THE CHROMOSPHERIC EMISSION AGE RELATION FOR STARS OF THE LOWER MAIN-SEQUENCE AND ITS IMPLICATIONS FOR THE STAR FORMATION RATE

We examine the existence and nature of the relationship between chromospheric emission (CE) and age in late-type dwarfs. To augment Ca II H and K data for the Sun and open clusters, we have observed solar-type stars in visual binaries whose ages were determined from Stromgren photometry of the primary, which is typically an A or early F star. We have also observed single, slightly evolved F dwarfs. These data, together with other evidence, strongly indicate that a CE-age relation indeed exists, and that this relation is deterministic, not just statistical. The functional form that best represents the relation of age to CE is less clear. Power laws provide a reasonable fit to the data, with exponents from -2 (corresponding to a Skumanich relation) to -4/3. However, power-law fits necessarily lead to an apparent excess of young solar-type stars (age less-than-or-similar-to 1 Gyr) in the solar neighborhood, given the observed distribution of HK emission strengths in the Vaughan-Preston survey. At the same time, a curve that corresponds to a constant star formation rate also fits these data well and so cannot yet be ruled out. Thus further observations should be able to demonstrate either that there is a real excess of young stars near the Sun, or that the evolution of CE for a low-mass star goes through a slow initial decline, a rapid decline at intermediate ages (approximately 1-2 Gyr), and, finally, a slow decline for old stars like the Sun. The lower resolution HK data of Barry are also consistent with a constant star formation rate, which means that attempts to use chromospheric data to estimate accurate ages of solar-type stars are at present premature, and that the physically simplest relationship that is compatible with the observations is a star formation rate that has not varied significantly over the past 9 Gyr.