Interstellar dust models

Interstellar dust models, previously constrained only from the extinction curve, have been radically changed with the arrival of IRAS observations of the dust infrared emission. An important component of interstellar dust is likely to be made of small particles that show a fluctuating temperature upon impinging single photons and which can produce large near and mid infrared excesses ubiquitously observed in the Galaxy and external galaxies. The analysis of COBE data should soon improve our understanding of dust infrared emissivity and particularly for big grains in the submillimeter domain. We will discuss the key observations (spectral features, broad-band colors, correlations with gas tracers…) which put the best constraints on any dust models and show that the next generation of IR/submm satellites (ISO, SIRTF…) should improve our knowledge of interstellar dust composition and the dust redistribution of the stellar energy inside galaxies.     

Olbers an interplanetary probe to study visible and infrared diffuse backgrounds

The visible extragalactic background (though as yet undetected) is insufficient to explain the abundance of heavy elements in galaxies: either there should be some diffuse extragalactic light in the near infrared (from 1 to 10 m) and/or in the far infrared (100 m) if dust has reprocessed the star light. We propose a new space mission to be dedicated to the search and mapping of primordial stellar light from the visible to the mid-infrared (20 m). In this spectrum range, detectors have reached such a sensitivity that the mission should aim at being (source) photon noise limited, and not any longer background photon noise limited. For that purpose, a small passively cooled telescope with large format CCDs and CIDs could be sent beyond the zodiacal dust cloud (which is absent beyond a solar distance of about 3 AU). In that case, the only remaining foregrounds before reaching the extragalactic background, is due to the Milky Way integrated emission from stars and the diffuse galactic light due to scattering and emission by interstellar dust, which are all unavoidable. Maps of the extragalactic light could be obtained at the arcminute resolution with high signal to noise ratio. This mission is the next logical step after IRAS, COBE and ISO for the study of extragalactic IR backgrounds. It has been proposed as a possible medium-sized mission for the post-horizon 2000 ESA program that could be a piggy back of a planetary mission.     

Background limited infrared and submillimeter instruments

The temperature and emissivity of infrared and submillimeter telescopes are basic parameters that drive the optical and thermal design of astronomical space projects. They determine also, among other parameters, the self-emission of the instrument and the photon noise produced by this radiation on the detectors. By comparing the telescope brightness with that of the sky in the 1m–1 cm wavelength range, general conditions for background limited photometry are derived. For <0.4 mm, temperature is the driving parameter, and for >0.4 mm, temperature and emissivity have equivalent importances. It can be shown on actual projects that these two regimes determine different optical and thermal concepts. Although based on a simplistic approach, this work intends to help designers to handle some basic system parameters of infrared and submillimeter instruments.