2,584 research outputs found
Dense matter equation of state and neutron star properties from nuclear theory and experiment
The equation of state of dense matter determines the structure of neutron
stars, their typical radii, and maximum masses. Recent improvements in
theoretical modeling of nuclear forces from the low-energy effective field
theory of QCD has led to tighter constraints on the equation of state of
neutron-rich matter at and somewhat above the densities of atomic nuclei, while
the equation of state and composition of matter at high densities remains
largely uncertain and open to a multitude of theoretical speculations. In the
present work we review the latest advances in microscopic modeling of the
nuclear equation of state and demonstrate how to consistently include also
empirical nuclear data into a Bayesian posterior probability distribution for
the model parameters. Derived bulk neutron star properties such as radii,
moments of inertia, and tidal deformabilities are computed, and we discuss as
well the limitations of our modeling.Comment: 9 pages, 5 figures. To appear in the AIP Proceedings of the
Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era
of Gravitational Wave Astronomy, Jan. 3-7, Xiamen, Chin
Proton pairing in neutron stars from chiral effective field theory
We study the proton pairing gap in beta-equilibrated neutron star
matter within the framework of chiral effective field theory. We focus on the
role of three-body forces, which strongly modify the effective proton-proton
spin-singlet interaction in dense matter. We find that three-body forces
generically reduce both the size of the pairing gap and the maximum density at
which proton pairing may occur. The pairing gap is computed within BCS theory,
and model uncertainties are estimated by varying the nuclear potential and the
choice of single-particle spectrum in the gap equation. We find that a
second-order perturbative treatment of the single-particle spectrum suppresses
the proton pairing gap relative to the use of a free spectrum. We
estimate the critical temperature for the onset of proton superconductivity to
be K, which is consistent with previous
theoretical results in the literature and marginally within the range deduced
from a recent Bayesian analysis of neutron star cooling observations.Comment: 8 pages, 9 figure
Molecular shells in IRC+10216: Evidence for non-isotropic and episodic mass loss enhancement
We report high angular-resolution VLA observations of cyanopolyyne molecules
HCN and HCN from the carbon rich circumstellar envelope of IRC+10216.
The observed low-lying rotational transitions trace a much more extended
emitting region than seen in previous observations at higher frequency
transitions. We resolve the hollow quasi-spherical distribution of the
molecular emissions into a number of clumpy shells. These molecular shells
coincide spatially with dust arcs seen in deep optical images of the IRC+10216
envelope, allowing us to study for the first time the kinematics of these
features. We find that the molecular and dust shells represent the same density
enhancements in the envelope separated in time by 120 to 360 yrs.
From the angular size and velocity spread of the shells, we estimate that each
shell typically covers about 10% of the stellar surface at the time of
ejection. The distribution of the shells seems to be random in space. The good
spatial correspondance between HCN and HCN emissions is in qualitative
agreement with a recent chemical model that takes into account the presence of
density-enhanced shells. The broad spatial distribution of the cyanopolyyne
molecules, however, would necessitate further study on their formation.Comment: 16 pages, 5 figures, accepted for publication in Ap
Dense molecular clumps in the envelope of the yellow hypergiant IRC+10420
The circumstellar envelope of the hypergiant star IRC+10420 has been traced
as far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrast
with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes
of giant and supergiant stars. Here, we present an observation of the
circumstellar envelope in SiO J=1-0 that, when combined with the previous
observation in {\sioii}, provide more stringent constraints on the density of
the SiO-emitting gas than hitherto possible. The emission in SiO peaks at a
radius of 2\arcsec\ whereas that in SiO J=2-1 emission peaks at a smaller
radius of 1\arcsec, giving rise to their ring-like appearances. The ratio
in brightness temperature between SiO J=1-0 and SiO J=2-1 decreases from a
value well above unity at the innermost measurable radius to about unity at
radius of 2\arcsec, beyond which this ratio remains approximately
constant. Dividing the envelope into three zones as in models for the CO J =
1-0 and CO J = 2-1 emission, we show that the density of the SiO-emitting gas
is comparable with that of the CO-emitting gas in the inner zone, but at least
an order of magnitude higher by comparison in both the middle and outer zones.
The SiO-emitting gas therefore originates from dense clumps, likely associated
with the dust clumps seen in scattered optical light, surrounded by more
diffuse CO-emitting interclump gas. We suggest that SiO molecules are released
from dust grains due to shock interactions between the dense SiO-emitting
clumps and the diffuse CO-emitting interclump gas.Comment: Accepted for publication in Ap
Evidence for Infalling Gas of Low Angular Momentum towards the L1551 NE Keplerian Circumbinary Disk
We report follow-up observations of the Class I binary protostellar system
L1551 NE in the C18O (3--2) line with the SMA in its compact and subcompact
configurations. Our previous observations at a higher angular resolution in the
extended configuration revealed a circumbinary disk exhibiting Keplerian
motion. The combined data having more extensive spatial coverage (~140 - 2000
AU) verify the presence of a Keplerian circumbinary disk, and reveals for the
first time a distinct low-velocity (~< +-0.5 km s-1 from the systemic velocity)
component that displays a velocity gradient along the minor axis of the
circumbinary disk. Our simple model that reproduces the main features seen in
the Position-Velocity diagrams comprises a circumbinary disk exhibiting
Keplerian motion out to a radius of ~300 AU, beyond which the gas exhibits pure
infall at a constant velocity of ~0.6 km s-1. The latter is significantly
smaller than the expected free-fall velocity of ~2.2 km s-1 onto the L1551 NE
protostellar mass of ~0.8 Msun at ~300 AU, suggesting that the infalling gas is
decelerated as it moves into regions of high gas pressure in the circumbinary
disk. The discontinuity in angular momenta between the outer infalling gas and
inner Keplerian circumbinary disk implies an abrupt transition in the
effectiveness at which magnetic braking is able to transfer angular momentum
outwards, a result perhaps of the different plasma beta and ionization
fractions between the outer and inner regions of the circumbinary disk.Comment: 23 pages, 5 figure
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