ar X iv : a st ro - p h / 99 06 21 7 v 1 1 2 Ju n 19 99 Phases and Structures of Interstellar Gas Bruce

The thermal and chemical phases of the cool component of interstellar gas are discussed. Variations with galactocentric radius and from galaxy to galaxy are mostly the result of changes in the ambient interstellar pressure and radiation field. Interstellar structure that is hierarchical or fractal in the cloudy parts and has large and connected empty regions between these clouds is probably the result of turbulence. Such structure opens up the disk to the transmission of OB star light into the halo, and it provides for a diffuse ionized component that tapers away gradually from each dense HII region. Fractal cloud structure may also produce the cloud and clump mass functions, and perhaps even the star cluster mass function. to be published in ”The Physics and Chemistry of the Interstellar Medium,” 3rd Cologne-Zermatt Symposium, ed. G. Winnewisser, J. Stützki, and V. Ossenkopf, Aachen: Shaker-Verlag, 1999, in press 1. Phases of Interstellar Gas Interstellar gas in the solar neighborhood has a variety of thermal and chemical phases, including cold atomic gas inside or mixed with cold molecular clouds (e.g., Sato & Fukui 1978), cool and warm atomic gas on the envelopes of molecular clouds (Chromey et al. 1989; Andersson & Wannier 1993; Wannier et al. 1993) and in the diffuse, low density medium (Kulkarni & Heiles 1987), cold molecules in dark and dense clouds (Combes 1991) and in translucent clouds (Magnani, Blitz & Mundy 1985), warm molecules in dense clouds near embedded luminous stars (e.g., Cesaroni et al. 1994), dense photo-ionized gas in the neighborhoods of O and B-type stars, low-density ionized gas between these stars (Walterbos & Braun 1994; Reynolds 1995; Ferguson et al. 1996), and hot, shock-heated gas near supernovae and windy stars (Ostriker & McKee 1988). There is also considerable variety in the types of molecules that are present inside dark clouds, ranging from primarily H2 in translucent clouds (Spitzer & Jenkins 1975; Magnani et al. 1998), with a possible trace of PAH or long-chain molecules (Leger & Puget 1984; Tulej et al. 1998), to a mixture of complex molecules in dense clouds (e.g., Langer et al. 1997; see reviews by van Dishoeck and Thaddeus, this conference). The thermal and chemical states of the cold and cool gas have such a great variety because the clouds shield themselves to different levels from photodissociative radiation and radiant heating. This makes the molecular abundance and temperature depend on density, column density, metallicity, age, and local radiation field, all of which vary from cloud to cloud and with radius in the Galaxy. The nature of cloud structure does not change this much, because it is largely the result of turbulence, self-gravity, and local explosions, which seem to act the same way everywhere. Here we discuss the dominant processes that affect the molecular and thermal states of the gas, and we review the some of the aspects of cloud structure that are likely to arise from turbulence. IBM Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598, USA, [email protected]