WIYN OPEN CLUSTER STUDY - I - DEEP PHOTOMETRY OF NGC-188

We have employed precise and carefully calibrated V- and I-band photom etry of NGC 188 at WIYN Observatory to explore the cluster luminosity function (LF) and study the cluster white dwarfs. Our photometry is of fset by V = 0.052 (fainter) from that of Sandage and Eggen & Sandage. All published photometry for the past three decades has been based on these two calibrations, which are in error by 0.05 +/- 0.01. We employ the Pinsonneault et al. fiducial open cluster main sequence to derive a distance modulus of 11.43 +/- 0.08 and E(B-V) = 0.09, with the larg est single source of error caused by uncertainty in the cluster metall icity. We report observations that are greater than or equal to 50% co mplete along the main sequence to V = 24.6. We find that the NGC 188 c entral-field LF peaks at M-I similar to 3 to 4. This is unlike the sol ar neighborhood LF and unlike the LFs of dynamically unevolved portion s of open and globular clusters, all of which typically rise continuou sly until M-I approximate to 9.5. Although we find that greater than o r equal to 50% of the unresolved objects in this cluster are multiple systems with mass ratios greater than or equal to 0.3, their presence cannot account for the shape of the NGC 188 LF. For theoretical reason s having to do with the long-term survivability of NGC 188, we believe the cluster is highly dynamically evolved and that the low-luminosity stars missing from the central cluster LF are either in the cluster o utskirts or have left the cluster altogether. We identify nine-candida te white dwarfs (WDs) in NGC 188, of which we expect at least three, a nd perhaps six, are bona fide cluster WDs. The luminosities of the fai ntest likely WD indicate an age of 1.14 +/- 0.09 Gyr, where the error in age includes the cluster distance uncertainty and we assume the WD has a hydrogen atmosphere. This age is a lower limit to the cluster ag e, and observations probing to V = 27 or 28 will be necessary to find the faintest duster WDs and independently determine the cluster age. W hile our lower age-limit is not surprising for this approximate to 6 G yr-old cluster, our result demonstrates the value of the WD age techni que with its very low internal errors.