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Takuya Ohtani, Shigeo S. Kimura, Toru Tsuribe, Eduard I. Vorobyov, The role of the inner disk in mass accretion to the star in the early phase of star formation, Publications of the Astronomical Society of Japan, Volume 66, Issue 6, December 2014, 112, https://doi.org/10.1093/pasj/psu098
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Abstract
A physical mechanism that drives FU Orionis-type outbursts is reconsidered. We study the effect of the inner part of a circumstellar disk covering a region from near the central star to a radius of approximately 5 au (hereafter, the inner disk). Using the fluctuating mass accretion rate onto the inner disk |$\dot{M}\,_{\rm out}$|, we consider the viscous evolution of the inner disk and the time variability of the mass accretion rate onto the central star |$\dot{M}\,_{\rm in}$| 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 |$\dot{M}\,_{\rm out}$|. It is shown that the time variability of |$\dot{M}\,_{\rm in}$| does not coincide with |$\dot{M}\,_{\rm out}$| due to viscous diffusion. Second, we investigate the properties of spontaneous outbursts with temporally constant |$\dot{M}\,_{\rm out}$|. Outbursts occur only in a limited range of mass accretion rates onto the inner disk of |$10^{-10}<\dot{M}\,_{\rm out}< 3\times 10^{-6}{\,M}_{\odot }\:$|yr−1 due to the gravo-magneto limit cycle (GML). Finally, we discuss the case with a combination of episodic |$\dot{M}\,_{\rm out}$| and accretion outbursts caused by the GML in the inner disk. The GML can drive accretion outbursts onto the star even for the case of fluctuating |$\dot{M}\,_{\rm out}$|, although fluctuations of |$\dot{M}\,$| decay while transmitting the inner disk inwards. We have newly identified two modes of outburst, one spontaneous and one stimulated. In the stimulated mode of outburst, |$\dot{M}\,_{\rm out}$| does appear directly in |$\dot{M}\,_{\rm in}$| (the latter defining the stellar accretion luminosity). In the spontaneous mode of outburst, |$\dot{M}\,_{\rm out}$| appears as the interval between outbursts.
