Evidence for enhanced mixing on the super-meteoritic Li-rich red giant HD 233517^⋆

Leibniz-Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: kstrassmeier@aip.de; tcarroll@aip.de; mweber@aip.de; tgranzer@aip.de

Received: 5 May 2014
Accepted: 7 July 2014

Abstract

Context. HD 233517 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.

Aims. We revisit the physical parameters of the system and aim to map its surface temperature distribution.

Methods. 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 échelle spectra and six Doppler images are presented.

Results. Light and radial-velocity variations suggest a stellar rotation period of 47.6±0.3 d. 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 –0.4 dex 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 ±65 K with respect to the effective temperature. Doppler maps from Li  i 670.78 reveal practically identical surface morphology, with a higher average contrast of ±160 K 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 ±4 K.

Conclusions. HD 233517 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 HD 233517 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.