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 ( $ 0:30 AE 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 V 555 ¼ 19:10 AE 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 ¼ 0:008 (consistent with the most recent determination of the cluster metallicity, ½M/H ¼ À0:37 dex) and a mild overshooting efficiency (Ã os ¼ 0:1). By adopting this theoretical set an age of ¼ 1:9 AE 0:1 Gyr has been obtained.

Age as a major factor in the onset of multiple populations in stellar clusters

“…It is indeed wellknown from previous studies (Geisler & Hodge 1980;Mucciarelli et al 2007) that NGC 1978 shows a non-negligible ellipticity. Then, we defined a background reference region having the same area as the cluster region in order to statistically subtract field stars from the cluster CMD in U −B vs. B space.…”

“…1 as a red solid curve. (Mucciarelli et al 2007) report an age of t = 1.9 ± 0.1 Gyr for NGC 1978 by using several sets of different isochrones (Padua, BaSTI, PEL). Our slightly different choice of age might be due to the different set of isochrones used in this work, i.e.…”

A study of AGB stars in LMC clusters

“…The O-rich stars and the C-rich stars would then have different masses and ages. The existence of two populations has been suggested by Alcaino et al (1999), but a more recent detailed study by Mucciarelli et al (2007) could not confirm this.…”

“…Both clusters have a similar metallicity around [Fe/H] = − 0.4 (Grocholski et al 2006;Mucciarelli et al 2007). The AGB stars of NGC 1846 have a mass of about 1.8 M (Lebzelter & Wood 2007), while the colour-magnitude diagram of NGC 1978 indicates a somewhat lower mass around 1.4-1.5 M (Mucciarelli et al 2007).…”

“…The AGB stars of NGC 1846 have a mass of about 1.8 M (Lebzelter & Wood 2007), while the colour-magnitude diagram of NGC 1978 indicates a somewhat lower mass around 1.4-1.5 M (Mucciarelli et al 2007). NGC 1978 is elongated, but no indication for a second population (resulting from a merging of two clusters) could be found (Mucciarelli et al 2007). On the opposite Mackey & Broby Nielsen (2007) claim the detection of two populations in NGC 1846 separated in age by about 300 Myr.…”