Nitrogen isotopic ratios in Barnard 1: a consistent study of the N₂H⁺, NH₃, CN, HCN, and HNC isotopologues

Abstract

Context. The15 N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to 15 N-fractionation effects that would have occurred in the protosolar nebula. Aims. The present study aims at putting constraints on the degree of 15 N-fractionation that occurs during the prestellar phase, through observations of D, 13 C, and 15 N-substituted isotopologues towards B1b. Molecules both from the nitrogen hydride family, i.e. N 2 H + , and NH 3 , and from the nitrile family, i.e. HCN, HNC, and CN, are considered in the analysis. Methods. As a first step, we modelled the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H 2 density. These parameters were subsequently used as input in a non-local radiative transfer model to infer the radial abundance profiles of the various molecules. Results. Our modelling shows that all the molecules are affected by depletion onto dust grains in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with 14 N/ 15 N ∼ 300, a value representative of the elemental atomic abundances of the parental gas. Conclusions. The inefficiency of the 15 N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ∼17 K, which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we cannot exclude the possibility that the molecules were previously enriched in 15 N, earlier in the B1b history and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility.

Nascent bipolar outflows associated with the first hydrostatic core candidates Barnard 1b-N and 1b-S

“…Models discussed by Commerçon et al (2012a) predicted the mass within 250 AU to be M 250 AU = 0.16 M for a total core mass of about 1 M within 4000 AU. These figures are compatible with the total mass of B1b, ∼2.5 M (Daniel et al 2013) and the mass of the compact mm sources, 0.36 M each. The outflow masses, velocities, and mechanical luminosities predicted by the MHD models depend on μ, with M out = 3.3 × 10 −3 M , V max = 4.7 km s −1 , L mech = 4.1 × 10 −3 L for μ = 2.…”

“…The H 2 densities derived from MADEX range from 3 × 10 5 to 9 × 10 5 cm −3 . They are comparable with the densities in the B1b envelope determined from the dust continuum emission by Daniel et al (2013). Although higher than the kinetic temperature of the quiescent gas, 12 K (Lis et al 2010), the kinetic temperatures in the shocked gas remain moderate, between 20 and 70 K. Using the average properties adapted to each outflow, [CH 3 OH] = 7 × 10 −8 in B1b-S, and 10 −8 for B1b-N, we computed the mass in each lobe of the outflowing gas by integrating the emission in the velocity intervals listed in Table 3.…”

“…We did not detect any high-velocity emission that might be associated with B1b-W or with the high-velocity CO knot detected by Hirano & Liu (2014). The region between B1b-N and B1b-S presents lower CH 3 OH emission despite the high dust column density (Daniel et al 2013), a behavior characteristic of depletions due to freezing of molecules onto grains, and supported by the detection of high column densities of ices including CH 3 OH towards B1b-W (Boogert et al 2008).…”

THE C 14 N/C 15 N RATIO IN DIFFUSE MOLECULAR CLOUDS

“…The authors interpreted these results as evidence of fractionation and presented model calculations that they propose can explain the observations. Daniel et al (2013) studied a suite of nitrogen-bearing molecules and their isotopologues in the dark cloud Barnard 1, finding no evidence for nitrogen fractionation, which they suggest may be linked to the relatively high gas kinetic temperature (T kin ≈ 17 K) in the innermost part of the B1b cloud. Their molecular abundances were found to be consistent with a 14 N/ 15 N ratio of ∼300 for the parental molecular cloud, in agreement with the Adande & Ziurys (2012) result obtained for warmer sources in the local ISM.…”

Characterising the physical and chemical properties of a young Class 0 protostellar core embedded in the Orion B9 filament

“…Moreover, the detected N-bearing species are often found to show spatial distributions comparable to the dust emission of dense cores (e.g. Caselli et al 2002a;Lai et al 2003;Daniel et al 2013). It is still possible, however, that the assumption of unity filling factor is not correct.…”