A PHOTOMETRIC AND RADIAL-VELOCITY ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER NGC-752

Using all available proper-motion and radial-velocity data, including new radial-velocity observations obtained for this investigation, prob able members of the open cluster NGC 752 have been identified. Photoel ectric data on six systems have been transformed and collated to form an internally consistent sample on the BV system. Binaries have been i dentified using photometric and radial-velocity data, including a phot ographic survey for variability and the radial-velocity observations o f this study. Analysis of the data leads to the following cluster para meters and their probable errors: E(B - V) = 0.035 +/- 0.005 mag, [Fe/ H] = -0.15 +/- 0.05 dex, and (m - M) = 8.25 +/- 0.10 mag. The spread i n color among stars in the color-magnitude diagram (cmd) along the mai n sequence from the turnoff to the unevolved main sequence is the cons equence of a rich population of binaries. Due to its age and the compr ehensive data available for the cluster, NGC 752 provides an ideal tes t of a variety of evolutionary phenomena. Comparison with theoretical isochrones normalized in an internally consistent manner leads to the conclusion that the morphology and distribution of stars in the cmd ca n best be matched using models that include convective overshoot, part icularly those of Schaller et al. (1992, A&AS, 96, 269). Despite their differences, the traditional and the overshoot isochrones both imply very similar ages, 1.9 +/- 0.2 Gyr and 1.7 +/- 0.1 Gyr, respectively, for the cluster. The Li abundances for the giants confirm that the gia nt branch is dominated by clump stars and first-ascent giants below th e luminosity of the clump. The position and size of the Li dip among t he main-sequence stars, compared to the Hyades, is readily explained b y stellar evolution with convective overshoot. It is predicted that am ong turnoff stars in the intermediate-age range Li will cease to be a unique function of age at a given color. Chromospheric flux is shown t o be a well-defined function of color for single, unevolved stars, ide ntical to that found for the Hyades, and the relation for NGC 752 fall s within the Vaughan-Preston gap. However, the slope of the relation r equires that increasing color implies increasing age for the bluer por tion of the weak-emission boundary. The combined effect of small sampl es, random errors, emission limits, a possible selection bias in favor of turnoff stars, and metallicity corrections is to render highly que stionable any interpretation of time-variable star formation within th e Galaxy based upon chromospheric ages.