Solar-type giants : new X-ray detections from EXOSAT observations.

Abstract

The authors report new X-ray detections of HD 1671 (F5 III) and HD 12929 (K2 III). From an analysis of X-ray count rates in three broadband filters from HD 203387, they derive an estimate of its coronal temperature of T ≡ 3×10⁶K. The data show that HD 12929 (K2 III) differs from the earlier type stars which the authors observed in the following respect: its chromospheric radiative losses and X-ray flux are one and two orders of magnitude lower, respectively, than those of the other giants observed and no Si IV and C IV emission is visible in its low resolution IUE spectrum. From their observations and previous X-ray flux measurements, the authors confirm that no simple correlation between X-ray flux and rotational rate alone can be expected for late-type giants beyond spectral type G5 III. It is speculated that convection might play a role in the change of the structure of the outer atmosphere of K giants.