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Identifying the massive progenitor stars that give rise to core‐collapse supernovae (SNe) is one of the main pursuits of supernova and stellar evolution studies. Using ground‐based images of recent, nearby SNe obtained primarily with the Katzman Automatic Imaging Telescope, astrometry from the Two Micron All Sky Survey, and archival images from the Hubble Space Telescope, we have attempted the direct identification of the progenitors of 16 Type II and Type Ib/c SNe. We may have identified the progenitors of the Type II SNe 1999br in NGC 4900, 1999ev in NGC 4274, and 2001du in NGC 1365 as supergiant stars with M⁰_V ≈ -6 mag in all three cases. We may have also identified the progenitors of the Type Ib SNe 2001B in IC 391 and 2001is in NGC 1961 as very luminous supergiants with M⁰_V ≈ -8 to −9 mag, and possibly the progenitor of the Type Ic SN 1999bu in NGC 3786 as a supergiant with M⁰_V ≈ -7.5 mag. Additionally, we have recovered at late times SNe 1999dn in NGC 7714, 2000C in NGC 2415, and 2000ew in NGC 3810, although none of these had detectable progenitors on pre‐supernova images. In fact, for the remaining SNe only limits can be placed on the absolute magnitude and color (when available) of the progenitor. The detected Type II progenitors and limits are consistent with red supergiants as progenitor stars, although possibly not as red as we had expected. Our results for the Type Ib/c SNe do not strongly constrain either Wolf‐Rayet stars or massive interacting binary systems as progenitors.

The results of a search for cool subdwarfs are presented. Kinematic (U, V, and W) and stellar parameters (T_eff, log g, [Fe/H], and ξ_t) are derived for 134 candidate subdwarfs based on high‐resolution spectra. The observed stars span 4200 K < T_eff < 6400 K and −2.70 < [Fe/H] < 0.25 including only eight giants (log g < 4.0). Of the sample, 100 stars have MgH bands present in their spectra. The targets were selected by their large reduced proper motion, by the offset from the solar‐metallicity main sequence, or from the literature. We confirm the claims made by Ryan that the NLTT catalog is a rich source of subdwarfs and verify the success of the reduced proper‐motion constraint in identifying metal‐poor stars.

We give mean spectra and report orbital periods P_orb based on radial velocities taken near minimum light for five dwarf novae, all of which prove to have P_orb<2 hr. The stars and their periods are KX Aql, 0.06035(3) day; FT Cam, 0.07492(8) day; PU CMa, 0.05669(4) day; V660 Her, 0.07826(8) day; and DM Lyr, 0.06546(6) day. The emission lines in KX Aql are notably strong and broad, and the other stars' spectra appear generally typical for short‐period dwarf novae. We observed FT Cam, PU CMa, and DM Lyr on more than one observing run and constrain their periods accordingly. Differential time‐series photometry of FT Cam shows strong flickering but rules out deep eclipses. Although dwarf novae in this period range generally show the superhumps and superoutbursts characteristic of the SU UMa subclass of dwarf novae, none of these objects have well‐observed superhumps.

Photoelectric B and V photometry of RR Lyrae was obtained over a 77 day interval in 1996. In addition to the 0.5668 day primary period, RR Lyr displayed a prominent 40 day Blazhko cycle during this time span. The observed light curve can be well but not perfectly described by frequency triplets of the form kf₀ + jf_B, where k = 0, 1, 2, ..., j = -1, 0, 1, f₀ is the primary pulsation frequency, and f_B is the Blazhko frequency.

A number of blue stars that appear to be similar to Population I B stars in the star‐forming regions of the Galactic disk are found more than 1 kpc from the Galactic plane. Uncertainties about the true distances and masses of these high‐latitude B stars have fueled a debate as to their origin and evolutionary status. The eclipsing binary IT Lib is composed of two B stars, is approximately 1 kpc above the Galactic plane, and is moving back toward the plane. Observations of the light and velocity curves presented here lead to the conclusion that the B stars in this system are massive young main‐sequence stars. While there are several possible explanations, it appears most plausible that the IT Lib system formed in the disk about 30 million years ago and was ejected on a trajectory taking it to its present position.

We have used self‐correlation—a simple form of variogram analysis—to study 33 RV Tauri and related stars in the LMC, using MACHO data. We confirm the periods and classifications of Alcock et al. and discuss a few stars of special interest. We find that self‐correlation is a useful adjunct to Fourier analysis, especially for stars whose classification is based on their cycle‐to‐cycle behavior. In particular, it can identify stars whose behavior is more complicated than the standard "alternating deep and shallow minima" and begin to investigate the question of whether the Population II Cepheids, the RV Tauri variables, and the SRd variables form a continuous sequence from periodicity to irregularity. Our results also emphasize that the RV Tauri phenomenon has two dimensions: the relative depths of adjacent minima and the number of cycles over which the alternating minima persist.

Up to now, 10 planetary nebulae have been regarded as halo planetary nebulae (halo PNe). We obtained low‐ and moderate‐dispersion spectra of the spatially resolved halo planetary nebula H4‐1 in the wavelength region of 3700–6800 Å with the Cassegrain Spectrograph of the 74 inch telescope. At moderate dispersions, we were able to observe [Oiii] λλ4959, 5007 and Hα of H4‐1 at several position angles (P.A.). We assumed that the widths of these emission‐line profiles were due to Doppler broadening and that these emission lines were composed of several Gaussian components. Multiple Gaussian deconvolution analysis of such emission lines indicated H4‐1 has broad wing components (∼600 km s⁻¹) at P.A. 135° and 180°. We think H4‐1 has a "bipolar flow" between P.A. 135° and 180°. From low‐dispersion spectra, we confirm that H4‐1 is a metal‐poor PN. In spite of the lack of spatially resolved images at present, we believe that H4‐1 has a bipolar flow and that its axis is almost parallel to the line of sight, with a small inclination angle.

This paper is the fourth in a series whose purpose is to study the interstellar abundances of sulfur, chlorine, and argon in the Galaxy using a sample of 86 planetary nebulae. Here we present new high‐quality spectrophotometric observations of 20 Galactic planetary nebulae with spectral coverage from 3700 to 9600 Å. A major feature of our observations throughout the entire study has been the inclusion of the near‐infrared lines of [Siii] λλ9069, 9532, which allows us to calculate accurate S⁺² abundances and to either improve upon or convincingly confirm results of earlier sulfur abundance studies. For each of the 20 objects here, we calculate ratios of S/O, Cl/O, and Ar/O and find average values of S/O = (1.1 ± 1.1) × 10^-2, Cl/O = (4.2 ± 5.3) × 10^-4, and Ar/O = (5.7 ± 4.3) × 10^-3. For six objects, we are able to compare abundances of S⁺³ calculated directly from available [Siv] 10.5 μm measurements with those inferred indirectly from the values of the ionization correction factors for sulfur. In the final paper of the series, we will compile results from all 86 objects, search for and evaluate trends, and use chemical evolution models to interpret our results.

Analysis of low‐resolution spectra of K giants in the old open clusters NGC 188 and NGC 6791 yields [Fe/H] = 0.075 ± 0.045 ± σ_sys for NGC 188 and [Fe/H] = 0.320 ± 0.023 ± σ_sys for NGC 6791. The term σ_sys represents the drift between our underlying star catalog's abundance scale and the true abundance scale. Star R23 in NGC 6791 has [Fe/H] > 0.6 according to our analysis and deserves further study.

The Princeton Variability Survey (PVS) is a robotic survey that makes use of readily available "off‐the‐shelf" type hardware products, in conjunction with a powerful set of commercial software products, in order to monitor and discover variable objects in the night sky. The main goal of the PVS has been to devise an automated telescope and data reduction system requiring only moderate technical and financial resources to assemble, which may be easily replicated by the dedicated amateur, a student group, or a professional and used to study and discover a variety of variable objects, such as stars. This paper describes the hardware and software components of the PVS device as well as observational results from the initial season of the PVS, including the discovery of a new bright variable star.

Having an accurate distortion correction is critical to doing accurate astrometry with the Wide Field Planetary Camera 2. Currently available solutions have serious systematic errors (∼0.2 pixel) of a skewing nature in their linear terms. We use the inner calibration field of ω Centauri to find an improved cubic solution that is accurate to ±0.01 pixel in the WF chips and to ±0.02 pixel in the PC chip. Improvements beyond this will be difficult, as there are many factors that perturb the solution at the ∼0.01 pixel level: breathing, variations with filter, and the physical movement of the chips. There are probably not enough data available to constrain the solution to better than ∼0.01 pixel. Nevertheless, our solution is a major improvement and will allow many astrometric projects to obtain near‐optimal results.

The observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow‐up observations of gamma‐ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next‐generation instrument, ROTSE‐III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.