96 research outputs found
On excess entropy and latent heat in crystallizing white dwarfs
Based on the linear mixing approach, we calculate the latent heat for
crystallizing fully-ionized C/O and O/Ne mixtures
in white dwarf (WD) cores for two different parametrizations of the corrections
to the linear-mixing energies and with account of ion quantum effects. We
report noticeable composition-dependent deviations of the excess entropy in
both directions from the standard value of 0.77 per ion. Within the same
framework, we evaluate the excess entropy and released or absorbed heat
accompanying the exsolution process in solidified WD layers. The inclusion of
this effect is shown to be important for reliable interpretation of WD cooling
data. We also analyze the latent heat of crystallizing eutectic
C/Ne mixture, where we find a qualitative dependence of both the
phase diagram and the latent heat behaviour on ion quantum effects. This may be
important for the model with Ne distillation in cooling C/O/Ne WD
proposed as a solution for the ultramassive WD multi-Gyr cooling anomaly.
Astrophysical implications of our findings for crystallizing WD are discussed.Comment: 5 pages, 2 figures. Letter to MNRAS, in pres
Phase diagrams of binary ionic mixtures and white dwarf cooling
Phase diagrams of fully ionized binary ionic mixtures are considered within
the framework of the linear mixing formalism taking into account recent
advances in understanding quantum one-component plasma thermodynamics. We have
followed a transformation of azeotropic phase diagrams into peritectic and
eutectic types with increase of the charge ratio. For solid C/O
and O/Ne mixtures, we have found extensive miscibility gaps.
Their appearance seems to be a robust feature of the theory. The gaps evolve
naturally into two-solid regions of eutectic phase diagrams at higher
. They do not depend on thermodynamic fit extensions beyond their
applicability limits. The gaps are sensitive to binary mixture composition and
physics, being strongly different for C/O and O/Ne mixtures and for the three
variants of corrections to linear-mixing solid-state energies available in the
literature. When matter cools to its miscibility gap temperature, the
exsolution process takes place. It results in a separation of heavier and
lighter solid solutions. This may represent a significant reservoir of
gravitational energy and should be included in future white dwarf (WD) cooling
simulations. Ion quantum effects mostly resulted in moderate modifications,
however, for certain , these effects can produce qualitative
restructuring of the phase diagram. This may be important for the model with
Ne distillation in cooling C/O/Ne WD proposed as a solution for the
ultramassive WD cooling anomaly.Comment: 13 pages, 8 figures, accepted in MNRA
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