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Session 14 - Various Stellar Surveys.
Display session, Wednesday, January 07
Exhibit Hall,
We use HIPPARCOS data for a few thousand nearby F stars with uvby photometry to look for effects of age, metallicity, and helium abundance in the F stars Main Sequence (MS) band. The age effect in metallicity groups of stars have been found to manifest itself in larger velocity dispersion in the middle of the MS band (locus of turnoff points), with the dispersion being larger at same effective temperature for metal poor groups, and increasing toward cooler stars within a group. The MS of the metal poor groups reveals a distinct blue cutoff suggestive of the current turnoff for the most massive stars among the oldest MS stars. The MS lower envelope is found to be essentially the same for metallicity groups ranging in [Fe/H] from \sim -0.5 to \sim +0.2. This may be consistent with larger helium abundance in metal rich stars, although the amount of required helium enrichment may be uncomfortably large. A spread of helium abundance at a given metallicity may alleviate the problem, however necessity of a ``fourth parameter'' (additional to age, metallicity and helium abundance) cannot be ruled out. All metallicity groups display a wide MS band exceeding 1 mag. Most of the band width is obviously attributable to an age spread within the groups, implying that there is a considerable age spread at a given metallicity. A large width of the MS of metal poor stars at the red end may be indicative of an additional factor (``fourth parameter''?) to widen the MS band because both the kinematics and the isochrone turnoff age for the stars here look to be incompatible with age widening. This may have a bearing on age determination of old metal poor stellar populations like globular clusters.
