2013ApJ...776....8I

We present ground-based observations of the ν₁ and ν₃fundamental bands of H₂ O toward the massive protostar AFGL 2136 IRS 1, identifying absorption features due to 47 different ro-vibrational transitions between 2.468 µm and 2.561 µm. Analysis of these features indicates the absorption arises in warm (T = 506±25 K), very dense (n(H₂) > 5x10⁹/cm3) gas, suggesting an origin close to the central protostar. The total column density of warm water is estimated to be N(H₂O) = (1.02±0.02)x10¹⁹/cm2, giving a relative abundance of N(H₂O)/N(H₂) ~ 10^–4. Our study represents the first extensive use of water vapor absorption lines in the near infrared, and demonstrates the utility of such observations in deriving physical parameters.