An Estimation of the Interstellar Contribution to the Zodiacal Thermal Emission

Abstract

Impact data from the Ulysses dust detector at 5 AU from the Sun have been interpreted as a Ñux of submicron interstellar dust particles arriving from ecliptic longitude 252¡ and ecliptic latitude By 2¡ .5. following the motions of these particles under the inÑuence of solar gravity, radiation pressure, and electromagnetic forces, we derive a model of the thermal emission from the resultant particle cloud. Since the distributions of the particles are time variable depending on the solar cycle, calculations are performed for the years 1984 and 1990, corresponding, respectively, to the times of the IRAS and COBE observations. We also illustrate how the distributions vary with particle size (or, at a more basic level b, the ratio of the radiation pressure to gravitational force) by presenting results for three di †erent particle sizes. Patches of emission from our test cloud reach peak levels of 0.1 MJy sr~1 in the 12 km wave band. This represents 10% of the average brightness asymmetry around the sky between the trailing/leading telescope pointing directions seen in the IRAS and COBE data sets. Some of these patches occur at high ecliptic latitudes where the contribution from the Galaxy is negligible and emission from the smooth zodiacal background is low compared to that at low ecliptic latitudes. A strong seasonal variation in the predicted interstellar emission trailing/leading asymmetry is the most obvious signature of the interstellar source, and, in addition, the time variability of the emission will produce di †erent features in the IRAS and COBE data sets and in any subsequent infrared mission. For these reasons, a search of the data for the predicted signatures is certainly justiÐable.