[Mesa-users] Units of use_other_torque

Tue Nov 13 18:34:49 EST 2018

Hi Pablo,

Whoops, when I sent you the inlist for the single star I meant to have Mdot_T1 set to extra_jdot(k). I was trying to clean up my routines so they would be easier to read for everyone.  Equation 3 is M_dot*sqr(G*M*R) in the paper, but later on in their paper they explain they substitute Rcor into their equation (3),  I will adjust it so it won't have this problem. I do keep having issue with understanding specific physical quantities used in MESA, such as specific angular momentum. I'll try to be more careful and use print statements if values appear outrageous like you recommend. l will try your solutions and thank you for the advice!

Thank you again,

Thomas

________________________________
From: Pablo Marchant <pamarca at gmail.com>
Sent: Tuesday, November 13, 2018 2:32:45 PM
To: Gehrman, Thomas Charles
Cc: mesa-users
Subject: Re: [Mesa-users] Units of use_other_torque

Hi Thomas, by looking through your inlists and run_star_extras it appears you have various mismatching units likely causing unrealistic values. First of all

          M_dot = s% x_ctrl(1)*msol

in your inlists you define x_ctrl(1) as a mass accretion rate in solar masses per year. You then need to divide this quantity by the number of seconds in a year (the constant called secyer in MESA) to get a value in CGS. Next you have

! Equation from Dervisoglu, Tout, & Ibanoglu 2010, eqn 3.
Mdot_T1 = M_dot*sqrt(G*M*R*(omega_surf/omega_surf_crit)**(-2.0/3.0))

Not sure what you're doing here, but this is not Eq. (3) of that paper. Moreover, it's an ill defined expression because it has a division by zero when omega_surf is zero. Also, this expression has units of torque, thats angular momentum per unit time. You apply this torque by doing this:

do k =1, 2
    if(log_time_step > 7.5 .and. star_age > 8.45d07) then
               s% extra_omegadot(k) = Mdot_T1
       s% extra_jdot(k) = 0
            else
       s% extra_omegadot(k) = 0
       s% extra_jdot(k) = 0
    end if
end do

There are various issues with this:
- You are applying a torque as an omegadot term, this is dimensionally incorrect.
- You are applying a torque that is meant to operate through the outer layers of the star to just the two outermost cells. The physical size of those cells is not a well defined physical quantity, you need to make a physical choice for how the angular momentum is actually distributed, not through an unphysical number of pre-defined cells but rather through a certain lengthscale or mass.
- If you apply the torque as an extra_jdot term, you have to be careful with units as well. jdot is the time derivative of the SPECIFIC angular momentum of each cell.

An issue that is perhaps more important, is that you are ignoring that MESA by default does not add layers as non-rotating. By default mass is accreted with the same omega as the surface. So your computation will be inconsistent in the way you're currently doing it. For what you're trying to do, I'd recommend you use this option instead of other_torque

3818          !### use_accreted_material_j
3819
3820          ! Angular momentum of accreted material.
3821
3822       use_accreted_material_j = .false.
3823
3824          ! If false, then accreted material is given j so that it
3825          ! is rotating at the same angular velocity as the surface.
3826          ! If true, then accreted material is given j = `accreted_material_j`.
3827
3828       accreted_material_j = 0

if you set use_accreted_material_j = .true., then you can directly specify the SPECIFIC angular momentum j with which material will be added to the star. You can adjust accreted_material_j in your run_star_extras simply by doing s% accreted_material_j = something, the best place to do it would be by making use of the "other" subroutine available with use_other_adjust_mdot = .true.. This subroutine lets you adjust things before MESA adds extra material to the star, so if you specify accreted_material_j in there the new layers will be added wtih the value you want.

In general, as you work through this I'd recommend you to be careful with the units. Be careful about which quantities are "specific", ie. per unit mass. Use write statements to see what values you have obtained and whether or not they are consistent. I often find I've missed a simple factor and I'm computing something 30 orders of magnitude off, in which case its no surprise that MESA struggles.

Cheers

On Tue, Nov 13, 2018 at 12:25 PM Gehrman, Thomas Charles <thomas.gehrman at mnsu.edu<mailto:thomas.gehrman at mnsu.edu>> wrote:

I have attached my inlist for the actual binary model with all equations I'm trying to model. I have also attached my approximation to this model using only the gainer star and one equation to simplify it. In each case my lg_dt~(-2) and retries and backups are implemented because it fails in angular momentum mixing which is found in the solve_omega_mix module in the star/private directory. The reason I asked about the units is because I believe my torques are large and MESA does a retry to a smaller time step to accommodate for it.

Thank you,

Thomas

________________________________
From: Pablo Marchant <pamarca at gmail.com<mailto:pamarca at gmail.com>>
Sent: Monday, November 12, 2018 11:50:07 PM
To: Gehrman, Thomas Charles
Cc: mesa-users
Subject: Re: [Mesa-users] Units of use_other_torque

Hi Gehrman, units are in CGS. If you're running into issues, you can also share your inlists/run_star_extras to better assess your problem.

Cheers

On Mon, Nov 12, 2018 at 4:34 PM Gehrman, Thomas Charles <thomas.gehrman at mnsu.edu<mailto:thomas.gehrman at mnsu.edu>> wrote:

Hello all,

I have a simple question. For the use_other_torque found in the controls inlist are the units for extra_jdot in cgs or solar?

Thank you kindly,

Thomas

Thomas C. Gehrman Jr.

M.S. Physics & Graduate Teaching Assistant

Department of Physics and Astronomy

Minnesota State University, Mankato

Office: N144

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--
Pablo Marchant Campos
M.Sc on Astrophysics, Universidad Católica de Chile
PhD on Astrophysics, Argelander-Institut für Astronomie, Universität Bonn

--
Pablo Marchant Campos
M.Sc on Astrophysics, Universidad Católica de Chile
PhD on Astrophysics, Argelander-Institut für Astronomie, Universität Bonn
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