A uvbyCaHβ CCD Analysis of the Open Cluster Standard, M67, and Its Relation to NGC 752

Abstract

Precision CCD uvbyCaHβ photometry is presented of the old cluster, M67, covering one square degree with typical internal precision at the 0.005–0.020 mag level to V ∼ 17. The photometry is calibrated using standards over a wide range in luminosity and temperature from NGC 752 and zeroed to the standard system via published photoelectric observations. Relative to NGC 752, differential offsets in reddening and metallicity are derived using astrometric members, supplemented by radial velocity information. From single-star members, offsets in the sense (M67−NGC 752) are δ E(b − y) = −0.005 ± 0.001 (sem) mag from 327 F/G dwarfs and δ[Fe/H] = 0.062 ± 0.006 (sem) dex from the combined m 1 and hk indices of 249 F dwarfs, leading to E(b − y) = 0.021 ± 0.004 (sem) and [Fe/H]M67 = +0.030 ± 0.016 (sem) assuming [Fe/H]Hyades = +0.12. With probable binaries eliminated using c 1, (b − y) indices, 83 members with (π/σ π ) > 50 generate (m − M)0 = 8.220 ± 0.005 (sem) for NGC 752 and an isochronal age of 1.45 ± 0.05 Gyr. Using the same parallax restriction for 312 stars, M67 has (m − M) = 9.77 ± 0.02 (sem), leading to an age tied solely to the luminosity of the subgiant branch of 3.70 ± 0.03 Gyr. The turnoff color spread implies ±0.1 Gyr, but the turnoff morphology defines a younger age/higher mass for the stars, consistent with recent binary analysis and broadband photometry indicating possible missing physics in the isochrones. Anomalous stars positioned blueward of the turnoff are discussed.