Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL

TL;DR

This paper presents Herschel/HIFI observations of water and its isotopologues in Orion KL, revealing detailed spectral profiles and deriving water abundances in different regions, indicating enhanced sublimation due to warm dust.

Contribution

First detection of numerous velocity-resolved water lines in Orion KL, with a simple spectral fit and abundance analysis providing new insights into water distribution in star-forming regions.

Findings

Detected over 48 water lines, most for the first time.

Derived water abundances for different regions, showing higher values in hot and warm areas.

Indicated enhanced water-ice sublimation in warmer regions.

Abstract

We report the detection of more than 48 velocity-resolved ground rotational state transitions of H2(16)O, H2(18)O, and H2(17)O - most for the first time - in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured H2(18)O line fluxes, which are less affected by line opacity than their H2(16)O counterparts, and an escape probability method, the column densities of H2(18)O associated with each emission component are derived. We infer total water abundances of 7.4E-5, 1.0E-5, and 1.6E-5 for the plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources.