[mesa-users] The meaning of photosphere_r

[Warrick Ball]

Hi,

I'm currently considering models with the atmospheric structure included 
in the stellar model by placing the outer boundary condition at a smaller 
optical depth using tau_factor in &star_job.  I had a look at the variable 
s% photosphere_r, which is the radius of the photosphere, and set by 
the function get_r_phot in star/private/star_utils.f90.

Judging by that code (appended below because it's quite short), it looks 
like photosphere_r is the depth at which tau=2/3.  Is that the desired 
behaviour?  I would personally say that the photospheric radius is where 
T=Teff.  For an Eddington grey atmosphere, this is tau=2/3, but it isn't 
for other T(tau) relations.  The T=Teff values are already in the function 
atm_tau_base in atm/public/atm_lib.f90, which returns the optical depth of 
T=Teff for the T(tau) relations: about 0.312 and 0.412 for krishna_swamy 
and solar_Hopf, respectively.

Now that I know what photosphere_r really means, I can (quite easily!) 
write my own routine for run_star_extras.f.  But I thought I'd bring this 
up because this definition of the photosphere isn't what I expected, so 
it's possible that it's not what other user expect(ed) either.

Cheers,
Warrick


       real(dp) function get_r_phot(s) ! return r where optical depth = 2/3
          type (star_info), pointer :: s

          integer :: k
          real(dp) :: tau00, taup1, dtau, r003, rp13, r3

          real(dp), parameter :: tau_phot = 2d0/3d0

          include 'formats'

          tau00 = 0
          taup1 = 0
          get_r_phot = s% r(1)
          if (s% tau_factor >= 1) return
          tau00 = s% tau_factor*s% tau_base
          if (tau00 >= tau_phot) return
          do k = 1, s% nz-1
             dtau = s% dm(k)*s% opacity(k)/(4*pi*s% rmid(k)*s% rmid(k))
             taup1 = tau00 + dtau
             if (taup1 >= tau_phot .and. dtau > 0d0) then
                r003 = s% r(k)*s% r(k)*s% r(k)
                rp13 = s% r(k+1)*s% r(k+1)*s% r(k+1)
                r3 = r003 + (rp13 - r003)*(tau_phot - tau00)/dtau
                get_r_phot = pow_cr(r3,1d0/3d0)
                return
             end if
             tau00 = taup1
          end do

       end function get_r_phot




------------
Warrick Ball
Postdoc, Institut für Astrophysik Göttingen
wball at astro.physik.uni-goettingen.de
+49 (0) 551 39 5069