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

Abstract

Context. Complex organic molecules with three carbon atoms are found in the earliest stages of star formation. In particular, propenal (C 2 H 3 CHO) is a species of interest due to its implication in the formation of more complex species and even biotic molecules. Aims. This study aims to search for the presence of C 2 H 3 CHO and other three-carbon species such as propylene (C 3 H 6) in the hot corino region of the low-mass protostellar binary IRAS 16293-2422 to understand their formation pathways. Methods. We use ALMA observations in Band 6 and 7 from various surveys to search for the presence of C 3 H 6 and C 2 H 3 CHO towards the protostar IRAS 16293-2422 B (IRAS 16293B). The identification of the species and the estimates of the column densities and excitation temperatures are carried out by modeling the observed spectrum under the assumption of local thermodynamical equilibrium. Results. We report the detection of both C 3 H 6 and C 2 H 3 CHO towards IRAS 16293B, however, no unblended lines were found towards the other component of the binary system, IRAS 16293A. We derive column density upper limits for C 3 H 8 , HCCCHO, n-C 3 H 7 OH, i-C 3 H 7 OH, C 3 O, and cis-HC(O)CHO towards IRAS 16293B. We then use a three-phase chemical model to simulate the formation of these species in a typical prestellar environment followed by its hydrodynamical collapse until the birth of the central protostar. Different formation paths, such as successive hydrogenation and radical-radical additions on grain surfaces, are tested and compared to the observational results in a number of different simulations, to assess which are the dominant formation mechanisms in the most embedded region of the protostar. Conclusions. The simulations reproduce the abundances within one order of magnitude from those observed towards IRAS 16293B, with the best agreement found for a rate of 10 −12 cm 3 s −1 for the gas-phase reaction C 3 + O → C 2 + CO. Successive hydrogenations of C 3 , HC(O)CHO, and CH 3 OCHO on grain surfaces are a major and crucial formation route of complex organics molecules, whereas both successive hydrogenation pathways and radical-radical addition reactions contribute to the formation of C 2 H 5 CHO.

O-bearing complex organic molecules at the cyanopolyyne peak of TMC-1: Detection of C2H3CHO, C2H3OH, HCOOCH3, and CH3OCH3

“…C 2 H 3 CHO has two conformers. The most stable, and the only one reported in space (Hollis et al 2004;Requena-Torres et al 2008;Manigand et al 2021), is the trans form, which is the one reported here as well. Level energies and transition frequencies were obtained from the rotational constants derived by Daly et al (2015).…”

“…Here we report the detection of four O-bearing COMs toward TMC-1(CP). Propenal (C 2 H 3 CHO) had previously been reported toward massive star-forming regions in the Galactic center (Hollis et al 2004;Requena-Torres et al 2008) and in the hot corino IRAS 16293-2422 B (Manigand et al 2021). Vinyl alcohol (C 2 H 3 OH) had only been seen toward Sagittarius B2(N), where the high spectral density complicates its identification (Turner & Apponi 2001).…”

Chemical evolution during the formation of a protoplanetary disk

“…Molecules recently detected for the first time in such objects (e.g., CH 3 Cl, HONO, Fayolle et al 2017;Coutens et al 2019) are not included either. A chemical network is currently being developed to include more complex species (up to three-carbon species) and grain-surface reactions (e.g., Manigand et al 2020a). The results presented here for the most complex species should consequently be seen more qualitatively than quantitatively.…”

Formation of the Alma Molecule Hoch2cn and Related Species From the Reaction of C+ With HCN and HNC in Icy Grain Mantles

“…The study by Doty et al predicts a copious amount of H atoms relative to HCN. Finally, various observational studies toward IRAS 16293-2422 B have inferred that ice chemistry may account for the presence of many complex organics in the source (e. g., Persson et al 2018;Manigand et al 2020aManigand et al , 2020b. In fact, the components required to form glycolonitrile by cation-ion chemistry are comparably plentiful in other sources, so it may be widely distributed in protostellar systems if either of the proposed pathways account for most of its formation.…”

“…In recent years, detections of molecules in protostellar nebulae and disks have emerged as important subsets of all deep space detections, with the lists of detections growing for protostellar disks such as TW Hydrae and DM Tauri and for the low-mass, solar-type protostar IRAS 16293-2422 B. For the latter, recent detections include CH3Cl (Fayolle et al 2017), trans-HONO (Coutens et al 2019), HOCH2CN or glycolonitrile (Zeng et al 2019), and, most recently, propanal and propylene (Manigand et al 2020a). In addition to detecting new species, it is of great interest to try to account for how they form.…”