Formation of interstellar propanal and 1-propanol ice: a pathway involving solid-state CO hydrogenation

Abstract

Context. 1-propanol (CH 3 CH 2 CH 2 OH) is a three carbon-bearing representative of the primary linear alcohols that may have its origin in the cold dark cores in interstellar space. To test this, we investigated in the laboratory whether 1-propanol ice can be formed along pathways possibly relevant to the prestellar core phase. Aims. We aim to show in a two-step approach that 1-propanol can be formed through reaction steps that are expected to take place during the heavy CO freeze-out stage by adding C 2 H 2 into the CO + H hydrogenation network via the formation of propanal (CH 3 CH 2 CHO) as an intermediate and its subsequent hydrogenation. Methods. Temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) was used to identify the newly formed propanal and 1-propanol. Reflection absorption infrared spectroscopy (RAIRS) was used as a complementary diagnostic tool. The mechanisms that can contribute to the formation of solid-state propanal and 1-propanol, as well as other organic compounds, during the heavy CO freeze-out stage are constrained by both laboratory experiments and theoretical calculations. Results. Here it is shown that recombination of HCO radicals formed upon CO hydrogenation with radicals formed via C 2 H 2 processing -H 2 CCH and H 3 CCH 2 -offers possible reaction pathways to solid-state propanal and 1-propanol formation. This extends the already important role of the CO hydrogenation chain to the formation of larger complex organic molecules (COMs). The results are compared with ALMA observations. The resulting 1-propanol:propanal ratio concludes an upper limit of < 0.35 − 0.55, which is complemented by computationally derived activation barriers in addition to the experimental results.

The ALMA-PILS survey: first detection of the unsaturated 3-carbon molecules Propenal (C2H3CHO) and Propylene (C3H6) towards IRAS 16293–2422 B

“…Previous detections of chemically related species, namely acetaldehyde (CH 3 CHO), acetone (CH 3 COCH 3 ), ethyl methyl ether (C 2 H 5 OCH 3 ), C 2 H 5 CHO and CH 3 CCH, reported from the same dataset by Lykke et al (2017) and Calcutt et al (2019), are also indicated in the table and have been used for a comparison with the chemical model. Qasim et al (2019) already reported a column density upper limit for Ga-n-C 3 H 7 OH of < 1.2 × 10 15 cm −2 at 300 K and < 7.4×10 14 cm −2 at 125 K, using the Band 6 dataset of Taquet et al (2018). Given their same dataset and the correction factor to include the contribution of the other conformers, this estimate is consistent with the results presented here.…”

“…The final abundances show a decrease compared to the simulation where the molecules are produced through ice-surface reactions, among others. The updated activation energies, taken from the study of Qasim et al (2019) and tested in the simulation E1, do not change the final abundances of the C 3 -species significantly.…”

“…The addition reaction of HCO and C 2 H on grains is not contributing to the formation of HCCCHO in our chemical network because the amount of available C 2 H on the ice surfaces is limited by the very high activation barrier of the hydrogen abstraction of C 2 H 2 (>56 000 K, Zhou et al 2008 Qasim et al (2019) are, in general, lower than those used in the simulation runs A1 to D1. • A2 to E2: These runs are similar to the runs A1 to E1, but with a lower reaction rate of 10 −16 cm 3 s −1 for reaction (2).…”

“…This demonstrates the possibility of finding the unsaturated precursors of C 2 H 5 CHO and provide new constraints on the formation of these species in low-mass protostellar environments. It also suggests the need to develop models to describe these threecarbon species (C 3 -species hereafter) using laboratory measurements and theoretical calculations both for the gas phase and grain surfaces (Loison et al 2014(Loison et al , 2017Hickson et al 2016b;Qasim et al 2019).…”

Laboratory experiments on cosmic dust and ices

“…The formation of sugars and their derivates under interstellar conditions was reported by Fedoseev et al (2017) and Nuevo et al (2018). Very recently, propanal and prebiotically relevant 1-propanol could be synthesized at a temperature of 10 K by Qasim et al (2019). The formation of peptide-like molecules, which are possible precursors of amino acids, were synthesized in CH 4 :HNCO ice mixtures upon UV processing (Ligterink et al 2018).…”