Episodic accretion in magnetically layered protoplanetary discs

Abstract

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 AU such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion onto the protostar occurs in bursts with an accretion rate of 10^{-5} solar masses / yr, separated by quiescent intervals where the accretion rate is 10^{-8} solar masses / yr. Such bursts could lead to repeated episodes of strong mass outflow in Young Stellar Objects. The transition to this episodic mode of accretion occurs at an early epoch (t < 1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at around 1 AU. The predicted rate of low mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1-3 AU.Comment: MNRAS, in pres

Protostellar accretion traced with chemistry

“…Furthermore, for gravitational instabilities to operate, disk masses 0.05 M are needed (Vorobyov 2013;Kratter & Lodato 2016). Although magnetic and thermal instabilities may continue to work in the inner disk independently of the total disk mass, gravitational instabilities in the outer disk are needed to feed this region (Armitage et al 2001;Zhu et al 2009). Consequently, there is some reason to believe that protostars encircled by massive disks are the most likely burst candidates.…”

“…Theoretically, such outbursts can be tied to accretion instabilities in the circumstellar disk encircling the YSO (e.g. Bell & Lin 1994;Armitage et al 2001;Vorobyov & Basu 2005;Zhu et al 2009), leading to a lot of material being dumped onto the central object over a short period of time. It is not known if deeply embedded protostars un- (a) Per-emb XX names follow the naming scheme of Enoch et al (2009).…”

Abundance Analysis of Planetary Host Stars. I. Differential Iron Abundances

“…Accretion disks have been observed around T Tauri stars (e.g., Hartmann et al 1998) and have been modeled by, e.g., Armitage, Livio, & Pringle (2001), with magnetohydrodynamic turbulence as the source for angular momentum transport. The presence of accretion disks could alter the stellar surface abundance if we consider the following assumptions: (1) accretion is still taking place at a significant rate when solid bodies large enough to be decoupled from the gas have formed; (2) the convection zone is already small enough, so that abundances within it are changed by adding material; (3) the accreted gas is depleted in heavy elements, because they are locked up in the solid bodies.…”

Testing the locality of transport in self-gravitating accretion discs

“…In the case of low mass (proto)stars, disc mass estimates are uncertain, but discs in the upper end of the mass distribution for T Tauri stars (Launhardt & Sargent 2001;Natta et al 2004) can have a mass ≈ 30% of the central star. In addition, there have also been theoretical suggestions that the disc self-gravity might be important in the outer disc of FU Orionis objects (Bell & Lin 1994;Armitage et al 2001;Lodato & Bertin 2003b). ple, Boss 2002;Rice et al 2003 or, for similar arguments on the galactic scale, Goodman & Tan 2004).…”