EDP Sciences
Free access
Volume 403, Number 1, May III 2003
Page(s) 225 - 237
Section Formation and evolution of planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:20030192

A&A 403, 225-237 (2003)
DOI: 10.1051/0004-6361:20030192

The red tail of carbon stars in the LMC: Models meet 2MASS and DENIS observations

P. Marigo1, L. Girardi2 and C. Chiosi1

1  Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
2  Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy

(Received 9 December 2002 / Accepted 4 February 2003 )

Carbon stars are known to exhibit systematically redder near-infrared colours with respect to M-type stars. In the near-infrared colour-magnitude diagrams provided by the 2MASS and DENIS surveys, the LMC C-type stars draw a striking "red tail", well separated from the sequences of O-rich giants. So far, this conspicuous feature has been absent from any set of available isochrones, even the few existing ones that include the TP-AGB evolution of low- and intermediate-mass stars. To investigate such issue we simulate the complete 2MASS $K_{\rm s}$ vs. $(J-K_{\rm s})$ data towards the LMC by means of a population synthesis approach, that relies on extended libraries of published stellar evolutionary tracks, including the TP-AGB phase. The simulations provide quite a detailed description of the several vertical "fingers" and inclined sequences seen in 2MASS data, due to both galactic foreground and LMC O-rich stars. Instead, as mentioned, the red tail of C-stars sets a major difficulty: we find that TP-AGB models with solar-scaled molecular opacities, the usual assumption of existing AGB calculations, do not succeed in reproducing this feature. Our tests indicate that the main reason for this failure should not be ascribed to empirical $T_{\rm eff}$- (J-K) transformations for C-type stars. Instead, the discrepancy is simply removed by adopting new evolutionary models that account for the changes in molecular opacities as AGB stars get enriched in carbon via the third dredge-up (Marigo 2002). In fact, simulations that adopt these models are able to reproduce, for the first time, the red tail of C-stars in near-infrared CMDs. Finally, we point out that these simulations also provide useful indications about the efficiency of the third dredge-up process, and the pulsation modes of long-period variables.

Key words: stars: AGB and post-AGB -- stars: evolution -- stars: carbon -- stars: fundamental parameters -- stars: mass loss

Offprint request: P. Marigo, marigo@pd.astro.it

© ESO 2003