Chromospheric activity as age indicator

TL;DR

This paper investigates the effectiveness of chromospheric activity as an age indicator, concluding it is reliable mainly for stars younger than about 2 Gyr, and challenges the assumption that activity follows the Skumanich law.

Contribution

It defines the age range where chromospheric activity is a robust age indicator and questions the applicability of the Skumanich law for older stars.

Findings

Chromospheric activity does not decay significantly after about 2 Gyr.

Field star data confirm previous results from open clusters.

Uncertainty in age determination limits calibration for old stars.

Abstract

Chromospheric activity has been calibrated and widely used as an age indicator. However, it has been suggested that the viability of this age indicator is, in the best case, limited to stars younger than about 1.5 Gyr. I aim to define the age range for which chromospheric activity is a robust astrophysical clock. I collected literature measurements of the S-index in field stars, which is a measure of the strength of the H and K lines of the Ca II and a proxy for chromospheric activity, and exploited the homogeneous database of temperature and age determinations for field stars provided by the Geneva-Copenhagen survey of the solar neighbourhood. Field data, inclusive data previously used to calibrate chromospheric ages, confirm the result found using open cluster data, i.e. there is no decay of chromospheric activity after about 2 Gyr. The only existing indication supporting the viability of chromospheric ages older than 2 Gyr is the similarity of chromospheric activity levels in the components of 35 dwarf binaries. However, even in the most optimistic scenario, uncertainty in age determination for field stars and lack of sufficient data in open clusters make any attempt of calibrating an age activity relationship for old stars premature. The hypothesis that chromospheric activity follows the Skumanich law, i.e. that it is proportional to the inverse square root of the age, should be relaxed.