Search for surface magnetic fields in Mira stars. First detection in chi Cyg

In order to complete the knowledge of the magnetic field and of its influence during the transition from Asymptotic Giant Branch to Planetary Nebulae stages, we have undertaken a search for magnetic fields at the surface of Mira stars. We used spectropolarimetric observations, collected with the Narval instrument at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman signature in the visible part of the spectrum. We present the first spectropolarimetric observations of the S-type Mira star chi Cyg, performed around its maximum light. We have detected a polarimetric signal in the Stokes V spectra and we have established its Zeeman origin. We claim that it is likely to be related to a weak magnetic field present at the photospheric level and in the lower part of the stellar atmosphere. We have estimated the strength of its longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the variation of the magnetic field strength across the circumstellar envelope of chi Cyg. This is the first detection of a weak magnetic field at the stellar surface of a Mira star and we discuss its origin in the framework of shock waves periodically propagating throughout the atmosphere of these radially pulsating stars. At the date of our observations of chi Cyg, the shock wave reaches its maximum intensity, and it is likely that the shock amplifies a weak stellar magnetic field during its passage through the atmosphere. Without such an amplification by the shock, the magnetic field strength would have been too low to be detected. For the first time, we also report strong Stokes Q and U signatures (linear polarization) centered onto the zero velocity (i.e., at the shock front position). They seem to indicate that the radial direction would be favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21 November 2013

Shock-Induced Polarized Hydrogen Emission Lines in the Mira Star omicron Ceti

In the spectra of pulsating stars, especially Mira stars, the detection of intense hydrogen emission lines has been explained by a radiative shock wave, periodically propagating throughout the atmosphere. Previous observation of the Mira star omicron Ceti around a bright maximum of light led to the detection of a strong linear polarization associated to Balmer emissions, although the origin of this phenomenon is not fully explained yet. With the help of spectropolarimetry, we propose to investigate the nature of shock waves propagating throughout the stellar atmosphere and present, for omicron Ceti (the prototype of Mira stars), a full observational study of hydrogen emission lines formed in the radiative region of such a shock. Using the instrument NARVAL, we performed a spectropolarimetric monitoring of omicron Ceti during three consecutive pulsation cycles. All Stokes parameters were systematically collected, with a particular emphasis on the maxima of luminosity, i.e. when a radiative shock wave is supposed to emerge from the photosphere and propagate outward. On Balmer lines, over a large part of the luminosity cycle, we report detections in Stokes spectra which are evolving with time. These signatures appear to be strongly correlated to the presence of an intense shock wave responsible for the hydrogen emission lines. We establish that those lines are polarized by a process inherent to the mechanism responsible for the emission line formation: the shock wave itself. Two mechanisms are considered: a global one that implies a polarization induced by convective cells located around the photosphere and a local one that implies a charge separation due to the passage of the shock wave, inducing an electrical current. Combined with the existing turbulence, this may generate a magnetic field, hence polarization.Comment: 11 pages, 9 figures; Astronomy and Astrophysics 2011, preprint onlin

Science cases for a visible interferometer

High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized on the 15th and 16th of January, 2015 in Nice with the support of the Action Specific in Haute Resolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la Cote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41

First detection of helium emissions in RR Lyrae

Context. During the past 100 years, helium emission lines have never been observed in RR Lyrae stars, especially in RRab-type ones where strong shock waves are nevertheless present in their atmosphere. Only recently have the observation of He I emission lines in 10 RRab stars and the detection of very weak He II emission in 3 of them been reported. All these stars are Blazhko stars, while helium emission has never been detected in non-Blazhko stars. Aims. We investigate the physical origin of the presence (or absence) of helium emission in the spectra of RR Lyrae stars, using a recent explanation of the Blazhko effect based on the interplay between two shocks occurring during each pulsation cycle. Methods. From high-resolution spectra collected with ESPaDOnS at CFHT, we report for the first time the observation of He I emission lines in the brightest RR Lyrae star of the sky, RR Lyr. Our observation was done just after a Blazhko maximum. We have also very likely detected He II emission. Results. The presence of He I and He II emission lines is directly connected to the shock intensity. Helium emissions only occur during the strongest amplitude phase of Blazhko stars in which an amplification mechanism of the shock intensity arises. This kind of hypersonic shock has not yet been reported in non-Blazhko stars because in this picture the intensity of their shocks is not amplified. Conclusions. The occurrence of helium emission is a direct consequence of the Blazhko effect. The greater the Blazhko amplitude, the greater the shock intensity, and therefore emission lines of helium increase. Considering the spectroscopic features observed (or not) in the RR Lyrae stars, we can define three hydrodynamic regimes: (a) the supersonic regime in which only hydrogen emissions are visible such as in non-Blazhko stars; (b) the weak hypersonic regime where the He I emission lines are also observed; and (c) the strong hypersonic regime with the additional observation of He II emission lines. These two last regimes only occur in Blazhko stars

Hydrogen Lines in Mira Stars Through Interferometry and Polarimetry

Balmer lines in emission are the most prominent features in Mira stars spectra and have a strong potential as a proxy to study the lower atmosphere's dynamics. In Fabas et al. ([1]), we accumulated spectropolarimetric observations of Balmer lines in emission. As the shock is propagating outwards, linear polarization rate increases and the angle of this polarization evolves. Assuming that linear polarization arises from anisotropic scattering, it has the potential of telling us about the geometric structure of the shock as it propagates and the study of such atmospheric structures can typically be performed with interferometry. In 2012, AMBER data on the Mira star omicron Ceti were collected in which the Brackett γ line is studied. The data show signatures in the interferometric observables around this line. Olivier Chesneau was in the jury evaluating the PhD thesis of N. Fabas and he was seduced by the idea to study these shock waves with interferometry and use polarimetry as a complementary study

A helium P-Cygni profile in RR Lyrae stars?

Context. Until 2006, helium emission lines had never been observed in RR Lyrae stars. For the first time, a pre-maximum helium emission in 11 RRab stars was observed during rising light (around the pulsation phase 0.92) and the reappearance of helium emission near maximum light (phase 0.0) in one RRab star: RV Oct. This post-maximum emission has been only observed in the He I λ5875.66 (D3) line. Its intensity is very weak, and its profile mimics a P-Cygni profile with the emission peak centered at the laboratory wavelength. The physical explanation for this unexpected line profile has not been proposed yet. Aims. Using new observations of RR Lyr, we investigate the physical origin of the presence of a P-Cygni profile in the He I λ5875.66 (D3) line. Methods. High-resolution spectra of RR Lyr, collected with a spectrograph eShel/C14 at the Oukaïmeden Observatory (Morocco) in 2013, were analyzed to understand the origin of the observed P-Cygni profile at D3. Results. When the shock intensity is moderate, helium emission cannot be produced in the shock wake, and consequently, the two consecutive helium emissions (pre- and post-maximum light emissions) are not observed. This is the most frequent case. When the shock intensity becomes high enough, a pre-maximum He I emission first occurs, which can be followed by the appearance of a P-Cygni profile if the shock intensity is still strong in the high atmosphere. The observation of a P-Cygni profile means that the shock wave is already detached from the photosphere. It is shown that the shock strongly first decelerates between the pulsation phases 0.90 and 1.04 from 130 km s-1 to 60 km s-1, probably before accelerating again to 80 km s-1  near phase 1.30. Conclusions. The presence of the P-Cygni profile seems to be a natural consequence of the large extension of the expanding atmosphere, which is induced by strong (radiative) shock waves propagating toward the high atmosphere. This kind of P-Cygni profile has already been observed in the Hα line of some RR Lyrae stars and long-period Cepheids

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

International audienceBetelgeuse is an M supergiant that harbors spots and giant granules at its surface and presents linear polarization of its continuum. 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. 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. 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 \ion{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 years to study the evolution of the spots. 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