Complex organic molecules in low-mass protostars on Solar System scales: II. Nitrogen-bearing species

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 (similar to 1 mm) and Band 5 (similar to 2 mm) are studied at similar to 0.5 resolution, complemented by Band 3 (similar to 3 mm) data in a similar to 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, (HNCO)-C-13, 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 (similar to 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.

Automated summary

The study found similarities in the abundances of nitrogen-bearing complex organic molecules compared to oxygen-bearing species in two deeply embedded protostars, suggesting a shared chemical history but with some variations due to local conditions like temperature sensitivity. Future data will enable a direct comparison between ice and gas abundances of smaller and larger nitrogen-bearing species.