Why Do Some Cores Remain Starless?

Abstract

Prestellar cores, by definition, are gravitationally bound but starless pockets of dense gas. 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 ∼ 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 supercritical state does not ensure that a core will necessarily become protostellar, (b) potentially star-forming cores (the VeLLO L1521F here), could be experiencing coagulation of dust-grains that must enhance the gasdust 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. Gas temperature in these purely hydrodynamic calculations was calculated by explicitly solving the respective equations of thermal-balance for gas and dust; the attenuation factor for the interstellar radiation-field, χ, which in literature has been well constrained to ∼ 10 −4 for putative star-forming clumps and cores was adopted in these calculations.

Core formation via filament fragmentation and the impact of ambient pressure on it

“…In the collect-and-collapse mode of core formation/evolution, we envisage such a core steadily acquires mass from its natal filament and then slowly proceeds to collapse. A core that forms in this manner will remain starless for as long as its mass is less than the Jeans mass, or as long as its interior does not cool sufficiently via some mechanism such as dust coagulation (e.g., Anathpindika 2016). In this manner, we can reconcile the dynamical stability of cores over several free-fall times, or equivalently, the starlessness of these cores without invoking an additional buoyant force such as a strong magnetic field.…”