The wavelength dependence of interstellar extinction from 1.25 to 8.0 μm using glimpse data

We determine and tabulate A([lambda]) /A(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 mum less than or equal to lambda less than or equal to 8.0 mum along two lines of sight: l = 42degrees and 284degrees. 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 mum by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A([lambda]) - AK)/(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([lambda]) /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([lambda]) /A(K) show a flattening across the 3 - 8 mum wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center.