The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core

A Faure,B Lefloch,P Hily-Blant,V Magalhaes,R Bachiller,C Vastel

doi:10.1093/mnras/sty1562

Abstract

The origin of the heavily fractionated reservoir of nitrogen in comets remains an issue in the theory of their formation and hence of the solar system. Whether the fractionated reservoir traced by comets is inherited from the interstellar cloud or is the product of processes taking place in the protostar, or in the protoplanetary disk, remains unclear. So far, observations of nitrogen isotopic ratios in protostars or prestellar cores have not securely identified such a fractionated reservoir owing to the intrinsic difficulty of direct isotopic ratios measurements. In this article, we report the detection of 5 rotational lines of \ce{HC3N}, {including the weaker components of the hyperfine multiplets}, and two rotational lines of its $^{15}$N isotopologue, towards the L1544 prestellar core. Based on a MCMC/non-LTE multi-line analysis at the hyperfine level, we derive the column densities of \ce{HC3N} ($8.0\pm0.4\tdix{13}$\cc) and \ce{HC3^{15}N} ($2.0\pm0.4\tdix{11}$\cc) and derive an isotopic ratio of 400$\pm$20(1$\sigma$). This value suggests that \ce{HC3N} is slightly depleted in $^{15}$N in L1544 with respect to the elemental $^{14}$N/$^{15}$N ratio {of $\approx$330} in the present-day local interstellar medium. Our study also stresses the need for radiative calculations at the hyperfine level. Finally, the comparison of the derived ratio with those obtained in CN and HCN in the same core seems to favor CN+C$_2$H$_2$ as the dominant formation route to HC$_3$N. However, uncertainties in the isotopic ratios preclude definitive conclusions.

Highlights

Isotopic ratios are so far the best tool to follow the volatile content of molecular clouds in their evolution towards the formation of stars, planets, and planetary systems bodies
Observations of sublimating ices from comet 67P/C-G with spectrometers on board the ESA/ROSETTA satellite has provided in situ measurements of isotopic ratios of hydrogen, and relative abundances of S2 and O2 molecules which consistently support a presolar origin for cometary ices (Altwegg et al 2015; Rubin et al 2015; Calmonte et al 2016)
We provide a direct measurement of the nitrogen isotopic ratio of the HC3N cyanopolyyne towards the L1544 prestellar core

Summary

Introduction

Isotopic ratios are so far the best tool to follow the volatile content of molecular clouds in their evolution towards the formation of stars, planets, and planetary systems bodies. Observations of sublimating ices from comet 67P/C-G with spectrometers on board the ESA/ROSETTA satellite has provided in situ measurements of isotopic ratios of hydrogen, and relative abundances of S2 and O2 molecules which consistently support a presolar origin for cometary ices (Altwegg et al 2015; Rubin et al 2015; Calmonte et al 2016). Herschel observations of water towards protostars suggest that less than 10–20% of interstellar ices sublimate (van Dishoeck et al 2014), leaving important amounts of pristine interstellar material available to build up planetary systems. Laboratory or in situ measurements of cosmomaterial samples have accuracy at the percent level (Bonal et al 2010)

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Conclusion