Radiative Transfer Modeling of the Winds & Circumstellar Environments of Hot And Cool Massive Stars

We present modeling research work of the winds and circumstellar environments of prototypical hot and cool massive stars using advanced radiative transfer (RT) calculations. This research aims at unraveling the detailed physics of various mass-loss mechanisms of luminous stars in the upper H-R diagram. Very recent 3-D RT calculations, combined with hydrodynamic simulations, show that radiatively-driven winds of OB supergiants are structured due to large-scale density- and velocity-fields caused by rotating bright spots. The mass-loss rates computed from matching DACs in HD 64760 (B Ib) do not reveal appreciable changes from the rates of smooth wind models. Intermediate yellow supergiants (such as Rho Cas, F-G Ia0), on the other hand, show prominent spectroscopic signatures of strongly increased mass-loss rates during episodic outbursts. Long-term spectroscopic monitoring of hypergiants near the Yellow Evolutionary Void reveals that their mass-loss rates and wind-structure are dominated by photospheric eruptions and large-amplitude pulsations that impart mechanical momentum to the circumstellar environment by propagating shock waves. In massive red supergiants, however, clear evidence for mechanical wave propagation from the sub-photospheric convection zones is lacking. Recent spatially resolved HST-STIS observations inside Betelgeuse's (M Iab) very extended chromosphere and dust envelope show evidence of warm chromospheric gas far beyond the dust condensation radius of RT models. Models for these long-term spectroscopic observations demonstrate that the chromospheric pulsations are not spherically symmetric. The STIS observations point to the importance of mechanical wave propagation for heating and sustaining chromospheric conditions in the extended winds of red supergiants.Comment: 6 pages, 5 figures. Hot And Cool: Bridging Gaps in Massive Star Evolution : proceedings of an international conference held at Caltech Campus, Pasadena, USA, 10-12 Nov. 2008. Eds. C. Leitherer, P.D. Bennett, P.W. Morris & J.T. van Loon, ASP Conf. Ser. 2009. Talk presentation & animations available at http://alobel.freeshell.org/conf.htm

Modeling DACs in UV Lines of Massive Hot Stars

We apply the 3-dimensional radiative transport code Wind3D to 3D hydrodynamic models of Corotating Interaction Regions to fit the detailed variability of Discrete Absorption Components observed in Si IV UV resonance lines of HD 64760 (B0.5 Ib). We discuss important effects of the hydrodynamic input parameters on these large-scale equatorial wind structures that determine the detailed morphology of the DACs computed with 3D transfer. The best fit model reveals that the CIR in HD 64760 is produced by a source at the base of the wind that lags behind the stellar surface rotation. The non-corotating coherent wind structure is an extended density wave produced by a local increase of only 0.6 % in the smooth symmetric wind mass-loss rate.Comment: 3 pages, 3 figures. To appear in Clumping in Hot Star Winds, Proc. of Int. Workshop, 18-22 June 2007, Potsdam, Universitats-Verlag, 2007. URN at the German National Library, Eds. W.-R. Hamann, A. Feldmeier, and L. Oskinova. Publ. of the Univ. of Potsdam, Germany. More information about Wind3D at http://alobel.freeshell.org/conference.htm

3-D Radiative Transfer Modelling of Massive-Star UV Wind Line Variability

We present detailed semi-empiric models for rotational modulations observed in ultraviolet wind lines of B0.5 supergiant HD 64760. We model the Rotational Modulation Regions (RMRs) with advanced 3-D radiative transfer calculations in the stellar wind and quantitatively fit the time-evolution of the Si IV 1395 resonance line. We find that the RMRs are due to linearly-shaped narrow sector-like density enhancements in the equatorial wind. Unlike the Co-rotating Interaction Regions (CIRs) which produce Discrete Absorption Components in the line, the RMRs do not spread out with larger distance above the stellar surface. The detailed best fit shows that the RMRs of HD 64760 have maximum density enhancements of ~17% above the surrounding smooth wind density, about twice smaller than the hydrodynamic models of CIRs that warp around the star. The semi-empiric 3-D transfer modelling reveals that the narrow spoke-like RMRs must co-exist with broader and curved large-scale CIR wind density structures in the equatorial plane of this fast rotating Ib-supergiant.Comment: 6 pages, 3 figures. To appear in Proceedings of the 38th Liege International Astrophysical Colloquium: Evolution and Pulsation of Massive Stars on the Main Sequence and close to it, Liege, July 7-11 2008. Comm. in Asteroseismology. Ed. M. Breger, Austrian Academy of Sciences Press. Talk presentation & animations available at http://alobel.freeshell.org/conf.htm

SpectroWeb: An Interactive Graphical Database of Digital Stellar Spectral Atlases

SpectroWeb is an online maintained interactive graphical database of digital spectral atlases of spectral standard stars at http://spectra.freeshell.org . It is an efficient and user-friendly research tool for accurate analyses of stellar spectra observed with large spectral resolution, including the solar spectrum. The web-interface displays observed and theoretical stellar spectra, and comprehensively provides detailed atomic and molecular line information via user interaction. It fully integrates interactive spectrum visualization tools for the analysis, management, and maintenance of large volumes of spectral line-identification, -transition, and -property data. SpectroWeb 1.0 currently offers optical (3300-6800 A) flux normalized high-resolution spectra of Betelgeuse (M2), Arcturus (K1), The Sun (G2), Beta Aqr (G0), Procyon (F5), and Canopus (F0). The provided line identifications are based on state of the art spectrum synthesis calculations. The graphical database is under permanent development as an online repository of identified (absorption) lines in spectral standard reference stars, covering a broad range of stellar spectral types. Its object-oriented (Java) implementation offers future expansion capabilities to link and read stellar spectral atlases from various public internet sites.Comment: 6 pages, 3 figures. To appear in The Ultraviolet Universe: Stars from Birth to Death, 26th meeting of the IAU, Joint Discussion 4, 16-17 August 2006, Prague, Czech Republic, p. 167. Eds. Ana I. Gomez de Castro and Martin A. Barstow, Publ. of Univ. Complutense de Madrid. More information about SpectroWeb at http://spectra.freeshell.org/spectroweb.htm

Spatially Resolved STIS Spectra of Betelgeuse's Upper Chromosphere and Circumstellar Dust Envelope

The Hubble Space Telescope observed red supergiant Betelgeuse with the Space Telescope Imaging Spectrograph to investigate the outer atmosphere from spatially resolved spectra. We present a new set of seven high-resolution near-UV spectra observed with HST-STIS in fall 2002 and spring 2003, by scanning at chromospheric intensity peak-up position and six off-limb target positions up to three arcseconds away from the star. A small aperture is used to study and determine the thermal conditions and flow dynamics in its upper chromosphere and inner circumstellar dust envelope. We provide the first evidence for the presence of warm chromospheric plasma at least 3 arcseconds away from Betelgeuse at ~120 R* (1 R* ~= 700 Rsun) based on detailed spectroscopic observations of the Mg II h & k emission lines. Many other weak chromospheric emission lines as Fe II lam2716, C II lam2327, Al II] lam2669, and Fe I lam2823 are detected out to at least 1 arcsecond in the spatially resolved STIS observations. The profiles of the Mg II h & k and the Si I resonance emission lines reveal a strong increase of asymmetry by scanning off-limb, signaling the outward acceleration of wind expansion in Betelgeuse's upper chromosphere beyond 200 mas (~8 R*). We discuss detailed radiative transfer models that fit the STIS observations showing that the local kinetic gas temperature in the upper chromosphere exceeds 2600 K. Our radiation transport models for the IR silicate dust emission at 9.8 um in the upper chromosphere show however that the ambient gas temperature remains below 600 K to sustain the presence of dust grains. Hence, the STIS spectra of Betelgeuse's upper chromosphere directly demonstrate that warm chromospheric plasma must co-exist with cool dusty plasma in its outer atmosphere.Comment: 8 pages, 12 figures. Submitted for publication in the Proceedings of the 13th Cool Stars, Stellar Systems and the Sun workshop, Hamburg, July 2004, ESA SP-560, Eds. Favata, F., et al., (in press). Presentation contributed talk at http://www.hs.uni-hamburg.de/cs13/day5/04_Lobel.ppt . More information on Betelgeuse at http://alobel.freeshell.or

Three Dimensional Radiative Transfer in Winds of Massive Stars: Wind3D

We discuss the development of the new radiative transfer code Wind3D. It solves the non-LTE radiative transport problem in moving stellar atmosphere models in three geometric dimensions. The code accepts arbitrary 3D velocity fields in Cartesian geometry without assumptions of axial symmetry. Wind3D is currently implemented as a fully parallelized (exact) accelerated lambda iteration scheme with a two level atom formulation. The numerical transfer scheme is efficient and very accurate to trace small variations of local velocity gradients on line opacity in strongly scattering dominated extended stellar winds. We investigate the detailed formation of P Cygni line profiles observed in ultraviolet spectra of massive stars. We compute the detailed shape of these resonance lines to model local enhancements of line opacity that can for instance be caused by clumping in supersonically expanding winds. Wind3D will be applied to hydrodynamic models to investigate physical properties of discrete absorption line components.Comment: 6 pages, 4 figures. To appear in The Ultraviolet Universe: Stars from Birth to Death, 26th meeting of the IAU, Joint Discussion 4, 16-17 August 2006, Prague, Czech Republic, p. 119. Eds. Ana I. Gomez de Castro and Martin A. Barstow, Publ. of Univ. Complutense de Madrid. More information about Wind3D at http://alobel.freeshell.org/conference.htm

3-D Radiative Transfer Modeling of Structured Winds in Massive Hot Stars with Wind3D

We develop 3-D models of the structured winds of massive hot stars with the Wind3D radiative transfer (RT) code. We investigate the physical properties of large-scale structures observed in the wind of the B-type supergiant HD 64760 with detailed line profile fits to Discrete Absorption Components (DACs) and rotational modulations observed with IUE in Si IV {\lambda}1395. We develop parameterized input models Wind3D with large-scale equatorial wind density- and velocity-structures, or so-called `Co-rotating Interaction Regions' (CIRs) and `Rotational Modulation Regions' (RMRs). The parameterized models offer important advantages for high-performance RT calculations over ab-initio hydrodynamic input models. The acceleration of the input model calculations permits us to simulate and investigate a wide variety of physical conditions in the extended winds of massive hot stars. The new modeling method is very flexible for constraining the dynamic and geometric wind properties of RMRs in HD 64760. We compute that the modulations are produced by a regular pattern of radial density enhancements that protrude almost linearly into the equatorial wind. We find that the modulations are caused by narrow `spoke-like' wind regions. We present a hydrodynamic model showing that the linearly shaped radial wind pattern can be caused by mechanical wave action at the base of the stellar wind from the blue supergiant.Comment: 6 pages, 6 figures, contributed paper for "The multi-wavelength View of Hot, Massive Stars", 39th Li\`ege Int. Astroph. Coll., held 12-16 July 2010, in Li\`ege, Belgium. To appear in Special Issue of the Bulletin of the Li\`ege Royal Scientific Society, http://www.srsl-ulg.ne

Mass-loss and Recent Spectral Changes in the Yellow Hypergiant Rho Cassiopeiae

The yellow hypergiant Rho Cassiopeiae (F-G Ia0) has recently become very active with a tremendous outburst event in the fall of 2000. During the event the pulsating supergiant dimmed by more than a visual magnitude, while its effective temperature decreased from 7000 K to below 4000 K over about 200 d, and we directly observed the largest mass-loss rate of about 5% of the solar mass in a single stellar outburst so far. Over the past three years since the eruption we observed a very prominent inverse P Cygni profile in Balmer H alpha, signaling a strong collapse of the upper atmosphere, also observed before the 2000 event. Continuous spectroscopic monitoring reveals that the H alpha line profile has transformed into a P Cygni profile since June 2003, presently (Sept 2004) signaling supersonic expansion velocities up to ~120 km/s in the extended upper atmosphere. Based on the very recent unique spectral evolution we observed the far-UV spectrum with the FUSE satellite in July 2004. The FUSE spectrum reveals that high-temperature plasma emission lines of O VI and C III are absent in the hypergiant, also observed for the red supergiant Alpha Ori (M2 Iab). On the other hand, we observe prominent transition region emission lines in the smaller (less luminous) classical Cepheid variable Beta Dor (F-G Iab-Ia), indicating that the mean atmospheric extension and surface gravity acceleration (as compared to effective temperature and atmospheric pulsation) play a major role for the formation of high-temperature stellar atmospheric plasmas. We present an overview of the recent spectral variability phases of Rho Cas with enhanced mass-loss from this enigmatic cool star.Comment: 4 pages, 5 figures. Submitted for publication in the Proceedings of the 13th Cool Stars, Stellar Systems and the Sun workshop, Hamburg, July 2004, ESA SP-560, Eds. Favata, F., et al., (in press). More information on Rho Cas at http://alobel.freeshell.or

Chromospheric Dynamics of Betelgeuse from STIS Spectra

We present a high-resolution spectral analysis of Betelgeuse (M2 Iab). Between 1998 January and 1999 March four spatially resolved raster scans (http://cfa-www.harvard.edu/cfa/ep/pressrel/alobel0100.html) have been obtained with the STIS on the Hubble Space Telescope. The near-UV echelle spectra reveal double-peaked permitted emission lines of neutral and singly ionized metals, with self-absorbed line cores. We observe reversals in the intensity of both emission line components when scanning across the UV disk, for four unsaturated lines of Si I, Fe I, Al II], and Fe II. We model the Si I lam2516 resonance line with detailed non-LTE radiative transport calculations in spherical geometry, and constrain the mean velocity structure in the projected aperture area, for each scan position on the chromospheric disk. We infer the spatial velocity structure of Betelgeuse's extended chromosphere, which reveals localized upflows in the western front hemisphere in 1998 September, that expand further toward the eastern hemisphere in 1999 March. The spatial scans exhibit simultaneous up- and downflows across the lower chromosphere with mean velocities of ~2 km/s. We infer non-radial (or non-coherent) mass movements during certain phases of the stellar variability cycle from these subsonic flows. We present a discussion of constructing semi-empiric models for the chromosphere of this cool supergiant, and of its temporal variability

Semi-empiric Radiative Transfer Modeling of FUSE Stellar Spectra

We present an overview of radiative transfer modeling efforts to interpret spectra of a variety of stellar objects observed with FUSE. Detailed radiative transfer modeling of high ion emission line profiles of C III and O VI observed in the far-UV spectrum provides a powerful means to probe the thermal and dynamic properties of high-temperature plasmas in the atmospheres of stars. We model asymmetric emission lines of C III lam977 observed in spectra of luminous cool stars such as Alpha Aqr, to infer the wind- and microturbulence velocity structures of the upper chromosphere. Semi-empiric radiative transfer models that include transition region temperature conditions, are further developed based on detailed fits to O VI resonance emission lines in the supergiant Alpha Aqr, the classical Cepheid variable Beta Dor, and to self-absorbed O VI emission lines in the cataclysmic variable SW UMa. We observe that the C III resonance line profile of Alpha Aqr assumes a remarkable asymmetric shape, reminiscent of P Cygni type profiles observed in hot luminous supergiants. The model calculations indicate outflow velocities above ~140 km/s at kinetic temperatures of 65 kK and higher. Based on detailed model fits to the narrow red-shifted and self-absorbed O VI emission lines of SW UMa we compute that the gas- and electron-density exceed the density conditions of the upper solar transition region by about three orders of magnitude. We discuss how detailed semi-empiric fits to emission lines observed with the high spectral resolution of FUSE can provide reliable constraints on the mass-loss or mass-accretion rates in these objects.Comment: 3 pages, 3 figures, Submitted for publication in the Proceedings of "Astrophysics in the Far Ultraviolet", 2nd FUSE Science and Data Workshop, Victoria, BC, Canada, Aug 2 - 6, 2004, ASP Conf. Ser., Eds. G. Sonneborn, W. Moos, & B.-G. Andersson, (in press