Isochrone computations and horizontal branch (IB) models with an original helium content Y(MS) = 0.23 and two values of metallicity (Z = 0.0001, 0.0004) are used to derive constraints on age indicators and HE morphology of metal-poor star clusters with age (t in Gyr) in the range of 1 to 20 Gyr. It is found that the luminosity L(3.83) of the zero age horizontal branch (ZAHB) at the mean effective temperature (logT(e) = 3.83) of the RR Lyrae gap depends on metallicity as well as age, and that the resulting difference in visual magnitude to the main sequence turn-off (TO) may be calibrated in terms of age as log t = -0.186 + 0.40 Delta V(TO-ZAHB) and log t = -0.332 + 0.45 Delta V(TO-ZAHB) at Z = 0.0001 and 0.0004, respectively. Synthetic horizontal branch models confirm that for fixed metallicity and age the mean luminosity of RR Lyrae depends on HE morphology. Moreover, the mean luminosity of purely red horizontal branches turns out to depend also on the amount of mass loss during the red giant phase, an occurrence which should be properly taken into account when the cluster age is derived from the observed difference in magnitude between main sequence TO and actual horizontal branch (rather than ZAHB). Finally, it is shown that for a given metallicity the observed HE morphology could provide an easy estimate of the upper limit of the cluster age. Since such a limit increases significantly with the adopted metallicity, while the absolute age which is derived from the difference Delta V(TO-HB) is roughly constant, useful constraints to the cluster metallicity could be also derived.
HB MORPHOLOGY AND AGE INDICATORS FOR METAL-POOR STELLAR-SYSTEMS WITH AGE IN THE RANGE OF 1 TO 20 GYR
DEGL'INNOCENTI, SCILLA
1995-01-01
Abstract
Isochrone computations and horizontal branch (IB) models with an original helium content Y(MS) = 0.23 and two values of metallicity (Z = 0.0001, 0.0004) are used to derive constraints on age indicators and HE morphology of metal-poor star clusters with age (t in Gyr) in the range of 1 to 20 Gyr. It is found that the luminosity L(3.83) of the zero age horizontal branch (ZAHB) at the mean effective temperature (logT(e) = 3.83) of the RR Lyrae gap depends on metallicity as well as age, and that the resulting difference in visual magnitude to the main sequence turn-off (TO) may be calibrated in terms of age as log t = -0.186 + 0.40 Delta V(TO-ZAHB) and log t = -0.332 + 0.45 Delta V(TO-ZAHB) at Z = 0.0001 and 0.0004, respectively. Synthetic horizontal branch models confirm that for fixed metallicity and age the mean luminosity of RR Lyrae depends on HE morphology. Moreover, the mean luminosity of purely red horizontal branches turns out to depend also on the amount of mass loss during the red giant phase, an occurrence which should be properly taken into account when the cluster age is derived from the observed difference in magnitude between main sequence TO and actual horizontal branch (rather than ZAHB). Finally, it is shown that for a given metallicity the observed HE morphology could provide an easy estimate of the upper limit of the cluster age. Since such a limit increases significantly with the adopted metallicity, while the absolute age which is derived from the difference Delta V(TO-HB) is roughly constant, useful constraints to the cluster metallicity could be also derived.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
