Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations

TL;DR

This study uses Herschel/HIFI observations to analyze water vapor in translucent clouds, revealing low excitation temperatures, constant water abundance relative to hydrogen, and insights into the thermal history and formation conditions of water molecules.

Contribution

First detailed analysis of water absorption in translucent clouds using Herschel/HIFI, providing insights into water abundance, excitation, and thermal history in these environments.

Findings

Water abundance relative to hydrogen is about 1×10^{-8}.

Water excitation temperature is below 10 K, indicating low excitation conditions.

Water ortho-to-para ratio mostly at 3, with some below, suggesting cold formation or thermalization history.

Abstract

We present Herschel/HIFI observations of nine transitions of \hho and \hheo towards six high-mass star-forming regions, obtained as part of the PRISMAS Key Program. Water vapor in translucent clouds is detected in absorption along every sightline. We derive the column density of \hho or \hheo for the lower energy level of each transition observed. The total water column density is about a few $10^{13} \rm{cm^{-2}}$. We find that the abundance of water relative to hydrogen nuclei is $1\times10^{-8}$ in agreement with models for oxygen chemistry with high cosmic ray ionization rates. Relative to \hh, the abundance of water is remarkably constant at $5\times10^{-8}$. The abundance of water in excited levels is at most 15%, implying that the excitation temperature $T_{ex}$ in the ground state transitions is below 10 K. The column densities derived from the two ortho ground state transitions indicates that $T_{ex}\simeq5$ K and that the density $n($\hh$)$ in the clouds is $\le10^4 \rm{cm^{-3}}$. For most clouds we derive a water ortho-to-para ratio consistent with the value of 3 expected in thermodynamic equilibrium in the high temperature limit. Two clouds with large column densities exhibit a ratio significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains, or when they later become at least partially thermalized with the cold gas ($\sim25$ K) in the shielded, low temperature regions of the clouds; evidently, they have not yet fully thermalized with the warmer ($\sim50$ K) translucent portions of the clouds.