Multicolor CCD Photometry of the Poorly Studied Globular Cluster M80

Abstract

We present results of the rst multicolor (UBV I) CCD photometry of 4899 stars in a wide (4@ ] 7@) eld of the poorly studied globular cluster M80. Our results feature both a wide magnitude range (11.5 \ V \ 21.5) and a relatively large number of stars belonging to brighter sequences of the colormagnitude diagram (CMD). A comparison between the V -(B[V ) CMDs of M80 and M79 (NGC 1904) shows a notable similarity in morphology and similar slope of the red giant branches (RGBs) of both clusters. Moreover, the V magnitude of the "" bump in the luminosity function of the RGB compared with the position of the horizontal branch (HB) and of the turno point corresponds to a most probable metallicity value of [Fe/H] \ 1.70 ^0.15 for M80. A comparison of the CMDs and of the HB luminosity functions of the two clusters demonstrates that the two HBs span an almost identical range in V and have a similar uneven (fragmented) distribution of stars along this branch. However, we nd a di erence (earlier noted for NGC 6541 and M79) between M79 and M80 in the position of the HB relative to the RGB, both in the U-(B[V ) CMD and in the two-color (U[B)-(B[V ) diagram. Our photometry of M80 yields a number of very probable UV-bright star and blue straggler candidates. The latter can be grouped, by photometric characteristics and position in the cluster, into two distinct classes of apparently di erent nature. The di erence in V magnitude between the HB and the turno point of M80 is *V \ 3.60 ^0.09. The helium abundance estimated from the R-method is Y \ 0.21 ^0.02. The age of the cluster, determined by comparing the di erence in color between the turno point and the lower RGB in the V -(B[V ) diagram with that for the oxygen-enhanced isochrones, is 17 ^2 Gyr. However, similar di erences in other colors do not lead to a single age value.