MNRAS 430, 2774–2788 (2013) doi:10.1093/mnras/stt079
The star formation history of the Large Magellanic Cloud star clusters NGC 1846 and NGC 1783
Stefano Rubele,1† Leo´ Girardi,1 Vera Kozhurina-Platais,2 Leandro Kerber,3 Paul Goudfrooij,2 Alessandro Bressan4 and Paola Marigo5 1Osservatorio Astronomico di Padova – INAF, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy 2Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 3Universidade Estadual de Santa Cruz, Rodovia Ilheus-Itabuna,´ km. 16 – 45662-000 Ilheus,´ Bahia, Brazil 4SISSA, via Bonomea 265, I-34136 Trieste, Italy 5Dipartimento di Fisica e Astronomia, Universita` di Padova, Vicolo dell’Osservatorio 2, I-35122 Padova, Italy Downloaded from
Accepted 2013 January 11. Received 2013 January 10; in original form 2012 September 3
ABSTRACT http://mnras.oxfordjournals.org/ NGC 1846 and NGC 1783 are two massive star clusters in the Large Magellanic Cloud, hosting both an extended main-sequence turn-off and a dual clump of red giants. They present similar masses but differ mainly in angular size. Starting from their high-quality ACS data in the F435W, F555W and F814W filters, and updated sets of stellar evolutionary tracks, we derive their star formation rates as a function of age, SFR(t), by means of the classical method of colour–magnitude diagram reconstruction which is usually applied to nearby galaxies. The method confirms the extended periods of star formation derived from previous analysis of the same data. When the analysis is performed for a finer resolution in age, we find clear evidence for a ∼50-Myr long hiatus between the oldest peak in the SFR(t), and a second prolonged at Space Telescope Science Institute on April 16, 2013 period of star formation, in both clusters. For the more compact cluster NGC 1846, there seems to be no significant difference between the SFR(t) in the cluster centre and in an annulus with radii between 20 and 60 arcsec (from 4.8 to 15.4 pc). The same does not occur in the more extended NGC 1783 cluster, where the outer ring (between 33 and 107 arcsec, from 8.0 to 25.9 pc) is found to be slightly younger than the centre. We also explore the best-fitting slope of the present-day mass function and binary fraction for the different cluster regions, finding hints of a varying mass function between centre and outer ring in NGC 1783. These findings are discussed within the present scenarios for the formation of clusters with multiple turn-offs. Key words: stars: evolution – Hertzsprung–Russell and colour–magnitude diagrams.
also seem to present a dual clump of red giants, in similarity to the 1 INTRODUCTION Small Magellanic Cloud (SMC) cluster NGC 419 (Girardi, Rubele The star clusters NGC 1846 and NGC 1783 represent prototypes & Kerber 2009) and the LMC’s NGC 1751 (Rubele et al. 2011). of massive intermediate-age star clusters in the Large Magellanic The presence of MMSTOs is commonly interpreted as the signa- Cloud (LMC) containing multiple main-sequence turn-offs (MM- ture of continued star formation, or multiple events of star forma- STO; Mackey & Broby Nielsen 2007; Mackey et al. 2008; Goud- tion, spanning a few 100 Myr in time (e.g. Mackey & Broby Nielsen frooij et al. 2009; Milone et al. 2009). They have masses of about 2007; Mackey et al. 2008; Goudfrooij et al. 2009, 2011b; Conroy & 1.5 and 1.7 × 105 M, respectively, which locate them among the Spergel 2011; Girardi, Eggenberger & Miglio 2011; Keller, Mackey most massive LMC clusters except for the old globulars (see e.g. & Da Costa 2011).1 For a well-defined age range between 1.2 and fig. 13 in Girardi et al. 1995). In addition to the MMSTOs, they 1.7 Gyr, star clusters with a turn-off age spanning a few 100 Myr