Formation of ethylene glycol and other complex organic molecules in star-forming regions^⋆

¹ Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italia
e-mail: rivilla@arcetri.astro.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

Received: 23 February 2016
Accepted: 23 August 2016

Abstract

Context. The detection of complex organic molecules related with prebiotic chemistry in star-forming regions allows us to investigate how the basic building blocks of life are formed.

Aims. Ethylene glycol (CH₂OH)₂ is the simplest sugar alcohol and the reduced alcohol of the simplest sugar glycoladehyde (CH₂OHCHO). We study the molecular abundance and spatial distribution of (CH₂OH)₂, CH₂OHCHO and other chemically related complex organic species (CH₃OCHO, CH₃OCH₃, and C₂H₅OH) towards the chemically rich massive star-forming region G31.41+0.31.

Methods. We analyzed multiple single-dish (Green Bank Telescope and IRAM 30 m) and interferometric (Submillimeter Array) spectra towards G31.41+0.31, covering a range of frequencies from 45 to 258 GHz. We fitted the observed spectra with a local thermodynamic equilibrium (LTE) synthetic spectra, and obtained excitation temperatures and column densities. We compared our findings in G31.41+0.31 with the results found in other environments, including low- and high-mass star-forming regions, quiescent clouds and comets.

Results. We report for the first time the presence of the aGg’ conformer of (CH₂OH)₂ towards G31.41+0.31, detecting more than 30 unblended lines. We also detected multiple transitions of other complex organic molecules such as CH₂OHCHO, CH₃OCHO, CH₃OCH₃, and C₂H₅OH. The high angular resolution images show that the (CH₂OH)₂ emission is very compact, peaking towards the maximum of the 1.3 mm continuum. These observations suggest that low abundance complex organic molecules, like (CH₂OH)₂ or CH₂OHCHO, are good probes of the gas located closer to the forming stars. Our analysis confirms that (CH₂OH)₂ is more abundant than CH₂OHCHO in G31.41+0.31, as previously observed in other interstellar regions. Comparing different star-forming regions we find evidence of an increase of the (CH₂OH)₂/CH₂OHCHO abundance ratio with the luminosity of the source. The CH₃OCH₃/CH₃OCHO and (CH₂OH)₂/C₂H₅OH ratios are nearly constant with luminosity. We also find that the abundance ratios of pairs of isomers (CH₂OHCHO/CH₃OCHO and C₂H₅OH/CH₃OCH₃) decrease with the luminosity of the sources.

Conclusions. The most likely explanation for the behavior of the (CH₂OH)₂/CH₂OHCHO ratio is that these molecules are formed by different chemical formation routes not directly linked, although different formation and destruction efficiencies in the gas phase cannot be ruled out. The most likely formation route of (CH₂OH)₂ is by combination of two CH₂OH radicals on dust grains. We also favor that CH₂OHCHO is formed via the solid-phase dimerization of the formyl radical HCO. The interpretation of the observations also suggests a chemical link between CH₃OCHO and CH₃OCH₃, and between (CH₂OH)₂ and C₂H₅OH. The behavior of the abundance ratio C₂H₅OH/CH₃OCH₃ with luminosity may be explained by the different warm-up timescales in hot cores and hot corinos.