[mesa-users] Accretion: max_star_mass_for_gain and eps_grav

[Philip D. Hall]

Hello,

Short version:
I want to use MESA 7624 to model accretion which stops when a chosen mass
is exceeded. I find strange behaviour when the max_star_mass_for_gain
parameter is used to achieve this. The strange behaviour seems to be
because the eps_grav calculation still thinks the star is accreting even
after the maximum mass is exceeded. One problem could be the following line
in mesa/star/private/eps_grav.f90:
G_m_q_mdot = s% cgrav(k)*mmid*qmid*s% mstar_dot

Long version:
I want to model the merger of two helium white dwarfs. I attach a model of
0.3Msol He white dwarf (hewd_03.mod). I want to accrete 0.3Msol at a rate
of 1e-5 Msol yr^-1 and follow the post-accretion evolution. I would expect
the following two methods to give the same results.

Method A. Stop accretion when the star exceeds 0.6Msol but continue
evolution.
Key parameters in inlist_A:
mass_change = 1d-5
max_star_mass_for_gain = 0.6d0
After core He burning the star sits at high luminosity and effective
temperature for 1e35 years (green line in attached plot).

Method B. Stop the run when the star exceeds 0.6Msol (inlist_B1). Restart
the run with accretion switched off (inlist_B2).
Key parameters in inlist_B1:
mass_change = 1d-5
star_mass_max_limit = 0.6d0
After core He burning the star becomes a white dwarf, as expected (e.g.,
fig. 5 of Zhang & Jeffery 2012, 2012MNRAS.419..452Z
<http://adsabs.harvard.edu/abs/2012MNRAS.419..452Z>; blue line in the
attached plot).

I find the two methods give the same result if in
mesa/star/private/eps_grav.f90 I replace the line
G_m_q_mdot = s% cgrav(k)*mmid*qmid*s% mstar_dot
with
if (s% star_mass >= 0.6d0) then
    G_m_q_mdot = 0d0
end if

Thanks,

Philip D. Hall
Armagh Observatory

OS X 10.10.4
MESA 7624
mesasdk-x86_64-osx-10.10-20150210
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