Antifreeze in the hot core of Orion

First detection of ethylene glycol in Orion-KL^⋆,⋆⋆

¹ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
e-mail: brouillet@obs.u-bordeaux1.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France
⁴ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁵ Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁶ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France

Received: 11 July 2014
Accepted: 4 February 2015

Abstract

Context. Ices are present in comets and in the mantles of interstellar grains. Their chemical composition has been indirectly derived by observing molecules released in the gas phase, when comets approach the sun and when ice mantles are sublimated or destroyed, e.g. in the hot cores present in high-mass, star-forming regions. Comparison of these chemical compositions sheds light on the formation of comets and on the evolution of interstellar matter from the molecular cloud to a protoplanetary disk, and it shows, to first order, a good agreement between the cometary and interstellar abundances. However, a complex O-bearing organic molecule, ethylene glycol (CH₂OH)₂, seems to depart from this correlation because it was not easily detected in the interstellar medium (Sgr B2) although it proved to be rather abundant with respect to other O-bearing species in comet C/1995 O1 (Hale-Bopp). Ethylene glycol thus appears, together with the closely related molecules glycolaldehyde CH₂OHCHO and ethanol CH₃CH₂OH, as a key species in the comparison of interstellar and cometary ices as well as in any discussion on the formation of cometary matter.

Aims. It is important to measure the molecular abundances in various hot cores to see if the observed differences between the interstellar medium and the comets are general. We focus here on the analysis of ethylene glycol in the nearest and best studied hot core-like region, Orion-KL.

Methods. We use ALMA interferometric data because high spatial resolution observations allow us to reduce the line confusion problem with respect to single-dish observations since different molecules are expected to exhibit different spatial distributions. Furthermore, a large spectral bandwidth is needed because many individual transitions are required to securely detect large organic molecules. Confusion and continuum subtraction are major issues and have been handled with care.

Results. We have detected the aGg′ conformer of ethylene glycol in Orion-KL. The emission is compact and peaks towards the hot core close to the main continuum peak, about 2″ to the south-west; this distribution is notably different from other O-bearing species. Assuming optically thin lines and local thermodynamic equilibrium, we derive a rotational temperature of 145 ± 30 K and a column density of 4.6 ± 0.8 × 10¹⁵ cm^-2. The limit on the column density of the gGg′ conformer is five times lower.