De-Hua Wen, W. G. Newton, Bao-An Li
Using a simple model of a neutron star with a perfectly rigid crust
constructed with a set of crust and core equations of state that span the range
of nuclear experimental uncertainty in the symmetry energy, we calculate the
instability window for the onset of the Chandrasekhar-Friedmann-Schutz (CFS)
instability in r-mode oscillations for canonical neutron stars ($1.4
M_{\odot}$) and massive neutron stars ($2.0 M_{\odot}$). The crustal thickness
is calculated consistently with the core equation of state (EOS). The EOSs are
calculated using a simple model for the energy density of nuclear matter and
probe the dependence on the symmetry energy by varying the slope of the
symmetry energy at saturation density $L$ from 25 MeV (soft symmetry energy and
EOS) to 115 MeV (stiff symmetry energy and EOS) while keeping the EOS of
symmetric nuclear matter fixed. The instability window is reduced by a
frequency of up to $\approx150Hz$ from the softest to the stiffest EOSs and by
$\approx 100$ Hz from $1.4 M_{\odot}$ to $2.0 M_{\odot}$ stars for a fixed EOS.
Where temperature estimates are available, the observed neutron stars in low
mass X-ray binaries (LMXBs) have frequencies below the instability window for
the $1.4 M_{\odot}$ models, while some LMXBs fall within the instability window
for $2.0 M_{\odot}$ stars if the symmetry energy is relatively stiff,
indicating that a softer symmetry energy is more consistent with observations
within this model. The critical temperature, below which no star can reach the
instability window without exceeding its Kepler frequency, varies by nearly an
order of magnitude from soft to stiff symmetry energies. When the crust
thickness and core EOS are treated consistently, a thicker crust corresponds to
a lower critical temperature, the opposite result to previous studies in which
the transition density was independent of the core EOS.
View original:
http://arxiv.org/abs/1110.5985
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