P. Abraham , Ch. Leinert , & D.
Lemke
Max-Planck-Institut für Astronomie, Heidelberg, Germany
In the 3-70 micron wavelength regime the infrared sky is dominated by thermal emission from interplanetary dust particles. ISO has successfully performed an extensive observing programme on the zodiacal light. In this paper we summarize the first published and some unpublished results derived from ISOPHOT measurements, and review those fields of zodiacal light research where important contribution is expected from ISO.
1. Spectrum of the zodiacal light. The thermal spectrum of the zodiacal light cannot be observed from the ground. We performed 3.6-200 micron multifilter photometry and 6-12 micron spectrophotometry with ISOPHOT at several positions, and found that the spectrum is remarkably well fitted by a Planck curve of about 270 K over the observed wavelength range. No obvious spectral features were detected in the mid-infrared low resolution spectrum. The measurements place a strong constrain on models of interplanetary dust concerning constituents and size distributions. The observed slight dependence of the temperature of the zodiacal light on ecliptic position will be modelled in the context of the three-dimensional density and temperature distribution of the interplanetary dust cloud.
2. Arcminute structure of the zodiacal light. Although the zodiacal light is smooth on large scale, little is known on the graininess of its brightness at small spatial scales. We mapped 5 fields of 0.5x0.5 deg at low, intermediate and high ecliptic latitudes at 25mum with ISOPHOT in order to look for fluctuations in the zodiacal light. We derived an upper limit for the fluctuations of 0.2% of the total brightness level, which supports the concept of a generally smooth zodiacal light distribution.
3. Large scale structure of the Interplanetary Dust Cloud. For many ISO observations, including the search for the very faint infrared extragalactic background, a precise subtraction of the zodiacal light is crucial. The all-sky surveys of IRAS and COBE are resulted in a picture of the 3-dimension structure of the Interplanetary Dust Cloud. In order to predict the brightness of the zodiacal light for any ISO observation, we have to adapt this picture (particularly the COBE/DIRBE model) to the ISO filters. For this purpose we observed 18 carefully selected dark areas distributed on the sky in the 3.6-200 micron wavelength range using absolute photometry mode. The first results indicate that the adaptation of the COBE/DIRBE model to ISOPHOT can be done with high accuracy.
4. Origin of the zodiacal dust: asteroidal bands and cometary trails. The asteroidal bands, discovered by IRAS, are probably the results of collisions in the asteroidal belt, and provide important information on the asteroidal contribution to the interplanetary dust. With ISOPHOT we performed multifilter scans across the Ecliptic, in order to study the brightness and temperature profiles of the bands. Similar multifilter scans were performed across the dust trail of P/Comet Kopff at various distances from the nucleus. The trail was clearly observed at 0.25 deg behind the comet but only marginally detected at larger separations. From our 12/25/60 micron photometry we want to determine the temperature of the trail particles and to confirm the expected temperature increase towards the comet.