Ca Ii Triplet Spectroscopy of Small Magellanic Cloud Red Giants. Iii. Abundances and Velocities for a Sample of 14 Clusters

Abstract

We obtained spectra of red giants in 15 Small Magellanic Cloud (SMC) clusters in the region of the CaII lines with FORS2 on the Very Large Telescope (VLT). We determined the mean metallicity and radial velocity with mean errors of 0.05 dex and 2.6 km s −1 , respectively, from a mean of 6.5 members per cluster. One cluster (B113) was too young for a reliable metallicity determination and was excluded from the sample. We combined the sample studied here with 15 clusters previously studied by us using the same technique, and with 7 clusters whose metallicities determined by other authors are on a scale similar to ours. This compilation of 36 clusters is the largest SMC cluster sample currently available with accurate and homogeneously determined metallicities. We found a high probability that the metallicity distribution is bimodal, with potential peaks at -1.1 and -0.8 dex. Our data show no strong evidence of a metallicity gradient in the SMC clusters, somewhat at odds with recent evidence from CaT spectra of a large sample of field stars (Dobbie et al. 2014). This may be revealing possible differences in the chemical history of clusters and field stars. Our clusters show a significant dispersion of metallicities, whatever age is considered, which could be reflecting the lack of a unique AMR in this galaxy. None of the chemical evolution models currently available in the literature satisfactorily represents the global chemical enrichment processes of SMC clusters.A major objective is to measure many clusters spanning as wide a range of age, abundance and location as possible in order to maximize our leverage on the chemical evolution as traced by the age-metallicity relation (AMR), metallicity gradient, kinematics and any variation of SFH with location and/or environment. It is also paramount to definitively test for the existence of any bursts of cluster formation (e.g., Rich et al. 2000).The many free parameters inherent in any realistic chemical evolution model, e.g., that allows radial variations, demands as large a cluster sample as possible. In addition, one requires a homogeneous technique with sufficient precision and accuracy in both age and metallicity. The brightest common stars in clusters older than ≈1 Gyr are red giants.

SMC west halo: a slice of the galaxy that is being tidally stripped?

“…Neither Correnti et al nor Rich et al used a statistical fitting for the metallicities as we did to derive photometric abundances, therefore we can not use them as references. In Paper I, however, we compared our metallicity derivation for HW 40 with that derived using CaII triplet of individual stars by Parisi et al (2015), and they were compatible. Therefore we can assume that our age and metallicity scales are satisfactory.…”

“…The opposite pattern is found for metallicity (Parisi et al 2015), that is, values decrease until a < 4.5…”

“…On the other hand, these works revealed no strong evidence of a metallicity radial gradient. Furthermore, the metallicity distribution presented by Parisi et al (2015) is clearly bimodal, with peaks at [Fe/H] = −1.1 and −0.8 dex. This result do not find a counterpart in the field stars, as demonstrated by the analysis of ∼3000 red giant stars performed by Dobbie et al (2014), who found a single peak at [Fe/H] ∼ −1.0.…”

“…Glatt et al 2008a,b;Girardi et al 2013), the VLT (Parisi et al 2014), and 4 m class telescopes (Piatti et al 2005b(Piatti et al , 2007b(Piatti et al , 2011Piatti 2011a,b;Dias et al 2014), as well as CaII triplet spectroscopy (Parisi et al 2009(Parisi et al , 2015. These analyses also showed how difficult it is to assign a unique AMR because the metallicity for a given age is highly dispersed.…”

The Ca ii triplet in red giant spectra: [Fe/H] determinations and the role of [Ca/Fe]

“…These include Galactic globular clusters (e.g., Armandroff & Da Costa 1991;Da Costa & Armandroff 1995;Rutledge et al 1997;Saviane et al 2012), Galactic open clusters (e.g., Cole et al 2004;Warren & Cole 2009;Carretta et al 2009;Carrera et al 2015) and stars in the Galactic Bulge (e.g., Vásquez et al 2015). It has also been used to study both field stars and star clusters in the LMC (e.g., Olszewski et al 1991;Cole et al 2005;Carrera et al 2008a) and SMC (e.g., Da Costa & Hatzidimitriou 1998;Carrera et al 2008b;Parisi et al 2010;Dobbie et al 2014;Parisi et al 2015), as well as red giants in dwarf spheroidal (e.g., Armandroff & Da Costa 1991;Pont et al 2004;Tolstoy et al 2004;Battaglia et al 2008Battaglia et al , 2011 and dwarf irregular galaxies (e.g., Leaman et al 2013). …”