[mesa-users] resending - Questions about physical model and numericalimplementation of binary star evolution

[Anastasios Fragkos]

Hi Bill,

You mention in your email below that “ it is often possible to let the binary go through a period of extremely high mass loss caused by unstable transfer.
the timesteps get small during that period, but mesa/star manages to shed an envelope okay at rates over 1Msun/year.” In my experience the hydro solver gives up much sooner, when the MT rate approaches 10^-2 or 10^-1 Msun/year, usually with the error message that the timestep dropped below the minimum timestep of 10^-6 seconds. Although I fully understand that the physics of common envelope cannot be modeled correctly that way, I would be very interested to hear any insights you may have about which parameters I should play with in the inlist files so that I make my runs more tolerating to high MT rates.

Cheers,
Tassos

On 11 Dec 2014, at 17:49, Bill Paxton <paxton at kitp.ucsb.edu<mailto:paxton at kitp.ucsb.edu>> wrote:



my previous response said we don't do common envelope modeling in mesa.
that's true, but it is often possible to let the binary go through a period of extremely high mass loss caused by unstable transfer.
the timesteps get small during that period, but mesa/star manages to shed an envelope okay at rates over 1Msun/year.
once things have restabilized the mass loss rate will drop back down to normal levels again.

that is a very rough form of dealing with a common envelope, a "stellar engineering" solution to a very difficult problem.
in some situations it may be good enough -- at least it let's you get beyond the common envelope stage.
but the details of what comes out cannot be expected to be highly accurate.  it is just a work-around.

-bill





On Dec 11, 2014, at 8:33 AM, Bill Paxton wrote:


On Dec 11, 2014, at 7:34 AM, Dean Townsley wrote:

I am unsure what the mesa binary module does once both stars touch their Roche lobes.  From your inlist, it appears that is a stopping condition, I don't know what might happen after that if it doesn't stop.


Once both stars fill their Roche lobes you've entered the strange and wonderful world of common envelopes.

Lot's of very smart people have spent lots of time exploring that world, and continue to do so.

mesa/star deals with stars; mesa/binary deals with pairs of stars; a common envelope object is neither of those.

so we stop when we form one and hand the problem off to a specialist.

once the common envelope object settles down to form a new star, then we can work with it again.

-B



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