X-ray emission and low-mass stars in the young open cluster NGC 2547

Abstract

We present the results of deep ROSAT High Resolution Imager X-ray observations and broad-band BVIC CCD photometry of the young open cluster NGC 2547. We have been able to find cluster counterparts for the majority of the 102 significant X-ray sources detected, confirming the power of high spatial resolution X-ray observations to find low-mass, magnetically active members of distant open clusters. The age of the cluster has been determined by fitting the main-sequence turn-off and the positions of low-mass stars on their pre-main-sequence tracks. The two methods yield 55 ± 25 and 14 ± 4 Myr respectively, but we consider the lower age to be more reliable because of the comparatively large number of low-mass stars that constrain the fit. We deduce that any age spread among the low-mass stars is 20 km s−1. If this is confirmed then, even with no angular momentum loss, the fastest rotating NGC 2547 stars could not evolve into the fastest rotating stars in the α Per cluster. These results cast doubt on the assumption that rotation rates and magnetic activity seen in one cluster are representative of similar stars at the same age. We hypothesize that the solar-type stars of NGC 2547 either still possess, or have recently lost, circumstellar accretion discs which regulate their angular momentum. This would require longer disc lifetimes than commonly measured or assumed for other young stars, and we speculate on possible reasons for this.