On the width of the theoretical lower main sequence.
Consequences for the determination of the ΔY/ΔZ ratio in the solar neighbourhood.

J. M. Fernandes, Y. Lebreton, A. Baglin

We discuss the relation between the main sequence thickness and chemical composition variations. With the help of theoretical zero-age main sequence models between 0.7 and 1.0Msun, computed with an advanced physical description of the stellar interiors, in a range of metallicities typical from Population I stars (0.008<=Z<=0.034), we establish a theoretical relation between the relative helium to metal enrichment, ΔY/ΔZ, and the main-sequence broadening, represented in this work by ΔMbol. The locus in the Hertzsprung-Russell diagram of 69 low mass stars closer than 25pc is analyzed, taking into account the present available metallicities for 25 of them. Models show that ΔY/ΔZ should be higher than 2.0 in the solar neighbourhood to account for the observational main sequence width. As a consequence an helium abundance lower limit of Y=0.246 is found in the solar neighbourhood. This value is presently loosely constrained by the observational uncertainties. A new step towards the ΔY/ΔZ determination is expected in the very near future, when accurate values of the stellar luminosities as determined by Hipparcos will permit more definite conclusions.

Astronomy and Astrophysics
Volume 311, Page 127
July 1996

ADS Bibliographic code: 1996A&A...311..127F