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

Abstract

Context. The chemical inventory of planets is determined by the physical and chemical processes that govern the early phases of star formation. Nitrogen-bearing species are of interest as many provide crucial precursors in the formation of life-related matter. Aims. The aim is to investigate nitrogen-bearing complex organic molecules towards two deeply embedded Class 0 low-mass protostars (Perseus B1-c and Serpens S68N) at millimetre wavelengths with the Atacama Large Millimeter/submillimeter Array (ALMA). Next, the results of the detected nitrogen-bearing species are compared with those of oxygen-bearing species for the same and other sources. The similarities and differences are used as further input to investigate the underlying formation pathways. Methods. ALMA observations of B1-c and S68N in Band 6 (~1 mm) and Band 5 (~2 mm) are studied at ~0.5′′ resolution, complemented by Band 3 (~3 mm) data in a ~2.5′′ beam. The spectra are analysed for nitrogen-bearing species using the CASSIS spectral analysis tool, and the column densities and excitation temperatures are determined. A toy model is developed to investigate the effect of source structure on the molecular emission. Results. Formamide (NH2CHO), ethyl cyanide (C2H5CN), isocyanic acid (HNCO, HN13CO, DNCO), and methyl cyanide (CH3CN, CH2DCN, and CHD2CN) are identified towards the investigated sources. Their abundances relative to CH3OH and HNCO are similar for the two sources, with column densities that are typically an order of magnitude lower than those of oxygen-bearing species. The largest variations, of an order of magnitude, are seen for NH2CHO abundance ratios with respect to HNCO and CH3OH and do not correlate with the protostellar luminosity. In addition, within uncertainties, the nitrogen-bearing species have similar excitation temperatures to those of oxygen-bearing species (~100–300 K). The measured excitation temperatures are larger than the sublimation temperatures for the respective species. Conclusions. The similarity of most abundances with respect to HNCO for the investigated sources, including those of CH2DCN and CHD2CN, hints at a shared chemical history, especially the high D-to-H ratio in cold regions prior to star formation. However, some of the variations in abundances may reflect the sensitivity of the chemistry to local conditions such as temperature (e.g. NH2CHO), while others may arise from differences in the emitting areas of the molecules linked to their different binding energies in the ice. The excitation temperatures likely reflect the mass-weighted kinetic temperature of a gas that follows a power law structure. The two sources discussed in this work add to the small number of sources that have been subjected to such a detailed chemical analysis on Solar System scales. Future data from the James Webb Space Telescope will allow a direct comparison between the ice and gas abundances of both smaller and larger nitrogen-bearing species.

Which molecule traces what: chemical diagnostics of protostellar sources

“…First, it shows that the relative abundances are remarkably similar on both sides of the outflow, well within the uncertainties. Second, most abundances relative to CH 3 OH are found to be comparable to the S68N hot core within our uncertainties and those of van Gelder et al (2020) and Nazari et al (2021), noting that due to lack of optically thin CH 3 OH line detected in the outflow, our uncertainties are larger. The greatest difference is seen for CH 3 CHO and H 2 CCO, which could be attributed to the additional gas-phase formation in shocks.…”

“…There is also a molecular ridge present in SO close to the disk-envelope interface. HNCO and HN 13 CO are detected toward B1-c and S68N peaking on source in higher E up transitions (Nazari et al 2021). For lines with E up <90 K an extended component is also detected in the outflow.…”

“…The inner regions of young protostellar systems are characterized by high temperatures which result in a rich chemistry as molecules that form efficiently in ices on grains in cold clouds sublimate into the gas phase. Complex organic molecules (COMs) detected in these datasets are discussed quantitatively in detail elsewhere, for both O-bearing by van Gelder et al (2020) and for N-bearing species by (Nazari et al 2021) In this section we focus on smaller molecules that also trace the innermost hot core regions and therefore are likely abundant in ices. This includes several small S-bearing molecules.…”

“…Therefore, we compare abundance ratios between different ice mantle species and methanol at three positions for S68N: one on the source, obtained from van Gelder et al ( 2020) and Nazari et al (2021), and one each in the blueshifted and redshifted part of the outflow, which are calculated from the fluxes obtained in this work. We measure the abundance of the species in a 3 region centered on CH 3 OH peak on the blueshifted and redshifted regions.…”

“…Covering a broad range of protostellar properties within the lowmass regime, the aim is to identify and describe key molecular tracers of future Sun-like stars and what physical components of star-forming sources they correspond to. Parts of the ALMA datasets presented here have been already published, focusing on different aspects: complex organic molecules (van Gelder et al 2020;Nazari et al 2021); Class I disks temperature structure (van 't Hoff et al 2020a); outflows and high-velocity jets in Serpens (Hull et al 2016;Tychoniec et al 2019), molecular emission associated with magnetic fields (Hull et al 2017;Le Gouellec et al 2019). In this work we make a comprehensive overview of these different datasets, making full use of those observations, with uniform analysis methods.…”