Astronomy and Astrophysics, volume 574A, 31-31 (2015/2-1)
Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517.
STRASSMEIER K.G., CARROLL T.A., WEBER M. and GRANZER T.
Abstract (from CDS):
HD233517 is among the most Li-rich stars in the sky. It is a rapidly rotating, single K giant thought to be on its first ascent on the red giant branch. The star has also the highest known infrared excess among any of the known first-ascent giants. We revisit the physical parameters of the system and aim to map its surface temperature distribution.New time-series photometry and high-resolution spectroscopy were obtained with our robotic facilities STELLA and Amadeus Automatic Photoelectric Telescope (APT) in 2007-2011. Inverse line-profile modelling is performed on a total of 167 echelle spectra and six Doppler images are presented.Light and radial-velocity variations suggest a stellar rotation period of 47.6±0.3d. The atmospheric parameters agree with previous studies and verify a super-meteoritic log 7Li abundance of 4.29±0.10 with undetected 6Li, while the metals are generally deficient by -0.4dex with respect to the Sun. We determine a lower than normal isotopic carbon ratio of 12C/13C=9+4–2. Our Doppler images indicate warm and cool spots with an average temperature contrast of just ±65K with respect to the effective temperature. Doppler maps from LiI 670.78 reveal practically identical surface morphology, with a higher average contrast of ±160K and errors that are five times larger. Reconstructions with simultaneously 1617 and 3007 spectral lines showed both a signal degradation with respect to our 56-line final image. An error analysis indicates an average temperature error per surface pixel of just ±4K. HD233517 appears to be an old (≃10-Gyr) single 0.95-M☉ giant currently undergoing mild mass loss in the form of a wind. The cool and warm photospheric features are interpreted to be merely locations of suppressed and enhanced convection, respectively, probably intermingled by a yet undetected weak magnetic field. The low carbon-isotope ratio is indicative of extra mixing rather than of an engulfing event. We tentatively conclude that HD233517 operates an enhanced non-axisymmetric mixing process that leads to an inhomogeneous super-granulation pattern on the surface in form of large cool and warm features.