Magnetic field structure in single late-type giants: The weakG-band giant 37 Comae from 2008 to 2011

Abstract

Aims. This work studies the magnetic activity of the late-type giant 37 Com. This star belongs to the group of weak Gband stars that present very strong carbon deficiency in their photospheres. The paper is a part of a global investigation into the properties and origin of magnetic fields in cool giants. Methods. We use spectropolarimetric data, which allows the simultaneous measurement of the longitudinal magnetic field B l , line activity indicators (Hα, Ca ii IRT, S-index) and radial velocity of the star, and consequently perform a direct comparison of their time variability. Mean Stokes V profiles are extracted using the least squares deconvolution (LSD) method. One map of the surface magnetic field of the star is reconstructed via the Zeeman Doppler imaging (ZDI) inversion technique. Results. A periodogram analysis is performed on our dataset and it reveals a rotation period of 111 days. We interpret this period to be the rotation period of 37 Com. The reconstructed magnetic map reveals that the structure of the surface magnetic field is complex and features a significant toroidal component. The time variability of the line activity indicators, radial velocity and magnetic field B l indicates a possible evolution of the surface magnetic structures in the period from 2008 to 2011. For completeness of our study, we use customized stellar evolutionary models suited to a weak G-band star. Synthetic spectra are also calculated to confirm the peculiar abundance of 37 Com. Conclusions. We deduce that 37 Com is a 6.5 M⊙ weak G-band star located in the Hertzsprung gap, whose magnetic activity is probably due to dynamo action.

The magnetic strip(s) in the advanced phases of stellar evolution

“…At the effective temperature of one of the lowest-mass stars observed by Aurière et al (2015, HD 203387; T eff = 5012 K, log L/L = 1.87) the theoretical period of the 2 M model varies between 10 and 60 days (which implies a factor of 6 variation on the theoretical Rossby between the two models) depending on the initial rotation velocity, while the observed period for this star is 68 days. While the overall agreement is relatively good, comparison with the magnetic and rotational properties of individual stars requires tailor-made models with different initial rotation rates for all individual sample stars (e.g., Aurière et al 2009Aurière et al , 2011Aurière et al , 2012Konstantinova-Antova et al 2010Tsvetkova et al 2013;Borisova et al 2016;Tsvetkova et al 2017), which is out of the scope of the present paper.…”

“…stars). Recently, magnetic fields have been unambiguously detected via Zeeman signatures in a large sample of single G-K giants observed with the spectropolatimeters TBL/Narval and CFHT/ESPaDOnS (Konstantinova-Antova et al 2013;Aurière et al 2015;Borisova et al 2016;Tsvetkova et al 2017). Interestingly, the cool intermediate-mass evolved stars with surface magnetic fields are found to cluster in certain regions of the HRD that correspond to precise moments of their evolution where convective envelopes make up a significant fraction of the stellar mass.…”

A decade-long magnetic monitoring of Vega

“…Period searches using ZDI were mainly performed on cool stars (e.g. Petit et al 2008;Tsvetkova et al 2017;Aurière et al 2021), and proved to be a powerful approach to isolating the stellar rotation signal.…”

Li-rich giant stars under scrutiny: binarity, magnetic activity, and the evolutionary status after Gaia DR2

“…The coefficients used were adjusted for cool giants as described by Aurière et al (2015) and Tsvetkova et al (2017). The errors were computed by using the standard error propagation from the observation spectrum.…”