Synthesis of solid-state Complex Organic Molecules through accretion of simple species at low temperatures

TL;DR

This study demonstrates that complex organic molecules like propanal and 1-propanol can form on icy grains at 10 K through non-energetic processes, with implications for astrochemistry and astrobiology.

Contribution

It provides experimental evidence for solid-state formation of COMs at low temperatures, including molecules not yet detected in space, supported by astronomical observations and computational analysis.

Findings

Propanal forms at 10 K in laboratory simulations.

1-Propanol is produced from propanal in experiments.

Astronomical observations set upper limits on 1-propanol abundance.

Abstract

Complex organic molecules (COMs) have been detected in the gas-phase in cold and lightless molecular cores. Recent solid-state laboratory experiments have provided strong evidence that COMs can be formed on icy grains through 'non-energetic' processes. In this contribution, we show that propanal and 1-propanol can be formed in this way at the low temperature of 10 K. Propanal has already been detected in space. 1-propanol is an astrobiologically relevant molecule, as it is a primary alcohol, and has not been astronomically detected. Propanal is the major product formed in the C2H2 + CO + H experiment, and 1-propanol is detected in the subsequent propanal + H experiment. The results are published in Qasim et al. (2019c). ALMA observations towards IRAS 16293-2422B are discussed and provide a 1-propanol:propanal upper limit of < 0.35 - 0.55, which are complemented by computationally-derived activation barriers in addition to the performed laboratory experiments.

Full text

††volume: 350††journal: Laboratory Astrophysics: from Observations to Interpretation††editors: Farid Salama, Helen Jane Fraser, & Harold Linnartz, eds.

1 Introduction

One of the earliest stages of the star formation cycle is the dense cloud stage. As the density increases to 104-5 cm*-3*, it becomes increasingly difficult for external UV photons to penetrate the cloud, and also causes the temperature to decrease to about 10-20 K. From these conditions, it would appear that the level of chemical activity in these clouds is negligible. Yet, observations show that complex organic molecules are formed in these clouds and must have a solid-state origin ([Soma et al. (2018), Soma et al. 2018]).

The combination of observations ([Boogert et al. (2015), Boogert et al. 2015]), laboratory experiments ([Watanabe et al. (2002), Watanabe et al. 2002]; [Fuchs et al. (2009), Fuchs et al. 2009]), and computational simulations ([Cuppen et al. (2007), Cuppen et al. 2007]) shows that such dense clouds are further characterized by at least two distinct chemical phases: the H2O-rich and CO-rich ice phases. Very recent laboratory experiments have shown that a number of alcohols have the potential to be formed during the H2O-rich ice phase ([Qasim et al. (2019a), Qasim et al. 2019a]). COM formation relevant to the CO-rich ice phase has additionally been investigated, and includes the synthesis of glycolaldehyde, ethylene glycol, and methyl formate ([Chuang et al. (2016), Chuang et al. 2016]). This is displayed in the middle of Figure 1.

Here, we discuss the addition of hydrocarbon radicals to the CO + H reaction network to form the COMs, propanal and 1-propanol. The resulting extension of the reaction network is illustrated in Figure 1 and links to the well-studied CO hydrogenation chain. More details on this study, which also include ALMA observations and computationally-derived activation barriers, are found in [Qasim et al. (2019c), Qasim et al. (2019c)].

2 Experimental

Experiments were performed with SURFRESIDE2, an ultrahigh vacuum (UHV) apparatus, and details of the setup can be found in [Ioppolo et al. (2013), Ioppolo et al. (2013)]. On a 10 K surface, hydrogen atoms produced from a hydrogen atom beam source were used to hydrogenate C2H2 into C2Hx radicals and CO into CHxO radicals, which can combine to form propanal and its hydrogenation product, 1-propanol. As 1-propanol was not clearly detected, also a propanal + H experiment was performed to unambiguously show 1-propanol formation. Detection techniques include temperature programmed desorption-quadrupole mass spectrometry (TPD-QMS) and reflection absorption infrared spectroscopy (RAIRS).

3 Results and discussion

Figure 2 shows the desorption of deposited propanal (bottom panel) and propanal formed in the C2H2 + CO + H experiment (top panel). Since the most abundant fragment of propanal is the parent ion, m/z = 58 was probed, as well as the fragment, m/z = 57, for extra confirmation. The peak desorption temperatures of 125 K are found in both experiments, which provides confirmation that propanal is formed in the C2H2 + CO + H experiment.

Figure 3 shows the desorption of deposited 1-propanol (bottom panel), deposited propanal (middle panel), and 1-propanol formed in the propanal + H experiment (top panel). The TPD data in the propanal + H experiment shows that m/z values of 29, 31, 59, and 60 have peak desorptions at 155 K. This is also the peak desorption temperature of pure 1-propanol, as shown in the bottom panel, fully in-line with the idea that propanal is partially converted to 1-propanol upon hydrogenation. These results also agree with the computationally-derived activation barriers of $\sim$2500-2600 K from [Zaverkin et al. (2018), Zaverkin et al. (2018)], in that the reaction can still proceed, although other possibly more efficient pathways to 1-propanol formation in the interstellar medium may exist, such as the pathways proposed in [Qasim et al. (2019a), Qasim et al. (2019a)]. The underlying chemical network is shown in Figure 1. The important conclusion here is that the CO + H reaction chain, which is known to form many molecules identified in space already, additionally offers a starting point for other molecules to form, depending on the new radicals that are included.

4 Astrochemical highlights and conclusions

ALMA observations towards the low-mass protostar, IRAS 16293-2422B, yields a 1-propanol:propanal upper limit of $<0.35-0.55$, where the 1-propanol column is from [Qasim et al. (2019c), Qasim et al. (2019c)] and the propanal column is from [Lykke et al. (2017), Lykke et al. (2017)]. By only taking into account the activation barrier(s) of propanal + H, it is expected that there is less 1-propanol in comparison to propanal if 1-propanol originates from propanal in those regions.

In essence, solid-state propanal and 1-propanol have the potential to be formed in the CO freeze-out stage starting from CHxO and C2Hx radicals. ALMA observations and computationally-derived activation barriers provide additional support that interstellar propanal can act as a precursor for 1-propanol. Although 1-propanol is currently undetected, its link to astrobiology, in that it is a primary alcohol, makes it a worthy target for future observational surveys. The chemical network presented here shows that such surveys should be performed in regions where propanal has been observed.

5 Acknowledgements

This research is part of the Dutch Astrochemistry Network II (DANII).