23,849 research outputs found
Higher-order symmetry energy and neutron star core-crust transition with Gogny forces
We study the symmetry energy and the core-crust transition in neutron stars
using the finite-range Gogny nuclear interaction and examine the deduced
crustal thickness and crustal moment of inertia. We start by analyzing the
second-, fourth- and sixth-order coefficients of the Taylor expansion of the
energy per particle in powers of the isospin asymmetry for Gogny forces. These
coefficients provide information about the departure of the symmetry energy
from the widely used parabolic law. The neutron star core-crust transition is
evaluated by looking at the onset of thermodynamical instability of the liquid
core. The calculation is performed with the exact (i.e., without Taylor
expansion) Gogny EoS for the core, and also with its Taylor expansion in order
to assess the influence of isospin expansions on locating the inner edge of
neutron star crusts. It is found that the properties of the core-crust
transition derived from the exact EoS differ from the predictions of the Taylor
expansion even when the expansion is carried through sixth order in the isospin
asymmetry. Gogny forces, using the exact EoS, predict the ranges for the transition
density and for the transition pressure. The transition densities show an
anticorrelation with the slope parameter of the symmetry energy. The
transition pressures are not found to correlate with . Neutron stars
obtained with Gogny forces have maximum masses below and
relatively small moments of inertia. The crustal mass and moment of inertia are
evaluated and comparisons are made with the constraints from observed glitches
in pulsars.Comment: 24 pages, 15 figures, discussions and bibliography updated, to appear
in Physical Review
Consistent Approximations for the Optimal Control of Constrained Switched Systems
Though switched dynamical systems have shown great utility in modeling a
variety of physical phenomena, the construction of an optimal control of such
systems has proven difficult since it demands some type of optimal mode
scheduling. In this paper, we devise an algorithm for the computation of an
optimal control of constrained nonlinear switched dynamical systems. The
control parameter for such systems include a continuous-valued input and
discrete-valued input, where the latter corresponds to the mode of the switched
system that is active at a particular instance in time. Our approach, which we
prove converges to local minimizers of the constrained optimal control problem,
first relaxes the discrete-valued input, then performs traditional optimal
control, and then projects the constructed relaxed discrete-valued input back
to a pure discrete-valued input by employing an extension to the classical
Chattering Lemma that we prove. We extend this algorithm by formulating a
computationally implementable algorithm which works by discretizing the time
interval over which the switched dynamical system is defined. Importantly, we
prove that this implementable algorithm constructs a sequence of points by
recursive application that converge to the local minimizers of the original
constrained optimal control problem. Four simulation experiments are included
to validate the theoretical developments
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
On the origin of the X-ray emission from a narrow-line radioquasar at z>1
We present new XMM-Newton X-ray observations of the z=1.246 narrow-line
radioquasar RX J1011.2+5545 serendipitously discovered by ROSAT. The flat X-ray
spectrum previously measured by ROSAT and ASCA is shown to be the result of a
steep Gamma~1.8 power law spectrum seen through a moderate intrinsic absorbing
column NH~4E21 cm^-2. The position of the X-ray source is entirely coincident
with the nucleus of the radio source that we have resolved in new sensitive VLA
observations at 3.6 and 6 cm, implying that scattering in the radio lobes is
not responsible for the bulk of X-ray emission. In the EPIC pn image, a faint
patch of X-ray emission is apparent 14'' to the NE of the main X-ray source.
The former is positionally coincident with an apparently extended optical
object with R~21.9, but there is no associated radio emission, thus ruling out
the possibility that this represents a hotspot in a jet emanating from the
primary X-ray source. No reflection features are detected in the X-ray spectrum
of the narrow-line radioquasar, although an Fe line with equivalent width of up
to 600 eV cannot be ruled out.Comment: 7 pages, 6 figures, MNRAS in the pres
New Gogny interaction suitable for astrophysical applications
The D1 family of parametrizations of the Gogny interaction commonly suffers from a rather soft neutron matter equation of state that leads to maximal masses of neutron stars well below the observational value of two solar masses. We propose a reparametrization scheme that preserves the good properties of the Gogny force but allows one to tune the density dependence of the symmetry energy, which, in turn, modifies the predictions for the maximum stellar mass. The scheme works well for D1M, and leads to a new parameter set, dubbed D1Mâ. In the neutron-star domain, D1Mâpredicts a maximal mass of two solar masses and global properties of the star in harmony with those obtained with the SLy4 Skyrme interaction. By means of a set of selected calculations in finite nuclei, we check that D1Mâperforms comparably well to D1M in several aspects of nuclear structure in nucleiThe work of LMR was supported by Spanish Ministry of Economy and Competitiveness (MINECO) Grants No.FPA2015-65929-P and FIS2015-63770-P. C.G., M.C., and X.V. were partially sup-ported by Grant FIS2014-54672-P from MINECO and FEDER, Grant 2014SGR-401 from Generalitat de Catalunya, and Project MDM-2014-0369 of ICCUB (Unidad de Excelencia MarĂa de Maeztu) from MINECO. C.G. also acknowledges Grant BES-2015-074210 from MINEC
- âŠ