Water Absorption from Gas Very Near the Massive Protostar AFGL 2136 IRS 1

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We present ground-based observations of the nu₁ and nu₃ fundamental bands of H2O toward the massive protostar AFGL 2136 IRS 1, identifying absorption features due to 47 different ro-vibrational transitions between 2.468 micron and 2.561 micron. Analysis of these features indicates the absorption arises in warm (T=506+-25 K), very dense (n(H2)>5*10⁹ cm-3) 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)*10¹9 cm-2, 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.