Black Holes & Stars: Can Stars Rotate BHs Stably?

[danihel]

Is it possible for a star and a black hole to rotate each other in a relatively stable way without the BH feeding on the star and if yes could that star have planets rotating around it?

 

[Ich]

It is possible, but I don't know real world examples.

 

[qraal]

Is it possible for a star and a black hole to rotate each other in a relatively stable way without the BH feeding on the star and if yes could that star have planets rotating around it?

Astrophysical black holes usually mass as much as stars, thus they can have companions in stable orbits without any Roche lobe overflow and mass exchange. Plenty of regular stellar binaries exist without mass exchange and plenty exist with exchange - it's what causes novae and Type Ia Supernovae, as well as mini-QSOs and the like.

If the stars were sufficiently far apart then they can have planets in stable orbits - the further apart, the bigger the spread of planets that remain stable long enough to form. Too close and a planetesimal disk never forms and the early dust-disk gets blown away.

 

[Frame Dragger]

Even in this scenario the BH is going to be accreting a certain amount of the other other star's ejecta, and any incoming object that would normally burn in a star would be added to the BH's mass.

This is essentialy the two body problem, with the BH constantly increasing in mass (and therefore gravitational range). A lot of mass is usually lost in Nova events with two stars... such would not be the case for a star and a BH. Would the BH be able to start accreting its partner as it grows, or would the orbit around the barycenter become unstable?

 

[qraal]

Even in this scenario the BH is going to be accreting a certain amount of the other other star's ejecta, and any incoming object that would normally burn in a star would be added to the BH's mass.

This is essentialy the two body problem, with the BH constantly increasing in mass (and therefore gravitational range). A lot of mass is usually lost in Nova events with two stars... such would not be the case for a star and a BH. Would the BH be able to start accreting its partner as it grows, or would the orbit around the barycenter become unstable?

You'd have to run a pretty advanced simulation to see how the system evolves, but past a few dozen stellar radii and there shouldn't be much accretion by the BH from a regular star. Once it goes red-giant, then the picture becomes seriously complicated, though the mass-flow is slow enough to avoid major instability. Energy from the accretion disk can also strip a red-giant down to its fusioning core, thus creating a rather odd looking binary system. AFAIK there's a couple of possible examples in the literature.

 

[Frame Dragger]

You'd have to run a pretty advanced simulation to see how the system evolves, but past a few dozen stellar radii and there shouldn't be much accretion by the BH from a regular star. Once it goes red-giant, then the picture becomes seriously complicated, though the mass-flow is slow enough to avoid major instability. Energy from the accretion disk can also strip a red-giant down to its fusioning core, thus creating a rather odd looking binary system. AFAIK there's a couple of possible examples in the literature.

Hmmm... that makes sense. I didn't think about the accretion disc, but that's a good point. I wonder what a naked fusing core 'looks' like? Hmmmm...

 

[Chronos]

Hmmm... that makes sense. I didn't think about the accretion disc, but that's a good point. I wonder what a naked fusing core 'looks' like? Hmmmm...

Short lived would be my guess. The radiation pressure of the core must be balanced by resistance of the stellar atmosphere to remain stable.

 

[Frame Dragger]

Short lived would be my guess. The radiation pressure of the core must be balanced by resistance of the stellar atmosphere to remain stable.

In that case wouldn't the core begin to expand as the star lost its gas envelope, and the scenario of total accretion becomes real again.