[mesa-users] Radiation pressure at zero optical depth

[Bill Paxton]

Hi Konstantin,

Thanks very much for your report.  Here are the lines of code in atm/private/mod_atm that you are referring to:

         ! P = (tau*g/kap)*(1 + 1.6d-4*kap*(L/Lsun)/(M/Msun))
         ! the factor comes from including nonzero Prad at tau=0
         ! see, e.g., Cox & Giuli, Section 20.1
         P0 = tau*g/kap
         Pextra = 1.6d-4*kap*(L/Lsun)/(M/Msun)

What precisely do you suggest for a fix -- what should replace the 1.6d-4 factor?

Bill


On May 3, 2014, at 2:31 PM, Konstantin Pavlovskii wrote:

> Dear MESA developer(s),
> 
> I'd like to report a couple of issues in simple grey atmosphere boundary condition in MESA/atm.
> 
> Prescription from Cox & Giuli is used there to take into account radiation pressure at zero optical depth -- 1.6d-4*kap*(L/Lsun)/(M/Msun). First, the numerical factor in this prescription is designed by Cox & Giuli to contain tau=2/3 embedded into it, whereas tau is supposed to be a parameter of the boundary condition in MESA. Second, the value of this factor -- 1.6d-4 is apparently not correct for tau=2/3 due to error in Cox & Giuli, it should be approximately two times smaller.
> 
> Kind regards. Konstantin.
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