2,057 research outputs found
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
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
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
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
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