The Search for Protostars Using Millimeter/Submillimeter Dust Emission as a Tracer

Abstract

In the disk of our Galaxy, about four solar masses of interstellar matter (ISM) per year are transformed into stars. Observations combined with evolutionary models (see also, e.g., Shu et al., 1987) suggest the following sequence of events for the formation of medium- and high-mass stars, which is illustrated by the Orion Giant Molecular Cloud (OMC; see Figure 1):

• 1. In an extended GMC of medium density (Figure 1a; n_H ∼ 10² to 10³ cm^-3) a filament or ridge structure of increased density (Figure 1b; n_H ∼ 10⁴ cm^-3) has developed which is usually investigated using transitions of molecules with high dipole moment (e.g., CS isotopes).

• 2. Elongated cloud cores form in this filament (Figure 1 c) with densities n_H ∼ 10⁵ to 10⁶ cm^-3, which should be gravitationally unstable according to the Jeans criterion but may be supported by frozen-in magnetic fields and (non-thermal) turbulence. These cores can be best investigated using transitions of molecules with high dipole moment (e.g., CS isotopes) or dust emission.

• 3. Rotating protostellar condensations with mean densities ≥ 10⁸ cm^-3 form along the major axis of such an elongated core like beads on a string (see, e.g., Figures 3 and 6c). In most cases these condensations can only be observed through their dust emission.

• 4. They collapse in free-fall until a stellar core forms at mean densities of n_H∼ 10¹¹ to l0¹² cm^-3.

• 5. A rotating disk forms as a consequence of conservation of angular momentum, through which the stellar core accretes additional matter.

• 6. Molecular outflow perpendicular to the accretion disk begins at a relatively early evolutionary stage — perhaps as early as the formation of a stellar core.

• 7. These pre-MS stars evolve to MS stars through the states of dust embedded star and T-Tauri star (medium-mass stars) or cocoon star (high-mass stars; see Figure 1 c), respectively.

Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.