Antisolar differential rotation with surface lithium enrichment on the single K-giant V1192 Orionis

Abstract

Context. Stars with about 1−2 solar masses at the red giant branch (RGB) represent an intriguing period of stellar evolution, i.e. when the convective envelope interacts with the fast-rotating core. During these mixing episodes freshly synthesized lithium can come up to the stellar surface along with high angular momentum material. This high angular momentum may alter the surface rotation pattern. Aims. The single rapidly rotating K-giant V1192 Ori is revisited to determine its surface differential rotation, lithium abundance, and basic stellar properties such as a precise rotation period. The aim is to independently verify the antisolar differential rotation of the star and possibly find a connection to the surface lithium abundance. Methods. We applied time-series Doppler imaging to a new multi-epoch data set. Altogether we reconstructed 11 Doppler images from spectroscopic data collected with the STELLA robotic telescope between 2007-2016. We used our inversion code iMap to reconstruct all stellar surface maps. We extracted the differential rotation from these images by tracing systematic spot migration as a function of stellar latitude from consecutive image cross-correlations. Results. The position of V1192 Ori in the Hertzsprung-Russell diagram suggests that the star is in the helium core-burning phase just leaving the RGB bump. We measure A(Li) NLTE = 1.27, i.e. a value close to the anticipated transition value of 1.5 from Li-normal to Li-rich giants. Doppler images reveal extended dark areas arranged quasi-evenly along an equatorial belt. No cool polar spot is found during the investigated epoch. Spot displacements clearly suggest antisolar surface differential rotation with α = −0.11 ± 0.02 shear coefficient. Conclusions. The surface Li enrichment and the peculiar surface rotation pattern may indicate a common origin.

Enhanced Stellar Activity for Slow Antisolar Differential Rotation?

“…The phenomenon of antisolar DR is well known from theoretical models of solar/stellar convective dynamos in spherical shells. So far, antisolar DR has only been observed in some K giants (Strassmeier et al 2003;Weber et al 2005;Kővári et al 2015Kővári et al , 2017 and subgiants (Harutyunyan et al 2016), but not yet in dwarfs. Our work is compatible with the interpretation that the enhanced activity at large Rossby numbers (slow rotation) is a manifestation of antisolar DR. Our results are suggestive of a bifurcation into two groups of stars: those which undergo reduced braking and become inactive at P rot /τ ≈ 2 (van Saders et al 2016), and those that enter the regime of antisolar rotation and continue to brake at enhanced activity, although with chaotic time variability.…”

Advances in mean-field dynamo theory and applications to astrophysical turbulence

“…So far, antisolar DR has only been observed in some K giants (Strassmeier, Kratzwald & Weber 2003; Weber, Strassmeier & Washuettl 2005; Kővári et al. 2015, 2017) and subgiants (Harutyunyan et al. 2016).…”

Superflares on the late-type giant KIC 2852961

“…Still, other mechanisms could also be con-sidered to spin up the surface of an evolved star on the red giant branch. A certain fraction of red giants have undergone such spin up phases (e.g., Carlberg et al 2011;Ceillier et al 2017), which may involve mixing processes (Simon & Drake e.g., 1989, but see also Kriskovics et al 2014;Kővári et al 2017b), planet engulfment (Siess & Livio 1999;Kővári et al 2016), binary mergers (Webbink 1976;Strassmeier et al 1998) or other, less known mechanisms. However, the lack of systematic radial velocity measurements does not enable to know whether KIC 2852961 is a member of a wide binary system or rather a close binary.…”