2013ApJ...776....7G

Based on photometric data from the Spitzer/SAGE survey, using red giants as extinction tracers, the mid-infrared (MIR) extinction laws in the Large Magellanic Cloud (LMC) are derived for the first time in the form of A_λ/A_Ks. This quantity refers to the extinction in the four Infrared Array Camera (IRAC) bands (i.e., [3.6], [4.5], [5.8], and [8.0] µm) relative to the Two Micron All Sky Survey K_S band at 2.16 µm. We obtain the near-infrared extinction coefficient to be E(J - H)/E(H - K_S) ~ 1.29±0.04 and E(J - K_S)/E(H - K_S) ~ 1.94±0.04. The wavelength dependence of the MIR extinction A_λ/A_Ks in the LMC varies from one sightline to another. The overall mean MIR extinction is A_[3.6]/A_Ks ≈ 0.72 ± 0.03, A_[4.5]/A_Ks ≈ 0.94 ± 0.03, A_[5.8]/A_Ks ≈ 0.58 ± 0.04, and A_[8.0]/A_Ks ≈ 0.62 ± 0.05. Except for the extinction in the IRAC [4.5] µm band, which may be contaminated by the 4.6 µm CO gas absorption of red giants used to trace LMC extinction, the extinction in the other three IRAC bands show a flat curve, close to the Milky Way R_V = 5.5 model extinction curve, where R_V is the optical total-to-selective extinction ratio. The possible systematic bias caused by the correlated uncertainties of K_S- λ and J - K_S is explored in terms of Monte Carlo simulations. We find that this bias could lead to an overestimation of A_λ/A_Ks in the MIR.