A. R. Klemola

The size, shape, density, and Albedo of Ceres from its occultation of BD+8°471

The occultation of BD+8°471 by Ceres on 13 November 1984 was observed photoelectrically at 13 sites in Mexico, Florida, and the Caribbean. These observations indicate that Ceres is an oblate spheroid having an equatorial radius of 479.6±2.4 km and a polar radius of 453.4±4.5 km. The mean density of this minor planet is 2.7 g/cm3±5%, and its visual geometric albedo is 0.073. While the surface appears globally to be in hydrostatic equilibrium, firm evidence of real limb irregularities is seen in the data.

Hubble Space Telescope Fine Guidance Sensor Astrometric Parallaxesfor Three Dwarf Novae: SS Aurigae, SS Cygni, and U Geminorum

We report astrometric parallaxes for three well-known dwarf novae obtained using the Fine Guidance Sensors on the Hubble Space Telescope (HST). We found a parallax for SS Aurigae of π=5.00±0.64 mas, for SS Cygni we found π=6.02±0.46 mas, and for U Geminorum we obtained π=10.37±0.50 mas. These represent the first true trigonometric parallaxes of any dwarf novae. We briefly compare these results with previous distance estimates. This program demonstrates that with a very modest amount of HST observing time, the Fine Guidance Sensors can deliver parallaxes with sub-milliarcsecond precision.

The diameter of Juno from its occultation of AG + 0 deg 1022

The occultation on Dec. 11, 1979, of AG + 0 deg 1022 by Juno was observed photoelectrically from 15 sites distributed across the occultation track. The observations are well represented by a mean elliptical limb profile having semimajor and semiminor axes of 145.2 + or 0.8 and 122.8 + or - 1.9 km, respectively. The corresponding effective diameter of Juno is 267 + or - 5 km, where the uncertainty has been conservatively increased to reflect the presence of limb irregularities clearly seen in the observations. Published radiometric and polarimetric diameters for Juno are 6% to 7% smaller than the occultation result. No secondary occultations attributable to possible satellites of Juno were recorded at any of 23 photoelectrically equipped observing sites.

Predicted occultations by Uranus - 1981-1984

Predictions are presented for 11 occultations by and appulses to Uranus and its ring system for ten stars from 1981 through 1984. The brightest stars are occulted on April 26, 1981 (BD - 19 deg 4222) and on April 22, 1982 (Hyd - 20 deg 51699). The ring system occults the same star twice during March 1983 (Hyd - 21 deg 64352).

Optical Observations of the Binary Pulsar System PSR B1718–19: Implications for Tidal Circularization

We report on Keck and Hubble Space Telescope optical observations of the eclipsing binary pulsar system PSR B1718-19, in the direction of the globular cluster NGC 6342. These reveal a faint star (mF702W = 25.21 ± 0.07; Vega system) within the pulsars 05 radius positional error circle. This may be the companion. If it is a main-sequence star in the cluster, it has radius RC 0.3 R☉, temperature Teff 3600 K, and mass MC 0.3 M☉. In many formation models, however, the pulsar (spun-up by accretion or newly formed) and its companion are initially in an eccentric orbit. If so, for tidal circularization to have produced the present-day highly circular orbit, a large stellar radius is required, i.e., the star must be bloated. Using constraints on the radius and temperature from the Roche and Hayashi limits, we infer from our observations that RC 0.44 R☉ and Teff 3300 K. Even for the largest radii, the required efficiency of tidal dissipation is larger than expected for some prescriptions.

A Preliminary Discussion of the Kinematics of BHB and RR Lyrae Stars Near the North Galactic Pole

The radial velocity dispersion of 67 RR Lyrae variable and blue horizontal branch (BHB) stars that are more than 4 kpc above the galactic plane at the North Galactic Pole is 110 km/sec and shows no trend with Z (the height above the galactic plane). Nine stars with Z 4 kpc have a Galactic V motion that is < -200 km/sec and which is characteristic of the halo. Thus the stars that have a flatter distribution are really halo stars and not members of the metal-weak thick-disk.

WIYN Open Cluster Study. VIII. The Geometry and Stability of the NOAO CCD Mosaic Imager

The field of direct imaging is experiencing a considerable growth in the number of available CCD mosaic imagers, especially on large telescopes. To fully exploit the astrometric potential of these imaging devices, we develop a technique, utilizing an astrometric standard, for precision transformation of pixel coordinates into a global coordinate system. We have constructed a new astrometric standard set of 1863 stars in the field of NGC 188 to derive the CCD chip constants for the NOAO CCD Mosaic Imager. The multiple-epoch data on the Mosaics metrics indicate that this CCD mosaic device may have experienced a one-time nonelastic expansion. We also present a new determination of the pixel scale and the optical field angle distortion constants for the KPNO Mayall 4 m telescope prime focus field corrector. To establish a reliable history of the CCD mosaic imager metrics for current and potential future astrometric applications, we recommend obtaining astrometric calibrations for CCD mosaic imagers on a regular basis. Apart from mechanical positioning of the CCD mosaic camera on the telescope, noticeable changes in the thermal environment of CCD mosaic chips should also prompt new astrometric calibrations. It is shown that, following all precautions, the NOAO CCD Mosaic Imager can produce excellent astrometric results on the Mayall 4 m telescope.

Predicted occultations by Neptune - 1981-1984

Predictions are presented for eight occultations by and appulses to Neptune from 1981 through 1984. Of special interest are a near-central occultation of Hyd - 22 deg 58794 on June 15, 1983, and a July 22, 1984 occultation of SAO 186001.

The Distance to the Hyades Cluster Based on Hubble Space Telescope Fine Guidance Sensor Parallaxes

Trigonometric parallax observations made with the Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) 3 of seven Hyades members in six fields of view have been analyzed along with their proper motions to determine the distance to the cluster. Knowledge of the convergent point and mean proper motion of the Hyades is critical to the derivation of the distance to the center of the cluster. Depending on the choice of the proper-motion system, the derived cluster center distance varies by 9%. Adopting a reference distance of 46.1 pc or m - M = 3.32, which is derived from the ground-based parallaxes in the General Catalogue of Trigonometric Stellar Parallaxes (1995 edition), the FK5/PPM proper-motion system yields a distance 4% larger, while the Hanson system yields a distance 2% smaller. The HST FGS parallaxes reported here yield either a 14% or 5% larger distance, depending on the choice of the proper-motion system. Orbital parallaxes (Torres et al.) yield an average distance 4% larger than the reference distance. The variation in the distance derived from the HST data illustrates the importance of the proper-motion system and the individual proper motions to the derivation of the distance to the Hyades center; therefore, a full utilization of the HST FGS parallaxes awaits the establishment of an accurate and consistent proper-motion system.

Optical and radio observations of the binary pulsar 1855+09: evolution of pulsar magnetic fields and low-mass white dwarf cooling

New radio and optical observations of the 5.4 ms binary pulsar 1855 + 09 are reported. The visual extinction of the system is shown to be about 1.5 mag. Only one star brighter than R about 24.6 is found that, on the basis of positional coincidence, can be plausibly the optical counterpart of the secondary star. The spectrum of this candidate is inconsistent with a low-mass main-sequence star. Neither is it a white dwarf because the spectroscopic distance modulus is inconsistent with the lower limit on distance obtained from timing observations. It is concluded that the companion of PSR 1855 + 09 must be a low-mass cold white dwarf. The inferred cooling age supports the hypothesis that magnetic field strengths of millisecond pulsars are essentially constant and that millisecond pulsars are long-lived objects. 40 refs.