We discuss the reliability of one of the most used method to determine the Helium-to-metals enrichment ratio, Delta Y / Delta Z, i.e. the photometric comparison of a selected data set of local disk low Main Sequence (MS) stars observed by HIPPARCOS and a new grid of stellar models with up-to-date input physics. Most of the attention has been devoted to evaluate the effects on the final results of different sources of uncertainty (observational errors, evolutionary effects, selection criteria, systematic uncertainties of the models, numerical errors). As a check of the result the procedure has been repeated using another, independent, data set: the low-MS of the Hyades cluster. The obtained of Delta Y/ Delta Z for the Hyades, together with spectroscopic determinations of [Fe/H] ratio, have been used to obtain the Y and Z values for the cluster. Isochrones have been calculated with the estimated chemical composition, obtaining a very good agreement between the predicted position of the Hyades MS and the observational data in the Color - Magnitude Diagram (CMD).
Galactic Helium-to-Metals Enrichment Ratio (Delta Y/ Delta Z) from the Analysis of Local Main Sequence Stars Observed by HIPPARCOS
PRADA MORONI, PIER GIORGIO;DEGL'INNOCENTI, SCILLA
2009-01-01
Abstract
We discuss the reliability of one of the most used method to determine the Helium-to-metals enrichment ratio, Delta Y / Delta Z, i.e. the photometric comparison of a selected data set of local disk low Main Sequence (MS) stars observed by HIPPARCOS and a new grid of stellar models with up-to-date input physics. Most of the attention has been devoted to evaluate the effects on the final results of different sources of uncertainty (observational errors, evolutionary effects, selection criteria, systematic uncertainties of the models, numerical errors). As a check of the result the procedure has been repeated using another, independent, data set: the low-MS of the Hyades cluster. The obtained of Delta Y/ Delta Z for the Hyades, together with spectroscopic determinations of [Fe/H] ratio, have been used to obtain the Y and Z values for the cluster. Isochrones have been calculated with the estimated chemical composition, obtaining a very good agreement between the predicted position of the Hyades MS and the observational data in the Color - Magnitude Diagram (CMD).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
