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

[Bill Paxton]

Hi Tassos,

As they say, "your mileage may vary".   I'm recalling runs I did a year or more ago, and I wasn't using the implicit mdot back then.  I don't have a case in hand to back-up my statement, so it will take some work to create one.  If you send me one of your cases that quit at lower mdots, I'll take a look and see if anything comes to mind as a workaround.

Cheers,
Bill


On Dec 12, 2014, at 6:02 AM, Anastasios Fragkos wrote:

> 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> 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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