Table of contents

     We present a deep BVrIK multicolor catalog of galaxies in the field of the high-redshift (z = 4.7) quasar BR 1202-0725. Reliable colors have been measured for galaxies selected down to R = 25. Taking advantage of the wide spectral coverage of the galaxies in the field, we compare the observed colors with those predicted by spectral synthesis models including UV absorption by the intergalactic medium and dust reddening. The choice of the optical filters has been optimized to define a robust multicolor selection of galaxies at 3.8 ≤ z ≤ 4.5. Within this interval the surface density of galaxy candidates with z ∼ 4 in this field is 1 arcmin^-2. Photometric redshifts have been derived for the galaxies in the field with the maximum likelihood analysis using the GISSEL library of ∼10⁶ synthetic spectra. The accuracy of the method used has been discussed and tested using galaxies in the Hubble Deep Field with known spectroscopic redshifts and accurate photometry. A peak in the redshift distribution is present at z ≃ 0.6 with relatively few galaxies at z > 1.5. At variance with brighter surveys (I < 22.5) is a tail in the distribution toward high redshifts up to z ∼ 4. The luminosity function at z ∼ 0.6 shows a steepening for M_B > -19. This increase is reminiscent of that found in the most recent estimates of the local luminosity function, where a similar volume density is reached about 2 mag fainter. The observed cosmological UV luminosity density is computed in the overall redshift interval z = 0.3–4.5, reaching a value ∼2 × 10¹⁹ W Hz^-1 Mpc^-3 at z ∼ 0.8. Including recent local estimates, it appears that the UV luminosity density changes by a factor of ∼2.5 in the overall redshift interval z = 0.1–4, not including correction for fainter undetected galaxies. Thus we find that the evidence of a marked maximum in the luminosity density at z ∼ 1–1.5 for galaxies with R ≤ 25 is weak. Using the GISSEL libraries we have derived in a homogeneous way the physical parameters connected with the fitted spectral energy distributions. Thanks to this new approach, the problem of the star formation history of the universe is dealt with in a consistent way, taking into account the dust and metallicity distributions derived from the spectrophotometric properties of each galaxy in the sample. The bulk of the blue intermediate-redshift population with z = 0.4–1 mostly consists of very young star-forming galaxies with a median starburst age of the order of a few times 10⁸ yr and typical mass in luminous stars ∼2 × 10⁸M_☉. The presence of this young population is in contrast with the pure luminosity evolutionary model based on a single high-formation redshift. The cosmological mass in formed stars per unit comoving volume at z ∼ 3 is already ∼20% of that formed at z = 0.5 in our magnitude-limited sample. Predictions based on the standard hierarchical clustering models are smaller, although not far from that derived from the observations.

     Metal line ratios in a sample of 13 quasar spectra obtained with the HIRES spectrograph on the Keck I Telescope have been analyzed to characterize the evolution of the metagalactic ionizing flux near a redshift of 3. The evolution of Si IV/C IV has been determined using three different techniques: using total column densities of absorption-line complexes, as in the approach of Songaila & Cowie, using the column densities of individual Voigt profile components within complexes, and using direct optical depth ratios. All three methods show that Si IV/C IV changes abruptly at z ∼ 3, requiring a jump in value of about a factor of 3.4 and indicating a significant change in the ionizing spectrum that occurs rapidly between z = 2.9 and z = 3, just above the redshift at which Reimers et al. detected patchy He II Lyα absorption. At lower redshifts, the ionization balance is consistent with a pure power-law ionizing spectrum, but at higher redshifts the spectrum must be very soft, with a large break at the He⁺ edge. An optical depth ratio technique is used to measure the abundances of ions whose transitions lie within the forest, and C III, Si III, and O VI are detected in this way. The presence of a significant amount of O VI at z > 3 suggests either a considerable volume of He III bubbles embedded in the more general region in which the ionizing flux is heavily broken or the addition of collisional ionization to the simple photoionization models.

     Absorption due to He II Lyα has now been detected at low resolution in the spectra of four quasars between redshifts z = 2.74 and z = 3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud diffuse emission and ionization physics, and statistical fluctuations. We use high-resolution observations of H I absorption toward quasars to derive an improved model for the opacity of the intergalactic medium (IGM) from the distribution of absorbing clouds in column density and redshift. We use these models to calculate the H I and He II photoionization history, the ratio η = He II/H I in both optically thin and self-shielded clouds, and the average line-blanketing contribution of the clouds to He II absorption. The derived ionization rate, Γ_HI = (1–3) × 10^-12 s^-1 (z = 2–4), is consistent with the ionizing background intensity inferred from the "proximity effect," but it remains larger than that inferred by N-body hydrodynamic simulations of the Lyα absorber distribution. The He II observations are consistent with line blanketing from clouds having N_H ≥ 10¹² cm^-2, although a contribution from a more diffuse IGM would help to explain the observed opacity. We compute the expected He II optical depth, τ_{He I}(z), and examine the implications of the sizable fluctuations that arise from variations in the cloud numbers and ionizing radiation field. We assess how He II absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution ultraviolet observations can distinguish between absorption from the diffuse IGM and the Lyα forest clouds and determine the source of the ionizing background.

     In 1989, Peebles showed that in the gravitational instability picture galaxy orbits can be traced back in time from a knowledge of their current positions, via a variational principle. We modify this variational principle so that galaxy redshifts can be input instead of distances, thereby recovering the distances. As a test problem, we apply the new method to a Local Group model. We infer M = (4–8) × 10¹²M_☉ depending on cosmology, implying that the dynamics of the outlying Local Group dwarfs are consistent with the timing argument. Some algorithmic issues need to be addressed before the method can be applied to recover nonlinear evolution from large redshift surveys, but there are no more difficulties in principle.

     We present near-infrared (observed frame) spectra of the high-redshift quasar S4 0636+68 at z = 3.2, which was previously thought to be one of a group of "strong" Fe II emitters [i.e., F(Fe II λλ4434–4684)/F(Hβ) > 1]. Our K-band spectrum clearly shows emission lines of Hβ and [O III] λλ4959, 5007, as well as optical Fe II emission. Our computed value of F(Fe II λλ4434–4684)/F(Hβ) ≃ 0.8 for S4 0636+68 is less than previously thought and, in fact, is comparable to values found for radio-loud, flat-spectrum, low-z quasars. Therefore S4 0636+68 appears not to be a strong optical Fe II emitter. Although more than half () of the high-z quasars observed to date are still classified as strong optical Fe II emitters, their Fe II/Hβ ratios, for the most part, follow the same trend as do those of low-z quasars, i.e., an anticorrelation in EW(Fe II)/EW(Hβ) versus EW([O III])/EW(Hβ), with radio-loud quasars having a mean value of EW(Fe II)/EW(Hβ) approximately half that of radio-quiet quasars at comparable values of EW([O III])/EW(Hβ).

     We present BVRI observations of BL Lacertae during its recent outburst. These observations, made during a 3 month period, cover a significant portion of the optical flare. The reduced data are displayed as light curves and broadband spectra. Changes in the spectral index are analyzed, and the results are compared with previous BL Lac observations. We find that the variations are simultaneous in the optical bands, but the higher frequency bands show a higher amplitude of variability. The spectral index is variable during the active period, and there is marginal evidence that the spectrum flattens as the source gets brighter.

     Recent analysis of the data from the Southern Sky Redshift Survey extension (SSRS2) based on cell counts and the two-point correlation function has shown that very luminous galaxies are much more strongly clustered than fainter galaxies. In fact, the amplitude of the correlation function of very luminous galaxies (L > L*) asymptotically approaches that of R ≥ 0 clusters. In this paper we investigate the properties of the most luminous galaxies, with blue absolute magnitudes M_B ≤ -21. We find that (1) the population mix is comparable to that in other ranges of absolute magnitude; (2) only a small fraction are located in bona fide clusters; (3) the bright galaxy–cluster cross-correlation function is significantly higher on large scales than that measured for fainter galaxies; (4) the correlation length of galaxies brighter than M_B ∼ -20.0, expressed as a function of the mean interparticle distance, appears to follow the universal dimensionless correlation function found for clusters and radio galaxies; (5) a large fraction of the bright galaxies are in interacting pairs, while others show evidence for tidal distortions and some appear to be surrounded by faint satellite galaxies. We conclude that very luminous optical galaxies differ from the normal population of galaxies in both clustering and other respects. We speculate that this population is a highly biased tracer of mass, being associated with dark halos with masses more comparable to clusters than to typical loose groups.

     Magnitude-limited samples of spiral galaxies drawn from the Ursa Major and Pisces Clusters are used to determine their extinction properties as a function of inclination. Imaging photometry is available for 87 spirals in the B, R, I, and K' bands. Extinction causes systematic scatter in color-magnitude plots. A strong luminosity dependence is found. Relative edge-on to face-on extinction of up to 1.7 mag is found at B for the most luminous galaxies but is unmeasurably small for faint galaxies. At R the differential absorption with inclination reaches 1.3 mag, at I it reaches 1.0 mag, and at K' the differential absorption can in the extreme be as great as 0.3 mag. The luminosity dependence of reddening can be translated into a dependence on rotation rate, which is a distance-independent observable. Hence, corrections can be made that are useful for distance measurements. The strong dependence of the corrections on luminosity act to steepen luminosity–line width correlations. The effect is greatest toward the blue, with the consequence that luminosity–line width slope dependencies are now only weakly a function of color.

     We measure the extent of 100 μm galactic emission in two independent galaxy samples using the IRAS 100 μm Sky Survey images and constrain the distribution of dust at large (≲30 kpc) radii. The first sample consists of 90 nearby (v < 6000 km s^-1) galaxies from the Third Reference Catalogue of Bright Galaxies with similar angular sizes and absolute luminosities (5' ≤ D₂₅ ≤ 10' and -22.5 ≤ M_B ≤ -18) that are isolated in the 100 μm images. The second sample consists of 24 local galaxies (v < 1500 km s^-1, 10' ≤ D₂₅ ≤ 30'). We rescale the 100 μm images of these galaxies using their optical diameters, D₂₅, rotate the images using their optical major-axis position angle, construct the mean and median images, and rebin the final images into polar coordinates to study the 100 μm emission as a function of radius and azimuthal angle. We find that the 100 μm emission extends at least to radii of 33 kpc (2 σ detection) for the typical galaxy in the 5'–10' sample and to 21 kpc (2 σ detection) in the 10'–30' sample (H₀ = 75 km s^-1 Mpc^-1). In both samples, the emission is preferentially elongated along the optical major axis. We fit an exponential to the 100 μm emission along the major axis and measure a scale length of 2.5 ± 0.8 kpc (90% confidence interval). Using a simple model that relates the far-IR emission to the stellar distribution, we examine the range of acceptable dust mass distributions allowed by our data and conclude that the dust is more extended than the starlight.

We construct dynamical models for a sample of 36 nearby galaxies with HubbleSpaceTelescope (HST) photometry and ground-based kinematics. The models assume that each galaxy is axisymmetric, with a two-integral distribution function, arbitrary inclination angle, a position-independent stellar mass-to-light ratio Υ, and a central massive dark object (MDO) of arbitrary mass M_•. They provide acceptable fits to 32 of the galaxies for some value of M_• and Υ; the four galaxies that cannot be fitted have kinematically decoupled cores. The mass-to-light ratios inferred for the 32 well-fitted galaxies are consistent with the fundamental-plane correlation Υ ∝ L^0.2, where L is galaxy luminosity. In all but six galaxies the models require at the 95% confidence level an MDO of mass M_• ∼ 0.006M_bulge ≡ 0.006ΥL. Five of the six galaxies consistent with M_• = 0 are also consistent with this correlation. The other (NGC 7332) has a much stronger upper limit on M_•. We predict the second-moment profiles that should be observed at HST resolution for the 32 galaxies that our models describe well.

We consider various parameterizations for the probability distribution describing the correlation of the masses of these MDOs with other galaxy properties. One of the best models can be summarized thus: a fraction f ≃ 0.97 of early-type galaxies have MDOs, whose masses are well described by a Gaussian distribution in log (M_•/M_bulge) of mean -2.28 and standard deviation ∼0.51. There is also marginal evidence that M_• is distributed differently for "core" and "power law" galaxies, with core galaxies having a somewhat steeper dependence on M_bulge.

     In this paper, the third of a series dedicated to the investigation of the nuclear properties of spiral galaxies, we (1) model the Wide Field Planetary Camera 2 F606W nuclear surface brightness profiles of 41 spiral galaxies presented in Papers I and II with the analytic law introduced by Lauer et al. and (2) deconvolve these surface brightness profiles and their analytic fits, so as to estimate the nuclear stellar densities of bulges of spiral galaxies. We find that the nuclear stellar cusps (quantified by the average logarithmic slope of the surface brightness profiles within 01–05) are significantly different for R^1/4 law and exponential bulges. The former have nuclear properties similar to those of early-type galaxies, i.e., similar values of nuclear cusps for comparable luminosities, and increasingly steeper stellar cusps with decreasing luminosity. By contrast, exponential bulges have (underlying the light contribution from photometrically distinct, central compact sources) comparatively shallower stellar cusps, and likely lower nuclear densities, than R^1/4 law bulges.

     We present the results of optical broadband and Hα imaging studies of the infrared-bright galaxy NGC 972. The broadband images and B/R color map reveal the peculiar morphology and complex dust distribution. Massive star formation activity in this galaxy is evident from the continuum-subtracted Hα image, which shows the presence of circumnuclear activity and disk star formation within a radius of 3.4 kpc. The circumnuclear star-forming regions are distributed in a ring of radius 630 pc and closely associated with an inner dust ring. Aperture photometry has been performed on the individual H II regions, and we estimate an age ≤5.4 Myr for the nuclear starburst using evolutionary synthesis models. The Hα luminosity of the nucleus is comparable to that of starburst nuclei. The emission-line ratios of NGC 972 are also indicative of a nuclear starburst, which is powered by photoionization by a large number of hot, massive stars. The enhancement of low-ionization lines compared with normal H II regions can be attributed to the influence of dust on the thermal properties of the nebula or to contributions by shocks from supernova remnants. The star formation rate is estimated as 0.32 M_☉ yr^-1 for the nucleus and 2.1–2.7 M_☉ yr^-1 in the inner 3.6 kpc of the galaxy.

     Deep CCD images of the Leo Triplet in the B and I bands were obtained with the Burrell-Schmidt telescope at Kitt Peak National Observatory to study the ∼80 kpc stellar tail, or "plume," extending from NGC 3628, which resulted from the tidal effects of its companion galaxies NGC 3627 and NGC 3623. This plume consists of clumpy star formation regions that are near previously measured high-resolution H I peaks. Comparison of our B-I colors and B luminosities with star cluster evolution models indicates star-forming ages of a few times 10⁸ years and complex masses of 10⁶M_☉, corresponding to a star formation efficiency of 3% overall in the plume, or about 26% at the H I peaks.

     Optical multislit spectra have been obtained for 47 globular clusters surrounding the brightest Virgo elliptical galaxy, NGC 4472 (M49). Including data from the literature, we analyze velocities for a total sample of 57 clusters and present the first tentative evidence for kinematic differences between the red and blue cluster populations that make up the bimodal color distribution of this galaxy. The redder clusters are more centrally concentrated and have a velocity dispersion of 240 km s^-1, compared with 320 km s^-1 for the blue clusters. The origin of this difference appears to be a larger component of systematic rotation in the blue cluster system. The larger rotation in the more extended blue cluster system is indicative of efficient angular momentum transport, as provided by galaxy mergers. Masses estimated from the globular cluster velocities are consistent with the mass distribution estimated from X-ray data and indicate that the M/L_B rises to 50 (M/L)_☉ at 2.5R_e.

     The Fornax Cluster galaxy FCC 35 shows an unusual multiply peaked integrated H I profile. We have now observed FCC 35 with the Australia Telescope Compact Array and have found a compact H I source with M_HI = 2.2 × 10⁸M_☉ and a spatially overlapping complex of H I gas with the same mass. By combining optical observations with the H I data, we are able to identify FCC 35 as a young compact source of star formation with a nearby intergalactic H I cloud that is devoid of stars. We classify FCC 35 as a blue compact dwarf or H II galaxy, having large amounts of neutral hydrogen, very blue colors (U - V = 0.1), and a low-metallicity spectrum with strong narrow emission lines. Together with the presence of the H I cloud, this suggests that FCC 35 is the result of a recent interaction within the Fornax Cluster.

     We present spectroscopy of two clusters in the dwarf irregular galaxy NGC 6822. From these we deduce an age for cluster VII of 11 Gyr and [Fe/H] = -1.95 ± 0.15 dex. Cluster VII appears to be an analog of the metal-poor Galactic globular clusters. Cluster VI is found to be much younger and more metal-rich, with an age of approximately 2 Gyr. Its derived metallicity, [Fe/H] ≈ -1.0 dex, is comparable to that of the gas seen today in NGC 6822. The existence of a metal-poor old cluster in NGC 6822 rules out models for the chemical evolution of this galaxy with significant prompt initial enhancement. We find that a star formation rate that is constant with time and is within a factor of 2 of the present star formation rate can reproduce the two points on the age-metallicity relationship for NGC 6822 over the past 10 Gyr defined by these two clusters.

     We discuss a Johnson-Cousins V- and I-band CCD mosaic survey of the Ursa Minor dwarf spheroidal galaxy (dSph) to V ∼ 22. We covered UMi with 27 105 × 105 overlapping CCD frames, each frame consisting of two 300 s exposures in each of V and I. We also observed several regions ∼3° from UMi to obtain an estimate of contamination by galaxies and Galactic stars. We report the first H-R diagram of an entire dSph. Separation of dwarf stars from foreground stars by color allows a robust estimation of the structural parameters of UMi. We examine earlier evidence of two lumps in the UMi stellar distribution. We detect a statistically significant asymmetry in the stellar distribution of UMi along the major axis. Structure in the stellar distribution of UMi might indicate a tidal origin for UMi's high observed mass-to-light ratio. We demonstrate a technique for obtaining the absolute magnitude of UMi from horizontal-branch star counts compared with M92; this method of luminosity estimation is independent of the distance to UMi. We obtain M_V = -8.87 ± σ , where σ = 0.14 if M92 is a perfect calibrator and σ ≲ 0.35 is an upper bound on the error arising from differences in stellar populations. Our value for M_V is consistent with earlier measurements and has a smaller uncertainty.

     We plot the globular clusters of the Fornax galaxy and those associated with the Sagittarius dwarf spheroidal galaxy in the horizontal-branch type versus metallicity diagram. The horizontal-branch types for the Fornax clusters include corrections for red horizontal-branch stars from the field and are based on our recent work and new results in the literature. Fornax globular clusters continue to stand out as having red horizontal branches for their low ([Fe/H] ∼ -2) metallicities, with no counterparts in either the outer Galactic halo or the Magellanic Clouds. The clusters associated with Sagittarius lie to the blue of the Fornax clusters, except for the metal-rich cluster Ter 7. Although the metallicities of the three metal-poor Sagittarius globular clusters are similar to those of the Fornax clusters, their horizontal branches are bluer and they lie in a region also populated by the old LMC and old halo clusters. Neither cluster system resembles the younger Galactic halo globular clusters, often suggested to have been accreted from disrupted dwarf spheroidal galaxies. Except for Ter 7, both the Fornax and Sagittarius globular clusters are metal-poor compared with their Galactic counterparts of the same horizontal-branch type. We find no correlation between HB type and other cluster properties such as central concentration, luminosity, central surface brightness, and estimated collision rate.

     Images recorded through broadband (JHK) and narrowband (CO and 2.2 μm continuum) filters are used to investigate the photometric properties of bright (K ≤ 13.5) stars in a 6' × 6' field centered on the Sgr A complex. The giant branch ridge lines in the (K, J - K) and (K, H - K) color-magnitude diagrams are well matched by the Baade's window (BW) M giant sequence if the mean extinction is A_K ∼ 2.8 mag. Extinction measurements for individual stars are estimated using the M_K versus infrared color relations defined by M giants in BW, and the majority of stars have A_K between 2.0 and 3.5 mag. The extinction is locally high in the Sgr A complex, where A_K ∼ 3.1 mag. Reddening-corrected CO indexes, CO₀, are derived for over 1300 stars with J, H, and K brightnesses and for over 5300 stars with H and K brightnesses. The distribution of CO₀ values for stars with K₀ between 11.25 and 7.25 can be reproduced using the M_K-CO₀ relation defined by M giants in BW. The data thus suggest that the most metal-rich giants in the central regions of the bulge and in BW have similar photometric properties and 2.3 μm CO strengths. Hence, it appears that the central region of the bulge does not contain a population of stars that are significantly more metal-rich than what is seen in BW.

     The velocity distribution f(v) of nearby stars is estimated, via a maximum likelihood algorithm, from the positions and tangential velocities of a kinematically unbiased sample of 14,369 stars observed by the Hipparcos satellite. The distribution f shows rich structure in the radial and azimuthal motions, v_R and v_φ, but not in the vertical velocity, v_z: there are four prominent and many smaller maxima, many of which correspond to well-known moving groups. While samples of early-type stars are dominated by these maxima, also up to about a quarter of red main-sequence stars are associated with them. These moving groups are responsible for the vertex deviation measured even for samples of late-type stars; they appear more frequently for ever redder samples, and as a whole they follow an asymmetric drift relation, in the sense that those only present in red samples predominantly have large |v_R| and lag in v_φ with respect to the local standard of rest (LSR). The question arises, how did these old moving groups get on their eccentric orbits? A plausible mechanism known from solar system dynamics that is able to manage a shift in orbit space is sketched. This mechanism involves locking into an orbital resonance; in this respect is intriguing that Oort's constants, as derived from Hipparcos data, imply a frequency ratio between azimuthal and radial motion of exactly Ω:κ = 3:4. Apart from these moving groups, there is a smooth background distribution, akin to Schwarzschild's ellipsoidal model, with axis ratios σ_R:σ_φ:σ_z ≈ 1:0.6:0.35. The contours are aligned with the v_R-direction, but not with respect to the v_φ- and v_z-axes: the mean v_z increases for stars rotating faster than the LSR. This effect can be explained by the stellar warp of the Galactic disk. If this explanation is correct, the warp's inner edge must not be within the solar circle, while its pattern rotates with frequency ≳13 km s^-1 kpc^-1 retrograde with respect to the stellar orbits.

     Several samples of nearby stars with the most accurate astrometric and photometric parameters are searched for clues to their evolutionary history. The main samples are (1) the main-sequence stars with b - y between 0.29 and 0.59 mag (F3 to K1) in the Yale parallax catalog, (2) a group of high-velocity subgiants studied spectroscopically by Ryan & Lambert, and (3) high-velocity main-sequence stars in the extensive investigation by Norris, Bessel, & Pickles. The major conclusions are as follows: (1) The oldest stars (halo), t ≥ 10–12 Gyr, have V-velocities (in the direction of Galactic rotation and referred to the Sun) in the range from about -50 to -800 km s^-1 and have a heavy-element abundance [Fe/H] of less than about -0.8 dex. The age range of these objects depends on our knowledge of globular cluster ages, but if age is correlated with V-velocity, the youngest may be M22 and M28 (V ∼ -50 km s^-1) and the oldest NGC 3201 (V ∼ -500 km s^-1) and assorted field stars. (2) The old disk population covers the large age range from about 2 Gyr (Hyades, NGC 752) to 10 or 12 Gyr (Arcturus group, 47 Tuc), but the lag (V) velocity is restricted to less than about 120 km s^-1 and [Fe/H] ≥ -0.8 or -0.9 dex. The [Fe/H] ∼ -0.8 dex division between halo and old disk, near t ∼ 10–12 Gyr, is marked by a change in the character of the CN index (C_m) and of the blanketing parameter K of the DDO photometry. (3) The young disk population, t < 2 Gyr, is confined exclusively to a well-defined area of the (U, V) velocity plane. The age separating young and old disk stars is also that separating giant evolution of the Hyades (near main-sequence luminosity) and M67 (degenerate helium cores and a large luminosity rise) kinds. The two disk populations are also separated by such indexes as the g-index of Geveva photometry. There appears to be no obvious need to invoke exogeneous influences to understand the motion and heavy-element abundance distributions of the best-observed stars near the Sun. Individual stars of special interest include the parallax star HD 55575, which may be an equal-component binary, and the high-velocity star HD 220127, with a well-determined space velocity near 1000 km s^-1.

     With a luminosity zero point fixed by the kinematics of old disk superclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups (Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colors of evolved old disk stars, especially red horizontal branch (RHB), early asymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch [AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71, M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) The RHB stars in the old disk all have M_V = +0.7 ± 0.1 (M_K = -1.3 ± 0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stable periods and light curves are old disk stars on AGB(2). (3) AGB(1) objects include CH stars and semiregular (SRa) variables. (4) The populous and overabundant cluster NGC 6791 may be the only disk cluster with sdOB stars, populating the lower portion of the bifurcated extended horizontal branch that is usual in most "blue tailed" and high-density halo clusters. (5) Post–red giant branch (RGB) stars in old disk clusters show a B - V (b - y) defect when compared with RGB stars, possibly because of a change in the character of the atmospheres. (6) If the bulk of the LARVs are pulsating in the fundamental mode, R Vir (P = 145 days) is possibly a first-overtone pulsator. (7) The overabundant old disk clusters are within the solar circle, with Liller 1 being at the Galactic center. (8) Several probable RHB stars at the south Galactic pole are identified. (9) The period-age relation, combined with the known spatial distribution of Galactic LARVs, leads to a relation between age and scale height of distribution that monotonically increases with age, leaving no obvious reason for a bifurcation of the population.

     The evidence for the existence of the important HR 1614 supercluster and group of rare rich CN stars is examined with the use of Hipparcos astrometry. Group membership is supported by Hipparcos parallaxes except for two stars for which available evidence suggests uncertainty in the Hipparcos values.

     Derived from first physical principles, a few simple rules are presented that can help in both the planning and interpretation of CCD and IR-array camera observations of resolvable stellar populations. These rules concern the overall size of the population sampled by a frame as measured by its total luminosity, and allow us to estimate the number of stars (in all evolutionary stages) that are included in the frame. The total luminosity sampled by each pixel (or resolution element) allows us, instead, to estimate the depth to which meaningful stellar photometry can be safely attempted, and below which crowding makes it impossible. Simple relations also give the number of pixels (resolution elements) in the frame that will contain an unresolved blend of two stars of any kind. It is shown that the number of such blends increases quadratically with both the surface brightness of the target and with the angular size of the pixel (or resolution element). A series of examples is presented illustrating how the rules are practically used in concrete observational situations. Application of these tools to existing photometric data for the inner parts of the bulge of M31, M32, and NGC 147 indicates that no solid evidence has yet emerged for the presence of a significant intermediate-age population in these objects.

     We report the measurement of the angular expansion rate of the compact planetary nebula Vy 2-2. Four epochs of radio continuum maps over a total time baseline of 15 years (1982–1997) and covering a wide band of frequencies are used to deduce the expansion rate. The variations in the observed flux densities at 1.5, 4.9, 8.4, 15, and 23 GHz and the variations in the observed radius deduced from the 15 GHz visibility data are used to compare with various simple models. The two methods provide a clear and consistent detection of the expansion. The resulting value of the angular expansion rate is 1.13 ± 0.12 mas yr^-1. A self-consistent solution from the flux density models indicates that the ionized shell is a density-bounded region. The kinematic age of the nebula, derived from the angular expansion, of 213 ± 26 yr confirms that Vy 2-2 is an extremely young planetary nebula.

We report the first direct detection of molecular hydrogen emission in the interstellar medium in the vicinity of a Wolf-Rayet (W-R) star. The spatial distribution of the excited molecular gas associated with NGC 2359 is filamentary and lies mainly on the border of the ionized gas, as traced by optical emission lines such as Hα or [O III] λ5007. The typical 1–0 S(1) H₂ brightness in the filaments is 5 × 10^-5 ergs s^-1 cm^-2 sr^-1 and the total 1–0 S(1) H₂ luminosity detected is ∼4 L_☉. The detected line flux in the 1–0 S(1) transition of H₂ at λ = 2.122 μm could equally be explained by shock excitation or by fluorescence from the strong ultraviolet flux of the W-R star. The morphological distribution of the H₂ filaments is not inconsistent with either mode of excitation. Although the ubiquity of this phenomenon needs to be confirmed, the relatively high level of 1–0 S(1) H₂ emission detected in this W-R nebula indicates that hot stars could potentially contribute a significant fraction of the total H₂ emission of young starburst galaxies.

Viewed at 1' resolution at 1420 MHz, the source G74.5+0.9 appears as a double lobe structure of total length 20' with a moderately strong compact source located slightly off-center. Although its morphology is reminiscent of a typical extragalactic radio source with a central jet-core source, its infrared-to-radio ratio is more typical of an H II region. Furthermore, neutral hydrogen observations show the presence of an H I cavity at an LSR velocity of about -70 km s^-1. Combining continuum radio, infrared, and optical observations, as well as H I data, we show that the source is compatible with the model of a bipolar H II region. If at its kinematic distance (about 12 kpc), the bipolar source has an overall length of about 60 pc and is powered by an O6 star. The possibility that the source is at a much closer distance of about 2 kpc, associated with material expanding toward us and away from the Cyg OB1/OB3 association, is also considered, in which case its overall dimension is only 12 pc and a B0 star is sufficient to power it. The nature of the central source is also discussed.

This paper describes the first optical spectroscopic survey of Class I sources (also known as embedded sources and protostars) in the Taurus-Auriga dark cloud. We detect 10 of the 24 known Class I sources in the cloud at 5500–9000 Å. All detected Class I sources have strong Hα emission; most also have strong [O I] and [S II] emission. These data—together with high-quality optical spectra of T Tauri stars in the Taurus-Auriga cloud—demonstrate that forbidden emission lines are stronger and more common in Class I sources than in T Tauri stars. Our results also provide a clear discriminant in the frequency of forbidden line emission between weak-emission and classical T Tauri stars. In addition to strong emission lines, three Class I sources have prominent TiO absorption bands. The M-type central stars of these sources mingle with optically visible T Tauri stars in the H-R diagram and lie somewhat below both the birth line for spherical accretion and the deuterium burning sequence for disk accretion.

     We have mapped the central parsec of the H II region W33 in the infrared fine-structure lines of [Ne II] (12.8 μm) and [Ar III] (8.99 μm), and have measured spectra of [Ar II] (6.99 μm) at selected locations. This is the first observation of the 6.99 μm line from the ground. The spatial distribution of the infrared lines resembles that of the radio continuum, except for one region where the extinction is apparently very high (even at 12.8 μm). The abundances of neon and argon in W33 are a factor of 4 lower than solar. We have analyzed the ratios of the mapped lines to derive the types of the exciting stars, their ages, and other features of the source. The cluster contains three distinct subsources, one of which appears to be a cometary and another a shell-like ultracompact H II region, although they are larger and less dense than most objects of this class. The stars are of types O6 to O7 and less than a few million years old. We suggest that star formation in this object has been proceeding simultaneously in at least three centers, and that the H II regions have not yet expanded to form one source.

     In an attempt to better understand the conditions under which molecules condense onto grains in the envelopes of evolved stars, we have searched for the presence of H₂O ice in the circumstellar envelopes of several evolved (OH/IR) stars. The sample of stars observed was selected on the basis of mass-loss rates, luminosities, and outflow velocities in order to cover a range of physical conditions that might affect the amount of ice present in stellar envelopes. Despite the clear presence of H₂O ice around other, previously observed, evolved stars, our search in six OH/IR stars has resulted in only one clear detection, in OH 26.5+0.6, and the tentative detection in one other, OH 26.4-1.9. We provide column densities or upper limits for the amount of ice that is present on the grains around these stars and explore the possibility that there could be a relationship between _* or _*/L_* and the H₂O ice column density to explain the observations.

     CS 22966-043 is an ultra–short-period pulsating star with high velocity (RV = -266 km s^-1) discovered during the course of a search for spectroscopic binaries among blue metal-poor field stars, in progress since 1992. With respect to period (0.0374 days), mean color (⟨B - V⟩ = 0.24), and metal abundance ([Fe/H] ≈ -2.4), it closely resembles the SX Phoenicis stars found among the blue stragglers in NGC 5053. CS 22966-043 also is the primary of a spectroscopic binary with (probable) period of 430 days. Light-travel time across the projected orbit, as large as 0.0037 days, must be added to the times of observation to combine data obtained in different years with minimal phase dispersion. If CS 22966-043 is, indeed, a blue straggler formed by binary interaction as is now generally believed, then it seems most probable that the interaction was one of mass transfer from the present-day secondary during its post–main-sequence evolution rather than merger of a close binary. The latter option would require that this rare field star was, in addition, a member of a primordial triple system.

     Outbursts averaging 0.6 mag in amplitude and 10 days in width are described in five old novae and nova-like cataclysmic variables: UU Aqr, Q Cyg, CP Lac, X Ser, and RW Sex. These stars are thought to be high mass transfer rate systems for which the accretion disk is expected to be stable against the thermal instability responsible for dwarf nova outbursts. The widths and spacings of these events are similar to those of dwarf nova eruptions, but the amplitudes are significantly smaller, or "stunted." The outbursts are sometimes accompanied by dips. These dips have amplitudes that are similar to the outbursts' but have shapes that scatter significantly more than the shapes of the outbursts. The outbursts and dips sometimes occur as pairs and are sometimes isolated. We are not able at this time to determine a single common mechanism for this behavior, or even to conclude that some mechanisms are preferred. Rather, we characterize these phenomena with regard to outburst shapes and frequency of occurrence and explore a range of possible causes, including truncated disks, mass transfer modulations, and Z Camelopardalis type behavior. Arguments are assembled for and against such possible mechanisms, and key observations are suggested. It appears unlikely that accretion disk instabilities are the single common cause of these phenomena, and we are left with either a combination of accretion disk and mass transfer events or a situation in which mass transfer events are somehow responsible for all these varied behaviors.

     We have used the results of recent smoothed particle hydrodynamic simulations of colliding stars to create models appropriate for input into a stellar evolution code. In evolving these models, we find that little or no surface convection occurs, precluding angular momentum loss via a magnetically driven stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers that have been formed by collisions. Angular momentum transfer to either a circumstellar disk (possibly collisional ejecta) or a nearby companion are plausible mechanisms for explaining the observed low rotation velocities of blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397 and 47 Tuc have been created solely by collisions, we find that the majority of blue stragglers cannot have been highly mixed by convection or meridional circulation currents at any time during their evolution. Also, on the basis of the agreement between the predictions of our nonrotating models and the observed blue straggler distribution, the evolution of blue stragglers is apparently not dominated by the effects of rotation.

     We present the results of searches for EUV emission from neutron stars conducted with the ExtremeUltravioletExplorer Deep Survey and scanner telescopes. To date, 21 fields containing known neutron stars have been observed in the Lexan/boron (40–190 Å) band. Of these, 11 fields were simultaneously observed in the aluminum/carbon (160–385 Å) band. Five neutron stars that have been detected in the EUV have been reported previously; no new detections have been made in the studies reported here. For those sources not detected, we have used the observations to obtain limits on the spectral flux from the neutron stars in these bands. We provide means to convert these fluxes into intrinsic source fluxes for blackbody and power-law spectra for varying levels of absorption by the interstellar medium.

     We have obtained new radial velocities of the double-lined spectroscopic binary star 12 Persei, whose period is 331 days, and which has been resolved in recent years by speckle interferometry. We derive a solution for the orbital elements from the speckle and radial velocity data simultaneously, and find from that solution masses of 1.306 ± 0.035 and 1.172 ± 0.030 M_☉ for the components, and an orbital parallax of 004224 ± 000056. We also determine spectroscopically the difference in magnitude between the components and, hence, their absolute magnitudes. We estimate their spectral types and individual colors by fitting standard-star colors to those observed and use those results to find effective temperatures, bolometric corrections, and radii, and hence surface gravities and mean densities for the components. Finally, we find projected rotational velocities. The stars seem to be well above the zero-age main sequence, but appear to have metallicities greater than the Sun; if so, they would lie closer to one appropriate to their metal content.

     We have used spectra taken between 1992 and 1997 to derive the spectroscopic orbit of the eclipsing double-lined spectroscopic binary HD 197770. This binary has a period of 99.69 ± 0.02 days and K amplitudes of 31.2 ± 0.8 and 47.1 ± 0.4 km s^-1 for components A and B, respectively. The m sin³i values for A and B are 2.9 and 1.9, respectively, and are close to the actual masses because of the eclipsing nature of this binary. Both components of HD 197770 have spectral types near B2 III. This means that both components are undermassive by about a factor of 5 and, thus, are evolved stars. Additional evidence of the evolved nature of HD 197770 is found in 25, 60, and 100 μm IRAS images of HD 197770. These images show two apparent shells centered on HD 197770, a bright 60 μm shell with a 28' diameter and a larger (08 diameter) bubble-like feature. At least one of the components of HD 197770 is likely to be a post–asymptotic giant branch star.

     We present high-quality spectra of the Hα emission line of the nova-like variable V Sge, obtained during a photometric bright state during 1986 July. The profiles have several components, including (1) a very broad, flat-topped emission line that forms in the optically thin wind of one of the stars, (2) a central, narrow peak that probably originates in a circumbinary shell, and (3) a moving component that attains an extreme velocity of approach just prior to secondary eclipse (when the brighter and lower mass Roche lobe–filling primary star is in the foreground). We review models for the emission based on the expected collision of the stellar winds of both stars, and suggest that the moving component originates in a high-density region that extends from between the stars to above the leading hemisphere of the primary. The example of V Sge shows that interacting binaries may cease mass transfer in favor of mass ejection from the system at some stage in their evolution.

     We present our data set of same-hemisphere, or fixed-phase, observations of five RS Canum Venaticorum and BY Draconis binaries made with the Solar-Stellar Spectrograph at Lowell Observatory. The purpose of this study is to evaluate the level of intrinsic variability in the components of these systems, and to compare it with variability arising from rotational modulation and from long-term, activity cycle–related variability. We obtain fixed-phase observations by observing tidally locked binaries with near–integral-day periods. The fixed-phase observations reveal that the intrinsic variability of the stellar hemispheres is significantly less than the variability induced by rotational modulation or by long-term, activity-cyclic evolution, but onlywhenseveralspectralfeaturesareconsideredintandem. Fixed-phase "flickering" of the stellar hemispheres is often apparent at a higher level than the rotational or long-term modulation in individual activity indicators. Interpretation of rotational or long-term modulation in active, late-type stars using data that do not allow quantification of the fixed-phase variability of the stars (i.e., data that do not span several rotational periods) is extremely risky.

     Three-epoch observations of the M1 V–plus–brown dwarf system Gliese 229AB using the HubbleSpaceTelescope's Wide Field Planetary Camera 2 (WFPC2) are reported. Relative astrometric measurements spanning 1 year confirm the common proper motion of the pair and reveal the first evidence of orbital motion. The radial and azimuthal components of Gl 229B's relative motion are -0087 and +0049, respectively, indicating an elliptical orbit rather than an inclined circular orbit. The absolute WFPC2 magnitudes of Gl 229B are M₁₀₄₂ = 16.37, M₈₁₄ = 20.76, and M₆₇₅ = 24.60, assuming a distance of 5.774 pc. The detection of Gl 229B through the F675W bandpass is the first reported at R-band wavelengths. The measured flux through F1042M (λ_c ≈ 1 μm) is well matched by the latest models of dust-free brown dwarf photospheres by Tsuji et al. These models severely overestimate the broadband fluxes shortward of 0.8 μm, however, which indicates that a strong source of optical continuum opacity exists in the photosphere of Gl 229B. A search for fainter companions to Gl 229A in the Planetary Camera was negative to limiting absolute magnitudes of M₁₀₄₂ ≈ 19 and M₈₁₄ ≈ 24.5 beyond 7'' of Gl 229A. Nondetection limits for the Wide Field Cameras are 1.5 mag greater than those for the Planetary Camera.

     We have carried out a survey of the entire Einstein IPC database (a total of 5934 sources) to select objects with strong components of ultrasoft X-ray emission (i.e., emission below 0.5 keV). The selection criteria are based on ratios of counts within three broad energy bands (i.e., X-ray "colors"). A total of 516 objects have been selected, with 202 (39%) having been identified through correlations with other databases. Of the currently identified objects, 45% are active galactic nuclei, 28% are stellar objects, 6% are "normal" galaxies and galaxy clusters, and the remaining 21% are "nonstellar" Galactic objects such as cataclysmic variables, pulsars, and white dwarfs. We present a discussion of the survey selection procedure, the list of selected sources, and discussions of interesting objects from the aforementioned groups. We discuss the remarkable success of our rather "low resolution" method of estimating spectral shape and present plans for follow-up studies of the as yet unidentified sources.

     We report J, H, and K photometry of 86 stars in 40 fields in the Northern Hemisphere. The fields are smaller than or comparable to a 4' × 4' field of view and are roughly uniformly distributed over the sky, making them suitable for a homogeneous broadband calibration network for near-infrared panoramic detectors. K magnitudes range from 8.5 to 14 and J-K colors, from -0.2 to 1.2. The photometry is derived from a total of 3899 reduced images; each star has been measured, on average, 26.0 times per filter on 5.5 nights. Typical errors on the photometry are ∼0.012 mag.

     Asteroid 3753 (1986 TO) is in a 1:1 mean motion resonance with Earth, on a complex horseshoe-type orbit. Numerical experiments are performed to determine its medium-term stability and the means by which it may have entered its current orbit. Though 3753 moves primarily under the influence of the Sun and Earth, the giant planets (and Jupiter especially) play an important role by influencing, through torque-induced precession, the position of the asteroid's nodes. Variations in the nodal distance strongly affect the interaction of 3753 with Earth and may change or destroy the horseshoe-like behavior currently seen. This precession of the nodes provides a mechanism for placing minor planets into, or removing them from, a variety of horseshoe-type orbits. The chaotic nature of this asteroid's orbit makes predictions difficult on timescales longer than its Lyapunov time (∼150 yr); therefore, ensembles of particles on orbits near that of 3753 are considered. The asteroid has a high probability of passing close to Venus and/or Mars on 10⁴ yr timescales, pointing to a dynamical age much shorter than that of the solar system.