[Mesa-users] Making uniform composition of CO white dwarfs

[Francis Timmes]

hi harish,

at zeroth order, oxygen is heavier than carbon, so all else being equal, 
one might expect that the more oxygen there is the smaller the radius would be. 
this is what your plot shows.

slightly deeper, degenerate electrons dominate the total pressure P in 
the interior of a white dwarf. these expressions depend on the composition
through the electron fraction Y_e = <Z>/<A>. but carbon and oxygen both
have Y_e = 1/2. this means the degenerate electrons cannot be the cause 
of the radius being composition dependent.  

but a white dwarf is composed of both electrons and ions. for ideal ions
the total pressure would be P = P_e + N_A/<A> * rho * k * T. this expression 
alone says that ions generate a composition dependence to the thermodynamics,
hence to the radius. in addition, the electrons and ions interact in a coulomb plasma, 
and these contributions to the thermodynamics also have composition dependence.

clearer?

fxt






> On Oct 6, 2021, at 9:00 PM, harish sorrout via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Respected MESA users,
> 
> I want to study the accretion effect on CO white dwarfs. For this I have generated white dwarf models of mass 0.6 solar mass using test_suite make_co_WD.  Then I changed carbon oxygen abundances to 0.3 C, 0.5 C and 0.7 C  by making set_uniform_xa_from_file = .true.
> All models with different carbon oxygen abundances are terminated at the same temperature 52000 K effective temperature. But when I compare the radius of white dwarf Models of different C and O abundances, I found a radius -1.856382 R_sun for 0.3 C and -1.85159 R_sun for 0.7 C.
>     Here I am confused why the radius is different for different compositions of white dwarf models?  Plots of variation in effective temperature and radius are attached.
> 
> you all are requested to kindly help  me in this regard. What is the reason behind this?
> 
> <R_age_all.png><Teff_age_all.png>_______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>