Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998mnras.300..331j&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 300, Issue 2, pp. 331-351.
Astronomy and Astrophysics
Astronomy
37
Stars: Activity, Stars: Coronae, Stars: Late-Type, Stars: Rotation, Open Clusters And Associations: Individual: Ngc 2547, X-Rays: Stars
Scientific paper
We present the results of deep ROSAT High Resolution Imager X-ray observations and broad-band BVI_C 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+/-4Myr 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 <10Myr and any small spread that exists could be attributable to binarity and starspots. The distribution of X-ray activity levels in the solar-type stars of NGC 2547 lies considerably above our threshold of sensitivity, and it is probable that our X-ray-selected sample is complete at these masses. For lower mass stars our sample is likely to be incomplete. Comparison with initial 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 stars in the older (age 52Myr) alpha Per cluster that are less active than their least active counterparts in NGC 2547. This is consistent with the current rotation-activity paradigm if there is modest angular momentum loss between 14 and 52Myr. There are no G or early K stars in NGC 2547 that reach the saturated level L_x/L_bol=10^-3, seen in older clusters, whereas saturated late K and M stars are observed. From this, we deduce that there are no fast-rotating G and early K stars in NGC 2547 with equatorial velocities >20 kms^-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 alpha 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.
Jeffries Robin D.
Tolley Andrew J.
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