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 (?~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 (?os=0.1). By adopting this theoretical set an age of ?=1.9+/-0.1 Gyr has been obtained.
Mucciarelli A., Ferraro F.R., Origlia L., Fusi Pecci F. (2007). The Globular Cluster NGC 1978 in the Large Magellanic Cloud. THE ASTRONOMICAL JOURNAL, 133, 2053-2060 [10.1086/513076].
The Globular Cluster NGC 1978 in the Large Magellanic Cloud
MUCCIARELLI, ALESSIO;FERRARO, FRANCESCO ROSARIO;
2007
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 (?~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 (?os=0.1). By adopting this theoretical set an age of ?=1.9+/-0.1 Gyr has been obtained.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
