Previous
abstract 
Next
abstract
We report on a survey of chromospheric emission (CE) in a large sample of southern solar-type stars. To date, we have observed more than 700 stars within $\sim$50 pc at the Ca II H and K lines, which can be used to measure stellar activity, and presumably age. This survey is intended to complement the long-term work continuing at Mount Wilson by Baliunas et al. , with a combined goal to observe a volume-limited sample of 5000 F, G and K dwarfs.
An important product of the Mount Wilson effort is the classic paper of Vaughan and Preston (1980) who reported on CE for a sample of 500 northern late-type dwarfs within 25 pc. They observed a bimodal distribution for 185 F and G dwarfs in which 75\% of the stars had weak CE (as the Sun does), some active ones had high levels of CE, and very few had intermediate levels. This ``gap'' suggested that either the star formation rate has been non-uniform (so that stars with ages corresponding to moderate CE are missing from the solar neighborhood), or that the CE-age relation has several phases (so that stars spend little time in the phase corresponding to intermediate CE). At the present time, it is not possible to distinguish between these two explanations. A survey of CE among an independent sample with different instrumentation provides a means of ensuring that the Mount Wilson result was not a fluke of a modest sample or an artifact of instrumentation or data analysis.
We find from our larger southern sample that the two populations of stars are again evident. Roughly 75\% fall in the relatively inactive group, corresponding to ages greater than a few Gyr. We have also identified a few ($\sim$5\%) very active, young, nearby stars that can be targeted for future in-depth study.
