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R D Gehrz

Publications and source records attributed to R D Gehrz.

5 recordsLinked to original sources
PubMed1990

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 Halley's 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.

Astronomical Phenomena↗
PubMed1990

Confirmation of dust condensation in the ejecta of supernova 1987a.

Shortly after its outburst, we suggested that supernova 1987a might condense a dust shell of substantial visual optical thickness as many classical novae do and predicted that dust might form within a year after the explosion. A critical examination of recent optical and infrared observations reported by others confirms that dust grains had begun to grow at a temperature of 1000 K after 300 days and that the dust shell had become optically thick by day 600. After day 600, the infrared luminosity closely followed the intrinsic luminosity expected for thermalized 56Co gamma rays, demonstrating that the luminosity is powered by radioactivity and that the dust is outside the radioactivity zone. The infrared luminosity sets an upper limit to the soft intrinsic bolometric luminosity of a pulsar central engine. This upper limit for the pulsar in supernova 1987a is the same luminosity as the Crab pulsar has today 936 years after its formation. It is unlikely that the rotation rate for a pulsar in supernova 1987a can be much higher than approximately 30 revolutions per sec. The relatively long time required for the shell to grow to maximum optical depth as compared with the dust in nova shells may be related to the relatively low outflow velocity of the condensible ejecta.

Journal Article↗
PubMed1987

On the possibility of dust condensation in the ejecta of supernova 1987a.

We suggest that supernova 1987a may condense dust of substantial visual optical thickness as do many novae. The dust will act as a calorimeter of the photon luminosity of any central engine that is dominant at the time of dust formation. Observations of novae suggest that dust formation may occur when the expanding ejecta reach a temperature of 1000 K. The early luminosity of the supernova may be dominated by radioactivity that is unrelated to the central engine that determines the energy balance for the long-term development of the supernova. We discuss the possibility that a constant luminosity central power source such as a pulsar dominates the luminosity of the supernova ejecta by the time that dust can condense and argue that, if a shell mass of more than a few tenths of one solar mass was ejected, emission from dust may be observable in the thermal infrared spectral region. Maximum dust optical depth should occur by late 1987 or early 1988. If the dust becomes optically thick, the visual light from the supernova may drop precipitously. The characteristics of an optically thick dust shell as a calorimeter of the luminosity of the central engine are discussed and are related to previous observations of dust formation in type II supernovae. It is suggested that dust of several chemical compositions may form at different epochs.

Journal Article↗
PubMed1980

Optical polarization and infrared spectrum of a possible protostar in a reflection nebula.

The star HD 200775 illuminates the reflection nebula NGC 7023. This star is one of a small group of Ae and Be stars associated with nebulosity and proposed by Herbig as candidates for newly formed stars of moderately large mass. The system is a part of a large dust complex more than a degree in extent. Structural features in the nebulosity can be traced very near to the star (several are seconds away) and some of these structures are highly polarized in the sense expected for scattered light from the central object. Although the star is classified as an early spectral type (B2-B8) of moderate luminosity (class IV or V), its energy distribution does not resemble a black body characteristic of a stellar photosphere. Instead, the radiated flux has the appearance of free free radiation (thermal bremsstrahlung) usually considered to arise in astrophysical situations involving optically thin plasmas of ions and free electrons. Our observations are consistent with the belief that HD 200775 is a protostar and that the surrounding material is the cocoon from which it formed. The complex polarized structures probably originate in material ejected by radiation pressure and plasma processes as the star rose in luminosity after its gravitational collapse.

Journal Article↗