A NEW CLASS MAGNETICALLY-ACTIVE STARS UNVEILED BY INTERNATIONAL
TEAM WITH BULGARIAN LEADING PARTICIPATION
December 13, 2011
Observations of fifty six single late-type giant stars have been carried out in the period 2007-2011. The 2m TBL telescope at Pic du Midi, France and 3.6m CFHT telescope, Hawaii (www.cfht.hawaii.edu) equipped with the new generation spectropolarimeters NARVAL and ESPADONS have been used. Both instruments provide high-accuracy magnetic field strength measurements, thus, very weak stellar magnetic fields can be detected. Work on this project was granted and funded under the competition basis by the EU Consortium OPTICON (3 sessions, PI Dr. Renada Konstantinova-Antova), the French PNPS program (PI Prof. Michel Auriere), Bulgarian NSF (PI Dr. Konstantinova-Antova) and a Canadian research program (PI Dr. Gregg Wade). The topic «Magnetic fields in single late giants» is also funded under Bulgarian NSF grants DO 02-85, DSAB 02/3 and in the framework of the Bulgarian-French exchange program RILA/EGIDE (project RILA 05/10). The interntional team with team-liders Assoc. Prof. Dr. Konstantinova-Antova and Prof. Auriere are recognized as world leader in the topic. They are included in the large internetional consortium for magnetic fields and dynamo in cools stars, Bcool and Dr. Konstantinova-Antova is the Thread Coordinator in evolved stars there. The study of magnetic activity in single late giants has been started in 90s at the Bulgarian observatories BNAO-Rozhen and AO-Belogradchik.
The results obtained shed light on the magnetic activity and stellar dynamo operation at the advanced stages of stellar evolution– one poorly understood topic yet. A new class of magnetically-active stars is unveiled. This kind of objects are at an evolutionary stage after the He-burning phase (the so-called Assymptotic Giants Branch). The results are listed by the IAU authorities among the best scientific achievements for the period 2009-2011 and presented in the IAU Comm. 25 report (arxiv.org/abs/1111.0589). We bring to your attention the appropriate excerpt of this report.
IAU Transactions, 2011, Volume XXVIIIA
Reports on Astronomy 2009-2011, Ian Corbett, ed., in press
4.3. Magnetic fields in red giants (C. Charbonnel)
The red giant branch is a crucial phase for understanding the interplay of magnetic fields and stellar evolution. During the last 3 years, a pilot study of more than 50 red giant stars (mainly selected as presenting evidence of magnetic activity) was carried out with the spectropolarimeters NARVAL@OHP and ESPaDOnS@CFHT. The first direct detection of the magnetic field of Betelgeuse was made; this magnetic field may be associated to the giant convection cells that could enable a “local dynamo (Auriåre et al. 2010). Almost all the Zeeman detected sample stars are located at the base of the RGB or at the clump. Importantly the strength of the magnetic fields was found to be related to the rotational period. The fast rotators host magnetic fields of several to tens Gauss (Konstantinova-Antova et al, 2008, 2009). Magnetic fields as weak as one-half G were also detected, e.g.in Pollux (Auriåre et al. 2009); they correspond to the activity level of solar twins, or even weaker (Petit et al. 2008). For EK Eri, it was concluded that the outstanding magnetic field of about 200 Gauss originates from the remnant of a strongly magnetic Ap star (Auriåre et al. 2008, 2011).
Konstantinova-Antova et al. (2010) measured the magnetic fields at a level of around a Gauss on several M-type giants among which EK Boo which is either at the onset of the thermal pulse phase on the asymptotic giant branch, or at the tip of the first red giant branch. Some of these M stars are known to be rotating fast for their class, and they are presumably also intermediate-mass AGB stars. In this way, a new class of magnetically active stars was unveiled. More observations like these will answer the question whether these stars are a special case, or magnetic activity is,rather, more common among M giants than expected. An unusually high lithium content was reported in HD 232 862, a field giant classified as a G8II star that hosts a magnetic field (Låbre et al. 2009).
References:
Auriåre, M., Konstantinova-Antova R., Petit P. et al., 2008, A&A, 491, 499
Auriåre, M., Wade, G., Konstantinova-Antova R., et al., 2009, A&A, 504, 231
Auriåre, M., Donati, J.F., Konstantinova-Antova R., et al., 2010, A&A, 516, L2
Auriåre, M., Konstantinova-Antova R., Petit, P., et al., 2011, A&A, in press
Konstantinova-Antova, R., Auriåre M., Iliev I ., et al., 2008, A&A, 480, 475
Konstantinova-Antova R., Auriåre M., Schrîder K.-P., Petit P., 2009, in Proc. IAU Symp. ”Cosmic Magnetic Fields: From Planets, to Stars and Galaxies”, K.G. Strassmeier,A.G. Kosovichev, J.E. Beckman, eds., 433
Konstantinova-Antova, R., Auriåre M., Charbonnel, C. et al., 2010, A&A, 524, A57
Låbre, A., Palacios, A., Do Nascimento, J.D., et al., 2009, A&A, 504, 1011
For more information: Assoc. Prof. Dr. Renada Konstantinova-Antova, renada@astro.bas.bg.