Deep wide-field spectrophotometry of the open cluster M67

We present nine color CCD intermediate-band spectrophotometry of a two square deg(2) field centered on the old open cluster M67, from 3890 Angstrom to nearly 1 mu m. These observations are taken as a part of the BATC (Beijing-Arizona-Taipei-Connecticut) Color Survey of the Sky, for both scientific and calibration reasons. The BATC program uses a dedicated 60/90 cm Schmidt telescope with a 2048 x 2048 CCD and 15 specially-designed intermediate-band filters to be applied to both galactic and extragalactic studies. With these data we show that the BATC survey can reach its goal of obtaining spectrophotometry to a zero point accuracy of 0.01 mag, and down to V = 21 with 0.3 mag random error. Nine-band spectrophotometry of 6558 stars is presented. Systematic issues studied include the effect of image undersampling, astrometric accuracy and transformation from BATC photometric system to broad-band systems. We fit the color-magnitude diagrams (CMDs) with Worthey's theoretical models. The net result is the excellent fit of the 4.0 Gyr, [Fe/H] = -0.10 model to our data, including a good fit to the main sequence (MS) turnoff, This fit predicts (m - M)(0) = 9.47 +/- 0.16 and E(B - V) between 0.015 and 0.052, We show that 16% of stars in M67 are binaries with mass-ratio larger than 0.7. Our data are consistent with a toy model with 50% of the stars in M67 being binaries and a random distribution of binary mass-ratios, although other models with different mass-ratio distributions cannot be ruled out. The spatial distribution and mass function (MF) of stars in M67 show marked effects of dynamical evolution and evaporation of stars from the cluster. Blue stragglers and binary stars are the most condensed within the cluster, with degree of condensation depending on mass. The inner part of M67 is missing most of the lower mass MS stars. We find M67 to have an elongated shape, oriented at an angle of 15 degrees relative to the galactic plane. Within its tidal radius, the observed MF of M67 between 1.2 M. and 0.8 M. has a Salpeter slope eta = -1.93 +/- 0.66. For stars of mass below 0.8 M., eta similar to 0. It is plausible that the leveling-off of the MF at lower masses is a result of evaporation of lower mass stars in this mass range at a rate of one every similar to 10(7) years. If so, it is plausible that the IMF of M67 has the canonical field value of eta = -2.0. Overall, we find the stellar distribution as a function of mass within M67, and the observed MF, are in good agreement with theoretical predictions of dynamical evolution and evaporation of an old galactic cluster. Moreover, the fraction of binary stars and inferred IMF for higher mass main sequence stars for this old galactic cluster are consistent with known field star values. This implies a similarity of IMF that persists for at least 4 Gyr in the disk of our Galaxy. (C) 1995 American Astronomical Society.