Asymmetries around Luminous Red Variables

Magalhães, A. M.; Nordsieck, K. H.

doi:10.1017/s0074180900002709

Cited by 2 publications

(3 citation statements)

References 22 publications

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“…Hayes 1984;Nordsieck et al 1994;Magalhaes & Nordsieck 2000, for the most recent studies). This is a clear sign of the existence of anisotropy in the radiation field: it is generally believed that the linear polarization of the continuum is mainly due to photospheric Rayleigh scattering and Mie scattering by circumstellar dust with an anisotropy induced by one or several brightness spots Doherty 1986;Magalhaes & Nordsieck 2000). From our high S/N, high resolution linearly polarized spectra of Betelgeuse, we observe significant linear polarization signal associated with most of the relatively strong lines.…”

Section: Linear Polarization Of the Continuum Of Betelgeuse And Its Dmentioning

confidence: 99%

“…Betelgeuse is known to present linear polarization of its continuum beyond the 0.5 % level in the blue (e.g. Hayes 1984;Nordsieck et al 1994;Magalhaes & Nordsieck 2000, for the most recent studies). This is a clear sign of the existence of anisotropy in the radiation field: it is generally believed that the linear polarization of the continuum is mainly due to photospheric Rayleigh scattering and Mie scattering by circumstellar dust with an anisotropy induced by one or several brightness spots Doherty 1986;Magalhaes & Nordsieck 2000).…”

Section: Linear Polarization Of the Continuum Of Betelgeuse And Its D...mentioning

confidence: 99%

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Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum

Aurière

Mathias

et al. 2016

A&A

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Context. Betelgeuse is an M supergiant that harbors spots and giant granules at its surface and presents linear polarization of its continuum. Aims. We have previously discovered linear polarization signatures associated with individual lines in the spectra of cool and evolved stars. Here, we investigate whether a similar linearly polarized spectrum exists for Betelgeuse. Methods. We used the spectropolarimeter Narval, combining multiple polarimetric sequences to obtain high signal-to-noise ratio spectra of individual lines, as well as the least-squares deconvolution (LSD) approach, to investigate the presence of an averaged linearly polarized profile for the photospheric lines. Results. We have discovered the existence of a linearly polarized spectrum for Betelgeuse, detecting a rather strong signal (at a few times 10 −4 of the continuum intensity level), both in individual lines and in the LSD profiles. Studying its properties and the signal observed for the resonant Na i D lines, we conclude that we are mainly observing depolarization of the continuum by the absorption lines. The linear polarization of the Betelgeuse continuum is due to the anisotropy of the radiation field induced by brightness spots at the surface and Rayleigh scattering in the atmosphere. We have developed a geometrical model to interpret the observed polarization, from which we infer the presence of two brightness spots and their positions on the surface of Betelgeuse. We show that applying the model to each velocity bin along the Stokes Q and U profiles allows the derivation of a map of the bright spots. We use the Narval linear polarization observations of Betelgeuse obtained over a period of 1.4 yr to study the evolution of the spots and of the atmosphere. Conclusions. Our study of the linearly polarized spectrum of Betelgeuse provides a novel method for studying the evolution of brightness spots at its surface and complements quasi-simultaneous observations obtained with PIONIER at the VLTI.

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Section: Linear Polarization Of the Continuum Of Betelgeuse And Its Dmentioning

confidence: 99%

Section: Linear Polarization Of the Continuum Of Betelgeuse And Its D...mentioning

confidence: 99%

Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum

Aurière

Mathias

et al. 2016

A&A

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“…One piece of evidence showing that departures from spherical symmetry exist around AGB stars is that light from these objects may show some degree of linear polarization (Coyne & Magalhães 1977, 1979Magalhães et al 1986aMagalhães et al , 1986bKahane et al 1997;Magalhães & Nordsieck 2000). Further evidence of asphericity in the envelopes of late-type giant and supergiant values observed in red giants (Brown et al 1989;de la Reza & Drake 1995).…”

Section: Introductionmentioning

confidence: 99%

Polarimetry of Li-rich giants

Pereyra

Castilho

Magalhães

2006

A&A

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Context. Protoplanetary nebulae typically present non-spherical envelopes. The origin of such geometry is still controversial. There are indications that it may be carried over from an earlier phase of stellar evolution, such as the AGB phase. But how early in the star's evolution does the non-spherical envelope appear? Aims. Li-rich giants show dusty circumstellar envelopes that can help answer that question. We study a sample of fourteen Li-rich giants using optical polarimetry in order to detect non-spherical envelopes around them. Methods. We used the IAGPOL imaging polarimeter to obtain optical linear polarization measurements in V band. Foreground polarization was estimated using the field stars in each CCD frame. Results. After foreground polarization was removed, seven objects presented low intrinsic polarization (0.19−0.34)% and two (V859 Aql and GCSS 557) showed high intrinsic polarization values (0.87−1.16)%. This intrinsic polarization suggests that Li-rich giants present a nonspherical distribution of circumstellar dust. The intrinsic polarization level is probably related to the viewing angle of the envelope, with higher levels indicating objects viewed closer to edge-on. The correlation of the observed polarization with optical color excess gives additional support to the circumstellar origin of the intrinsic polarization in Li-rich giants. The intrinsic polarization correlates even better with the IRAS 25 µm far infrared emission. Analysis of spectral energy distributions for the sample show dust temperatures for the envelopes that tend to be between 190 and 260 K. We suggest that dust scattering is indeed responsible for the optical intrinsic polarization in Li-rich giants. Conclusions. Our findings indicate that non-spherical envelopes may appear as early as the red giant phase of stellar evolution.

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Asymmetries around Luminous Red Variables

Cited by 2 publications

References 22 publications

Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum

Discovery of a complex linearly polarized spectrum of Betelgeuse dominated by depolarization of the continuum

Polarimetry of Li-rich giants

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