13,654 research outputs found
Neutron-Rich Nuclei in Heaven and Earth
An accurately calibrated relativistic parametrization is introduced to
compute the ground state properties of finite nuclei, their linear response,
and the structure of neutron stars. While similar in spirit to the successful
NL3 parameter set, it produces an equation of state that is considerably softer
-- both for symmetric nuclear matter and for the symmetry energy. This
softening appears to be required for an accurate description of several
collective modes having different neutron-to-proton ratios. Among the
predictions of this model are a symmetric nuclear-matter incompressibility of
K=230 MeV and a neutron skin thickness in 208Pb of Rn-Rp=0.21 fm. Further, the
impact of such a softening on the properties of neutron stars is as follows:
the model predicts a limiting neutron star mass of Mmax=1.72 Msun, a radius of
R=12.66 km for a ``canonical'' M=1.4 Msun neutron star, and no (nucleon) direct
Urca cooling in neutrons stars with masses below M=1.3 Msun.Comment: 4 pages, 3 tables, and no figure
Relativistic Jets and Long-Duration Gamma-ray Bursts from the Birth of Magnetars
We present time-dependent axisymmetric magnetohydrodynamic simulations of the
interaction of a relativistic magnetized wind produced by a proto-magnetar with
a surrounding stellar envelope, in the first seconds after core
collapse. We inject a super-magnetosonic wind with ergs
s into a cavity created by an outgoing supernova shock. A strong
toroidal magnetic field builds up in the bubble of plasma and magnetic field
that is at first inertially confined by the progenitor star. This drives a jet
out along the polar axis of the star, even though the star and the magnetar
wind are each spherically symmetric. The jet has the properties needed to
produce a long-duration gamma-ray burst (GRB). At s after core bounce,
the jet has escaped the host star and the Lorentz factor of the material in the
jet at large radii cm is similar to that in the magnetar wind
near the source. Most of the spindown power of the central magnetar escapes via
the relativistic jet. There are fluctuations in the Lorentz factor and energy
flux in the jet on second timescale. These may contribute to
variability in GRB emission (e.g., via internal shocks).Comment: 5 pages, 3 figures, accepted in MNRAS letter, presented at the
conference "Astrophysics of Compact Objects", 1-7 July, Huangshan, Chin
O VI and Multicomponent H I Absorption Associated with a Galaxy Group in the Direction of PG0953+415: Physical Conditions and Baryonic Content
We report the discovery of an O VI absorption system at z(abs) = 0.14232 in a
high resolution FUV spectrum of PG0953+415 obtained with the Space Telescope
Imaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet and
multicomponent H I Lya absorption are detected, but the N V doublet and the
strong lines of C II and Si III are not apparent. We examine the ionization
mechanism of the O VI absorber and find that while theoretical considerations
favor collisional ionization, it is difficult to observationally rule out
photoionization. If the absorber is collisionally ionized, it may not be in
equilibrium due to the rapid cooling of gas in the appropriate temperature
range. Non-equilibrium collisionally ionized models are shown to be consistent
with the observations. A WIYN survey of galaxy redshifts near the sight line
has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/s
from this absorber, and three additional galaxies are found within 130 km/s of
this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc.
Combining the STIS observations of PG0953+415 with previous high S/N GHRS
observations of H1821+643, we derive a large number of O VI absorbers per unit
redshift, dN/dz ~20. We use this sample to obtain a first estimate of the
cosmological mass density of the O VI systems at z ~ 0. If further observations
confirm the large dN/dz derived for the O VI systems, then these absorbers
trace a significant reservoir of baryonic matter at low redshift.Comment: Accepted for publication in Ap.J., vol. 542 (Oct. 10, 2000
Brownian dynamics simulations of planar mixed flows of polymer solutions at finite concentrations
Periodic boundary conditions for planar mixed flows are implemented in the
context of a multi-chain Brownian dynamics simulation algorithm. The effect of
shear rate , and extension rate , on the size of
polymer chains, \left, and on the polymer contribution to
viscosity, , is examined for solutions of FENE dumbbells at finite
concentrations, with excluded volume interactions between the beads taken into
account. The influence of the mixedness parameter, , and flow strength,
, on \left and , is also examined, where
corresponds to pure shear flow, and
corresponds to pure extensional flow. It is shown that there exists a critical
value, , such that the flow is shear dominated for , and extension dominated for .Comment: 18 pages, 12 figures, to appear in Chemical Engineering Scienc
The Replication Argument for Incompatibilism
In this paper, I articulate an argument for incompatibilism about moral responsibility and determinism. My argument comes in the form of an extended story, modeled loosely on Peter van Inwagen’s “rollback argument” scenario. I thus call it “the replication argument.” As I aim to bring out, though the argument is inspired by so-called “manipulation” and “original design” arguments, the argument is not a version of either such argument—and plausibly has advantages over both. The result, I believe, is a more convincing incompatibilist argument than those we have considered previously
Nitrogen Oxide Concentrations in Natural Waters on Early Earth
A key challenge in origins-of-life studies is estimating the abundances of
species relevant to the chemical pathways proposed to have contributed to the
emergence of life on early Earth. Dissolved nitrogen oxide anions
(NO), in particular nitrate (NO) and nitrite
(NO), have been invoked in diverse origins-of-life chemistry, from
the oligomerization of RNA to the emergence of protometabolism. Recent work has
calculated the supply of NO from the prebiotic atmosphere to the
ocean, and reported steady-state [NO] to be high across all plausible
parameter space. These findings rest on the assumption that NO is
stable in natural waters unless processed at a hydrothermal vent. Here, we show
that NO is unstable in the reducing environment of early Earth. Sinks
due to UV photolysis and reactions with reduced iron (Fe) suppress
[NO] by several orders of magnitude relative to past predictions. For
pH and C, we find that it is most probable that
NO]M in the prebiotic ocean. On the other hand, prebiotic
ponds with favorable drainage characteristics may have sustained
[NO]M. As on modern Earth, most NO on prebiotic
Earth should have been present as NO, due to its much greater
stability. These findings inform the kind of prebiotic chemistries that would
have been possible on early Earth. We discuss the implications for proposed
prebiotic chemistries, and highlight the need for further studies of
NO kinetics to reduce the considerable uncertainties in predicting
[NO] on early Earth.Comment: In review for publication at Geochemistry, Geophysics, and Geosystems
(G-cubed). Comments, questions, and criticism solicited; please contact
corresponding author at [email protected]. SI at:
https://web-cert.mit.edu/sukrit/Public/nox_si.pdf. GitHub at:
https://github.com/sukritranjan/no
The United States Chiropractic Workforce: An alternative or complement to primary care?
UnlabelledBackgroundIn the United States (US) a shortage of primary care physicians has become evident. Other health care providers such as chiropractors might help address some of the nation's primary care needs simply by being located in areas of lesser primary care resources. Therefore, the purpose of this study was to examine the distribution of the chiropractic workforce across the country and compare it to that of primary care physicians.MethodsWe used nationally representative data to estimate the per 100,000 capita supply of chiropractors and primary care physicians according to the 306 predefined Hospital Referral Regions. Multiple variable Poisson regression was used to examine the influence of population characteristics on the supply of both practitioner-types.ResultsAccording to these data, there are 74,623 US chiropractors and the per capita supply of chiropractors varies more than 10-fold across the nation. Chiropractors practice in areas with greater supply of primary care physicians (Pearson's correlation 0.17, p-value < 0.001) and appear to be more responsive to market conditions (i.e. more heavily influenced by population characteristics) in regards to practice location than primary care physicians.ConclusionThese findings suggest that chiropractors practice in areas of greater primary care physician supply. Therefore chiropractors may be functioning in more complementary roles to primary care as opposed to an alternative point of access
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