Wind variability of B supergiants

We present the most suitable data sets available in the International Ultraviolet Explorer (IUE) archive for the study of time-dependent stellar winds in early B supergiants. The UV line profile variability in 11 B0 to B3 stars is analysed, compared and discussed, based on 16 separate data sets comprising over 600 homogeneously reduced high-resolution spectrograms. The targets include ``normal'' stars with moderate rotation rates and examples of rapid rotators. A gallery of grey-scale images (dynamic spectra) is presented, which demonstrates the richness and range of wind variability and highlights different structures in the winds of these stars. This work emphasises the suitability of B supergiants for wind studies, under-pinned by the fact that they exhibit unsaturated wind lines for a wide range of ionization. The wind activity of B supergiants is substantial and has highly varied characteristics. The variability evident in individual stars is classified and described in terms of discrete absorption components, spontaneous absorption, bowed structures, recurrence, and ionization variability and stratification. Similar structures can occur in stars of different fundamental parameters, but also different structures may occur in the same star at a given epoch. We discuss the physical phenomena that may be associated with the spectral signatures. The diversity of wind patterns evident likely reflects the role of stellar rotation and viewing angle in determining the observational characteristics of azimuthally extended structure rooted at the stellar surface. In addition, SEI line-synthesis modelling of the UV wind lines is used to provide further information about the state of the winds in our program stars. Typically the range, implied by the line profile variability, in the product of mass-loss rate and ion fraction (mdot q_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ; it can however be several times larger over localised velocity regions. At a given effective temperature the mean relative ion ratios can differ by a factor of 5. The general excess in predicted (forward-scattered) emission in the low velocity regime is discussed in terms of structured outflows. Mean ion fractions are estimated over the B0 to B1 spectral classes, and trends in the ionic ratios as a function of wind velocity are described. The low values obtained for the ion fractions of UV resonance lines may reflect the role of clumping in the wind

Massive Star Formation

This chapter reviews progress in the field of massive star formation. It focuses on evidence for accretion and current models that invoke high accretion rates. In particular it is noted that high accretion rates will cause the massive young stellar object to have a radius much larger than its eventual main sequence radius throughout much of the accretion phase. This results in low effective temperatures which may provide the explanation as to why luminous young stellar objects do not ionized their surroundings to form ultra-compact H II regions. The transition to the ultra-compact H II region phase would then be associated with the termination of the high accretion rate phase. Objects thought to be in a transition phase are discussed and diagnostic diagrams to distinguish between massive young stellar objects and ultra-compact H II regions in terms of line widths and radio luminosity are presented.Comment: 21 pages, 6 figures, chapter in Diffuse Matter from Star Forming Regions to Active Galaxies - A Volume Honouring John Dyson, Edited by T.W. Hartquist, J. M. Pittard, and S. A. E. G. Falle. Series: Astrophysics and Space Science Proceedings. Springer Dordrecht, 2007, p.6

Variability in the UV resonance lines of the cataclysmic variable V795 Herculis (PG-1711 + 336)

Original article can be found at: http://adsabs.harvard.edu/basic_search.html Copyright Royal Astronomical Society [Full text of this article is not available in the UHRA]Variability in the UV resonance lines from the cataclysmic variable V795 Her is investigated on the basis of a total of 28 IUE SWP low-resolution spectra. Significant line profile fluctuations are evident in the C IV 1549 and Si IV 1400 doublets on time-scales down to less than about 30 min. There is little or no evidence for changes in N V 1240. Only C IV 1549 is occasionally seen in emission; the morphology of this profile changes from an absorption-dominated line to an emission-dominated one in a few hours. Although no correlations are evident between changes in the Si IV and C IV line strengths and the mooted orbital period of V795 Her (about 2.6 hr), the present time-series analyses of the UV data suggest an approximately 4.8-hr periodic modulation in the line profile variability. Constraints on the origin and geometry of the line formation are discussed on the basis of these UV data. Alternative specifications for departures from axial symmetry are suggested.Peer reviewe

Phase-resolved Hubble Space Telescope ultraviolet spectroscopy of V795 Her

‘The definitive version is available at www.blackwell-synergy.com.’ Copyright Blackwell Publishing. DOI: 10.1046/j.1365-8711.1998.01844.x [Full text of this article is not available in the UHRA]We present highly time-resolved HST FOS UV spectroscopy of the nova-like binary V795 Her. Several key results emerge. For the first time we find a strong 2.6-h signature in the variability of the UV lines. The HST data reveal no evidence of a 4.8-h ‘period’, in contrast to our previous IUE observations. This, and differences in the spectral line characteristics, suggests that HST found the system in a different state from earlier IUE observations. The C IV line alone contains a fairly stable, asymmetric, extended blueward absorption trough which we associate with a wind outflow. The 2.6-h variations of the line profiles are largely confined to an interval of about 0.4 in phase and to the velocity regime −1500 < v < 0 km s−1, the changes being dominated by the apparent decline and re-emergence of a blueshifted emission peak. The complex profiles permit many empirical interpretations, but the simplest attributes the variability to a narrow (FWHM∼1000 km s−1) emission component which is always blueshifted with a mean velocity of around –600 km s−1. This interpretation, however, is not readily related to any obvious source within the binary. An alternative picture, which attempts to relate the UV and (simultaneously observed) optical line behaviour, invokes a more stable, broad (FWHM∼2000 km s−1) emission feature, the intrinsic morphology of which is disguised by superposed constant and variable absorption components. One tentative physical explanation of such a decomposition involves an accretion stream that overflows the accretion disc. However, several problems with this model remain to be resolved. We also draw attention to similarities between the velocity-restricted behaviour in the UV lines of V795 Her and that in the optical lines of T Tauri stars. This might indicate a connection between V795 Her and the magnetically influenced inflow/outflow characteristics associated with the central star in T Tauri systems. If such a connection were eventually demonstrated, it would reopen the question of whether the 2.6-h period in V795 Her is really the binary period and whether the system is in fact related to the intermediate polars.Peer reviewe

Ultraviolet studies of interacting binaries

Interacting Binaries consist of a variety of stellar objects in different stages of evolution and those containing accreting compact objects still represent a major challenge to our understanding of not only close binary evolution but also of the chemical evolution of the Galaxy. These end-points of binary star evolution are ideal laboratories for the study of accretion and outflow processes, and provide insight on matter under extreme physical conditions. One of the key-questions of fundamental relevance is the nature of SN Ia progenitors. The study of accreting compact binary systems relies on observations over the entire electromagnetic spectrum and we outline here those unresolved questions for which access to the ultraviolet range is vital, as they cannot be addressed by observations in any other spectral region

X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description

International audienceObservations of individual massive stars, super-luminous supernovae, gamma-ray bursts, and gravitational-wave events involving spectacular black-hole mergers, indicate that the low-metallicity Universe is fundamentally different from our own Galaxy. Many transient phenomena will remain enigmatic until we achieve a firm understanding of the physics and evolution of massive stars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbits to observe 250 massive stars at low Z in the ultraviolet (UV) with the COS and STIS spectrographs under the ULLYSES program. The complementary ``X-Shooting ULLYSES'' (XShootU) project provides enhanced legacy value with high-quality optical and near-infrared spectra obtained with the wide-wavelength coverage X-shooter spectrograph at ESO's Very Large Telescope. We present an overview of the XShootU project, showing that combining ULLYSES UV and XShootU optical spectra is critical for the uniform determination of stellar parameters such as effective temperature, surface gravity, luminosity, and abundances, as well as wind properties such as mass-loss rates in function of Z. As uncertainties in stellar and wind parameters percolate into many adjacent areas of Astrophysics, the data and modelling of the XShootU project is expected to be a game-changer for our physical understanding of massive stars at low Z. To be able to confidently interpret James Webb Space Telescope (JWST) spectra of the first stellar generations, the individual spectra of low Z stars need to be understood, which is exactly where XShootU can deliver