203 research outputs found
Quark Matter in Neutron Stars: An apercu
The existence of deconfined quark matter in the superdense interior of
neutron stars is a key question that has drawn considerable attention over the
past few decades. Quark matter can comprise an arbitrary fraction of the star,
from 0 for a pure neutron star to 1 for a pure quark star, depending on the
equation of state of matter at high density. From an astrophysical viewpoint,
these two extreme cases are generally expected to manifest different
observational signatures. An intermediate fraction implies a hybrid star, where
the interior consists of mixed or homogeneous phases of quark and nuclear
matter, depending on surface and Coulomb energy costs, as well as other finite
size and screening effects. In this brief review article, we discuss what we
can deduce about quark matter in neutron stars in light of recent exciting
developments in neutron star observations. We state the theoretical ideas
underlying the equation of state of dense quark matter, including color
superconducting quark matter. We also highlight recent advances stemming from
re-examination of an old paradigm for the surface structure of quark stars and
discuss possible evolutionary scenarios from neutron stars to quark stars, with
emphasis on astrophysical observations.Comment: 15 pages, 1 figure. Invited review for Modern Physics Letters
Constraining phases of quark matter with studies of r-mode damping in neutron stars
The r-mode instability in rotating compact stars is used to constrain the
phase of matter at high density. The color-flavor-locked phase with kaon
condensation (CFL-K0) and without (CFL) is considered in the temperature range
10^8K < T <10^{11} K. While the bulk viscosity in either phase is only
effective at damping the r-mode at temperatures T > 10^{11} K, the shear
viscosity in the CFL-K0 phase is the only effective damping agent all the way
down to temperatures T > 10^8 K characteristic of cooling neutron stars.
However, it cannot keep the star from becoming unstable to gravitational wave
emission for rotation frequencies f ~ 56-11 Hz at T ~ 10^8-10^9 K. Stars
composed almost entirely of CFL or CFL-K0 matter are ruled out by observation
of rapidly rotating neutron stars, indicating that dissipation at the
quark-hadron interface or nuclear crust interface must play a key role in
damping the instability.Comment: 8 pages, 2 figure
High-density Skyrmion matter and Neutron Stars
We examine neutron star properties based on a model of dense matter composed
of B=1 skyrmions immersed in a mesonic mean field background. The model
realizes spontaneous chiral symmetry breaking non-linearly and incorporates
scale-breaking of QCD through a dilaton VEV that also affects the mean fields.
Quartic self-interactions among the vector mesons are introduced on grounds of
naturalness in the corresponding effective field theory. Within a plausible
range of the quartic couplings, the model generates neutron star masses and
radii that are consistent with a preponderance of observational constraints,
including recent ones that point to the existence of relatively massive neutron
stars with mass M 1.7 Msun and radius R (12-14) km. If the existence of neutron
stars with such dimensions is confirmed, matter at supra-nuclear density is
stiffer than extrapolations of most microscopic models suggest.Comment: 27 pages, 5 figures, AASTeX style; to be published in The
Astrophysical Journa
Surface structure of Quark stars with magnetic fields
We investigate the impact of magnetic fields on the electron distribution in
the electrosphere of quark stars. For moderately strong magnetic fields G, quantization effects are generally weak due to the large number
density of electrons at surface, but can nevertheless affect the spectral
features of quark stars. We outline the main observational characteristics of
quark stars as determined by their surface emission, and briefly discuss their
formation in explosive events termed Quark-Novae, which may be connected to the
-process.Comment: 9 pages, 3 figures. Contribution to the proceedings of the IXth
Workshop on High Energy Physics Phenomenology (WHEPP-9), Bhubaneswar, India,
3-14 Jan. 200
Numerical Simulation of the Hydrodynamical Combustion to Strange Quark Matter
We present results from a numerical solution to the burning of neutron matter
inside a cold neutron star into stable (u,d,s) quark matter. Our method solves
hydrodynamical flow equations in 1D with neutrino emission from weak
equilibrating reactions, and strange quark diffusion across the burning front.
We also include entropy change due to heat released in forming the stable quark
phase. Our numerical results suggest burning front laminar speeds of 0.002-0.04
times the speed of light, much faster than previous estimates derived using
only a reactive-diffusive description. Analytic solutions to hydrodynamical
jump conditions with a temperature dependent equation of state agree very well
with our numerical findings for fluid velocities. The most important effect of
neutrino cooling is that the conversion front stalls at lower density (below
approximately 2 times saturation density). In a 2-dimensional setting, such
rapid speeds and neutrino cooling may allow for a flame wrinkle instability to
develop, possibly leading to detonation.Comment: 5 pages, 3 figures (animations online at
http://www.capca.ucalgary.ca/~bniebergal/webPHP/research.php
The Strange Star Surface: A Crust with Nuggets
We reexamine the surface composition of strange stars. Strange quark stars
are hypothetical compact stars which could exist if strange quark matter was
absolutely stable. It is widely accepted that they are characterized by an
enormous density gradient ( g/cm) and large electric fields at
surface. By investigating the possibility of realizing a heterogeneous crust,
comprised of nuggets of strange quark matter embedded in an uniform electron
background, we find that the strange star surface has a much reduced density
gradient and negligible electric field. We comment on how our findings will
impact various proposed observable signatures for strange stars.Comment: 4 pages, 2 figure
Dispersion in a relativistic degenerate electron gas
Relativistic effects on dispersion in a degenerate electron gas are discussed
by comparing known response functions derived relativistically (by Jancovici)
and nonrelativistically (by Lindhard). The main distinguishing feature is
one-photon pair creation, which leads to logarithmic singularities in the
response functions. Dispersion curves for longitudinal waves have a similar
tongue-like appearance in the relativistic and nonrelativistic case, with the
main relativistic effects being on the Fermi speed and the cutoff frequency.
For transverse waves the nonrelativistic treatment has a nonphysical feature
near the cutoff frequency for large Fermi momenta, and this is attributed to an
incorrect treatment of the electron spin. We find (with two important provisos)
that one-photon pair creation is allowed in superdense plasmas, implying
relatively strong coupling between transverse waves and pair creation.Comment: 17 pages, 9 figures. Submitted to Physical Review
Direct Urca neutrino rate in colour superconducting quark matter
If deconfined quark matter exists inside compact stars, the primary cooling
mechanism is neutrino radiation via the direct Urca processes d->u+e+antinu_e
and u+e->d+nu_e. Below a critical temperature, T_c, quark matter forms a colour
superconductor, one possible manifestation of which is a condensate of
quark Cooper pairs in an electric-charge neutralising background of electrons.
We compute the neutrino emission rate from such a phase, including charged
pair-breaking and recombination effects, and find that on a material
temperature domain below T_c the pairing-induced suppression of the neutrino
emission rate is not uniformly exponential. If gapless modes are present in the
condensed phase, the emissivity at low temperatures is moderately enhanced
above that of completely unpaired matter. The importance of charged current
pair-breaking processes for neutrino emission both in the fully gapped and
partially gapped regimes is emphasised.Comment: 5 pages, 2 figures; to appear in Phys. Rev. C (Rapid Comm.
Bremsstrahlung neutrinos from electron-electron scattering in a relativistic degenerate electron plasma
We present a calculation of neutrino pair bremsstrahlung due to
electron-electron scattering in a relativistic degenerate plasma of electrons.
Proper treatment of the in-medium photon propagator, i.e., inclusion of Debye
screening of the longitudinal part and Landau damping of the transverse part,
leads to a neutrino emissivity which is several orders of magnitude larger than
when Debye screening is imposed for the tranverse part. Our results show that
this in-medium process can compete with other sources of neutrino radiation and
can, in some cases, even be the dominant neutrino emission mechanism. We also
discuss the natural extension to quark-quark bremsstrahlung in gapped and
ungapped quark matter.Comment: 15 pages, 7 figure
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