The Globular Cluster NGC 1978 in the Large Magellanic Cloud
Abstract
We have used deep, high-resolution Hubble Space Telescope ACS observations to image the cluster NGC 1978 in the Large Magellanic Cloud. This high-quality photometric data set allowed us to confirm the high ellipticity (ɛ~0.30+/-0.02) of this stellar system. The derived color-magnitude diagram allowed a detailed study of the main evolutionary sequences; in particular, we have detected the so-called bump along the red giant branch (at V555=19.10+/-0.10). This is the first detection of this feature in an intermediate-age cluster. Moreover, the morphology of the evolutionary sequence and the population ratios have been compared with the expectations of different theoretical models (namely, BaSTI, Pisa Evolutionary Library [PEL], and Padua) in order to quantify the effect of convective overshooting. The best agreement (in terms of both morphology and star counts) has been found within the PEL isochrone, with Z=0.008 (consistent with the most recent determination of the cluster metallicity, [M/H]=-0.37 dex) and a mild overshooting efficiency (Λos=0.1). By adopting this theoretical set an age of τ=1.9+/-0.1 Gyr has been obtained.
- Publication:
-
The Astronomical Journal
- Pub Date:
- May 2007
- DOI:
- 10.1086/513076
- arXiv:
- arXiv:astro-ph/0701649
- Bibcode:
- 2007AJ....133.2053M
- Keywords:
-
- globular clusters: individual: NGC 1978;
- Magellanic Clouds;
- techniques: photometric;
- Astrophysics
- E-Print:
- 26 pages, 8 figures, accepted to AJ