Evolution of Gravitationally Unstable Protostellar DIsks

Details Evolution of Gravitationally Unstable Protostellar DIsks Evolution of Gravitationally Unstable Protostellar DIsks

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.1407s&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #14.07; Bulletin of the American Astronomical Society, Vol. 24, p.1142

Astronomy and Astrophysics

Astronomy

Scientific paper

The formation of single stars from rotating interstellar clouds generally requires that the angular momentum of the cloud be redistributed in such a way that a major fraction of the mass (eventually residing in the star) retains only a minor fraction of the angular momentum, most of the latter being deposited in a circumstellar disk. Gravitational instabilities in a growing disk would promote such a redistribution by the action of self-excited density waves. Such instabilites are predicted to occur early in the formation stage of these disks and may play a primary role in their development. We have attempted to quantify this process by conducting two-dimensional N-body simulations of the nonlinear development of long-wavelength instabilities predicted to occur early in the formation stage of the disk, under conditions of controlled energy loss and for different disk/protostar mass ratios. The results are quantified in terms of an empirical relationship between the local thermodynamic state of the disk, as characterized by Toomre's stability parameter Q and the imposed cooling rate, measured in terms of the local orbital frequency. Dynamical evolution rates are derived from the magnitude of the instability-generated torques as a function of the cooling rates. It is confirmed that uncooled disks heat to the point of stability and cease to produce density waves, as found in previous studies. While gravitationally unstable cooled disks also heat up, the instability is never entirely removed; they evolve through the continued action of density waves. Thus it is demonstrated that the rate of evolution of gravitationally unstable disks is controlled by the rate at which thermal energy can escape the disk, just as in the case of convectively unstable disks and Hayashi track stars.

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