Complex organic molecules in low-mass protostars on Solar System scales

Abstract

Context. Complex organic molecules (COMs) are thought to form on icy dust grains in the earliest phase of star formation. The evolution of these COMs from the youngest Class 0/I protostellar phases toward the more evolved Class II phase is still not fully understood. Since planet formation seems to start early, and mature disks are too cold for characteristic COM emission lines, studying the inventory of COMs on Solar- System scales in the Class 0/I stage is relevant. Aims. Our aim is to determine the abundance ratios of oxygen-bearing COMs in Class 0 protostellar systems on scales of ~100 AU radius. We aim to compare these abundances with one another, and to the abundances of other low-mass protostars such as IRAS 16293-2422B and HH 212. Additionally, using both cold and hot COM lines, the gas-phase abundances can be tracked from a cold to a hot component, and ultimately be compared with those in ices to be measured with the James Webb Space Telescope (JWST). The abundance of deuterated methanol allows us to probe the ambient temperature during the formation of this species. Methods. ALMA Band 3 (3 mm) and Band 6 (1 mm) observations are obtained for seven Class 0 protostars in the Perseus and Serpens star-forming regions. By modeling the inner protostellar region using local thermodynamic equilibrium models, the excitation temperature and column densities are determined for several O-bearing COMs including methanol (CH3OH), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and dimethyl ether (CH3OCH3). Abundance ratios are taken with respect to CH3OH. Results. Three out of the seven of the observed sources, B1-c, B1-bS (both Perseus), and Serpens S68N (Serpens), show COM emission. No clear correlation seems to exist between the occurrence of COMs and source luminosity. The abundances of several COMs such as CH3OCHO, CH3OCH3, acetone (CH3COCH3), and ethylene glycol ((CH2OH)2) are remarkably similar for the three COM-rich sources; this similarity also extends to IRAS 16293-2422B and HH 212, even though collectively these sources originate from four different star-forming regions (i.e., Perseus, Serpens, Ophiuchus, and Orion). For other COMs like CH3CHO, ethanol (CH3CH2OH), and glycolaldehyde (CH2OHCHO), the abundances differ by up to an order of magnitude, indicating that local source conditions become important. B1-c hosts a cold (Tex ≈ 60 K), more extended component of COM emission with a column density of typically a few percent of the warm/hot (Tex ~ 200 K) central component. A D/H ratio of 1–3% is derived for B1-c, S68N, and B1-bS based on the CH2DOH/CH3OH ratio (taking into account statistical weighting) suggesting a temperature of ~15 K during the formation of methanol. This ratio is consistent with other low-mass protostars, but is lower than for high-mass star-forming regions. Conclusions. The abundance ratios of most O-bearing COMs are roughly fixed between different star-forming regions, and are presumably set at an earlier cold prestellar phase. For several COMs, local source properties become important. Future mid-infrared facilities such as JWST/MIRI will be essential for the direct observation of COM ices. Combining this with a larger sample of COM-rich sources with ALMA will allow ice and gas-phase abundances to be directly linked in order to constrain the routes that produce and maintain chemical complexity during the star formation process.

Formation of complex organic molecules in molecular clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via the “energetic” processing of C2H2 ice

“…As reported in Chuang et al (2020), ethylene oxide, with the highest internal energy among the three C 2 H 4 O isomers, is absent in the present work. This result also confirms the proposed formation channel of ethylene oxide, which can be only produced in C 2 H 4 containing ice reacting with O-atoms under ISM-like conditions (Bennett et al 2005b;Ward & Price 2011;Bergner et al 2019). The hydrogen saturated product ethanol is identified by its nonoverlapped IR peaks at 1046 and 1088 cm −1 that correspond to the CO stretching modes (ν 11 ) and CH 3 rocking mode (ν 10 ), respectively (Barnes & Hallam 1970;Mikawa et al 1971;Boudin et al 1998).…”

“…7), such as B5, and later stages (righthanded panel of Fig. 7)), such as low-mass protostellar shock region L1157 B1, protostars IRAS 16293-2422A, B, HH 212, B1c, and S68N, as well as comet C/2014 Q2(Lovejoy) (Lefloch et al 2017;Taquet et al 2017;Jørgensen et al 2018;Biver & Bockelée-Morvan 2019;Lee et al 2019;van Gelder et al 2020;Manigand et al 2020), assuming the net transfer from the solid to gas phase is 1:1 ratio for all COMs. The simultaneous and accumulative effects of non-energetic and energetic processing require further astrochemical modeling in order to simulate solid-state reactions occurring on surfaces and in ice bulks containing realistic chemical compositions covering an astronomical timescale.…”

Thermal desorption of interstellar ices. A review on the controlling parameters and their implications fromsnowlines to chemical complexity

“…In the warm inner regions where the temperature is higher than about 100 K, many molecules such as water and complex organic molecules are detected in the gas phase. [58][59][60] Whereas the solid state formation of many smaller species, such as H 2 O, NH 3 and CH 4 is well established, and therefore such species are expected to have directly desorbed from the grains, for COMs both solid state and gas phase formation pathways (involving desorbed species) have been proposed. [61][62][63][64][65][66][67] These regions are called hot cores and hot corinos for the high-mass and low-mass starforming regions, respectively 11,14,[68][69][70] .…”

Study of Fischer–Tropsch-type reactions on chondritic meteorites

“…The synthesis of interstellar complex organic molecules (iCOMs) in the solar nebula and during the process of star formation has been extensively studied during recent years (Herbst 2017). The simplest organic compounds, such as formaldehyde (H 2 CO) and methanol (CH 3 OH), and other iCOMS, including acetaldehyde (CH 3 CHO) and formamide (NH 2 CHO) for instance, have been repeatedly observed from starless cores to star forming regions (Vasyunina et al 2014;López-Sepulcre et al 2015;Bianchi et al 2018;McGuire 2018;van Gelder et al 2020). Robust pieces of evidence indicate that the formation of iCOMs takes place on the surface of interstellar grains (Kress & Tielens 2001;Sekine et al 2006;Linnartz et al 2015;Oberg 2016, Enrique-Romero et al 2019Zamirri et al 2019;Martín-Doménech et al 2020), which with time will coagulate and form the embryos of rocky planets and minor bodies.…”