The origin of organic emission in NGC 2071^⋆

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: kempen@strw.leidenuniv.nl
² Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
³ Joint Astronomy Center, 660 North A’ohoku Place, University Park, Hilo, HI 96720, USA
⁴ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Rd, Victoria, BC, V9E 2E7, Canada
⁵ Department of Physics & Astronomy, University of Victoria, Victoria, BC, V8P 1A1, Canada

Received: 17 March 2014
Accepted: 8 July 2014

Abstract

Context. The physical origin behind organic emission lines in embedded low-mass star formation has been fiercely debated over the last two decades. A multitude of scenarios have been proposed, from a hot corino to PDRs on cavity walls to shock excitation.

Aims. The aim of this paper is to determine the location and the corresponding physical conditions of the gas responsible for organics emission lines. The outflows around the small protocluster NGC 2071 are an ideal testbed that can be used to differentiate between various scenarios.

Methods. Using Herschel-HIFI and the Submillimeter Array, observations of CH₃OH, H₂CO, and CH₃CN emission lines over a wide range of excitation energies were obtained. Comparisons to a grid of radiative transfer models provide constraints on the physical conditions. Comparison to H₂O line shape is able to trace gas-phase synthesis versus a sputtered origin.

Results. Emission of organics originates in three separate spots: the continuum sources IRS 1 (“B”) and IRS 3 (“A”) and a new outflow position (“F”). Densities are above 10⁷ cm^-3 and temperatures between 100 K and 200 K. CH₃OH emission observed with HIFI originates in all three regions and cannot be associated with a single region. Very little organic emission originates outside of these regions.

Conclusions. Although the three regions are small (<1500 AU), gas-phase organics likely originate from sputtering of ices as a result of outflow activity. The derived high densities (>10⁷ cm^-3) are likely a requirement for organic molecules to survive from being immediately destroyed by shock products after evaporation. The lack of spatially extended emission confirms that organic molecules cannot (re-)form through gas-phase synthesis, as opposed to H₂O, which shows strong line wing emission. The lack of CH₃CN emission at “F” is evidence for a different history of ice processing because of the absence of a protostar at that location and recent ice mantle evaporation.