Strömgren four-color photometry of X-ray active late-type stars: Evidence for activity-induced deficiency in the index ^*

¹ Istituto e Osservatorio Astronomico di Palermo, Palazzo dei Normanni, 90134 Palermo, Italy
² Astrophysics Division, European Space Agency – Postbus 299, 2200 AG Noordwijk, The Netherlands

Send offprint request to: F. Morale

Received: 30 October 1995
Accepted: 9 February 1996

Abstract

We present the results of a uvby-β photometric study of a sample of active late-type stars (F–K) selected from the Einstein Extended Medium Sensitivity Survey. Our work shows the presence in the sample of a star population with the photometric index c₁ typical of main sequence stars and an unexpected deficiency in m₁ index. Stars with more anomalous values of m₁ have also very high values of and X-ray surface fluxes, near the “saturation” limit observed in the most active stars and similar to the flux observed in solar active regions. We discuss these results in the light of similar results found in the Sun comparing m₁ indices in quiet and active regions and in other samples of active stars.