Structure of the main sequence of F and G stars - Effects of age, metallicity, and helium abundance

Using uvby photometry data for about 5500 F stars within 80 pc of the sun, it is shown that the MS lower envelopes for groups of stars with different metallicities coincide. This means that helium abundance varies with metallicity as Delta Y = about 1/3 Delta lg Z, if the effect of metallicity on the lower envelope is compensated by an opposite effect of the helium abundance. In addition, the width of the MS band in the F stars for any given value of F/H is quite large and does not differ from the MS width for a complete sample of F stars. In different parts of the MS stars of a given metallicity differ in their kinematics. The kinematic structure of the MS suggests that its great width is due mainly to the age spread of the stars. Comparison of the kinematics of stars in narrow ranges of b-y with the ages derived from isochrones reveals that when Z is less than solar Z, isochrone ages are likely to be increasingly overestimated as b-y increases. MS characteristics for stars later than G3 seem to differ from those of the MS of stars F2-G3. For a given metallicity they have a narrower main sequence, and the positions of their lower envelopes depend on metallicity.