Magnetic field structure in single late-type giants:βCeti in 2010–2012

Abstract

Aims. We study the behavior of the magnetic field and the line activity indicators of the single late-type giant β Ceti. Using spectropolarimetric data, we aim to reconstruct the magnetic field structure on the star's surface and to present the first magnetic maps for β Ceti. Methods. The data were obtained using two spectropolarimeters -Narval at the Bernard Lyot Télescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight circularly-polarized spectra have been collected in the period June 2010-January 2012. The least square deconvolution method was applied for extracting high signal-to-noise ratio line profiles, from which we measured the surface-averaged longitudinal magnetic field B l . Chromospheric activity indicators CaII K, Hα, CaII IR (854.2 nm), and radial velocity were simultaneously measured, and their variability was analyzed along with the behavior of B l . The Zeeman Doppler imaging (ZDI) inversion technique was employed for reconstructing the large-scale magnetic field and two magnetic maps of β Ceti are presented for two periods (June 2010-December 2010 and June 2011-January 2012). Results. The B l stays with a same positive polarity for the whole observational period and shows significant variations in the interval 0.1-8.2 G. The behavior of the line activity indicators is in good agreement with the B l variations. Searching for periodic signals in the Stokes V time series, we found a possible rotation period of 215 days. The two ZDI maps show a mainly axisymmetric and poloidal magnetic topology and a simple surface magnetic field configuration dominated by a dipole. Little evolution is observed between the two maps, in spite of a 1 yr interval between both subsets. We also use state-of-the-art stellar evolution models to constrain the evolutionary status of β Ceti. We derive a mass of 3.5 M and propose that this star is already in the central helium-burning phase. Conclusions. Considering all our results and the evolutionary status of the star, we suggest that dynamo action alone may not be efficient enough to account for the high magnetic activity of β Ceti. As an alternate option, we propose that it is a descendant of an Ap star presently undergoing central helium-burning and still exhibiting a remnant of the Ap star magnetic field.

Detection of ultra-weak magnetic fields in Am stars:βUrsae Majoris andθLeonis

“…A number of cool active stars were reported to display weak net circular polarization after integration over LSD line profiles (Petit et al 2005;Aurière et al 2008Aurière et al , 2011Morgenthaler et al 2012;Tsvetkova et al 2013;Lèbre et al 2014). However, no similar findings have been reported so far in strongly magnetic massive stars or intermediate-mass stars, and the very subtle effect reported for cool stars is nowhere near the extreme stituation reported here.…”

Planets around evolved intermediate-mass stars

“…Those stars present a good correlation between the strength of the magnetic field, the chromospheric activity (S -index) and the rotational period. On the other hand, stars such as EK Eri (Aurière et al 2011) or β Ceti (Tsvetkova et al 2013) with P rot of 308.8 and 215 days, respectively, probably represent the prototype of descendants of magnetic Ap-Bp stars, hosting fossil magnetic fields. They have strong magnetic fields (|B l | max > 10 G) with lower than expected chromospheric activity.…”

Magnetic Field Structure and Activity of the He-burning Giant 37 Comae

“…37 Com shows simpler surface magnetic structure than the fast rotators V390 Aur (Konstantinova-Antova et al 2012) and HD 232862 (Aurière et al in prep. ) and shows more complex structure than the slow rotators EK Eri (Aurière et al 2011) and β Ceti (Tsvetkova et al 2013), which are suspected of being descendants of Ap-stars.…”