KIC 9821622: An interesting lithium-rich giant in the Kepler field

JOFRÉ, E., PETRUCCI, R., GARCÍA L., GOMEZ, M.

San Juan

Congreso; 59 Reunión Anual de la Asociación Argentina de Astronomía; 2016

Asociación Argentina de Astronomía

The standard stellar evolution theory indicates that the depth of the convective envelope increases as a star leaves the main-sequence. As soon as the star evolves into the red giant branch it experiences the first dredge-up, where the deepening of the convection zone brings high-temperature material from deep internal layers to the stellar surface. Given the fragility of lithium (Li), this mixing process causes a rapid destruction of most of the surface Li. Thus, red giant stars are expected to present low Li abundances (A(Li) . 1.5 dex; Iben 1967, ApJ, 147, 624). However, challenging these predictions, results from different surveys reveal that about 1% of all observed giants are Li-rich (A(Li) > 1.5 dex). Although several scenarios have been proposed to explain these unexpected objects the actual mechanism operating is still unknown. In this contribution we report the discovery of a new exceptional Li-rich giant, KIC 9821622, in the Kepler field. From high-resolution spectra obtained with GRACES at Gemini North, we derive fundamental parameters and detailed chemical abundances of 23 elements from equivalent widths and synthesis analysis. By combining atmospheric stellar parameters with available asteroseismic data, we obtain the stellar mass, radius, and age. The data analysis reveals that KIC 9821622 is a Li-rich (A(Li) = 1.80 dex) intermediate-mass giant star (M = 1.64 M) located at the red giant branch near the luminosity bump, and presents elevated abundances of Fe-peak and r-process elements. In addition, as recently reported by Martig et al. (2015, MNRAS, 451, 2230), we find that this is a young star (2.37 Gyr) with unusually high abundances of alpha-elements ([alpha/Fe] = 0.31). We explore several mechanisms proposed to explain Li-rich giants. The evolutionary status of KIC 9821622 suggests that its Li-rich nature is probably the result of internal fresh Li that is synthesized through the Cameron-Fowler mechanism near the luminosity bump. However, its peculiar enhancement of alpha, Fe-peak, and r-process elements opens the possibility of external contamination by material enriched by a supernova explosion. Although it is less likely, planet accretion cannot be ruled out (Jofr´e et al. 2015, A&A, 584L, 3).