[mesa-users] mesa-users Digest, Vol 41, Issue 33

Sat Jan 19 13:56:12 EST 2013

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
>
>
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: inlist_dynamo
> Type: application/octet-stream
> Size: 17812 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: surf_avg_v_rot.pdf
> Type: application/pdf
> Size: 17767 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: H_R.pdf
> Type: application/pdf
> Size: 23082 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: surf_avg_v_rot.pdf
> Type: application/pdf
> Size: 16205 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: H_R.pdf
> Type: application/pdf
> Size: 21276 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
> -------------- next part --------------
> A non-text attachment was scrubbed...
> Name: inlist_project
> Type: application/octet-stream
> Size: 10884 bytes
> Desc: not available
> -------------- next part --------------
> An HTML attachment was scrubbed...
>
> ------------------------------
>
> ------------------------------------------------------------------------------
> Master Visual Studio, SharePoint, SQL, ASP.NET, C# 2012, HTML5, CSS,
> MVC, Windows 8 Apps, JavaScript and much more. Keep your skills current
> with LearnDevNow - 3,200 step-by-step video tutorials by Microsoft
> MVPs and experts. SALE $99.99 this month only -- learn more at:
> http://p.sf.net/sfu/learnmore_122912
>
> ------------------------------
>
> _______________________________________________
> mesa-users mailing list
> mesa-users at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/mesa-users
>
>
> End of mesa-users Digest, Vol 41, Issue 33
> ******************************************
>