Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy)

Abstract

The presence of numerous complex organic molecules (COMs; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. These COMs may be part of the material from which planetesimals and, ultimately, planets formed. Comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. Molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few COMs, such as ethylene glycol and glycine. We report the detection of 21 molecules in comet C/2014 Q2 (Lovejoy), including the first identification of ethyl alcohol (ethanol, C2H5OH) and the simplest monosaccharide sugar glycolaldehyde (CH2OHCHO) in a comet. The abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar-type protostars. Overall, the high abundance of COMs in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk.

Keywords: astronomy; comets; complex organic molecules; space science.

Fig. 1. Spectra of organics in comet C/2014 Q2 (Lovejoy).

The observations were obtained with the IRAM 30m radio telescope in the 211–272 GHz band between 13 and 25 January 2015. The velocity scale is in the nucleus rest frame. Intensity is given in the main beam temperature scale. Spectra, from top left, are glycolaldehyde (CH₂OHCHO, average of two lines), ethyl alcohol (ethanol, C₂H₅OH, average of 13 lines), aGg′ ethylene glycol [(CH₂OH)₂, average of 14 lines], methyl formate (HCOOCH₃, average of two groups of blends of several lines, whose positions are marked by blue arrows), formic acid (HCOOH, average of six lines), acetaldehyde (CH₃CHO, average of 40 lines), isocyanic acid [HNCO(11_0,11–10_0,10) line at 241.774 GHz], and formamide (NH₂CHO, average of 10 lines). The signal-to-noise ratio is 6 for glycolaldehyde, 10 for ethanol, and higher than 7 for the other molecules.

Fig. 2. Abundances of complex organics in comets and protostars.

The abundances measured in comets Lovejoy (Table 1) and Hale-Bopp (table S2) are compared with those measured in the low-mass protostar IRAS 16293-2422(B) (19, 33) and in the high-mass protostar Orion-KL (18) and with results from chemical modeling in a protoplanetary disk (25). Abundances are given relative to CH₃OH, the most abundant organic molecule in these sources. H₂CO values may be less relevant because, for comets, a significant fraction is released from grains (34), whereas for IRAS 16293-2422, the plotted value pertains to the sum of the contributions to the two components A and B of the binary source. For CH₂CO, the black triangle is a 3σ upper limit obtained in comet Lovejoy.