Shocks and UV radiation around low-mass protostars: the Herschel-PACS legacy

TL;DR

This study uses Herschel-PACS far-infrared spectroscopy of 90 low-mass protostars to analyze gas heating, shocks, and UV radiation effects, revealing high-temperature CO components and UV influence on molecular abundances.

Contribution

It provides a comprehensive observational analysis of shock and UV effects in low-mass protostars using Herschel-PACS data, comparing results with shock models to understand physical conditions.

Findings

CO rotational temperatures around 300 K, some up to 1000 K

Low H2O/CO and H2O/OH flux ratios suggest UV dissociation effects

Good agreement with UV-illuminated shock models at specific densities and UV fields

Abstract

Far-infrared spectroscopy reveals gas cooling and its underlying heating due to physical processes taking place in the surroundings of protostars. These processes are reflected in both the chemistry and excitation of abundant molecular species. Here, we present the Herschel-PACS far-IR spectroscopy of 90 embedded low-mass protostars from the WISH (van Dishoeck et al. 2011), DIGIT (Green et al. 2013), and WILL surveys (Mottram et al. 2017). The $5\times5$ spectra covering the $\sim50''\times50''$ field-of-view include rotational transitions of CO, H$_2$O, and OH lines, as well as fine-structure [O I] and [C II] in the $\sim$50-200 $\mu$m range. The CO rotational temperatures (for $J_\mathrm{u}\geq14)$ are typically $\sim$300 K, with some sources showing additional components with temperatures as high as $\sim$1000 K. The H$_2$O / CO and H$_2$O / OH flux ratios are low compared to stationary shock models, suggesting that UV photons may dissociate some H$_2$O and decrease its abundance. Comparison to C shock models illuminated by UV photons shows good agreement between the line emission and the models for pre-shock densities of $10^5$ cm$^{-3}$ and UV fields 0.1-10 times the interstellar value. The far-infrared molecular and atomic lines are the unique diagnostic of shocks and UV fields in deeply-embedded sources.