An analysis of the Vaughan-Preston survey of chromospheric emission

Abstract

The statistical properties of the Vaughan-Preston survey of chromospheric emission of nearby stars have been investigated using models of the decay of chromospheric activity with increasing age. It has been suggested that the " gap " in the distribution of emission observed in the survey indicates a discontinuous change in chromospheric behavior at a critical value of age (or rotation). However, this interpretation of the data has been made without considering two crucial effects. First, photospheric light maintains a minimum value of the Vaughan-Preston Ca n emission index (S) even in the absence of any chromospheric activity. Second, solar-type stars in the Pleiades do not follow the Skumanich relation for chromospheric decay if the nuclear age of the cluster is used. The emission levels of the Pleiades F-G stars are lower than the extrapolation of the t~ 112 emission variation between the Hyades cluster and the Sun, suggesting that there may be a saturation of chromospheric emission in very young stars. This tendency toward an upper limit of chromospheric emission, combined with the lower limit from photospheric flux, creates concentrations of strong and weak emission stars in the Vaughan-Preston diagram, enhancing the impression of a " gap. " We show that the observed large-scale distribution of chromospheric emission in solar-type stars can be modeled using a smoothly varying decay of chromospheric activity with age. A statistical analysis shows that no discontinuous change in magnetic activity is required to fit the data. The survey shows small-scale structure which appears to be statistically significant. Although fine discontinuities in chromospheric behavior cannot be ruled out, it seems more plausible to interpret this structure with reasonable fluctuations in the local birthrate of solar-type stars on time scales of a few times 10 8 yr.