G. Bono et al., ADVANCED EVOLUTIONARY PHASES OF LOW-MASS STARS - THE ROLE OF THE ORIGINAL HELIUM, Astronomy and astrophysics, 297(1), 1995, pp. 115-126
We present evolutionary He burning models which cover both the HE and
AGE evolutionary phases, as computed for the metallicities Z = 10(-4),
10(-3) and for two assumptions on the value of the original He conten
t Y = 0.20, 0.27. We found that both the difference in luminosity betw
een the AGE and the HE, and the number ratio of AGE to HE stars appear
quite unaffected by the amount of original He, allowing a relevant te
st for the efficiency of internal mixing during the central He burning
phase. Data concerning HE evolution have been connected with the evol
utionary times of the corresponding RGB stars to recalibrate the param
eter R as a function of Y for the two investigated metallicities. Comp
arisons with previous calibrations disclose variations which are not n
egligible, and which should be taken into account when R is used to de
rive constraints on the amount of cosmological helium. The critical ro
le of the HE luminosity on the evaluation of R is briefly discussed.