The Benchmark Cluster Reddening Project. I. Reddening Values for the Hyades, Coma, and Praesepe

From histories of reddening studies for the Hyades, Praesepe, and nearby field stars, it is concluded that reddening scholarship requires fundamental reform. The adopted protocol for the reformed version of scholarship includes (1) rigorous published foundations for reddening values, (2) explicit quality control of reddening techniques, (3) use of techniques with limited metallicity sensitivity, (4) explicit zero-point control for both input data and derived reddening values, (5) an explicit choice of a reddening zero point, and (6) adherence to statistical norms. It is argued that only limited use should be made of reddening values from (1) spectral types for F0 and later type stars, (2) UBV analysis of blanketed stars, and (3) certain published relations between reddening and distance. However, reddening values may be based safely on (1) polarization measurements, (2) comparisons of β and (R - I)C for F stars, and (3) Strömgren β analysis of A stars. For the Hyades and Coma, upper 95% confidence limits on E(B - V) are found to be ≤1.0 and ≤3.2 mmag, respectively. For Praesepe, the derived value of E(B - V) is 27 ± 4 mmag. The latter result is obtained after an allowance is made for the effects of an anomalous relation between b - y and (R - I)C in Praesepe A stars. A test of the possibility that differential stellar rotation causes the anomaly yields an inconclusive result. Adoption of the revised reddening value instead of the canonical value of zero yields an increase in the derived Praesepe distance modulus by 0.14 mag and a decrease in the derived cluster age by about 11%. In addition, by using published high-dispersion results, it is found that the value of Fe/H for Praesepe that is consistent with the derived reddening value is +0.01 ± 0.04 dex. Further results for Praesepe stars will be required for a definitive comparison of the Hyades and Praesepe metallicities.