Table of contents

We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 ≤ z ≤ 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters: the Hubble constant (H₀), the mass density (Ω_M), the cosmological constant (i.e., the vacuum energy density, Ω_Λ), the deceleration parameter (q₀), and the dynamical age of the universe (t₀). The distances of the high-redshift SNe Ia are, on average, 10%–15% farther than expected in a low mass density (Ω_M = 0.2) universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Ω_Λ > 0) and a current acceleration of the expansion (i.e., q₀ < 0). With no prior constraint on mass density other than Ω_M ≥ 0, the spectroscopically confirmed SNe Ia are statistically consistent with q₀ < 0 at the 2.8 σ and 3.9 σ confidence levels, and with Ω_Λ > 0 at the 3.0 σ and 4.0 σ confidence levels, for two different fitting methods, respectively. Fixing a "minimal" mass density, Ω_M = 0.2, results in the weakest detection, Ω_Λ > 0 at the 3.0 σ confidence level from one of the two methods. For a flat universe prior (Ω_M + Ω_Λ = 1), the spectroscopically confirmed SNe Ia require Ω_Λ > 0 at 7 σ and 9 σ formal statistical significance for the two different fitting methods. A universe closed by ordinary matter (i.e., Ω_M = 1) is formally ruled out at the 7 σ to 8 σ confidence level for the two different fitting methods. We estimate the dynamical age of the universe to be 14.2 ± 1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely effect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these effects appear to reconcile the data with Ω_Λ = 0 and q₀ ≥ 0.

We report on sensitive radio observations made with the VLA at 8.5 GHz centered on the Hubble Deep Field (HDF). We collected data in the A, CnB, C, DnC, and D configurations corresponding to angular resolutions ranging from 03 to 10''. We detected 29 radio sources in a complete sample within 46 of the HDF center and above a flux density limit of 9.0 μJy (5 σ). Seven of these sources are located within the HDF itself, while the remaining 22 sources are covered by the Hubble flanking fields (HFFs) or ground-based optical images. All of the sources in the HDF are identified with galaxies with a mean magnitude R = 21.7, while the mean magnitude of the identifications outside the HDF is R = 22.1. One radio source in the HDF, which is just below our formal completeness level but is confirmed by independent 1.4 GHz observations, has no optical counterpart above the HDF limit of R = 29. Three radio sources outside the HDF have no optical counterparts to R = 27. Based on a radio and optical positional coincidence, we detected an additional 19 radio sources in this field (seven of which are contained in the HDF) with 6.3 μJy ≤S_ν < 9.0 μJy (3.5 σ ≤ S_ν < 5 σ) and R ≤ 25 but which are not included in the complete sample.

The microjansky radio sources are distributed over a wide range of redshifts (0.1 < z < 3) and have a typical monochromatic luminosity of about 10²³ W Hz^-1. The majority of the optical identifications are with luminous (L > L_*) galaxies at modest redshifts (0 ≲ z ≲ 1), many with evidence for recent star formation. The remainder are composed of a mixture of bright field elliptical galaxies and late-type galaxies with evidence of nuclear activity (LINERs and Seyfert galaxies) and nearby (z < 0.2) field spiral galaxies. None of the radio sources in this survey are identified with quasars or galactic stars.

     The Hubble Deep Field South (HDFS) has been recently selected, and the observations are planned for 1998 October. We present a high-resolution (FWHM ≃ 14 km s^-1) spectrum of the quasar J2233-606 (z_em ≃ 2.22), which is located 51 east of the HDFS. The spectrum obtained with the New Technology Telescope redward of the Lyα emission line covers the spectral range 4386–8270 Å. This range corresponds to redshift intervals for C IV and Mg II intervening systems of z = 1.83–2.25 and z = 0.57–1.95, respectively. The data reveal the presence of two complex intervening C IV systems at redshift z = 1.869 and z = 1.943, and two complex associated (z_abs ≈ z_em) systems. Two other C IV systems at z = 1.7865 and z = 2.077, suggested by the presence of strong Lyα lines in low-resolution ground-based and HubbleSpaceTelescope (HST) observations, have been identified. The system at z = 1.943 is also responsible for the Lyman limit absorption seen in the HST Space Telescope Imaging Spectrograph spectrum. The main goal of the present work is to provide astronomers interested in the HDFS program with information related to absorbing structures at high redshift, which are distributed along the nearby QSO line of sight. For this purpose, the reduced spectrum, obtained from 3 hr of integration time, has been released to the astronomical community.

     We apply the Lyman absorption signature to search for galaxies at redshifts z ≈ 6–17 using optical and infrared images of the Hubble Deep Field. The infrared images are sensitive to a point-source 5 σ detection threshold of AB(22,000) = 23.8, which, adopting plausible assumptions to relate rest-frame ultraviolet flux densities to unobscured star formation rates, is easily sufficient to detect the star formation rates expected for massive elliptical galaxy formation to quite high redshifts. For q₀ = 0.5, the infrared images are sensitive to an unobscured star formation rate of = 100 h^-2M_⊙ yr^-1 to redshifts as large as z = 17, and for q₀ = 0, the infrared images are sensitive to an unobscured star formation rate of = 300 h^-2M_⊙ yr^-1 to redshifts as large as z = 14. The primary result of the analysis is that only one extremely high redshift galaxy candidate is identified at the 5 σ level of significance (and four at the 4 σ level). This implies a strict upper limit to the surface density of extremely high redshift galaxies of less than 1.5 arcmin^-2 to a limiting magnitude threshold AB(22,000) = 23.8. This also implies a strict upper limit to the volume density of extremely high redshift galaxies if (and only if) such galaxies are not highly obscured by dust.

A method of obtaining approximate redshifts and spectral types of galaxies using a photometric system of six broad-bandpass filters is developed. The technique utilizes a smallest maximum difference approach rather than a least-squares approach, and does not consider a galaxy's apparent magnitude in the determination of its redshift. In an evalution of its accuracy using two distinct galaxy samples, the photometric redshifts are found to have a root mean square deviation of ±0.05 from spectroscopically determined redshifts. Possible systematic errors of the method are investigated, including the effects of post-starburst ("E+A") galaxies and attempts to measure redshifts with incomplete color information. Applications of the technique are discussed.

     A complete sample of 659 field galaxies with 17.0 < U ≤ 21.1, each with UBVR-I7500-I8600 photometry, has been selected from a deep field survey that covers 0.83 deg² along six lines of sight. Each galaxy's spectral type and redshift have been estimated using a multicolor photometric technique of Liu & Green. Total number counts of the galaxies in the U band give a count slope d(log N)/dM = 0.55 ± 0.05, consistent with previous studies. The 545 galaxies in the sample classified as spectral type Sbc or bluer are analyzed for signs of evolution with redshift and for unusual star formation histories. The U-band luminosity function of these blue galaxies at 0.02 < z < 0.15 has a steep α ≃ -1.85 down to M(B) ≃ -14. The luminosity functions at 0.15 ≤ z < 0.3 and 0.3 ≤ z ≤ 0.5 show significant evolution in M* and ϕ* at levels consistent with those found in the Canada-France and Autofib redshift surveys. A significant population of very blue (rest-frame U - B < -0.35) galaxies, with spectral energy distributions indicating strong starburst activity, is observed at z ≳ 0.3 but not at z < 0.3. This population is confirmed via spectroscopy of part of the sample. These may be galaxies temporarily brightened by global starbursts, which subsequently fade and redden at lower redshifts.

     A search for preferential galaxy alignments in the Pisces-Perseus Supercluster (PPS) is made using the Minnesota Automated Plate Scanner Pisces-Perseus Survey (MAPS-PP). The MAPS-PP is a catalog of ∼1400 galaxies with a (roughly) isophotal diameter greater than 30'' constructed from digitized scans of the blue and red plates of the Palomar Observatory Sky Survey covering the PPS. This is the largest sample of galaxies applied to a search of galaxy alignments in this supercluster, and it has been used in combination with previously published redshifts to construct the deepest PPS galaxy luminosity function to date. While previous studies have relied extensively on catalogs with visually estimated parameters for both sample selection and determination of galaxy orientation, the MAPS-PP uses selection criteria and measurements that are entirely machine and computer based. Therefore, it is not susceptible to some of the biases, such as the diameter inclination effect, known to exist in some other galaxy catalogs. The presence of anisotropic galaxy distributions is determined by use of the Kuiper statistic, a robust alternative to the χ² statistic more traditionally used in these studies. Three statistically significant anisotropic distributions are observed. The reddest galaxies are observed to be oriented preferentially perpendicular to the local large-scale structure. The bluest galaxies near the supercluster plane are observed to have an anisotropic position angle distribution. Finally, a weak trend for the median position angle of color-selected galaxy subsamples to "twist" with increasing distance from the PPS plane is observed. These position angle distribution anisotropies are weak and are not consistent with any single primordial or modern-era galaxy alignment mechanism, although a mixture of such mechanisms is not ruled out.

     We derive I-band luminosity functions for galaxies in Abell 426 (Perseus and Abell 539, two rich, low Galactic latitude clusters at moderate redshift. Cluster members are selected via the color-magnitude relation for bright galaxies. We find α = -1.56 ± 0.07 for Perseus over the range -19.4 < M_I < -13.4 (15 < I < 21) and α = -1.42 ± 0.14 for -18.5 < M_I < -14 (17 < I < 21.5) for A539. These luminosity functions are similar to those derived in Virgo and Fornax, weakly supporting claims for the existence of a universal luminosity function for galaxies in clusters.

     A study of mass distributions in 22 spiral galaxies, with rotation curves taken from the literature and Hubble types ranging from Sa to Sd, is presented. It is demonstrated that a relation of the form between the mass distribution of the luminous components M(r) and that of the dark halo M_d(r) is consistent with the observed kinematics with only the dark-to-visible mass ratio displaying significant variations between galaxies. The parameter γ, which is correlated with M_dyn/M_lum, characterizes the relative importance of the dark-to-luminous mass. The model M/L ratio of the stellar component, M/L_B(stars), is correlated with the observed B - V color. In our sample the average model ⟨M/L_B(stars)⟩ of Sb galaxies is 2.7 M_⊙L for an average color ⟨(B - V)₀⟩ = 0.71, that of the Sc's is ⟨M/L_B(stars)⟩ = 1.5 M_⊙L for ⟨(B - V)₀⟩ = 0.54, both values being in very good agreement with population synthesis models. A large part of the intrinsic scatter in the Tully-Fisher relation is due to the spread in γ. The relation of coupling accounts for the disk-halo conspiracy in HSB spirals and is also valid in LSB dwarf galaxies. Its universality in disk rotating systems implies the existence of a physical mechanism responsible for the continuity between visible and dark mass distributions.

     We present optical and near-infrared surface photometry of four early-type galaxies considered to have resonance ring phenomena on the basis of morphology. We also present kinematics and rotation curves for three of the galaxies. The four galaxies are exceptional examples of resonance ring galaxies, and from these observations we derive the characteristic light distributions, the colors of the rings, and the near-infrared disk mass-to-light ratios. We also deproject the galaxies to examine intrinsic ring shapes and bar/ring alignments. From models of the rotation curves, we compute the Lindblad precession frequencies to examine resonance locations in the disks of the galaxies. Under the assumption that the outer rings and pseudorings seen in these galaxies are linked to the outer Lindblad resonance, we find that the inner rings of the sample galaxies lie near the inner 4:1 resonance and that the sharp ends of the bars lie on average at only 0.65 times the predicted radius of corotation.

     We quantify the mean asymmetry of 54 face-on, early-type disk galaxies (S0 to Sab) using the amplitude of the m = 1 azimuthal Fourier component of the R-band surface brightness. We find that the median lopsidedness, ⟨A₁/A₀⟩, of our sample is 0.11 and that the most lopsided 20% of our galaxies have ⟨A₁/A₀⟩ ≥ 0.19. Asymmetries in early-type disks appear to be of similar frequency and strength as in late-type disk galaxies. We have observed our early-type disks in a bandpass (R band) in which the light is dominated by stars with ages greater than 10⁹ yr and therefore are seeing azimuthal asymmetries in the stellar mass distribution. The similar degree of lopsidedness seen in disks of very different star formation rates indicates that the lopsidedness in all galactic disks is primarily due to azimuthal mass asymmetries. Hence, 20% of all disk galaxies (regardless of Hubble type) have azimuthal asymmetries, ⟨A₁/A₀⟩ ≥ 0.19, in their stellar disk mass distribution, confirming lopsidedness as a dynamical phenomenon.

     We have used the Nançay decimetric radio telescope to obtain high-resolution, high signal-to-noise, global H I spectra of 30 southern, extreme late-type spiral galaxies with V_h ≤ 3000 km s^-1. For all of the galaxies we measure new precise radial velocities and H I velocity widths. Optical imaging and photometry of these galaxies was recently published by Matthews & Gallagher. In Appendix B we present new H I spectra for four additional late-type galaxies. The H I spectra of the extreme late-type spirals have velocity widths characteristic of true, rotationally supported disk galaxies, and most are double-peaked. Nonetheless, the morphologies of the H I profiles are diverse, indicating that disk galaxies with similar H I contents and optical luminosities can have very different H I distributions and/or velocity fields. H I profile type and Hubble type are generally well-correlated for our sample. Seventy-seven percent of our H I profiles show at least some degree of asymmetry; in many cases the stellar disk of these galaxies is also asymmetric. Several of our spectra have an additional H I peak near the systemic velocity, perhaps indicating a gas enhancement in the central regions of the galaxy disk.

     We present the results of high spatial resolution H I observations of five intrinsically compact dwarf galaxies that are currently experiencing a strong burst of star formation. The H I maps indicate that these systems have a complex and clumpy interstellar medium. Unlike typical dwarf irregular galaxies, these blue compact dwarf galaxies (BCDs) exhibit strong central concentrations in their neutral gas distributions that may provide a clue to the origin of their strong starburst activity. Furthermore, while all of the systems appear to be rotating, based on observed velocity gradients, the kinematics are complex. All systems have nonordered kinematic structure at some level; some of the extended gas is not necessarily kinematically connected to the main system. The observed gas distributions and kinematics place constraints on evolutionary scenarios for BCDs. Evolutionary links between BCDs, dwarf irregular galaxies, and dwarf elliptical galaxies have been postulated to explain their high star formation rates and low-luminosity, low-metallicity nature. The BCDs appear to have higher central mass concentrations in both gas and stellar content than the dwarf irregulars, indicating that evolutionary scenarios connecting these two classes will require mass redistribution. In addition, the fact that BCDs are rotationally supported systems indicates that they are unlikely to evolve into dwarf ellipticals without substantial loss of angular momentum. Thus, while such evolutionary scenarios may still be possible with the aid of mergers or tidal interactions, the isolated nature of BCDs suggests that the majority of BCDs will not fade to become objects similar to the present-day dwarf ellipticals.

     A Goddard High Resolution Spectrograph archival study of the interstellar absorption lines in the line of sight to the H I–rich, starburst dwarf galaxy NGC 1705 in the 1170 to 1740 Å range at ∼120 km s^-1 resolution is presented. The absorption features arising because of photospheric lines are distinctly different from the interstellar lines: the photospheric lines are weak, broad (equivalent widths >1 Å), asymmetric, and centered around the systemic LSR velocity of NGC 1705 (∼610 km s^-1). The interstellar lines consist of three relatively narrow components at LSR velocities of -20, 260, and 540 km s^-1, and include absorption by neutral atoms (N I λ1200 triplet and O I λ1302), singly ionized atoms (Si II λλ1190, 1193, 1260, 1304, and 1526, S II λ1253, C II λ1334, C II* λ1336, Fe II λ1608, and Al II λ1670), and atoms in higher ionization states (Si III λ1206, Si IV λλ1393, 1402, and C IV λλ1548, 1550). The Si IV and C IV absorption features have both interstellar and photospheric contributions. In an earlier study, Sahu & Blades identified the absorption system at -20 km s^-1 with Milky Way disk/halo gas, and the 260 km s^-1 system with a small, isolated high-velocity cloud HVC 487, which is probably associated with Magellanic Stream gas. The 540 km s^-1 absorption system is associated with a kiloparsec-scale expanding, ionized supershell centered on the super–star cluster NGC 1705-1. The analysis presented in this paper consists of (1) a list of all interstellar absorption features with greater than 3 σ significance and their measured equivalent widths, (2) plots of the lines in the various atomic species together with the results of nonlinear least-squares fit profiles to the observed data, and (3) unpublished 21 cm maps from the Wakker & van Woerden survey showing the large-scale H I distribution in the region near the NGC 1705 sight line and HVC 487. Furthermore, weak N I λ1200 triplet absorption for the supershell component is reported, which in the absence of dust depletion and ionization corrections implies a low N abundance. A low N abundance for the supershell is consistent with an interpretation of nucleosynthetic enrichment by time-delayed primary nitrogen production, the age estimate of (10–20) × 10⁶ yr for the central super–star cluster NGC 1705-1 of Heckman & Leitherer, and the underabundance of Fe reported by Sahu & Blades. However, using the N I λ1200 triplet alone to estimate the total N abundance could result in a severe underestimation of this quantity: although N does not deplete onto interstellar dust grains, photoionization and collisional ionization effects could increase the fraction of N found in higher ionization stages. Uncertainties in the total N abundance caused by photoionization and collisional ionization effects can only be addressed by future observations of the higher ionization lines, namely, N II λ1084 and N III λ989.

     We present VLA images of the radio continuum emission of the starburst in NGC 5253 at wavelengths of 6, 3.6, and 2 cm, with resolutions of 1''–2'', and at 20 cm with resolution of ∼9''. A spectral index map constructed from the 6 and 2 cm images indicates that thermal free-free emission from ionized gas, most of it from a single source, dominates the radio emission. The unusually high fraction of free-free emission suggests that the starburst is very young. Regions of diffuse nonthermal synchrotron emission and probable supernova remnants surround the central radio source. We detect several regions of rising spectral index in NGC 5253, suggesting that the free-free emission is optically thick at 2 cm. These nebulae are situated to the north of the central cluster/main radio source. The turnover frequencies require an emission measure (EM) ∼ 10⁸–10⁹ cm^-6 pc, and ⟨n⟩^1/2 ∼ 10⁴–10⁵ cm^-3. From the observed intensities, we infer diameters of a few tenths of a parsec for these regions, which is an order of magnitude larger than the diameters of H II regions of comparable density in the Galaxy. Lyman continuum rates of over N_Lyc ∼ 10⁵¹ s^-1 are required to ionize these high densities and volumes. Unlike Galactic compact H II regions, which contain no more than a few O stars, the optically thick H II regions in NGC 5253 are excited by an estimated 200–1000 O stars each. This is consistent with the idea that very large clusters are the mode of star formation in the central region of NGC 5253. The optically thick H II regions, located to the north of the dominant optical and radio sources, may well trace the very youngest parts of the starburst.

     We use high-resolution near-infrared (J- and K-band) images of the nuclear starburst galaxy NGC 6946 to compare stellar morphology with the previously published distribution of CO in the central regions. We find a small stellar bar with a length of 210 pc, which lies perpendicular to an 1860 pc long stellar oval distortion. The small bar has a position angle of 140°, which coincides with a barlike feature in CO, as well as with the kinematic minor axis. It has a flat light distribution and a strong bar-interbar contrast of 1 mag; it may play a role in fueling the central starburst despite its small size. A biconical structure about 80 pc long, perpendicular to the bar, is evident in the (J - K) color map. The (J - K) contours match published high-resolution CO contours very well.

     We have used the HubbleSpaceTelescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands: F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modified version of DoPHOT. Artificial star tests showed the data to be 50% complete to V = 26.3, B = 25.4, and I = 25.2. The color-magnitude diagrams contain well-defined populations, including a very young main sequence (MS) (<10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of μ = 26.75 ± 0.35 (2.2 Mpc), which we adopted. An extended stellar "halo" was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 M_⊙ Myr^-1 kpc^-2 with up to 60% variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100–200 pc), which lasted ∼100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies: Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger M_{H I}/L_B. Also, the star formation pattern is similar in all four galaxies. Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.

     The unprecedented detail of the Wide Field Planetary Camera 2 (WFPC2) color-magnitude diagrams (CMDs) of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops, and the tip of the red giant branch, we obtain a distance modulus, m - M = 24.2 ± 0.2, or 690 ± 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low-metallicity (Z = 0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e., less than 2 Gyr old. A major episode of star formation 900–1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occurred in this galaxy. Leo A provides a nearby laboratory for studying young metal-poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

Was star formation in the OB associations, LH 51 and LH 54, triggered by the growth of the superbubble DEM 192? To examine this possibility, we investigate the stellar contents and star formation history and model the evolution of the shell. H-R diagrams constructed from UBV photometry and spectral classifications indicate highly coeval star formation, with the entire massive star population having an age of ≲2–3 Myr. However, LH 54 is constrained to an age of ∼3 Myr by the presence of a W-R star, and the initial mass function (IMF) for LH 51 suggests a lower mass limit implying an age of 1–2 Myr. There is no evidence of an earlier stellar population to create the superbubble, but the modeled shell kinematics are consistent with an origin due to the strongest stellar winds of LH 54. It might therefore be possible that LH 54 created the superbubble, which in turn may have triggered the creation of LH 51. Within the errors, the spatial distribution of stellar masses and IMF appear uniform within the associations.

We reinvestigate the estimates for stellar wind power L_w(t) during the H-burning phase and note that revised mass-loss rates yield a significantly different form for L_w(t) and may affect stellar evolution timescales. We also model superbubble expansion into an ambient medium with a sudden, discontinuous drop in density and find that this can easily reproduce the anomalously high-shell expansion velocities seen in many superbubbles.

We present UBV photometry of four fields within Shapley Constellation III and one field on the edge of the shell. Our fields cover roughly 20% of the region, mostly in the southern half. Determinations are made of ages of the fields, the star formation densities, and the initial mass function (IMF) slopes. The field-age determinations inside the constellation show ages between 12 and 16 Myr uncorrelated with distance from the center, while the age of the field on the edge of the constellation shows an age of around 6–7 Myr. The southern part of the constellation shows star formation densities and IMF slopes typical of OB associations and giant H II regions, while the northern part shows significantly fewer intermediate-mass stars and a steeper IMF slope. We compare these properties of Constellation III with those of 30 Doradus, another LMC star-forming region of comparable size to Constellation III. Although the regions formed from roughly the same amount of gas, we estimate that 30 Doradus formed a few times more stars than Constellation III.

     We have examined the [Na/Fe] and [Mg/Fe] ratios in a sample of 68 field halo giants with -3 ≲ [Fe/H] ≲ -1. We recalculated the Galactic (U, V, W) velocity components for these stars, using Hipparcos proper motions and a new Hipparcos-based distance scale. We used these data to see how the abundance ratios may relate to kinematical substructure in the Galactic halo. To isolate a set of true halo stars, we eliminated metal-weak thick-disk stars, about 10% of our sample. The field halo giants show the expected correlation of Na and Mg abundances, so we can use Na as a surrogate for Mg and the α-elements. The most metal-poor stars show a wider dispersion of [Na/Fe] ratios than do the less metal-poor stars; the difference is most striking for stars on retrograde galactic orbits. Some 20% of our retrograde giants and 13% of all our halo giants have [Na/Fe] ≤ -0.35 and may be significantly younger than the oldest halo objects. Halo giants considered "young" by this Na abundance criterion show a preference for retrograde orbits. Giants in some globular clusters (e.g., M13) do not exhibit the Mg versus Na correlation found among halo field giants. Instead, they have very large [Na/Fe] ratios and widely scattered [Mg/Fe] ratios, probably induced by deep mixing, which field halo giants apparently do not experience.

     We present VI color-magnitude diagrams (CMDs) of the globular cluster NGC 6558 and its surrounding field, obtaining cluster parameters. The cluster has a blue extended horizontal branch and a depleted red giant branch, characteristics already observed in other post–core-collapse clusters in the bulge, such as NGC 6522, NGC 6540, and HP 1. These clusters do not belong to the metal-rich stellar population, and they may define a distinct class in the bulge. We derive a reddening of E(B - V) = 0.50 and a distance d_⊙ ≈ 6.3 kpc. The CMD morphology is consistent with a metallicity range of -1.6 < [Fe/H] < -1.2. We also report photometry of the bulge field population (b = -6° on the minor axis) in which NGC 6558 is imbedded. As is the case for other bulge fields, there is a prominent red clump and a strong descending red giant branch, similar to that seen in lower latitude bulge fields; we estimate the metallicity of the bulge population to lie between that of 47 Tuc and the nearly solar-metallicity cluster NGC 6553 (i.e., approximately -0.3 dex).

We examine images of the field of X1832-330, the luminous (L_X ∼ 10³⁶ ergs s^-1) X-ray burst source near the center of the globular cluster NGC 6652, in order to identify the optical counterpart for further study. U and B ground-based images allow us to set a limit M ≳ 3.5 for the counterpart at the time of those observations, provided that the color is (U - B)₀ ∼ -1, similar to the sources known in other clusters. Archival HubbleSpaceTelescope observations survey most, but not all, of the 1 σ X-ray error circle and allow us to set limits M > 5.9 and M > 5.2 in the Wide Field Planetary Camera 1 (WF/PC-1) and Wide Field Planetary Camera 2 (WFPC2) regions, respectively.

In the WF/PC-1 images we do weakly detect a faint object with UV excess, but it is located 117 from the ROSAT X-ray position. This considerable (2.3 σ) discrepancy in position suggests that this candidate be treated with caution, but it remains the only reasonable one advanced thus far. We measure for this star m₄₃₉ = 20.2 ± 0.2 and m₃₃₆ - m₄₃₉ = -0.5 ± 0.2 and estimate M = 5.5 and (U - B)₀ = -0.9, similar to other known optical counterparts. If this candidate is not the identification, our limits imply that the true counterpart, not yet identified, is probably the optically faintest cluster source yet known or, alternatively, that it did not show significant UV excess at the time of these observations. Finally, we assess the outlook for the identification of the remaining luminous globular cluster X-ray sources.

     I examine the influence of planets on the location of stars on the Hertzsprung-Russel diagram as the stars turn to the horizontal branch. As stars that have planetary systems evolve along the red giant branch and expand, they interact with the close planets, with orbital separations of ≲5 AU. The planets deposit angular momentum and energy into the red giant stars' envelopes, both of which are likely to enhance mass loss on the red giant branch. The enhanced mass loss causes the star to become bluer as it turns to the horizontal branch. I propose that the presence of planetary systems, through this mechanism, can explain some anomalies in horizontal branch morphologies. In particular, planetary systems may be related to the "second parameter," which determines the distribution of horizontal branch stars on the Hertzsprung-Russel diagram. The presence of planets, though, cannot be the only factor that influences the "second parameter." The distribution of planets' properties (e.g., mass, orbital separation, prevalence) in a specific globular cluster depends on several properties of the globular cluster itself (e.g., shape, density). This dependence may explain some of the anomalies and variations in the horizontal branch morphologies between different globular clusters. The proposed scenario predicts that surviving massive planets or brown dwarfs orbit some of the extreme blue horizontal branch stars, at orbital periods of ∼10 days.

     The Sco OB2 complex is a member of the Local Association. The association contains pre–main-sequence stars in addition to objects some 3 × 10⁷ yr old. If it is assumed that stars in wide binaries are coeval, then the He-weak variables in the supercluster are pre–main-sequence, elevated above that sequence by an amount similar to that of the 3 × 10⁷ yr old, normal stars but contracting toward the sequence, not evolving from it. The apparent great depth of the association is probably a result of some foreground supercluster members, superposed on Sco OB2, and a rift in the absorption cloud between Scorpius and Ophiuchus. The adjoining Chamaeleon complex also appears to be a member of the supercluster.

     We present the results of VLA observations in the ground-state hydroxyl (OH) transition at 1720 MHz toward 20 supernova remnants (SNRs). We detect compact emission from four objects. For three of these objects (G32.8-0.1, G337.8-0.1, and G346.6-0.2), we argue that the emission results from masers that are shock-excited due to the interaction of the SNR and an adjacent molecular cloud. We observe a characteristic Zeeman profile in the Stokes V spectrum, which allows us to derive a magnetic field of 1.5 and 1.7 mG for G32.8-0.1 and G346.6-0.2, respectively. The velocity of the masers also allows us to determine a kinematic distance to the supernova remnants (SNRs). Our criteria for a maser to be associated with an SNR along the line of sight are that the position and velocity of the maser and SNR must agree and the OH (1720) emission must be unaccompanied by other OH lines.

     We present results of a recent 2–12 μm ground-based infrared (IR) imaging study of Eta Carinae. We discuss the comparative spatial morphology of the bipolar lobes, the "skirt" defined by the equatorial ejecta, and the bright features in the core. We derive separate color temperature values of 200, 240, and 420 K for the bipolar lobes, skirt, and core, respectively. The mass of the Homunculus Nebula estimated from thermal dust emission and a standard gas-to-dust ratio is 2.5 M_⊙. Roughly one-fifth of the total mass is contained in the fast-moving equatorial ejecta. These mass estimates, combined with expansion velocities in the lobes and the skirt, lead us to conclude that the 1890 outburst and the Great Eruption of 1843 were almost comparable in terms of total kinetic energy output. We argue that it is unlikely that these eruptions were caused by tidal interactions with a companion star.

     The structure and conditions within the Helix Nebula have been determined from emission-line images in Hβ, [O III], and He II and using spectra tracing a radial from the central star almost out to the northern boundary of the optical object. The nebula is approximately like a thick disk, rather than the torus suggested by low-ionization ions. The central region, previously thought to be a cavity, is filled with He⁺² with a total gas density comparable to the main ring. The electron temperature in the outer part was determined from [N II] line ratios to be 9400 ± 200 K, while [O III] lines gave 11700 ± 700 K for the bulk of the main ring of intermediate-ionization material. The Hα/Hβ ratio is anomalously low in the central portion, suggesting that the electron temperature there is very high. The likely source of this elevated central temperature is heating by photoelectric electrons from grains mixed in with the nebular gas, since this process depends directly on the distance from the central star and becomes relatively more important than photoionization heating at the low nebular densities that apply for the helix. However, it is unlikely that the two-phase condition that can exist when photoelectric heating dominates has produced the cometary knots that freely populate this nearby planetary nebula.

     First results from a HubbleSpaceTelescope Wide Field Planetary Camera 2 Hα imaging survey of young planetary nebulae (PNs) are reported. The PNs have been selected on the basis of their low excitation characteristics. All objects imaged so far show highly aspherical morphology, with a majority characterized by multipolar bubbles distributed roughly point-symmetrically around the central star. In some objects, bipolar ansae or collimated radial structures are seen, indicating the presence of jets, whereas in others bright structures near the minor axes indicate the presence of disks or torii. The complexity, organization, and symmetry of the above structures lead us to propose that the primary agent for shaping PNs is high-speed collimated outflows or jets that operate during the late asymptotic galactic branch (AGB) and/or early post-AGB evolutionary phase, and not a preexisting equatorial density enhancement as envisioned in the currently popular model. These outflows carve out a complex imprint within an intrinsically spherical AGB circumstellar envelope (CSE). Subsequent expansion of a hot, tenuous stellar wind from the post-AGB star inside the imprinted AGB CSE then produces the observed PN, whose shape and structure depend in detail on how the characteristics of the jets change with time.

     We present high-resolution, spatially resolved spectra of the nebula surrounding Sakurai's object. We find the expansion velocity of the nebula to be 30.9 ± 0.7 km s^-1, and the nebula extent to be 44'' in diameter. We discuss the wide range of reported distances to Sakurai's object and the dilemma that these present, since some evidence strongly favors a short distance (1.1 kpc) while other data strongly favor a longer distance (5–8 kpc). We also present spectra of Sakurai's star centered near the λ4216 CN band that suggests the star had cooled significantly between 1996 October and 1997 May, but had not cooled further by 1997 July.

     We performed a near-infrared imaging survey toward 23 Bok globules in the southern sky containing IRAS point sources. Visual examination of the images revealed that 15 globules showed evidence of nebular emission or very red stellar objects located at the position of the sources. Analysis of the near-infrared nebulosities present in the images revealed that (1) these nebulosities generally contain one or more stellar-like sources surrounded by a more or less extended component; (2) a couple of possible binaries with separations of about 4'' were found to reside in common infrared nebulosity; (3) infrared reflection nebulae, seen at 2.2 μm, are usually associated with class I sources. The nature of the southern sky sample of Bok globules seems to be similar to that of the northern sample of globules, with similar star formation properties: they tend to form a single or a few stars, and in some cases, they seem to be lodging small aggregates of young stellar objects. We confirm that, in general, the value of the 12/25 μm spectral index is a good indicator of the evolutionary stage of a young stellar object. Large negative 12/25 μm indices seem to indicate younger objects deeply embedded in their clouds (by showing nebulosities mostly in the K band, having associated molecular outflows, and no optical counterparts). As these objects reach later stages of their pre–main-sequence evolution (becoming optically visible), their 12/25 μm indices increase and become positive.

     The physical characteristics of the Cepheus E "embedded" outflow are analyzed using ISOCAM images in the v = 0–0 S(5) 6.91 μm and S(3) 9.66 μm molecular hydrogen lines. We find that the morphology of the Cep E outflow in the ground vibrational H₂ lines is similar to that in the near-infrared v = 1–0 2.12 μm line. At these mid-infrared wavelengths, we detect neither the second H₂ outflow that is almost perpendicular to the Cep E 2.12 μm flow nor traces of H₂ emission along the second ¹²CO J = 2–1 outflow, at an angle of ∼52°, down to a surface brightness of 12–46 μJy arcsec^-2. We do detect at 6.91 μm the likely source of the main H₂ and CO outflows, IRAS 23011+6126, and show that the source is easily seen in all IRAS bands using HIRES images. The source is not detected at 9.66 μm, but we think this agrees with the interstellar extinction curve, which has a minimum at ∼7 μm but rises at ∼9.7 μm as a result of the strong absorption silicate feature, enhanced in this case by a cocoon surrounding the Class 0 object. This idea is supported by our models of the spectral energy distribution (SED) of the central object. The models assume that the main source of opacity is due to bare silicates, and our best fit for the SED yields a total envelope mass of 17 M_⊙ and a dust temperature of 18 K.

     We have discovered a large number of Herbig-Haro (HH) flows associated with the embedded infrared cluster surrounding the IRAS source 05338-0624, the driving source of the L1641-N molecular outflow. The best collimated HH flow in the region, HH 303, stretches in two groups of knots along a well-defined axis from the VLA source that coincides with the IRAS source. This HH flow is coaxial with the blue northern lobe of the molecular flow. To the north-northwest of the cluster we find a long series of very large and finely shaped bow shocks, which stretch on a well-defined axis away from the cluster. The most distant HH object, HH 310, is 6.3 pc in projection from the center of the cluster, making it the largest HH lobe known from a low-mass star. We identify the counterlobe with an already known chain of infrared H₂ emission knots. Additionally, we find two very large fragmented bow shocks, HH 403 and 404, to the northeast of and facing away from the cluster. They lie on an axis that passes through the cluster at projected distances of 4.2 and 5.4 pc. In the opposite direction, to the southwest we identify the previously known object HH 127 at a projected distance of 5.2 pc from the cluster and within an angle of 10° of the HH 403/404 axis. HH 127 consists of three faint knots also on a line through the cluster. Precisely south of the VLA source and on the axis of the HH 303 jet, we find a large bow shock, HH 61, which is 5.9 pc in projection from the cluster, and which we believe is part of the counterflow to the HH 303 jet. Our CCD images and ¹³CO maps clearly show that HH 61 and HH 127 are located just beyond the well-defined edge of the L1641 cloud, which explains why they are optically visible. Our extensive C¹⁸O map of the region shows that the cluster is associated with a prominent cloud core. Our ¹²CO map reveals, in addition to the known L1641-N outflow, a large pair of compact low-velocity outflow lobes, a southeastern blueshifted lobe, and a northwestern redshifted lobe, separated by 2.7 pc and centered on the infrared cluster. The numerous outflows in the region provide evidence that a large number of the young stars in the loose cluster are presently in active mass-losing phases. The L1641-N cluster is clearly the presently most active site of low-mass star formation in the L1641 molecular cloud.

     The evolved bipolar nebula OH 231.8+4.2 is host to a Mira variable star, QX Pup, at its core. We used the diffraction-limited near-infrared imaging (DLIRIM) system at Kitt Peak to obtain the first direct images of this star in the near-infrared. In subarcsecond resolution DLIRIM images at K (2.2 μm) and L' (3.8 μm) the star is clearly detected and is well resolved from the surrounding nebulosity. The star lies midway between the lobes of the OH 231.8+4.2 reflection nebula, confirming previous inferences based on polarimetric imaging and nebular colors. In addition to betraying the location of the central Mira, these images reveal the detailed structure of OH 231.8+4.2, including a point symmetric system of jetlike features protruding from its polar lobes. The K - L' color distribution of the nebula is consistent with reflection and extinction of near-infrared radiation from the Mira by dust grains larger than those characteristic of the interstellar medium. From the apparent K magnitude of the star, combined with its relatively well-determined distance (1300 pc) and the circumstellar extinction we infer from its measured K - L' color, we estimate a mean absolute magnitude M_K ∼ -10.2, comparable to Mira variables of similar (∼700 day) period in the Large Magellanic Cloud. In this and other respects, the central Mira appears remarkably "normal" given its position at the heart of such an unusual object.

     We report some results of a long-term photometric program called ROTOR being carried out at Maidanak Observatory in the Republic of Uzbekistan. The goal of the program is to search for periodic components in the light of T Tauri stars (TTSs), Herbig Ae/Be stars (HAEBEs), FU Orionis stars, and related objects. In this paper we analyze results obtained for TTSs in the vicinity of the ρ Oph dark clouds obtained between 1986 and 1993. Our sample includes 6 weak-emission TTSs (WTTSs) and 11 classical TTSs (CTTSs). All of the WTTSs investigated have significant photometric periods that are attributed to spottedness of the stellar photospheres. Two of them, SR 9 and SR 12, maintained stable periods and phases during the entire span of observation. The periods are between 1 and 4 days, except for SR 9, which has a period of 6.53 days; its relatively large Hα equivalent width (for a WTTS) also suggests it may be a transition object between the WTTSs and CTTSs. Among the CTTSs in our sample, we found only three stars with definite periods, two of which were longer than 8 days. Generally speaking, therefore, our observations are in accordance with the canonical view of WTTSs and CTTSs, in which stars with more active accretion disks spin slower due to disk locking and show greater irregular variations due to accretion hot spots. The exception is V895 Sco (Haro 1-1), which seems to be a typical CTTS with a rotation period of less than 4 days. The stability of the periods and epochs of minima of some or most WTTSs is noted and discussed.

     The M dwarf binary, Wolf 1062 (Gliese 748), has been observed with the HubbleSpaceTelescope (HST) Fine Guidance Sensor 3 in the transfer function scan mode to determine the apparent orbit. This is the first orbit defined fully and exclusively with HST, and is the most accurate definitive orbit for any resolved, noneclipsing system. The orbital period is 2.4490 ± 0.0119 yr and the semimajor axis is 01470 ± 00007—both quantities are now known to better than 1%. Using the weighted mean of seven parallax measurements and these HST data, we find the system mass to be 0.543 ± 0.031 M_⊙, where the error of 6% is due almost entirely to the parallax error. An estimated fractional mass from the infrared brightness ratio and infrared mass-luminosity relation yields a mass for the primary of 0.37 M_⊙, and the secondary falls in the regime of very low mass stars, with a mass of only 0.17 M_⊙.

     The M dwarf star L722-22 (LHS 1047, GJ 1005) was discovered to be a binary in 1979. Analysis of ground-based data indicated a mass near 0.06 M_⊙ for the secondary star, well below the nominal stellar mass limit of 0.08 M_⊙. The close, faint binary was near the limit for ground-based astrometry and was approved for HubbleSpaceTelescope Fine Guidance Sensor (FGS) observations in 1992. The relative orbital motion of the binary has been monitored using FGS "transfer" mode measurements. The trigonometric parallax and motion of the primary about the center of mass were determined from the FGS "position" mode observations. All possible background reference stars in the FGS field-of-view were used. The relative orbit and fractional masses have been determined with far higher precision and accuracy than possible with ground-based techniques for this close, faint binary. The FGS observations definitely eliminate the possibility that the secondary star is a candidate for having a substellar mass, and place its mass and lower mass error range well above the stellar mass limit. Masses of 0.179 and 0.112 M_⊙ have been found for the two components, with formal random errors as low as 1.5%. The mass errors resulting from the correction from relative to absolute parallax are somewhat larger.

     Light curves of the Algol-type binary system, RZ Cassiopeiae, in the near-IR wavelengths J and K are obtained for the first time. The light curves are analyzed using the Wilson-Devinney model. UBV light curves of RZ Cas obtained by Chambliss are also reanalyzed using the same program. In the J and K bands, the bolometric albedo of the secondary of RZ Cas exhibited values above 0.7, whereas the theoretically expected value for such a star is 0.5. Also, the values of the secondary temperature derived from the J and K light curves are found to be less than that derived from our analysis of the optical light curves as well as from the previous studies in the optical photometric bands. We have attempted to model these effects with a dark spot on the secondary of RZ Cas. The J-band light curve gave a better fit with a cool dark spot on the secondary. Another possible reason for the above mentioned effects is a gas stream from the lobe-filling secondary to the primary star. The magnitudes and colors of the individual components are derived from the observed light curves and the light contributions from the stars derived from the light curve analysis. The primary is found to be an A3 V star as observed by previous investigators. The secondary is classified as K0–K4 IV from the derived colors. Seven epochs of primary minima and 3 epochs of secondary minima are obtained from the observations. Because of the increased depths of the secondary eclipse in the infrared bands, the moments of minima are calculated with nearly the same accuracy as that of the primary minima. All the secondary minima are found to occur at phase 0.5. None of the observed primary minima are flat as found by many observers before at optical wavelengths. The colors of the system at the minima obtained by us confirm that the system is partially eclipsing.

     This paper presents 316 new equatorial positions for the planets Uranus, Neptune, and Pluto obtained in 1996–1997 with the Flagstaff Astrometric Scanning Transit Telescope (FASTT). All the positions were determined in the extragalactic reference frame using CCD observations and are accurate to ∼±02 in each coordinate. When these positions are compared with the latest JPL ephemerides (DE405), there is generally good agreement between the observed and calculated positions for these planets. In particular, the offset in the declination of Pluto (Δδ ∼ 01) is no longer evident, and consequently, the DE405 is obviously an improvement over its predecessor, the DE403, in that respect. There appears to be a small offset, ∼009, however, in the right ascensions of Uranus, which might be accidental or caused by errors in the FASTT observed or DE405 calculated ephemeris positions; future observations are planned to clarify this issue. With the release of the Hipparcos, Tycho, and ACT catalogs of extremely accurate reference star positions, it is now possible for telescopes, such as the FASTT, to determine the positions of planets and their satellites directly in the international celestial reference frame (ICRF) with greatly improved accuracy (∼±006) in each coordinate. All future FASTT observations will be made in this manner, and the FASTT observing program will be expanded to include satellites of planets as well.

Two UK Schmidt plates of comet Hale-Bopp dust tail taken in 1996 May are analyzed by means of the inverse dust tail model. The dust tail fits are the only available tools providing estimates of the ejection velocity, the dust-loss rate, and the size distribution of the dust grains ejected during years preceding the comet discovery. These quantities describe the comet dust environment driven by CO sublimation between 1993 and 1996, when the comet approached the Sun from 13 to 4 AU. The outputs of the model are consistent with the available coma photometry, quantified by the Afρ quantity. The dust mass loss rate increases from 500 to 8000 kg s^-1, these values being inversely proportional to the dust albedo, assumed here to be 10%. Therefore, the mass ratio between icy grains and CO results is at least 5. Higher values of the dust-to-gas ratio are probable, because the model infers the dust-loss rate over a limited size range, up to 1 mm sized grains, and because the power-law index of the differential size distribution ranges between -3.5 and -4.0, so that most of the dust mass was ejected in the largest boulders that Hale-Bopp was able to eject. The dust ejection velocity close to the observations, between 7 and 4 AU, was close to 100 m s^-1 for grains 10 μm in size, much higher than that predicted by R. F. Probstein's theory, thus confirming previous results of Neck-Line photometry. This result is an indicator of CO superheating with respect to a free sublimating CO ice, in agreement with the high observed CO velocity. The fundamental result of the paper is that such a high dust velocity remained constant between 13 and 4 AU, thus providing a strong constraint to all models of the CO-driven activity of the comet during its approach to the Sun: CO superheating must have been active since 13 AU from the Sun. It might be provided by the abundant dust itself, or by seasonal effects heating the subsurface layers, as was suggested for comet 29P/Schwassmann-Wachmann 1. Another similarity between the two comets is provided by the power-law index of the time-averaged size distributions: -3.6 ± 0.1 for C/1995O1 and -3.3 ± 0.3 for 29P/SW1. However, other characteristics of the dust environments are very different, so that, in general, it is impossible to distinguish a CO-driven comet from a typical water-driven one.

Photometric observations with the JCMT at millimeter and submillimeter wavelengths have been made for the asteroids 1 Ceres, 4 Vesta, 6 Hebe, 7 Iris, 16 Psyche, 18 Melpomene, and 216 Kleopatra. The measurements reported here include a careful estimation of the possible systematic biases that may be present in the photometry. Whenever possible, the flux densities have been averaged over a complete rotational light curve to eliminate rotational phase as a source of uncertainty. Combining our measurements with those from the literature at other wavelengths, we present spectral energy distributions (SEDs) for the thermal emission from these asteroids spanning the infrared and radio ranges.

The effective emissivity e_EFF is defined as the ratio of the observed flux density to that which would have been observed from a nonrotating, spherical blackbody with the same size, distance from the Earth, and distance from the Sun, as though viewed at opposition. The physical properties that influence e_EFF are discussed qualitatively, using our SEDs to illustrate the importance of each effect. In this way, the effective emissivity is demonstrated to be a useful means to present the SED of an asteroid over the whole range of wavelengths for which thermal emission dominates the observable flux density.

The most important physical properties that distinguish the SEDs of the nonmetallic asteroids (Ceres, Vesta, Hebe, Iris, and Melpomene) from each other appear to be (1) the optical depth through the layer of warm material that has been heated by the Sun on the day side of the asteroid; (2) the density of the surface materials; and (3) the rotation period of the asteroid. For Ceres the warm surface layer is partially opaque at wavelengths near 1 mm, while for Vesta it is transparent at all wavelengths longer than 0.35 mm. We attribute the transparency of Vesta's warm surface layer to its low density. In contrast, Iris appears to have relatively dense materials on its surface that transport heat effectively from its surface to its deeper layers, reducing the infrared beaming compared to Ceres and increasing the optical depth of its warm surface layer compared to Vesta. The effectiveness of rotation in suppressing the infrared beaming phenomenon is illustrated by Vesta, a rapid rotator with a weak infrared beaming effect, and by Melpomene, a slow rotator with a strong infrared beaming effect.

The SEDs of the M-type (metallic) asteroids (Psyche and Kleopatra) have a distinctive shape, with a steep decrease from infrared to radio wavelengths. The effective emissivities at wavelengths near 1 mm are too low to correspond to physical temperatures in the asteroids' surfaces but are consistent with the presence of large metal fractions in their surface minerals, which would make their surfaces reflective rather than emissive at long wavelengths. This is the first clear mineralogical distinction that we have been able to make based on the shape of an asteroid's SED.

     This paper deals with the influence of Jupiter's "Great Inequality" (GI) on the orbital evolution of 2:1 resonant asteroids. This perturbation of Jupiter's motion enhances the chaotic diffusion of orbits and the depletion of asteroids in the Hecuba gap. The failure of models that adopt for Jupiter an elliptic motion with only secular perturbations is explained. We also show the dependence of the diffusion rates on the GI period, and the maximum diffusion rates that would take place if the GI period were closer to the libration period. Significant similar effects are absent in the 3:2 asteroidal resonance.

     No significant difference is found between the systems of the Hipparcos and Allegheny Observatory MAP parallax catalogs. The correlation of the parallaxes of 63 stars common to both programs is 0.9995 ± 0.0001, with an average standard deviation of the difference of 00023. While there is no indication of systematic difference in the two programs, our study suggests that the formal errors in one or both catalogs are somewhat underestimated.

     Results of the lunar occultations observed from 1984 to 1997 at the Engelhardt Astronomical Observatory are reported. The times of lunar occultation for 63 stars, with a precision of a few milliseconds, and diameters for 12 stars have been determined.