[Mesa-users] High rate accretion to a MS star

[Ebraheem Farag]

Hello Ealeal,

Thanks for reaching out!

>I am using "mass_change"  and commenting the wind as recommended.
>I am encountering a lot of problems (the run terminates due to dt to small:
"stopping because of problems dt < min_timestep_limit =1d-6s" (as in the
default)).

It's hard to know exactly what's going on without more information about
your specific model, like some inlists or a plot, as a variety of things
could cause your model timestep to dive.

My best guess is that you have a low mass star and you are trying to
accrete too much mass, too quickly, on too low of a mass star, which is
making the surface go bonkers.

There's a couple solutions I can think of:
------------
(1) Avoid the surface entirely by relaxing the surface inward, say to ~1d4
tau or higher.

This can be done setting some thing like the following in your &starjob:

    relax_to_this_tau_factor = 1d4
    dlogtau_factor = 0.1d0
    relax_tau_factor = .true.

or (2) Gradually increase mass_change as opposed to dumping 1d-3 Msun/yr
onto your star, the star needs time to adjust its thermal structure.
------------
To illustrate options (2), I've attached an example work directory
'1Msun_accrete.tar.gz' for a 1Msun star in r22.11.1 (should work in
r22.05.1) that uses the run_star_extras 'subroutine extras_check_model' to
employ option (2):

by setting 'x_logical_ctrl(1) = .true.' in inlist_project, this will toggle
a very, very crude scheme I wrote that will slowly increase the mass_change
over ~ 200 timesteps from 5d-6 ---> 1d-3 in steps of 5d-6 Msun/yr.

This model works by evolving a 1Msun star from ZAMS and turning on
mass_change when center_h1 drops below 0.6. The model then accretes mass
until the final mass of the star is 20Msun. Then the model evolves the star
until center_h1 < 1d-4 (H depletion). You can download the pgstar movie
here: 1Msun_acc_20Msun.mp4
<https://drive.google.com/file/d/169HHHlx7Dy8UY24gA8FwWib1c6CK7Oxd/view?usp=sharing>
which
illustrates this in action.

-You can also test option (1) in this directory by commenting
out  'x_logical_ctrl(1) = .true.'  or setting it to  'x_logical_ctrl(1) =
.false.' in inlist_project, as well as uncommenting the tau relaxation
controls in &starjob. Here is a movie of what this looks like: '
1Msun_acc_20Msun_1d4tau.mp4
<https://drive.google.com/file/d/1wd-hro1nySrprcyj2QYecuxP1_4AKJrr/view?usp=sharing>
'
------------

> I was looking for parameters related to mass_change, and I found:
> lgT_lo_for_set_new_abundances = 5.2d0

> lgT_hi_for_set_new_abundances = 5.5d0

If adjusting the composition on the fly, these controls set a minimum
threshold above which the composition of material added does not include
light elements like H2, Li7 or else the high temperatures will induce
explosive burning which will prevent the model from converging. If you're
interested, you can check out the subroutine set_new_abundances in
$MESA_DIR/star/private/adjust_xyz,f90, to read further.

good luck!

-EbF

On Thu, May 4, 2023 at 11:05 PM Bear Ealeal via Mesa-users <
mesa-users at lists.mesastar.org> wrote:

> Dear MESA users,
>
> I am trying to accrete mass on a MS star at high rate 10^-3Mo/yr or so. I
> tried this in MESA 10398 and MESA 22.5.1
>
> I am using "mass_change"  and commenting the wind as recommended.
>
> I am encountering a lot of problems (the run terminates due to dt to
> small: "stopping because of problems dt < min_timestep_limit =1d-6s" (as in
> the default)).
>
> I was looking at parameters related to the mass change, and found:
>
> lgT_lo_for_set_new_abundances = 5.2d0
>
> lgT_hi_for_set_new_abundances = 5.5d0
>
> I am not sure what they do or how these numbers were chosen, I will
> appreciate any insight on these parameters or other parameters that might
> help me accrete mass at high rates.
>
> Thank you,
>
> Ealeal
>
> _______________________________________________
> mesa-users at lists.mesastar.org
>
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>
>
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