implications of these results for the early chemical evolution of the Galaxy, including such issues as the number of contributing supernovae and the sizes of typical protogalactic fragments in which they were born. In addition, we have identified a very metal-poor star in our sample that appears to represent the result of the s-process chain, operating in a very metal-poor environment, and exhibits extremely enhanced C, Ba, and Pb and somewhat enhanced Sr.

The conclusion published in 1992 by Garmany & Stencel from a study of northern OB associations, that the absolute magnitudes of the O stars show "a large scatter ... intrinsic to the classification system," is critically examined. It is found that the differences between their derived absolute magnitudes of O stars and this author's 1973 calibration exhibit large systematic effects in several associations, ranging from -0.74 to +1.02 mag with substantially smaller dispersions. Of course, when these results are combined, the scatter equals the full range of the systematic effects. To investigate the possibility of distance errors, the Garmany & Stencel B0–B2.5 stars in the same associations are subjected to the same analysis. The results for the B stars show no significant systematic differences, eliminating errors in the association distances derived by Garmany & Stencel from the B stars as the source of the differences found for the O stars. It is noteworthy that the dispersions in the absolute magnitudes of the B stars within a given association are similar to or larger than those of the O stars. An examination of the distribution on the sky of the stars shows that the O and B stars in the discrepant associations are generally not colocated; such was already known to be the case for the important Perseus OB1 association. It is suggested that despite their efforts to improve them, significant problems remain with the association memberships adopted by Garmany & Stencel; the relatively small dispersions of the O star absolute magnitudes even in the discrepant cases indicate that they belong to different, usually more distant associations near the lines of sight to the B associations with which they have been mistakenly connected. Several individual cases of unrecognized multiple systems and classification errors are also found in the Garmany & Stencel sample. It is concluded that the scatter in the absolute magnitudes of the O stars is not as large as found by Garmany & Stencel, and not larger than that of the B stars.

We carried out a search for M-type Vega-like stars by correlating the IRAS Faint Source Catalog with Hipparcos-selected M-type stars. Three stars with apparent IRAS 25 μm excess emission are shown instead to be non–IR-excess stars from ground-based 11.7 and 17.9 μm photometry. Two stars previously suggested to have Vega-like mid-IR excess are also shown to be nonexcess stars. These results imply that other suggested mid–IR-excess stars in the literature may also be false excess stars. Detection threshold bias is apparently responsible for these bogus IR excesses. Sixty micron excess emission from a previously known M-type Vega-like star (GJ 803) is identified again.

We present high-resolution echelle spectroscopy of 39 dwarfs with spectral types between M6.5 and L0.5. With one exception, those dwarfs were selected from the Two Micron All Sky Survey database using photometric criteria, (J-K_S) ≥ 1.1 and K_S ≤ 12.0, and therefore should provide a sample free of the kinematic biases that can affect proper-motion–selected samples. Two of the stars, 2MASSI J0253202+271333 and 2MASSW J0952219-192431, are double-lined spectroscopic binaries. We have used our observations to search for Li I 6708 Å absorption, characteristic of substellar mass; estimate the level of chromospheric activity through measurement of Hα emission fluxes; measure rotational velocities via line broadening; and determine radial velocities and Galactic space motions. Two dwarfs have strong lithium absorption, the previously known brown dwarf LP 944-20 and 2MASSI J0335020+234235, which we identify as a probable 0.06 M_⊙ brown dwarf with an age of ∼1 Gyr. We have investigated the prospect of using the observed frequency of lithium absorption among ultracool M dwarfs (M7 to M9.5) as a probe of the initial mass function, comparing the observed frequency against predictions based on recent theoretical models of low-mass dwarfs and an assumed star formation history. Our results show that the conclusions drawn are vulnerable both to systematic differences between the available models and to incomplete local sampling of the most recent star formation events (ages less than 10⁸ yr). The latter consideration stems from the mass-dependent rate of evolution of brown dwarfs. Even given those caveats, however, the available observations are difficult to reconcile with Salpeter-like power-law mass functions (α ≥ 2) for masses below 0.1 M_⊙. A comparison between the rotational velocities and Hα fluxes shows no evidence for significant correlation. The mean activity level of the ultracool dwarfs lies almost a factor of 10 below that of early- and mid-type M dwarfs. The relative number of dwarfs with v sin i < 20 km s^-1 with respect to greater than 20 km s^-1 is independent of spectral type. Finally, velocity dispersions derived for our photometrically selected sample of ultracool dwarfs are significantly lower than those measured for nearby M dwarfs but show remarkable similarity to results for earlier type emission-line (dMe) dwarfs. The latter are generally assigned ages of less than ∼3 Gyr.

We review the systematic variation between optical- and infrared-wavelength angular diameters reported for stars in the approximate range of spectral types K0–M6. We show that there is a correlation between the ratio of angular diameters and the depth of TiO absorption, in the sense that the optical diameters are larger. We argue that this validates a recent proposal by Houdashelt et al. that TiO absorption affects certain, but not all, optical-wavelength angular diameters significantly. Those authors pointed out that the infrared angular diameters appear to yield better effective temperatures than do the optical diameters, even though the latter are of higher precision. The observed angular diameter differences may arise either from limb darkening, atmospheric extension, or a combination of these two processes. Model atmosphere calculations of limb-darkening coefficients are needed to see whether the diameter discrepancy may be resolved. These models need to contain the correct opacity sources and a realistic estimate of the atmospheric geometry and dynamics. A comparison with observations such as those described in this paper will be useful for testing the validity of atmosphere models.

We examine the early angular momentum history of stars in young clusters via 197 photometric periods in fields flanking the Orion Nebula cluster (ONC), 81 photometric periods in NGC 2264, and 202 measurements of v sin i in the ONC itself. We show that pre–main-sequence stars spanning an age range from 0.1 to 3 Myr do not appear to conserve stellar angular momentum as they evolve down their convective tracks, but instead preserve the same range of periods even though they have contracted by about a factor of 3. This result seems to require a mechanism that regulates the angular velocities of young stars. We discuss several candidate mechanisms. The most plausible appears to be disk locking, though most of our stars do not have I_C-K_s excesses suggestive of disks. However, a decisive test of this hypothesis requires a more sensitive diagnostic than the I_C-K_s excesses used here.

We report new, more sensitive observations of nine of the 11 extrastatistical signals in the Megachannel Extraterrestrial Assay (META). These extrastatistical signals had all the expected characteristics of a transmission from an extraterrestrial transmitter, except that they did not repeat. Cordes, Lazio, & Sagan showed that this lack of repeatability could be explained by the high detection thresholds used in the reobservations of these candidates, combined with interstellar scintillation of intrinsically steady sources. We use the Cordes et al. methodology, correcting an error in the original presentation, and our new observations to rule out this scintillation hypothesis at a confidence level of at least 97.8% (for the case of an intrinsically weak source) and to a level in excess of 99% (if the source strengths are comparable to the value favored by Cordes et al.). We also demonstrate that gravitational microlensing cannot account for the initial detection of these candidate signals, nor is gravitational lensing likely to play a role in future SETI programs. We conclude that the META candidates do not reflect a large population of powerful, strong beacons.

We describe a high-precision radial velocity search for Jovian-mass companions to main-sequence stars in the Hyades. The Hyades provides an extremely well controlled sample of stars of the same age, the same metallicity, and a common birth and early dynamical environment. This sample allows us to explore the dependence of the process of planet formation on only a single independent variable: the stellar mass. In this paper, we describe the survey and summarize results for the first 5 years.

The Hyades constitute a homogeneous sample of stars ideal for investigating the dependence of planet formation on the mass of the central star. Because of their youth, Hyades members are much more chromospherically active than stars traditionally surveyed for planets using high-precision radial velocity techniques. Therefore, we have conducted a detailed investigation of whether magnetic activity of our Hyades target stars will interfere with our ability to make precise radial velocity (v_rad) searches for substellar companions. We measure chromospheric activity (which we take as a proxy for magnetic activity) by computing the equivalent of the R activity index (which is corrected for photospheric contributions) from the Ca II K line. The value of ⟨R⟩ is not constant in the Hyades: we confirm that it decreases with increasing temperature in the F stars and also find it decreases for stars cooler than mid K. We examine correlations between simultaneously measured R and radial velocities using both a classical statistical test and a Bayesian odds ratio test. We find that there is a significant correlation between R and the radial velocity in only five of the 82 stars in this sample. Thus, simple R-v_rad correlations will generally not be effective in correcting the measured v_rad values for the effects of magnetic activity in the Hyades. We argue that this implies long-timescale activity variations (of order a few years; i.e., magnetic cycles or growth and decay of plage regions) will not significantly hinder our search for planets in the Hyades if the stars are closely monitored for chromospheric activity. The trends in the radial velocity scatter (σ) with ⟨R⟩, v sin i, and P_rot for our stars is generally consistent with those found in field stars in the Lick planet search data, with the notable exception of a shallower dependence of σ on ⟨R⟩ for F stars.

We present data for main-belt asteroids serendipitously observed by the Midcourse Space Experiment (MSX) in the course of its long-scan astronomy survey observations. Based upon the orbital elements for 26,791 asteroids, we have identified 325 sightings (920 observations) of 168 different asteroids in the MSX data, 20 of which were observed only once in a single band and are labeled singletons. Of the 168 different asteroids, 30 (including six singletons) were not previously observed by IRAS. Diameters and albedos were derived on the IRAS system. We present the entire MSX Infrared Minor Planet Survey data set, describe how it was created, and compare it with the Supplemental IRAS Minor Planet Survey data set.

We explore the possibility that extrasolar planets might be found in the 1 : 1 mean motion resonance, in which a pair of planets share a time-averaged orbital period. There are a variety of stable co-orbital configurations, and we specifically examine three different versions of the 1 : 1 resonance. In the first configuration, the two planets and the star participate in tadpole-type librations about the vertices of an equilateral triangle. The dynamics of this situation resemble the orbits of Jupiter's Trojan asteroids. We show analytically that an equilateral configuration consisting of a star and two equal-mass planets is linearly stable for mass ratios μ = 2m_pl/(2m_pl + M_*) < 0.03812. When the equilateral configuration is subjected to larger perturbations, a related 1 : 1 resonance occurs. In this second family of configurations, the planet pair executes horseshoe-type orbits in which the librating motion in the corotating frame is symmetric about a 180° separation. The Saturnian satellites Janus and Epimetheus provide a solar system example of this phenomenon. In the case of equal-mass planets, a numerical survey indicates that horseshoe configurations are stable over long periods for mass ratios μ < 0.0004, indicating that a pair of Saturn-mass planets can exist in this resonance. The third configuration that we examine is more exotic and involves a pair of planets that exchange angular momentum in a manner that allows them to indefinitely avoid close encounters. An illustrative example of this resonance occurs when one planet has a highly eccentric orbit while the other planet moves on a nearly circular orbit; the periapses are in alignment, and conjunctions occur near periapse. All three of these resonant configurations can be stable over timescales comparable to or longer than stellar lifetimes. We show that pairs of planets in 1 : 1 resonance yield characteristic radial velocity signatures that are not prone to the sin i degeneracy. Indeed, Keplerian fits to the radial velocities cannot reveal the presence of two planets in the 1 : 1 resonance. We discuss a dynamical fitting method for such systems and illustrate its use with a simulated data set. Finally, we argue that hydrodynamic simulations and torqued three-body simulations indicate that 1 : 1 resonant pairs might readily form and migrate within protostellar disks.

The field of direct imaging is experiencing a considerable growth in the number of available CCD mosaic imagers, especially on large telescopes. To fully exploit the astrometric potential of these imaging devices, we develop a technique, utilizing an astrometric standard, for precision transformation of pixel coordinates into a global coordinate system. We have constructed a new astrometric standard set of 1863 stars in the field of NGC 188 to derive the CCD chip constants for the NOAO CCD Mosaic Imager. The multiple-epoch data on the Mosaic's metrics indicate that this CCD mosaic device may have experienced a one-time nonelastic expansion. We also present a new determination of the pixel scale and the optical field angle distortion constants for the KPNO Mayall 4 m telescope prime focus field corrector. To establish a reliable history of the CCD mosaic imager metrics for current and potential future astrometric applications, we recommend obtaining astrometric calibrations for CCD mosaic imagers on a regular basis. Apart from mechanical positioning of the CCD mosaic camera on the telescope, noticeable changes in the thermal environment of CCD mosaic chips should also prompt new astrometric calibrations. It is shown that, following all precautions, the NOAO CCD Mosaic Imager can produce excellent astrometric results on the Mayall 4 m telescope.

Many of the International Celestial Reference Frame (ICRF) sources are not pointlike, as shown by the 2 GHz and 8 GHz radio maps. The size scale reaches up to a few tens of milliarcseconds for extended sources. Also, although the optical images are not resolved, the centers of emission are not necessarily coincident with the radio centroids. Here we search for indications of such noncoincidence. We divide the sources into two sets, extended and compact, according to the radio structure index given in the ICRF extension. The optical positions are from recent determinations, to obtain the highest precision and evenness of accuracy. The ICRF radio positions are of milliarcsecond precision or better. The average of the absolute values of the differences between the lengths of the optical and radio arcs joining pairs of sources taken within each of the sets is found to be about 7.9 mas larger for the extended sources than for the compact sources. This is interpreted as evidence of noncoincidence between the radio and optical centers, at least for the extended sources. Additional checks made with larger, different sets using the optical source positions from the USNO-A2.0 catalog support this conclusion.

The Las Campanas Infrared Survey Camera is a near-infrared (1.0–2.5 μm), wide-area instrument used to detect and measure the photometric properties of galaxies out to large redshifts, z > 2. The camera, a modified Offner 1 : 1 reimaging optical system, is mounted at the f/7.5 focus of the 2.5 m du Pont Telescope. The detectors are four Rockwell 1024 × 1024 HgCdTe (HAWAII) arrays operating at a scale of 020 pixel^-1. With four telescope pointings, the instrument produces a pipelined mosaic of J, H, or K_s images 13' × 13' on the sky, with a measured point-spread function as good as 038 FWHM. The good imaging quality results in part from fast tip-tilt guiding on stars within a 9' × 9' field centered on the optical axis of the telescope. Appropriately bright guide stars are found within 2 s from a catalog of 5 × 10⁷ stars and presented as a "finding chart" to the observer. The optical, mechanical, and thermal design choices and their associated engineering implementations are discussed in some detail. The detector readout electronics, the automatic data acquisition and control system, and our data reduction pipeline are also described. The design goals of the camera—excellent imaging quality and throughput, low flexure and internal background, and 5 Hz on-axis guiding, are all realized and quantified.