The Mid-IR Extinction Law in the LMC

The mid-infrared (MIR) extinction law in the Large Magellanic Cloud (LMC) at four IRAC bands is derived using the data of the Spitzer/SAGE Program. The derived mean extinctions are A _[3.6]/A _Ks = 0.68±0.03, A _[4.5]/A _Ks = 0.97±0.03, A _[5.8]/A _Ks = 0.54±0.04, and A _[8.0]/A _Ks = 0.58±0.07. The results show that: (1) The extinctions at [3.6], [5.8] and [8.0] of the LMC consist a flat curve, similar to that of the Milky Way (MW) predicted by the interstellar grain model at Rv = 5.5; (2) The extinction at [4.5] is clearly higher than the other three bands, which may be caused by the additional absorption of the 4.27μm CO₂ ice and/or the 4.67μm CO ice in the LMC molecular clouds; (3) As far as individual sightlines are concerned, the MIR interstellar extinction law A _λ/A _Ks in the LMC varies with sightlines as the MW does.