Episodic outbursts during brown dwarf formation

Abstract

There is evidence that stars and browns dwarfs grow through episodic rather than continuous gas accretion. However, the role of episodic accretion in the formation of brown dwarfs remains mostly unexplored. We investigate the role of episodic accretion, triggered by the magnetorotational instability in the inner disk regions, resulting in episodic outbursts during the formation of brown dwarfs, and its implications for their early formation stages. We use hydrodynamical simulations coupled with a sub-grid accretion model to investigate the formation of young proto-brown dwarfs and protostars, taking into account the effects of episodic accretion resulting in episodic radiative feedback, i.e. in luminosity outbursts. The formation timescale for proto brown dwarfs is at least one order of magnitude shorter than that of protostars. Episodic accretion leads to a shorter main accretion phase compared to continuous accretion in brown dwarfs, whereas the opposite is true for low-mass stars. Episodic accretion can accelerate early mass accretion in proto-brown dwarfs and protostars, but it results in less massive objects by the end of the main phase compared to continuous accretion. We find an approximately linear correlation between an object's mass at the end of the main accretion phase and the timing of the last episodic outburst: later events result in more massive brown dwarfs but less massive low-mass stars. Episodic outbursts have a stronger effect on brown dwarf-forming cloud cores, with the last outburst essentially splitting the brown dwarf evolution into a short high-accretion and a much longer low-accretion phase.