Transport-driven super-Jeans fragmentation in dynamical star-forming regions

Abstract

The Jeans criterion is one cornerstone in our understanding of gravitational fragmentation. A critical limitation of the Jeans criterion is that the background density is assumed to be a constant, which is often not true in dynamic conditions such as star-forming regions. For example, during the formation phase of the high-density gas filaments in a molecular cloud, a density increase rate $\dot \rho$ implies a mass accumulation time of $t_{\rm acc}= \rho / \dot \rho= - \rho (\nabla \cdot (\rho \vec{v}))^{-1}$. The system is non-stationary when the mass accumulation time becomes comparable to the free-fall time $t_{\rm ff} = 1 / \sqrt{G \rho}$. We study fragmentation in non-stationary settings, and find that accretion can significantly increase in the characteristic mass of gravitational fragmentation ( $\lambda_{\rm Jeans,\; aac}= \lambda_{\rm Jeans} (1 + t_{\rm ff} / t_{\rm acc})^{1/3}$, $m_{\rm Jeans,\, acc} = m_{\rm Jeans} (1 + t_{\rm ff} / t_{\rm acc})$). In massive star-forming regions, this mechanism of transport-driven super-jeans fragmentation can contribute to the formation of massive stars by causing order-of-magnitude increases in the mass of the fragments.