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Astron. Astrophys. 354, 150-156 (2000)

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1. Introduction

Hipparcos parallaxes for clumping He burning Red Giants in the solar neighborhood have recently raised a large interest in the astronomical community, providing a new valuable (though controversial) tool to approach the problem of Magellanic Clouds distances (see, e.g., Udalski et al. 1998, Stanek et al. 1998, Cole 1998, Girardi et al. 1998). However, on theoretical grounds one has to notice that these parallaxes provide us for the first time with direct observational evidences for the luminosity of He burning stars whose Red Giant progenitors experienced electron degeneracy in the stellar core. Therefore providing a relevant test for the rather sophisticated input physics supporting current theoretical predictions concerning galactic globulars and, more in particular, concerning some relevant issues as the luminosity of RR Lyrae stars and the ages of halo stellar clusters.

Such a test appears now of particular interest because of the growing rumor about a possible overluminosity of theoretical models for He burning stars with degenerated progenitors. As a matter of example, Pols et al. (1998) found that the He clump luminosity in the open cluster M67 appears 0.2 - 0.3 mag fainter than predicted on the basis of their evolutionary computations. Such a discrepancy appears larger than the expected uncertainties on the bolometric correction and it appears further supported by independent evolutionary computations recently presented by Castellani et al. (1999), hereinafter C99, which have discussed the severe difficulties in fitting the CM diagram of that clusters. However, in the meantime, Girardi et al. (1998) presented a careful discussion on the luminosities of Hipparcos He burning giants, which were found in splendid agreement with the adopted theoretical predictions.

To throw light on such a scenario, in the next section we will discuss current predictions about the luminosity of He burning stars in a suitable mass range, disclosing the occurrence of sensitive differences among various authors. Sect. 3 will be devoted to a preliminary investigation of the uncertainties in theoretical results due to uncertainties on both the amount of extramixing from convective cores and the amount of mass loss in the pre-He burning phases. On this basis, in Sect. 4 we will compare theoretical results with the observed mean luminosity of the red giant clump in the Hipparcos sample, concluding for the possible need of a revision of the "most updated" input physics used in recent evolutionary models. The origin of differences in the predicted luminosities are finally discussed in Sect. 5, by comparing the results of selected evolutionary codes to the light of the adopted physical ingredients. A short section of concluding remarks will close the paper.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000