THE GALACTIC GLOBULAR-CLUSTER NGC-5897 AND ITS POPULATION OF BLUE STRAGGLERS

UBVI CCD photometry for 2709 stars in the loose Galactic globular cluster NGC 5897 is presented. The measurements carried out in the very central regions reach below the main-sequence tum-off. From analysis of the colour-magnitude diagrams the following conclusions are drawn. (i) The overall morphology of the main branches closely resembles that of the similarly low-concentration cluster NGC 288, including also the presence of a clear gap along the almost identical horizontal branch (whose level is estimated to be V(HB) = 16.35 +/- 0.10) and a population of blue stragglers. (ii) Since the mean adopted metallicity [from (B - V)0,g, DELTA-V, S, delta(U - B)0.6 and (V - I)0.-3] is [Fe/H] = -1.75 +/- 0.20 [in the Zinn & West scale, with E(B - V) = 0.10 +/- 0.03], whilst that of NGC 288 is -1.40 (or even larger), the pair NGC 5897-NGC 288, showing identical HBs, is a typical representative of the so-called 'second parameter' effect. (iii) The BS candidates (34 plus about 10 at lower confidence) are more centrally concentrated than the cluster subgiant stars of similar brightness at the 2-sigma level of confidence. Based also on a wide forthcoming review of the CMDs of all the clusters containing BS, arguments are presented to suggest the detection on the red HB extreme of a very small BS progeny.