A&A 423, 353–362 (2004) Astronomy DOI: 10.1051/0004-6361:20034504 & c ESO 2004 Astrophysics Chemical composition and evolutionary status of nine UV-bright stars in five globular clusters from VLT/UVES spectra,, G. Jasniewicz1,P.deLaverny2, M. Parthasarathy3,A.Lèbre1, and F. Thévenin2 1 UMR 5024 CNRS/UMII, Université Montpellier II, CC 72, 34095 Montpellier Cedex 5, France e-mail: [email protected] 2 Dpt. Cassiopée, UMR 6202, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France 3 Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India Received 13 October 2003 / Accepted 17 April 2004 Abstract. We have derived the chemical composition of nine UV-bright stars belonging to five Galactic globular clusters of various metallicities ([Fe/H] from −1.0to−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). Key words. stars: abundances – stars : AGB and post-AGB – Galaxy: globular clusters : general 1. Introduction 1974). The presence/absence of supra-HB stars in globulars is strongly correlated with the presence/absence of hot HB stars UV-bright stars are found in several Milky Way globular clus- (Zinn 1974; Moehler 2001). ters, but there are at most a few per cluster. The first sur- The main question we address in this paper is: what is the vey was done by Zinn et al. (1972, ZNG) who introduced the evolutionary status of the UV-bright stars? term “UV-bright stars” for those lying above the horizontal From a theoretical point of view, Dorman et al. (1993) show branch (HB) and bluer than red giants. A first compilation of that there is a critical envelope mass ∼0.05 M for HB stars, these stars was done by Harris et al. (1983). The contribution of above which the fuel supply is adequate to enable the evolu- UV-bright stars to globular cluster luminosities has been tack- tion to reach the later stages of AGB evolution. The normal led by de Boer (1985, 1987). AGB sequence evolves through a thermal pulsing stage to a In addition, observers distinguish among UV-bright stars high post-AGB luminosity log L/L ∼> 3.5. those that are the supra-horizontal-branch stars (supra-HB) that Stars on this post-AGB branch are bona fide post-AGB stars are about one magnitude above the HB, and high-luminosity (see Fig. 1 in Dorman et al. 1993). Dorman et al. (1993) use stars lying several magnitudes above the HB (see Sweigart et al. the appelation “extreme horizontal branch” (EHB) to refer to HB sequences of constant mass that do not reach the thermally Based on data collected at Paranal Observatory (ESO, Chile), pro- gram identifier ID 69.D-0081. pulsing stage on the AGB. These models evolve after core he- Full Table 6 is only available in electronic form at the CDS via lium exhaustion into post-early AGB (PEAGB) stars, which anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via leave the AGB before thermal pulsing, and AGB-manqué http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/353 stars, which never reach the AGB. EHB stars have envelope Appendix is only available in electronic form at masses that are too small to reach the end stages of normal http://www.edpsciences.org AGB evolution. AGB-manqué stars are the fate of HB stars Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20034504 354 G. Jasniewicz et al.: Chemical composition of UV-bright stars with the less massive envelopes (<0.02 M and log Teff > 4.3) 2. Selection of globular clusters and stars that do not support hydrogen shell burning. 2.1. Selection of globular clusters Globular clusters with populous EHBs are expected to be deficient in post-AGB stars; however, they are expected to show The clusters NGC 5986, NGC 6218 (M 12), NGC 6656 (M 22), a substantial population of less luminous (1.8 < log L/L < NGC 6712, and NGC 7078 (M 15) contain UV-bright star 3) UV-bright stars, which can be either post-EHB stars or candidates and could be observed during our scheduled tele- PEAGB stars. The population of post-EHB stars is expected scope time. The cluster metallicities cover the range between to be about 15−20 percent of the population of EHB stars roughly −2and−1 dex (see Table 1). Hereafter we list some (Dorman et al. 1993). published results on the HB morphology of the selected clus- We can suggest some answers to the question raised just ters and provide some discussion of the chemical abundance above. The most luminous UV-bright stars can be PAGB stars; patterns of their stars in the framework of evolutionary or pri- less luminous ones can be PEAGB or AGB manqué stars evolv- mordial scenarios. ing from hot-HB stars, or evolved blue stragglers; mergers of stars; first ascent giants that have lost their outer envelopes, etc. – NGC 5986: the horizontal branch here is predominantly Thus the location of UV-bright stars in the HR diagram is not blue (Alves et al. 2001). According to Kravtsov et al. sufficient at all for discriminating their origin, and high resolu- (1997) the luminosity function of the horizontal branch tion spectroscopy is necessary for useful constraints (for exam- shows systematic changes as one goes from the central re- ple signatures of the third dredge-up, spectral binarity, etc.). gions of the cluster to its periphery; – NGC 6218 (M 12): the color–magnitude diagram (CMD) is Only five UV-bright stars have detailed chemical com- very similar to the one for M 10 (Von Braun et al. 2002), position determinations based on high resolution spectra: exhibiting an extremely blue horizontal branch (very high Barnard 29 in M 13 (Conlon et al. 1994; Moehler et al. 1998), value of the morphology parameter in Table 1); ZNG 4 in M 13 (Ambika et al. 2004), ROA 24 in ω Cen – NGC 6656 (M 22): this cluster shows the anomalous abun- (Gonzalez & Wallerstein 1994), ROA 5701 in ω Cen (Moehler dance patterns of ω Cen (Norris & Freeman 1983). Giants et al. 1998), and ZNG 1 in M 10 (Mooney et al. 2001). To display variations in Ca, Na, and Fe abundance, and these this list we can add the young planetary nebula K 648 in M 15 variations correlate with variations in CH and CN band (Bianchi et al. 2001). These studies indicate that some UV- strength (Lehnert et al. 1991). Brown & Wallerstein (1990) bright stars show evidence for third dredge-up and overabun- discuss the possibility that winds from massive WR stars dance of carbon and s-process elements (for example ROA 24 or, more likely, cool supergiants provided self-enrichment in ω Cen and K 648 in M 15), while others show severe carbon during the star-formation period in M 22; deficiency, indicating that they left the AGB before the third – NGC 6712: this cluster is a small and sparse system of in- dredge-up occured (for example ZNG 1 in M 10, ROA 5701 termediate metallicity, but is suspected to have been much in ω Cen, Barnard 29 in M 13). more massive and concentrated at some early epoch of The chemical analysis of UV-bright stars must be done its history (Paltrinieri et al. 2001). The extension of the while keeping in mind the general chemical peculiarities (ini- blue HB tail is small. The bulk of the horizontal branch tially devoted to stars from the main sequence to the red gi- stars populate the red side of the instability strip (Paltrinieri ant branch) of the host clusters in the framework of evolution- et al. 2001); / ary and or primordial scenarios (see the review by Kraft 1994; – NGC 7078 (M 15): this cluster shows an extended blue tail and Sneden 1999). These discussions have stimulated investi- and a gap in the distribution of stars along the HB. Stellar gations toward two directions: on one hand, deep extra-mixing rotation rates and photospheric compositions of HB stars mechanisms in giants, and on the other hand yields from con- vary strongly as a function of effective temperature (Behr tamination by AGB stars (Ventura et al.