CCD PHOTOMETRY OF THE GLOBULAR-CLUSTER NGC 5897 - MORPHOLOGY OF THE COLOR MAGNITUDE DIAGRAM

We present CCD photometry in the B and V filter bands extending below the main sequence turnoff for the Galactic globular cluster NGC 5897. From the color-magnitude diagram (CMD), we draw the following conclusions: (1) The horizontal branch (HB) is predominantly blue with V(HB) = 16.35 +/- 0.15 at the blue edge of the RR Lyrae gap. (2) From our adopted value of E(B-V) = 0.07 +/- 0.04 mag, the color of the red giant branch (RGB) at the level of the HB, and other published data, we determine [Fe/H]= - 1.66 +/- 0.10 dex for NGC 5 897, placing it at the metallicity of the well-known globular cluster M3. However, the HB of M3 is not predominantly blue; instead it contains stars on the red and blue sides of the RR Lyrae gap, making NGC 5897 and M3 a "second parameter" pair, in which another parameter, in addition to metallicity, is controlling the morphology of the HB. (3) The relative age technique based on the color difference between the turnoff and the RGB indicates that NGC 5897 is almost-equal-to 2 Gyr older than M3, which suggests that, in this case, age is the second parameter. (4) The RGB luminosity function exhibits a significant enhancement of stars near the level of the HB. The difference in V mag between this enhancement and the HB, which is primarily dependent on the metal abundance, is essentially identical in NGC 5897 and M3. (5) The CMD for NGC 5897 also reveals a significant population of blue straggler stars (BSS); these are shown to be more centrally concentrated than the cluster subgiant branch (SGB) stars of similar magnitude, indicating the BSS are more massive than the SGB stars. In addition, a pseudo-main sequence, which includes the BSS and extends to the faintest regions of the unevolved main sequence, is constructed for NGC 5897 and the previously studied globular cluster NGC 6101. After appropriately combining these two sequences, luminosity functions of this composite photometry are constructed and reveal the possible existence of a gap in the distribution of stars near the main sequence turnoff.