ON THE WIDTH OF THE THEORETICAL LOWER MAIN-SEQUENCE - CONSEQUENCES FOR THE DETERMINATION OF THE DELTA-Y DELTA-Z RATIO IN THE SOLAR NEIGHBORHOOD/

We discuss the relation between the main sequence thickness and chemic al composition variations. With the help of theoretical zero-age main sequence models between 0.7 and 1.0 M(.), computed with an advanced ph ysical description of the stellar interiors, in a range of metalliciti es typical from Population I stars (0.008 less than or equal to Z less than or equal to 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 lo cus in the Hertzsprung-Russell diagram of 69 low mass stars closer tha n 25 pc is analyzed, taking into account the present available metalli cities for 25 of them. Models show that Delta Y/Delta Z should be high er than 2.0 in the solar neighbourhood to account for the observationa l main sequence width. As a consequence an helium abundance lower limi t of Y=0.246 is found in the solar neighbourhood. This value is presen tly loosely constrained by the observational uncertainties. A new step towards the Delta Y/Delta Z determination is expected in the very nea r future, when accurate values of the stellar luminosities as determin ed by Hipparcos will permit more definite conclusions.