4.3.1 Radiative transfer

Baschek, B.; Butler, K.; Scholz, M.; Ulmschneider, P.; Wehrsě, R.

doi:10.1007/10057805_11

Cited by 47 publications

(75 citation statements)

References 8 publications

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“…For a more detailed discussion see e.g. Höflich (1987Höflich ( , 1990Höflich ( , 1995Baschek, Scholz, and Wehrse 1991) where it has been shown that the Doppler shift of the absorption of weak lines provides a good measurement of the last scattering radius. For non-spherical geometries the situation is even more complicated since the radial direction is not orthogonal to the shells of constant density.…”

Section: Methodsmentioning

confidence: 99%

Analysis of the Polarization and Flux Spectra of SN 1993J

Hoêflich

Wheeler²,

Hines

et al. 1996

ApJ

105

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Synthetic polarization and flux spectra are presented for aspherical, electron scattering-dominated photospheres of Type II supernovae (SN II) in general and the specific case of SN 1993J. Monte Carlo calculations are based on the following assumptions: a) ellipsoidal envelopes with power law density profiles; b) occupation numbers given by local thermodynamical equilibrium (LTE); c) pure electron scattering for continuum opacities; d) lines treated in a Sobolev approximation with an assumed constant thermalization fraction; e) line transitions result in depolarization; and f) the temperature structure is given by a gray extended atmosphere.The observed luminosity of a Type II supernova depends on the unknown inclination angle i. Spectral analysis alone will fail to detect even strong deviations from spherical symmetry. Line scattering depolarizes incident polarized light, but the residual intrinsic polarization does not completely vanish because of electron scattering effects that depend on the electron density distribution. By combining results on the polarization and velocity structure of the emission lines, and the degree of polarization in the continuum, we place strong constraints on the degree of asphericity, the inclination of the system and the electron density distribution. In addition, since the problem is over constrained, we can independently test for the contribution to the polarization caused by aligned interstellar grains between the SN and earth.These modelling techniques have been applied to SN1993J. Both the flux spectra and the percentage of polarization as a function of wavelength can be reproduced by an aspherical model with an axis ratio of 0.6, radial electron densities proportional to r −5 and an effective temperature of 4800 K. In an oblate model, SN 1993J is seen almost equator on. The line forming region is still within the hydrogen rich part of the envelope 3 weeks after the explosion, although the continuum may form in deeper layers. Although the residual polarization across Hα is not zero, the interstellar component derived herein is consistent with that deduced by Trammell et al. (1993). The power of this technique for investigating the structure of other SN II, as a method for independently deriving the interstellar polarization, and the implications of these results on the use of SN II to determine distances through the Baade-Wesselink method are discussed.-2 -

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Section: Methodsmentioning

confidence: 99%

Analysis of the Polarization and Flux Spectra of SN 1993J

Hoêflich

Wheeler²,

Hines

et al. 1996

ApJ

105

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“…The observed apparent diameter changes dramatically within the given wavelength range due to the strong wavelength dependent opacity of the atmospheric constituents (Baschek et al 1991;Scholz 2001). For the O-rich stars, the measured diameters in the optical are sensitive to TiO bands.…”

Section: Wavelength Dependence Of the Diametermentioning

confidence: 97%

“…The consequence of a very extended atmosphere is a not clearly definable and measurable photospheric radius R phot 1 (e.g. Mennesson et al 2002;Tej et al 2003;Ohnaka 2004;Woodruff et al 2004;Fedele et al 2005) and observations at different wavelengths probe different atmospheric layers (Baschek et al 1991;Scholz 2001). The measured apparent diameters are correlated to the absorption and emission features of the most abundant and radiatively important molecular species (e.g.…”

Section: Introductionmentioning

confidence: 99%

Dust and molecular shells in asymptotic giant branch stars

Zhao-Geisler

Quirrenbach

Köhler

et al. 2012

A&A

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Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods. Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbonrich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H 2 O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al 2 O 3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al 2 O 3 in oxygenrich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.

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“…Various definitions for the radius can be found (cf. reviews by Baschek et al 1991;Scholz 2003), where the most commonly used radius in atmospheric modelling is the Rosseland radius. It is defined by the distance between the centre of the star and the layer having Rosseland optical depth τ ross = 2 3 .…”

Section: Effective Temperaturementioning

confidence: 99%

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Klotz

Paladini

Hron

et al. 2013

A&A

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Context. Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. Aims. We analyse archive spectra obtained with the Short Wavelength Spectrometer (SWS) onboard ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. Methods. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the "classic" interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. Results. We find good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a "quiet" phase compared to the subsequent strong-wind regime. This agrees with the C/O ratio being only slightly greater than one.

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4.3.1 Radiative transfer

Cited by 47 publications

References 8 publications

Analysis of the Polarization and Flux Spectra of SN 1993J

Analysis of the Polarization and Flux Spectra of SN 1993J

Dust and molecular shells in asymptotic giant branch stars

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

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