How are stars in the galactic center related to the prior AGN phase?

Jun 8, 2022 09:24 · 391 words · 2 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 fourteenth post in a series describing some of the ideas I’ve accumulated.

What’s the idea?

We think the Milky Way used to host an AGN disk. Stellar evolution in AGN disks is expected to produce massive stars, and indeed the galactic center hosts a population of massive stars with coplanar orbits. These are clearly related to the disk in some fashion, and could either have been around coincident with the AGN disk or could have formed from the last remnants of the disk.

These stars provide a direct probe of something to do with the AGN disk, but precisely what is unclear. The idea would be to figure out if these stars formed from the last remnants of the disk or if they evolved in the disk, and then use that to make inferences about the disk properties and/or stellar evolution in AGN disks.

Why is this interesting?

The stars in the galactic center are the only observationally-accessible population of stars that could have been embedded in an AGN disk. If they indeed were embedded they provide a unique probe of AGN star evolution and of AGN disk structure. If they weren’t embedded and instead formed from the remnants of the disk they provide a measurement of the composition of the disk, which is likewise hard to measure in other ways.

How can I get started?

I’d start by compiling a list of the coplanar galactic center stars, then look for any archival observations of these. Of particular interest are measurements that can be used to determine how old they are (e.g. $T_{\rm eff}$, $L$, $M$) and spectroscopic measurements (particularly of He, C, N, and O, as these are what we expect to be most enhanced in the AGN environment).

From that point it becomes detective work. Are the ages consistent with each other (suggesting a common formation episode)? Are the compositions unusual? How massive are these stars? Are they consistent with end-points of AGN star evolution with the AGN shut off, or do they look more like normal massive stars with potentially unusual compositions?

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