Evolution of a Viscous Protoplanetary Disk with Convectively Unstable Regions. II. Accretion Regimes and Long-Term Dynam
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ution of a Viscous Protoplanetary Disk with Convectively Unstable Regions. II. Accretion Regimes and Long-Term Dynamics L. A. Maksimovaa, *, Ya. N. Pavlyuchenkova, and A. V. Tutukova aInstitute
of Astronomy, Russian Academy of Sciences, Moscow, 119017 Russia *e-mail: [email protected]
Received April 29, 2020; revised June 11, 2020; accepted June 30, 2020
Abstract—In this article, we proceed to study convection as a possible factor of episodic accretion in protoplanetary disks. Within the model presented in Article I, the accretion history is analyzed at different rates and areas of matter inflow from the envelope onto the disk. It is shown that the burst-like regime occurs in a wide range of parameters. The long-term evolution of the disk is also modeled, including the decreasingwith-time matter inflow from the envelope. It is demonstrated that the disk becomes convectively unstable and maintains burst-like accretion onto the star for several million years. Meanwhile, the instability expands to an area of several tens of astronomical units and gradually decreases with time. It is also shown that at early stages in the disk evolution, conditions arise for gravitational instability in the outer parts of the disk and for dust evaporation in the convectively unstable inner regions. The general conclusion of the study is that convection can serve as one of the mechanisms of episodic accretion in protostellar disks, but this conclusion needs to be verified using more consistent hydrodynamic models. DOI: 10.1134/S1063772920110050
1. INTRODUCTION The formation and evolution of protoplanetary disks (PDs) around young stars is one of the most intriguing topics in astrophysics. In the earliest evolution stages, many young stellar systems show signs of episodic accretion; these objects are known as FUors and EXors (see, e.g., [1, 2]). There are theoretical agruments to believe that, all PDs go through a phase of episodic accretion at initial stages in their evolution, which explains the variable luminosity of young stellar objects [3]. However, the physical mechanisms of this variability remain debatable. The variability problem is closely related to the more general question about the mechanisms of angular momentum transfer in accretion disks. Vigorous discussions have taken place regarding the possible mechanisms that not only ensure the transfer of angular momentum in disks but also cause the nonsteady accretion pattern and involve such factors as gravitational, magnetorotational, and thermal instabilities (see [2]). Thus, the nonsteady pattern of accretion caused by gravitational instability is associated with the clump formation and their falling onto the star, leading to luminosity bursts (see, e.g., [4]). The burst-like pattern caused by magnetorotational instability is due to the positive feedback link of this instability to the ionization degree of matter [5]. Thermal instability is caused by an increase in gas
opacity with increasing temperature in partially ionized gas [6]. Convection is also considered
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