Externally Fed Star Formation: A Numerical Study

TL;DR

This study uses numerical simulations to explore how dense, unmagnetized gas clouds evolve during star formation, revealing that observed subsonic infall is difficult to reproduce without magnetic effects, which may help regulate accretion.

Contribution

The paper demonstrates that purely hydrodynamic models produce unrealistic supersonic infall and accretion rates, suggesting magnetic tension may be essential for realistic star formation simulations.

Findings

Infall remains subsonic for extended periods, matching observations.

Supersonic infall and high accretion rates occur shortly before star formation.

Magnetic tension may be necessary to prevent excessive infall and accretion.

Abstract

We investigate, through a series of numerical calculations, the evolution of dense cores that are accreting external gas up to and beyond the point of star formation. Our model clouds are spherical, unmagnetized configurations with fixed outer boundaries, across which gas enters subsonically. When we start with any near-equilibrium state, we find that the cloud's internal velocity also remains subsonic for an extended period, in agreement with observations. However, the velocity becomes supersonic shortly before the star forms. Consequently, the accretion rate building up the protostar is much greater than the benchmark value c_s^3/G, where c_s is the sound speed in the dense core. This accretion spike would generate a higher luminosity than those seen in even the most embedded young stars. Moreover, we find that the region of supersonic infall surrounding the protostar races out to engulf much of the cloud, again in violation of the observations, which show infall to be spatially confined. Similar problematic results have been obtained by all other hydrodynamic simulations to date, regardless of the specific infall geometry or boundary conditions adopted. Low-mass star formation is evidently a quasi-static process, in which cloud gas moves inward subsonically until the birth of the star itself. We speculate that magnetic tension in the cloud's deep interior helps restrain the infall prior to this event.