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For a star with a given Zero-Age Main Sequence (ZAMS) mass, as a function of metallicity how strongly is the star's core coupled to its envelope?

I understand that the core-envelope boundary is only (semi-) well defined for Main Sequence stars that do have a core, and that in principle the final spin (angular velocity) of the core should depend on the spin of the star prior to losing its envelope in a mass transfer event. I've read, here for instance, that the core should be nearly completely uncoupled from the envelope, however in this recent paper they assume that the coupling is sufficient enough to determine the natal spin of the stellar core (see figure 1).

Is there a simple way to understand the strength of core-envelope coupling in terms of the ZAMS quantities of the star, without performing full stellar population synthesis?

I asked this question at the Physics SE but it has received very little interest.

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    $\begingroup$ The problem is likely that few know the answer, indeed it might not be known by anyone. It's certainly an ongoing area of research, but it is hard to make progress purely theoretically without knowing all the variables, like magnetic field and evolutionary stage and so on. We don't even have models for star formation that could tell you the initial rotation state. So I think the question requires observations, such as asteroseismology and rotation of stellar remnants, a huge dataset with probably lots of different stories to tell. You might need to narrow down the question quite a bit! $\endgroup$ – Ken G Dec 30 '18 at 6:36

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