J. B. Holberg

Ultraviolet Spectrometer Observations of Neptune and Triton

Results from the occultation of the sun by Neptune imply a temperature of 750 � 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman α (340 � 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Tritons atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Tritons upper atmosphere is 95 � 5 kelvins, and the surface pressure is roughly 14 microbars.

CALIBRATION OF SYNTHETIC PHOTOMETRY USING DA WHITE DWARFS

We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Str?mgren uvby filters, the Two Micron All Sky Survey JHKs, and the Sloan Digital Sky Survey ugriz filters. Synthetic magnitudes are calculated from model white dwarf spectra folded through the published filter response functions; these magnitudes in turn are absolutely calibrated with respect to the HST flux scale. Effective zero-magnitude fluxes and zero-point offsets of each system are determined. In order to verify the external observational consistency, as well as to demonstrate the applicability of these definitions, the synthetic magnitudes are compared with the respective observed magnitudes of larger sets of DA white dwarfs that have well-determined effective temperatures and surface gravities and span a wide range in both of these parameters.

Extreme ultraviolet observations from the Voyager 2 encounter with Saturn

Combined analysis of helium (584 angstroms) airglow and the atmospheric occultations of the star δ Scorpii imply a vertical mixing parameter in Saturns upper atmosphere of K (eddy diffusion coefficient) ∼ 8 x 107 square centimeters per second, an order of magnitude more vigorous than mixing in Jupiters upper atmosphere. Atmospheric H2 band absorption of starlight yields a preliminary temperature of 400 K in the exosphere and a temperature near the homopause of ∼ 200 K. The energy source for the mid-latitude H2 band emission still remains a puzzle. Certain auroral emissions can be fully explained in terms of electron impact on H2, and auroral morphology suggests a link between the aurora and the Saturn kilometric radiation. Absolute optical depths have been determined for the entire C ring andparts of the A and B rings. A new eccentric ringlet has been detected in the C ring. The extreme ultraviolet reflectance of the rings is fairly uniform at 3.5 to 5 percent. Collisions may control the distribution of H in Titans H torus, which has a total vertical extent of ∼ 14 Saturn radii normal to the orbit plane.

ULTRAVIOLET SPECTROMETER OBSERVATIONS OF URANUS.

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 1029 per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 1020 per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x1011 watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10-7 at a vertical column abundance of molecular hydrogen of 1023 per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

The White Dwarf Distance to the Globular Cluster NGC 6752 (and Its Age) with the HUBBLE SPACE TELESCOPE

Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WD). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster:

The True Incidence of Magnetism Among Field White Dwarfs

(m-M)_\circ=13.05

A Determination of the Local Density of White Dwarf Stars

, with an uncertainty less than

Kepler-21b: A 1.6 R Earth Planet Transiting the Bright Oscillating F Subgiant Star HD?179070

\pm0.1

A NEW LOOK AT THE LOCAL WHITE DWARF POPULATION

mag which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance and

The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age

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