Volume 552, April 2013
|Number of page(s)||10|
|Section||Galactic structure, stellar clusters and populations|
|Published online||17 April 2013|
1 GEPI – Observatoire de Paris, 64 avenue de l’Observatoire, 75014 Paris, France
2 UPJV – Université de Picardie Jules Verne, 80000 Amiens, France
3 ESO – European Southern Observatory, Karl-Schwarzschild-Str. 3, 85748 Garching bei München, Germany
4 INAF – Capodimonte Astronomical Observatory, via Moiariello 16, 80131 Naples, Italy
Received: 19 December 2012
Accepted: 20 February 2013
Context. Lithium is a fundamental element for studying the mixing mechanisms acting in the stellar interiors, for understanding the chemical evolution of the Galaxy and the Big Bang nucleosynthesis. The study of Li in stars of open clusters (OC) allows a detailed comparison with stellar evolutionary models and permits us to trace its galactic evolution. The OC NGC 2243 is particularly interesting because of its low metallicity ([Fe/H] = −0.54 ± 0.10 dex).
Aims. We measure the iron and lithium abundance in stars of the metal-poor OC NGC 2243. The first aim is to determine whether the Li dip extends to such low metallicities, the second is to compare the results of our Li analysis in this OC with those present in 47 Tuc, a globular cluster of similar metallicity.
Methods. We performed a detailed analysis of high-resolution spectra obtained with the multi-object facility FLAMES at the ESO VLT 8.2 m telescope. Lithium abundance was derived through line equivalent widths and the OSMARCS atmosphere models. Iron abundances from Fe i and Fe ii lines have also been measured and used to check the atmospheric model parameters.
Results. The Li line is detected in 27 stars confirmed as likely cluster members by repeated radial velocity measurements. We determine a Li dip center of 1.06 M⊙, which is much smaller than that observed in solar metallicity and metal-rich clusters. This finding confirms and strengthens the conclusion that the mass of the stars in the Li dip strongly depends on stellar metallicity. The mean Li abundance of the cluster is log n(Li) = 2.70 dex, which is substantially higher than that observed in 47 Tuc. We estimated an iron abundance of [Fe/H] = −0.54 ± 0.10 dex for NGC 2243, which is similar (within the errors) to previous findings. The [α/Fe] content ranges from 0.00 ± 0.14 for Ca to 0.20 ± 0.22 for Ti, which is low when compared to thick disk stars and to Pop II stars, but compatible with thin disk objects. We found a mean radial velocity of 61.9 ± 0.8 km s-1 for the cluster.
Conclusions. We confirm a correlation between the Li dip cool-side position in mass as a function of the cluster’s [Fe/H]. The Li abundance found in the metal-poor OC NGC 2243 agrees well with results obtained for the interstellar medium in the Small Magellanic Cloud having similar low metallicity. This value of Li is comparable to the primordial Li abundance deduced from WMAP measurements, therefore putting strong constraints on the models of Li enrichment during the early history of our Galaxy.
Key words: stars: late-type / open clusters and associations: individual: NGC 2243 / techniques: spectroscopic
Tables 2 and 3 are available in electronic form at http://www.aanda.org
© ESO, 2013
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