On the determination of oxygen abundances in chromospherically active stars ^*,**

Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail: morel@astropa.unipa.it

Received: 2 March 2004
Accepted: 29 April 2004

Abstract

We discuss oxygen abundances derived from [] λ6300 and the  triplet in stars spanning a wide range in chromospheric activity level, and show that these two indicators yield increasingly discrepant results with higher chromospheric/coronal activity measures. While the forbidden and permitted lines give fairly consistent results for solar-type disk dwarfs, spuriously high  triplet abundances are observed in young Hyades and Pleiades stars, as well as in individual components of RS CVn binaries (up to 1.8 dex). The distinct behaviour of the []-based abundances which consistently remain near-solar suggests that this phenomenon mostly results from large departures from LTE affecting the  triplet at high activity level that are currently unaccounted for, but also possibly from a failure to adequately model the atmospheres of K-type stars. These results suggest that some caution should be exercised when interpreting oxygen abundances in active binaries or young open cluster stars.