On the width of the theoretical lower main sequence. Consequences for the determination of the {DELTA}Y/{DELTA}Z ratio in the solar neighbourhood.

Abstract

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 1.0M_sun_, 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, {DELTA}Y/{DELTA}Z, and the main-sequence broadening, represented in this work by {DELTA}M_bol_. 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 {DELTA}Y/{DELTA}Z 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 {DELTA}Y/{DELTA}Z determination is expected in the very near future, when accurate values of the stellar luminosities as determined by Hipparcos will permit more definite conclusions.