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Astron. Astrophys. 360, 120-132 (2000)

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1. Introduction

The colour-magnitude diagrams of metal-poor globular clusters show a large variety of horizontal-branch (HB) morphologies, including "gaps" along the blue HB and long "blue tails" that extend towards higher effective temperatures. It has been suggested that the gaps might separate stars with different evolutionary origins. Spectroscopic analyses of stars along the blue HB and blue tails in a number of globular clusters (Moehler 1999 and references therein) yielded the following results:

  1. Most of the stars analysed above and below any gaps are horizontal branch B type (HBB) stars ([FORMULA] K). Their surface gravities are significantly lower (up to more than 0.5 dex, see Fig. 4 of Moehler 1999) than expected from canonical HB evolution theory while their masses are lower than expected by about a factor of 2. For most clusters the problem of the masses may be solved by new globular cluster distances derived from HIPPARCOS data (see Reid 1999and references therein, Heber et al. 1997, Moehler 1999).

  2. Only in NGC 6752 and M 15 have spectroscopic analyses verified the presence of stars that could be identified with the subdwarf B stars known in the field of the Milky Way ([FORMULA] K, [FORMULA] [FORMULA] 5). In contrast to the cooler HBB stars these stars show gravities and masses that agree well with the expectations of canonical stellar evolution for extreme HB stars (Moehler et al. 1997a, 1997b).

There are currently two scenarios for explaining these apparent contradictions:

  • Helium mixing:

    The dredge-up of nuclearly processed material to the stellar surface of red giant branch (RGB) stars has been invoked to explain the abundance anomalies (in C, N, O, Na, and Al) observed in such stars in many globular clusters (e.g. Kraft 1994, Norris & Da Costa 1995a, Kraft et al. 1997). Since substantial production of Al in these low-mass stars only seems to occur inside the hydrogen shell (Langer & Hoffman 1995, Cavallo et al. 1996, 1998), any mixing process which dredges up Al will also dredge up helium. Possible dredge-up mechanisms include rotationally induced mixing (Sweigart & Mengel 1979, Zahn 1992, Charbonnel 1995) and hydrogen shell instabilities (Von Rudloff et al. 1988, Fujimoto et al. 1999). Such dredge-up would increase the helium abundance in the red giant's hydrogen envelope and thereby increase the luminosity (and the mass loss) along the RGB (Sweigart 1997a, 1997b). The progeny of these stars on the horizontal branch would then have less massive hydrogen envelopes than unmixed stars. As the temperature of an HB star increases with decreasing mass of the hydrogen envelope, "mixed" HB stars would be hotter than their canonical counterparts. The helium enrichment would also lead to an increased hydrogen burning rate and thus to higher luminosities (compared to canonical HB stars of the same temperature). The luminosities of stars hotter than about 20,000 K are not affected by this mixing process because these stars have only inert hydrogen shells. In this framework the low gravities of hot HB stars would necessarily be connected to abundance anomalies observed on the RGB, thereby explaining both of these puzzles at once.

  • Radiative levitation of heavy elements: Caloi (1999) and Grundahl et al. (1999) suggested that the low surface gravities of the HBB stars are related to a stellar atmospheres effect caused by the radiative levitation of heavy elements. Such an enrichment in the metal abundance would change the temperature structure of the stellar atmosphere and thereby affect the flux distribution and the line profiles (Leone & Manfrè 1997). This scenario would also account for the fact that there is no evidence for deep mixing amongst field red giants (e.g. Hanson et al. 1998, Gratton et al. 2000) even though field HBB stars show the same low surface gravities as globular cluster stars (Saffer et al. 1997, Mitchell et al. 1998). Behr et al. (1999, 2000b) have recently reported slightly super-solar iron abundances for HBB stars in M 13 and M 15, in agreement with the radiative levitation scenario.

NGC 6752 is an ideal test case for these scenarios: Its distance modulus is very well determined from both white dwarfs (Renzini et al. 1996) and HIPPARCOS parallaxes (Reid 1997), and thus any mass discrepancies cannot be explained by a wrong distance modulus. Spectroscopic analyses of the faint blue stars in NGC 6752 showed them to be subdwarf B (sdB) stars. As mentioned above, their mean mass agrees well with the canonical value of 0.5 [FORMULA]. However, almost no stars in the sparsely populated region above the sdB star region have been analysed. If these stars show low surface gravities and canonical masses, then the combination of deep mixing and the long distance scale (for the other globular clusters) would resolve the discrepancies described above. If they show low surface gravities and low masses, diffusion may indeed play a rôle when analysing these stars for effective temperature and surface gravity. Then the low surface gravities found for HBB stars could be artifacts from the use of inappropriate model atmospheres for the analyses. We therefore decided to observe stars in this region of the colour-magnitude diagram and to derive their atmospheric parameters. First results, which strongly support radiative levitation of heavy elements as the explanation, have been discussed by Moehler et al. (1999a). Here we describe the observations and their reductions, provide the detailed results of the spectroscopic analyses (temperatures, surfaces gravities, helium and partly iron and magnesium abundances, masses), and discuss the consequences of our findings in more detail.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000