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

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

A past discharges assimilation system for ensemble streamflow forecasts over France – Part 1: Description and validation of the assimilation system

International audienceTwo Ensemble Streamflow Prediction Systems (ESPSs) have been set up at M´et´eo-France. They are based on the French SIM distributed hydrometeorological model. A deterministic analysis run of SIM is used to initialize the two ESPSs. In order to obtain a better initial state, a past discharges assimilation system has been implemented into this analysis SIM run, using the Best Linear Unbiased Estimator (BLUE). Its role is to improve the model soil moisture by using streamflow observations in order to better simulate streamflow. The skills of the assimilation system were assessed for a 569-day period on six different configurations, including two different physics schemes of the model (the use of an exponential profile of hydraulic conductivity or not) and, for each one, three different ways of considering the model soil moisture in the BLUE state variables. Respect of the linearity hypothesis of the BLUE was verified by assessing of the impact of iterations of the BLUE. The configuration including the use of the exponential profile of hydraulic conductivity and the combination of the moisture of the two soil layers in the state variable showed a significant improvement of streamflow simulations. It led to a significantly better simulation than the reference one, and the lowest soil moisture corrections. These results were confirmed by the study of the impacts of the past discharge assimilation system on a set of 49 independent stations

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 Role of Helium Stars in the Formation of Double Neutron Stars

We have calculated the evolution of 60 model binary systems consisting of helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star companion to investigate the formation of double neutron star systems.Orbital periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe overflow starting from the helium main sequence to after the ignition of carbon burning in the core. We have also examined the evolution into a common envelope phase via secular instability, delayed dynamical instability, and the consequence of matter filling the neutron star's Roche lobe. The survival of some close He-star neutron-star binaries through the last mass transfer episode (either dynamically stable or unstable mass transfer phase) leads to the formation of extremely short-period double neutron star systems (with P<~0.1days). In addition, we find that systems throughout the entire calculated mass range can evolve into a common envelope phase, depending on the orbital period at the onset of mass transfer. The critical orbital period below which common envelope evolution occurs generally increases with M_He. In addition, a common envelope phase may occur during a short time for systems characterized by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun). The existence of a short-period population of double neutron stars increases the predicted detection rate of inspiral events by ground-based gravitational-wave detectors and impacts their merger location in host galaxies and their possible role as gamma-ray burst progenitors. We use a set of population synthesis calculations and investigate the implications of the mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592 n1 July 20, 200

A past discharge assimilation system for ensemble streamflow forecasts over France – Part 2: Impact on the ensemble streamflow forecasts

International audienceThe use of ensemble streamflow forecasts is developing in the international flood forecasting services. Ensemble streamflow forecast systems can provide more accurate forecasts and useful information about the uncertainty of the forecasts, thus improving the assessment of risks. Nevertheless, these systems, like all hydrological forecasts, suffer from errors on initialization or on meteorological data, which lead to hydrological prediction errors. This article, which is the second part of a 2-part article, concerns the impacts of initial states, improved by a streamflow assimilation system, on an ensemble streamflow prediction system over France. An assimilation system was implemented to improve the streamflow analysis of the SAFRAN-ISBAMODCOU (SIM) hydro-meteorological suite, which initializes the ensemble streamflow forecasts at M´et´eo-France. This assimilation system, using the Best Linear Unbiased Estimator (BLUE) and modifying the initial soil moisture states, showed an improvement of the streamflow analysis with low soil moisture increments. The final states of this suite were used to initialize the ensemble streamflow forecasts of M´et´eo-France, which are based on the SIM model and use the European Centre for Medium-range Weather Forecasts (ECMWF) 10-day Ensemble Prediction System (EPS). Two different configurations of the assimilation system were used in this study: the first with the classical SIM model and the second using improved soil physics in ISBA. The effects of the assimilation system on the ensemble streamflow forecasts were assessed for these two configurations, and a comparison was made with the original (i.e. without data assimilation and without the improved physics) ensemble streamflow forecasts. It is shown that the assimilation system improved most of the statistical scores usually computed for the validation of ensemble predictions (RMSE, Brier Skill Score and its decomposition, Ranked Probability Skill Score, False Alarm Rate, etc.), especially for the first few days of the time range. The assimilation was slightly more efficient for small basins than for large ones

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

Multilevel and multiscale drought reanalysis over France with the Safran-Isba-Modcou hydrometeorological suite

Physically-based droughts can be defined as a water deficit in at least one component of the land surface hydrological cycle. The reliance of different activity domains (water supply, irrigation, hydropower, etc.) on specific components of this cycle requires drought monitoring to be based on indices related to meteorological, agricultural, and hydrological droughts. This paper describes a high-resolution retrospective analysis of such droughts in France over the last fifty years, based on the Safran-Isba-Modcou (SIM) hydrometeorological suite. The high-resolution 1958–2008 Safran atmospheric reanalysis was used to force the Isba land surface scheme and the hydrogeological model Modcou. Meteorological droughts are characterized with the Standardized Precipitation Index (SPI) at time scales varying from 1 to 24 months. Similar standardizing methods were applied to soil moisture and streamflow for identifying multiscale agricultural droughts – through the Standardized Soil Wetness Index (SSWI) – and multiscale hydrological droughts, through the Standardized Flow Index (SFI). Based on a common threshold level for all indices, drought event statistics over the 50-yr period – number of events, duration, severity and magnitude – have been derived locally in order to highlight regional differences at multiple time scales and at multiple levels of the hydrological cycle (precipitation, soil moisture, streamflow). Results show a substantial variety of temporal drought patterns over the country that are highly dependent on both the variable and time scale considered. Independent spatio-temporal drought events have then been identified and described by combining local characteristics with the evolution of area under drought. Summary statistics have finally been used to compare past severe drought events, from multi-year precipitation deficits (1989–1990) to short hot and dry periods (2003). Results show that the ranking of drought events depends highly on both the time scale and the variable considered. This multilevel and multiscale drought climatology will serve as a basis for assessing the impacts of climate change on droughts in France

Exceptionally Bright Type Ib Supernova 1991D

Photometric and spectroscopic observations of the peculiar Type Ib Supernova 1991D are presented. SN 1991D was exceptionally bright for a Type Ib supernova. The He I lines were rather weak and the velocity at the photosphere as a function of time was unusually low. Comparison of the observed and synthetic spectra indicates that either hydrogen was ejected with a minimum velocity of 12,000 km/s or the spectrum contained features caused by lines of Ne I. Light curve modelling suggests that the progenitor probably had a very large radius (~10^14 cm) and that a considerable amount of 56Ni was synthesized during the explosion (~0.7 Msolar). We suggest a progenitor model of SN 1991D that involves a binary system.Comment: 7 pages, 9 figures, MNRAS in pres

A New Formation Channel for Double Neutron Stars Without Recycling: Implications for Gravitational Wave Detection

We report on a new evolutionary path leading to the formation of close double neutron stars (NS), with the unique characteristic that none of the two NS ever had the chance to be recycled by accretion. The existence of this channel stems from the evolution of helium-rich stars (cores of massive NS progenitors), which has been neglected in most previous studies of double compact object formation. We find that these non-recycled NS-NS binaries are formed from bare carbon-oxygen cores in tight orbits, with formation rates comparable to or maybe even higher than those of recycled NS-NS binaries. On the other hand, their detection probability as binary pulsars is greatly reduced (by about 1000) relative to recycled pulsars, because of their short lifetimes. We conclude that, in the context of gravitational-wave detection of NS-NS inspiral events, this new type of binaries calls for an increase of the rate estimates derived from the observed NS-NS with recycled pulsars, typically by factors of 1.5-3 or even higher.Comment: Accepted for publication in ApJ Letters; 5 pages, 1 figure, 2 tables. Two new paragraphs and one formula adde