Does the Parker mechanism spin down AGN stars?

May 31, 2022 07:15 · 439 words · 3 minute read astronomy science research

I sometimes have research ideas that I think are cool, but that don’t make sense for me to pursue. I generally just make a note of them and move on. This is the fifth post in a series describing some of the ideas I’ve accumulated.

Does the Parker mechanism spin down AGN stars?

What’s the idea?

Collaborators and I previously predicted that stars embedded in AGN disks should end up spinning really fast (near breakup). One effect we explicitly neglected was magnetic spindown associated with mass loss (the Parker mechanism). Yuri Levin pointed out to me that magnetic spindown could be important, so it would be good to understand how big this effect is.

Why is this interesting?

We think that the spins of AGN stars map pretty directly onto the spins of black holes in the disk, which in turn might be observable via gravitational waves during binary black hole mergers. This is one of the few observational handles we have on AGN stars, and so being able to predict their spins is crucial for connecting theory and observations.

Also, fast-spinning massive stars are expected to produce Gamma Ray Bursts (GRB’s) whereas slow-spinning ones should not. GRB’s are potentially observable even in AGN disks, so another observational handle is tied in with stellar spins.

How can I get started?

I think there are two basic inputs to this calculation:

  1. How strong is the magnetic field at the surface of an AGN star?
  2. How much mixing happens between the escaping stellar wind and the incoming accretion stream?

The first I’d guess is set by the magnetic field in the AGN disk being compressed in the accretion stream and brought to the surface of the star. Either that or the field is generated in the star by dynamo processes, in which case it’s probably in rough equipartition with the turbulent kinetic energy in the relevant layers.

The second seems more complicated. Here’s why it matters though: the Parker mechanism relies on field lines transmitting torques from the Alfven radius to the bulk of the star, which requires that the field in the wind be connected to the field in the star. If there is substantial mixing in the wind, e.g. because of interaction with the accretion stream, there could be reconnection events and the field in the wind could end up disconnected from that in the star. That would mean that torques are only communicated from a much smaller radius (the typical radius of the first reconnection events), which reduces the spindown substantially.

I’m not sure how to estimate this effect without running MHD simulations, so that’s probably necessary here.

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