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POSYDON-code / POSYDON

POSYDON is a next-generation single and binary-star population synthesis code incorporating full stellar structure and evolution modeling with the use of MESA.
BSD 3-Clause "New" or "Revised" License
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BH spin realignment due to accretion #422

Open tassos25 opened 2 days ago

tassos25 commented 2 days ago

In teh v2 paper, section 5.4, we say:

We assume that mass transfer is inefficient at realigning the spin of the first-born compact object between SN1 and SN2, allowing us to calculate the cumulative misalignment between its spin and the orbital angular momentum across both SNe. However, we assume that tides and mass transfer are efficient at realigning the non-degenerate companion star to the post-SN1 orbital plane.

In Fragos et al. 2010, we had performed the necessary integration in order to estimate the spin alignment. We should implement the same the sam thing in a computationally efficient way in POSYDON as well.

@ZepeiX had already experimented with some implementations but found it to be too slow. @ZepeiX can you comment?

ZepeiX commented 2 days ago

The integration for BH spin alignment due to accretion is too slow. In my NSHB paper, I did a simplified calculation. I assumes the BH accretes the angular momentum of a chunk of mass at the ISCO only at the direction of the orbital plane (Y axis), and the angular momentum at X axis remains unchanged. Then, we have a new spin magnitude and a new direction. Issues: I used IF, so I assumed all the mass is thrown only once. This could potentially induce a final spin greater than one. The condition at ISCO changed step by step, and we didn't consider this.

tassos25 commented 2 days ago

An additional complication is that the specific AM of teh ISCO, does depend also on the angle between the plane of the assumed accretion disk and the BH spin, which is also evolving. In Fragos et al. (2010), we followed the evolution self-consistently, but there is a root finder involved in the solution, that slows down things. I had dug out the IDL scripts i used to do the calculation, which Zepei had translated to python.

This is a subtle but important process that I do not think any other BPS does correctly. It becomes especially important when looking at the misalignment angles of BH spins. It's one of these things that people assume it is doen correctly, but in most cases it is not. I suggest that we work on a better implementation before the v2 code release.