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 1.25 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] - 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 sight lines and fit the reddening as a function of magnitude (distance) to determine AJ kpc-1, AK kpc-1, and AJ/AK. Our values of A[λ]/AK 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.