FORS2/VLT survey of Milky Way globular clusters

Abstract

Context. Globular clusters trace the formation and evolution of the Milky Way and surrounding galaxies, and outline their chemical enrichment history. To accomplish these tasks it is important to have large samples of clusters with homogeneous data and analysis to derive kinematics, chemical abundances, ages and locations. Aims. We obtain homogeneous metallicities and α-element enhancement for 51 Galactic bulge, disc, and halo globular clusters that are among the most distant and/or highly reddened in the Galaxy's globular cluster system. We also provide membership selection based on stellar radial velocities and atmospheric parameters. The implications of our results are discussed. Methods. We observed R ∼ 2000 spectra in the wavelength interval 456-586 nm for over 800 red giant stars in 51 Galactic globular clusters. We applied full spectrum fitting with the code ETOILE together with libraries of observed and synthetic spectra. We compared the mean abundances of all clusters with previous work and with field stars. We used the relation between mean metallicity and horizontal branch morphology defined by all clusters to select outliers for discussion. Results. [Fe/H], [Mg/Fe], and [α/Fe] were derived in a consistent way for almost one-third of all Galactic globular clusters. We find our metallicities are comparable to those derived from high-resolution data to within σ = 0.08 dex over the interval −2.5 < [Fe/H] < 0.0. Furthermore, a comparison of previous metallicity scales with our values yields σ < 0.16 dex. We also find that the distribution of [Mg/Fe] and [α/Fe] with [Fe/H] for the 51 clusters follows the general trend exhibited by field stars. It is the first time that the following clusters have been included in a large sample of homogeneous stellar spectroscopic observations and metallicity derivation: BH 176, Djorg 2, Pal 10, NGC 6426, Lynga 7, and Terzan 8. In particular, only photometric metallicities were available previously for the first three clusters, and the available metallicity for NGC 6426 was based on integrated spectroscopy and photometry. Two other clusters, HP 1 and NGC 6558, are confirmed as candidates for the oldest globular clusters in the Milky Way. Conclusions. Stellar spectroscopy in the visible at R ∼ 2000 for a large sample of globular clusters is a robust and efficient way to trace the chemical evolution of the host galaxy and to detect interesting objects for follow-up at higher resolution and with forthcoming giant telescopes. The technique used here can also be applied to globular cluster systems in nearby galaxies with current instruments and to distant galaxies with the advent of ELTs.

Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569

“…These values are consistent with previous spectroscopic and photometric estimates that ranged from [Fe/H]∼−0.75 to −0.90 dex (Zinn 1980;Bica & Pastoriza 1983;Ortolani et al 2001;Valenti et al 2005Valenti et al , 2011Dias et al 2016) and indicate that NGC 6569 is a relatively metalrich globular cluster. Valenti et al (2011) prevented a more detailed analysis, and they were not able to confirm whether the cluster exhibits the usual light-element abundance correlations.…”

Chemical characterisation of the globular cluster NGC 5634 associated to the Sagittarius dwarf spheroidal galaxy

“…For member stars, we found an average RV of −16.07 km s −1 (with σ = 3.98). This value is in excellent agreement with the average RV (−16.7 km s −1 , σ = 5.5) found by S15 from two stars, but not with the older literature value (−45.1 km s −1 ) listed in Harris (1996Harris ( , 2010, as already discussed in S15, or with the value of −29.6 (σ = 39.1) km s −1 in Dias et al (2016), that was however obtained on much-lower-resolution spectra.…”

“…Finally, Dias et al (2016) analysed spectra of nine stars in the wavelength range 4560-5860 Å, at a resolution approximately 2000. They measured radial velocities (RV) and found that eight of the stars are members of the cluster; they also determined atmospheric parameters, iron and Mg (or alpha) abundance ratios using a comparison with stellar libraries.…”

“…Except A118, page 4 of 13 Table 11, the mean abundances in NGC 5634 are summarised. We cross-matched our stars with the nine objects in Dias et al (2016) and found only three in common. Stars # 9, 54, and 87 in our sample are within 1 of stars # 11, 8 and 7 in their list and have reasonably similar V magnitudes (theirs are instrumental values) and atmospheric parameters.…”

The vertical Na–O relation in the bulge globular cluster NGC 6553

“…We have three stars in common with Tang (Table 2) -their results for these stars are compatible with our results taking into account the errors. Dias et al (2016) studied the low-resolution optical spectra of the same stars from Saviane et al (2012). They used a full-spectrum fitting technique to derive the abundance.…”