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[73.01] Measuring the Slope of the Galactic Dust Extinction Law, R_V, in the Direction of Differentially-Reddened Globular Clusters

J. Melbourne, P. Guhathakurta (UCSC)

We present two new methods for determining the value of R_V in the direction of differentially-reddened globular clusters. The methods work on the premise that the width of the blue horizontal branch will be minimized when a reddening correction with the correct extinction law is applied. CCD observations in BVI of the low-latitude Galactic globular cluster NGC 4833 are used in this study. The cluster has a mean reddening of \langle{E(B-V)}\rangle = 0.3 mag, and differential reddening across the field. In the first method, the A_V value for a star is determined in two ways, using the (B-V, V) CMD and the (V-I, V) CMD. A star-by-star comparison of these two A_V estimates is carried out. The RMS difference in the two estimates shows a distinct minimum at R_V ~3. In the second method the (B-V, V) CMD of NGC 4833 is used in conjunction with the Schelgel et al. (1998) dust map to perform a reddening correction. The scatter in the horizontal branch is minimized for R_V between 3.0 and 3.5. Monte Carlo simulations are run to determine the accuracy of the two methods. While NGC 4833 gives the canonical value of R_V ~q 3.0, the methods are sensitive to a wide range of potential R_V values.

The author(s) of this abstract have provided an email address for comments about the abstract: jmel@ucolick.org

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.