Why do some cores remain starless ?

TL;DR

This study investigates why some starless cores in the local Universe do not form stars, finding that thermal state, dust grain coagulation, and dynamic conditions influence star formation potential.

Contribution

It demonstrates that thermally super-critical cores do not necessarily become protostellar and highlights the role of dust grain coagulation in core collapse.

Findings

Super-critical cores may remain starless due to thermal and dynamic factors.

Dust grain coagulation can promote collapse by lowering gas temperature.

Pressure confinement and virial binding are not definitive indicators of star formation.

Abstract

Physical conditions that could render a core starless(in the local Universe) is the subject of investigation in this work. To this end we studied the evolution of four starless cores, B68, L694-2, L1517B, L1689, and L1521F, a VeLLO. The density profile of a typical core extracted from an earlier simulation developed to study core-formation in a molecular cloud was used for the purpose. We demonstrate - (i) cores contracted in quasistatic manner over a timescale on the order of $\sim 10^{5}$ years. Those that remained starless did briefly acquire a centrally concentrated density configuration that mimicked the density profile of a unstable Bonnor Ebert sphere before rebounding, (ii) three of our test cores viz. L694-2, L1689-SMM16 and L1521F remained starless despite becoming thermally super-critical. On the contrary B68 and L1517B remained sub-critical; L1521F collapsed to become a VeLLO only when gas-cooling was enhanced by increasing the size of dust-grains. This result is robust, for other cores viz. B68, L694-2, L1517B and L1689 that previously remained starless could also be similarly induced to collapse. Our principle conclusions are : (a) acquiring the thermally super-critical state does not ensure that a core will necessarily become protostellar, (b) potentially star-forming cores (VeLLO L1521F here), could be experiencing coagulation of dust-grains that must enhance the gas-dust coupling and in turn lower the gas temperature, thereby assisting collapse. This hypothesis appears to have some observational support, and (c) depending on its dynamic state at any given epoch, a core could appear to be pressure-confined, gravitationally/virially bound, suggesting that gravitational/virial boundedness of a core is insufficient to ensure it will form stars, though it is crucial for gas in a contracting core to cool efficiently so it can collapse further to become protostellar.