MULTICOLOR CCD PHOTOMETRY OF THE POORLY STUDIED GLOBULAR-CLUSTER M80

We present results of the first multicolor (UBVI) CCD photometry of 48 99 stars in a wide (4' x 7') field of the poorly studied globular clus ter M80. Our results feature both a wide magnitude range (11.5 < V < 2 1.5) and a relatively large number of stars belonging to brighter sequ ences 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 mo rphology and similar slope of the red giant branches (RGBs) of both cl usters. Moreover, the V magnitude of the ''bump'' in the luminosity fu nction of the RGB compared with the position of the horizontal branch (HB) and of the turnoff point corresponds to a most probable metallici ty value of [Fe/H] = 1.70 +/- 0.15 for M80. A comparison of the CMDs a nd of the HE luminosity functions of the two clusters demonstrates tha t the two HBs span an almost identical range in V and have a similar u neven (fragmented) distribution of stars along this branch. However, w e find a difference (earlier noted for NGC 6541 and M79) between M79 a nd M80 in the position of the HE relative to the RGB, both in the U-(B -V) CMD and in the two-color (U-B)-(B-V) diagram. Our photometry of M8 0 yields a number of very probable W-bright star and blue straggler ca ndidates. The latter can be grouped, by photometric characteristics an d position in the cluster, into two distinct classes of apparently dif ferent nature. The difference in V magnitude between the HE and the tu rnoff point of M80 is Delta V = 3.60 +/- 0.09. The helium abundance es timated 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 oxyge n-enhanced isochrones, is 17 +/- 2 Gyr. However, similar differences i n other colors do not lead to a single age value.