Astron. Astrophys. 351, 519-525 (1999)

1. Introduction

The metal-rich globular clusters play a fundamental rôle in determining the formation history of our Galaxy (e.g. Minniti 1995, 1996; Ortolani et al. 1995; Zinn 1996; Barbuy et al. 1998; Rich 1998). Moreover, they provide a crucial template for interpreting the integrated spectra of elliptical galaxies and for understanding the evolution of old metal-rich stellar systems. The horizontal branch (HB) stars in the metal-rich clusters are particularly important, since they provide a standard candle for determining distances (and hence ages) and are believed to be the major contributors to the UV-upturn phenomenon (e.g. Caloi 1989; Greggio & Renzini 1990, 1999; Bressan et al. 1994; Dorman et al. 1995; Yi et al. 1998).

Recent Hubble Space Telescope (HST) observations have found that the metal-rich globular clusters NGC 6388 (C1732-447) and NGC 6441 (C1746-370) ( and -0.53, respectively; Harris 1996) contain an unexpected population of hot HB stars and therefore exhibit the well-known second parameter effect (Rich et al. 1997). Most surprisingly, the mean HB luminosity at the top of the blue HB tail is roughly 0.5 mag brighter in V than the red HB "clump," which itself is strongly sloped as well. Differential reddening cannot be the cause of these sloped HB's (Piotto et al. 1997; Sweigart & Catelan 1998; Layden et al. 1999).

The second parameter effect has often been attributed to differences in age or mass loss on the red giant branch (RGB). However, canonical HB simulations show that increasing the assumed age or RGB mass loss moves an HB star blueward in the V,  plane but does not increase its luminosity. Thus some other second parameter(s) must be causing the sloped HB's in NGC 6388 and NGC 6441 (Sweigart & Catelan 1998, hereafter SC98).

Three non-canonical scenarios have been suggested to explain the sloped HB's and long blue HB tails in these metal-rich globular clusters (SC98):

1. High cluster helium abundance scenario : Red HB stars evolve along blue loops during most of their HB lifetime. For larger than "standard" helium abundances Y, these loops become considerably longer, reaching higher effective temperatures and deviating more in luminosity from the zero-age HB (ZAHB). If the cluster stars form with sufficiently high Y (), the HB will slope upward (Catelan & de Freitas Pacheco 1996), as observed in NGC 6388 and NGC 6441. However, this scenario also predicts a much larger value for the number ratio R of HB to RGB stars than the value recently obtained by Layden et al. (1999) for NGC 6441. Thus a high primordial helium abundance seems to be ruled out as the cause of the sloped HB's and will not be considered further in this paper.

2. Rotation scenario : Rotation during the RGB phase can delay the helium flash, thereby increasing both the final helium-core mass and the amount of mass loss near the tip of the RGB. The net effect is to shift a star's HB location towards higher effective temperatures and luminosities, depending on the amount of rotation. This scenario predicts a sloped HB with the shift towards higher luminosities (and hence lower gravities) increasing with effective temperature (Rood & Crocker 1989).

3. Helium-mixing scenario : The observed abundance variations in globular cluster RGB stars show that these stars are able to mix nuclearly processed material from the vicinity of the hydrogen-burning shell out to the surface (e.g. Kraft 1994). In particular, the observed Al enhancements indicate that the mixing can penetrate into the H-shell, thus leading to the dredge-up of helium. The resulting increase in the envelope helium abundance produces an HB morphology that slopes upward towards brighter luminosities with increasing effective temperature (Sweigart 1997; SC98). The shift towards lower than canonical gravities due to helium mixing reaches a maximum between 10,000 K and 20,000 K. No shift is predicted for the hottest HB stars (20,000 K), where the H-shell is inactive and the luminosity is therefore unaffected by helium mixing (Sweigart 1997). This scenario might also help to explain the low gravities of the blue HB stars found in several metal-poor globular clusters (Moehler 1999 and references therein), although Grundahl et al. (1999) have recently provided some caveats about it. They do note, however, that "helium mixing stands out as the best candidate to explain the anomalous HB morphology of the metal-rich globular clusters NGC 6388 and NGC 6441".

These scenarios make different predictions for the surface gravities of HB stars which can be tested observationally. We started a program to obtain spectroscopic observations with this goal in mind and report here on our first results. In Sect. 2 we describe our observations and the employed data reduction techniques; in Sect. 3 the procedure adopted to derive the atmospheric parameters of the programme stars is outlined. Finally, we discuss our results and their consequences in Sect. 4.

Table 1. Target list. Positions and photometry are from Piotto (priv. comm.)

© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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