A study of the dust distribution and extinction law in Mon R2

The authors have observed infrared hydrogen recombination lines with a number of different beam sizes toward the H II region Mon R2 IRS 1. Two distinctly different models with variations in the extinction can fit the data: (i) If the variations are confined to scales comparable to or to larger than our smallest beam (~4"), the exponent in the extinction law (τ∝λ^-β) must be 1.4±0.1, implying that grains have grown considerably larger in the dense cloud than in the diffuse interstellar medium. (ii) If the extinction varies on a much smaller scale (~0.3"), the authors can fit the data with β=1.85. The clumps must be quite small and dense [r (clump)~1.4×10^-3 pc; M (clump)~6×10^-4M_⊙; n(H₂) (clump)~1.5×10⁶ cm^-3].