Abstract
CONTEXT: The terrestrial planets, comets, and meteorites are significantly enriched in (15)N 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 (15)N-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 (15)N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases. AIMS: The aim of this study is to measure the (14)N/(15)N ratio around three nearby, embedded low- to intermediate-mass protostars. METHODS: Isotopologues of HCN and HNC were used to probe the (14)N/(15)N ratio. A selection of J = 3-2 and 4-3 transitions of H(13)CN, HC(15)N, HN(13)C, and H(15)NC was observed with the Atacama Pathfinder EXperiment telescope (APEX). The (14)N/(15)N ratios were derived from the integrated intensities assuming a standard (12)C/(13)C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting. RESULTS: Two sources, IRAS 16293A and R CrA IRS7B, show (15)N-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 (14)N/(15)N ratios with increasing outer envelope temperature. CONCLUSIONS: The enhanced (15)N abundances in HCN and HNC found in two Class 0 sources ((14)N/(15)N ~ 160-290) and the tentative trend toward a temperature-dependent (14)N/(15)N ratio are consistent with the chemical fractionation scenario, but (14)N/(15)N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.