A semi-analytic model for the temporal evolution of the episodic disc-to-star accretion rate during star formation

Abstract

We develop a semi-analytic formalism for the determination of the evolution of the stellar mass accretion rate for specified density and velocity profiles that emerge from the runaway collapse of a prestellar cloud core. In the early phase, when the infall of matter from the surrounding envelope is substantial, the star accumulates mass primarily because of envelope-induced gravitational instability in a protostellar disc. In this phase, we model the envelope mass accretion rate from the isothermal free-fall collapse of a molecular cloud core. The disc gains mass from the envelope, and also transports matter to the star via a disc accretion mechanism that includes episodic gravitational instability and mass accretion bursts according to the Toomre -criterion. In the early phase the envelope accretion is dominant, whereas in the late phase the disc accretion is dominant. In the disc accretion phase, mass is accreted on to the star due to gravitational torques within the spiral structures in the disc, in a manner that analytic theory suggests has a mass accretion rate ∝ −6/5 . Our model provides a self-consistent evolution of the mass accretion rate by joining the spherical envelope accretion with the disc accretion and accounts for the presence of episodic accretion bursts at appropriate times. We show using a simple example that the burst mode is essential to explain the long-standing 'luminosity problem' of young stellar objects. The bursts are needed to provide a good match to the observed distribution of bolometric luminosities. In contrast, a smoothly time-dependent mass accretion rate, whether monotonically increasing or decreasing, is unable to do so. Our framework reproduces key elements of detailed numerical simulations of disc accretion and can aid in developing intuition about the basic physics as well as in comparing theory with observations.