The Infrared Extinction Law at Extreme Depth in a Dark Cloud Core

We combined sensitive near-infrared data obtained with ground-based imagers on the ESO NTT and VLT telescopes with space mid-infrared data acquired with the IRAC imager on the Spitzer Space Telescope to calculate the extinction law Aλ/AKs as a function of λ between 1.25 and 7.76 μm to an unprecedented depth in Barnard 59, a star-forming, dense core located in the Pipe Nebula. The ratios Aλ/AKs were calculated from the slopes of the distributions of sources in color-color diagrams λ-Ks versus H-Ks. The distributions in the color-color diagrams are fit well with single slopes to extinction levels of AKs~7 (AV~59 mag). Consequently, there appears to be no significant variation of the extinction law with depth through the B59 line of sight. However, when slopes are translated into the relative extinction coefficients Aλ/AKs, we find an extinction law that departs from the simple extrapolation of the near-infrared power-law extinction curve, and agrees more closely with a dust extinction model for a cloud with a total to selective absorption RV=5.5 and a grain size distribution favoring larger grains than those in the diffuse interstellar medium. Thus, the difference we observe could possibly be due to the effect of grain growth in denser regions. Finally, the slopes in our diagrams are somewhat less steep than those from the study of Indebetouw et al. for clouds with lower column densities, and this indicates that the extinction law between 3 and 8 μm might vary slightly as a function of environment.