[mesa-users] mesa-users Digest, Vol 41, Issue 33
Bill Paxton
paxton at kitp.ucsb.edu
Sat Jan 19 14:08:53 EST 2013
Hi Ben,
Thanks for the analysis! It is interesting that for massive stars,
mesa can survive pushing all the way up to critical rotation without
seeing the instabilities that show up with the 1Msun case.
There are of course instabilities lurking for the massive stars;
I love Matteo's case in Fig 45 of the new instrument paper.
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But the behavior of the 1M looks more like a numerical problem.
Perhaps not a mesa "bug" but an unwanted feature from
a breakdown of the numerics.
Cheers,
Bill
On Jan 19, 2013, at 10:56 AM, Ben Brown wrote:
> Bill & Dengkai,
> At 300 km/s, a 1M_sol star is rotating about 150 times the current
> solar rotation rate (2 km/s). At these rotational speeds, centrifugal
> force is greater than the Sun's self-gravity. We can get an estimate of
> this by comparing the kinetic energy of rotation to the gravitational
> potential:
>
> v_rot^2/(G M_sun/R)
>
> I'm going a bit fast and loose here, and there may be a better estimate,
> but this will get us in the ballpark. Putting in cgs values:
>
> (300e5**2)/(6.67e-8 * 2e33/7e10) ~ 0.5
>
> At 300 km/s, this star is on the verge of breaking up (in comparison,
> for the at 2 km/s this term is ~ 1e-5 and is tiny). And that's pretty
> wildly unstable. So, I'm not sure this is a "bug" per-se in MESA :)
>
>
> As another way to look at this, let's figure out how deep in the star we
> need to go to have centrifugal effects be less than 1% at 300 km/s at the
> photosphere... the solid body rotation rate (assuming r_sun~7e10 cm) is
> about Omega_rot = 4e-4. In terms of Omega, KE/GPE is:
>
> Omega_rot^2 r^3/(G M(r))
>
> or, setting this equal to 0.01 (1%) and solving for r:
>
> r_1% = (0.01 G M(r)/Omega_rot^2)^(1/3)
>
> Plugging in cgs values and assuming (erroneously) that M(r)~M_sun, this
> gives:
>
> r_1%/R_sun ~ (0.01*G*2e33/Omega_rot**2)**(1./3.)/7e10 ~ 0.3
>
>> From this rough estimate, assuming I didn't flub something obvious,
> centrifugal forces are more than 1% of gravity in the outer 70% of this
> 300 km/s rotating model.
>
>
> Maybe this is the source of the wildly unstable behaviour. Altenatively,
> the spruit-tayler dynamo prescription or the various rotational mixing
> prescriptions may be going nutso. I'm impressed that MESA is handling
> the 200 km/s structure (where this is still a reasonably large effect)
> without problems.
>
> Cheers,
> --Ben
>
>
>
>
>> ------------------------------
>>
>> Message: 4
>> Date: Fri, 18 Jan 2013 22:07:36 -0800
>> From: Bill Paxton <paxton at kitp.ucsb.edu>
>> Subject: Re: [mesa-users] Problem of fast rotation stars
>> To: "dengkai" <dengkai at ynao.ac.cn>
>> Cc: mesa-users group <mesa-users at lists.sourceforge.net>
>> Message-ID: <C0836F48-11C0-43F8-A423-42F7EA22DF03 at kitp.ucsb.edu>
>> Content-Type: text/plain; charset="gb2312"
>>
>> Hello Dengkai,
>>
>> I'm glad you are getting started using MESA.
>> I encourage you to join the mesa-users email list and use it for future emails like this one;
>> there are lots of experienced mesa users who will be able to help you and who know a lot
>> more than I do about many questions you might have, especially if they are questions
>> about astrophysics!
>>
>> For example, until I got your email I had never tried running
>> a 1Msun model with fast rotation. So when I tried I got surprised.
>> I started with the test_suite/15M_dynamo inlist and made some changes.
>> Here's the inlist I ended up with.
>>
>>
>>
>> This seems to run fine with zams surface rotational velocities up to 200 km/sec.
>>
>>
>> But when I try 300 km/sec things go wildly unstable in a manner I don't understand at all.
>> While it is still on the main sequence the surface goes crazy. This is probably a bug in mesa,
>> but for the moment it is just a mystery.
>>
>>
>>
>> So, welcome to MESA! ;-) Somethings work and somethings don't. There's no magic.
>> If you can find interesting problems for which it works, then you can get some good work done.
>> For mysteries like this one, we might get lucky and find a solution, but there's no way to know.
>> It is all an adventure. You are welcome to join in.
>>
>> Good luck,
>> Bill
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> On Jan 18, 2013, at 7:24 PM, dengkai wrote:
>>
>>> Dear Bill,
>>>
>>> My name is Dengkai Jiang, and I have a problem about rotation stars.
>>> 1. Aim: to study the evolution of very fast rotation stars (~1Msun)
>>> 2. Problem: the rotation velocity (sur_v_rot) will ?try to? jump to 1028km/s when the star evolves.
>>> 3. MESA version: 4631
>>> 4. inlist: attachment (to clear, I use a very simple inlist with rotation)
>>> 5. notes:
>>> (1) At first, I used the inlist given by Chatzopoulos (2012.748.42). When I found this, I used a very simple inlist as attached in this email and this problem also happens.
>>> (2) This jump seems to happen at ?~0.8?crit (initial ?~0.5?crit)
>>> (3) If use a larger initial rotation velocity (new_surface_rotation_v = 400), the star can evolve through. But the rotation velocity (sur_v_rot) is always about 1028km/s.
>>> (4) If use a larger inital mass star, this jump will happen lately.
>>> (5) I tested in another computer with MESA 4028, and it also happen.
>>> (6) This problem seems like the problem found by Roberto (ZAMS rotation models problem), which has been solved by your suggestion (set mesh_delta_coeff = 0.5). I have try this setting (0.5) and other sets (0.3, 0.8, 1.5), but the problem is still there.
>>>
>>> I am a new user of MESA, and this problem can be caused by my wrong set which I don't noticed. Please help me and thank you very much.
>>>
>>> In addition, I am very grateful to you for MESA code, which code can make me do some very interest study on the evolution of single and binary stars.
>>>
>>> Best regards,
>>> Dengkai
>>
>>
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>> End of mesa-users Digest, Vol 41, Issue 33
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