[mesa-users] Fixed rate accretion onto WD with full structure

[Dean Townsley]

Hi Manos,

I think the "wd" test suite problem is almost exactly what you are 
trying to do.

Getting any given WD model started on accretion can sometimes be tricky 
because of the structure in the near-surface layers left from the last 
flashes on the AGB.  Also the beginning surface luminosity may be very 
different from what your accretion rate implies, so that a fair amount 
of short-timescale evolution needs to occur before the integrator can 
start to take large timesteps.  (by large I mean 100 years or more.)  
You did, after all, effectively just instantaneously turn on accretion.  
One common workaround is to run several short (say 100 year) segments 
with increasing accretion rate.  Another is to strip some of the outer 
layers to remove some of the structure that the integrator is trying its 
best to preserve.

If you want to start from any arbitrary WD, you may need to be willing 
to wait several thousand steps or more for the initial part of the 
evolution.  Once the timestep rises back above 100 years or so, you are 
usually good.  until...

If you are accreting at 10^-9 Msun/yr, the hydrogen shell will undergo 
thermonuclear flashes (nova outburst), including mass ejection and all 
that, every 10^5 years or so.  So getting to a gigayear will take quite 
a lot of evolution, through thousands of flashes, each of which may take 
hundreds or a thousand timesteps.

The "wd" test suite problem runs accretion on a WD, stopping as the 
first H shell flash is happening but before any mass loss.
I believe the "wd2" test suite problem runs through a flash, stopping on 
the second flash.

My advice would be to start from one of the test suite problems that 
already do what you're trying to do.  Or some other source of an initial 
WD that is already known to start up accretion without too much hassle.  
Typically somebody has already done some fiddling with the WD, or made a 
good choice of model.

The "wd" test suite problem starts from an approximately 1.0 Msun WD.  
If you want a 0.6 Msun WD, a good option is the one used in the second 
MESA paper, available here:
http://mesastar.org/results/mesa2/5.-evolution-beyond-the-main-sequence-and-white-dwarfs/5.3-compressional-heating-and-accretion

If you want to start from an arbitrary WD, I think you will just need to 
get used to the messy process of getting accretion started. Trying a few 
different cases may give the best insight into how well this plays out 
with the current version of MESA.

Hopefully that helps,
Dean



On 05/20/2015 10:55 AM, Manos Chatzopoulos wrote:
> Dear all,
>
>  I am setting up a relatively simple MESA problem where I want to 
> accrete mass at a constant rate (1d-9 Msun/yr) onto a white dwarf. I 
> know there are numerous binary test_suite problems that deal with 
> accretion onto a WD as a point mass, but in my case I want to include 
> the full
> structure of the WD in the calculation because I want to see how 
> several parameters change (including angular momentum profile) as mass
> is accreted. The problem I am dealing with is that the time-step 
> becomes tiny after a few time-steps not allowing me to complete a full
> simulation for ~ 1bn years. That only happens when I accrete mass onto 
> the WD at a rate higher than ~ 1d-14 Msun/year. I need to use
> a rate of 1d-9 Msun/year, however. I am using the "final.mod" WD 
> profiles from the test_suite problem agb_to_wd and wd_cool to start
> the calculation. An example model and inlist (and 
> profiles_columns.list)  are attached. I am using the latest version of 
> MESA (7503).
>  Any help on this would be appreciated. Thank you very much for your 
> time!
>
> Best,
>
> Manos
>
>
>
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