Comets are thought to contain relatively pristine material from the origin of the solar system, having condensed directly out of the pre-solar nebula (e.g., Mumma & Charnley 2011). It is postulated that comets may have even delivered some of the water and organic matter found on the Earth via impacts (e.g., Hartogh et al. 2011). Over 22 molecules have been identified in comets via radio observations (Crovisier et al. 2004), including organic species such as formamide (NH$_2$CHO, Biver et al. 2014). Formamide and methyl isocyanate are particularly interesting for their potential role in prebiotic chemistry (Saladino et al. 2012).
Formamide has been detected in a large variety of star-forming environments, as well as in Solar System comets, thus supporting the hypothesis that molecules with a strong prebiotic potential could have been delivered to Earth by comets after being synthesized in prestellar environments (e.g. Caselli & Ceccarelli, 2013).
Recently, methyl isocyanate (CH$_3$NCO) has been tentatively detected by the Rosetta spacecraftʼs Philae lander in the comet 67P/Churyumov–Gerasimenko (Goesmann et al. 2015). Methyl isocyanate has been detected for the first time recently towards SgrB2(N) (Halfen et al. 2015) and most recently towards Orion KL (Cernicharo et al. 2015). Finally, using all the available ALMA data, CH$_3$NCO has been detected for the first time towards a low-mass proto-star, IRAS16293$-$2422 (Martín-Doménech et al. 2017; Ligterink et al. 2017).
The chemistry of formamide and methyl isocyanate in the interstellar medium, and of its precursors, is highly uncertain. This chemistry has theoretically been explored only for massive hot cores at high temperatures (Garrod et al. 2013) but only a few modellings has been done for the chemistry of these molecules under the physical conditions found in pre-stellar cores or low-mass proto-stars. There is increased evidence that chemical processes unaccounted for in past theoretical modelling (e.g. UV photo-desorption, cosmic-ray-induced diffusion, and/or chemical reactive desorption) are required to explain the formation, and detection, of complex organics in those regions.
In this talk, I will present a detailed modelling of the chemistry of formamide and methyl isocyanate in star-forming regions such as pre-stellar cores (L1544) and hot corinos (IRAS16293$-$2422). This chemical modelling aims at fully characterising the main formation/destruction routes of these two species, establishing their expected abundances, and compare them to available observations. This study identifies their precursors and other related species, providing good molecular targets to test our models against observations. We finally use the established chemical network to predict the emission of formamide around gravitationally unstable discs, based on Smoothed Particles Hydrodynamics (SPH) simulations.
Final abundances as a function of time for NH$_2$CHO and two parent species for the four environments. The time-scale for which we obtain the best agreement between the modelling and observations is shown in vertical grey-scale. Observational constraints are shown in horizontal coloured area or in dashed lines for upper limits. For each source, the observed abundances are taken from the literature. Top left: IRAS 16293$-$2422 B hot corino: NH$_2$ (Hily-Blant et al. 2010), H$_2$CO (Ceccarelli et al. 2000), and NH$_2$CHO (López-Sepulcre et al. 2015). Top right: IRAS 16293$-$2422 cold envelope: same references as for the hot corino. Bottom left: L1544 core centre: NH$_3$ (Crapsi et al. 2007), H$_2$CO (Bacmann et al. 2003), and NH$_2$CHO (Jiménez-Serra et al. 2016). Bottom right: L1544 methanol peak: NH$_3$ (Crapsi et al. 2007), H$_2$CO: assumed the same as that of the core centre; NH$_2$CHO (Jiménez-Serra et al. 2016).