14,786 research outputs found

    Starburst-driven galactic winds: I. Energetics and intrinsic X-ray emission

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    We have performed an extensive hydrodynamical parameter study of starburst-driven galactic winds, motivated by the latest observation data on the best-studied starburst galaxy M82. We study how the wind dynamics, morphology and X-ray emission depend on the host galaxy's ISM distribution, starburst star formation history and strength, and presence and distribution of mass-loading by dense clouds. We find that the soft X-ray emission from galactic winds comes from low filling factor (ff < 2 per cent) gas, which contains only a small fraction (f < 10 per cent) of the mass and energy of the wind, irrespective of whether the wind models are strongly mass-loaded or not. X-ray observations of galactic winds therefore do not directly probe the gas that contains the majority of the energy, mass or metal-enriched gas in the outflow. The soft X-ray emission comes from gas at a wide range different temperatures and densities. Estimates of the physical properties of the hot gas in starburst galaxies, based on fitting the standard simple spectral models to existing X-ray spectra, should therefore be treated with extreme suspicion. The majority of the thermal and kinetic energy of these winds is in a volume filling hot, T approx 10^7 K, component which is extremely difficult to probe observationally due to its low density and hence low emissivity. Most of the total energy is in the kinetic energy of this hot gas, a factor which must be taken into account when attempting to constrain wind energetics observationally. We also find that galactic winds are efficient at transporting large amounts of energy out of the host galaxy, in contrast to their inefficiency at transporting mass out of star-forming galaxies. (Abridged)Comment: Accepted for publication in MNRAS. Letter page size postscript available from http://adcam.pha.jhu.edu/~dks/dks_published.htm

    The dominant X-ray wind in massive star binaries

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    We investigate which shocked wind is responsible for the majority of the X-ray emission in colliding wind binaries, an issue where there is some confusion in the literature, and which we show is more complicated than has been assumed. We find that where both winds rapidly cool (typically close binaries), the ratio of the wind speeds is often more important than the momentum ratio, because it controls the energy flux ratio, and the faster wind is generally the dominant emitter. When both winds are largely adiabatic (typically long-period binaries), the slower and denser wind will cool faster and the stronger wind generally dominates the X-ray luminosity.Comment: 4 pages, 1 figure, accepted by A&A Letter

    GMRT Low Frequency Observations of Extrasolar Planetary Systems

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    Extrasolar planets are expected to emit detectable low frequency radio emission. In this paper we present results from new low frequency observations of two extrasolar planetary systems (Epsilon Eridani and HD 128311) taken at 150 MHz with the Giant Metrewave Radio Telescope (GMRT). These two systems have been chosen because the stars are young (with ages < 1 Gyr) and are likely to have strong stellar winds, which will increase the expected radio flux. The planets are massive (presumably) gas giant planets in longer period orbits, and hence will not be tidally locked to their host star (as is likely to be the case for short period planets) and we would expect them to have a strong planetary dynamo and magnetic field. We do not detect either system, but are able to place tight upper limits on their low frequency radio emission, at levels comparable to the theoretical predictions for these systems. From these observations we have a 2.5sigma limit of 7.8 mJy for Epsilon Eri and 15.5 mJy for HD 128311. In addition, these upper limits also provide limits on the low frequency radio emission from the stars themselves. These results are discussed and also the prospects for the future detection of radio emission from extrasolar planets.Comment: 6 pages, 2 figures, accepted for publication in MNRA

    Probing the wind-wind collision in Gamma Velorum with high-resolution Chandra X-ray spectroscopy: evidence for sudden radiative braking and non-equilibrium ionization

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    We present a new analysis of an archived Chandra HETGS X-ray spectrum of the WR+O colliding wind binary Gamma Velorum. The spectrum is dominated by emission lines from astrophysically abundant elements: Ne, Mg, Si, S and Fe. From a combination of broad-band spectral analysis and an analysis of line flux ratios we infer a wide range of temperatures in the X-ray emitting plasma (~4-40 MK). As in the previously published analysis, we find the X-ray emission lines are essentially unshifted, with a mean FWHM of 1240 +/- 30 km/s. Calculations of line profiles based on hydrodynamical simulations of the wind-wind collision predict lines that are blueshifted by a few hundred km/s. The lack of any observed shift in the lines may be evidence of a large shock-cone opening half-angle (> 85 degrees), and we suggest this may be evidence of sudden radiative braking. From the R and G ratios measured from He-like forbidden-intercombination-resonance triplets we find evidence that the Mg XI emission originates from hotter gas closer to the O star than the Si XIII emission, which suggests that non-equilibrium ionization may be present.Comment: 22 pages, 14 figures. Accepted for publication in MNRA

    A three dimensional model of the Venusian thermosphere with superrotation

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    An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory

    Variations of the amplitudes of oscillation of the Be star Achernar

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    We report on finding variations in amplitude of the two main oscillation frequencies found in the Be star Achernar, over a period of 5 years. They were uncovered by analysing photometric data of the star from the SMEI instrument. The two frequencies observed, 0.775 c/d and 0.725 c/d, were analysed in detail and their amplitudes were found to increase and decrease significantly over the 5-year period, with the amplitude of the 0.725 c/d frequency changing by up to a factor of eight. The nature of this event has yet to be properly understood, but the possibility of it being due to the effects of a stellar outburst or a stellar cycle are discussed.Comment: 6 pages, 6 figures, 1 table, to be published in MNRA

    Strange Hadron Spectroscopy with a Secondary KL Beam at GlueX

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    We propose to create a secondary beam of neutral kaons in Hall D at Jefferson Lab to be used with the GlueX experimental setup for strange hadron spectroscopy. A flux on the order of 3 x 10^4 KL/s will allow a broad range of measurements to be made by improving the statistics of previous data obtained on hydrogen targets by three orders of magnitude. Use of a deuteron target will provide first measurements on the neutron which is {\it terra incognita}. The experiment will measure both differential cross sections and self-analyzed polarizations of the produced {\Lambda}, {\Sigma}, {\Xi}, and {\Omega} hyperons using the GlueX detector at the Jefferson Lab Hall D. The measurements will span c.m. cos{\theta} from -0.95 to 0.95 in the c.m. range above W = 1490 MeV and up to 3500 MeV. These new GlueX data will greatly constrain partial-wave analyses and reduce model-dependent uncertainties in the extraction of strange resonance properties (including pole positions), and provide a new benchmark for comparisons with QCD-inspired models and lattice QCD calculations. The proposed facility will also have an impact in the strange meson sector by providing measurements of the final-state K{\pi} system from threshold up to 2 GeV invariant mass to establish and improve on the pole positions and widths of all K*(K{\pi}) P-wave states as well as for the S-wave scalar meson {\kappa}(800).Comment: 97 pages, 63 figures, Proposal for JLab PAC45, PR12-17-001; v3 missed citation in Sec 9 (pg 22

    Asteroseismology of red giants: photometric observations of Arcturus by SMEI

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    We present new results on oscillations of the K1.5 III giant Arcturus (alpha Boo), from analysis of just over 2.5 yr of precise photometric observations made by the Solar Mass Ejection Imager (SMEI) on board the Coriolis satellite. A strong mode of oscillation is uncovered by the analysis, having frequency 3.51+/-0.03 micro-Hertz. By fitting its mode peak, we are able offer a highly constrained direct estimate of the damping time (tau = 24+/-1 days). The data also hint at the possible presence of several radial-mode overtones, and maybe some non-radial modes. We are also able to measure the properties of the granulation on the star, with the characteristic timescale for the granulation estimated to be 0.50+/-0.05 days.Comment: 6 pages, 5 figures; accepted for publication in MNRAS Letter

    Energy-dependent evolution in IC10 X-1: hard evidence for an extended corona and implications

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    We have analyzed a ~130 ks XMM-Newton observation of the dynamically confirmed black hole + Wolf-Rayet (BH+WR) X-ray binary (XB) IC10 X-1, covering ~1 orbital cycle. This system experiences periodic intensity dips every ~35 hr. We find that energy-independent evolution is rejected at a >5σ level. The spectral and timing evolution of IC10 X-1 are best explained by a compact disk blackbody and an extended Comptonized component, where the thermal component is completely absorbed and the Comptonized component is partially covered during the dip. We consider three possibilities for the absorber: cold material in the outer accretion disk, as is well documented for Galactic neutron star (NS) XBs at high inclination; a stream of stellar wind that is enhanced by traveling through the L1 point; and a spherical wind. We estimated the corona radius (r ADC) for IC10 X-1 from the dip ingress to be ~106 km, assuming absorption from the outer disk, and found it to be consistent with the relation between r ADC and 1-30 keV luminosity observed in Galactic NS XBs that spans two orders of magnitude. For the other two scenarios, the corona would be larger. Prior BH mass (M BH) estimates range over 23-38 M ☉, depending on the inclination and WR mass. For disk absorption, the inclination, i, is likely to be ~60-80°, with M BH ~ 24-41 M ☉. Alternatively, the L1-enhanced wind requires i ~ 80°, suggesting ~24-33 M ☉. For a spherical absorber, i ~ 40°, and M BH ~ 50-65 M ☉
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