What would happen if two AGN stars merged?

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 twelfth post in a series describing some of the ideas I’ve accumulated.

What would happen if two AGN stars merged?

What’s the idea?

We think that stars in AGN disks live in a balance between accretion and radiation-driven mass-loss, and that this is a stable equilibrium. If the star becomes more massive, its luminosity increases and drives a faster wind, and this is a stronger effect than the increase in its accretion rate caused by its greater mass.

So what happens if two of these stars merge?

Why is this interesting?

Stars in AGN disks likely live in the plane of the disk, and migrate over time. Because the migration rate is mass-dependent, AGN stars may well run into each other as they migrate, though the rates of these encounters depend on lots of poorly-understood details about the structure of the disk and the masses of the stars.

If two AGN stars merge the new, merged object would be far over the equilibrium mass and must shed material, but it’s possible other things (like explosions) happen first.

How can I get started?

I’d start by thinking about the time-scales involved. Mass-loss gets driven by the luminosity of the star, which adjusts to the new mass on a thermal time-scale. By contrast accretion responds to the gravitational pull of the star, which adjusts immediately upon merger. So it’s possible the merger remnant actually accretes at an increased rate for a while before the winds turn up, and this could cause lots of interesting things to happen.

I’d also think about the possibility of mass-loss during the merger. Much of the material in AGN stars is loosely bound and radiation-dominated, so there could be substantial losses during the merger.

It likewise seems possible that nuclear burning could be dramatically enhanced in the first thermal time after the merger, and so the core of the star might well undergo significant chemical evolution before the transient dies down and the excess mass is shed. This could lead to explosions and/or chemical pollution of the disk.

After thinking about those considerations, I’d probably try to model a merger in MESA using a temporary enhancement of the accretion rate, with a surface entropy comparable to what I’d expect in the bulk of a star, then see what happens. This could be a tricky model to get running and converged, but probably includes enough of the relevant physics to answer some of the questions above.