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

We determine and tabulate A_[λ]/A_K, the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 μm ≤ λ ≤ 8.0 μm along two lines of sight: l = 42^° and 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 3-8 μm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A_[λ] - A_K)/(A_J - A_K) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., A_J/A_K, is required to calculate A_[λ]/A_K from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine A_J kpc^-1, A_K kpc^-1, and A_J/A_K. Our values of A_[λ]/A_K show a flattening across the 3-8 μm wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.