Table of contents

Images of the Hubble Deep Field (HDF) at 3.2 μm, taken with the 10 m Keck Telescope, are presented. The images cover a total area of ∼2.5 arcmin². To a 5 σ limit of [3.2]_tot ≈ 17.5 mag (Vega relative), 11 sources are detected, nine of which are extragalactic. The integrated galaxy number counts are therefore ∼1.3 × 10⁴ deg^-2 to this depth. The galaxies detected at 3.2 μm have a median redshift of z = 0.56. All 3.2 μm sources have 1.6 μm, 1.1 μm, and visual counterparts, all of fairly regular morphology; several also have 6.7 μm, 15 μm, 850 μm, 8.5 GHz, or 1.4 GHz counterparts. No sources are found that are either anomalously red or anomalously blue in their H-[3.2] color, and there are significant detections of most of the known near-infrared HDF sources for which detection in these 3.2 μm data seemed likely.

We report on two programs to address differential evolution between the radio-loud and radio-quiet quasar populations at high (z > 4) redshift. Both programs entail studying the radio properties of optically selected quasars. First, we have observed 32 optically selected, high-redshift (z > 4) quasars with the VLA at 6 cm (5 GHz). These sources represent a statistically complete and well-understood sample. We detect four quasars above our 3 σ limit of ≈0.15 mJy, which is sufficiently sensitive to detect all radio-loud quasars at the probed redshift range. Second, we have correlated 134 z > 4 quasars, comprising all such sources that we are aware of as of mid-1999, with the FIRST survey and the NRAO VLA Sky Survey. These two recent 1.4 GHz VLA sky surveys reach 3 σ limits of approximately 0.6 and 1.4 mJy, respectively. We identify a total of 15 z > 4 quasars, of which six were not previously known to be radio-loud. The depth of these surveys does not reach the radio-loud/radio-quiet demarcation luminosity density (L_{1.4 GHz} = 10^32.5h ergs s^-1 Hz^-1) at the redshift range considered; this correlation therefore provides only a lower limit to the radio-loud fraction of quasars at high redshift. The two programs together identify eight new radio-loud quasars at z > 4, a significant increase over the seven currently in the published literature. We find no evidence that the radio-loud fraction depends on optical luminosity for -25 > M_B > -28 at z ≃ 2 or for -26 > M_B > -28 at z > 4. Our results also show no evolution in the radio-loud fraction between z ≃ 2 and z > 4 (for -26 > M_B > -28).

The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing fluctuations during the observations, in the differential light curves used to study optical microvariability. Repeated CCD observations during two consecutive nights were used to obtain time-resolved aperture photometry for the BL Lac object PKS 2316-423, which is located at the center of a conspicuous elliptical galaxy, and for several field stars. The blazar seems to be variable according to standard variability criteria; however we show that the observed differential magnitude variations are strongly correlated with seeing fluctuations during the nights. Moreover, another galaxy within the same CCD field shows nearly identical variations, clearly indicating that such variations are artifacts of the aperture photometry. Simulated observations of quasars within host galaxies of different morphologies and spanning a wide range of luminosities were also used to evaluate the effects of changing seeing conditions. The results show that spurious differential magnitude variations due to seeing fluctuations are larger for active nuclei within brighter hosts, particularly when small photometric apertures (about the seeing FWHM in radius) are used. According to our results, several recommendations are given to future observers.

We present optical polarimetry of the violently variable BL Lacertae object S5 0716+714, obtained over a three year period using the 6 m telescope at the Special Astrophysical Observatory in Russia. The degree of polarization is high and variable throughout the observations. With a minimum time resolution of 1 minute, significant variations on a timescale of 10–15 minutes are observed. The variations are complex and not fully sampled by this data, but they are characterized by large amplitude changes on a timescale of about 1 day superposed on smooth variations with a timescale of about 1 week. Possible periods of 12.5, 2.5, and 0.14 days have been discovered. S5 0716+714 is a highly compact radio source with nonthermal emission observed across the electromagnetic spectrum out to gamma rays. The optical polarization variations are consistent with relativistically beamed synchrotron emission viewed at a very small angle to the line of sight.

We present gri CCD photometry of 44 Abell clusters and four cluster candidates. Twenty-one clusters in our sample have spectroscopic redshifts. Fitting a relation between mean g, r, and i magnitudes and redshift for this subsample, we have calculated photometric redshifts for the remainder with an estimated accuracy of approximately 0.03. The resulting redshift range for the sample is 0.03 < z < 0.38. Color-magnitude diagrams are presented for the complete sample and used to study evolution of the galaxy population in the cluster environment. Our observations show a strong Butcher-Oemler effect, with an increase in the fraction of blue galaxies (f_B) with redshift that seems more consistent with the steeper relation estimated in 1995 by Rakos and Schombert than with the original one by Butcher and Oemler in 1984. However, in the redshift range between about 0.08 and 0.2, where most of our clusters lie, there is a wide range of f_B-values, consistent with no redshift evolution of the cluster galaxy population. A large range of f_B-values is also seen between about 0.2 and 0.3, when we add X-ray clusters from Smail et al. to our sample. The discrepancies between samples underscore the need for an unbiased sample to understand how much of the Butcher-Oemler effect is due to evolution and how much to selection effects. We also tested the idea proposed by Garilli et al. in 1996 that there is a population of unusually red galaxies that could be associated either with the field or clusters, but we find that these objects are all near the limiting magnitude of the images (20.5 < r < 22) and have colors that are consistent with those expected for stars or field galaxies at z ∼ 0.7.

We reexamine the disk-halo decompositions of the rotation curves of low surface brightness (LSB) galaxies with V_max ≥ 80 km s^-1, taking full account of the effects of beam smearing. We show that the spatial resolution of the data is not sufficient to put any meaningful constraints on the density profiles of the dark halos, or on cosmological parameters. This is in strong contrast to claims made in the literature that these LSB rotation curves are only consistent with dark matter halos with shallow central cusps, and it has important implications regarding the halos of LSB galaxies, such as the self-similarity of their rotation curves, and their inconsistency with certain cosmological models or with cold dark matter altogether. Only in one case are the data of sufficient spatial resolution to obtain reliable constraints on the slope of the central density distribution of the dark matter halo. For this single case, we find a central cusp ρ ∝ r^-α with 0.55 < α < 1.26 at the 99.73% confidence level. This contrasts strongly with the results for two dwarf galaxies (V_max < 70 km s^-1) that we analyze, which yield α < 0.5 at the same level of confidence. This possibly suggests that halos with constant-density cores are restricted to low-mass systems. We show that violent outflows of baryonic matter by supernova feedback can reproduce this mass dependence of halo cusp slopes.

Radio continuum emission at 20 cm has been observed in a sample of 16 edge-on galaxies, using the VLA in its A configuration. These galaxies were observed recently at lower resolution by Irwin et al., who found evidence for extraplanar disk-halo features in 15 of the 16 galaxies. Twelve of the galaxies are detected in the new high-resolution observations. Of these, only two, which were previously known Seyferts, show convincing evidence for AGNs. For six of the galaxies, we provide improved positions for the galaxy nuclei. In four galaxies, radio continuum loops extending perpendicular to the major axis have been discovered, confirming previous conclusions from lower resolution observations that extraplanar emission is present, as well as providing further evidence that radio "halos" consist, at least in part, of underlying discrete features. For several galaxies, the resolution is sufficient to reveal individual star-forming regions in the disk. We examine NGC 3556 in detail since this galaxy has been shown by King & Irwin to display extremely large H I supershells. The estimated input energy from supernovae in the brightest radio component of this galaxy is insufficient to drive the observed H I supershells. We explore possible resolutions to this energy deficit.

The [O II] λ3727 emission lines and absorption features from stellar Balmer and Ca H and K lines are the most accessible kinematic diagnostics in galaxies at z ∼ 1. We investigate the kinematics of 22 local late-type galaxies using these spectral features, and we compare the results with 21 cm neutral hydrogen spectra in order to assess the utility of each diagnostic for measuring galaxy masses. In order to simulate data at high redshift, where only one-dimensional velocity profiles are normally available, we study spatially integrated, as well as spatially resolved, spectra. Although the studied galaxies span a wide range of morphological types, inclinations, and star formation rates, we find that the gaseous and stellar kinematic tracers yield comparable kinematic line widths and systemic velocities. The [O II] and H I line widths correlate most strongly, showing an intrinsic dispersion of ∼20 km s^-1, or ∼10% for a typical galaxy with a kinematic width of 200 km s^-1. In a few extreme cases, the [O II] line widths underestimate the neutral hydrogen width by 50%. Reliable velocity widths can also be obtained from the stellar Balmer and Ca H and K absorption lines, even for some of the very late type galaxies that have strong emission lines. The intrinsic dispersion is ≤10% between the stellar absorption and H I line widths. We provide a prescription for using these strong stellar absorption and [O II] emission features to measure the kinematics, and thus masses, of galaxies in the distant universe.

We report on a blind neutral hydrogen survey for galaxies using the 21 cm multibeam receiver on the Parkes 64 m telescope. The surveyed region covers |b| ≤ 5° in the zone of avoidance (ZOA) from Galactic longitude 308° to 332°. The survey represents the first phase of a blind H I survey covering the southern ZOA (l = 212° to 36°). We have detected H I in 42 galaxies above a 3 σ limit of 60 mJy. The galaxies detected in this survey have velocities out to 6000 km s^-1 and H I masses in the range 4 × 10⁷ to 3 × 10¹⁰M_⊙ (h). Only eight of the 42 galaxies have velocities previously measured. A further nine galaxies appear to have optical counterparts in the Woudt's 1998 catalog. In total, 16 of the galaxies appear to be associated with IRAS sources, although only three of these are without optical counterparts. The estimated median extinction for the 20 galaxies with optical or IR counterparts is A_B = 3.8 mag. For the 22 galaxies with no counterparts, the estimated median extinction is A_B = 5.6 mag. The distribution of galaxies is suggestive of a connection between the Centaurus supercluster above the Galactic plane and the Pavo-Indus supercluster beneath the plane. No previously hidden concentrations of galaxies were found.

We present accurate optical surface photometry in the U, B, V, R, and I passbands for 11 disk galaxies. The sample has been selected in order to study the different morphological structures present in disk galaxies and includes all morphological types. For each galaxy, we present surface brightness, ellipticity, and position angle radial profiles from ellipse fits to the isophotes. Color index images and color index profiles in U-B, B-V, and B-I are also shown. The photometric information obtained is crucial to understanding the different morphological structures presented in all these galaxies, and to obtain their mass distributions. The latter topic will be the subject of a forthcoming paper.

This paper commences a series of investigations into the stellar populations of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evolution histories of present-day elliptical galaxies. The primary galaxy sample analyzed is that of González, which consists of 39 elliptical galaxies drawn primarily from the local field and nearby groups, plus the bulge of Messier 31. Single-burst stellar population (SSP)–equivalent ages, metallicities, and abundance ratios are derived from Hβ, Mg b, and ⟨Fe⟩ line strengths using an extension of the Worthey models that incorporates nonsolar line-strength "response functions" by Tripicco & Bell. These functions account for changes in the Lick/IDS indices caused by nonsolar abundance ratios, allowing us to correct the Worthey models for the enhancements of Mg and other α-like elements relative to the Fe-peak elements.

SSP-equivalent ages of the González elliptical galaxies are found to vary widely, 1.5 Gyr ≲ t ≲ 18 Gyr, while metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked around ⟨[Z/H]⟩ = +0.26 and ⟨[E/Fe]⟩ = +0.20 (in an aperture of radius r_e/8). The enhancement ratios [E/Fe] are milder than previous estimates because of the application of nonsolar abundance corrections to both Mg b and ⟨Fe⟩ for the first time. While [E/Fe] is usually greater than zero, it is not the "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken ⟨Fe⟩ but also to strengthen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al.), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the E group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age increasing by 25%, metallicity decreasing by ⟨[Z/H]⟩ = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius r_e/2 for nearly all systems.

Our ages have an overall zero-point uncertainty of at least ∼25% because of uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] ≠ 0 on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hβ by blue stragglers or hot horizontal-branch stars, or by fill-in of Hβ by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.

We present Hubble Space Telescope (HST) imagery and photometry of the active galaxy Markarian 421 and its companion galaxy 14'' to the east-northeast. The HST images indicate that the companion is a morphological spiral rather than elliptical, as previous ground-based imaging has concluded. The companion has a bright, compact nucleus, appearing unresolved in the HST images. This is suggestive of Seyfert activity, or possibly a highly luminous compact star cluster. We also report the results of high dynamic range long-slit spectroscopy with the slit placed to extend across both galaxies and nuclei. We detect no emission lines in the companion nucleus, though there is evidence for recent star formation. Velocities derived from a number of absorption lines visible in both galaxies indicate that the two systems are probably tidally bound and thus in close physical proximity. Using the measured relative velocities, we derive a lower limit on the Mrk 421 mass within the companion orbit (R ∼ 10 kpc) of 5.9 × 10¹¹ solar masses, and a mass-to-light ratio of ≥ 17. Our spectroscopy also shows for the first time the presence of Hα and [N II] emission lines from the nucleus of Mrk 421, providing another example of the appearance of new emission features in the previously featureless spectrum of a classical BL Lac object. We see both broad and narrow line emission, with a velocity dispersion of several thousand kilometers per second evident in the broad lines.

It is concluded that faint members found in the vicinities of compact groups (CGs) are gravitationally bound with corresponding groups. All members of CGs, faint and bright, move preferentially in the direction of the elongation of the group. We suppose that the number of faint members may be much larger than previously assumed. These two circumstances should apparently change the conclusion about the short lifetime of CGs. Consequently, many puzzles related to CGs, such as their very existence, the absence of strong radio and infrared sources, and the absence of strong signs of interaction and merging will probably vanish.

We report the discovery of a low surface brightness (LSB) object serendipitously found during deep CCD imaging of a compact group of galaxies, Seyfert's Sextet, in the VR and I bands. The LSB object is located 23 southwest from the group's center. Its surface brightness within the angular effective radii of r_eff(VR) = 48 and r_eff(I) = 48 is very low: μ_eff(VR) = 25.28 mag arcsec^-2 and μ_eff(I) = 24.47 mag arcsec^-2, respectively. The apparent magnitudes are m_AB(VR) = 19.87 and m(I) = 19.06. The object is most likely an LSB dwarf galaxy, but other possibilities are also discussed.

We present very long baseline interferometry (VLBI) images of the core emission from a nearby bright FR II radio galaxy, Pictor A, revealing its parsec-scale jet structure and evolution for the first time. These data constitute a significant addition to our knowledge of powerful radio galaxies on the smallest scales, effectively doubling the number studied at this resolution. The jet, 14 h^-1 pc in projected extent, is directed west of the core for the first 5 h^-1 pc and then appears to bend approximately 40° to the north. Apparent motions for three of the five parsec-scale jet components have been estimated, 0.5 ± 0.4, 1.1 ± 0.5, and 0.4 ± 0.7 h^-1c, indicating that subluminal motion is likely. No parsec-scale counterjet has been detected, allowing only lower limits on the jet-to-counterjet surface brightness ratio to be estimated. Two models, one describing the apparent 40° bend in the parsec-scale jet as an intrinsic deflection of the jet and one describing it as the effect of jet precession, may each be plausible and should be testable with future VLBI observations. By adopting the jet deflection model to describe the apparent 40° bend, we estimate that the Pictor A jet is initially inclined to our line of sight by less than 51°. Comparing this result with VLBI observations of Cygnus A suggests that, while the components in both jets are consistent with at least mildly relativistic speeds, the Pictor A jet lies significantly closer to our line of sight than the Cygnus A jet. This conclusion is consistent with both the parsec-scale radio structures and the kiloparsec-scale orientations of the host galaxies as well as the "unified model" interpretation of the optical spectra from these two objects.

We present H-band images obtained with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) of a field 75'' (5 kpc) above the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based observations have shown that NGC 5907 has a luminous halo with a shallow radial profile between 4 and 8 kpc that roughly traces the dark matter distribution of the galaxy deduced from its rotation curve. Our NICMOS observations were designed to resolve bright giants in the halo of NGC 5907 to constrain its stellar composition with the goal of understanding its nature and origin. More than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in the halo of NGC 5907 is consistent with that expected from standard stellar initial mass functions and if ground-based estimates of the distance to NGC 5907 and the integrated colors of its halo are correct. Instead we observe only one candidate giant star. This apparent discrepancy can be resolved by assuming either a significantly larger distance than that suggested by several studies or a halo metallicity much lower than that suggested by ground-based colors and as low as that in metal-poor Galactic globular clusters. If previous distance and halo color estimates for NGC 5907 are correct, our NICMOS results suggest that its extended light is composed of stars that formed with an initial mass function different than that observed locally, leading to a much higher ratio of dwarfs to giants. We describe how these three possible explanations for the absence of bright giants in our NICMOS images of the halo of NGC 5907 might be constrained by future observations.

We analyze the 2''-resolution, high-sensitivity radio continuum observations of the nearby spiral galaxy NGC 6946 at 6 and 20 cm by Lacey, Duric, & Goss to derive a luminosity function at each wavelength for 43 H II region candidates and for 15 supernova remnant (SNR) candidates. The H II region radio luminosity functions are best fitted by power laws consistent with the shape of the optical luminosity function of Bonnarel, Boulsteix, & Marcelin. The SNR luminosity functions are fitted by considerably steeper power laws than the H II region luminosity functions. Extinctions to the H II regions are estimated from a comparison of radio flux densities with new optical fluxes derived from the Hα observations of Van Dyk. A dependence of extinction on galactocentric radius is investigated, but none is found.

We present 38''resolution Very Large Array 21 cm continuum and H I line emission observations of the spiral galaxy IC 342, at an adopted distance of 2 Mpc. Kinematic evidence exists for a m = 2 spiral density wave in the inner disk with a corotation radius located at 4 kpc and a possible four-arm pattern in the outer disk. On smaller scales, outside of the central depression in H I column density, H I is organized into a complex pattern of relatively short (∼2–5 kpc), interconnected, spiral arm segments. Numerous "holes" are distributed throughout the H I disk. By considering the effects of shear, structures that are not self-gravitating, such as holes and voids, cannot be long-term phenomena. The timescale, combined with the total energy required to evacuate holes, leads us to reject wind and supernovae origins for the large-scale pattern of H I holes in IC 342. Gravitational instabilities in the disk form on a timescale that is short compared with the rotation period of the disk. The pattern of H I spiral arm segments exists on a scale that is consistent with their being material arms that result from gravitational instabilities. The H I cavities are a natural remnant of the process.

We describe an objective method for the identification of stellar OB associations in the Large Magellanic Cloud under the assumption that they are loose, unbound stellar systems with a young OB stellar component. The method is based on star counts and spectral classification. First we detect the areas where an enhancement of star number density occurs above 3 σ of the average field density in large regions. The boundaries at 3 σ provide the size and morphology of the detected stellar concentrations. Further examination at different magnitude ranges allows us to select the systems with a bright stellar component within the detected areas. In the second step, star counts around the peak density of each detected stellar concentration provide a typical value of the projected half-mass radius, in order to calculate the central density using the appropriate mass function slope. The central density, being a crucial parameter for the bound and unbound systems, has been used as a tentative criterion for the distinction between open clusters and associations. Finally, spectral classification from objective-prism plates provides further evidence for the existence of OB-type stars in these concentrations. The faintest magnitude at which the various systems were detected is found to be independent of the presence or absence of gas and varies by up to 4 mag. An explanation for this effect is the possible existence of pre–main-sequence stars that are not visible in the optical region.

We have carried out Wide Field Planetary Camera 2 (WFPC2) F160BW, F555W, and F656N imaging of four young populous clusters: NGC 330, in the Small Magellanic Cloud, and NGC 1818, 2004, and 2100, in the Large Magellanic Cloud. We report photometric results for these four clusters, including identification using photometric colors of the cluster Be star population. We present theoretical WFPC2 and broadband colors and bolometric corrections for LMC and SMC metallicities. The use of the far-UV F160BW filter enables accurate determination of the effective temperatures for stars in the vicinity of the main-sequence turnoff and on the unevolved main sequence.

We present deep VI-band photometry of the globular cluster M54, a nearby field in the Sagittarius dwarf galaxy, and a control field. The color-magnitude diagrams reach well below the oldest main-sequence turnoffs, thus enabling an analysis of the galaxy's age-metallicity relation with unprecedented clarity. We also study the variable stars in the direction of M54. From 67 RR Lyrae variables, we confirm and improve on our previous estimates of the cluster horizontal branch magnitude, foreground reddening, and horizontal branch morphology. These values are used in determining the ages of M54 and the Sagittarius field populations. We confirm our previous result that M54 is the same age as Galactic globular clusters of similar metallicity. We also derive ages on a self-consistent scale for the other three globular clusters in Sagittarius. We find strong evidence for multiple episodes of star formation (or continuous star formation with a variable rate) in the field of Sagittarius. We characterize the principal episodes with the ages 11, 5, and 0.5 through 3 Gyr and with [Fe/H] values of -1.3, -0.7, and -0.4, respectively. On this scale, M54 has an age of 15 Gyr. Surprisingly, the age-metallicity relation we have derived for the galaxy as a whole is described quite well by a closed-box chemical evolution model. We also find that the populations associated with the Sgr field are clumped spatially around M54, and we consider several explanations for this phenomenon. We again speculate that Sagittarius is a nucleated dwarf elliptical galaxy with M54 as its nucleus.

We present CCD photometry in the BVI-filter passbands for the Galactic globular cluster NGC 6144. This cluster is located in the direction of the ρ Ophiuchi dust cloud, approximately 30' northwest of the bright star α Scorpii (Antares). Our color-magnitude diagrams (CMDs) of NGC 6144 extend from above the tip of the first-ascent red giant branch (RGB) to ∼3 mag below the horizontal branch (HB). The morphology of the HB is predominantly blueward of the RR Lyrae instability strip, while the RGB is relatively steep, signifying a low-to-intermediate metal abundance. Our CMD also reveals the presence of three candidate post-asymptotic giant branch stars. We find V_HB = 16.30 ± 0.07; coupling the measured RGB color at the level of the HB with a polynomial describing the shape of the RGB, we have utilized the simultaneous reddening and metallicity method of Sarajedini to estimate a metallicity of [Fe/H] = -1.81 ± 0.12 (on the Zinn & West scale) and a mean reddening of E(B-V) = 0.41 ± 0.02. In addition to this mean level, our observations reveal that the reddening has a spatially variable component generally increasing from north to south consistent with the expected density variations in the ρ Ophiuchi dust cloud.

Sixteen photometric variable stars with mean V magnitudes between 15.2 and 20.3 have been discovered in a 167 × 167 field centered on the metal-rich globular cluster M71. The identifications are the result of a systematic time-series CCD imaging survey of the cluster using the Dominion Astrophysical Observatory 1.8 m telescope. Light curves and finding charts are presented for the newly identified stars (eight of which are definite variables, three probable, and five possible). These 16 stars, when combined with the seven faint variable stars discovered recently by Hodder et al. and Yan & Mateo, bring to 23 the number of faint variables now known in M71. The variable stars are of two basic types: W UMa– or Algol-like eclipsing binaries and SX Phoenicis radially pulsating variable blue straggler stars. Additional photometry is required to confirm the nature of the variability of several of the stars.

We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster ω Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over ΔS and other techniques in determining the metallicity of RR Lyrae stars and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the ΔS observations of Butler, Dickens, &Epps and Gratton, Tornambe, &Ortolani, we have found that our metallicities, [Fe/H]_hk, deviate systematically from their ΔS metallicity, while our [Fe/H]_hk for well-observed, field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is because of the larger errors associated with the previous ΔS observations for this cluster. The M_V(RR)-[Fe/H] and period shift–[Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Y.-W. Lee, confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period-amplitude diagram, we have also identified highly evolved RRab stars in the range - 1.9 ≤ [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.

We report on the results of our analyses of high-quality spectra of M supergiants in the h and χ Persei double cluster. Our temperature estimates range from 3500 to 4000 K for a sample of 14 stars. There is a wide range in the lithium abundances, from [Li/Fe] = -1.7 to [Li/Fe] = -0.10. The ¹²C/¹³C ratio ranges from 10 to 16 and displays a weak trend with luminosity, such that the more luminous stars have a slightly smaller ratio. We find a significant excess of Na, which may be due to its production by proton capture by ²²Ne. However, there is no evidence for the O deficiency that usually accompanies a Na excess in globular cluster stars.

Photometric CCD UBV measurements have been obtained in the field of the young Cygnus cluster NGC 6910. The observations reach down to magnitude V ≃ 18 for 206 stars measured in all three UBV bands, uncovering the region of the color-magnitude diagram where pre–main-sequence (PMS) stars are expected. The transformation of our instrumental magnitudes to the standard system is discussed on the basis of two different sets of standard stars: 22 stars from six fields in the Landolt catalogs and 48 stars with published photoelectric photometry in common with ours in the four clusters observed in this campaign. The latter set is preferred, since it enables the effect of systematic errors in the U-B colors to be minimized. The adopted cluster parameters are E(B-V) = 1.02 ± 0.13, V₀ - M_V = 11.2 ± 0.2, and age = (6.5 ± 3) × 10⁶ yr. The search for PMS cluster members on the basis of the calculated color excess and distance modulus results in the proposed membership for 11 PMS stars of spectral types from A to G.

Several dozen stars are identified as probable previously unrecognized members of the association Vel OB1, including the Vela X-1 binary pulsar system. Based on a variable extinction analysis of 70 stars, the distance and ratio of total to selective absorption of the association are determined to be 1750 pc and 3.70, respectively. Vel OB1 may be significantly larger than previously appreciated, perhaps up to 600 by 300 pc in the sense of longitude by latitude. This physical extent, and the fact that the H-R diagram includes both low-mass evolved yellow supergiants and massive, early-type supergiants, indicates that Vel OB1 is probably not coeval.

We present near-infrared J, H, and K images and K-band spectroscopy in the giant H II region W42. A massive star cluster is revealed; the color-color plot and K-band spectroscopic emission features of two of the brighter objects suggest the presence of young stellar objects. The spectrum of the bright central star is similar to unobscured stars with MK spectral types of O5–O6.5. If this star is on the zero-age main sequence, then the derived spectrophotometric distance is considerably smaller than previous estimates. The Lyman continuum luminosity of the cluster is a few times that of the Trapezium. The slope of the K-band luminosity function is similar to that for the Trapezium cluster and significantly steeper than that for the massive star cluster in M17 or in the Arches cluster near the Galactic center.

We present new velocity-resolved Fabry-Perot images in [S II] λ6731 and Hα of the blueshifted portion of the protostellar outflow L1551. These new data isolate the line emission from the reflected continuum and make it possible to visualize the kinematics of the shock waves in the outflow clearly for the first time on large scales with subarcsecond spatial resolution. Velocity images of the L1551 jet confirm that a fainter, slower jet lies a few arcseconds below the main jet. Emission from the main jet decreases sharply in radial velocity and the emission-line width increases suddenly as the jet encounters bright knot 3, in agreement with bow shock models. This knot must move into previously ejected material to account for the observed radial velocities, supporting the idea that shocks in Herbig-Haro (HH) flows form as the result of variable velocity ejections from the embedded protostar. However, a velocity gradient observed along the entire edge of the bow shock is spatially larger than expected if the bow shock alone were responsible for all the line emission. Deviations from the simple model are most easily explained if a Mach disk alters the emission and kinematics within the bow shock region, though a precursor to the bow shock is an alternate possibility. The spatial distribution of radial velocities and emission-line widths across HH 29 implies that this object is a slower portion of the outflow currently being overtaken by faster material. New proper-motion images of HH 29 independently confirm this result. The large-scale velocity structure of the L1551 outflow is complex but can generally be understood if faster material drives shock waves into slower material around the edge of a cavity. A striking circular feature whose center lies near the intersection of the axis of the bright jet and the cavity resembles similar structures in the Orion Nebula and could define a hole through which a fast jet has penetrated. The velocity structure along a string of HH objects to the southwest of HH 29 is consistent with the recent result of Devine et al. that L1551 NE drives this portion of the outflow. Linear features that cross the L1551 flow may be associated with separate, unrelated jets from the HH 30 region or elsewhere within L1551.

We present a spectral analysis of 30 T Tauri stars observed with the Hamilton echelle spectrograph over more than a decade. One goal is to test magnetospheric accretion model predictions. Observational evidence previously published supporting the model, such as emission-line asymmetry and a high frequency of redshifted absorption components, are considered. We also discuss the relation between different line-forming regions and search for good accretion rate indicators. In this work we confirm several important points of the models, such as the correlation between accretion and outflow, broad emission components that are mostly central or slightly blueshifted, and only the occasional presence of redshifted absorption. We also show, however, that the broad emission components supposedly formed in the magnetospheric accretion flow only partially support the models. Unlike the predictions, they are sometimes redshifted and are mostly found to be symmetric. The published theoretical profiles do not have a strong resemblance to our observed ones. We emphasize the need for accretion models to include a strong turbulent component before their profiles will match the observations. The effects of rotation, as well as the outflow components, will also be needed to complete the picture.

The Robotic Optical Transient Search Experiment I (ROTSE-I) experiment has generated CCD photometry for the entire northern sky in two epochs nightly since 1998 March. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first results of a search for periodic variable stars derived from ROTSE-I observations. Variable identification, period determination, and type classification are conducted via automatic algorithms. In a set of nine ROTSE-I sky patrol fields covering roughly 2000 deg², we identify 1781 periodic variable stars with mean magnitudes between m_v = 10.0 and m_v = 15.5. About 90% of these objects are newly identified as variable. Examples of many familiar types are presented. All classifications for this study have been manually confirmed. The selection criteria for this analysis have been conservatively defined and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will include more than 32,000 periodic variable stars.

The 14.4 day period eclipsing binary SS Lacertae in the open cluster NGC 7209 is unusual in that the eclipses stopped completely about the middle of this century, giving rise to considerable speculation as to the cause. Disruption of the binary through a collision with another member of the cluster and, more likely, gravitational perturbations from a third star in the system have been proposed to explain the phenomenon. We present here the results of our intensive radial velocity monitoring of the object, which show clearly that there is indeed a third star in the system in a slightly eccentric orbit with a period of about 679 days. We also reanalyze the historical light curves of SS Lac to determine the properties of the system early in the century. We discuss the implications of our findings in terms of changes in the inclination angle of the close pair. An analysis of available times of eclipse for SS Lac together with the new velocity information has also revealed apsidal motion in the system, = 00137 cycle^-1, corresponding to an apsidal period of about 1000 yr. We argue that the third star is probably responsible for this effect also.

We present coordinated Extreme Ultraviolet Explorer (EUVE) and Rossi X-Ray Timing Explorer (RXTE) target of opportunity observations of the 1998 August high state of AM Herculis. The EUVE light curve sampled phases ∼0.0–0.3 and ∼0.5–0.8 and showed AM Her to be uncharacteristically bright during these phases but lacking the expected counts at phase ≈0.5, possibly because of absorption. The RXTE proportional counter array (PCA) 2–20 keV and HEXTE 15–60 keV light curves clearly show the 3.09 hr orbital modulation. No correlations between the soft and hard bands predicted in the simple one-pole accretion model were detected in the ≈1600 s of true, simultaneous EUVE and RXTE data. Quasi-periodic oscillations (QPOs) of 1–3 s found in optical data for several magnetic cataclysmic variables were not detected in PCA high time resolution data of AM Her with a 90% confidence upper limit for the rms amplitude of 3%. No significant QPOs were detected in the 10s–100s of seconds range with EUVE or RXTE. The EUVE spectrum was well fitted with a 19 eV blackbody. The phase-averaged PCA spectrum was well fitted with a 17.6 keV thermal bremsstrahlung (TB) continuum plus a broad iron line centered at 6.5 keV and absorption edge at 9.2 keV. HEXTE detections extended to 100 keV and preferred a highly absorbed TB continuum with temperature of 37 keV. The thermal bremsstrahlung plus emission-line (E_line = 6.7 keV) and edge model fit to PCA spectra as a function of phase could be described as a single TB temperature of 18.9 ± 0.2 keV with a variable column density of 7.1–53.5 × 10²¹ cm^-2 and did not require more complex continuum models. The ratio of EUV to X-ray luminosity showed no soft excess and was more in accord with the simple one-pole model than previous observations.

We report new radial velocity measurements made with the Harvard-Smithsonian Center for Astrophysics Digital Speedometers of the F-type double-lined eclipsing binary EI Cep (P = 8.4 days), in which the secondary is a metallic-line star. Using these and previous measurements from the literature, we redetermine the masses of the components to a precision of 0.4%, 4 to 5 times better than previously achieved. We also reanalyze UBV light curves for the system from the literature with modern methods to determine the radii and the effective temperatures. The resulting stellar parameters are M_A = 1.7716 ± 0.0066 M_⊙, M_B = 1.6801 ± 0.0062 M_⊙, R_A = 2.896 ± 0.048 R_⊙, R_B = 2.329 ± 0.044 R_⊙, T = 6750 ± 100 K, and T = 6950 ± 100 K. Further properties of the system are given in this paper. Both components are near the end of their main-sequence evolution, and a comparison of the observational results with theoretical evolutionary models gives excellent agreement with models using two somewhat different treatments of overshooting from the convective cores (but not with standard models). An age of 1.4 Gyr is derived for the roughly solar metallicity indicated by our low signal-to-noise ratio spectra. A precise spectroscopic determination of the metallicity of EI Cep would be very useful to place even tighter constraints on the models. The measured projected rotational velocities of the stars (13 and 17 km s^-1 for EI Cep A and B, respectively) are very close to, but not quite synchronized with, the orbital motion. This is in agreement with predictions from current tidal theories, which, however, also predict that orbital circularization has not yet occurred, although we detect no significant eccentricity.

Spectral energy distributions from recent model atmospheres covering a broad range of temperature and metallicity are used to synthesize colors for low-mass stars. Color-color diagrams can therefore be prepared in which each point corresponds to a particular combination of T_eff and M/H. Comparison of observed with synthetic colors yields estimates of those parameters generally, but not necessarily under the assumption that log g_cgs = 5. Results are presented for twelve stars of astrophysical interest.

In situ measurements of the light locally scattered by cometary dust, as well as the local dust spatial density, are only available for one comet, 1P/Halley. These data were returned from the Optical Probe Experiment (OPE) and the Dust Impact Detection System (DID) aboard the European Space Agency spacecraft Giotto. Due to a detailed calibration of OPE at the time of Giotto's encounter with comet 26P/Grigg-Skjellerup, as well as improved analysis techniques, the similarities and correlation between the OPE and DID data sets can be reassessed. In this paper, we utilize this opportunity to compare these unique observations with a cometary coma dynamical model. Our results indicate that, to first order, the data can be fitted by a coma model that incorporates a grain size distribution index (at the nucleus), which need not be time dependent. Further, we find that the general shape of both the OPE and DID data can largely be explained by Keplerian dynamics alone, without recourse to fragmentation processes. The model is used to constrain the cometary dust bulk density, and a likely range of 50 < ρ < 500 kg m^-3 is found, although a value of ρ = 100 kg m^-3 is favored. In addition, the corresponding favored solution of the geometric albedo A_p(α = 0°) is found to be 0.04. Within the quoted density range, the ratio between density and albedo remains constant at ρ/A_p(α = 0°) ≈ 2500 kg m^-3. The modeling also indicates that the grain size distribution power index at the nucleus is constant (in the range 10^-12 < m < 10^-3 kg) and has a likely value k = -2.6 ± 0.2 (i.e., cumulative mass distribution index b = -0.53 ± 0.1).

Quasi satellites are bodies in a particular configuration of a 1 : 1 mean motion resonance, one in which they librate about the longitude of their associated planet. We investigate numerically the stability of such orbits around the giant planets of our solar system. We find that test particles can remain on quasi-satellite orbits around Uranus and Neptune for times up to 10⁹ yr in some cases, though only at low inclinations relative to their accompanying planet and over a restricted range of heliocentric eccentricities. These stable areas are well outside the traditional satellite region. Based on these results, we conclude that a primordial population of such objects may still exist in our solar system.

This paper presents 1155 accurate equatorial positions for the outer planets Uranus, Neptune, and Pluto and 17 satellites of Jupiter–Neptune. Additional positions for Jupiter, Saturn, Uranus, and Neptune can be determined from the planetocentric motions of their satellites given in this paper. All the positions were determined in the International Celestial Reference Frame (ICRF), the IAU standard for all future astrometric reductions, from CCD observations taken with the Flagstaff Astrometric Scanning Transit Telescope (FASTT) and reduced differentially using the ACT reference stars. The methods used to determine these positions are fully described. Accuracies of ±008 to ±025 were obtained in each coordinate, depending on the signal-to-noise ratio observed for each object. In many cases, planets and satellites were imaged in the same CCD field of view, thereby giving excellent relative astrometry (±004) for well-exposed images. Moreover, 424 older FASTT positions determined in 1995–1997 for Uranus, Neptune, and Pluto were converted to the ICRF and are given also in this paper. When FASTT positions are compared with modern Jet Propulsion Laboratory (JPL) ephemerides for the planets and satellites considered herein, there is generally good agreement (less than 005) between observation and theory. For the planets, FASTT observations and DE405 ephemerides all agree to better than 005 and, in most cases, less than 003 when mean differences are formed. There are some exceptions for the satellites. Namely, the ephemerides for the outer satellites of Jupiter and Saturn considered in this paper (Himalia, Elara, Pasiphae, and Phoebe) and the Uranian satellites Titania and Oberon probably need improvement as indicated from the FASTT observational data. The former show systematic trends when (FASTT - JPL ephemeris) coordinate differences are plotted against either coordinate position or orbital phase, and the latter show a possible offset between the right ascension of Uranus and those of the two satellites. Future FASTT observations of these planets and satellites are planned in the coming years.

We have completed a search for candidates for occultations by Pluto and Charon over the years 2000 through 2009. Using the DE405 ephemeris and the Plu006 orbit model for Pluto and Charon, we searched for stars that lie within 10 of their respective apparent motions. The images used were CCD strip scans taken at the George R. Wallace Astrophysical Observatory, reaching a depth of 15th to 17th magnitude. As Pluto-Charon enter the Galactic plane, a dramatic increase in the number of stellar appulses will occur. We identify 486 appulses by Pluto and 479 appulses by Charon over this period, with the frequency peaking in 2007. Further astrometry will be necessary to determine which of these appulses will have minimum separations small enough (less than 034 for Pluto or 031 for Charon) to produce an occultation visible from Earth.

The Flagstaff Astrometric Scanning Transit Telescope was used to scan the sky in search of occultation candidates through which the Centaur 10199 Chariklo (formerly 1997 CU₂₆) will pass in 1999–2005. Positions for 96,397 stars (magnitudes 7.5 < V < 17.3) were determined using differential reductions to the ACT catalog of star positions and proper motions, and accuracies of ±30 mas were achieved for well-exposed images. The ephemeris for 10199 Chariklo was improved by including the new positions presented in this paper and, second, by correcting older positions taken from the literature for systematic errors. During this time period, 10199 Chariklo passes within 2'' of 117 of these stars, which are identified as occultation candidates in this paper. Among these, 28 candidates have magnitudes V < 15.0, making them the best choices for observing programs using portable telescopes. Circumstances for each occultation are given. Because the angular radius of 10199 Chariklo is only about 15 mas, the astrometry provided in this paper is only accurate enough for identifying possible occultation events and is not sufficient for predicting individual shadow paths across the surface of Earth, although representative cases are given. Last-minute astrometry with a large-aperture telescope will be needed to refine each prediction.

Color-temperature relations and bolometric corrections in the Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer F1110W, F160W, and F222M and the Wide Field Planetary Camera 2 F439W, F555W, and F814W photometric systems, using two different sets of model atmospheres, have been derived. This database of homogeneous, self-consistent transformations between the theoretical and observational planes also allows combinations of visual and infrared quantities without any further transformation between the two different photometric systems. The behavior of the inferred quantities under variation of the stellar parameters, the adopted model atmospheres, and the instrumental configurations is investigated. Suitable relations to transform colors and bolometric corrections from HST to ground-based photometric systems are also provided.