Herschel observations of deuterated water towards Sgr B2(M)

TL;DR

This study uses Herschel observations of HDO in Sgr B2(M) to map its abundance profile, revealing a steep increase with temperature that informs water formation processes in different astrophysical environments.

Contribution

It provides the first detailed HDO abundance profile in Sgr B2(M) using radiative transfer modeling of Herschel data, highlighting temperature-dependent abundance variations.

Findings

HDO abundance increases with temperature from 2.5e-11 to 3.5e-9

Abundance profile suggests different formation mechanisms in envelope and core

Modeling confirms steep abundance gradient with temperature

Abstract

Observations of HDO are an important complement for studies of water, because they give strong constraints on the formation processes -- grain surfaces versus energetic process in the gas phase, e.g. in shocks. The HIFI observations of multiple transitions of HDO in Sgr~B2(M) presented here allow the determination of the HDO abundance throughout the envelope, which has not been possible before with ground-based observations only. The abundance structure has been modeled with the spherical Monte Carlo radiative transfer code RATRAN, which also takes radiative pumping by continuum emission from dust into account. The modeling reveals that the abundance of HDO rises steeply with temperature from a low abundance ($2.5\times 10^{-11}$) in the outer envelope at temperatures below 100~K through a medium abundance ($1.5\times 10^{-9}$) in the inner envelope/outer core, at temperatures between 100 and 200~K, and finally a high abundance ($3.5\times 10^{-9}$) at temperatures above 200~K in the hot core.