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

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4. The Hipparcos constraints

In Sect. 3 we have explored the influence of physical effects which are badly understood and the treatment of which is usually done in parametrized form in stellar evolution codes. Having discussed the scenario of these theoretical uncertainties, let us here attempt to use Hipparcos constraints to test the already quoted different predictions concerning the luminosities of He burning stars. Following the careful discussion given by G98 about the clump population, one finds that for any reasonable assumption about the age spread of field stars one predicts the bulk of the clump to be populated by stars below the transition mass, whose luminosity is practically insensitive to assumptions about the efficiency of overshooting. Coming back to Fig. 2, one finds that at a solar metallicity C99 predictions give for these stars a luminosity larger than G98 by about [FORMULA]logL=0.1. More in general, comparison of data in G98 and C99 shows that the two evolutionary scenarios have a rather similar dependence of luminosities on the chemical composition, thus with the quoted systematic difference at any given metallicity.

"Sic stantibus rebus", the already quoted evidence that G98 evolutionary scenario appears able to nicely fit the Hipparcos mean magnitude of clumping He burning stars, implies that C99 must predict too luminous giants, running against the Hipparcos evidence. This has been confirmed by independent simulation of the clump population based on C99 evolutionary tracks, as transferred into the CM diagram by adopting model atmospheres by Castelli et al.(1997a,b). Data in Fig. 1 gives the additional evidence that reasonable variations in the assumed original He content cannot decrease the C99 predicted luminosity by the required amount. Therefore one concludes that G98 evolutionary scenario works better.

However, for the sake of the discussion one has to notice that there is - at least in principle - a way to reconcile C99 prediction with Hipparcos observations. Fig. 4 in this paper shows that a substantial amount of mass loss could lower the C99 predictions by the required amount of about [FORMULA]logL[FORMULA]0.1. As an example, one would require a mass loss by about 0.9 [FORMULA] for a 2.0 [FORMULA], and by about 0.6 [FORMULA] for a 1.5 [FORMULA] model. This, however, appear a too large requirement vis-a-vis current estimates for mass loss. Taking also into account the uncertainties on evolutionary parameters of the field population in the solar neighborhood, we regard the previous discussion not as a proof, but at least as a suggestion that the most updated models, as C99 are, when dealing with the progeny of degenerated RG tend to give too luminous He burning models.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
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