Chemical composition and evolutionary status of nine UV-bright stars in five globular clusters from VLT/UVES spectra

Abstract

Abstract.We have derived the chemical composition of nine UV-bright stars belonging to five Galactic globular clusters of various metallicities ([Fe/H] from −1.0 to −2.4 dex). The analyses are based on high resolution spectra obtained with the UV-Visual Echelle Spectrograph (UVES) at VLT-UT2. The evolutionary status of the stars is assessed from the chemical analysis and location in the H-R diagram. The star ID7 in NGC 5986 is confirmed as a bona fide post-asymptotic giant branch star (post-AGB) whereas the highluminosity star ID6 has probably left the AGB before the third dredge-up. ZNG 1 in NGC 6712 shows an overabundance of sodium, oxygen, and silicon similar to overabundances we find in the UV-bright star ID6 in NGC 5986; both stars could be in a post-early-AGB (PEAGB) phase of evolution. The UV-bright star ZNG 7 in NGC 6218 seems to be an AGB star. The stars V-4 and ZNG 5 in NGC 6656 are in a post-horizontal-branch phase of evolution, with V-4 being significantly overabundant in heavy elements. The origin of these overabundances is discussed in the context of the evolutionary versus primordial scenario. The three UV-bright stars K 260, K 996 and K 1082 observed in the very metal-deficient globular cluster NGC 7078 are post-horizontal-branch stars, one of them being slightly enriched in s-elements but with a luminosity too low for third dredge-up to have occured. The abundance patterns of K 1082 in NGC 7078 seem to indicate the presence of mild diffusion and a radiative levitation process, already reported in the blue HB stars of M 13 (Behr et al. 1999, ApJ, 517, L135) and NGC 6752 (Moehler et al. 1999, A&A, 339, 537).

Chemical Abundances in Giant Stars of the Tidally Disrupted Globular Cluster NGC 6712 from High‐Resolution Infrared Spectroscopy

“…For each star, we obtained a radial velocity measure from the H band and the K band and the velocities measured from each region were in good agreement for a given star. In Table 3, we report the radial Jasniewicz et al (2004) for their post-AGB star, V rad ¼ À116:4 km s À1 . The stellar parameters were derived in the following way.…”

“…Therefore, NGC 6712 has almost certainly contributed stars to the disk and/or halo. Previous abundance analyses of NGC 6712 only considered one post-AGB star (Jasniewicz et al 2004;Mooney et al 2004), whose composition may not reflect the composition of the cluster due to the rich nucleosynthesis occurring in the late phases of stellar evolution. In this paper, we present the first detailed chemical abundance analysis of bright red-giant stars in this tidally disrupted globular cluster.…”

Evolved A-type stars

“…There are several types of evolved A-type stars in globular clusters including blue straggler stars, interacting binaries, A and B type horizontal branch (HB) stars (Moehler 2005), A-type post-HB and post-AGB stars , Jasniewicz et al 2004, Mooney et al 2004. In earlier surveys some were classified as UV-bright stars (Zinn et al 1972).…”

“…Chemical composition studies of B, A, and F type UV-bright stars in globular clusters reveal that some of them are post-AGB A and F stars showing evidence for third dredgeup and overabundance of carbon and s-process elements for example ROA 24 in ω Cen (Gonzalez & Wallerstein 1994), while some show severe carbon deficiency indicating that they left the AGB before the third dredgeup occurred for example ZNG 1 in M10, ROA 5701 in ω Cen and Barnard 29 in M13 , Jasniewicz et al 2004, Mooney et al 2004.…”

“…Recently , Jasniewicz et al (2004), and Mooney et al (2004) derived chemical composition of few A and B type evolved stars in several globular clusters. Some are in the post-HB and the post-AGB stages of evolution.…”