When I think about "episodic accretion", I imagine material from an envelope accreting only sometimes. Is that the right meaning of episodic accretion? or does it mean that the accretion rate is irregular?

  • $\begingroup$ That sounds right, but if you could link to the context where you read this phrase, we might be able to answer more carefully. $\endgroup$ – rob Mar 3 '18 at 21:13
  • $\begingroup$ It's in the context of star formation, FU Orionis objects are believed to be evidence for episodic accretion. $\endgroup$ – APORIL Mar 3 '18 at 21:16
  • $\begingroup$ Can you give a link? $\endgroup$ – rob Mar 3 '18 at 21:28

"Episodic accretion" is the term that has been given to a hypothesis to explain the luminosity distribution of "class I" embedded protostars.

The idea is that these stars accrete via a disk from a more spherically distributed envelope. The envelope adds mass to the disk, which then become unstable on an irregular basis. The instability dumps extra material onto the protostar and the accretion rate and luminosity increase by an order of magnitude or more.

The mass of the star is therefore built up largely through bursts of accretion with a small duty cycle.

Some papers that address this subject can be found here: Baraffe et al. 2009; Vorobyov & Basu 2015.

Whilst the episodic accretion of matter onto protostars seems widely accepted, the physical and observational effects of that accretion are debated. My interest in this debate is whether the accreted material is "hot" or "cold" - that is whether it's gravitational potential energy is absorbed by the protostar or promptly radiated away. Hot accretion could give rise to FU Orionis type outbursts, while cold accretion may cause a dispersion in the Hertzsprung-Russell diagram that is later interpreted as an age spread in coeval populations.

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  • $\begingroup$ Thank you very much for the clarification! According to a model proposed in the paper of Hartmann & Kenyon, 1996, ARAA, 34, 207, only a small fraction (~0.1) of the accreted material is blown away. This is when it has to lose angular momentum while falling onto the protostar. $\endgroup$ – APORIL Mar 4 '18 at 1:31

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