HIFI observations of warm gas in DR21: Shock versus radiative heating

TL;DR

This study uses Herschel/HIFI observations to distinguish between shock and radiative heating in the DR21 star formation region, revealing that radiative heating dominates despite evidence of shocks and outflows.

Contribution

It provides a quantitative analysis of heating mechanisms in DR21 by sampling the excitation ladder of molecular tracers with new high-frequency observations.

Findings

All observed lines up to > 2 THz are explained by radiative heating.

Identification of hot, dense clumps near the cluster affected by outflows.

Detection of widespread cooler, dense molecular clumps.

Abstract

The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. HIFI observations of mid-J transitions of CO and HCO+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. In spite of the known shock-excitation of H2 and the clearly visible strong outflow, we find that the emission of all lines up to > 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.