Wind variability of B supergiants (original) (raw)
Related papers
The Astrophysical Journal, 1999
Empirical ionization fractions of C IV, N V, Si IV, and empirical ionization plus excitation fractions of C III* and N IV* in the winds of 34 O stars and one B star have been derived. We combine the mass-loss rates derived from radio measurements and Ha with the line Ðtting of ultraviolet resonance lines and subordinate lines using the Sobolev plus exact integration (SEI) method. The dependence of the empirical ionization fractions, SqT, on the stellar e †ective temperature and on the mean wind density is dis
Measuring the Ionization of O Star Winds
The Astrophysical Journal, 2000
We present an analysis of wind line profiles from Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of two O7 supergiants in the Large and Small Magellanic Clouds (Sk Ϫ67Њ111 and AV 232, respectively). Model fits yield the column densities of S iv, S vi, P iv, P v, N iii, and N iv, providing the first direct measurement of the ionization balance in stellar winds. The ratios of S iv/S vi and P iv/P v are consistently lower in the LMC star. IUE and Hubble Space Telescope archival spectra are also used to measure N iv and N v, but the much higher optical depth makes the N v measurements inconclusive. The velocity and optical depth distributions in the wind are qualitatively similar between the two stars, when scaled to their terminal velocities. The terminal velocities are different, with AV 232 being lower (as found previously in SMC stars and linked to lower metallicity). These are the first results from a program to investigate wind ionization and velocity structure among hot stars in local galaxies, and they demonstrate the higher accuracy in measuring column densities of less abundant ions, such as phosphorus and sulfur, observable in the FUSE range. Subject headings: Magellanic Clouds-stars: early-type-stars: winds, outflows-ultraviolet: stars 1 Observer with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE), which is operated by Johns Hopkins University under NASA contract NAS5-32985.
Ultraviolet Ionization Stratification in Wolf-Rayet Winds
Acta Astronomica Warsaw and Cracow, 1997
Ionization stratification can be used to study the structure of WR winds. In an accelerated outflow the stratification is observable through the Doppler effect as an inverse correlation of ionization potential (IP) with line width (FWHM). However, not only the average line widths of different ions, but also the line widths of one series of HeII show stratification. The evidence of both effects is demonstrated here as a part of ongoing study. The ultraviolet spectra of WR stars obtained from the IUE archive are used to get the IP vs. FWHM diagrams as well as the principal quantum number n of HeII (n ? 3) transitions vs. FWHM velocity relations. A systematic insight into stratification in HeII lines is provided on the basis of observations.
Wind variability in early-type stars
1993
We present a quantitative study of the variability in ultraviolet resonance lines of N v, Si IV, and CIV of the Be star 7 Cas. For this purpose we used IUE spectra obtained over a period of eleven years. Variability occurs in the form of discrete absorption components (DACs), which are formed in the fast-outflowing radiatively driven part of the stellar wind. We constructed a template spectrum from spectra containing no or minor extra absorption due to DACs and modelled the isolated DACs in the obtained quotient spectra. Besides the frequently observed narrow components (v t typically < 250 km s" 1) at high velocity, we found several broad components occurring at low and intermediate wind velocities. We confirm the finding of Doazan et al. (1987) who reported that the number of observed DACs is associated with the cyclic V/R variability of the Balmer-emission lines. We show that when V/R<1 the central optical depth of DACs is significantly lower than when V/R>1. In our interpretation this is due to a correlation between the column density associated with the DACs and the phase of the V/R cycle. We find that the H/3 observations of Doazan et al. are consistent with a model in which the cyclic V/R variability is due to a global, one-armed oscillation moving through an equatorial disc. We suggest that the higher column density of DACs in phases of V/R>1 is the result of the higher density in the region of their origin, namely close to or in the part of the equatorial disc which is rotating towards the observer.
HST/COS Spectra of the Wind Lines of VFTS 102 and 285
The Astrophysical Journal
Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively driven winds that can lead to enhanced mass loss at the poles (with little angular momentum loss) and/or equator (with maximal angular momentum loss). Here we present an examination of the stellar wind lines of the two O-type stars with the fastest known equatorial velocities, VFTS 102 (V sin i = 610 ± 30 km s −1 ; O9: Vnnne+) and VFTS 285 (V sin i = 609 ± 29 km s −1 ; O7.5 Vnnn) in the Large Magellanic Cloud. Ultraviolet spectra of both stars were obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. The spectrum of VFTS 285 displays a fast outflow in N V and a much slower wind in Si IV, and we argue that there is a two-wind regime in which mass loss is strong at the poles (fast and tenuous wind) but dominant at the equator (slow and dense winds). These ions and wind lines are not present in the spectrum of the cooler star VFTS 102, but the double-peaked Hα emission in its spectrum implies equatorial mass loss into a circumstellar disk. The results suggest that in the fastest rotating O-stars, most mass is lost as an equatorial outflow, promoting angular momentum loss that contributes to a spin down over time.
Space Science Reviews, 1996
The solar wind carves a cavity in the flow of interstellar H atoms through the solar system by charge-exchange ionization. The resulting Ly-ot ,~ky pattern depends on the latitude distribution of the solar wind flux and velocity. We review how the solar wind characteristics (mass flux latitude distribution) can be retriew.'d from Ly-a observations, yielding a new remote sensing method of solar wind studies~ through UV optical measurements.
Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740
Astronomy & Astrophysics, 2013
Context. The light variability of hot magnetic chemically peculiar stars is typically caused by the flux redistribution in spots with peculiar abundance. This raises the question why some stars with surface abundance spots show significant rotational light variability, while others do not. Aims. We study the Bp star HD 64740 to investigate how its remarkable inhomogeneities in the surface distribution of helium and silicon, and the corresponding strong variability of many spectral lines, can result in one of the faintest photometric variabilities among the Bp stars. Methods. We used model atmospheres and synthetic spectra calculated for the silicon and helium abundances from surface abundance maps to predict the ultraviolet and visual light and line variability of HD 64740. The predicted fluxes and line profiles were compared with the observed ones derived with the IUE, HST, and Hipparcos satellites and with spectra acquired using the FEROS spectrograph at the 2.2 m MPG/ESO telescope in La Silla. Results. We are able to reproduce the observed visual light curve of HD 64740 assuming an inhomogeneous distribution of iron correlated with silicon distribution. The light variations in the ultraviolet are hardly detectable. We detect the variability of many ultraviolet lines of carbon, silicon, and aluminium and discuss the origin of these lines and the nature of their variations. Conclusions. The maximum abundances of helium and silicon on the surface of HD 64740 are not high enough to cause significant light variations. The detected variability of many ultraviolet lines is most likely of atmospheric origin and reflects the inhomogeneous elemental surface distribution. The variability of the C iv resonance lines of carbon is stronger and it probably results from the dependence of the wind mass-loss rate on the chemical composition and magnetic field orientation. We have not been able to detect a clear signature of the matter trapped in the circumstellar clouds.
The impact of accretion disc winds on the optical spectra of cataclysmic variables
Monthly Notices of the Royal Astronomical Society
Many high-state non-magnetic cataclysmic variables (CVs) exhibit blueshifted absorption or P-Cygni profiles associated with ultraviolet (UV) resonance lines. These features imply the existence of powerful accretion disc winds in CVs. Here, we use our Monte Carlo ionization and radiative transfer code to investigate whether disc wind models that produce realistic UV line profiles are also likely to generate observationally significant recombination line and continuum emission in the optical waveband. We also test whether outflows may be responsible for the single-peaked emission line profiles often seen in high-state CVs and for the weakness of the Balmer absorption edge (relative to simple models of optically thick accretion discs). We find that a standard disc wind model that is successful in reproducing the UV spectra of CVs also leaves a noticeable imprint on the optical spectrum, particularly for systems viewed at high inclination. The strongest optical wind-formed recombination lines are H α and He II λ4686. We demonstrate that a higher density outflow model produces all the expected H and He lines and produces a recombination continuum that can fill in the Balmer jump at high inclinations. This model displays reasonable verisimilitude with the optical spectrum of RW Trianguli. No single-peaked emission is seen, although we observe a narrowing of the double-peaked emission lines from the base of the wind. Finally, we show that even denser models can produce a single-peaked H α line. On the basis of our results, we suggest that winds can modify, and perhaps even dominate, the line and continuum emission from CVs.
Another look at the BL Lacertae flux and spectral variability
Astronomy & Astrophysics, 2010
Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with related inverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the whole earth blazar telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability. Results. The GASP-WEBT observations show an optical flare in 2008 February-March, and oscillations of several tenths of mag on a few-day time scale afterwards. The radio flux is only mildly variable. The UV data from both XMM-Newton and Swift seem to confirm a UV excess that is likely caused by thermal emission from the accretion disc. The X-ray data from XMM-Newton indicate a strongly concave spectrum, as well as moderate (∼4-7%) flux variability on an hour time scale. The Swift X-ray data reveal fast (interday) flux changes, not correlated with those observed at lower energies. We compare the spectral energy distribution (SED) corresponding to the 2008 low-brightness state, which was characterised by a synchrotron dominance, to the 1997 outburst state, where the inverse-Compton emission was prevailing. A fit with an inhomogeneous helical jet model suggests that two synchrotron components are at work with their self inverse-Compton emission. Most likely, they represent the radiation from two distinct emitting regions in the jet. We show that the difference between the source SEDs in 2008 and 1997 can be explained in terms of pure geometrical variations. The outburst state occurred when the jet-emitting regions were better aligned with the line of sight, producing an increase of the Doppler beaming factor. Conclusions. Our analysis demonstrates that the jet geometry can play an extremely important role in the BL Lacertae flux and spectral variability. Indeed, the emitting jet is probably a bent and dynamic structure, and hence changes in the emitting regions viewing angles are likely to happen, with strong consequences on the source multiwavelength behaviour.
2010
Aims. In a previous study we suggested that the broad-band emission and variability properties of BL Lacertae can be accounted for by a double synchrotron emission component with related inverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range. Methods. The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical frequencies to follow its flux behaviour. During this period, high-energy observations were performed by XMM-Newton, Swift, and Fermi. We analyse these data with particular attention to the calibration of Swift UV data, and apply a helical jet model to interpret the source broad-band variability.