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Krticka J.,Masaryk University | Kubat J.,Astronomicky ustav AV CR
Monthly Notices of the Royal Astronomical Society | Year: 2012

Different diagnostics of hot star wind mass-loss rates provide results that are difficult to reconcile with each other. The widely accepted presence of clumping in hot star winds implies a significant reduction of observational mass-loss rate estimates from diagnostics that depend on the square of the density. Moreover, the ultraviolet Pv resonance lines indicate a possible need for an even stronger reduction of hot star mass-loss rates, provided that Pv is a dominant ionization stage of phosphorus, at least in some hot stars. The latter assumption is challenged by the possible presence of extreme ultraviolet (XUV) radiation. Here, we study the influence of XUV radiation on the Pv ionization fraction in hot star winds. Using a detailed solution of the hydrodynamical radiative transfer and statistical equilibrium equations, we confirm that a sufficiently strong XUV radiation source might decrease the Pv ionization fraction, possibly depreciating the Pv lines as a reliable mass-loss rate indicator. However, the XUV radiation also influences the ionization fraction of heavier ions that drive the wind, leading to a decrease of the wind terminal velocity. Consequently, we conclude that XUV radiation alone cannot bring theory and observations into agreement. We fit our predicted wind mass-loss rates using a suitable formula and we compare the results with the observational mass-loss rate diagnostics. We show that for supergiants and giants the theoretical predictions do not contradict the mass-loss rate estimates based on X-ray line profiles or density-squared diagnostics. However, for main-sequence stars, the predicted mass-loss rates are still significantly higher than those inferred from Pv or X-ray lines. This indicates that the 'weak wind problem' recently detected in low-luminosity main-sequence stars might also occur, to some extent, for stars with higher luminosity. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Hannah I.G.,University of Glasgow | Hudson H.S.,University of California at Berkeley | Battaglia M.,University of Glasgow | Christe S.,NASA | And 4 more authors.
Space Science Reviews | Year: 2011

This review surveys the statistics of solar X-ray flares, emphasising the new views that RHESSI has given us of the weaker events (the microflares). The new data reveal that these microflares strongly resemble more energetic events in most respects; they occur solely within active regions and exhibit high-temperature/nonthermal emissions in approximately the same proportion as major events. We discuss the distributions of flare parameters (e.g., peak flux) and how these parameters correlate, for instance via the Neupert effect. We also highlight the systematic biases involved in intercomparing data representing many decades of event magnitude. The intermittency of the flare/microflare occurrence, both in space and in time, argues that these discrete events do not explain general coronal heating, either in active regions or in the quiet Sun. © 2011 Springer Science+Business Media B.V.

Kontar E.P.,University of Glasgow | Brown J.C.,University of Glasgow | Emslie A.G.,Oklahoma State University | Emslie A.G.,Western Kentucky University | And 12 more authors.
Space Science Reviews | Year: 2011

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future. © 2011 Springer Science+Business Media B.V.

Kawka A.,Astronomicky ustav AV CR | Vennes S.,Astronomicky ustav AV CR | O'Toole S.,Australian Astronomical Observatory | Nemeth P.,University of Bamberg | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We have measured the orbital parameters of seven close binaries, including six new objects, in a radial velocity survey of 38 objects comprising a hot subdwarf star with orbital periods ranging from ~0.17 to 3 d. One new system, GALEX J2205-3141, shows reflection on an M dwarf companion. Three other objects show significant short-period variations, but their orbital parameters could not be constrained. Two systems comprising a hot subdwarf paired with a bright main-sequence/giant companion display short-period photometric variations possibly due to irradiation or stellar activity and are also short-period candidates. All except two candidates were drawn from a selection of subluminous stars in the Galaxy Evolution Explorer ultraviolet sky survey. Our new identifications also include a low-mass subdwarf B star and likely progenitor of a low-mass white dwarf (GALEX J0805-1058) paired with an unseen, possibly substellar, companion. The mass functions of the newly identified binaries imply minimum secondary masses ranging from 0.03 to 0.39M⊙. Photometric time series suggest that, apart from GALEX J0805-1058 and J2205-3141, the companions are most likely white dwarfs. We update the binary population statistics: close to 40 per cent of hot subdwarfs have a companion. Also, we found that the secondary mass distribution shows a lowmass peak attributed to late-type dwarfs, and a higher mass peak and tail distribution attributed to white dwarfs and a few spectroscopic composites. Also, we found that the population kinematics imply an old age and include a few likely halo population members. © 2015 The Authors.

Kawka A.,Astronomicky ustav AV CR | Vennes S.,Astronomicky ustav AV CR
Monthly Notices of the Royal Astronomical Society | Year: 2016

We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT 19868. The spectra obtained with X-shooter on the Very Large Telescope-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near-infrared spectral energy distribution to constrain the atmospheric parameters of NLTT 19868. Our analysis shows that NLTT 19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT 19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT 888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: in the case of NLTT 888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT 19868 it is close to mantle composition depleted by subsequent chemical separation at the bottom of the convection zone. © 2016 The Authors.

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