[Mesa-users] Problems with white dwarf accretion

[Liam Kroll]

Hello Mesa-users,

I'm currently attempting to simulate accretion of brown-dwarf-like material
onto a series of CO white-dwarfs and a single ensuing thermonuclear runaway
using mesa-r11701 with the latest MESA-SDK (20190503) on a machine running
Ubuntu 18.04.2 LTS. The ultimate goal of this project is to obtain full
temperature/density profile trajectories from the beginning of
thermonuclear runaway to a (for now) arbitrary time when nuclear burning in
the expanding envelope has subsided. So far, I have been using the
make_co_wd test suite inlists to create the WD model I am starting with. I
have been using this test_suite example completely unchanged save for the
initial mass (2.75 Msun) of all inlists and the stopping condition
(log_center_temp_lower_limit
= 7.18 (~15MK)) of inlist_part3, which outputs the final WD model. To model
the accretion and nova I am using inlists which are mostly based off of
test_suite examples which model accretion onto a WD. Inlist_init loads the
WD model created by make_co_wd and changes to a custom reaction network.
Inlist_nova then begins accreting material onto the model. However I am
running into some issues when trying to simulate a nova outburst. When the
WD starts accreting material the timesteps have trouble increasing over
roughly 2<lg_dt_yr<3 because MESA seems to be stuck in a vicious cycle of
max_increase/retry. I have tried a few things which I thought might help,
some of which did help but not eliminate the problem completely:
-Increasing grid resolution, especially where there is burning occurring.
-Turning on the report_ierr control, I received a message very similar to
the one reported in the MESA-users archives:
https://lists.mesastar.org/pipermail/mesa-users/2019-April/010002.html
Following
the suggestions therein and playing with which eos/form of the energy
equation is being used.
-relaxing the tau factor to both low and high values (0.0001-300). Many of
the test_suite examples modelling novae do this, and as far as I can tell
MESA proceeds smoothest for my models at a value of ~0.01. As I understand
it, this value means more of the atmosphere is included in the model when
compared to the default value of 1.
I have included the starting model and inlists as well as some plots of the
output from inlist_nova. To me it looks like MESA runs into trouble around
model number 125, where both the number of retries and newton iterations
spike. Looking at the entropy profile we can see that there is a
discontinuity around this time in the outer layers of the model. I took a
look at the mass fraction of h1 and indeed this discontinuity in the
entropy coincides with the region where material is being accreted. Perhaps
the composition gradient is an issue here? I was wondering if anyone might
have any input or experience with similar problems? I apologise if similar
issues have been discussed on the archives, but I am unable to find them. I
am relatively inexperienced with MESA and certainly a novice when it comes
to the theory of stellar modelling, so I'm hoping the hurdle here is just
my ignorance and not a fundamental issue with the problem I am trying to
model.

Thanks for any and all feedback,

Liam Kroll

P.S. I modified run_star_extras but I don't think that is the issue here,
as the problem persists even when I do not call the added routines.
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