Nitrogen abundance in turn-off stars of NGC 6397 and NGC 6752^*

¹ European Southern Observatory, Garching bei München, Germany e-mail: lpasquin@eso.org
² Institut d'Astrophysique, Paris, France
³ CIFIST Marie Curie Excellence Team, France
⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁵ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy

Received: 14 April 2008
Accepted: 12 June 2008

Abstract

Aims. Our goal is to measure the nitrogen abundance in 5 turn off (TO) stars of the two globular clusters NGC 6397 and NGC 6752, and to compare the cluster abundances with those of field stars of comparable metallicity. In addition, we investigate the variations in nitrogen abundance and its connections to variations in the abundances of other light elements.

Methods. We determine the nitrogen abundance from the compact band-head of the (0-0) vibrational band of the NH system at 3360 Å, using spectra of resolution R~ 45 000 acquired using the UVES spectrograph on the ESO Kueyen-VLT 8.2 m telescope, analysed with spectrum synthesis based on plane parallel LTE model atmospheres. We apply a method previously applied to field stars, to enable a direct comparison to be made between the results.

Results. The nitrogen abundances of two stars in NGC 6397 are found to be almost identical, even though there is a difference of one order of magnitude in their oxygen abundances. In a third star of the same cluster, the nitrogen abundance is slighty lower, although consistent, within the calculated margins of error, with the other two measurements. All stars in NGC 6397 are N-rich with respect to field objects of similar metallicity. The two stars in NGC 6752 show a difference in nitrogen abundance of over one order of magnitude. The same stars differ in the abundances of other elements such as Na, O, and Li, although only by a factor ~3–4.

Conclusions. NGC 6397 and NGC 6752 are, at present, the only two globular clusters in which N has been measured in TO stars in a way consistent with similar measurements for field stars. The behaviour of N is different in the two clusters: no variation is observed between stars in NGC 6397, while a large variation is observed in NGC 6752. This is consistent with a picture in which the stars in NGC 6752 have been formed by a mixture of “pristine” material and material which has been processed by an early generation of stars, referred to as “polluters”. The N abundances reported will help to constrain the properties of the polluters. In the case of NGC 6397, a simple pollution history is probably not viable, since the observed variations in O abundances are not accompanied by corresponding variations in N or Li.