AAS 200th meeting, Albuquerque, NM, June 2002
Session 73. Interim Stellar Materials: Gas, Dust and Clouds
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

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

J. Melbourne, P. Guhathakurta (UCSC)

We present two new methods for determining the value of RV 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 AV 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 AV estimates is carried out. The RMS difference in the two estimates shows a distinct minimum at RV ~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 RV 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 RV ~q 3.0, the methods are sensitive to a wide range of potential RV values.