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

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 aim to 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 have analyzed multiple single dish (Green Bank Telescope and IRAM 30m) and interferometric (Submillimeter Array) spectra towards G31.41+0.31, covering a range of frequencies from 45 to 258 GHz. We have fitted the observed spectra with a Local Thermodynamic Equilibrium synthetic spectra, and obtained excitation temperatures and column densities. We have compared our findings in G31.41+0.31 with the results found in other environments, including low-and high-mass star-forming regions, cold clouds and comets. Results. We have reported 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 have detected also 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 have also found 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 warm-up timescales effects and 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.

The complexity of Orion: an ALMA view

“…Both CH 3 COOH and (CH 2 OH) 2 are believed to mainly be formed on the icy surface of grains, although gas-phase formation routes cannot be ruled out (see, e.g., Laas et al 2011;Rivilla et al 2017). Interestingly enough, Garrod et al (2008) have shown that ethylene glycol in grain mantles is produced more efficiently than acetic acid by at least one order of magnitude.…”

“…Regarding gGg -ethylene glycol, this conformer has only been detected toward the Class 0 protostar IRAS 16293-2422 by Jørgensen et al (2016). Incidentally, the most stable conformer of ethylene glycol (aGg ) is detected toward low-, intermediate-, and high-mass sources, including Orion-KL (see, e.g., Fuente et al 2014;Lykke et al 2015;Brouillet et al 2015;Rivilla et al 2017, and references therein). In Sect.…”

Formation of Glycerol through Hydrogenation of CO Ice under Prestellar Core Conditions

“…Many of the basic chemical compounds required for the emergence of life consist of chains of carbon atoms with repeatable functional groups, such as (un)saturated fatty acids, simple sugars, and sugar alcohols. The two latter classes deserve special attention, as their smallest members, glycolaldehyde (HOCH 2 CHO) and ethylene glycol (HOCH 2 CH 2 OH), have been successfully observed in the gas phase toward our Galactic center (Hollis et al 2000(Hollis et al , 2002, in solar-mass protostars (Jørgensen et al 2012(Jørgensen et al , 2016Coutens et al 2015;Taquet et al 2015;Rivilla et al 2017), and in cometary ices (Biver et al 2014;Goesmann et al 2015;Le Roy et al 2015). It is possible that the mechanism responsible for the formation of glycolaldehyde and ethylene glycol also holds the potential to form the next larger members in these series, glyceraldehyde and glycerol.…”

Production of complex organic molecules:H-atom addition versus UV irradiation

“…In space, COMs have been unambiguously identified in very different environments , such as toward the Galactic Centre in hot cores and giant molecular clouds (Hollis, Lovas & Jewell 2000;Martín-Pintado et al 2001;Requena-Torres et al 2006Belloche et al 2013), toward low-and high-mass proto-stars (Blake et al 1987;Neill et al 2014;Tercero et al 2015;Jørgensen et al 2012Jørgensen et al , 2016Caselli & Ceccarelli 2012;Coutens et al 2015;Taquet et al 2015;Rivilla et al 2016), molecular outflows (Arce et al 2008;Codella et al 2015), photon-dominated regions (Guzmán et al 2013), as well as toward dark cloud cores and pre-stellar cores (Marcelino et al 2007;Bacmann et al 2012;Ö berg et al 2010Cernicharo et al 2012;Vastel et al 2014;Jiménez-Serra et al 2016). With eight and 10 atoms, the simplest sugar, glycolaldehyde (HC(O)CH2OH, hereafter GA), and the simplest sugar alcohol, ethylene glycol (H2C(OH)CH2OH, hereafter EG), can be considered amongst the largest organic molecules detected so far in the interstellar medium (ISM).…”

“…GA has been observed toward solarmass proto-stars (Jørgensen et al 2012(Jørgensen et al , 2016Coutens et al 2015) and in comets (Crovisier et al 2004;Biver et al 2014;Goesmann et al 2015;Le Roy et al 2015) together with chemically related species, such as EG or methyl formate (CH(O)OCH3, hereafter MF), one of the corresponding isomers. It is generally accepted that these species form in the solid state, on icy dust grains and are often found chemically linked to other O-bearing COMs in star-forming regions (Rivilla et al 2016). Recent work on the spatial distribution of COMs in the L1544 pre-stellar core (with T < 10 K) revealed that O-bearing gas-phase COMs are more abundant toward a low-density (Av ∼ 7.5-8 mag) shell, where gas-phase methanol also peaks, than toward the denser (Av ≥ 30 mag) continuum peak (Jiménez-Serra et al 2016).…”