Friday, January 25, 2013

1301.5651 (Giancarlo Pace)

Chromospheric activity as age indicator    [PDF]

Giancarlo Pace
Chromospheric activity has been calibrated and widely used as age indicator. However, it has been suggested that the viability of such an 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 larger 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 hypothesys that chromospheric activity follows the Skumanich law, i.e. that is proportional to the inverse of the square root of age, should be relaxed.
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