The complexity of Orion: an ALMA view

I. Data and first results^⋆,⋆⋆

¹ LERMA & UMR 8112 du CNRS, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
e-mail: laurent.pagani@obspm.fr
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: cfavre@arcetri.astro.it
⁴ JPL, Pasadena, California, USA
e-mail: paul.f.goldsmith@jpl.nasa.gov
⁵ Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109, USA
⁶ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁷ Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA

Received: 19 January 2017
Accepted: 22 May 2017

Abstract

Context. We wish to improve our understanding of the Orion central star formation region (Orion-KL) and disentangle its complexity.

Aims. We collected data with ALMA during cycle 2 in 16 GHz of total bandwidth spread between 215.1 and 252.0 GHz with a typical sensitivity of 5 mJy/beam (2.3 mJy/beam from 233.4 to 234.4 GHz) and a typical beam size of 1.̋7 × 1.̋0 (average position angle of 89°). We produced a continuum map and studied the emission lines in nine remarkable infrared spots in the region including the hot core and the compact ridge, plus the recently discovered ethylene glycol peak.

Methods. We present the data, and report the detection of several species not previously seen in Orion, including n- and i-propyl cyanide (C₃H₇CN), and the tentative detection of a number of other species including glycolaldehyde (CH₂(OH)CHO). The first detections of gGg′ ethylene glycol (gGg′ (CH₂OH)₂) and of acetic acid (CH₃COOH) in Orion are presented in a companion paper. We also report the possible detection of several vibrationally excited states of cyanoacetylene (HC₃N), and of its ¹³C isotopologues. We were not able to detect the ¹⁶O¹⁸O line predicted by our detection of O₂ with Herschel, due to blending with a nearby line of vibrationally excited ethyl cyanide. We do not confirm the tentative detection of hexatriynyl (C₆H) and cyanohexatriyne (HC₇N) reported previously, or of hydrogen peroxide (H₂O₂) emission.

Results. We report a complex velocity structure only partially revealed before. Components as extreme as −7 and +19 km s^-1 are detected inside the hot region. Thanks to different opacities of various velocity components, in some cases we can position these components along the line of sight. We propose that the systematically redshifted and blueshifted wings of several species observed in the northern part of the region are linked to the explosion that occurred ~500 yr ago. The compact ridge, noticeably farther south displays extremely narrow lines (~1 km s^-1) revealing a quiescent region that has not been affected by this explosion. This probably indicates that the compact ridge is either over 10 000 au in front of or behind the rest of the region.

Conclusions. Many lines remain unidentified, and only a detailed modeling of all known species, including vibrational states of isotopologues combined with the detailed spatial analysis offered by ALMA enriched with zero-spacing data, will allow new species to be detected.