Herschel observations of Extra-Ordinary Sources: Methanol as a probe of physical conditions in Orion KL

TL;DR

This study uses Herschel/HIFI observations of methanol emission in Orion KL to probe the physical conditions and thermal structure of different spatial components, revealing external heating of the compact ridge.

Contribution

First detailed analysis of methanol emission lines in Orion KL with Herschel, demonstrating their effectiveness in probing physical conditions and thermal structure.

Findings

Methanol lines reveal distinct physical conditions in Orion KL components.

The compact ridge is externally heated, as shown by methanol emission analysis.

Methanol transitions cover a broad energy range, enabling thermal structure mapping.

Abstract

We have examined methanol emission from Orion KL with of the {\em Herschel}/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated $\Delta$J$=$0, K$=-4\rightarrow$-3, v$_t$$=$0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the $\Delta$J$=$0, K$=7\rightarrow$6, v$_t$$=$0 Q branch transitions which are mostly blended. We have used the isolated E-type v$_t$$=$0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI's high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.