Herschel-HIFI detections of hydrides towards AFGL 2591 (Envelope emission versus tenuous cloud absorption)

TL;DR

This study uses Herschel-HIFI to observe hydrides in AFGL 2591, revealing complex absorption and emission features that inform about the chemical and physical conditions in the star-forming region's envelope and outflows.

Contribution

First high-resolution spectral observations of multiple hydrides in AFGL 2591, distinguishing envelope emission from cloud absorption and proposing a FUV irradiation model for molecular excitation.

Findings

Detection of CH, CH+, NH, OH+, H2O+ in absorption; NH+ and SH+ not detected.

CH and CH+ lines observed in emission at systemic velocity.

FUV irradiation of outflow walls explains observed molecular excitation.

Abstract

The Heterodyne Instrument for the Far Infrared (HIFI) onboard the Herschel Space Observatory allows the first observations of light diatomic molecules at high spectral resolution and in multiple transitions. Here, we report deep integrations using HIFI in different lines of hydrides towards the high-mass star forming region AFGL 2591. Detected are CH, CH+, NH, OH+, H2O+, while NH+ and SH+ have not been detected. All molecules except for CH and CH+ are seen in absorption with low excitation temperatures and at velocities different from the systemic velocity of the protostellar envelope. Surprisingly, the CH(JF,P = 3/2_2,- - 1/2_1,+) and CH+(J = 1 - 0, J = 2 - 1) lines are detected in emission at the systemic velocity. We can assign the absorption features to a foreground cloud and an outflow lobe, while the CH and CH+ emission stems from the envelope. The observed abundance and excitation of CH and CH+ can be explained in the scenario of FUV irradiated outflow walls, where a cavity etched out by the outflow allows protostellar FUV photons to irradiate and heat the envelope at larger distances driving the chemical reactions that produce these molecules.