Multicolor NTT CCD Photometry of the Post–Core-Collapse Globular Cluster M30

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Abstract
We have carried out multicolor (UBVI) CCD photometry for more than 1400 stars (13.5 < V < 21.5) in a 6' × 6' field of the globular cluster M30. We find the main-sequence turnoff at VTO = 18.65 ± 0.05, (B - V)TO = 0.46 ± 0.03, (V - I)TO = 0.62 ± 0.04, and (B - I)TO = 1.08 ± 0.05 (errors include uncertainties of the photometric zero points). From a comparison of the two-color (U - B)-(B - V) diagram for the cluster with the similar diagram for NGC 6397, a reddening value E(B - V) = 0.078 ± 0.043 is derived for M30. We have analyzed the positions of main-sequence stars, subgiants, and giants in the (U - B)-(B - V) diagram and revealed some deviations from theoretical predictions and some differences compared with NGC 6397. Our comparison of the color-magnitude diagram with isochrones shows that the cluster is approximately as old as NGC 6397. The isochrone fit is not quite consistent when using different color indexes. We have also found some deviations from the observed shape of the main sequence for existing theoretical isochrones. The upper limit for binary population in the studied cluster field (including visual double stars) is estimated as ~5% at r ~ 6' from the cluster's center.