## Interstellar Medium

PLANCK will provide all-sky maps of the emission from Galactic dust at sub-mm and mm wavelengths. At the two shortest wavelengths the PLANCK maps will have the same resolution as those from IRAS with an order of magnitude better sensitivity in terms of gas column density: N(H) of a few times 1018 H cm-2. At longer wavelengths the detection limit is 1 to H cm-2. This sensitivity allows the detection of emission from Galactic dust over the whole sky in all of the bolometric bands. One can thus expect that PLANCK will have an even more significant impact on Galactic studies than the successful IRAS mission. Within the Galactic plane the sub-mm data will allow a decomposition of the IR emission from the different gas components and, in particular, the detection of emission from cold clouds that are not heated by active star forming regions. The fraction of gas in such clouds is still a matter of debate, and an estimate of its value is essential in establishing the efficiency of star formation in our Galaxy.

The IRAS data have been used to derive the size distribution of interstellar grains (Desert et al. 1990). An important result of this work was the discovery that the abundance of small particles emitting in the mid-IR varies by one order of magnitude within nearby molecular complexes, such as Taurus or Chamaeleon, and among high latitude cirrus clouds (Boulanger et al. 1990). The abundance of small particles has been observed to drop over physical sizes of about 0.1 pc where dense molecular gas is present (Laureijs et al. 1991). The COBE mission has provided data on the overall emission spectrum of dust from the near-IR to mm wavelengths which allows a better characterization of the evolution of dust. However at the sub-mm wavelengths, the low angular resolution of FIRAS (7 degrees) smooths out most of the spatial variations revealed by IRAS. The analysis of the high latitude COBE data shows that the emission spectrum from dust associated with HI gas is well fitted by a single temperature grey body with an emissivity scaling as and a temperature T = 17.5 K. However it is clear from the spectra within the Galactic plane and in the direction of molecular complexes that a colder emission component is present there. The nature of this colder component is controversial. Is it a particular type of grain or does it only correspond to a change in dust emissivity? Is the cold dust component correlated with variations in the abundance of small particles and physical conditions in the gas? The angular resolution provided by PLANCK will allow a detailed investigation of these questions.

Hints suggesting a correlation between cold dust emission and dense gas opens the prospect of using the PLANCK data to systematically search for dense condensations in nearby clouds. Such a condensation with Td=10 K has recently been found in Orion with the Pronaos experiment (Ristorcelli 1996). For Td=10 K, the PLANCK detection limit at m corresponds to a gas mass of at a distance of 150 pc.

Dispersion measurements towards pulsars show that about a quarter of the gas in the nearby interstellar medium is ionized gas outside classical HII regions. The presence of such a large amount of ionized gas far away from ionizing stars is not understood (Reynolds 1990). The diffuse ionized gas is the dominant gas component at distances higher than 1kpc above the Galactic plane. Studies of this gas are important in establishing the physical processes responsible for expelling gas from the Galactic disk and in developing general models of the evolution of interstellar matter.

The wide frequency range of PLANCK will allow studies of this gas component from two distinct points of view: (1) the infrared emission from dust associated with this gas and (2) the free-free emission from the electrons. Attempts to identify dust emission from the diffuse ionizing gas using IRAS and COBE data have so far been unsuccessful. The difficulty here is to separate the emission from the HI and HII gas since the two are known to be spatially correlated to a significant extent (Reynolds et al. 1995). The free-free maps which will be extracted from the longer wavelength maps will help to spatially separate the two gas components. Dust in the low density HII gas is likely to have been processed by shocks. Detection of this dust would thus open the possibility of testing models of dust destruction in shocks.