Jay A. Frogel

Infrared standard stars

The results of an observational program aimed at setting up a network of faint near-infrared standards of sufficient accuracy are reported. The network covers both northern and southern hemispheres and includes standards red enough to provide at least a limited check on color transformations. The standards are set up at J (1.2 micron), H (1.6 micron), K (2.2 microns), and L (3.5 microns), and their H2O and CO molecular absorption indices are determined. The problem of color transformations between observatories is discussed briefly. All magnitudes presented are transformed to the natural system defined by the CIT observations.

Orbits of nine Uranian rings

Observations of a stellar occultation by Uranus and its nine rings are presented and used to examine the structures and kinematics of the rings. The observations of the occultation of the K giant star KM 12 were obtained in the K band with the 4-m CTIO telescope at a signal-to-noise ratio higher than any previously obtained. Ring occultation profiles reveal the alpha ring to possibly have a double structure and less abrupt boundaries than the gamma ring, which exhibits diffraction fringes, while the eta ring is a broad ring with an unresolved narrow component at its inner edge. The present timing data, as well as previous occultation timings, are fit to a kinematic model in which all nine rings are treated as coplanar eclipses of zero inclination, precessing due to the zonal harmonics of the Uranian gravitational potential to obtain solutions for the ring orbits. Analysis of the residuals from the fitted orbits reveals that the proposed model is a good representation of ring kinematics. The reference system defined by the orbit solutions has also been used to obtain a value of 0.022 + or - 0.003 for the ellipticity of Uranus and a Uranian rotation period of 15.5 h.

The thermal structure and energy balance of the Uranian upper atmosphere

Abstract Two occultation observations of the upper atmosphere of Uranus are reported: (i) the 15–16 August 1980 occultation of KM 12 from Cerro Tololo Inter-American Observatory, and (ii) the 26 April 1981 occultation of KME 13 from the Anglo-Australian Telescope and the 1-m telescope of the Australian National University. Mean atmospheric temperatures of 154 ± 15°K for the 1980 event and 132 ± 15°K for the 1981 event are derived from the lightcurves. A comparison of all available Uranus occultation data since March 1977 suggests that the mean atmospheric temperature of Uranus has changed significantly, with a typical variation of 15°K year−1. We investigate plausible energy sources that might account for such large temperature variations. We conclude that molecular and eddy diffusion, and atmospheric dynamics, are potentially as important as radiation to the upper atmospheric heat balance. There is evidence that no significant radiative cooling had occurred at two suboccultation points that had been in darkness for more than 0.5 terrestrial year, suggesting upper limits to the mixing ratios of CH4 and C2H2 of 6 × 10−5 and 5 × 10−7, respectively. The consistently close agreement of immersion and emersion mean temperatures for each occultation, in spite of apparently large secular changes in the atmospheric mean temperature, suggests that effective meridional transport occurs on Uranus. A continuing program of occultation observations in future years should reveal whether this pattern is significant.

The rings of Uranus - Occultation profiles from three observatories

Abstract Occultation profiles for the nine confirmed Uranian rings obtained from Las Campanas, the European Southern Observatory, and Cerro Tololo on 15–16 August 1980 are compared. The α ring shows a “double-dip” structure; the η ring shows a broad and narrow component (similar to Saturns F ring); and the e ring shows six features that appear in the data from all three observatories. Diffraction fringes appear at the edges of several of the occultation profiles.

Optical and Infrared Studies of Stellar Populations: The Galactic Nuclear Bulge

Giants in the Galactic nuclear bulge are different from solar neighborhood giants in terms of their observed photometric and spectroscopic characteristics and their mean metallicity. These bulge stars are the best available sample to use in stellar synthesis models of early-type galaxies. Observations from the main sequence to the top of the giant branch show no evidence for a component of the bulge population much younger than 10 Gyr. Several independent lines of evidence indicate a strong metallicity gradient in the bulge as would be expected if it formed dissipatively.

Photometry of occultation candidate stars. II - Uranus 1985-1990 and Saturn 1986-1991

Abstract V and I magnitudes are presented for 20 stars to be occulted by the Uranian rings during the period 1986–1990 and for 15 stars to be occulted by Saturns rings during 1986–1991. K magnitudes for seven Uranus stars are presented. The occultation stars are up to one magnitude brighter in the K bandpass than one would predict on the basis of V-I colors alone. This effect can be explained by typical amounts of interstellar reddening.

Characteristics and Space Distribution of Stars in the Bulge of the Milky Way

Stars in Baade’s Window at b = -4° are described. They are old, metal rich, and have a velocity dispersion greater than 100 km/sec. Their colors are unlike those of any other group of stars. Stars in other windows between b = -3 and -12° differ systematically from those in Baade’s Window. The differences are best understood if they are caused by a metallicity gradient in the bulge. The distribution of surface brightness or surface density over the bulge has a much steeper falloff than that seen in other galaxies or in the very center of the Milky Way itself. It is now possible to link together stars in the immediate vicinity of the nucleus with those that are several hundred parsecs out. Understanding this linkage will help in understanding the history and evolution of stars throughout the bulge and that part of the inner disk with which it overlaps.