CCD PHOTOMETRY OF GLOBULAR-CLUSTERS .4. NGC-7006

A new color-magnitude diagram (CMD) for the galactic globular cluster NGC 7006 is presented, based on B, V CCD photometry of approximately 3000 stars to a limiting magnitude of V approximately 24. From the analysis of the CMD the following conclusions have been drawn: (a) The overall morphology of the main branches reproduces in essence earlier results; in particular, the horizontal branch (HB) is unusually red for the cluster metallicity ([Fe/H] = - 1.77). This confirms that NGC 7006 is a genuine member of the group of clusters affected by the so-called "second parameter" syndrome. (b) The difference in luminosity between the HB and the main sequence turn-off (TO) is DELTA-HB(TO) = 3.50 +/- 0.12, totally consistent with the mean value-3.55 +/- 0.09-obtained by Buonanno et al. (1989) for 19 Galactic globulars. (c) The mean color of the HB in the inner regions (r less-than-or-equal-to 30 arcsec) appears to be significantly shifted towards bluer colors compared to the outer regions. (d) The combination of the "normal" location of the TO with the peculiar radial variation in HB morphology leads to exclude age as the second parameter in this cluster. A strong influence of the "environmental" conditions on HB morphology seems supported by these data, confirming our previous suggestions [Buonanno et al., A&A, 145, 97 (1985)] and a growing body of observational evidences. (e) Adopting the approach presented by Buonanno et al. [A&A, 216, 80 (1989)], the present data yield for this cluster: M(v)(HB) = 0.63, (m - M)v = 18.17 and, finally, d. = 40 +/- 2 kpc.