Physical conditions in dark clouds

Abstract

We discuss physical conditions in interstellar molecular clouds with visual extinction A_V ∼1–10 magnitudes, based primarily on observations of spectral lines of CO and NH₃. Diffuse clouds with A_V∼1 mag have kinetic energy density which is primarily nonthermal, and is comparable to that of the “intercloud” gas; diffuse clouds have too little mass to bind their internal motions by gravity; their incidence of low-mass cores and low-mass stars is rare. Dark clouds with mean A_V∼3 mag have kinetic energy density which is primarily nonthermal, like diffuse clouds; but much greater than that of the intercloud gas, unlike diffuse clouds. Dark clouds are generally gravitationally bound. They are often surrounded by a number of diffuse clouds; in dark clouds low-mass cores and low-mass stars are common. Low-mass cores have mean A_V∼10 mag. Their kinetic energy density is mainly thermal, unlike their surrounding dark or diffuse clouds, and much higher than that of the intercloud gas. Cores are generally bound, and are frequently associated with low-mass stars. Several lines of evidence indicate that the nonthermal energy density in most molecular clouds is largely magnetic.