AGB stars of the intermediate-age LMC cluster NGC 1846

II. Dredge up along the AGB

¹ Department of Astronomy, University of Vienna, Tuerkenschanzstrasse 17, A1180 Vienna, Austria e-mail: lebzelter@astro.univie.ac.at
² INAF, Osservatorio Astronomico di Collurania, 64100 Teramo, Italy
³ National Optical Astronomy Observatories (Operated by the Association of Universities for Research in Astronomy, under cooperative agreement with the National Science Foundation.) , PO Box 26732, Tucson, AZ 85726, USA
⁴ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy

Received: 8 January 2008
Accepted: 21 April 2008

Abstract

Aims. We investigate the change in the surface abundance of ¹²C during the evolution along the AGB, aiming to constrain third dredge-up models.

Methods. High-resolution, near-infrared spectra of a sample of AGB stars in the LMC cluster NGC 1846 were obtained. A cluster sample ensures a high level of homogeneity with respect to age, metallicity, and distance. The C/O ratio and the ratio of ¹²C/¹³C were measured and compared with our evolutionary models.

Results. For the first time, we show the evolution of the C/O and ¹²C/¹³C ratios along a cluster AGB. Our findings allow us to check the reliability of the evolutionary models and, in particular, the efficiency of the third dredge up. The increase in both C/O and ¹²C/¹³C in the observed O-rich stars is reproduced by the models well. However, the low carbon isotopic ratios of the two C-stars in our sample indicate the late occurrence of moderate extra mixing. The extra mixing affects the most luminous AGB stars and is capable of increasing the abundance of ¹³C, while leaving unchanged the C/O ratio, which has been fixed by the cumulative action of several third dredge-up episodes. We find indications that the F abundance also increases along the AGB, supporting an in situ production of this element.