The Open Cluster NGC 2516. I. Optical Photometry

We present UBVI CCD photometry of the intermediate-age open cluster NGC 2516. 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 of the Padua group. We find that the X-ray bright stars [log LX(ergs s-1)29.0] are systematically bluer in B-V and U-B. It was necessary to modify the photometric membership criterion used by Sung & Bessell due to the UV excess of X-ray bright stars and due to the abrupt increase in the slope of the (MV, B-V) ZAMS relation at B-V1.4. A minimum binary frequency 40% (±5%) was estimated from the distribution of distance moduli of unevolved photometric members. The photometric abundance obtained in this study is similar to that of the Pleiades and therefore the lower metallicity is not the cause of strong X-ray emission of F-type stars in NGC 2516. We believe that this results from them being close binaries, as suggested by their different distribution of distance moduli compared to other cluster members and surmise that the tidal forces between the two stars must increase their X-ray activity.

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