Table of contents

We report 2868 new multiobject spectroscopic measurements of galaxy redshifts in an area roughly 12° × 6° (right ascension × declination) centered on the Shapley Supercluster (SSC). These correspond to 2627 different galaxies. Including other measurements reported in the literature, the total number of galaxies with measured redshifts in a 19° × 16° area centered on the supercluster now reaches 5090. Of these, 2949 lie in the redshift range 9000–18,000 km s^-1, which we tentatively identify with the SSC. This unprecedented sample allows a quite detailed qualitative morphological study of the SSC. Based on the three-dimensional distribution of galaxies and clusters of galaxies, we identify several subcondensations of the supercluster, as well as walls and filaments connecting them. We also find another supercluster in the background, at redshift ∼23,000 km s^-1.

We present the first application of a spherical collapse model to a supercluster of galaxies. Positions and redshifts of ∼3000 galaxies in the Shapley supercluster (SSC) are used to define velocity caustics that limit the gravitationally collapsing structure in its central part. This is found to extend at least to 8 h^-1 Mpc of the central cluster, A3558, enclosing 11 ACO clusters. Infall velocities reach ∼2000 km s^-1. Dynamical models of the collapsing region are used to estimate its mass profile. An upper bound on the mass, based on a pure spherical infall model, gives M(<8 h^-1 Mpc) ≲ 1.3 × 10¹⁶h^-1M_⊙ for an Einstein–de Sitter (critical) universe and M(<8 h^-1 Mpc) ≲ 8.5 × 10¹⁵h^-1M_⊙ for an empty universe. The Diaferio & Geller model, based on estimating the escape velocity, gives a significantly lower value, M(<8 h^-1 Mpc) ≈ 2.1 × 10¹⁵h^-1M_⊙, very similar to the mass Geller et al. found around the Coma cluster by the same method and comparable to or slightly lower than the dynamical mass in the virialized regions of clusters enclosed in the same region of the SSC. In both models, the overdensity in this region is substantial, but it is far from the value required to account for the peculiar motion of the Local Group with respect to the cosmic microwave background.

Color gradients in elliptical galaxies in distant clusters (z = 0.37–0.56) are examined by using the archival deep imaging data of the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby elliptical galaxies, but predict significantly different gradients at redshift larger than ∼0.3. Comparison of the observed gradients and the model gradients favors the metallicity gradient much more than the age gradient as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field elliptical galaxies by using those at redshift 0.1–1.0 in the Hubble Deep Field North by Tamura et al. Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.

This paper presents photometric redshifts for 431 Abell clusters imaged as part of the Palomar Abell Cluster Optical Survey, of which 236 are new redshifts. We have obtained moderately deep, three-band (Gunn gri) imaging for this sample at the Palomar Observatory 60 inch (1.5 m) telescope, as part of the photometric calibration of the Digitized Second Palomar Sky Survey. Our data acquisition, reduction, and photometric calibration techniques are described, and photometric accuracy and consistency are demonstrated. An empirical redshift estimator is presented, utilizing background-corrected median g-r colors and mean g magnitudes for the ensemble of galaxies in each field. We present photometric redshift estimates for the clusters in our sample with an accuracy of σ_z = 0.038. These redshift estimates provide checks on single-galaxy cluster redshifts, as well as distance information for studies of the Butcher-Oemler effect, luminosity functions, mass-to-light ratios, and many other projects.

We investigate the rich cluster Abell 2029 (z ∼ 0.08) using optical imaging and long-slit spectral observations of 52 disk galaxies distributed throughout the cluster field. No strong emission-line galaxies are present within ∼400 kpc of the cluster center, a region largely dominated by the similarly shaped X-ray and low surface brightness optical envelopes centered on the giant cD galaxy. However, two-thirds of the galaxies observed outside the cluster core exhibit line emission. Hα rotation curves of 14 cluster members are used in conjunction with a deep I-band image to study the environmental dependence of the Tully-Fisher relation. The Tully-Fisher zero point of Abell 2029 matches that of clusters at lower redshifts, although we do observe a relatively larger scatter around the Tully-Fisher relation. We do not observe any systematic variation in the data with projected distance to the cluster center. We see no environmental dependence of Tully-Fisher residuals, R-I color, Hα equivalent width, or the shape and extent of the rotation curves.

We present imaging and spectropolarimetric observations of the ultraluminous infrared galaxy IRAS P09104+4109 using the Keck 10 m Telescope. We detect the clear presence of broad Hβ, Hγ, and Mg II λ2800 emission lines in the polarized flux spectra of the nucleus and of an extranuclear emission region ∼4'' away, confirming the presence of a hidden central quasar. The polarization of the broad Mg II emission line is high (∼29%), consistent with the remarkably high polarization (∼30%–40%) observed in the extended continuum emission. This indicates that the off-nuclear continuum is dominated by light scattered from the hidden quasar, most probably by dust mixed with the line-emitting gas. The high polarizations, combined with the "foreshortened" morphology of the polarized brightness distribution, allow us to constrain the scattering biconical structure to be at inclination i ≈ 50° with a half-opening cone angle θ_c ≈ 40°.

The narrow emission lines are polarized in a stratified fashion, with the high-ionization lines ([O III], [Ne V], [Fe VII]) being polarized 0.7%–1.7% and [O II] essentially unpolarized. The line polarizations are positively correlated with critical density, ionization potential, and velocity width of the emission lines. This indicates that, as is the case with the narrow-line radio galaxies, which also often contain powerful quasars, the narrow emission line region may be partially shadowed by the putative torus, with the higher ionization lines originating closer to the nucleus.

One notable characteristic of the extranuclear knot is that all species of Fe are markedly absent in its spectrum, while they appear prominently in the nucleus. In addition, narrow Mg II is observed to be much weaker than predicted by ionization models. Our favored interpretation is that there is a large amount of dust in the extranuclear regions, allowing gaseous refractory metals to deposit. Near the nucleus, dust is destroyed in the strong radiation field of the quasar, inhibiting metal depletion onto grains. The extended emission regions are most likely material shredded from nearby cluster members and not gas condensed from the cooling flow or expelled from the obscured quasar.

The higher temperature inferred from [O III] lines compared to that from [N II] and the general better agreement with models of line ratios, especially [O III] λ5007/λ4363 and He II/Hβ, provide strong evidence for matter-bounded clouds in addition to ionization-bounded clouds in the narrow-line region. Ionization by pure velocity shocks can be ruled out. Shocks with photoionizing precursors may be present but are probably not a dominant contributor to the energy input.

We present a catalog of extremely red objects (EROs) discovered using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) parallel imaging database and ground-based optical follow-up observations. Within an area of 16 arcmin², we detect 15 objects with R-F160W > 5 and F160W < 21.5. We have also obtained K-band photometry for a subset of the 15 EROs. All of the R-F160W selected EROs imaged at K-band have R-K > 6. Our objects have F110W-F160W colors in the range 1.3–2.1, redder than the cluster elliptical galaxies at z ∼ 0.8 and nearly 1 mag redder than the average population selected from the F160W images at the same depth. In addition, among only 22 NICMOS pointings, we detected two groups or clusters in two fields; each contains three or more EROs, suggesting that extremely red galaxies may be strongly clustered. At bright magnitudes with F160W < 19.5, the ERO surface density is similar to what has been measured by other surveys. At the limit of our sample, F160W = 21.5, our measured surface density is 0.94 ± 0.24 arcmin^-2. Excluding the two possible groups or clusters and the one apparently stellar object reduces the surface density to 0.38 ± 0.15 arcmin^-2.

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths—spiral arms, disks, rings, and bright knots of emission—are also seen in the mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 μm. In addition, the maps are quite similar at 6.75 and 15 μm, except for a few cases where a central starburst leads to lower I_ν(6.75 μm)/I_ν(15 μm) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I_ν(6.75 μm)/I_ν(15 μm) shows a distinct trend with the far-infrared color I_ν(60 μm)/I_ν(100 μm). The quiescent galaxies in our sample [I_ν(60 μm)/I_ν(100 μm) ≲ 0.6] show I_ν(6.75 μm)/I_ν(15 μm) near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of mid-infrared colors. The galaxies are mostly well resolved in these maps: almost half of them have <10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the flux at 4400 Å near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 μm (15 μm), comparable to observations of the diffuse quiescent regions of the Milky Way.

We present the first ground-based U' (λ_central = 3410 Å, Δ_λ = 320 Å) images of ultraluminous infrared galaxies (ULIGs). Two samples were observed: (1) "warm" ULIGs with mid-infrared colors characteristic of active galactic nuclei (f_{25 μm}/f_{60 μm} > 0.2), which are believed to represent a critical transition phase between cooler ULIGs and optically selected QSOs according to a previously proposed evolutionary model, and (2) the complementary "cool" ULIGs (f_{25 μm}/f_{60 μm} < 0.2), which in the evolutionary model are the progenitors of warm ULIGs and which have many characteristics associated with active star formation. Although in some cases there is also emission identified with an active galactic nucleus, the U' emission originates primarily in massive young stars and as such allows a direct examination of the sites of recent high-mass star formation. Strong U' emission (median total M_U = -20.8) is seen in all systems, and in some cases the extended tidal features (both the smooth stellar distribution and compact star-forming features) contribute up to 60%–80% of the total flux. The star-forming regions in both samples are found to have ages based on spectral synthesis models in the range 10–100 Myr, and most differences in color between them can be attributed to the effects of dust reddening. Additionally, it is found that star formation in compact knots in the tidal tails is most prominent in those ULIGs that have double nuclei, suggesting that the star formation rate in the tails peaks prior to the actual coalescence of the galaxy nuclei and diminishes quickly thereafter. Similar to results at other wavelengths, the observed star formation at U' can account for only a small fraction of the known bolometric luminosity of the ULIGs. Azimuthally averaged radial light profiles at U' are characterized by a Sérsic law with index n = 2, which is intermediate between an exponential disk and an r^-1/4 law and closely resembles the latter at large radii. The implications of this near-ultraviolet imaging for optical/near-infrared observations of high-redshift counterparts of ULIGs are discussed.

We present new observations of Hα emission in the Hickson compact group 18 (HCG 18), obtained with a scanning Fabry-Perot interferometer. The velocity field does not show motions of individual group members but instead a complex common velocity field for the whole group. The gas distribution is very asymmetric, with clumps of maximum intensity coinciding with the optically brightest knots. Comparing Hα and H I data, we conclude that HCG 18 is not a compact group but instead a large irregular galaxy with several star-forming clumps.

Hubble Space Telescope (HST) images of the galaxies NGC 2207 and IC 2163 show star formation and dust structures in a system that has experienced a recent grazing encounter. Tidal forces from NGC 2207 compressed and elongated the disk of IC 2163, forming an oval ridge of star formation along a caustic where the perturbed gas rebounded after its inward excursion. Gas flowing away from this ridge has a peculiar structure characterized by thin parallel dust filaments transverse to the direction of motion. The filaments become thicker and longer as the gas approaches the tidal arm. Star formation that occurs in the filaments consistently lags behind, as if the exponential disk pressure gradient pushes outward on the gas but not on the young stars. Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction. The arms stretch out into parallel filaments as the tidal tail forms. A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction. Gas at small-to-intermediate radii along this edge flows back toward the galaxy, while elsewhere in the tidal arm, the gas flows outward.

A spiral arm of NGC 2207 that is backlit by IC 2163 is seen with HST to contain several parallel, knotty filaments spanning the full width of the arm. These filaments are probably shock fronts in a density wave. The parallel structure suggests that the shocks occur in several places throughout the arm, or that the interarm gas is composed of spiral-like wisps that merge together in the arms. Blue clusters of star formation inside the clumps of these dust lanes show density-wave triggering in unprecedented detail. The star formation process seems to be one of local gravitational collapse, rather than cloud collisions. Spiral arms inside the oval of IC 2163 have a familiar geometry reminiscent of a bar, although there is no obvious stellar bar. The shape and orientation of these arms suggest they could be the result of inner Lindblad resonance–related orbits in the cos 2θ tidal potential that formed the oval. Their presence suggests that tidal forces alone may initiate a temporary nuclear gas flow and eventual starburst without first forming a stellar bar. Several emission structures resembling jets or conical flows that are 100–1000 pc long appear in these galaxies. In the western arm of NGC 2207, there is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip, and there is a conical emission nebula of the same length that points away from the cluster in the other direction. This region also coincides with a nonthermal radio continuum source that is ∼1000 times the luminosity of Cas A at λ = 20 cm. Surrounding clusters in arclike patterns may have been triggered by enormous explosions.

We report detailed evidence for multiple merger, extended massive star formation, galactic wind, and circular/noncircular motions in the luminous infrared galaxy NGC 3256, based on observations of high-resolution imaging (Hubble Space Telescope, ESO NTT), and extensive spectroscopic data (more than 1000 spectra, collected at Estación Astrofísica de Bosque Alegre, Complejo Astronómico el Leoncito, Cerro Tololo InterAmerican Observatory, and IUE observatories).

We find in a detailed morphological study (resolution ∼15 pc) that the extended massive star formation process detected previously in NGC 3256 shows extended triple asymmetrical spiral arms (r ∼ 5 kpc), emanating from three different nuclei. The main optical nucleus shows a small spiral disk (r ∼ 500 pc), which is a continuation of the external one and reaches the very nucleus. The core shows blue elongated structure (50 pc × 25 pc) and harbors a blue stellar cluster candidate (r ∼ 8 pc). We discuss this complex morphology in the framework of an extended massive star formation driven by a multiple merger process (models of Hernquist et al. and Taniguchi et al.).

We study the kinematics of this system and present a detailed Hα velocity field for the central region (40'' × 40''; r_max ∼ 30'' ∼ 5 kpc), with a spatial resolution of 1'' and errors of ±15 km s^-1. The color and isovelocity maps show mainly (1) a kinematic center of circular motion with "spider" shape, located between the main optical nucleus and the close (5'') mid-IR nucleus and (2) noncircular motions in the external parts. We obtained three "sinusoidal rotation curves" (from the Hα velocity field) around position angle (P.A.) ∼55°, ∼90°, and ∼130°. In the main optical nucleus we found a clear "outflow component" associated with galactic winds plus an "inflow radial motion." The outflow component was also detected in the central and external regions (r ≤ 5–6 kpc). The main axis of the inflow region (P.A. ∼ 80°) is practically perpendicular to the ouflow axis (at P.A. ∼ 160°).

We analyze in detail the physical conditions in the giant H II regions located in the asymmetric spiral arms, the two main optical nuclei, and the outflow component (using long-slit spectroscopy, plus standard models of photoionization, shocks, and starbursts). We present four detailed emission-line ratios (N II/Hα, S II/Hα, S II/S II), and FWHM (Hα) maps for the central region (30'' × 30''; r_max ∼ 22'' ∼ 4 kpc), with a spatial resolution of 1''. In the central region (r ∼ 5–6 kpc) we detected that the nuclear starburst and the extended giant H II regions (in the spiral arms) have very similar properties, i.e., high metallicity and low-ionization spectra, with T_eff = 35,000 K, solar abundance, a range of T_e ∼ 6000–7000 K, and N_e ∼ 100–1000 cm^-3. The nuclear and extended outflow shows properties typical of galactic wind/shocks, associated with the nuclear starburst. We suggest that the interaction between dynamical effects, the galactic wind (outflow), low-energy cosmic rays, and the molecular+ionized gas (probably in the inflow phase) could be the possible mechanism that generate the "similar extended properties in the massive star formation, at a scale of 5–6 kpc!"

We have also studied the presence of the close merger/interacting systems NGC 3256C (at ∼150 kpc, ΔV = -100 km s^-1) and the possible association between the NGC 3256 and 3263 groups of galaxies. In conclusion, these results suggest that NGC 3256 is the product of a multiple merger, which generated an extended massive star formation process with an associated galactic wind plus a nuclear inflow. Therefore, NGC 3256 is another example in which the relation between mergers and extreme starburst (and the powerful galactic wind, "multiple" Type II supernova explosions) play an important role in the evolution of galaxies (the hypothesis of Rieke et al., Joseph et al., Terlevich et al., Heckman et al., and Lípari et al.).

We present 6 and 4 cm radio images of the inner disks of the nearby merger pair NGC 4038 and NGC 4039 at resolutions between ∼ 1'' and 26, or ∼ 100 and 260 pc. We detect numerous compact radio sources embedded in more diffuse radio emission; the compact radio sources produce ∼ 12% and ∼ 25% of the 6 and 4 cm radio emission in the system, respectively. The strongest radio continuum emission occurs between the galaxies, at an optically unremarkable location near but not coincident with an extremely red cluster detected by Whitmore & Schweizer. The radio peak is at a location of intense star formation (as detected in the mid-IR) and of dense molecular gas. We identify 115 individual compact 6 cm radio sources and 63 individual 4 cm sources in the system, to a limiting luminosity of ∼ 2 × 10¹⁸ W Hz^-1, or ∼ 4 times the luminosity of Cas A; 90% of the 6 cm sources and 67% of the 4 cm sources are unresolved in these observations. Of the strongest sources, for which the flux densities are large enough to measure spectral indices with reasonable errors, one-third have nominally flat radio spectra, indicating that they are dominated by thermal radio emission from H II regions, and two-thirds have nominally steep spectra dominated by nonthermal emission from supernova remnants. For the compact sources dominated by thermal emission, we derive typical masses of 10⁴ to 10⁵M_⊙ in massive young stars. Using reasonable assumptions about source sizes, we estimate associated ionized gas masses of 10³ to 10⁴M_⊙. A comparison with the ∼ (2–5) × 10³M_⊙ typical optical clusters detected recently by Whitmore et al. suggests that the thermal radio sources detected here may be composed of tens of these typical young star clusters or else that they are more like the individual massive 10⁵–10⁶M_⊙ Whitmore et al. clusters. The strongest thermal radio source is associated with none of the Whitmore et al. clusters but is coincident with strong CO and 15 μm emission, implying that massive star clusters are still enshrouded by dust in this region. The second strongest thermal source is near several optical clusters, suggesting that these clusters are just now emerging from their birth cloud and are clearing away the surrounding material. Estimates of stellar ages and sound speeds indicate that we do not see star formation propagating in this system; rather, the star formation sequence is probably related to more global properties of the interaction. From the steep-spectrum sources, we derive a total system supernova rate of 0.2–0.3 yr^-1, about an order of magnitude larger than that expected from the currently observed O star population. This suggests that a large number of O stars may have formed in the system in a short-lived burst, less than 10⁶ yr in duration and about 3–4 Myr ago.

We report on large-scale regular morphological patterns found in the radio jet of the nearby radio galaxy NGC 6251. Investigating morphological properties of this radio jet from the nucleus to a radial distance of ∼300'' (≈140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, & Willis, we find three chains, each of which consists of five radio knots. We also find that eight radio knots in the first two chains consist of three small subknots (the triple-knotty substructures). We discuss the observational properties of these regular morphological patterns.

BVRI and Hα imaging and long-slit optical spectroscopic data are presented for four morphologically normal and relatively isolated Sa galaxies, NGC 3626, NGC 3900, NGC 4772, and NGC 5854. VLA H I synthesis imaging is presented for the first three objects. In all four galaxies, evidence of kinematic decoupling of ionized gas components is found in the long-slit spectroscopic data; the degree and circumstances of the distinct kinematics vary from complete counterrotation of all of the gas from all of the stars (NGC 3626) to nuclear gas disks decoupled from the stars (NGC 5854) to anomalous velocity central gas components (NGC 3900 and NGC 4772). In the three objects mapped in H I, the neutral gas extends far beyond the optical radius, R_{H I}/R₂₅ ≥ 2. In general, the H I surface density is very low, and the outer H I is patchy and asymmetric (NGC 3900) or found in a distinct ring, exterior to the optical edge (NGC 3626 and NGC 4772). While the overall H I velocity fields are dominated by circular motions, strong warps are suggested in the outer regions by bending of the minor axis isovelocity contours (NGC 3900) and/or systematic shifts in position angle between inner and outer rings (NGC 3626 and NGC 4772). In the interior, coincidence is found between the Hα and H I in rings, sometimes partial and crisscrossed by dust lanes. Optical imaging is also presented for NGC 4138 previously reported by Jore et al. to show counterrotating stellar components. The multiwavelength evidence is interpreted in terms of the kinematic "memory" of past minor mergers in objects that otherwise exhibit no morphological signs of interaction.

We propose a limit-cycle scenario of star formation history for any morphological type of spiral galaxy. It is known observationally that an early-type spiral sample has a wider range of present star formation rate (SFR) than a late-type sample. This tendency is understood in the framework of the limit-cycle model of the interstellar medium (ISM), in which the SFR changes cyclically in accordance with the temporal variation of the mass fraction of the three ISM components. When the limit-cycle model of the ISM is applied, the amplitude of variation of the SFR is expected to change with the supernova (SN) rate. Observational evidence indicates that early-type spiral galaxies should show smaller rates of present SNe than late-type ones. Combining this evidence with the limit-cycle model of the ISM, we predict that the early-type spiral galaxies will show larger amplitudes in their SFR variation than the late types. Indeed, this prediction is consistent with the observed wider range of the SFR in the early-type sample than in the late-type sample. Thus, in the framework of the limit-cycle model of the ISM, we are able to interpret the difference in the amplitude of SFR variation among the morphological classes of spiral galaxies.

The giant elliptical galaxy M86 in Virgo has a ∼28 kpc long dust trail inside its optical halo that points toward the nucleated dwarf elliptical galaxy VCC 882. The trail seems to be stripped material from the dwarf. Extinction measurements suggest that the ratio of the total gas mass in the trail to the blue luminosity of the dwarf is about unity, which is comparable to such ratios in dwarf irregular galaxies. The ram pressure experienced by the dwarf galaxy in the hot gaseous halo of M86 was comparable to the internal gravitational binding energy density of the presumed former gas disk in VCC 882. Published numerical models of this case are consistent with the overall trail-like morphology observed here. Three concentrations in the trail may be evidence for the predicted periodicity of the mass loss. The evaporation time of the trail is comparable to its age obtained from the relative speed of the galaxies and the trail length. Thus the trail could be continuously formed from stripped replenished gas if the VCC 882 orbit is bound. However, the high gas mass and the low expected replenishment rate suggest that this is only the first stripping event. Implications for the origin of nucleated dwarf ellipticals are briefly discussed.

The central regions of the early-type disk galaxies NGC 524 and NGC 6340 have been investigated with the Multipupil Field Spectrograph at the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We confirm the existence of chemically distinct stellar nuclei in these galaxies, which had been claimed earlier. The metallicity differences that we have found between the nuclei and the bulges, 0.5–1.0 kpc from the centers, reach 0.5–0.6 dex. Both nuclei are magnesium overabundant, but the bulges have different magnesium-to-iron ratios: it is solar in NGC 6340 and the same as the nuclear one in NGC 524. Kinematical and morphological analyses reveal the existence of inclined central disks in these galaxies. In NGC 524 the central disk consists of stars, dust, and ionized gas; its extension may be as large as up to R ≈ 3 kpc, and it is inclined by some 20° to the global galactic plane. In NGC 6340 only a gaseous polar disk is detected, the extension of which does not exceed R ≈ 500 pc.

Here we present the size and luminosity versus velocity dispersion empirical correlations for the giant H II regions in the large irregular galaxy NGC 4449. We show that correlations only hold for nebulae with a surface brightness higher than 2 × 10³⁵ ergs s^-1 pc^-2 in Hα and with a supersonic single-line Gaussian profile. The exponents of the fits are consistent with virial mechanisms. A comparison with the results from other studies that have only used first-ranked giant H II regions from a variety of star-forming galaxies is also given.

We present the results from an optical study of the stellar and star formation properties of NGC 925 using the WIYN 3.5 m telescope. Images in B, V, R, and Hα reveal a galaxy that is fraught with asymmetries. From isophote fits we discover that the bar center is not coincident with the center of the outer isophotes or with the dynamical center (from Paper I in this series). Cuts across the spiral arms reveal that the northern arms are distinctly different from the southern arm. The southern arm not only appears more coherent, but the peaks in stellar and Hα emission are found to be coincident with those of the H I distribution, while no such consistency is present in the northern disk. We also examine the gas surface density criterion for massive star formation in NGC 925, and we find that its behavior is more consistent with that for irregular galaxies than with late-type spirals. In particular, star formation persists beyond the radius at which the gas surface density falls below the predicted critical value for star formation for late-type spirals. Such properties are characteristic of Magellanic spirals but are present at a less dramatic level in NGC 925, a late-type spiral.

The properties of globular clusters in dwarf galaxies are key to understanding the formation of globular cluster systems and in particular in verifying scenarios in which globular cluster systems of larger galaxies formed (at least partly) from the accretion of dwarf galaxies. Here, we revisit the globular cluster system of the dE,N galaxy NGC 3115 DW1—a companion of the nearby S0 galaxy NGC 3115—adding Keck/LRIS spectroscopy and Hubble Space Telescope (HST) Wide Field Planetary Camera 2 imaging to previous ground-based photometry. Spectra for seven globular clusters reveal normal abundance ratios with respect to the Milky Way and M31 clusters, as well as a relatively high mean metallicity ([Fe/H] ≈ -1.0 ± 0.1 dex). Crude kinematics indicate a high velocity dispersion within 10 kpc that could be caused either by dark matter–dominated outer regions or by the stripping of outer globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass out to 3 and 10 kpc lies between 1 × 10¹⁰ and 1 × 10¹¹M_⊙ and 2 × 10¹⁰ and 4 × 10¹¹M_⊙, respectively, depending on the mass estimator used and the assumptions on cluster orbits and systemic velocity. The HST imaging allows measurement of sizes for two clusters, returning core radii around 2.0 pc, similar to the sizes observed in other galaxies. Spectroscopy allows an estimate of the degree of contamination by foreground stars or background galaxies for the previous ground-based photometry but does not require a revision of previous results: NGC 3115 DW1 hosts around N_GC = 60 ± 20 clusters, which corresponds to a specific frequency of S_N = 4.9 ± 1.9, on the high end for massive dE's. Given its absolute magnitude (M_V = -17.7 mag) and the properties of its cluster system, NGC 3115 DW1 appears to be a transition between a luminous dE and low-luminosity E galaxy.

Analysis of 80 ks ASCA and 60 ks ROSAT HRI observations of the face-on spiral galaxy NGC 6946 are presented. The ASCA image is the first observation of this galaxy above ∼2 keV. Diffuse emission may be present in the inner ∼4' extending to energies above ∼2–3 keV. In the HRI data, 14 pointlike sources are detected, the brightest two being a source very close to the nucleus and a source to the northeast that corresponds to a luminous complex of interacting supernova remnants (SNRs). We detect a point source that lies ∼30'' west of the SNR complex but with a luminosity ∼1/15 of the SNR complex. None of the point sources show evidence of strong variability; weak variability would escape our detection.

The ASCA spectrum of the SNR complex shows evidence for an emission line at ∼0.9 keV that could be either Ne IX at ∼0.915 keV or a blend of ion stages of Fe L-shell emission if the continuum is fitted with a power law. However, a two-component, Raymond-Smith thermal spectrum with no lines gives an equally valid continuum fit and may be more physically plausible given the observed spectrum below 3 keV. Adopting this latter model, we derive a density for the SNR complex of 10–35 cm^-3, consistent with estimates inferred from optical emission-line ratios. The complex's extraordinary X-ray luminosity may be related more to the high density of the surrounding medium than to a small but intense interaction region where two of the complex's SNRs are apparently colliding.

We have obtained V- and I-band photometry for 1886 stars down to I = 27 and V = 28 in the field of the dwarf irregular galaxy WLM, using deep Space Telescope Imaging Spectrograph (STIS) CL- and LP-band images taken in the parallel mode with the Hubble Space Telescope. The photometry is used to study the horizontal branch identified in WLM. The horizontal branch, extending blueward from the red giant clump, is an unambiguous signature of an old population. We demonstrate that it is possible to reach the horizontal branch of an old population at a distance of ≃1 Mpc using STIS, with relatively short exposure times. From the VI color-magnitude diagram, we obtain an accurate distance modulus (m - M)₀ = 24.95 ± 0.13 for WLM by using the V magnitude of the horizontal branch, and by adopting E(V-I) = 0.03. The implications are that (1) WLM formed stars at high redshift and (2) the old population of WLM can be representative of a protogalactic fragment, related to those accreted to form the Milky Way halo.

We investigate the influence of blending on the Cepheid distance scale. Blending is the close association of a Cepheid with one or more intrinsically luminous stars. High-resolution HST images are compared with our ground-based data, obtained as part of the DIRECT project, for a sample of 22 Cepheids in the M31 galaxy. The average (median) V-band flux contribution from luminous companions that are not resolved on the ground-based images is about 19% (12%) of the flux of the Cepheid. This is a large effect—at the 10% level for distances. The current Cepheid distance estimates to M31 are all ground based and are thus affected (underestimated). We discuss indirect methods to find which Cepheids are blended, e.g., by the use of well-sampled light curves in at least two optical bands. More generally, our ground-based resolution in M31 corresponds to the HST resolution at about 10 Mpc. Blending leads to systematically low distances in the observed galaxies and therefore to systematically high estimates of H₀; we discuss the issue and the implications.

Lying at a projected distance of 40' or 9 kpc from the center of M31, Andromeda IV is an enigmatic object first discovered during van den Bergh's search for dwarf spheroidal companions to M31. Since it is bluer, more compact, and has a higher surface brightness than other known dwarf spheroidals, it has been suggested that And IV is either a relatively old "star cloud" in the outer disk of M31 or a background dwarf galaxy. We present deep Hubble Space Telescope WFPC2 observations of And IV and the surrounding field, which, along with ground-based long-slit spectroscopy and Hα imagery, are used to decipher the true nature of this puzzling object. We find compelling evidence that And IV is a background galaxy seen through the disk of M31. The moderate surface brightness ( ∼ 24), very blue color (V-I ≲ 0.6), low current star formation rate (∼0.001 M_⊙ yr^-1), and low metallicity (∼10% Z_⊙) reported here are consistent with And IV being a small dwarf irregular galaxy, perhaps similar to Local Group dwarfs such as IC 1613 and Sextans A. Although the distance to And IV is not tightly constrained with the current data set, various arguments suggest it lies in the range 5 ≲ D ≲ 8 Mpc, placing it well outside the confines of the Local Group. It may be associated with a loose group of galaxies, containing major members UGC 64, IC 1727, and NGC 784. We report an updated position and radial velocity for And IV.

We present moderate-resolution (λ/Δλ = 1300–4800) K-band spectra of more than 110 M giants in Galactic bulge fields interior to -4° (560 pc) and as close as within 02 (28 pc) of the Galactic center. From the equivalent widths of three features in these spectra, EW(Na), EW(Ca), and EW(CO) we calculate metallicities for the stars with a new calibration derived by Stephens et al. from globular clusters. The mean metallicity for each field is in good agreement with the recent results from Frogel et al. based on the slope of the giant branch method. We find no evidence for a metallicity gradient along the minor or major axes of the inner bulge (R < 560 pc). The lack of a metallicity gradient in the inner bulge is not predicted by a theoretical model developed by Mollá et al. for a bulge formed by dissipative collapse. A metallicity gradient along the minor axis found by Frogel et al. only arises when fields located at larger galactic radius are included. However, these more distant fields are located outside of the infrared bulge defined by the COBE/DIRBE observations. We compute the metallicity distribution for the inner bulge and find a mean value of -0.21 dex with a dispersion of ±0.30 dex, close to the values found in 1996 by Sadler et al. for Baade's window. Unlike the absence of a gradient, these values are close to theoretical predictions for a bulge formed by dissipative collapse.

The proper motion of the Large Magellanic Cloud (LMC) relative to three background quasi-stellar objects has been determined using 125 CCD frames taken from 1989.0 to 1997.2 at the Cassegrain focus of the CTIO 1.5 m telescope. The observation and reduction methods are fully described. The results are compared with those obtained by other authors who use different methods and reference systems for the proper motions. This comparison results in a good agreement for μ_α cos δ and a rather large discrepancy for μ_δ. Our LMC proper motion seems to indicate that the LMC is not leading the Magellanic Stream. Finally, from the newly determined proper motion, the LMC spatial velocity vector is calculated, which in turn is used to determine the lower limit of the mass of the Galaxy contained within 50 kpc from its center. The possible consequences of our results on the dynamical status of the LMC are discussed.

Evidence for a bar at the center of the Milky Way triggered a renewed enthusiasm for dynamical modeling of the Galactic bar-bulge. Our goal is to compare the kinematics of a sample of tracers, planetary nebulae, widely distributed over the bulge with the corresponding kinematics for a range of models of the inner Galaxy. Three of these models are N-body barred systems arising from the instabilities of a stellar disk (Sellwood, Fux, and Kalnajs), and one is a Schwarzschild system constructed to represent the three-dimensional distribution of the COBE/DIRBE near-IR light and then evolved as an N-body system for a few dynamical times (Zhao). For the comparison of our data with the models, we use a new technique developed by Saha. The procedure finds the parameters of each model, i.e., the solar galactocentric distance R₀ in model units, the orientation angle ϕ, the velocity scale (in km s^-1 per model unit), and the solar tangential velocity that best fit the data.

The color-magnitude diagram and the main-sequence luminosity function of the globular cluster M13 have been investigated. While those clusters observed by the Hubble Space Telescope (HST) show a shallow luminosity function for low-mass stars, M13 has been known to have a very steep gradient toward the faint end. To understand this seemingly unique characteristic of M13, we carried out deep BV CCD observations. The observed field of nearly 56 arcmin² is located approximately 12' from the cluster center. Our (B-V)-V color-magnitude diagram has the main sequence extended to the detection limit at V ≈ 24.5 mag. It is apparent that there is a significant population of unexpected field stars below V ≈ 22.5, which cannot be accounted for by photometric errors. When these field stars are eliminated, the derived luminosity function of M13 shows a much shallower slope at the faint end, just like other Galactic globular clusters studied by HST observations.

We have observed the metal-rich globular clusters NGC 6624 and NGC 6637 (M69) using the planetary camera of the WFPC2 on the Hubble Space Telescope (HST). Observations of the Ca II triplet lines in giant stars in these clusters show that NGC 6624 and NGC 6637 have metallicities on the Zinn and West scale of [Fe/H] = -0.63 ± 0.09 and -0.65 ± 0.09, only slightly more metal rich than 47 Tuc [Fe/H] = -0.71 ± 0.07. For clusters of identical (or nearly so) metallicity, one can make a direct comparison of the color-magnitude diagrams to derive the relative ages of the clusters. From the color-magnitude diagrams derived from the HST photometry, we find that NGC 6624 and NGC 6637 differ in age by less than 0.5 Gyr. Their color-magnitude diagrams are also compared with those of 47 Tuc and NGC 6352, and while these latter diagrams are of somewhat lower quality, they are consistent with all of these clusters having the same ages. Adopting an apparent distance modulus of 13.40 and reddening E(B-V) = 0.04 for 47 Tuc, the new Yale isochrones yield an age for the clusters of 14 Gyr.

The positions of NGC 6624 and NGC 6637 in the Galaxy suggest that they belong to the bulge population of globular clusters. The only other bulge clusters that have been dated so far are the more metal rich clusters NGC 6528 and NGC 6553, which also appear to be very old. Consequently, the age-metallicity relation of the bulge may be very steep. The close similarity of the ages and metallicities of NGC 6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC 6352 indicates that the age-metallicity relations of these populations intersect. We briefly discuss the possibility that these populations had a common origin.

UBVRI and Hα CCD photometry has been performed in the southern region of NGC 2264 around the Cone Nebula. A nearly complete list of pre–main-sequence (PMS) members in the cluster has been made from the compilation of Hα emission stars identified in this study plus those selected in previous investigations, together with stars with X-ray emission. Using the H-R diagram, we tested a set of four PMS evolution models based on the PMS age and age spread estimate for the cluster and the mass-age relationship among individual stars. The resultant initial mass function (IMF) is in good agreement with the field star IMF of the solar neighborhood, with an IMF slope Γ = -1.7 in the mass range 0.3 ≤ log m ≤ 0.8.

We present images and spectra of the Cepheus E (Cep E) region at both optical and infrared wavelengths. Only the brightest region of the southern lobe of the Cep E outflow reveals optical emission, suggesting that the extinction close to the outflow source plays an important role in the observed difference between the optical and IR morphologies. Cep E is a unique object since it provides a link between the spectroscopic properties of the optical Herbig-Haro (HH) objects and those of deeply embedded outflows. The observed H₂ infrared lines allow us to determine an excitation temperature of ∼2300 K, an Ortho-to-Para ratio of ∼3, and an H₂ (1, 0)/(2, 1) S(1) line ratio of ∼9. These results are consistent with the values observed for HH objects with detected NIR emission lines, with shock excitation as the main mechanism for their formation, and also with the values observed for embedded, NIR flows.

The optical spectroscopic characteristics of Cep E (HH 377) appear to be similar to the ones of low-excitation HH objects. However, the electron density determined from the [S II] λλ6731/6717 line ratio for this object (n_e = 4100 cm^-3), and the [O I] λ6300/Hα, [S II] λλ(6717 + 6731)/Hα ratios are higher than the values of all of the previously studied low-excitation HH objects. This result is likely to be the consequence of an anomalously high environmental density in the HH 377 outflow. The ionization fraction obtained for HH 377 is x_e ∼ 1%. From this result, together with the observed [O I] λ6300/Hα line ratio, we conclude that the observed Hα line emission is collisionally excited. From a comparison with shock models, we also conclude that the extinction toward HH 377 is very low. Comparing the observed Hβ and Hα fluxes of HH 377 with model predictions, we determine a shock speed between 15 and 20 km s^-1, although somewhat higher velocities also produce spectra with line ratios that qualitatively agree with the observations of HH 377.

Multiepoch HST/WFPC2 images of η Carinae are used to investigate the relationship between the photometric variability of the circumstellar nebula and the variability of the central star. In the past few years, the central star has brightened considerably, and the response of the reflection nebula to this brightening has been surprisingly complex. While the central star has brightened by a factor of 2 at near-ultraviolet and optical wavelengths, bright dust condensations in the bipolar lobes have increased by only a factor of ∼1.3, and dark lanes between these dust condensations have brightened by factors of 1.5–2. Certain regions of the nebula have brightened much more than the star itself (as much as a factor of 8), and others have actually faded, despite the brightening of the star. Some of the anomalous fading can be attributed to contributions of intrinsic [S III] and [N II] line emission. The variations of the equivalent width of [S III] λ6312 and [N II] λ6583 as measured in the WFPC2 F631N and F658N filters follow the same trend of other high-excitation lines observed in ground-based spectra during η Car's 5.5 yr spectroscopic cycle, and the amplitude of the change accounts for the total change measured in ground-based spectra for these same lines. The WFPC2 images indicate, however, that these high-excitation lines are emitted by circumstellar gas at distances of a few hundred to a few thousand AU from the star, probably located in the equatorial plane. Thus, periodic ionization of gas at large distances from the central star appears to dominate the spectroscopic changes that define the 5.5 yr cycle. The fact that the variable high-excitation emission is extended places important constraints on models for η Car's 5.5 yr spectroscopic variability.

New simultaneous photometric and spectroscopic observations of the supersoft X-ray binary RX J0925.7-4758 (MR Vel) have been obtained. These confirm the unusually long orbital period near 3.8 days. An accurate period is determined based on several years of photometric observations. Short-term photometric variability (∼0.25 days) is also present, but it does not appear to be strictly periodic. The spectroscopic behavior reveals large-amplitude He II and H velocity curves, indicating that the system is viewed from a moderately high inclination angle. Masses for the component stars are derived. Evidence of a high-velocity jet (outflow velocity of 5350 km s^-1) is present at Hα. Its radial velocity curve is very similar to the disk emission lines, suggesting the jet is formed near the compact star. A new model for the system is presented.

We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron-star binary Aql X-1 in 1998 June while it was at quiescence. We find that its light curve is dominated by ellipsoidal variations, although the ellipsoidal variations are severely distorted and have unequal maxima. After we correct for the contaminating flux from a field star located only 046 away, the peak-to-peak amplitude of the modulation is ≈0.25 mag in the R band, which requires the orbital inclination to be greater than 36°. The orbital period we measure is consistent with the 18.95 hr period measured by Chevalier & Ilovaisky. During its outbursts the light curve of Aql X-1 becomes single-humped. The outburst light curve observed by Garcia et al. agrees in phase with our quiescent light curve. We show that the single-humped variation is caused by a "reflection effect," that is, by heating of the side of the secondary star facing toward the neutron star.

We present high spatial resolution HST and ground-based adaptive optics observations and high-sensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post– (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ≥30 AU from the primary stars. No such objects were found within 300 AU of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTSs and PTTSs in ScoCen reveal infrared excesses which are clearly above photospheric levels and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTSs in ScoCen compared with the younger classical and weak-line TTSs in Cha can be attributed to a deficiency of smaller size (0.1 to 1 μm) dust grains relative to larger size (≈5 μm) dust grains in the disks of the PTTSs. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to become dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.

We present a sample of 148 candidate RR Lyrae stars selected from Sloan Digital Sky Survey (SDSS) commissioning data for about 100 deg² of sky surveyed twice with Δt = 1.9946 days. Although the faint-magnitude limit of the SDSS allows us to detect RR Lyrae stars to large Galactocentric distances (∼100 kpc, or r* ∼ 21), we find no candidates fainter than r* ∼ 20, i.e., farther than ∼65 kpc from the Galactic center. On the assumption that all 148 candidates are indeed RR Lyrae stars (contamination by other species of variable star is probably less than 10%), we find that their volume density has roughly a power-law dependence on Galactocentric radius, R^-2.7±0.2, between 10 and 50 kpc and drops abruptly at R ∼ 50–60 kpc, possibly indicating a sharp edge to the stellar halo as traced by RR Lyrae stars. The Galactic distribution of stars in this sample is very inhomogeneous and shows a clump of over 70 stars at about 45 kpc from the Galactic center. This clump is also detected in the distribution of nonvariable objects with RR Lyrae star colors. When sources in the clump are excluded, the best power-law fit becomes consistent with the R^-3 distribution found from surveys of bright RR Lyrae stars. These results imply that the halo contains clumpy overdensities inhomogeneously distributed within a smooth R^-3 background, with a possible cutoff at ∼50 kpc.

A previous study of distinguishing between type I and type II Cepheids through the use of BV photometry found confusing results for RW Cam. This confusion is now found to be caused by RW Cam's having an unusually luminous binary companion, and the problem resolved. The criteria developed by Fernie & Ehlers are thus strengthened, and attention drawn to the role of companion stars in causing confusing results.

We present precise stellar radial velocity measurements for the Cepheid-type star Polaris taken in 1992–1993. The peak-to-peak amplitude of the pulsations was 1.555 ± 0.056 km s^-1 in epoch 1992.4 and 1.517 ± 0.047 km s^-1 in epoch 1993.2. These amplitudes are comparable to the one measured by Dinshaw et al. in 1987.75 and recent measurements by Kamper &Fernie for 1995–1997. Consequently, these do not support the continued drop in pulsational amplitude first extrapolated by Dinshaw et al. A periodogram analysis for our data yields a pulsational period of 39726769 ± 000011, slightly higher than the value found by Dinshaw et al. and consistent with the value derived from the Kamper &Fernie data. The pulsational period of Polaris may thus be increasing. Residual radial velocity measurements after removal of the component due to radial pulsations show the presence of a 40 day period with a 2K amplitude of about 400 m s^-1, first reported by Dinshaw et al. Spectral line bisectors of the Sc II λ5526 and Mg I λ5528 lines also show variations with the 40 day period, strongly indicating that this period is indeed real. The presence of line bisector variations excludes a low-mass companion object as a cause of the residual radial velocity variations. Two models are considered for the residual radial velocity and bisector span variations: starspots and nonradial pulsations. Although both can reproduce the amplitude and shape of the residual radial velocity variations, the spot models considered were unable to reproduce the overall shape and amplitude of the bisector span variations. The best-fit spot model was provided by a single spot with a temperature 500 K cooler than the photosphere with a macroturbulent velocity of zero in the spot. If surface features do exist on Polaris and are responsible for the residual radial variations for this star, then possibly they have a temperature structure and distribution more complicated than the simple models considered here. The best fit to all the observed variations was provided by a nonradial m = 4 mode, although these were not entirely satisfactory having a phase shift of about 0.25 between the observed and predicted bisector variations. Possible explanations for this shift include temperature effects, a source function originating in a dynamic stellar atmosphere, or a different pulsation mode than the one that was considered. Although the exact pulsational mode is yet to be identified, we believe that the residual radial velocity variations are due to a long-period pulsation mode. Supporting evidence comes from the fact that three different residual variations with periods near 40 days have now been seen in Polaris and it is unlikely that these variations are due to rotational modulation by surface features on a star that is differentially rotating.

Accurate mass determinations for Cepheids may be used to determine the degree of excess mixing in the interiors of their main-sequence progenitors: the larger the excess mixing, the larger the luminosity of the Cepheid of a given mass, or the smaller the mass of a Cepheid with given luminosity. Dynamical masses determined recently for a few Cepheid binaries indicate excess mixing somewhat stronger than that corresponding to the convective overshoot models by Schaller et al. Beat Cepheids can be used similarly to test main-sequence mixing in stellar interiors. The period ratios for beat Cepheids depend on luminosity, T_eff, heavy element abundance, and mass. By comparing pulsational models and the observationally derived luminosity, T_eff, metallicities, and period ratios it is possible to obtain masses for these stars, the so-called beat masses. With the old opacities masses much smaller than the evolutionary masses were obtained. With the new OPAL opacities a beat mass close to the dynamical mass was obtained for the binary beat Cepheid Y Carinae, showing that it is now possible to obtain reliable beat masses. In this paper, we determine beat masses for seven Galactic beat Cepheids for which photometric and spectroscopic data are available. We find an average mass around 4.2 ± 0.3 M_⊙ for these stars, though the actual error limits for each star may be larger mainly because of uncertainties in E(B-V) and the heavy element abundances. (As derived spectroscopically, beat Cepheids are in general metal-poor, with -0.4 ≲ [Fe/H] ≲ 0.0). The relation between the derived beat masses and the luminosities again indicates excess mixing that is somewhat larger than that corresponding to the models by Schaller et al.

The strength of the Hβ index is computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. For the first time, these models take into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the Hβ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the Hβ does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature at which the Hβ index becomes strongest. Our models indicate that the strength of the Hβ index increases as much as 0.75 Å because of the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way with those in the giant elliptical galaxies NGC 1399 and M87 shows a systematic shift in the Hβ-metallicity plane. Our models suggest that this systematic difference is explained if the mean age of globular cluster systems in giant elliptical galaxies is several billion years older than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable us to clarify whether this difference is indeed due to the age difference or whether other explanations are also possible.

We present new photometry and spectroscopy of 34 stars from a catalog of 38 nearby (d < 25 pc) G and K dwarfs selected as analogs to the early Sun. We report that the least active star in our sample is also slowly rotating and probably of solar age. Two other stars appear to be evolved objects that have recently acquired angular momentum. A fourth star may be a spectroscopic binary. Many of the other stars belong to previously identified common proper-motion groups. Space motions, lithium abundances, and Ca II emission of these stars suggest ages between 70 and 800 Myr.

The radial velocity behavior and chemical compositions of sixty-two blue metal-poor (BMP) stars have been established from more than 1200 echelle spectra obtained at Las Campanas Observatory from 1992 through 1999. Analysis of survey spectra provides abundances for this sample, which we use to calibrate the K line versus B-V relation. Forty-four of the stars have [Fe/H] < -1, while eighteen lie on -1 < [Fe/H] < 0. One star, the SX Phe variable CS 22966-043, appears to be the most extreme example of a rare abundance class characterized by α-element deficiencies, high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and [Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities, while 42 are definite or probable members of binary systems. The binary fraction of BMP stars appears to be independent of chemical composition.

The high binary fraction f_BMP ∼ 0.6 of BMP stars compared with that found for the F- and G-type stars near the Sun, the systematically low mass functions of these binaries, and the paucity of double-lined binaries among them lead us to suggest that at least half of the BMP binaries are blue stragglers and that these blue stragglers are formed by McCrea mass transfer rather than by the various merger processes that are currently believed to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long periods and small orbital eccentricities, properties these binaries share with McClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch evolution. Primaries of more widely separated binaries that survive this phase of stellar evolution may engage in mass transfer during subsequent asymptotic giant branch evolution to form s-process abundance enhanced carbon stars.

Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose from improper use of the globular cluster BS specific frequency, S_BS = n(BS)/n(HB)∼1, to estimate the halo BS space density. We use a simple model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction f_B. Our model calculations return values of S_BS ∼ 5 for f_B = 0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.

We consider the effects of recent NLTE gravities and Fe abundances on stellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce or eliminate the well-known discrepancy between CH- and C I–based C abundances in metal-poor stars and previously seen trends of atomic-based [C/Fe] and [O/Fe] with T_eff. With the NLTE parameters, the metal-poor molecular-based [C/Fe] ratio maintains its increase with declining [Fe/H]; this may also be demonstrated by the revised atomic-based ratios. [O/Fe] values derived from OH and O I features are considerably reduced and typically show improved agreement but are 0.1–0.2 dex larger than those exhibited by the Lick-Texas syndicate's recent [O I]–based giant determinations. The revised [O/Fe] ratios still show an increase down to at least [Fe/H] ∼ -2; we suggest that recent field giant data show an increase with similar slope. Even adopting uniform NLTE parameters, study-to-study abundance differences can be significant; moreover, different NLTE studies yield differing gravities and Fe abundances even after taking T_eff differences into account. Comparison of metal-poor giant gravities and cluster abundances with isochrones, trigonometric gravities, and near-turnoff cluster abundances yields conflicting indications about whether the evolved gravities might be underestimated as suggested for metal-poor dwarfs. Regardless, we argue that even extreme gravity revisions do not affect the [O/Fe]-[Fe/H] relation derived from the extant results. Combining what we believe the most reliable giant and dwarf data considered here, we find [O/Fe] = -0.184(±0.022) × [Fe/H] + 0.019 with an rms scatter of only 0.13 dex; there is no indication of a break or slope change at intermediate [Fe/H]. The gentle slope is in very reasonable agreement with some chemical evolution models employing yields with small mass and metallicity dependences. Finally, two notes are made concerning Na abundance-spatial position and element-to-element correlations in M13 giants.

We derive a B-V color-temperature relation for stars in the least model-dependent way employing the best modern data. The fit we obtained with the form T_eff = T_eff{(B-V)₀, [Fe/H], log g} covers stars in the range F0–K5 with metallicity [Fe/H] = -1.5 to +0.3 for both dwarfs and giants. The fit is well constrained, and the residual temperature of the fit is 62 K, which is consistent with what is expected from the quality of the input data. Metallicity and surface gravity effects are well separated from the color dependence. Dwarfs and giants match in a single family of the fit, differing only in log g. The fit also detects the interstellar extinction for nearby stars with the amount E(B-V) = 0.235 ± 0.03 mag kpc^-1. Taking our newly obtained relation as a reference, we examine a number of B-V color-temperature relations and atmosphere models available in the literature. We find with the Kurucz atmosphere a systematic error of 0.07 mag in B-V in the color-temperature relation across G–K5 dwarfs. On the other hand, the Bell-Gustafsson atmosphere gives colors in agreement with our empirical relation from F to G stars; for late-K stars, however, it gives colors that are too blue by 0.05 mag. We also argue for errors in the temperature scale adopted in popularly used stellar population synthesis models; synthetic colors from these models, based on the temperature calibration of Ridgway et al., may be too blue for aged elliptical galaxies. Finally, we derive the color index of the Sun to be (B-V)_⊙ = 0.626 ± 0.018 and discuss that redder colors (e.g., 0.65–0.67) often quoted in the literature are incompatible with the color-temperature relation for normal stars.

We have combined 2MASS and POSS II data in a search for nearby ultracool (later than M6.5) dwarfs with K_s < 12. Spectroscopic follow-up observations identify 53 M7–M9.5 dwarfs and seven L dwarfs. The observed space density is 0.0045 ± 0.0008 M8–M9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/substellar limit. We show the observed frequency of Hα emission peaks at ∼100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute terms, however, as measured by the ratio of Hα to bolometric luminosity, none of the ultracool M dwarfs can be considered very active compared to earlier M dwarfs, and we show that the decrease that begins at spectral type M6 continues to the latest L dwarfs. We find that flaring is common among the coolest M dwarfs and estimate the frequency of flares at 7% or higher. We show that the kinematics of relatively active (EW > 6 Å) ultracool M dwarfs are consistent with an ordinary old disk stellar population, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is near spectral types L2–L4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger field (brown) dwarfs are less active than many of the older, more massive field stellar dwarfs. Thus, contrary to the well-known stellar age-activity relationship, low activity in field ultracool dwarfs can be an indication of comparative youth and substellar mass.

We report the discovery of a bright (J = 13.83 ± 0.03) methane brown dwarf, or T dwarf, by the Two Micron All Sky Survey. This object, 2MASSI J0559191-140448, is the first brown dwarf identified by the newly commissioned CorMASS instrument mounted on the Palomar 60 inch (1.5 m) telescope. Near-infrared spectra from 0.9 to 2.35 μm show characteristic CH₄ bands at 1.1, 1.3, 1.6, and 2.2 μm, which are significantly shallower than those seen in other T dwarfs discovered to date. Coupled with the detection of an FeH band at 0.9896 μm and two sets of K I doublets at J band, we propose that 2MASS J0559-14 is a warm T dwarf, close to the transition between L and T spectral classes. The brightness of this object makes it a good candidate for detailed investigation over a broad wavelength regime and at higher resolution.

With fourteen epochs of fringe-tracking data spanning 1.7 yr from Fine Guidance Sensor 3 we have obtained a parallax (π_abs = 113.1 ± 0.3 mas) and perturbation orbit for Gl 791.2A. Contemporaneous fringe-scanning observations yield only three clear detections of the secondary on both interferometer axes. They provide a mean component magnitude difference, ΔV = 3.27 ± 0.10. The period (P = 1.4731 yr), from the perturbation orbit, and the semimajor axis (a = 0.963 ± 0.007 AU), from the measured component separations with our parallax, provide a total system mass M_A + M_B = 0.412 ± 0.009 M_⊙. Component masses are M_A = 0.286 ± 0.006 M_⊙ and M_B = 0.126 ± 0.003 M_⊙. Gl 791.2A and Gl 791.2B are placed in a sparsely populated region of the lower main-sequence mass-luminosity relation where they help define the relation because the masses have been determined to high accuracy, with errors of only 2%.

We present orbit-resolved spectroscopy and orbit-sampled photometry of the binary system PG 1114+187. Both photometry and radial velocity studies reveal a period P = 1.75992 days, which is taken to be the orbital period of the binary. Strong modulation of emission-line strength with the same period is also present. A preliminary mass ratio, M₂/M₁ ≈ 0.7, is found from primary- and secondary-star radial velocity amplitudes. No evidence is seen for either an accretion disk or mass transfer, leading to the conclusion that PG 1114+187 is not a cataclysmic variable (CV) but is in a pre-CV state, before the initiation of mass transfer. The short orbital period also leads to the conclusion that the system passed through a common-envelope phase at some time in the past. The current list of known post–common-envelope and precataclysmic binary stars is also reviewed and the general properties of this class of star are discussed.

The results of 1068 speckle interferometric observations of double stars, made with the 26 inch (0.66 m) refractor of the US Naval Observatory, are presented. These observations are averaged into 841 means of 815 binary stars. The systems range in separation from 022 to 601 with a mean separation of 221 and have a limiting secondary magnitude of V = 12.5. This is the sixth in a series of papers presenting measures obtained with this system, and it covers the period 1999 January 1 through 2000 January 9.

Radial velocity measurements and simple sine-curve fits to the orbital velocity variations are presented for the third set of 10 close binary systems: CN And, HV Aqr, AO Cam, YY CrB, FU Dra, RZ Dra, UX Eri, RT LMi, V753 Mon, and OU Ser. All systems except two (CN And and RZ Dra) are contact, double-line spectroscopic binaries, with four of them (YY CrB, FU Dra, V753 Mon, and OU Ser) being the recent discoveries of the Hipparcos satellite project. The most interesting object is V753 Mon with the mass ratio closest to unity among all contact systems (q = 0.970 ± 0.003) and large total mass [(M₁ + M₂) sin³i = 2.93 ± 0.06]. Several of the studied systems are prime candidates for combined light and radial velocity synthesis solutions.

We present a study of Jovian Trojan objects detected serendipitously during the course of a sky survey conducted at the University of Hawaii 2.2 m telescope. We used a 8192 × 8192 pixel charge-coupled device (CCD) mosaic to observe 20 deg² at locations spread over the L4 Lagrangian swarm and reached a limiting magnitude V = 22.5 (50% of maximum detection efficiency). Ninety-three Jovian Trojans were detected with radii 2 km ≤ r ≤ 20 km (assumed visual albedo 0.04). Their differential magnitude distribution has a slope of 0.40 ± 0.05, corresponding to a power-law size distribution index 3.0 ± 0.3 (1 σ). The total number of L4 Trojans with radii ≥1 km is of order 1.6 × 10⁵, and their combined mass (dominated by the largest objects) is ∼10^-4M_Earth. The bias-corrected mean inclination is 137 ± 05. We also discuss the size and spatial distribution of the L4 swarm.

The Tycho-2 Catalogue, released in February 2000, is based on the ESA Hipparcos space mission data and various ground-based catalogs for proper motions. An external comparison of the Tycho-2 astrometry is presented here using the first US Naval Observatory CCD Astrograph Catalog (UCAC1). The UCAC1 data were obtained from observations performed at Cerro Tololo Inter-American Observatory (CTIO) between 1998 February and 1999 November, using the 206 mm–aperture five-element lens astrograph and a 4K × 4K CCD. Only small systematic differences in position between Tycho-2 and UCAC1, up to 15 mas, are found, mainly as a function of magnitude. The standard deviations of the distributions of the position differences are in the 35 to 140 mas range, depending on magnitude. The observed scatter in the position differences is about 30% larger than that expected from the combined formal, internal errors, also depending on magnitude. The Tycho-2 Catalogue has the more precise positions for bright stars (V ≤ 10 mag, while the UCAC1 positions are significantly better at the faint end (11 mag ≤ V ≤ 12.5 mag) of the magnitude range in common. UCAC1 data go much fainter (to R ≈ 16 mag) than Tycho-2 data; however, complete sky coverage is not expected before mid-2003.

Far-UV observations in and near the Hubble Deep Fields demonstrate that the Space Telescope Imaging Spectrograph (STIS) may obtain unique and precise measurements of the diffuse far-ultraviolet background. Although STIS is not the ideal instrument for such measurements, high-resolution images allow Galactic and extragalactic objects to be masked to very faint magnitudes, thus ensuring a measurement of the truly diffuse UV signal. The programs we have analyzed were not designed for this scientific purpose, but they would be sufficient to obtain a very sensitive measurement if it were not for a weak but larger than expected signal from airglow in the STIS 1450–1900 Å bandpass. Our analysis shows that STIS far-UV crystal quartz observations taken near the limb during orbital day can detect a faint airglow signal, most likely from N I λ1493, that is comparable to the dark rate and inseparable from the far-UV background. Discarding all but the night data from these data sets yields the diffuse far-ultraviolet background measurement 501 ± 103 photons cm^-2 s^-1 sr^-1 Å^-1, along a line of sight with very low Galactic neutral hydrogen column (N_{H I} = 1.5 × 10²⁰ cm^-2) and extinction [E(B-V) = 0.01 mag]. This result is in good agreement with earlier measurements of the far-UV background and should not include any significant contribution from airglow. We present our findings as a warning to other groups who may use the STIS far-UV camera to observe faint extended targets and to demonstrate how this measurement may be properly obtained with STIS.