SOFIA/EXES Observations of Water Absorption in the Protostar AFGL 2591 at High Spectral Resolution

TL;DR

This study uses high-resolution infrared spectroscopy from SOFIA/EXES to detect and analyze water vapor absorption in the protostar AFGL 2591, revealing details about its water content and physical conditions.

Contribution

First high-resolution spectral observations of water absorption in AFGL 2591, providing new measurements of water column density and temperature in a massive protostar.

Findings

Detected ten water absorption features, including transitions in different vibrational states.

Estimated water column density as approximately 1.3 x 10^{19} cm^{-2}.

Determined rotational temperature of water as about 640 K.

Abstract

We present high spectral resolution (~3 km/s) observations of the nu_2 ro-vibrational band of H2O in the 6.086--6.135 micron range toward the massive protostar AFGL 2591 using the Echelon-Cross-Echelle Spectrograph (EXES) on the Stratospheric Observatory for Infrared Astronomy (SOFIA). Ten absorption features are detected in total, with seven caused by transitions in the nu_2 band of H2O, two by transitions in the first vibrationally excited nu_2 band of H2O, and one by a transition in the nu_2 band of H2{18}O. Among the detected transitions is the nu_2 1(1,1)--0(0,0) line which probes the lowest lying rotational level of para-H2O. The stronger transitions appear to be optically thick, but reach maximum absorption at a depth of about 25%, suggesting that the background source is only partially covered by the absorbing gas, or that the absorption arises within the 6 micron emitting photosphere. Assuming a covering fraction of 25%, the H2O column density and rotational temperature that best fit the observed absorption lines are N(H2O)=(1.3+-0.3)*10^{19} cm^{-2} and T=640+-80 K.