[mesa-users] Module binary_mdot.f, eddington accretion bug

[Mads Sorensen]

Hi,

In MESA-r7184 module binary_mdot.f line 282 states the Eddington accretion using the opacity of the donor star most outer layer which is probably a good solution for mass tranfers between two stars. When conducting mass transfer sequences involving compact objects like black holes, i.e. x-ray binaries, the current solution fails to account for the conditions of the accretion disc. Under these conditions a more likely formulation would be to use the Thompson scattering opacity and radiation efficiency $\eta$ = 0.1 assuming an accretion disc of mostly hydrogen.

We have tested a new formulation using eq. 1.4 and 1.9 of Frank, King, and Raine, 2002, Accretion power in astrophysics, against the previous model. In short the new formulation allows a higher accretion rate onto the compact object and will therefore have an effect on the overall mass transfer in the binary system relative to the current formulation.

Current line 282 in binary_mdot.f:
mdot_edd = 4*pi*b% s1% cgrav(1)*b% m(b% a_i)/(clight*b% s1% opacity(1))

New suggestion to line 282 in binary_mdot.f which we have used:

mdot_edd = 1.3d0*1.d38 * b% m(b% a_i)/msol  /(clight**2*0.1d0)

Enclosed are examples of runs using the current and new formulation of the Eddington accretion rate and a run with no limit to the accretion rate - see the plot title inside.
The run is the binary/test_suite donor only implicit mdot simulation with conservative mass transfer of a donor of 8 Msun, accretor of 6 Msun, and initial period of 1 day, inlist files are also attached.


With my best regards,

Mads Sørensen,
Candoc/Doctorant/PhD student
Observatoire Astronomique
Geneva University

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