THE MID-INFRARED EXTINCTION LAW IN THE LARGE MAGELLANIC CLOUD

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(lambda)/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] mu m) relative to the Two Micron All Sky Survey K-S band at 2.16 mu m. We obtain the near-infrared extinction coefficient to be E(J - H)/E(H - K-S) approximate to 1.29 +/- 0.04 and E(J - K-S)/E(H -K-S) approximate to 1.94 +/- 0.04. The wavelength dependence of the MIR extinction A(lambda)/A(KS) in the LMC varies from one sightline to another. The overall mean MIR extinction is A([3.6])/A(KS) approximate to 0.72 +/- 0.03, A([4.5])/A(KS) approximate to 0.94 +/- 0.03, A([5.8])/A(KS) approximate to 0.58 +/- 0.04, and A([8.0])/A(KS) approximate to 0.62 +/- 0.05. Except for the extinction in the IRAC [4.5] mu m band, which may be contaminated by the 4.6 mu m 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 - lambda and J - K-S is explored in terms of Monte Carlo simulations. We find that this bias could lead to an overestimation of A(lambda)/A(KS) in the MIR.