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

[harish sorrout]

Thankyou Timmes for your valuable comments on my queries.
        Sir, I still have some questions regarding the accretion on white
dwarfs. I have accreted helium on a cool (38000 K) and hot (52000 K) white
dwarfs of different carbon oxygen
abundances with accretion rate 5d-8. When I studied the plot of helium
luminosity with the accretion age for both cool and hot white dwarfs. I
found following facts form the figures attached:

1. For cool WD, helium ignites earlier in the white dwarf of 70 % carbon as
compared to the white dwarf of 30 % carbon abundance.
2. For hot WD,  helium ignites earlier in the white dwarf of 30 % carbon as
compared to the white dwarf of 70 % .
sir, i could not understand the physics behind this. I will be highly
thankful if you or mesausers help me in this regard.

Thankyou

with regards
 Harish Kumar



On Thu, Oct 7, 2021 at 12:24 PM Francis Timmes <fxt44 at mac.com> wrote:

> 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
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20211023/1af5bd87/attachment.htm>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: L_he_age_52000.png
Type: image/png
Size: 31131 bytes
Desc: not available
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20211023/1af5bd87/attachment.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Lhe_age_38000K.png
Type: image/png
Size: 31210 bytes
Desc: not available
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20211023/1af5bd87/attachment-0001.png>