Multicolor CCD Photometry of the Poorly Studied Globular Cluster M80

We present results of the first multicolor (UBVI) CCD photometry of 4899 stars in a wide (4' x 7') field 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 color-magnitude 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 turnoff 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 find a difference (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 different nature. The difference in V magnitude between the HB and the turnoff point of M80 is DeltaV = 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 difference in color between the turnoff point and the lower RGB in the V-(B-V) diagram with that for the oxygen-enhanced isochrones, is 17 +/- 2 Gyr. However, similar differences in other colors do not lead to a single age value.