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

By V. Rivilla, M. Beltr'an, R. Cesaroni, F. Fontani, C. Codella, Q. Zhang. 2016

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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 (CH 2 OH) 2 is the simplest sugar alcohol and the reduced alcohol of the simplest sugar glycoladehyde (CH 2 OHCHO). We study the molecular abundance and spatial distribution of (CH 2 OH) 2 , CH 2 OHCHO and other chemically related complex organic species (CH 3 OCHO, CH 3 OCH 3 , and C 2 H 5 OH) 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 (CH 2 OH) 2 towards G31.41+0.31, detecting more than 30 unblended lines. We also detected multiple transitions of other complex organic molecules such as CH 2 OHCHO, CH 3 OCHO, CH 3 OCH 3 , and C 2 H 5 OH. The high angular resolution images show that the (CH 2 OH) 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 (CH 2 OH) 2 or CH 2 OHCHO, are good probes of the gas located closer to the forming stars. Our analysis confirms that (CH 2 OH) 2 is more abundant than CH 2 OHCHO 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 (CH 2 OH) 2 /CH 2 OHCHO abundance ratio with the luminosity of the source. The CH 3 OCH 3 /CH 3 OCHO and (CH 2 OH) 2 /C 2 H 5 OH ratios are nearly constant with luminosity. We also find that the abundance ratios of pairs of isomers (CH 2 OHCHO/CH 3 OCHO and C 2 H 5 OH/CH 3 OCH 3 ) decrease with the luminosity of the sources. Conclusions. The most likely explanation for the behavior of the (CH 2 OH) 2 /CH 2 OHCHO 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 (CH 2 OH) 2 is by combination of two CH 2 OH radicals on dust grains. We also favor that CH 2 OHCHO is formed via the solid-phase dimerization of the formyl radical HCO. The interpretation of the observations also suggests a chemical link between CH 3 OCHO and CH 3 OCH 3 , and between (CH 2 OH) 2 and C 2 H 5 OH. The behavior of the abundance ratio C 2 H 5 OH/CH 3 OCH 3 with luminosity may be explained by the different warm-up timescales in hot cores and hot corinos.

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/1c295035f882ea105be827bbc60d29efed33dcba