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

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 ${\dot{M}}_{\mathrm{out}}$, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star ${\dot{M}}_{\mathrm{in}}$ 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 ${\dot{M}}_{\mathrm{out}}$. It is shown that the time variability of ${\dot{M}}_{\mathrm{in}}$ does not coincide with ${\dot{M}}_{\mathrm{out}}$ due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant ${\dot{M}}_{\mathrm{out}}$. Outburst occur only in a limited range of mass accretion rates onto the inner disk $10^{-10}<{\dot{M}}_{\mathrm{out}}<3\times 10^{-6}{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}$ due to gravo-magneto limit cycle (GML). Finally, we discuss the case with a combination of episodic ${\dot{M}}_{\mathrm{out}}$ 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 ${\dot{M}}_{\mathrm{out}}$, although fluctuations of $\dot{M}$ 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, ${\dot{M}}_{\mathrm{out}}$ does appear directly in ${\dot{M}}_{\mathrm{in}}$ (the latter defining the stellar accretion luminosity). In a spontaneous mode of outburst, ${\dot{M}}_{\mathrm{out}}$ appears as the interval between outbursts.

20 pages, 7 figures, accepted for publication in PASJ

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