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Cooling Evolution of Hybrid Stars
The cooling of compact isolated objects for different values of the
gravitational mass has been simulated for two alternative assumptions. One is
that the interior of the star is purely hadronic and second that the star can
have a rather large quark core. It has been shown that within a nonlocal chiral
quark model the critical density for a phase transition to color
superconducting quark matter under neutron star conditions can be low enough
for these phases to occur in compact star configurations with masses below 1.3
M_sun. For a realistic choice of parameters the equation of state (EoS) allows
for 2SC quark matter with a large quark gap ~ 100 MeV for u and d quarks of two
colors that coexists with normal quark matter within a mixed phase in the
hybrid star interior. We argue that, if in the hadronic phase the neutron
pairing gap in 3P_2 channel is larger than few keV and the phases with unpaired
quarks are allowed, the corresponding hybrid stars would cool too fast.
Even in the case of the essentially suppressed 3P_2 neutron gap if free
quarks occur for M < 1.3 M_sun, as it follows from our EoS, one could not
appropriately describe the neutron star cooling data existing by today.
It is suggested to discuss a "2SC+X" phase, as a possibility to have all
quarks paired in two-flavor quark matter under neutron star constraints, where
the X-gap is of the order of 10 keV - 1 MeV. Density independent gaps do not
allow to fit the cooling data. Only the presence of an X-gap that decreases
with increase of the density could allow to appropriately fit the data in a
similar compact star mass interval to that following from a purely hadronic
model.Comment: 10 pages, 5 figures, Talk given at Joint Meeting
Heidelberg-Liege-Paris-Rostock (HLPR 2004), Spa, Belgium, 16-18 Dec 200
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