Eccentricities of Double Neutron Star Binaries

Recent pulsar surveys have increased the number of observed double neutron stars (DNS) in our galaxy enough so that observable trends in their properties are starting to emerge. In particular, it has been noted that the majority of DNS have eccentricities less than 0.3, which are surprisingly low for binaries that survive a supernova explosion that we believe imparts a significant kick to the neutron star. To investigate this trend, we generate many different theoretical distributions of DNS eccentricities using Monte Carlo population synthesis methods. We determine which eccentricity distributions are most consistent with the observed sample of DNS binaries. In agreement with Chaurasia & Bailes (2005), assuming all double neutron stars are equally as probable to be discovered as binary pulsars, we find that highly eccentric, coalescing DNS are less likely to be observed because of their accelerated orbital evolution due to gravitational wave emission and possible early mergers. Based on our results for coalescing DNS, we also find that models with vanishingly or moderately small kicks (sigma < about 50 km/s) are inconsistent with the current observed sample of such DNS. We discuss the implications of our conclusions for DNS merger rate estimates of interest to ground-based gravitational-wave interferometers. We find that, although orbital evolution due to gravitational radiation affects the eccentricity distribution of the observed sample, the associated upwards correction factor to merger rate estimates is rather small (typically 10-40%).Comment: 9 pages, 8 figures, accepted by ApJ. Figures reduced and some content changed, references adde

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 nécessité de disposer sur toute la France de séries horaires de rayonnement visible descendant a mené à une reconstitution en deux étapes, à partir de paramètres facilement disponibles : * Une reconstitution en des points où sont disponibles la durée d'insolation, éventuellement des résultats de simulations Safran et des observations de nature et d'étendue des couches nuageuses. La méthode de reconstitution employée est basée sur des moyennes d'ensembles d'arbres de régression. Cette méthode permet de prendre en compte les non linéarités entre les divers intrants météorologiques. Elle est comparée avec d'autres ajustements non-linéaires. Les critères de choix entre les divers algorithmes statistiques sont : la qualité des résultats, la rapidité des apprentissages et la facilité des maintenances logicielles. On n'a pas rencontré de contradiction entre ces 3 critères. Des liaisons sont d'abord établies entre le rayonnement horaire et les autres paramètres météorologiques sur toutes les stations disposant de l'intégralité des données nécessaires. Elles sont d'abord testées par des méthodes de cross-validation sur ces mêmes stations, puis appliquées en tous les points disposant de mesure simultanées de durée d'insolation et de nébulosité (environ un par département). Ces pseudo-observations supplémentaires viennent compléter le réseau d'observations de rayonnement horaire, dont la répartition spatiale est très irrégulière. * La spatialisation à l'échelle de Safran (maille de 8'8km) est alors effectuée par krigeage ordinaire. La validation de ces traitements a porté sur 11 stations indépendantes, non gérées par Météo-France, et sur une période (2004) n'ayant servi à aucun réglage

Internal Combustion Engine

The goal of this project is to modify a Honda GX35 engine to improve its fuel efficiency and maximize its competitiveness in the Shell Eco-marathon competitions. This competition has tight restrictions on the type of fuel used to power the engine, but otherwise has very few rules limiting the type of modifications which may be made. Our primary goals are to maximize volumetric efficiency and maintain an efficient operating temperature. Future work might pursue more advanced technologies such as pseudo-atkinson cycle operation

A New Way to Detect Massive Black Holes in Galaxies: The Stellar Remnants of Tidal Disruption

We point out that the tidal disruption of a giant may leave a luminous (10^35-10^39 ergs/s), hot (10-100 eV) stellar core. The ``supersoft'' source detected by Chandra at the center of M31 may be such a core; whether or not it is, the observations have shown that such a core is detectable, even in the center of a galaxy. We therefore explore the range of expected observational signatures and how they may be used to (1) test the hypothesis that the M31 source is a remnant of tidal stripping and (2) discover evidence of black holes and disruption events in other galaxies.Comment: Four pages with 1 figure. Appeared in ApJL (2001, 551, L37

Modelling the water budget and the riverflows of the Maritsa basin in Bulgaria

International audienceA soil-vegetation-atmosphere transfer model coupled with a macroscale distributed hydrological model was used in order to simulate the water cycle for a large region in Bulgaria. To do so, an atmospheric forcing was built for two hydrological years (1 October 1995 to 30 September 1997), at an eight km resolution. It was based on the data available at the National Institute of Meteorology and Hydrology (NIMH) of Bulgaria. Atmospheric parameters were carefully checked and interpolated with a high level of detail in space and time (3-h step). Comparing computed Penman evapotranspiration versus observed pan evaporation validated the quality of the implemented forcing. The impact of the human activities on the rivers (especially hydropower or irrigation) was taken into account. Some improvements of the hydrometeorological model were made: for better simulation of summer riverflow, two additional reservoirs were added to simulate the slow component of the runoff. Those reservoirs were calibrated using the observed data of the 1st year, while the 2nd year was used for validation. 56 hydrologic stations and 12 dams were used for the model calibration while 41 rivergages were used for the validation of the model. The results compare well with the daily-observed discharges, with good results obtained over more than 25% of the rivergages. The simulated snow depth was compared to daily measurements at 174 stations and the evolution of the snow water equivalent was validated at 5 sites. The process of melting and refreezing of snow was found to be important on this region. The comparison of the normalized values of simulated versus measured soil moisture showed good correlation. The surface water budget shows large spatial variations due to the elevation influence on the precipitations, soil properties and vegetation variability. An inter annual difference was observed in the water cycle as the first year was more influenced by Mediterranean climate, while the second year was characterised by continental influence. Energy budget shows a dominating sensible heat component in summer, due to the fact that the water stress limits the evaporation. This study is a first step for the implementation of an operational hydrometeorological model that could be used for real time monitoring and forecast the water budget and the riverflow of Bulgaria

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

The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array

Most exoplanets have been discovered via radial velocity studies, which are inherently insensitive to orbital inclination. Interferometric observations will show evidence of a stellar companion if it sufficiently bright, regardless of the inclination. Using the CHARA Array, we observed 22 exoplanet host stars to search for stellar companions in low-inclination orbits that may be masquerading as planetary systems. While no definitive stellar companions were discovered, it was possible to rule out certain secondary spectral types for each exoplanet system observed by studying the errors in the diameter fit to calibrated visibilities and by searching for separated fringe packets.Comment: 26 pages, 5 tables, 8 figure

The late stages of evolution of helium star-neutron star binaries and the formation of double neutron star systems

With a view to understanding the formation of double neutron-stars (DNS), we investigate the late stages of evolution of helium stars with masses of 2.8 - 6.4 Msun in binary systems with a 1.4 Msun neutron-star companion. We found that mass transfer from 2.8 - 3.3 Msun helium stars and from 3.3 - 3.8 Msun in very close orbits (P_orb > 0.25d) will end up in a common-envelope (CE) and spiral-in phase due to the development of a convective helium envelope. If the neutron star has sufficient time to complete the spiraling-in process before the core collapses, the system will produce very tight DNSs (P_orb ~ 0.01d) with a merger timescale of the order of 1 Myr or less. These systems would have important consequences for the detection rate of GWR and for the understanding of GRB progenitors. On the other hand, if the time left until the explosion is shorter than the orbital-decay timescale, the system will undergo a SN explosion during the CE phase. Helium stars with masses 3.3 - 3.8 Msun in wider orbits (P_orb > 0.25d) and those more massive than 3.8 Msun do not go through CE evolution. The remnants of these massive helium stars are DNSs with periods in the range of 0.1 - 1 d. This suggests that this range of mass includes the progenitors of the galactic DNSs with close orbits (B1913+16 and B1534+12). A minimum kick velocity of 70 km/s and 0 km/s (for B1913+16 and B1534+12, respectively) must have been imparted at the birth of the pulsar's companion. The DNSs with wider orbits (J1518+4904 and probably J1811-1736) are produced from helium star-neutron star binaries which avoid RLOF, with the helium star more massive than 2.5 Msun. For these systems the minimum kick velocities are 50 km/s and 10 km/s (for J1518+4904 and J1811-1736, respectively).Comment: 16 pages, latex, 12 figures, accepted for publication in MNRA

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