The Interstellar Structures. I. Gas Clouds.

Abstract

In an effort to arrive at a dynamical model for the interstellar medium, its mechanical properties are discussed. Using a polytrope relation between the temperature and density within a gas cloud, the mechanical and then the statistical properties of an idealized model of the interstellar clouds are developed. One finds that the collision rate of clouds, computed in an elementary fashion from their diameters and random translational velocities, leads to several difficulties. First, the very inelastic nature of head-on collisions would rapidly reduce the translational velocities, and it would be difficult to explain the observed radial velocities. Second, there would be a strong tendency toward statistical equilibrium of the internal state of the clouds. The equilibrium is investigated, and the cloud is found to be either in a collapsed condition or completely dispersed, again contrary to observation. Ordering of the trans-lational velocities is proposed as one means of circumventing these difficulties. In any case, the tendency of clouds to collapse, possibly to form clusters of stars, is shown to be intrinsic in their nature. I. MECHANICAL PROPERTIES OF THE INTERSTELLAR MEDIUM This is the first of a series of papers attempting to investigate theoretically the mechanical and statistical state of the interstellar medium. This paper is devoted to the gas structures because they contain the great preponderance of matter, and the motion of such things as dust and smoke depend greatly upon the motion of the gas. We begin with a discussion of the mechanical properties of the gas, so that later we may consider its dynamical state. Observation shows that the interstellar gas is clumped rather than homogeneous. The clump or cloud diameters are often assumed to be of the order of 10-20 psc, and the interval between cloud centers of the order of 20-40 psc. Densities within the clouds usually range from 5 to 1000 hydrogen atoms per cubic centimeter. The gas density 1,2 outside the clouds is of the order of 10-1 or 10" 2 hydrogen atoms per cubic centimeter, so that the overall mean density is of the order of one hydrogen atom per cubic centimeter. We see, then, that the density within the clouds is 10 2 to 10 4 times greater than outside the clouds, and the clouds may be regarded as individual entities. The clouds apparently have more or less random translational velocities of the order of 5 km/sec. 3,4 > 6 There is some indication of equipartition of translational energy between clouds of different masses. 5 The escape velocity for atoms from a given cloud probably is somewhat less than the 5-10 km/sec translational velocity of the clouds. The observations may be supplemented by the following rather elementary theoretical considerations. The mean free path for collision of gas clouds is 20-40 psc. A random velocity of 5 km/sec implies a collision for a given cloud every f • 10 7 years. We see, then, that in two revolutions of the galaxy (i e., the lifetime of the galaxy) an individual cloud will undergo approximately 60 collisions. Hence, in any study of the interstellar medium, we must consider its dynamic equilibrium state in the presence of these collisions .