180 research outputs found
Approximate analytic expressions for circular orbits around rapidly rotating compact stars
We calculate stationary configurations of rapidly rotating compact stars in
general relativity, to study the properties of circular orbits of test
particles in the equatorial plane. We search for simple, but precise,
analytical formulae for the orbital frequency, specific angular momentum and
binding energy of a test particle, valid for any equation of state and for any
rotation frequency of the rigidly rotating compact star, up to the
mass-shedding limit. Numerical calculations are performed using precise 2-D
codes based on multi-domain spectral methods. Models of rigidly rotating
neutron stars and the space-time outside them are calculated for several
equations of state of dense matter. Calculations are also performed for quark
stars consisting of self-bound quark matter. At the mass-shedding limit, the
rotational frequency converges to a Schwarzschildian orbital frequency at the
equator. We show that orbital frequency for any orbit outside equator is also
approximated by a Schwarzschildian formula. Using a simple approximation for
the frame-dragging term, we obtain approximate expressions for the specific
angular momentum and specific energy on the corotating circular orbits in the
equatorial plane of neutron star, which are valid down to the stellar equator.
The formulae recover reference numerical values with typically 1% of accuracy
for neutron stars with M > 0.5 M_sun. They are less precise for quark stars
consisting of self-bound quark matter.Comment: 6 pages, 6 figures, A&A in pres
Climbing depth-bounded adjacent discrepancy search for solving hybrid flow shop scheduling problems with multiprocessor tasks
This paper considers multiprocessor task scheduling in a multistage hybrid
flow-shop environment. The problem even in its simplest form is NP-hard in the
strong sense. The great deal of interest for this problem, besides its
theoretical complexity, is animated by needs of various manufacturing and
computing systems. We propose a new approach based on limited discrepancy
search to solve the problem. Our method is tested with reference to a proposed
lower bound as well as the best-known solutions in literature. Computational
results show that the developed approach is efficient in particular for
large-size problems
Refined Neutron-Star Mass Determinations for Six Eclipsing X-Ray Pulsar Binaries
We present an improved method for determining the mass of neutron stars in
eclipsing X-ray pulsar binaries and apply the method to six systems, namely
Vela X-1, 4U 1538-52, SMC X-1, LMC X-4, Cen X-3, and Her X-1. In previous
studies to determine neutron star mass, the X-ray eclipse duration has been
approximated analytically by assuming the companion star is spherical with an
effective Roche lobe radius. We use a numerical code based on Roche geometry
with various optimizers to analyze the published data for these systems, which
we supplement with new spectroscopic and photometric data for 4U 1538-52. This
allows us to model the eclipse duration more accurately and thus calculate an
improved value for the neutron star mass. The derived neutron star mass also
depends on the assumed Roche lobe filling factor beta of the companion star,
where beta = 1 indicates a completely filled Roche lobe. In previous work a
range of beta between 0.9 and 1.0 was usually adopted. We use optical
ellipsoidal lightcurve data to constrain beta. We find neutron star masses of
1.77 +/- 0.08 M_{sun} for Vela X-1, 0.87 +/- 0.07 M_{sun} for 4U 1538-52
(eccentric orbit), 1.00 +/- 0.10 M_{sun} for 4U 1538-52 (circular orbit), 1.04
+/- 0.09 M_{sun} for SMC X-1, 1.29 +/- 0.05 M_{sun} for LMC X-4, 1.49 +/- 0.08
M_{sun} for Cen X-3, and 1.07 +/- 0.36 M_{sun} for Her X-1. We discuss the
limits of the approximations that were used to derive the earlier mass
determinations, and we comment on the implications our new masses have for
observationally refining the upper and lower bounds of the neutron star mass
distribution.Comment: 10 figures, accepted for publication in The Astrophysical Journa
Progenitor neutron stars of the lightest and heaviest millisecond pulsars
The recent mass measurements of two binary millisecond pulsars, PSR
J1614-2230 and PSR J0751+1807 with a mass M=1.97+/-0.04 Msun and M= 1.26 +/-
0.14 Msun, respectively, indicate a wide range of masses for such objects and
possibly also a broad spectrum of masses of neutron stars born in core-collapse
supernovae.
Starting from the zero-age main sequence binary stage, we aim at inferring
the birth masses of PSR J1614-2230 and PSR J0751+1807 by taking the differences
in the evolutionary stages preceding their formation into account.
Using simulations for the evolution of binary stars, we reconstruct the
evolutionary tracks leading to the formation of PSR J1614-2230 and PSR
J0751+1807. We analyze in detail the spin evolution due to the accretion of
matter from a disk in the intermediate-mass/low-mass X-ray binary. We consider
two equations of state of dense matter, one for purely nucleonic matter and the
other one including a high-density softening due to the appearance of hyperons.
Stationary and axisymmetric stellar configurations in general relativity are
used, together with a recent magnetic torque model and
observationally-motivated laws for the decay of magnetic field.
The estimated birth mass of the neutron stars PSR J0751+1807 and PSR
J1614-2230 could be as low as 1.0 Msun and as high as 1.9 Msun, respectively.
These values depend weakly on the equation of state and the assumed model for
the magnetic field and its accretion-induced decay.
The masses of progenitor neutron stars of recycled pulsars span a broad
interval from 1.0 Msun to 1.9 Msun. Including the effect of a slow Roche-lobe
detachment phase, which could be relevant for PSR J0751+1807, would make the
lower mass limit even lower. A realistic theory for core-collapse supernovae
should account for this wide range of mass.Comment: 13 pages, 10 figures, accepted in A&
Study of measured pulsar masses and their possible conclusions
We study the statistics of 61 measured masses of neutron stars (NSs) in
binary pulsar systems, including 18 double NS (DNS) systems, 26 radio pulsars
(10 in our Galaxy) with white dwarf (WD) companions, 3 NSs with main-sequence
companions, 13 NSs in X-ray binaries, and one undetermined system. We derive a
mean value of M = 1.46 +/- 0.30 solar masses. When the 46 NSs with measured
spin periods are divided into two groups at 20 milliseconds, i.e., the
millisecond pulsar (MSP) group and others, we find that their mass averages
are, respectively, M=1.57 +/- 0.35 solar masses and M=1.37+/- 0.23 solar
masses. In the framework of the pulsar recycling hypothesis, this suggests that
an accretion of approximately 0.2 solar mass is sufficient to spin up a neutron
star and place it in the millisecond pulsar group. An empirical relation
between the accreting mass and MSP spin period is \Delta M=0.43 (solar
mass)(P/1 ms)^{-2/3}. UNlike the standard recycling process, if a MSP is formed
by the accretion induced collapse (AIC) of a white dwarf with a mass less than
Chandrasekha limit, e.g. 1.35 solar mass, then the binary MSPs involved in AICs
is not be higher than 20%, which imposes a constraint on the AIC origin of
MSPs.Comment: 6 pages, 5 figures, in press, Astronomy and Astrophysics 2011, 527,
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Estimating the body weight of byzantine dogs from the theodosius harbour at yenikapı, istanbul
Changing Trends and Experience with Esophageal Cancer Surgery in a Single University Hospital: Are The Results Similar or Not?
Objective: The main treatment modality for esophageal cancer remains to be surgery. Over the last decades, surgical strategies have evolved remarkably. When neoadjuvant chemoradiotherapy became standard, discussions about the role, type, and timing of surgery began. In this study, we share results we obtained after operating our patients using various surgical techniques.Material and Methods:Reliable data from 51 esophageal cancer patients were evaluated retrospectively. Of the 51 cases, 31 were operable. These operable cases were further classified according to surgical method and neoadjuvant therapy status. Median survival time in months, complications, hospital mortality, length of hospital stay, and pathology results (total lymph nodes harvested and pathologic tumor node metastasis stage [p_TNM]) were documented for the different surgical approaches.Results: Open surgical methods were performed in 21 cases, while in 10 cases the Minimally Invasive Surgery (MIS) method was used. The MIS group received neoadjuvant therapy more frequently than the open surgical methods group (p=0.013). Although more complications were observed in the MIS group, the difference to the open esophagectomy methods group was not significant. Patients in the MIS group also had longer hospital stays, but again the difference was not significant. Although a pathologic complete response was seen in 8 of the 11 (72.7%) patients in our study who received chemoradiotherapy as neoadjuvant treatment, the surgical results of patients who received chemoradiotherapy were worse, although not to a statistically significant extent.Conclusion: Despite changing trends and treatment options in esophageal cancer surgery, we have yet to see the expected improved results
Filtering Algorithms for the Multiset Ordering Constraint
Constraint programming (CP) has been used with great success to tackle a wide
variety of constraint satisfaction problems which are computationally
intractable in general. Global constraints are one of the important factors
behind the success of CP. In this paper, we study a new global constraint, the
multiset ordering constraint, which is shown to be useful in symmetry breaking
and searching for leximin optimal solutions in CP. We propose efficient and
effective filtering algorithms for propagating this global constraint. We show
that the algorithms are sound and complete and we discuss possible extensions.
We also consider alternative propagation methods based on existing constraints
in CP toolkits. Our experimental results on a number of benchmark problems
demonstrate that propagating the multiset ordering constraint via a dedicated
algorithm can be very beneficial
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