Based in part on observations made with the European Southern Observatory (ESO) telescopes obtained from the ESO/ST-ECF Science Archive Facility.
The influence of the C+N+O abundances on the determination of the relative ages of globular clusters: the case of NGC 1851 and NGC 6121 (M4)
Version of Record online: 14 SEP 2009
© 2009 The Authors. Journal compilation © 2009 RAS
Monthly Notices of the Royal Astronomical Society: Letters
Volume 399, Issue 1, pages L151–L155, October 2009
How to Cite
D'Antona, F., Stetson, P. B., Ventura, P., Milone, A. P., Piotto, G. and Caloi, V. (2009), The influence of the C+N+O abundances on the determination of the relative ages of globular clusters: the case of NGC 1851 and NGC 6121 (M4). Monthly Notices of the Royal Astronomical Society: Letters, 399: L151–L155. doi: 10.1111/j.1745-3933.2009.00756.x
This paper makes use of data obtained from the Isaac Newton Group Archive which is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge.
- Issue online: 1 OCT 2009
- Version of Record online: 14 SEP 2009
- Accepted 2009 August 11. Received 2009 July 31; in original form 2009 June 27
- Galaxy: formation;
- globular clusters: general;
- globular clusters: individual: NGC 1851;
- globular clusters: individual: NGC 6121
The colour–magnitude diagram (CMD) of NGC 1851 presents two subgiant branches (SGBs), probably due to the presence of two populations differing in total C+N+O (carbon+nitrogen+oxygen) content. We test the idea that a difference in total C+N+O may simulate an age difference when comparing the CMD of clusters to derive relative ages. We compare NGC 1851 with NGC 6121 (M4), a cluster of very similar [Fe/H]. We find that, with a suitable shift of the CMDs that brings the two red horizontal branches at the same magnitude level, the unevolved main sequence and red giant branch match, but the SGB of NGC 6121 and its red giant branch ‘bump’ are fainter than in NGC 1851. In particular, the SGB of NGC 6121 is even slightly fainter than the faint SGB in NGC 1851. Both these features can be explained if the total C+N+O in NGC 6121 is larger than that in NGC 1851, even if the two clusters are coeval. We conclude by warning that different initial C+N+O abundances between two clusters, otherwise similar in metallicity and age, may lead to differences in the turnoff morphology that can be easily attributed to an age difference.