E. P. Ney

0.7- to 23-μm photometric observations of P/Halley 1986 III and six recent bright comets

Abstract We report 0.7- to 23-μm observations of P/Halley 1986 III and six other recent bright comets. P/Halley was measured on 47 occasions between 1985 December 12 UT and 1986 May 6 UT, several times within hours of the perihelion passage on 1986 February 9 UT. Our data show that the strength of the 10-μm silicate emission feature and the temperature excess (superheat; S = T obs / T BB ) of the infrared continuum emission are strongly correlated. IR Type I comets have low continuum superheat and muted or undetectable silicate emission features, suggesting that the coma emission from these comets is produced by large grains with radii larger than 1 μm. IR Type II comets have superheated thermal infrared continua and high-contrast silicate emission features, indicating that the coma emission is from small grains with radii between 0.5 and 1 μm. Both types of behavior were exhibited by Comet P/Halley at various times. The relationship between superheat and 10-μm silicate emission may be complex, for although though the strength of these quantities was generally strongly correlated, several comets exhibited occasional episodes when superheat and silicate emission were not correlated. P/Halleys dust coma had an average albedo of 0.20 at a scattering angle of 130°. Our data show that the scattering phase function for typical comet dust is characterized by a moderately strong forward scattering peak, no appreciable backscattering peak, a mean bolometric albedo of ≈0.32, and an albedo of ≈0.15 for scattering angles between 120° and 180°. These characteristics are consistent with laboratory and theoretical results for nonspherical and “fluffy” core-mantle aggregate grains. P/Halleys 10-μm silicate signature showed significant variations in strength and was occasionally weak or absent at heliocentric distances both smaller and larger than 1 AU. Simultaneous measurements of P/Halley and Bradfield 1980 XV with different diaphragms are generally consistent with the steady-state model for nuclear ablation. P/Halleys coma luminosity fluctuated by a factor of nearly 10 on time scales of 1 to 2 days. These variations are consistent with jet-like activity probably associated with nuclear rotation. Dust mass loss rates for the comets studied here are estimated, and we conclude that P/Halley was losing ≥10 6 g sec −1 of dust at a heliocentric distance of 1 AU.

Comet West and the Scattering Function of Cometary Dust

Observations of Comet West (1975n) at wavelengths from 0.5 to 18 microns and at a variety of scattering angles are used to infer the scattering phase function for the cometary dust. This function is strongly peaked in the forward direction. The form of the function indicates that the particles are dielectric grains with radii of approximately 1 micron. Abrupt increases in the intrinsic brightness of the coma (both in scattered sunlight and in thermal emission) are consistent with the projected times of comet fragmentation.

A Null Method for the Comparison of Two Ion Currents in a Mass Spectrometer

A method of determining the ratio of two small currents which vary intermittently or continuously, but always have the same relative magnitudes, is described. Although applied only to the case of making precise measurement of ratio of two ion currents in a mass spectrometer for the purpose of measuring the relative abundance of isotopes, the method would be useful in other applications as well.

Multiband photometry of Comets Kohoutek, Bennett, Bradfield, and Encke

Abstract Observations of Comets Kohoutek (1973f), Bradfield (1974b), and P/Encke have been made at a number of wavelengths between 0.55 and 18 μm. The silicate feature first observed in Comet Bennett (1969i) seems to be a common characteristic of cometary material. The comas of these comets radiate infrared with an effective temperature higher than the black-body temperature at the given distance from the Sun. The albedo of the dust particles is between 0.10 and 0.20. The particles in the coma and tail are small (diameter less than 2 μm), but the particles in the anti-tail of Comet Kohoutek must be larger than about 10 μm diameter. The observations give an absolute upper limit to the diameter of Comet Kohoutek of 30 km. A consistent interpretation would indicate that Comets Kohoutek and Bradfield have nuclear diameters of 5 to 10km, that Bennett was several times larger, and that P/Encke is 10 times smaller. The peculiar behavior of Bradfield showed that the coma of a single comet can abruptly change its dust composition.

Satellite Observations of Lightning

Analysis of the positions of nighttime thunderstorms as determined from the detection of optical radiation by satellite OSO-B reveals that ten times as many lightning storms occur over land areas as over the sea.

The infrared spectrum of comet Bradfield (1987s) and the silicate emission feature

Infrared (1-20 micrometers) observations of comet Bradfield (1987s) from three observatories are reported. Silicate emission is prominent in all the data, from heliocentric distance r=0.87 to 1.45 AU. A CVF spectrum at r=1.45 AU shows a peak at 11.3 micrometers identified as crystalline olivine; the spectral shape is similar to that in Halley. Dust optical properties are similar to those of the grains in Halleys jets. Dust production rate near perihelion was approximately 10(6) g s-1 and varied in proportion to (r-2). We suggest that some differences in grain properties among comets could result from differences in the thermal history of the nuclear surface and the relative fraction of the dust particles originating in the subsurface icy layer versus the devolatilized mantle.

Ultraviolet spectroscopy of the recurrent nova U Scorpii during outburst

Observations of the recurrent nova U Sco during the 1979 outburst are presented, and the spectral evolution is found to differ from that of other recurrent novas. Spectra are dominated by emission lines, and the strong forbidden-line emission characteristic is conspicuously absent. A method to determine masses of nova shells is outlined, and an analysis of the emission lines shows an enrichment in N relative to C and O, and that the nova ejecta are rich in He relative to H. Optical spectra of U Sco obtained following its return to quiescence show predominantly He II emission lines, which suggests an enrichment of the preoutburst gas in He, and thus the presence of a highly evolved companion.

The isothermal dust condensation of Nova Vulpeculae 1976

Nova Vulpeculae 1976 has been followed with broad-band photometry at wavelengths of 0.5 to 12.5 microns from day 3 to day 235. Three definite phases are identified. They are expanding pseudophotosphere, free-free expansion, and dust condensation. A very long (day 80 to day 220) period of isothermal infrared emission (Tapprox. =900 K) is accompanied by an exponential decrease in infrared luminosity and is interpreted in terms of continuous mass ejection in the nova wind.

The evolution of the dust shell of Nova Serpentis 1978

Photometric measurements of Nova Serpentis 1978 from V to 19.5 microns covering the first 240 days of the novas development are reported. A free-free expansion phase was followed by the condensation of a dust shell that became optically thick at both visible and infrared wavelengths. Carbon grains grew to radii of approximately 0.3 microns during the condensation period. The grains appeared to sputter or evaporate for a period of approximately 100 days following infrared maximum. It is found that the condensed carbon comprised 2% of the ejected mass, which implies that carbon abundance was enhanced in the nova shell. The angular expansion rate of the shell yields a distance of approximately 5 kpc and a luminosity of approximately 3000 solar luminosities for Nova Serpentis 1978.

Observations of the solar corona from the limb of the sun to the zodiacal light, july 20, 1963

Absolute brightness, polarization and polarization direction of inner corona obtained during solar eclipse of july 20, 1963