X-RAY-EMISSION AND LOW-MASS STARS IN THE YOUNG OPEN CLUSTER NGC-2547

We present the results of deep ROSAT High Resolution Imager X-ray obse rvations and broadband BVIC CCD photometry of the young open cluster N GC 2547. We have been able to find cluster counterparts for the majori ty of the 102 significant X-ray sources detected, confirming the power of high spatial resolution X-ray observations to find low-mass, magne tically active members of distant open clusters. The age of the cluste r has been determined by fitting the main-sequence turn-off and the po sitions of low-mass stars on their pre-main-sequence tracks. The two m ethods yield 55 +/- 25 and 14 +/- 4 Myr respectively, but we consider the lower age to be more reliable because of the comparatively large n umber of low-mass stars that constrain the fit. We deduce that any age spread among the low-mass stars is < 10 Myr and any small spread that exists could be attributable to binarity and starspots. The distribut ion of X-ray activity levels in the solar-type stars of NGC 2547 lies considerably above our threshold of sensitivity, and it is probable th at our X-ray-selected sample is complete at these masses. For lower ma ss stars our sample is likely to be incomplete. Comparison with initia l mass functions indicates there are still of order 100, low-activity M dwarfs yet to be found. When X-ray activity is gauged in terms of X- ray to bolometric flux ratio, L-x/L-bol, we find that there are G star s in the older (age 52 Myr) alpha Per cluster that are less active tha n their least active counterparts in NGC 2547. This is consistent with the current rotation-activity paradigm if there is modest angular mom entum loss between 14 and 52 Myr. There are no G or early K stars in N GC 2547 that reach the saturated level L-x/L-bol = 10(-3), seen in old er clusters, whereas saturated late K and M stars are observed. From t his, we deduce that there are no fast-rotating G and early K stars in NGC 2547 with equatorial velocities > 20 km s(-1). If this is confirme d then, even with no angular momentum loss, the fastest rotating NGC 2 547 stars could not evolve into the fastest rotating stars in the or P er cluster. These results cast doubt on the assumption that rotation r ates and magnetic activity seen in one cluster are representative of s imilar stars at the same age. We hypothesize that the solar-type stars of NGC 2547 either still possess, or have recently lost, circumstella r accretion discs which regulate their angular momentum. This would re quire longer disc lifetimes than commonly measured or assumed for othe r young stars, and we speculate on possible reasons for this.