N₂H⁺and N¹⁵NH⁺toward the prestellar core 16293E in L1689N

Abstract

Context. Understanding the processes that could lead to an enrichment of molecules in 15 N atoms is of particular interest because this may shed light on the relatively strong variations observed in the 14 N/ 15 N ratio in various solar system environments. Aims. The sample of molecular clouds where 14 N/ 15 N ratios have been measured currently is small and has to be enlarged to allow statistically significant studies. In particular, the N 2 H + molecule currently shows the broadest spread of 14 N/ 15 N ratios in high-mass star-forming regions. However, the 14 N/ 15 N ratio in N 2 H + was obtained in only two low-mass star-forming regions (L1544 and B1b). We here extend this sample to a third dark cloud. Methods. We targeted the 16293E prestellar core, where the N 15 NH + J = 1−0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a nonlocal radiative transfer code. For this purpose, we computed specific collisional rate coefficients for the N 15 NH + -H 2 collisional system. As a first step of the analysis, the N 2 H + abundance profile was constrained by reproducing the N 2 H + J = 1−0 and 3−2 maps. A scaling factor was then applied to this profile to match the N 15 NH + J = 1−0 spectrum. Results. We derive a column density ratio N 2 H + /N 15 NH + = 330 +170 −100 . Conclusions. We performed a detailed analysis of the excitation of N 2 H + and N 15 NH + in the direction of the 16293E core with modern models that solve the radiative transfer and with the most accurate collisional rate coefficients available to date. We obtained the third estimate of the N 2 H + /N 15 NH + column density ratio in the direction of a cold prestellar core. The current estimate ∼330 agrees with the typical value of the elemental isotopic ratio in the local interstellar medium. It is lower than in some other cores, however, where values as high as 1300 have been reported.

Modelling the molecular composition and nuclear-spin chemistryof collapsing pre-stellar sources★

“…Observationally, both ratios are close to 3, within their uncertainties (Daniel et al 2016;Harju et al 2017). If hydrogenation and deuteration were determined by statistics only -as is presumably the case in ices -both these ratios would be exactly equal to 3.…”

“…However, as freeze-out occurs least rapidly at the densities (lower than 10 5 cm −3 ) prevailing in the outer envelope, Figure 5. Column densities of ammonia and its deuterated forms, as predicted from our L-P collapse model (full lines) and from observations (hashed regions) of the I16293E, Barnard-B1, and H-MM1 pre-stellar cores (Daniel et al 2016;Harju et al 2017) (see Table 6). The initial parameters are those of Table 1.…”

“…Column densities of specific isomers of ammonia and its deuterated forms have been measured towards starless and pre-stellar cores (Daniel et al 2016;Harju et al 2017). Fig.…”

Nitrogen and hydrogen fractionation in high-mass star-forming cores from observations of HCN and HNC

“…In the few low mass pre-stellar cores or protostellar envelopes observed so far, values of 14 N/ 15 N comparable to the value of the PSN have been measured from N 2 H + and NH 3 (330±150 in N 2 H + , Daniel et al 2016;350 -500, Gerin et al 2009;334±50, Lis et al 2010 in NH 3 ), or even larger (1000±200 in N 2 H + , Bizzocchi et al 2013). Conversely, through observations of the nitrile-bearing species HCN e HNC in lowmass sources, the 14 N/ 15 N ratio turns out to be significantly lower (140 -360, Hily-Blant et al 2013a;160-290, Wampfler et al 2014).…”

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

“…Significant effects on collisional properties occur mostly for H→D substitutions, especially for hydrides: DCO + -H 2 [138], ND-He [113], ND 3 -H 2 [137], D 2 O-H 2 [85], and N 2 D + -H 2 [139]. If isotopic substitution does not change the symmetry of the molecule at all, the effect on its collisional properties is small (<30%); this category includes 13 CN-H 2 and C 15 N-H 2 [140], and N 15 NH + -H 2 [141] This latter category is the least urgent to update because, for symmetry-conserving isotopic substitutions, simple scaling by (reduced) mass is quite accurate.…”