The Bimodality of Accretion In T Tauri Stars and Brown Dwarfs

We present numerical solutions of the collapse of prestellar cores that lead to the formation and evolution of circumstellar disks. The disk evolution is then followed for up to three million years. A variety of models of different initial masses and rotation rates allows us to study disk accretion around brown dwarfs and low-mass T Tauri stars, with central object mass $M_st < 0.2 Msun$, as well as intermediate and upper-mass T Tauri stars (0.2 Msun < M_st < 3.0 Msun). Our models include self-gravity and allow for nonaxisymmetric motions. In addition to the self-consistently generated gravitational torques, we introduce an effective turbulent \alpha-viscosity with \alpha = 0.01, which allows us particularly to model accretion in the low-mass regime where disk self-gravity is diminishing. A range of models with observationally-motivated values of the initial ratio of rotational to gravitational energy yields a correlation between mass accretion rate \dot{M} and M_st that is relatively steep, as observed. Additionally, our modeling reveals evidence for a bimodality in the \dot{M}--M_st correlation, with a steeper slope at lower masses and a shallower slope at intermediate and upper masses, as also implied by observations. We show that the neglect of disk self-gravity leads to a much steeper \dot{M}--M_st relation for intermediate and upper-mass T Tauri stars. This demonstrates that an accurate treatment of global self-gravity is essential to understanding observations of circumstellar disks.