CCD PHOTOMETRY OF STARS IN THE OLD OPEN CLUSTER NGC-188

We present CCD photometry for stars in three fields centered on the old open cluster NGC 188, with the aim of investigating the HR diagram distribution of cluster main-sequence stars. A sequence of subgiant, turnoff, and main-sequence stars has been detected, extending the observed lower main sequence down to about mv = 20 mag. By relying on theoretical evolutionary tracks and by adopting recent estimates of the cluster reddening and distance modulus, we find that the observed color-magnitude diagram appears well fitted by a theoretical isochrone for an age t≃6×109 yr, provided that Yale transformations from the theoretical (luminosity versus effective temperature) into the observational plane (visual magnitude versus color) are assumed. The same age would be derived by using Vanden-Berg [Astrophys. J. Suppl. 58, 711 (1985)] transformations, but with a discrepancy in color for the main-sequence stars, which grows for lower luminosities. From the distribution of the stellar luminosities one finds that the turnoff stars should have masses around 1.3μ⊙. Moreover, we find that the MS appears progressively depopulated in going toward lower masses. We finally discuss the cluster evolutionary scenario in connection with the reported luminosity of the clump of He-burning giants. A cluster original helium content of the order of Y≃0.28 is derived, suggesting a "solar" chemical composition for NGC 188.