Resolving the chemical substructure of Orion-KL^⋆,⋆⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: syfeng@mpia.de
² Centro de Radioastronomia y Astrofisica, Universidad Nacional Autonoma de Mexico, Morelia Michoacan, Mexico
³ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA

Received: 22 September 2013
Accepted: 12 March 2015

Abstract

Context. The Kleinmann-Low nebula in Orion (Orion-KL) is the nearest example of a high-mass star-forming environment. Studying the resolved chemical substructures of this complex region provides important insight into the chemistry of high-mass star-forming regions (HMSFRs), as it relates to their evolutionary states.

Aims. The goal of this work is to resolve the molecular line emission from individual substructures of Orion-KL at high spectral and spatial resolution and to infer the chemical properties of the associated gas.

Methods. We present a line survey of Orion-KL obtained from combined Submillimeter Array (SMA) interferometric and IRAM 30 m single-dish observations. Covering a 4 GHz bandwidth in total, this survey contains over 160 emission lines from 20 species (25 isotopologues), including 11 complex organic molecules (COMs). Spectra are extracted from individual substructures and the intensity-integrated distribution map for each species is provided. We then estimate the rotation temperature for each substructure, along with their molecular column densities and abundances.

Results. For the first time, we complement 1.3 mm interferometric data with single-dish observations of the Orion-KL region and study small-scale chemical variations in this region. (1) We resolve continuum substructures on ~3′′ angular scale. (2) We identify lines from the low-abundance COMs CH₃COCH₃ and CH₃CH₂OH, as well as tentatively detect CH₃CHO and long carbon-chain molecules C₆H and HC₇N. (3) We find that while most COMs are segregated by type, peaking either towards the hotcore (e.g., nitrogen-bearing species) or the compact ridge (e.g., oxygen-bearing species like HCOOCH₃ and CH₃OCH₃), the distributions of others do not follow this segregated structure (e.g., CH₃CH₂OH, CH₃OH, CH₃COCH₃). (4) We find a second velocity component of HNCO, SO₂, ³⁴SO₂, and SO lines, which may be associated with a strong shock event in the low-velocity outflow. (5) Temperatures and molecular abundances show large gradients between central condensations and the outflow regions, illustrating a transition between hot molecular core and shock-chemistry dominated regimes.

Conclusions. Our observations of spatially resolved abundance variations in Orion-KL provide the nearest reference source for hot molecular core and outflow chemistry, which will be an important example for interpreting the chemistry of more distant HMSFRs.