Exploring the Nature of Weak Chandra Sources near the Galactic Centre

We present results from the first near-IR imaging of the weak X-ray sources discovered in the Chandra/ACIS-I survey (Wang et al. 2002) towards the Galactic Centre (GC). These ~800 discrete sources, which contribute significantly to the GC X-ray emission, represent an important and previously unknown population within the Galaxy. From our VLT observations we will identify likely IR counterparts to a sample of the hardest sources, which are most likely X-ray binaries. With these data we can place constraints on the nature of the discrete weak X-ray source population of the GC.Comment: In Proc. of ``Interacting Binaries: Accretion, Evolution, and Outcomes'', eds. L. A. Antonelli et al., AIP, Cefalu, Sicily, 200

From climatological to small-scale applications: simulating water isotopologues with ICON-ART-Iso (version 2.3)

We present the new isotope-enabled model ICON-ART-Iso. The physics package of the global ICOsahedral Nonhydrostatic (ICON) modeling framework has been extended to simulate passive moisture tracers and the stable isotopologues HDO and H182O. The extension builds on the infrastructure provided by ICON-ART, which allows for high flexibility with respect to the number of related water tracers that are simulated. The physics of isotopologue fractionation follow the model COSMOiso. We first present a detailed description of the physics of fractionation that have been implemented in the model. The model is then evaluated on a range of temporal scales by comparing with measurements of precipitation and vapor. A multi-annual simulation is compared to observations of the isotopologues in precipitation taken from the station network GNIP (Global Network for Isotopes in Precipitation). ICON-ART-Iso is able to simulate the main features of the seasonal cycles in δD and δ18O as observed at the GNIP stations. In a comparison with IASI satellite retrievals, the seasonal and daily cycles in the isotopologue content of vapor are examined for different regions in the free troposphere. On a small spatial and temporal scale, ICON-ART-Iso is used to simulate the period of two flights of the IAGOS-CARIBIC aircraft in September 2010, which sampled air in the tropopause region influenced by Hurricane Igor. The general features of this sample as well as those of all tropical data available from IAGOS-CARIBIC are captured by the model. The study demonstrates that ICON-ART-Iso is a flexible tool to analyze the water cycle of ICON. It is capable of simulating tagged water as well as the isotopologues HDO and H182

Is our Sun a Singleton?

Most stars are formed in a cluster or association, where the number density of stars can be high. This means that a large fraction of initially-single stars will undergo close encounters with other stars and/or exchange into binaries. We describe how such close encounters and exchange encounters can affect the properties of a planetary system around a single star. We define a singleton as a single star which has never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, often leaving planets on tighter and more eccentric orbits. Thus planetary systems which initially resembled our own solar system may later more closely resemble some of the observed exoplanet systems.Comment: 2 pages, 1 figure. To be published in the proceedings of IAUS246 "Dynamical Evolution of Dense Stellar Systems". Editors: E. Vesperini (Chief Editor), M. Giersz, A. Sill

Green Bank Telescope Measurement of the Systemic Velocity of the Double Pulsar Binary J0737-3039 and Implications for its Formation

We report on the measurement at 820- and 1400-MHz of orbital modulation of the diffractive scintillation timescale from pulsar A in the double-pulsar system J0737-3039 using the Green Bank Telescope. Fits to this modulation determine the systemic velocity in the plane of the sky to be V_iss ~ 140.9 +/- 6.2 km/s. The parallel and perpendicular components of this velocity with respect to the line of nodes of the pulsar's orbit are V_plane ~ 96.0 +/- 3.7 km/s and V_perp ~ 103.1 +/- 7.7 km/s respectively. The large V_perp implies that pulsar B was born with a kick speed of >~ 100 km/s. Future VLBA determination of the angular proper motion in conjunction with improved V_iss measurements should provide a precise distance to the system. Using high-precision timing data and the V_iss model, we estimate a best-fit orbital inclination of i = 88.7 +/- 0.9 deg.Comment: 5 pages, 3 figures. To be submitted to ApJ Letters. A version with higher resolution figures is available upon request from S. Ranso

Detection Of KOI-13.01 Using The Photometric Orbit

We use the KOI-13 transiting star-planet system as a test case for the recently developed BEER algorithm (Faigler & Mazeh 2011), aimed at identifying non-transiting low-mass companions by detecting the photometric variability induced by the companion along its orbit. Such photometric variability is generated by three mechanisms, including the beaming effect, tidal ellipsoidal distortion, and reflection/heating. We use data from three Kepler quarters, from the first year of the mission, while ignoring measurements within the transit and occultation, and show that the planet's ephemeris is clearly detected. We fit for the amplitude of each of the three effects and use the beaming effect amplitude to estimate the planet's minimum mass, which results in M_p sin i = 9.2 +/- 1.1 M_J (assuming the host star parameters derived by Szabo et al. 2011). Our results show that non-transiting star-planet systems similar to KOI-13.01 can be detected in Kepler data, including a measurement of the orbital ephemeris and the planet's minimum mass. Moreover, we derive a realistic estimate of the amplitudes uncertainties, and use it to show that data obtained during the entire lifetime of the Kepler mission, of 3.5 years, will allow detecting non-transiting close-in low-mass companions orbiting bright stars, down to the few Jupiter mass level. Data from the Kepler Extended Mission, if funded by NASA, will further improve the detection capabilities.Comment: Accepted to AJ on October 4, 2011. Kepler Q5 Long Cadence data will become publicly available on MAST by October 23. Comments welcome (V2: minor changes, to reflect proof corrections

Exploring the Nature of Weak Chandra Sources near the Galactic Centre

We present early results from the first IR imaging of the weak X-ray sources discovered in a recent Chandra survey towards the Galactic Centre. From our VLT observations we will identify likely counterparts to a sample of the hardest sources in order to place constraints on the nature of this previously unknown population.Comment: To appear in RevMexAA(SC) Conference Series, Proc. of IAU Colloquium 194, Compact Binaries in the Galaxy and Beyond, La Paz, Mexico, 17-21 Nov., 200

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

Fragmentation of Collapsar Disks and the Production of Gravitational Waves

We argue that gravitational instability in the outer parts of collapsar disks may lead to fragmentation near the radius where helium photodisintegrates, because of the strong cooling provided by this process. This physics sets clear physical scales for the fragmentation conditions and the properties of gravitationally bound clumps. Collapse of a fragment proceeds until the neutrons become degenerate; a neutron star of mass ~0.1-1Msun may result. We find that tidal disruption of a fragment and accretion by the central black hole are too rapid to account for the durations of observed X-ray flares from long gamma-ray bursts. Prior to disruption, migration of the fragment is driven by gravitational radiation and disk viscosity, which act together to produce a unique gravitational-wave signature. Advanced LIGO may be able to detect such sources within ~100 Mpc.Comment: Accepted for publication in The Astrophysical Journal, 5 pages, 2 figures; added noise estimates for enhanced LIGO and expanded discussion of rate

The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks

We systematically examine how the presence in a binary affects the final core structure of a massive star and its consequences for the subsequent supernova explosion. Interactions with a companion star may change the final rate of rotation, the size of the helium core, the strength of carbon burning and the final iron core mass. Stars with initial masses larger than \sim 11\Ms that experiece core collapse will generally have smaller iron cores at the time of the explosion if they lost their envelopes due to a previous binary interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger helium and metal cores if they have a close companion, since the second dredge-up phase which reduces the helium core mass dramatically in single stars does not occur once the hydrogen envelope is lost. We find that the initially more massive stars in binary systems with masses in the range 8 - 11\Ms are likely to undergo an electron-capture supernova, while single stars in the same mass range would end as ONeMg white dwarfs. We suggest that the core collapse in an electron-capture supernova (and possibly in the case of relatively small iron cores) leads to a prompt explosion rather than a delayed neutrino-driven explosion and that this naturally produces neutron stars with low-velocity kicks. This leads to a dichotomous distribution of neutron star kicks, as inferred previously, where neutron stars in relatively close binaries attain low kick velocities. We illustrate the consequences of such a dichotomous kick scenario using binary population synthesis simulations and discuss its implications. This scenario has also important consequences for the minimum initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio