The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

Abstract

Abstract. High dispersion spectra (R ∼ > 40 000) for a quite large number of stars at the main sequence turn-off and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The [Fe/H] values we found are −2.03 ± 0.02 ± 0.04 and −1.42 ± 0.02 ± 0.04 for NGC 6397 and NGC 6752 respectively, where the first error bars refer to internal and the second ones to systematic errors (within the abundance scale defined by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the [Fe/H]'s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The [O/Fe] = 0.21 ± 0.05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the first time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

Sulfur in the globular clusters 47 Tucanae and NGC 6752

“…Star identifications, Na, and Mg abundances in Table 3 come from Gratton et al (2001). In Table 4, identifications, atmosphere parameters and abundances other than S come from Carretta et al (2004), with the exception of star # 952, described in Bonifacio et al (2007).…”

“…The atmospheric parameters have been taken from Gratton et al (2001) for the stars in NGC 6752 and from Carretta et al (2004) and Bonifacio et al (2007) (for star # 952) for the stars in 47 Tuc. The 47 Tuc stars have a typical temperature of 5800 K for the TO stars (log g = 4.05) and of 5100 K for the subgiants (log g = 3.84).…”

“…Observations, atmospheric parameters, and abundances for elements other than S have been presented in Gratton et al (2001), Carretta et al (2004), and Bonifacio et al (2007). We are only concerned here with the dichroic #2, CD4 cross-disperser UVES@VLT (Dekker et al 2000) spectra (setting centered at 760 nm).…”

Globular cluster abundances: the role of asymptotic giant branch stars

“…The star-to-star abundance variations in C, N, O and Na have been observed in stars at or near the main-sequence turnoff (e.g., Gratton et al 2001;Ramírez & Cohen 2003), supporting the hypothesis that the GC stars did not produce their abundances in situ. Instead, a previous generation of stars polluted the atmospheres of stars we observe today or provided part of the material from which those stars formed (Cottrell & Da Costa 1981).…”

“…Hence, these abundance patterns are somehow the result of the cluster environment. The variations of the elements follow a similar pattern: C-N, O-Na and Mg-Al are anticorrelated (Shetrone 1996;Kraft et al 1997;Cannon et al 1998;Gratton et al 2001;Cohen & Meléndez 2005). Furthermore, Smith et al (2005) noted that stars in M4 showed evidence for a Na-F anticorrelation (an F spread was also found in NGC 6712; Yong et al 2008a), while Pasquini et al (2005) and Bonifacio et al (2007) found a Na-Li anticorrelation in NGC 6752 and 47 Tucanae, respectively.…”