COLLAPSE AND FRAGMENTATION OF MOLECULAR CLOUD CORES. II. COLLAPSE INDUCED BY STELLAR SHOCK WAVES

摘要

The standard scenario for low-mass star formation involves the "inside-out" collapse of a dense molecular cloud core following loss of magnetic field support through ambipolar diffusion. However, isotopic anomalies in presolar grains and meteoritical inclusions imply that the collapse of the presolar cloud may have been triggered by a stellar shock wave. This paper explores "outside-in" collapse, that is, protostellar collapse initiated directly by the compression of quiescent dense cloud cores impacted by relatively slow stellar shock waves. A second-order accurate, gravitational hydrodynamics code has been used to study both the spherically symmetrical and three-dimensional evolution of initially centrally condensed, isothermal, self-gravitating, solar-mass cloud cores that are struck by stellar shock waves with velocities up to 25 km s~(-1) and postshock temperatures of 10 to 10,000 K. The models show that such mild shock waves do not completely shred and destroy the cloud, and that the dynamical ram pressure can compress the cloud to the verge of self-gravitational collapse. However, compression caused by a high postshock temperature is a considerably more effective means of inducing collapse. Shock-induced collapse produces high initial mass accretion rates ( > 10~(-4) solar mass yr~(-1) in a solar-mass cloud) that decline rapidly to much lower values, depending on the presence (～10~(-6) solar mass yr~(-1)) or absence (～10~(-8) to 10~(-7) solar mass yr~(-1)) of an infinite reservoir of mass. Stellar mass accretion rates ～10~(-7) solar mass yr~(-1) have been previously inferred from the luminosities of T Tauri stars; balanced mass accretion (stellar rate = envelope rate) at ～10~(-7) solar mass yr~(-1) could then be possible if accretion occurs from a finite mass reservoir. Fluid tracers are used to determine what fraction of the stellar shock material is incorporated into the resulting protostellar object and disk; roughly half the impinging material is injected into the collapsing cloud core when there is a high postshock temperature. The models are consistent with a scenario where an AGB star wind triggered the collapse of the presolar cloud while injecting about 0.01 solar mass of matter derived from the AGB star envelope, as has been separately inferred on the basis of nucleosynthesis calculations.