26 research outputs found
Dynamo-generated magnetic fields in fast rotating single giants
Red giants offer a good opportunity to study the interplay of magnetic fields
and stellar evolution. Using the spectro-polarimeter NARVAL of the Telescope
Bernard Lyot (TBL), Pic du Midi, France and the LSD technique, we began a
survey of magnetic fields in single G-K-M giants. Early results include 6
MF-detections with fast rotating giants, and for the first time a magnetic
field was detected directly in an evolved M-giant: EK Boo. Our results could be
explained in the terms of -- dynamo operating in these giants.Comment: 2 pages, 1 figure, proceedings of IAUS259: Cosmic Magnetic Field
The convection of close red supergiant stars observed with near-infrared interferometry
Our team has obtained observations of the photosphere of the two closest red
supergiant stars Betelgeuse ( Ori) and Antares ( Sco) using
near infrared interferometry. We have been monitoring the photosphere of
Betelgeuse with the VLTI/PIONIER instrument for three years. On Antares, we
obtained an unprecedented sampling of the visibility function. These data allow
us to probe the convective photosphere of massive evolved stars.Comment: 5 pages, 3 figures. Published in the proceedings of the Physics Of
Evolved Stars conference, dedicated to the memory of Olivier Chesneau (Nice,
France, 2015
The Hertzsprung-gap giant 31 Comae in 2013: Magnetic field and activity indicators
We have observed the giant star 31 Comae in April and May 2013 with the spectropolarimeter Narval at Pic du Midi Observatory, France. 31 Comae is a single, rapidly rotating giant with rotational period ~6.8 d and vsini ~ 67 km/s. We present measurements and discuss variability of the longitudinal magnetic field (Bl), spectral activity indicators Hα, CaII H&K, Ca II IR triplet and evolutionary status. Our future aim is to perform a Zeeman-Doppler imaging study for the sta
Pollux: a stable weak dipolar magnetic field but no planet?
Pollux is considered as an archetype of a giant star hosting a planet: its radial velocity (RV) presents sinusoidal variations with a period of about 590 d, which have been stable for more than 25 years. Using ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic field at the surface of Pollux and followed up its variations with Narval during 4.25 years, i.e. more than for two periods of the RV variations. The longitudinal magnetic field is found to vary with a sinusoidal behaviour with a period close to that of the RV variations and with a small shift in phase. We then performed a Zeeman Doppler imaging (ZDI) investigation from the Stokes V and Stokes I least-squares deconvolution (LSD) profiles. A rotational period is determined, which is consistent with the period of variations of the RV. The magnetic topology is found to be mainly poloidal and this component almost purely dipolar. The mean strength of the surface magnetic field is about 0.7 G. As an alternative to the scenario in which Pollux hosts a close-in exoplanet, we suggest that the magnetic dipole of Pollux can be associated with two temperature and macroturbulent velocity spots which could be sufficient to produce the RV variations. We finally investigate the scenarii of the origin of the magnetic field which could explain the observed properties of Pollu
Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?
We present our first results on a new sample containing all single G, K and M giants down to V = 4 mag in the Solar vicinity, suitable for spectropolarimetric (Stokes V) observations with Narval at TBL, France. For detection and measurement of the magnetic field (MF), the Least Squares Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present case enables detection of large-scale MFs even weaker than the solar one (the typical precision of our longitudinal MF measurements is 0.1-0.2 G). The evolutionary status of the stars is determined on the basis of the evolutionary models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and fundamental parameters given by Massarotti et al. (1998). The stars appear to be in the mass range 1-4 M ⊙, situated at different evolutionary stages after the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB). The sample contains 45 stars. Up to now, 29 stars are observed (that is about 64% of the sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the base of the Red Giant Branch (RGB) and in the He-burning phase are detected. Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB phase are detected (8 of 13 observed stars). For all Zeeman detected stars v sin i is redetermined and appears in the interval 2-3 km/s, but few giants with MF possess larger v sin
Monitoring of the magnetic field topology and activity of the core helium-burning giant beta Ceti in the period 2010-2013
Beta Ceti is a slowly rotating (v sin i = 3.5 kms−1) single giant. In our previous study (Tsvetkova et al. (2013)) we showed that it is in the core He-burning phase and we reconstructed two Zeeman Doppler imaging (ZDI) maps (using data from 2010 and 2011) revealing a simple large-scale magnetic field structure. We concluded that the magnetic field of beta Ceti could have a fossil field origin. In addition, the study of Aurière et al. (2015) about the properties and origin of the magnetism of late-type giants, where beta Ceti was a member of that sample, revealed that this star did not follow the general trends for dynamo-generated magnetic fields. Now, we present a new ZDI map of beta Ceti and compare the new results with our previous study. This monitoring for several years of the magnetic field topology and line activity indicators variability supports our previous conclusion about the fossil field origin of the magnetic field of beta Ceti