Depletion and fractionation of nitrogen in collapsing cores

Abstract

Measurements of the nitrogen isotopic ratio in Solar System comets show a constant value, ~140, which is three times lower than the protosolar ratio, a highly significant difference that remains unexplained. Observations of static starless cores at early stages of collapse confirm the theoretical expectation that nitrogen fractionation in interstellar conditions is marginal for most species. Yet, observed isotopic ratios in N2H+ are at variance with model predictions. These gaps in our understanding of how the isotopic reservoirs of nitrogen evolve, from interstellar clouds to comets, and, more generally, to protosolar nebulae, may have their origin in missing processes or misconceptions in the chemistry of interstellar nitrogen. So far, theoretical studies of nitrogen fractionation in starless cores have addressed the quasi-static phase of their evolution such that the effect of dynamical collapse on the isotopic ratio is not known. In this paper, we investigate the fractionation of 14N and 15N during the gravitational collapse of a pre-stellar core through gas-phase and grain adsorption and desorption reactions. The initial chemical conditions, which are obtained in steady state after typically a few Myr, show low degrees of fractionation in the gas phase, in agreement with earlier studies. However, during collapse, the differential rate of adsorption of 14N- and 15N-containing species onto grains results in enhanced 15N:14N ratios, in better agreement with the observations. Furthermore, we find differences in the behavior, with increasing density, of the isotopic ratio in different species. We find that the collapse must take place on approximately one free-fall timescale, based on the CO abundance profile in L183 [see the end in the PDF file]

arXiv commentAccepted for publication in A&A

References in corpus (45)

• Photodissociation and photoionisation of atoms and molecules of astrophysical interest
• Chemodynamical evolution of the Milky Way disk I: The solar vicinity
• Probing the evolutionary status of starless cores through N2H+ and N2D+ observations
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Reactive Desorption and Radiative Association as Possible Drivers of Complex Molecule Formation in the Cold Interstellar Medium
• A Parametric Modeling Approach to Measuring the Gas Masses of Circumstellar Disks
• Tracing the Mass during Low-Mass Star Formation. III. Models of the Submillimeter Dust Continuum Emission from Class 0 Protostars
• Depletion and low gas temperature in the L183 prestellar core : the N2H+ - N2D+ tool
• The evolution of CNO isotopes: a new window on cosmic star-formation history and the stellar IMF in the age of ALMA
• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• Nitrogen chemistry and depletion in starless cores
• Isotope selective photodissociation of N2 by the interstellar radiation field and cosmic rays
• Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
• The 15N-enrichment in dark clouds and Solar System objects
• Nitrogen hydrides and the H2 ortho-to-para ratio in dark clouds
• Nitrogen isotope fractionation in protoplanetary disks
• Detection of (15)NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate (14)N/(15)N values
• Freeze-out and coagulation in pre-protostellar collapse
• Detection of 15NH2D in dense cores: A new tool for measuring the 14N/15N ratio in the cold ISM
• Nitrogen Isotopic Fractionation in Interstellar Ammonia
• Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue
• Nitrogen fractionation in high-mass star-forming cores across the Galaxy
• Observations of nitrogen isotope fractionation in deeply embedded protostars
• The CN/C15 N isotopic ratio towards dark clouds
• Heterogeneity in ${}^{12}$CO/${}^{13}$CO Ratios Toward Solar-Type Young Stellar Objects
• Revised Models of Interstellar Nitrogen Isotopic Fractionation
• ${}^{14}$N/${}^{15}$N ratio measurements in prestellar cores with N${}_2$H${}^{+}$: new evidence of ${}^{15}$N-antifractionation
• Depletion of heavy nitrogen in the cold gas of star-forming regions
• Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation
• Physical structure and water line spectrum predictions of the Intermediate Mass protostar OMC2-FIR4
• Radial Migration of the Sun in the Milky Way: a Statistical Study
• Chemical nitrogen fractionation in dense molecular clouds
• Modelling the molecular composition and nuclear-spin chemistry of collapsing prestellar sources
• Abundance of HCN and its C and N isotopologues in L1498
• The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core
• An unbiased spectral line survey observation toward the low-mass star-forming region L1527
• First measurement of the 14N/15N ratio in the analogue of the Sun progenitor OMC-2 FIR4
• The C${}^{14}$N/C${}^{15}$N Ratio in Diffuse Molecular Clouds
• N${}_2$H${}^{+}$ and N${}^{15}$NH${}^{+}$ towards the prestellar core 16293E in L1689N
• Precise observations of the 12C/13C ratios of HC3N in the low-mass star-forming region L1527
• The ortho-to-para ratio of water in interstellar clouds
• First interferometric study of enhanced N-fractionation in N${}_2$H${}^{+}$: the high-mass star-forming region IRAS 05358+3543
• Stratified NH and ND emission in the prestellar core 16293E in L1689N
• First Detection of HC${}_5$${}^{15}$N in the Interstellar Medium
• Synthetic molecular line observations of the first hydrostatic core from chemical calculations

Cited by in corpus (13)

• Dust coagulation feedback on magnetohydrodynamic resistivities in protostellar collapse
• An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion
• CHEMOUT: CHEMical complexity in star-forming regions of the OUTer Galaxy III. Nitrogen isotopic ratios in the outer Galaxy
• Combined model for ${}^{15}\mathrm{N}$, ${}^{13}\mathrm{C}$, and spin-state chemistry in molecular clouds
• Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation
• Molecular Cloud Cores with High Deuterium Fractions: Nobeyama Mapping Survey
• First sample of ${\mathrm{N}}_2{\mathrm{H}}^{+}$ nitrogen isotopic ratio measurements in low-mass protostars
• The initial magnetic criticality of prestellar cores
• ALMA-IRDC II. First high-angular resolution measurements of the 14N/15N ratio in a large sample of infrared-dark cloud cores
• Seeds of Life in Space (SOLIS). XIII. Nitrogen fractionation towards the protocluster OMC-2 FIR4
• Radial variations in nitrogen, carbon, and hydrogen fractionation in the PDS 70 planet-hosting disk
• Gas-phase Elemental abundances in Molecular cloudS (GEMS) XI. The evolution of HCN, HNC, and N2H+ isotopic ratios in starless cores
• Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya