566 research outputs found
A source of high-velocity white dwarfs
We investigate whether the recently-observed population of high-velocity
white dwarfs can be derived from a population of binaries residing initially
within the thin disk of the Galaxy. In particular we consider binaries where
the primary is sufficiently massive to explode as a type II supernova. A large
fraction of such binaries are broken up when the primary then explodes as a
supernova owing to the combined effects of the mass loss from the primary and
the kick received by the neutron star on its formation. For binaries where the
primary evolves to fill its Roche lobe, mass transfer from the primary leads to
the onset of a common envelope phase during which the secondary and the core of
the primary spiral together as the envelope is ejected. Such binaries are the
progenitors of X-ray binaries if they are not broken up when the primary
explodes. For those systems which are broken up, a large number of the
secondaries receive kick velocities ~100 - 200 km/s and subsequently evolve
into white dwarfs. We compute trajectories within the Galactic potential for
this population of stars and relate the birthrate of these stars over the
entire Galaxy to those seen locally with high velocities relative to the LSR.
We show that for a reasonable set of assumptions concerning the Galactic
supernova rate and the binary population, our model produces a local number
density of high-velocity white dwarfs compatible with that inferred from
observations. We therefore propose that a population of white dwarfs
originating in the thin disk may make a significant contribution to the
observed population of high-velocity white dwarfs.Comment: 6 pages, 5 figures; revised version, MNRAS in pres
Reconstitution par arbres de régression du rayonnement visible descendant horaire sur la France continentale, à partir de données in situ et de simulations : Spatialisation et vérification sur des données indépendantes
49 p.La nĂ©cessitĂ© de disposer sur toute la France de sĂ©ries horaires de rayonnement visible descendant a menĂ© Ă une reconstitution en deux Ă©tapes, Ă partir de paramĂštres facilement disponibles : * Une reconstitution en des points oĂč sont disponibles la durĂ©e d'insolation, Ă©ventuellement des rĂ©sultats de simulations Safran et des observations de nature et d'Ă©tendue des couches nuageuses. La mĂ©thode de reconstitution employĂ©e est basĂ©e sur des moyennes d'ensembles d'arbres de rĂ©gression. Cette mĂ©thode permet de prendre en compte les non linĂ©aritĂ©s entre les divers intrants mĂ©tĂ©orologiques. Elle est comparĂ©e avec d'autres ajustements non-linĂ©aires. Les critĂšres de choix entre les divers algorithmes statistiques sont : la qualitĂ© des rĂ©sultats, la rapiditĂ© des apprentissages et la facilitĂ© des maintenances logicielles. On n'a pas rencontrĂ© de contradiction entre ces 3 critĂšres. Des liaisons sont d'abord Ă©tablies entre le rayonnement horaire et les autres paramĂštres mĂ©tĂ©orologiques sur toutes les stations disposant de l'intĂ©gralitĂ© des donnĂ©es nĂ©cessaires. Elles sont d'abord testĂ©es par des mĂ©thodes de cross-validation sur ces mĂȘmes stations, puis appliquĂ©es en tous les points disposant de mesure simultanĂ©es de durĂ©e d'insolation et de nĂ©bulositĂ© (environ un par dĂ©partement). Ces pseudo-observations supplĂ©mentaires viennent complĂ©ter le rĂ©seau d'observations de rayonnement horaire, dont la rĂ©partition spatiale est trĂšs irrĂ©guliĂšre. * La spatialisation Ă l'Ă©chelle de Safran (maille de 8'8km) est alors effectuĂ©e par krigeage ordinaire. La validation de ces traitements a portĂ© sur 11 stations indĂ©pendantes, non gĂ©rĂ©es par MĂ©tĂ©o-France, et sur une pĂ©riode (2004) n'ayant servi Ă aucun rĂ©glage
Search for an Near-IR Counterpart to the Cas A X-ray Point Source
We report deep near-infrared and optical observations of the X-ray point
source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have
identified a J=21.4 +/- 0.3 mag and Ks=20.5 +/- 0.3 mag source within the
1-sigma error circle, but we believe this source is a foreground Pop II star
with Teff=2600-2800 K at a distance of ~2 kpc, which could not be the X-ray
point source. We do not detect any sources in this direction at the distance of
Cas A, and therefore place 3-sigma limits of R >~ 25 mag, F675W >~ 27.3 mag, J
>~ 22.5 mag and Ks >~ 21.2 mag (and roughly H >~ 20 mag) on emission from the
X-ray point source, corresponding to M_{R} >~ 8.2 mag, M_{F675W} >~ 10.7 mag,
M_{J} >~ 8.5 mag, M_{H} >~ 6.5 mag, and M_{Ks} >~ 8.0 mag, assuming a distance
of 3.4 kpc and an extinction A_{V}=5 mag.Comment: 14 pages, 7 figures. Accepted by Ap
Evolution of Neutron-Star, Carbon-Oxygen White-Dwarf Binaries
At least one, but more likely two or more, eccentric neutron-star,
carbon-oxygen white-dwarf binaries with an unrecycled pulsar have been
observed. According to the standard scenario for evolving neutron stars which
are recycled in common envelope evolution we expect to observe \gsim 50 such
circular neutron star-carbon oxygen white dwarf binaries, since their formation
rate is roughly equal to that of the eccentric binaries and the time over which
they can be observed is two orders of magnitude longer, as we shall outline. We
observe at most one or two such circular binaries and from that we conclude
that the standard scenario must be revised. Introducing hypercritical accretion
into common envelope evolution removes the discrepancy by converting the
neutron star into a black hole which does not emit radio waves, and therefore
would not be observed.Comment: 25 pages, 1 figure, accepted in Ap
Parthenocarpic potential in Capsicum annuum L. is enhanced by carpelloid structures and controlled by a single recessive gene
<p>Abstract</p> <p>Background</p> <p>Parthenocarpy is a desirable trait in <it>Capsicum annuum </it>production because it improves fruit quality and results in a more regular fruit set. Previously, we identified several <it>C. annuum </it>genotypes that already show a certain level of parthenocarpy, and the seedless fruits obtained from these genotypes often contain carpel-like structures. In the <it>Arabidopsis bel1 </it>mutant ovule integuments are transformed into carpels, and we therefore carefully studied ovule development in <it>C. annuum </it>and correlated aberrant ovule development and carpelloid transformation with parthenocarpic fruit set.</p> <p>Results</p> <p>We identified several additional <it>C. annuum </it>genotypes with a certain level of parthenocarpy, and confirmed a positive correlation between parthenocarpic potential and the development of carpelloid structures. Investigations into the source of these carpel-like structures showed that while the majority of the ovules in <it>C. annuum </it>gynoecia are unitegmic and anatropous, several abnormal ovules were observed, abundant at the top and base of the placenta, with altered integument growth. Abnormal ovule primordia arose from the placenta and most likely transformed into carpelloid structures in analogy to the <it>Arabidopsis bel1 </it>mutant. When pollination was present fruit weight was positively correlated with seed number, but in the absence of seeds, fruit weight proportionally increased with the carpelloid mass and number. <it>Capsicum </it>genotypes with high parthenocarpic potential always showed stronger carpelloid development. The parthenocarpic potential appeared to be controlled by a single recessive gene, but no variation in coding sequence was observed in a candidate gene <it>CaARF8</it>.</p> <p>Conclusions</p> <p>Our results suggest that in the absence of fertilization most <it>C. annuum </it>genotypes, have parthenocarpic potential and carpelloid growth, which can substitute developing seeds in promoting fruit development.</p
Nova Sco and coalescing low mass black hole binaries as LIGO sources
Double neutron star binaries, analogous to the well known Hulse--Taylor
pulsar PSR 1913+16, are guaranteed-to-exist sources of high frequency
gravitational radiation detectable by LIGO. There is considerable uncertainty
in the estimated rate of coalescence of such systems, with conservative
estimates of ~1 per million years per galaxy, and optimistic theoretical
estimates one or more magnitude larger. Formation rates of low-mass black
hole-neutron star binaries may be higher than those of NS-NS binaries, and may
dominate the detectable LIGO signal rate.
We estimate the enhanced coalescence rate for BH-BH binaries due to weak
asymmetric kicks during the formation of low mass black holes like Nova Sco,
and find they may contribute significantly to the LIGO signal rate, possibly
dominating the phase I detectable signals if the range of BH masses for which
there is significant kick is broad enough. For a standard Salpeter IMF,
assuming mild natal kicks, we project that the R6 merger rate of BH-BH systems
is ~0.5, smaller than that of NS-NS systems. However, the higher chirp mass of
these systems produces a signal nearly four times greater, on average, with a
commensurate increase in search volume.
The BH-BH coalescence channel considered here also predicts that a
substantial fraction of BH-BH systems should have at least one component with
near-maximal spin (a/M ~ 1).The waveforms produced by the coalescence of such a
system should produce a clear spin signature, so this hypothesis could be
directly tested by LIGO.Comment: 16 pages, LaTeX/AASTeX, 5 figure
The Galactic Population of Low- and Intermediate-Mass X-ray Binaries
(abridged) We present the first study that combines binary population
synthesis in the Galactic disk and detailed evolutionary calculations of low-
and intermediate-mass X-ray binaries (L/IMXBs). We show that the formation
probability of IMXBs with initial donor masses of 1.5--4 Msun is typically >~5
times higher than that of standard LMXBs, and suggest that the majority of the
observed systems may have descended from IMXBs. Distributions at the current
epoch of the orbital periods, donor masses, and mass accretion rates have been
computed, as have orbital-period distributions of BMPs. Several significant
discrepancies between the theoretical and observed distributions are discussed.
The orbital-period distribution of observed BMPs strongly favors cases where
the envelope of the neutron-star progenitor is more easily ejected during the
common-envelope phase. However, this leads to a >~100-fold overproduction of
the theoretical number of luminous X-ray sources relative to the total observed
number of LMXBs. X-ray irradiation of the donor star may result in a dramatic
reduction in the X-ray active lifetime of L/IMXBs, thus possibly resolving the
overproduction problem, as well as the long-standing BMP/LMXB birthrate
problem.Comment: 12 pages, emulateapj, submitted to Ap
Equipotential Surfaces and Lagrangian points in Non-synchronous, Eccentric Binary and Planetary Systems
We investigate the existence and properties of equipotential surfaces and
Lagrangian points in non-synchronous, eccentric binary star and planetary
systems under the assumption of quasi-static equilibrium. We adopt a binary
potential that accounts for non-synchronous rotation and eccentric orbits, and
calculate the positions of the Lagrangian points as functions of the mass
ratio, the degree of asynchronism, the orbital eccentricity, and the position
of the stars or planets in their relative orbit. We find that the geometry of
the equipotential surfaces may facilitate non-conservative mass transfer in
non-synchronous, eccentric binary star and planetary systems, especially if the
component stars or planets are rotating super-synchronously at the periastron
of their relative orbit. We also calculate the volume-equivalent radius of the
Roche lobe as a function of the four parameters mentioned above. Contrary to
common practice, we find that replacing the radius of a circular orbit in the
fitting formula of Eggleton (1983) with the instantaneous distance between the
components of eccentric binary or planetary systems does not always lead to a
good approximation to the volume-equivalent radius of the Roche-lobe. We
therefore provide generalized analytic fitting formulae for the
volume-equivalent Roche lobe radius appropriate for non-synchronous, eccentric
binary star and planetary systems. These formulae are accurate to better than
1% throughout the relevant 2-dimensional parameter space that covers a dynamic
range of 16 and 6 orders of magnitude in the two dimensions.Comment: 12 pages, 10 figures, 2 Tables, Accepted by the Astrophysical Journa
Coronary artery assessment by multidetector computed tomography in patients with prosthetic heart valves
Objectives Patients with prosthetic heart valves may require assessment for coronary artery disease. We assessed whether valve artefacts hamper coronary artery assessment by multidetector CT. Methods ECG-gated or -triggered CT angiograms were selected from our PACS archive based on the presence of prosthetic heart valves. The best systolic and diastolic axial reconstructions were selected for coronary assessment. Each present coronary segment was scored for the presence of valve-related artefacts prohibiting coronary artery assessment. Scoring was performed in consensus by two observers. Results Eighty-two CT angiograms were performed on a 64-slice ( = 27) or 256-slice ( = 55) multidetector CT. Eighty-nine valves and five annuloplasty rings were present. Forty-three out of 1160 (3.7%) present coronary artery segments were non-diagnostic due to valve artefacts (14/82 patients). Valve artefacts were located in right coronary artery (15/43; 35%), left anterior descending artery (2/43; 5%), circumflex artery (14/43; 32%) and marginal obtuse (12/43; 28%) segments. All cobalt-chrome containing valves caused artefacts prohibiting coronary assessment. Biological and titanium-containing valves did not cause artefacts except for three specific valve types. Conclusions Most commonly implanted prosthetic heart valves do not hamper coronary assessment on multidetector CT. Cobalt-chrome containing prosthetic heart valves preclude complete coronary artery assessment because of severe valve artefacts. Key Points Most commonly implanted prosthetic heart valves do not hamper coronary artery assessment Prosthetic heart valve composition determines the occurrence of prosthetic heart valve-related artefacts Bjork-Shiley and Sorin tilting disc valves preclude diagnostic coronary artery segment assessmen
On the Formation and Progenitor of PSR J0737-3039: New Constraints on the Supernova Explosion Forming Pulsar B
We revisit the formation of PSR J0737-3039, taking into account the most
recent observational constraints. We show that the most likely kick velocity
and progenitor parameters depend strongly on the consideration of the full
five-dimensional PDF for the magnitude and direction of the kick velocity
imparted to pulsar B at birth, the mass of pulsar B's pre-supernova helium star
progenitor, and the pre-supernova orbital separation, and on the adopted prior
assumptions. The priors consist of the transverse systemic velocity, the age of
the system, and the treatment of the unknown radial velocity. Since the latter
cannot be determined from observation, we adopt a statistical approach and use
theoretical radial-velocity distributions obtained from population synthesis
calculations for coalescing double neutron stars. We find that the prior
assumptions about the pre-supernova helium star mass affect the derived most
likely parameters significantly: when the minimum helium star mass required for
neutron star formation is assumed to be 2.1Msun, the most likely kick velocity
ranges from 70-180km/s; when masses lower than 2.1Msun are assumed to allow
neutron star formation, the most likely kick velocity can be as low as a few
km/s, although the majority of the considered models still yield most likely
kick velocities of 50-170km/s. We also show that the proximity of the double
pulsar to the Galactic plane and the small proper motion do not pose stringent
constraints on the kick velocity and progenitor mass of pulsar B. Instead, the
constraints imposed by the orbital dynamics of asymmetric supernova explosions
turn out to be much more restrictive. We conclude that the currently available
observational constraints cannot be used to favor a specific core-collapse and
neutron star formation mechanism. (abridged)Comment: Accepted by Physical Review D. Revised version incorporates a
detailed rebuttal of the remarks made by Piran & Shaviv in astro-ph/0603649
(see Appendix A) and also minor clarifications requested by an anonymous
refere
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