T. Le Bertre 1, N. Mauron 2, G. Lagache 3, F. Boulanger 3, F.X. Désert 4, N. Epchtein 5, & P. Le Sidaner 1
1 Paris Observatory, France
2 GRAAL, CNRS and Montpellier University, France
3 Institut d'Astrophysique Spatiale, Orsay, France
4 Observatoire de Grenoble, Grenoble, France
5 Nice Observatory, France
We present the results of an ISOPHOT program (ISM-AGB) dedicated to study the structure and infrared emission of interstellar matter surrounding red giants. This emission is detectable within about 2 pc of these stars, and the dust is heated to temperatures of 30 K-50 K, significantly warmer than the average temperature of diffuse ISM dust. In most cases, the dust mass within this volume is dominated by interstellar dust rather than circumstellar dust.
Several of the reasons for doing such a program are the following. To the difference of early-type O-B stars, red giant do not emit ultraviolet radiation so that one can properly isolate the emission of the ISM big grains from the other components (small grains, PAH); the surrounding volume is also not ionized. Red giants do not have energetic winds which would significantly modify their interstellar environment; finally red giants can be found at large heights above the Galaxy plane, so that the ISM above the disk can be probed.
We present several ISOPHOT maps obtained at 60 and 90 microns. The fields are 7x7 arcmin, with pixel size 45 arcsec. These maps are centered at 4.7 arcmin from the central stars for which circumstellar envelopes and distances are particularly well known (through the period-luminosity relation). The typical size of the probed volume is 1 to 3 pc.
The majority of the maps show the expected gradient of brightness due to stellar heating of the ISM. Its structure at small scale, 0.1-0.3 pc, the heating properties and other characteristics such as the filling factor or differences with the height above the Galaxy plane will be investigated in the future with our complete sample of 18 two-color maps. We hope also that progresses can be made on the absolute photometric calibration of ISOPHOT data.
The first results of this investigation have been presented in : Le Bertre et al. 1998, AA 335, 287.