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 content Y = 0.20, 0.27. We found that both the difference in luminosity between 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 test for the efficiency of internal mixing during the central He burning phase. Data concerning HE evolution have been connected with the evolutionary times of the corresponding RGB stars to recalibrate the parameter R as a function of Y for the two investigated metallicities. Comparisons with previous calibrations disclose variations which are not negligible, and which should be taken into account when R is used to derive constraints on the amount of cosmological helium. The critical role of the HE luminosity on the evaluation of R is briefly discussed.
ADVANCED EVOLUTIONARY PHASES OF LOW-MASS STARS - THE ROLE OF THE ORIGINAL HELIUM
DEGL'INNOCENTI, SCILLA;
1995-01-01
Abstract
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 content Y = 0.20, 0.27. We found that both the difference in luminosity between 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 test for the efficiency of internal mixing during the central He burning phase. Data concerning HE evolution have been connected with the evolutionary times of the corresponding RGB stars to recalibrate the parameter R as a function of Y for the two investigated metallicities. Comparisons with previous calibrations disclose variations which are not negligible, and which should be taken into account when R is used to derive constraints on the amount of cosmological helium. The critical role of the HE luminosity on the evaluation of R is briefly discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
