Figure 15 shows the comparison between pairs of molecular abundances, HC(O)NH2 and HNCO, CH3NCO and HNCO, and CH3NCO and HC(O)NH2. It is already known from previous observations that there is a correlation between HNCO and HC(O)NH2 (López-Sepulcre et al. 2015, 2019; Allen et al. 2020). In the top-left panel of Fig. 15 the best power-law fits derived by López-Sepulcre et al. (2015), X[HC(O)NH2] = 0.04 × X[HNCO]0.93, and by Quénard et al. (2018), X[HC(O)NH2] = 32.14 × X[HNCO]1.29, are compared with the one derived here, X[HC(O)NH2] = 0.006 × X[HNCO]0.73 (with a Pearson coefficient of 0.99 and a P-value 0.05, indicating a strong positive correlation). Thus, the sample of sources discussed in this work, which also includes HMCs and a shock-dominated molecular cloud, is in agreement with the correlation found previously for low- and intermediate-mass pre-stellar and protostellar objects, which holds across several orders of magnitude in abundance. Based on this tight correlation, it has been proposed that the two species are chemically related and that the formation of HC(O)NH2 might occur through H-addition to solid-phase HNCO (e.g. Tielens & Hagen 1982; Charnley et al. 2004). Experimental works first suggested that this process is not efficient (Noble et al. 2015; Fedoseev et al. 2015), while recent works revised this possibility and found that a correlation between these two molecular species can be understood by H-abstraction and addition reactions (e.g. Nguyen et al. 2011; Haupa et al. 2019; Suhasaria & Mennella 2020). Moreover, hydrogenation of NO combined with UV-photon exposure and radical-radical reactions on grains has been suggested as the main formation pathways for both HNCO and HC(O)NH2 (e.g. Jones et al. 2011; Fedoseev et al. 2016; Ligterink et al. 2018; Dulieu et al. 2019). Coutens et al. (2016) found that the deuteration (D/H ratio) of HC(O)NH2 in IRAS 16293 B is similar to that of HNCO, in agreement with the hypothesis that both species are chemically related via grain-surface reactions. Gas-phase formation routes have also been proposed (see e.g. NH2 + H2CO, Barone et al. 2015; Skouteris et al. 2017). Laboratory experiments by Martín-Doménech et al. (2020) show that both HNCO and HC(O)NH2 could form upon UV photoprocessing or electron irradiation of ice samples, indicating that energetic processing (like UV photons and cosmic rays) of ISM CO-rich ices could form both species, without the need of a chemical link and/or a similar precursor between the two. This was predicted by the chemical modelling of Quénard et al. (2018), who showed that the formation of HC(O)NH2 at different temperature regimes is governed by different chemical processes. While at low temperatures the formation of HC(O)NH2 is driven by gas-phase formation via the reaction NH2 + H2CO → HC(O)NH2 + H, at high temperature its formation occurs on the surface of dust grains via radical-radical addition reactions. Moreover, they showed that for HNCO grain-surface and gas-phase reactions are equally efficient at low temperature, while at high temperatures the gas-phase formation predominates and the small fraction formed on grains is released into the gas phase via thermal desorption. Rimola et al. (2018) also showed via theoretical quantum chemical computations that HC(O)NH2 can form on grain surfaces starting from CN, which can quickly react with water-rich amorphous ices. Thus, the correlation between HNCO and HC(O)NH2 is mainly due to a similar response to the temperature of the two molecules, and not to a direct chemical link. In fact, the increase of the temperaturetriggers processes on the ice-mantle of grains, such as thermal evaporation. Moreover, as discussed above, other processes, like UV photons, cosmic rays, and shocks, could help both on the formation of these molecules on grain surfaces and on their desorption in the gas.
Citation: Coutens et al. (2016)
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