Near-Infrared Extinction in the Coalsack Globule 2

We have conducted J, H, and Ks imaging observations of the Coalsack Globule 2 with the SIRIUS infrared camera on the IRSF 1.4 m telescope at SAAO and determined the color excess ratio, EJ-H/EH − Ks. The ratio is determined in the same photometric system as our previous study of the ρ Oph and Cha clouds without any color transformation; this enables us to directly compare the near-infrared extinction laws among these regions. The current ratio, EJ-H/EH − Ks = 1.91 ± 0.01 for the extinction range 0.5 < EJ-H < 1.8, is significantly larger than the ratios for the ρ Oph and Cha clouds (EJ-H/EH − Ks = 1.60-1.69). This ratio corresponds to a large negative index α = 2.34 ± 0.01 when the wavelength dependence of extinction is approximated by a power law Aλ ∝ λ-α, which in this cloud might indicate little growth of dust grains, larger abundance of dielectric nonabsorbing components, such as silicates, or both. We also confirm that the color excess ratio for the Coalsack Globule 2 has a trend of increasing with decreasing optical depth, which is the same trend as for the ρ Oph and Cha clouds.