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Astron. Astrophys. 346, L1-L4 (1999)

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1. Introduction

The discovery of "gaps" along the blue horizontal branch (HB) in globular clusters as well as of long extensions towards higher temperatures has triggered several spectroscopic investigations (Moehler 1999 and references therein) yielding the following results:

  1. Most of the stars analysed above and below any gaps along the blue horizontal branch are "bona fide" blue HB stars ([FORMULA] K), which show significantly lower gravities than expected from canonical stellar evolution theory.

  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 blue HB (BHB) stars the gravities of these "extended HB" (EHB) stars agree well with the expectations of canonical stellar evolution.

Two scenarios have been suggested to account for the low gravities of BHB stars:

Abundance anomalies observed in red giant branch (RGB) stars in globular clusters (e.g., Kraft 1994, Kraft et al. 1997) may be explained by the dredge-up of nuclearly processed material to the stellar surface. If the mixing currents extend into the hydrogen-burning shell - as suggested by current RGB nucleosynthesis models and observed Al overabundances - helium can be mixed into the stellar envelope. This in turn would increase the luminosity (and mass loss) along the RGB (Sweigart 1999) and thereby create less massive (i.e. bluer) HB stars with helium-enriched hydrogen envelopes. The helium enrichment increases the hydrogen burning rate, leading to higher luminosities (compared to canonical HB stars of the same temperature) and lower gravities. The gravities of stars hotter than about 20,000 K are not affected by this mixing process because these stars have only inert hydrogen shells.
Grundahl et al. (1999) found a "jump" in the u, [FORMULA] colour-magnitude diagrams of 15 globular clusters, which can be explained if radiative levitation of iron and other heavy elements takes place over the temperature range defined by the "low-gravity" BHB stars. This assumption has been confirmed in the case of M 13 by the recent high resolution spectroscopy of Behr et al. (1999). Grundahl et al. argue that super-solar abundances of heavy elements such as iron should lead to changes in model atmospheres which may be capable of explaining the disagreement between models and observations over the "critical" temperature range [FORMULA] K.

NGC 6752 is an ideal test case for these scenarios, since it is a very close globular cluster with a long blue HB extending to rather hot EHB stars. While previous data already cover the faint end of the EHB, we now obtained new spectra for 32 stars in and above the sparsely populated region between the BHB and the EHB stars. In this Letter , we present atmospheric parameters derived for a total of 42 BHB and EHB stars and discuss the constraints they may pose on the scenarios described above.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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