53,974 research outputs found

    O-star mass-loss rates at low metallicity

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    Mass fluxes J are computed for the extragalactic O stars investigated by Tramper et al. (2011; TSKK). For one early-type O star, computed and observed rates agree within errors. However, for two late-type O stars, theoretical mass-loss rates underpredict observed rates by ~ 1.6 dex, far exceeding observational errors. A likely cause of the discrepancy is overestimated observed rates due to the neglect of wind-clumping. A less likely but intriguing possibility is that, in observing O stars with Z/Z_sun ~ 1/7, TSKK have serendipitously discovered an additional mass-loss mechanism not evident in the spectra of Galactic O stars with powerful radiation-driven winds. Constraints on this unknown mechanism are discussed. In establishing that the discrepancies, if real, are inescapable for purely radiation-driven winds, failed searches for high-J solutions are reported and the importance of a numerical technique that cannot spuriously create or destroy momentum stressed. The Z-dependences of the computed rates for Z/Z_sun in the interval (1/30, 2) show significant departures from a single power law, and these are attributed to curve-of-growth effects in the differentially-expanding reversing layers. The best-fitting power-law exponents range from 0.68-0.97.Comment: 6 pages, 2 figure

    The structure of line-driven winds

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    Following procedures pioneered by Castor, Abbott & Klein (1975, [CAK]), spherically-symmetric supersonic winds for O stars are computed for matching to plane-parallel moving reversing layers (RL's) from Paper I (Lucy 2007). In contrast to a CAK wind, each of these solutions is singularity-free, thus allowing its mass-loss rate to be fixed by the regularity condition at the sonic point within the RL. Moreover, information propagation in these winds by radiative-acoustic waves is everywhere outwardly-directed, justifying the implicit assumption in Paper I that transonic flows are unaffected by inwardly-directed wave motions.Comment: Accepted by A&A; 7 pages, 1 table, 4 figure

    Monte Carlo transition probabilities. II

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    The macroscopic quantizations of matter into macro-atoms and radiant and thermal energies into r- and k-energy packets initiated in Paper I is completed with the definition of transition probabilities governing energy flows to and from the thermal pool. The resulting Monte Carlo method is then applied to the problem of computing the hydrogen spectrum of a Type II supernova. This test problem is used to demonstrate the scheme's consistency as the number of energy packets N -> infinity, to investigate the accuracy of Monte Carlo estimators of radiative rates, and to illustrate the convergence characteristics of the geometry-independent, constrained Lambda-iteration method employed to obtain the NLTE stratifications of temperature and level populations. In addition, the method's potential, when combined with analytic ionization and excitation formulae, for obtaining useful approximate NLTE solutions is emphasized.Comment: 17 pages, 4 figure

    Bayesian inference for orbital eccentricities

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    Highest posterior density intervals (HPDI's) are derived for the true eccentricities of spectroscopic binaries with measured values e ~ 0. These yield upper limits when e is below the detection threshold e_th and seamlessly transform to upper and lower bounds when e > e_th. In the main text, HPDI's are computed with an informative eccentricity prior representing orbital decay due to tidal dissipation. In an appendix, the corresponding HPDI's are computed with a uniform prior and are the basis for a revised version of the Lucy-Sweeney test, with the previous outcome e = 0 now replaced by an upper limit. Sampling experiments with known prior confirm the validity of the HPDI's.Comment: 7 pages, 6 figures. Error in terminology corrected. Results unchanged. Accepted by Astronomy & Astrophysic

    Spectroscopic binaries with elliptical orbits

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    The radial velocity curves of many spectroscopic binaries (SBs) are perturbed by gas streams or proximity effects. For SBs with circular orbits, these perturbations can give rise to spurious orbital eccentricities of high statistical significance. But tests to identify such anomalous orbits can be constructed since perturbed velocity curves are in general no longer Keplerian. The derived tests are applied both to synthetic and to observed velocity curves.Comment: 9pages,3figures,accepted by A&

    Global Impacts Report 2017

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    Our fifth Global Impacts Report reflects on the progress of the MSC over the past 20 years, examines the sustainability performance of certified fisheries around the world and highlights areas of future interest

    Mass fluxes for hot stars

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    In an attempt to understand the extraordinarily small mass-loss rates of late-type O dwarfs, mass fluxes in the relevant part of (T_{eff}, g)-space are derived from first principles using a previously-described code for constructing moving reversing layers. From these mass fluxes, a weak-wind domain is identified within which a star's rate of mass loss by a radiatively-driven wind is less than that due to nuclear burning. The five weak-wind stars recently analysed by Marcolino et al. (2009) fall within or at the edge of this domain. But although the theoretical mass fluxes for these stars are ~ 1.4 dex lower than those derived with the formula of Vink et al. (2000), the observed rates are still not matched, a failure that may reflect our poor understanding of low-density supersonic outflows. Mass fluxes are also computed for two strong-wind O4 stars analysed by Bouret et al. (2005). The predictions agree with the sharply reduced mass loss rates found when Bouret et al. take wind clumping into account.Comment: Accepted by A&A; 6 pages, 5 figures; minor changes from v

    Monte Carlo transition probabilities

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    Transition probabilities governing the interaction of energy packets and matter are derived that allow Monte Carlo NLTE transfer codes to be constructed without simplifying the treatment of line formation. These probabilities are such that the Monte Carlo calculation asymptotically recovers the local emissivity of a gas in statistical equilibrium. Numerical experiments with one-point statistical equilibrium problems for Fe II and Hydrogen confirm this asymptotic behaviour. In addition, the resulting Monte Carlo emissivities are shown to be far less sensitive to errors in the populations of the emitting levels than are the values obtained with the basic emissivity formula.Comment: Improved text. Accepted for publication in A&

    High School Students at Risk: The Challenge of Dropouts and Pushouts

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    Analyzes the factors contributing to high dropout rates at schools in New York, Chicago, and Las Vegas. Explores the available alternatives and program reforms that have been implemented to improve graduation rates at urban schools
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