Na-O anticorrelation and horizontal branches

Abstract

We present the LTE abundance analysis of high resolution spectra for red giant stars in the peculiar bulge globular cluster NGC 6388. Spectra of seven members were taken using the UVES spectrograph at the ESO VLT2 and the multiobject FLAMES facility. We exclude any intrinsic metallicity spread in this cluster: on average, [Fe/H] = −0.44 ± 0.01 ± 0.03 dex on the scale of the present series of papers, where the first error bar refers to individual star-to-star errors and the second is systematic, relative to the cluster. Elements involved in H-burning at high temperatures show large spreads, exceeding the estimated errors in the analysis. In particular, the pairs Na and O, Al and Mg are anticorrelated and Na and Al are correlated among the giants in NGC 6388, the typical pattern observed in all galactic globular clusters studied so far. Stars in NGC 6388 shows an excess of α−process elements, similar to the one found in the twin bulge cluster NGC 6441. Mn is found underabundant in NGC 6388, in agreement with the average abundance ratio shown by clusters of any metallicity. Abundances of neutron-capture elements are homogeneously distributed within NGC 6388; the [Eu/Fe] ratio stands above the value found in field stars of similar metallicity.

A double stellar generation in the globular cluster NGC 6656 (M 22)

“…In Fig. 4, we compare the Na versus O trend for M 22 with those observed in other four GCs where multiple stellar populations have been identified: NGC 2808 (data from Carretta et al 2006), NGC 1851 (data from Yong & Grundahl 2008), NGC 6388 (data from Carretta et al 2007), and M 4 (data from Marino et al 2008).…”

“…Moreover, no split in either the MS or SGB has been iden- As in M 22, in NGC 1851 (Milone et al 2008) and NGC 6388 (Piotto 2008;Moretti et al 2009;Piotto 2009) the presence of multiple populations is inferred from a split in the SGB. Unfortunately, for both of these clusters, the available chemical measurements from high resolution spectroscopy are limited to seven RGB stars for NGC 6388 (Carretta et al 2007) and eight RGB stars for NGC 1851 (Yong & Grundahl 2008). In the case of NGC 6388, the stars are located in a portion of the NaO plane that is not populated by any M 22 star in our sample, since the stars in NGC 6388 are systematically O-poorer.…”

Multiple stellar populations in the globular cluster NGC 1851

“…The procedure for error estimates is the same as in Carretta et al (2010e) and results are given in the Appendix for UVES and GIRAFFE observations in Tables A.1 and A .2, respectively. As in our previous papers, oxygen abundances are obtained from the forbidden [O i] lines at 6300.3 and 6363.8 Å, after cleaning the former from telluric lines as described in Carretta et al (2007c). The prescription of Gratton et al (1999) was adopted to correct the derived Na abundances for non-LTE effects (from the 5682−88 and 6154−60 Å doublets).…”

Low-resolution spectroscopy of main sequence stars belonging to 12 Galactic globular clusters

“…It is very centrally concentrated and tightly bound, and has among the highest predicted escape velocity at the cluster center (McLaughlin & van der Marel 2005). Based on highresolution FLAMES-UVES spectra, Carretta et al (2007a) measured [Fe/H] = −0.44 ± 0.01(±0.03). These authors also detected the presence of Na-O and Mg-Al anti-correlations, and of a Na-Al correlation among RGB stars.…”

“…Its orbit is remarkably similar to that of 47 Tuc (Dinescu et al 2000), and it is therefore associated with the thick disc by some authors. Many high-resolution spectroscopic studies of this cluster provide the following iron abundance measurements: [Fe/H] = −1.48 ± 0.01 ± 0.06 dex by ) based on FLAMES-UVES spectra of 7 giants near the RGB bump, [Fe/H] = −1.42 based on UVES spectra of MS and sub-giant stars (Carretta et al 2007a(Carretta et al , 2009c.…”