[Mesa-users] Units of use_other_torque

Tue Nov 13 15:32:45 EST 2018

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> 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>
> *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> 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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