Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

TL;DR

This study uses Herschel/HIFI to measure the ortho/para ratio of water in interstellar clouds, revealing ratios close to statistical high-temperature values and a notably lower ratio in Sagittarius B2(M), indicating formation or relaxation at lower temperatures.

Contribution

First measurement of water's ortho/para ratio in specific interstellar regions using Herschel/HIFI, providing insights into molecular formation and thermal history.

Findings

Ortho/para ratio generally near 3, consistent with high-temperature equilibrium.

Lower ratio (~2.35) observed towards Sagittarius B2(M), indicating lower formation temperature.

Water molecules in certain regions reflect local temperature conditions.

Abstract

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H$_2^{16}$O and H$_2^{18}$O in absorption towards Sagittarius~B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of $2.35 \pm 0.35$, corresponding to a spin temperature of $\sim$27~K, towards Sagittarius~B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.