The Wavelength Dependence of Interstellar Extinction from 1.25 to 8.0 μm Using GLIMPSE Data

We determine and tabulate A[λ]/AK, the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25µm ≤ λ ≤ 8.0µm along two lines of sight: l = 42 ◦ and l = 284 ◦. The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49 as well as a “field” region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3–8µm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A[λ] − AK)/(AJ − AK) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g. AJ/AK, is required to calculate A[λ]/AK from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sightlines, and fit the reddening as a function of magnitude (distance) to determine AJ/kpc, AK/kpc, and AJ/AK. Our values of A[λ]/AK show a flattening across the 3–8µm wavelength range, roughly consistent with the Lutz et al. (1996) extinction measurements derived