Mudumba Parthasarathy

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

We have derived the chemical composition of nine UV-bright stars belonging to five Galactic globular clusters of various metallicities ((Fe/H) from −1. 0t o−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 anal- ysis 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 high- luminosity 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 overabun- dant 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).

The Evolutionary Status of HD 179821

HD 179821 (IRAS 19114+0002) is classified as a moderately metal-poor ([Fe/H] = − 0.5) post-AGB star of Teff = 5660K and also as a very massive supergiant of Teff = 6750K. Recent detailed studies of this star are not able to establish whether HD 179821 is a low mass star in post-AGB stage of evolution or a massive (30 Solar Mass) yellow supergiant caught in between RGB and Wolf Rayet phase (Reddy and Hrivnak (1999), Jura and Werner (1999), Thevenin et.al (2000)). Our LTE analysis of the spectrum of HD 179821 and the recent radio study of circumstellar envelope by Josselin and Lebre (2000) favours the hypothesis that it is a low mass star in post-AGB stage of evolution.