2004/03/31 by J. Melbourne, P. Guhathakurta · 6 citations
Physics and Astronomy · #Angular diameter #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Cluster (spacecraft) #Extinction (optical mineralogy) #Globular cluster #Photometry (optics) #Power law #Star cluster #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/421737
published in The Astronomical Journal 128(1), 271-286 (Institute of Physics) · 36 pages, 15 figures, 1 table. Accepted for publication in the Astronomical Journal (July 2004)
arxiv created 2004/03/31 · openalex publication_date 2004/07/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present three methods for measuring the slope of the Galactic dust extinction law, R V , and a method for measuring the fine-scale structure of dust clouds in the direction of differentially reddened globular clusters. We apply these techniques to BVI photometry of stars in the low-latitude Galactic globular cluster NGC 4833, which displays spatially variable extinction/reddening about a mean ⟨ A V ⟩ ≈ 1. An extensive suite of Monte Carlo simulations is used to characterize the efficacy of the methods. The essence of the first two methods is to determine, for an assumed value of R V , the relative visual extinction δ A V of each cluster horizontal-branch (HB) star with respect to an empirical HB locus; the locus is derived from the color-magnitude diagram (CMD) of a subset of stars in a small region near the cluster center for which differential extinction/reddening are relatively small. A star-by-star comparison of δ A V from the ( B - V , V ) CMD with that from the ( V - I , V ) CMD is used to find the optimal R V . In the third method R V is determined by minimizing the scatter in the HB in the ( B - V , V ) CMD after correcting the photometry for extinction and reddening using the dust maps of Schlegel, Finkbeiner, & Davis. The weighted average of the results from the three methods gives R V = 3.0 ± 0.4 for the dust along the line of sight to NGC 4833. The fine-scale structure of the dust is quantified via the difference, (Δ A V ) ij ≡ (δ A V ) i - (δ A V ) j , between pairs of cluster HB stars ( i , j ) as a function of their angular separation r ij . The variance (mean square scatter) of (Δ A V ) ij is found to have a power-law dependence on angular scale: var( r ) ∝ r β , with β = +0.9 ± 0.1. This translates into an angular power spectrum P (κ) ∝ κ α , with the index α = -1.9 ± 0.1 for r ∼ 1'–5', where κ ≡ 1/ r . The dust angular power spectrum on small scales (from optical data) matches smoothly onto the larger scale power spectrum derived from Schlegel et al.'s far-infrared map of the dust thermal emission.