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    ABSTRACT: Na+,K+/H+ antiporters are H+-coupled cotransporters that are crucial for cellular homeostasis. Populus euphratica, a well-known tree halophyte, contains six Na+/H+ antiporter genes (PeNHX1-6) that have been shown to function in salt tolerance. However, the catalytic mechanisms governing their ion transport remain largely unknown. Using the crystal structure of the Na+/H+ antiporter from the Escherichia coli (EcNhaA) as a template, we built the three-dimensional structure of PeNHX3 from P. euphratica. The PeNHX3 model displays the typical TM4-TM11 assembly that is critical for ion binding and translocation. The PeNHX3 structure follows the 'positive-inside' rule and exhibits a typical physicochemical property of the transporter proteins. Four conserved residues, including Tyr149, Asn187, Asp188, and Arg356, are indentified in the TM4-TM11 assembly region of PeNHX3. Mutagenesis analysis showed that these reserved residues were essential for the function of PeNHX3: Asn187 and Asp188 (forming a ND motif) controlled ion binding and translocation, and Tyr149 and Arg356 compensated helix dipoles in the TM4-TM11 assembly. PeNHX3 mediated Na+, K+ and Li+ transport in a yeast growth assay. Domain-switch analysis shows that TM11 is crucial to Li+ transport. The novel features of PeNHX3 in ion binding and translocation are discussed.
    PLoS ONE 08/2014; 9(8):e104147. DOI:10.1371/journal.pone.0104147
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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 a uniquely instructive exception, and we present a close-up view of its quiescent X-ray emission based on 3 mega-second 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 based on the lack of predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha 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: submitted to MNRAS We study the origin and properties of satellite galaxies within Milky Way sized haloes as predicted in Cold Dark Matter based models of galaxy formation, making use of dissipationless and hydrodynamic numerical N-body techniques as well as three different semi-analytic model (SAMs) galaxy formation codes. We extract merger trees from very high-resolution dissipationless N-body simulations of four Galaxy-sized DM haloes, and use these as common input for the semi-analytic models. We present a detailed comparison of our predictions with the observational data recently obtained on the Milky Way satellite luminosity function (LF). We find that semi-analytic models with rather standard astrophysical ingredients are able to reproduce the observed luminosity function over six orders of magnitude in luminosity, down to magnitudes as faint as MV = −2. We also perform a comparison with the actual observed number of satellites as a function of luminosity, by applying the selection criteria of the SDS survey to our simulations instead of correcting the observations for incompleteness.
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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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