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The Open Cluster NGC 2516. I. Optical Photometry
We present UBVI CCD photometry of the intermediate-age open cluster NGC2516. From this new photometry we derive the cluster parameters[V0-MV=7.77+/-0.11 mag, E(B-V)=0.112+/-0.024 mag],age [log(age)=8.2+/-0.1], and a photometric abundance([Fe/H]=-0.10+/-0.04) from a comparison with theoretical isochrones ofthe Padua group. We find that the X-ray bright stars[logLX(ergs s-1)>~29.0] are systematicallybluer in B-V and U-B. It was necessary to modify the photometricmembership criterion used by Sung & Bessell due to the UV excess ofX-ray bright stars and due to the abrupt increase in the slope of the(MV, B-V) ZAMS relation at B-V>~1.4. A minimum binaryfrequency 40% (+/-5%) was estimated from the distribution of distancemoduli of unevolved photometric members. The photometric abundanceobtained in this study is similar to that of the Pleiades and thereforethe lower metallicity is not the cause of strong X-ray emission ofF-type stars in NGC 2516. We believe that this results from them beingclose binaries, as suggested by their different distribution of distancemoduli compared to other cluster members and surmise that the tidalforces between the two stars must increase their X-ray activity.
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