Observations of nitrogen isotope fractionation in deeply embedded protostars

The terrestrial planets, comets, and meteorites are significantly enriched in ¹⁵N compared to the Sun and Jupiter. While the solar and jovian nitrogen isotope ratio is believed to represent the composition of the protosolar nebula, a still unidentified process has caused ¹⁵N-enrichment in the solids. Several mechanisms have been proposed to explain the variations, including chemical fractionation. However, observational results that constrain the fractionation models are scarce. While there is evidence of ¹⁵N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases.

The aim of this study is to measure the ¹⁴N/¹⁵N ratio around three nearby, embedded low- to intermediate-mass protostars.

Isotopologues of HCN and HNC were used to probe the ¹⁴N/¹⁵N ratio. A selection of J = 3-2 and 4-3 transitions of H¹³CN, HC¹⁵N, HN¹³C, and H¹⁵NC was observed with the Atacama Pathfinder EXperiment telescope (APEX). The ¹⁴N/¹⁵N ratios were derived from the integrated intensities assuming a standard ¹²C/¹³C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting.

Two sources, IRAS 16293A and R CrA IRS7B, show ¹⁵N-enrichment by a factor of ~1.5-2.5 in both HCN and HNC with respect to the solar composition. IRAS 16293A falls in the range of typical prestellar core values. Solar composition cannot be excluded for the third source, OMC-3 MMS6. Furthermore, there are indications of a trend toward increasing ¹⁴N/¹⁵N ratios with increasing outer envelope temperature.

The enhanced ¹⁵N abundances in HCN and HNC found in two Class 0 sources (¹⁴N/¹⁵N ~ 160-290) and the tentative trend toward a temperature-dependent ¹⁴N/¹⁵N ratio are consistent with the chemical fractionation scenario, but ¹⁴N/¹⁵N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.