The methanol lines and hot core of OMC2-FIR4, an intermediate-mass protostar, with Herschel-HIFI

TL;DR

This study presents the first detailed spectral analysis of an intermediate-mass protostar, OMC2-FIR4, revealing methanol lines, physical conditions, and potential infall motions, thereby bridging understanding between low- and high-mass star formation.

Contribution

It provides the first rich spectral dataset of an intermediate-mass protostar with Herschel-HIFI, analyzing methanol lines and physical conditions, and compares them with low-mass hot cores.

Findings

Detected over 100 methanol lines between 554 and 961 GHz.

Estimated methanol abundance in the hot core is up to 10^-6.

Observed asymmetric line profiles suggest infall motions.

Abstract

In contrast with numerous studies on the physical and chemical structure of low- and high-mass protostars, much less is known about their intermediate-mass counterparts, a class of objects that could help to elucidate the mechanisms of star formation on both ends of the mass range. We present the first results from a rich HIFI spectral dataset on an intermediate-mass protostar, OMC2-FIR4, obtained in the CHESS (Chemical HErschel SurveyS of star forming regions) key programme. The more than 100 methanol lines detected between 554 and 961 GHz cover a range in upper level energy of 40 to 540 K. Our physical interpretation focusses on the hot core, but likely the cold envelope and shocked regions also play a role in reality, because an analysis of the line profiles suggests the presence of multiple emission components. An upper limit of 10^-6 is placed on the methanol abundance in the hot core, using a population diagram, large-scale source model and other considerations. This value is consistent with abundances previously seen in low-mass hot cores. Furthermore, the highest energy lines at the highest frequencies display asymmetric profiles, which may arise from infall around the hot core.