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B. J. Taylor, M. D. Joner (BYU)
This paper describes the first step in deriving accurate, high-precision reddening values for a network of galactic clusters. Despite decades of work on the reddenings of these clusters and on other near-Sun reddening problems, there are no well-founded consensus reddening values for a number of galactic clusters. Moreover, there is no consensus about the dependence of reddening on distance near the Sun. To solve such problems, three standards of procedure are established: 1) acceptable reddening methods shall have small metallicity dependences that are explicitly allowed for (if necessary), 2) reddenings derived from polarization measurements shall be adopted as either definitive values or definitive lower limits, and 3) photometry used to derive reddening values shall have explicitly tested and internally consistent zero points. The third of these standards is applied to photometry on the Strömgren system and the Cousins R-I system. For the Hyades and Coma, photometric and polarimetric values of E(B-V) turn out to agree, with no differences from zero reddening at 95% confidence being encountered. The upper 95% confidence limit in E(B-V) for the Hyades is found from polarimetry to be 1.2 mmag (0.0012 mag). For Coma, the corresponding limit (again from polarimetry) is 3.5 mmag. For Praesepe, affairs are not so encouraging initially. The formal reddening value for the A stars turns out to be less than the polarization limit at 99% confidence, while the counterpart for the F stars exceeds the polarization limit at 99.99% confidence. The A-star reddening is therefore suspect, and one does in fact deduce (at 99.2% confidence) that the problem is posed by A-star values of b-y that are anomalously blue by about 11 mmag. When this problem is allowed for, the values of E(B-V) for the A and F stars agree, and their average value turns out to be 22.1 ±2.4 mmag.
This research has been supported by the BYU College of Physical and Mathematical Sciences.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.