First detection of surface magnetic fields in post-AGB stars: the cases of U Monocerotis and R Scuti

Sabin, L.; Wade, G. A.; Lèbre, A.

doi:10.1093/mnras/stu2227

Cited by 39 publications

(37 citation statements)

References 68 publications

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“…We also applied these criteria to the null profiles to assess whether they display any significant features. In case for which the statistics lead to a "no detection" in the null profiles we attribute a Zeeman origin for the Stokes V signals, based on the successful detection of very weak circularly polarised signatures in the RSG star α Ori (Aurière et al 2010) and in cool giants (see for instance Konstantinova-Antova et al 2013;Sabin et al 2015;Lèbre et al 2014;Aurière et al 2015). However, in the case of a "definite detection" of a signal in the null profiles we performed an in-depth study of this signal.…”

Section: Methodsmentioning

confidence: 99%

Measuring surface magnetic fields of red supergiant stars

Tessore

Lèbre

Morin

et al. 2017

A&A

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Context. Red supergiant (RSG) stars are very massive cool evolved stars. Recently, a weak magnetic field was measured at the surface of α Ori and this is so far the only M-type supergiant for which a direct detection of a surface magnetic field has been reported. Aims. By extending the search for surface magnetic field in a sample of late-type supergiants, we want to determine whether the surface magnetic field detected on α Ori is a common feature among the M-type supergiants. Methods. With the spectropolarimeter Narval at Télescope Bernard-Lyot we undertook a search for surface magnetic fields in a sample of cool supergiant stars, and we analysed circular polarisation spectra using the least-squares deconvolution technique. Results. We detect weak Zeeman signatures of stellar origin in the targets CE Tau, α 1 Her and µ Cep. For the latter star, we also show that cross-talk from the strong linear polarisation signals detected on this star must be taken into account. For CE Tau and µ Cep, the longitudinal component of the detected surface fields is at the Gauss-level, such as in α Ori. We measured a longitudinal field almost an order of magnitude stronger for α 1 Her. We also report variability of the longitudinal magnetic field of CE Tau and α 1 Her, with changes in good agreement with the typical atmospheric dynamics time-scales. We also report a non-detection of magnetic field at the surface of the yellow supergiant star ρ Cas. Conclusions. The two RSG stars of our sample, CE Tau and µ Cep, display magnetic fields very similar to that of α Ori. The nondetection of a magnetic field on the post-RSG star ρ Cas suggests that the magnetic field disappears, or at least becomes undetectable with present methods, at later evolutionary stages. Our analysis of α 1 Her supports the proposed reclassification of the star as an M-type asymptotic giant branch star.

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Section: Methodsmentioning

confidence: 99%

Measuring surface magnetic fields of red supergiant stars

Tessore

Lèbre

Morin

et al. 2017

A&A

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“…These observations suggest that the polarized signal is related to the lines of Betelgeuse. The linear polarization spectrum of cool evolved stars has already been detected using the LSD method in a number of Miras (Lèbre et al 2014 and RV Tauri stars (Sabin et al 2015;Lèbre et al 2015). In these cases, the polarization seems to be A119, page 3 of 12 A&A 591, A119 (2016) linked to the atmospheric shock wave since its shape changes with pulsation phase, being maximum near maximum brightness, which corresponds to the shock passage through the atmosphere.…”

Section: Stellar Origin Of the Linearly Polarized Spectrum Of Betelgeusementioning

confidence: 98%

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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“…We therefore proceeded with the detection of (longitudinal) magnetic fields at the surface of post-AGB stars using the high resolution spectropolarimeters (R= 65000) ESPaDOnS (CFHT, Hawaii) and Narval (TBL, France). Our first sample was composed of seven well known bright objects CY CMi, U Mon, V814 Her, 89 Her, AC Her, V4728 Sgr and R Sct (Sabin et al 2015). The observing log is presented in Table 1 and we emphasize the high signal-to-noise (above ∼500) which is needed to unveil any meaningful signal.…”

Section: Optical Spectropolarimetrymentioning

confidence: 99%

Detection and characteristics of magnetic fields in evolved intermediate mass stars

Sabin

2014

Proc. IAU

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Abstract. The role of magnetic fields in late type stars, such as Asymptotic Giant Branch stars (AGBs), Post-AGBs and Planetary Nebulae (PNe), is poorly known from an observational point of view. Magnetic fields are however believed to have a non-negligible influence on the dynamics (via mass loss control, outflows shaping) and even on the chemistry (e.g. extra mixing) of these stellar objects. We are therefore presenting two different types of investigation, both based on the use of polarimetry, which aim at filling the gap between the observations on the one hand and the theoretical predictions on the other hand.

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First detection of surface magnetic fields in post-AGB stars: the cases of U Monocerotis and R Scuti

Cited by 39 publications

References 68 publications

Measuring surface magnetic fields of red supergiant stars

Measuring surface magnetic fields of red supergiant stars

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

Detection and characteristics of magnetic fields in evolved intermediate mass stars

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