The CHESS survey of the L1157-B1 shock: the dissociative jet shock as revealed by Herschel--PACS

TL;DR

This study uses Herschel-PACS spectra to analyze the L1157-B1 shock, revealing high-temperature, high-density gas and identifying a dissociative J-type shock driven by protostellar outflows, with detailed spatial and spectral insights.

Contribution

First detailed Herschel-PACS spectral mapping of L1157-B1 revealing the nature of the dissociative shock and hot gas conditions in protostellar outflows.

Findings

High-excitation CO lines indicate temperatures around 210 K.

The shock is consistent with a dissociative J-type shock model.

The shock region is about 7 arcseconds in size, with high density (>10^5 cm^-3).

Abstract

Outflows generated by protostars heavily affect the kinematics and chemistry of the hosting molecular cloud through strong shocks that enhance the abundance of some molecules. L1157 is the prototype of chemically active outflows, and a strong shock, called B1, is taking place in its blue lobe between the precessing jet and the hosting cloud. We present the Herschel-PACS 55--210 micron spectra of the L1157-B1 shock, showing emission lines from CO, H2O, OH, and [OI]. The spatial resolution of the PACS spectrometer allows us to map the warm gas traced by far-infrared (FIR) lines with unprecedented detail. The rotational diagram of the high-Jup CO lines indicates high-excitation conditions (Tex ~ 210 +/- 10 K). We used a radiative transfer code to model the hot CO gas emission observed with PACS and in the CO (13-12) and (10-9) lines measured by Herschel-HIFI. We derive 200<Tkin<800 K and n>10^5 cm-3. The CO emission comes from a region of about 7 arcsec located at the rear of the bow shock where the [OI] and OH emission also originate. Comparison with shock models shows that the bright [OI] and OH emissions trace a dissociative J-type shock, which is also supported by a previous detection of [FeII] at the same position. The inferred mass-flux is consistent with the "reverse" shock where the jet is impacting on the L1157-B1 bow shock. The same shock may contribute significantly to the high-Jup CO emission.