Inner Disk in Mass Accretion into Host Star Early in Formation

The Role for the Inner Disk in Mass Accretion to the Star in the Early Phase of Star Formation

Authors:

Ohtani et al

Abstract:

A physical mechanism that drives FU Orionis-type outbursts is reconsidered. We study the effect of inner part of a circumstellar disk covering a region from near the central star to the radius of approximately 5 AU (hereafter, the inner disk). Using the fluctuated mass accretion rate onto the inner disk M˙out, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star M˙in by means of numerical calculation of an unsteady viscous accretion disk in a one-dimensional axisymmetric model. First, we calculate the evolution of the inner disk assuming an oscillating M˙out. It is shown that the time variability of M˙in does not coincide with M˙out due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant M˙out. Outburst occur only in a limited range of mass accretion rates onto the inner disk 10−10 greater than M˙out greater than 3×10−6 M⊙yr−1 due to gravo-magneto limit cycle (GML). Finally, we discuss the case with a combination of episodic M˙out and accretion outbursts cause by the GML in the inner disk. The GML can drive accretion outbursts onto the star even for the case of fluctuating M˙out, although fluctuations of M˙ decay during transmitting the inner disk inwards. We newly identified two modes of outburst which are spontaneous one and stimulated one. In a stimulated mode of outburst, M˙out does appear directly in M˙in (the latter defining the stellar accretion luminosity). In a spontaneous mode of outburst, M˙out appears as the interval between outbursts.