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    ABSTRACT: The existence of water in extrasolar planetary systems is of great interest because it constrains the potential for habitable planets and life. We have identified a circumstellar disk that resulted from the destruction of a water-rich and rocky extrasolar minor planet. The parent body formed and evolved around a star somewhat more massive than the Sun, and the debris now closely orbits the white dwarf remnant of the star. The stellar atmosphere is polluted with metals accreted from the disk, including oxygen in excess of that expected for oxide minerals, indicating that the parent body was originally composed of 26% water by mass. This finding demonstrates that water-bearing planetesimals exist around A- and F-type stars that end their lives as white dwarfs.
    Science 10/2013; 342(6155):218-220. DOI:10.1126/science.1239447
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    ABSTRACT: Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides an instructive exception, and we present a close-up view of its quiescent x-ray emission based on 3 megaseconds of Chandra observations. Although the x-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission on the basis of the lack of predicted iron (Fe) Kα emission. The extremely weak hydrogen (H)-like Fe Kα line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.
    Science 08/2013; 341(6149):981-3. DOI:10.1126/science.1240755
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    ABSTRACT: Gaia is a cornerstone European Space Agency astrometry space mission and a successor to the Hipparcos mission. Gaia will observe the whole sky for 5 years, providing a serendipitous opportunity for the discovery of large numbers of transient and anomalous events, e.g. supernovae, novae and microlensing events, gamma-ray burst afterglows, fallback supernovae, as well as theoretical or unexpected phenomena. In this paper, we discuss our preparations to use Gaia to search for transients at optical wavelengths, and briefly describe the early detection, classification and prompt publication of anomalous sources.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 06/2013; 371(1992):20120239. DOI:10.1098/rsta.2012.0239
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    ABSTRACT: We have analysed all the good quality XMM-Newton data publicly available for the bright ULXs Holmberg IX X-1 and NGC 1313 X-1, with the aim of searching for discrete emission or absorption features in the Fe K band that could provide observational evidence for the massive outflows predicted if these sources are accreting at substantially super-Eddington rates. We do not find statistically compelling evidence for any atomic lines, and the limits that are obtained have interesting consequences. Any features in the immediate Fe K energy band (6-7 keV) must have equivalent widths weaker than ~30 eV for Holmberg IX X-1, and weaker than ~50 eV for NGC 1313 X-1 (at 99 per cent confidence). In comparison to the sub-Eddington outflows observed in GRS 1915+105, which imprint iron absorption features with equivalent widths of ~30 eV, the limits obtained here appear quite stringent, particularly when Holmberg IX X-1 and NGC 1313 X-1 must be expelling at least 5-10 times as much material if they host black holes of similar masses. The difficulty in reconciling these observational limits with the presence of strong line-of-sight outflows suggests that either these sources are not launching such outflows, or that they must be directed away from our viewing angle.
    Monthly Notices of the Royal Astronomical Society 07/2012; 426(1). DOI:10.1111/j.1365-2966.2012.21727.x
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    ABSTRACT: Abell 2029 Walker et al. (2012a), MNRAS, 422, 3503 (arXiv:1203.0486) PKS 0745-191 Walker et al. (2012b), MNRAS in press (arXiv:1205.2276) Using Suzaku observations (Fig. 1) we explore the ICM of the galaxy cluster Abell 2029 (z=0.0767) to higher radius than before, with no statistically significant emission detected beyond 22′ (except for the northern pointing between Abell 2029 and Abell 2033), which is 1.9Mpc, and is equal to our measurement of r 200 . Excess emission between Abell 2029 and Abell 2033 appears to originate from Abell 2033 indicating we are seeing the overlap of the outskirts of the two clusters to the north, supported by the galaxy overdensity map (compare Figs. 1 and 3). We detect a cold feature to the SE extending out to the edge of the detected cluster (22′) where the ICM is significantly colder than in the other directions, consistent with the XMM-Newton findings of Bourdin & Mazzotta (2008) which found a temperature depression to the south east within the central 8′ (right column of Fig. 2). The lower temperature causes the entropy profile to be lower and to flatten, indicating that the ICM in this direction is significantly out of hydrostatic equilibrium, possibly the result of the accretion of galaxy groups along the SE direction, which has disturbed the ICM.
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    ABSTRACT: We present observational evidence that leakage of ionising photons from star-forming regions can affect the quantification of the star formation rate (SFR) in galaxies. This effect could partially explain the differences between the SFR estimates using the far ultraviolet (FUV) and the Halpha emission. We find that leakage could decrease the SFR(Ha)/SFR(FUV) ratio by up to a 25 per cent. The evidence is based on the observation that the SFR(Ha)/SFR(FUV) ratio is lower for objects showing a shell Halpha structure than for regions exhibiting a much more compact morphology. The study has been performed on three object samples: low luminosity dwarf galaxies from the Local Volume Legacy survey and star-forming regions in the Large Magellanic Cloud and the nearby Local Group galaxy M33. For the three samples we find differences (1.1-1.4sigma) between the SFR(Ha)/SFR(FUV) for compact and shell objects. Although leakage cannot entirely explain the observed trend of SFR(Ha)/SFR(FUV) ratios for systems with low SFR, we show the mechanism can lead to different SFR estimates when using Halpha and FUV luminosities. Therefore, further study is needed to constrain the contribution of leakage to the low SFR(Ha)/SFR(FUV) ratios observed in dwarf galaxies and its impact on the Halpha flux as a SFR indicator in such objects.
    Monthly Notices of the Royal Astronomical Society 04/2012; 423(3). DOI:10.1111/j.1365-2966.2012.21107.x
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    ABSTRACT: In the hot, dense plasma of solar and stellar interiors, the Coulomb interaction is screened by the surrounding plasma. Although the standard Salpeter approximation for static screening is widely accepted and used in stellar modeling, the question of dynamic screening has been revisited. In particular, Shaviv and Shaviv apply the techniques of molecular dynamics to the conditions in the solar core in order to numerically determine the dynamic screening effect. By directly calculating the motion of ions and electrons due to Coulomb interactions, they compute the effect of screening without the mean-field assumption inherent in the Salpeter approximation. Here we reproduce their numerical analysis of the screening energy in the plasma of the solar core and conclude that the effects of dynamic screening are relevant and should be included in the treatment of the plasma, especially in the computation of stellar nuclear reaction rates. Equation of state-Nuclear reactions-Nucleosynthesis-Abundances-Plasmas-Sun: general
    Astrophysics and Space Science 02/2012; 328(1):153-156. DOI:10.1007/s10509-009-0245-x
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    ABSTRACT: The core collapse supernova (CC SN) rate provides a strong lower limit for the star formation rate (SFR). Progress in using it as a cosmic SFR tracer requires some confidence that it is consistent with more conventional SFR diagnostics. We compare standard SFR measurements based on Halpha , Far Ultraviolet (FUV) and Total Infrared (TIR) galaxy luminosities with the observed CC SN rate in the same galaxy sample. The comparison can be viewed from two perspectives. Firstly, by adopting an estimate of the minimum stellar mass to produce a CC SN one can determine a SFR from SN numbers. Secondly, the radiative SFRs can be assumed to be robust and then the SN statistics provides a constraint on the minimum stellar mass for CC SN progenitors. We exploit the multi-wavelength data set from 11HUGS, a volume-limited survey designed to provide a census of SFR in the local Volume. There are 14 SNe discovered in this sample of galaxies within the last 13 years. Assuming a lower limit for CC SN progenitor of 8 M⊙, the CC SN rate matches the SFR from the FUV luminosity. However, the SFR based on mbox {M_{B}}ox{H\alpha} luminosity is lower than these two estimates by a factor of about 2. If we assume that the FUV or mbox {M_{B}}ox{H\alpha} based luminosities are a true reflection of the SFR, we find that the minimum mass for CC SN progenitors is 8 ±1 M⊙, and 6± 1 M⊙, respectively.
    01/2012;
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    ABSTRACT: We study the relationship between coronal X-ray emission and stellar age for late-type stars, and the variation of this relationship with spectral type. We select 717 stars from 13 open clusters and find that the ratio of X-ray to bolometric luminosity during the saturated phase of coronal emission decreases from 10^-3.1 for late K-dwarfs to 10^-4.3 for early F-type stars (across the range 0.29<(B-V)_0<1.41). Our determined saturation timescales vary between 10^7.6 and 10^8.3 years, though with no clear trend across the whole FGK range. We apply our X-ray emission - age relations to the investigation of the evaporation history of 121 known transiting exoplanets using a simple energy-limited model of evaporation and taking into consideration Roche lobe effects and different heating/evaporation efficiencies. We confirm that a linear cut-off of the planet distribution in the M^2/R^3 versus a^-2 plane is an expected result of population modification by evaporation and that the known transiting exoplanets display such a cut-off. We find that for an evaporation efficiency of 25% we expect around 1 in 10 of the known transiting exoplanets to have lost > 5% of their mass since formation. In addition we provide estimates of the minimum formation mass for which a planet could be expected to survive for 4 Gyrs for a range of stellar and planetary parameters. We emphasise the importance of the earliest periods of a planet's life for its evaporation history with 75% expected to occur within the first Gyr. This raises the possibility of using evaporation histories to distinguish between different migration scenarios. For planets with available spin-orbit angles no difference is found between the distributions of planets with misaligned orbits and those with aligned orbits. This suggests that misalignment occurs early in the life of the planetary system, though more data is needed.
    Monthly Notices of the Royal Astronomical Society 10/2011; 422(3). DOI:10.1111/j.1365-2966.2012.20657.x
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    ABSTRACT: We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle θ between them obeys cos θ < −Jd/2Jb, on a time short compared with the mass gain time of the central accretor(s). The magnitude Jb remains unchanged in this process. Counteralignment can promote the rapid merger of supermassive black hole binaries, and possibly the formation of coplanar but retrograde planets around stars in binary systems.
    Monthly Notices of the Royal Astronomical Society Letters 09/2011; 417(1):L66 - L69. DOI:10.1111/j.1745-3933.2011.01121.x
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