[mesa-users] Tau

[Aaron Dotter]

Hi Amber,

The optical depth tau that is reported in MESA profiles is a dimensionless
number.  It's calculated by subroutine get_tau in star_utils.f90:

         do k = 2, s% nz
            tau(k) = tau(k-1) + dtau
            dtau = s% dm(k)*s% opacity(k)/(4*pi*s% rmid(k)*s% rmid(k))
            ...
    enddo

dm has units of mass, rmid units of distance (it's a radius) and opacity is
measured in cm^2/g => dimensionless.

Typically we say that the "surface" is tau~2/3 (exactly 2/3 for the
Eddington T-tau relation) but note that this depends on how you are
treating the "atmosphere" attached to your stellar interior model.  Maybe
better to talk about the photosphere, i.e. the point in the model at which
T=Teff, than the surface.

Good luck!

Aaron



On Tue, Jan 12, 2016 at 12:04 PM, amber lauer <alauer1 at lsu.edu> wrote:

> Let me be more clear: what I'm trying to do is compare MESA abundance
> values to observations, so I need to sum the abundances over all the cells
> down to the surface optical depth, which is I think tau=1.
>
> On Mon, Jan 11, 2016 at 6:56 PM, amber lauer <alauer1 at lsu.edu> wrote:
>
>> Wait, I already know that's not right either. I'll look into it more
>> deeply on my own and see if I can figure it out.
>>
>> On Mon, Jan 11, 2016 at 6:52 PM, amber lauer <alauer1 at lsu.edu> wrote:
>>
>>> Of course I meant Solar Radii.
>>>
>>>
>>> --
>>> Amber Lauer. M.S. Physics
>>> PhD Candidate,
>>> Graduate School of Physics,
>>> Louisiana State University
>>>
>>
>>
>>
>> --
>> Amber Lauer. M.S. Physics
>> PhD Candidate,
>> Graduate School of Physics,
>> Louisiana State University
>>
>
>
>
> --
> Amber Lauer. M.S. Physics
> PhD Candidate,
> Graduate School of Physics,
> Louisiana State University
>
>
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