Detection of (15)NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate (14)N/(15)N values

Abstract

Samples of pristine Solar System material found in meteorites and interplanetary dust particles are highly enriched in (15)N. Conspicuous nitrogen isotopic anomalies have also been measured in comets, and the (14)N/(15)N abundance ratio of the Earth is itself larger than the recognised pre-solar value by almost a factor of two. Ion--molecules, low-temperature chemical reactions in the proto-solar nebula have been repeatedly indicated as responsible for these (15)N-enhancements. We have searched for (15)N variants of the N2H+ ion in L1544, a prototypical starless cloud core which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. The goal is the evaluation of accurate and reliable (14)N/(15)N ratio values for this species in the interstellar gas. A deep integration of the (15)NNH+ (1-0) line at 90.4 GHz has been obtained with the IRAM 30 m telescope. Non-LTE radiative transfer modelling has been performed on the J=1-0 emissions of the parent and (15)N-containing dyazenilium ions, using a Bonnor--Ebert sphere as a model for the source. A high-quality fit of the N2H+ (1--0) hyperfine spectrum has allowed us to derive a revised value of the N2H+ column density in L1544. Analysis of the observed N(15)NH+ and (15)NNH+ spectra yielded an abundance ratio N[N(15)NH+]/N[(15)NNH+] = 1.1 +/- 0.3. The obtained (14)N/(15)N ratio is ~ 1000 +/- 200, suggestive of a sizeable (15)N depletion in this molecular ion. Such a result is not consistent with the prediction of present nitrogen chemical models. As chemical models predict large (15)N fractionation of N2H+, we suggest that (15)N(14)N, or (15)N in some other molecular form, is preferentially depleted onto dust grains.

12 pages, 9 figures, manuscript accepted in A&A

References in corpus (6)

• Our astrochemical heritage
• From Prestellar to Protostellar Cores II. Time Dependence and Deuterium Fractionation
• The Different Structures of the Two Classes of Starless Cores
• The 15N-enrichment in dark clouds and Solar System objects
• Nitrogen superfractionation in dense cloud cores
• The Excitation of N${}_2$H${}^{+}$ in Interstellar Molecular Clouds. I - Models

Cited by in corpus (49)

• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• Deuterated methanol in the pre-stellar core L1544
• 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 Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
• High sensitivity maps of molecular ions in L1544: I. Deuteration of N2H+ and HCO+ and first evidence of N2D+ depletion
• Direct evidence of multiple reservoirs of volatile nitrogen in a protosolar nebula analogue
• Why does ammonia not freeze out in the center of pre-stellar cores?
• Nitrogen fractionation in high-mass star-forming cores across the Galaxy
• Nitrogen Fractionation in Protoplanetary Disks from the H13CN/HC15N Ratio
• Observations of nitrogen isotope fractionation in deeply embedded protostars
• Nitrogen and hydrogen fractionation in high-mass star forming cores from observations of HCN and HNC
• 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
• Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation
• Hyperfine excitation of N${}_2$H${}^{+}$ by H${}_2$: Toward a revision of N${}_2$H${}^{+}$ abundance in cold molecular clouds
• Depletion of heavy nitrogen in the cold gas of star-forming regions
• The cosmic-ray ionisation rate in the pre-stellar core L1544
• First measurements of 15N fractionation in N2H+ toward high-mass star forming cores
• A 1.3 cm Line Survey toward Orion KL
• Ion-molecule reactions involving HCO${}^{+}$ and N${}_2$H${}^{+}$: Isotopologue equilibria from new theoretical calculations and consequences for interstellar isotope fractionation
• The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core
• First measurement of the 14N/15N ratio in the analogue of the Sun progenitor OMC-2 FIR4
• N${}_2$H${}^{+}$ and N${}^{15}$NH${}^{+}$ towards the prestellar core 16293E in L1689N
• 15N Fractionation in Infrared-Dark Cloud Cores
• Precise observations of the 12C/13C ratios of HC3N in the low-mass star-forming region L1527
• First interferometric study of enhanced N-fractionation in N${}_2$H${}^{+}$: the high-mass star-forming region IRAS 05358+3543
• A sensitive 3 mm line survey of L483: a broad view of the chemical composition of a core around a Class 0 object
• A large (~1 pc) contracting envelope around the prestellar core L1544
• Depletion and fractionation of nitrogen in collapsing cores
• Deuterium and ${}^{15}$N fractionation in N${}_2$H${}^{+}$ during the formation of a Sun-like star
• Combined model for ${}^{15}\mathrm{N}$, ${}^{13}\mathrm{C}$, and spin-state chemistry in molecular clouds
• Chemistry and dynamics of the prestellar core L1544
• Infall/Expansion Velocities in the Low-Mass Dense Cores L492, L694-2, and L1521F: Dependence on Position and Molecular Tracer
• Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation
• Nitrogen fractionation in ammonia and its insights on nitrogen chemistry
• Modeling Nitrogen Fractionation in the Protoplanetary Disk around TW Hya: Model Constraints on Grain Population and Carbon-to-Oxygen Elemental Abundance Ratio
• Millimeter Spectral Line Mapping Observations Toward Four Massive Star Forming HII Regions
• First sample of ${\mathrm{N}}_2{\mathrm{H}}^{+}$ nitrogen isotopic ratio measurements in low-mass protostars
• Nitrogen Fractionation in External Galaxies
• Depletion of 15N in the center of L1544: Early transition from atomic to molecular nitrogen?
• No nitrogen fractionation on 600 au scale in the Sun progenitor analogue OMC-2 FIR4
• ALMA-IRDC II. First high-angular resolution measurements of the 14N/15N ratio in a large sample of infrared-dark cloud cores
• Tracing the contraction of the pre-stellar core L1544 with HC${}^{17}$O${}^{+}$ $J$ = 1-0 emission
• Laboratory spectroscopic study of the ${}^{15}$N isotopomers of cyanamide, H${}_2$NCN, and a search for them toward IRAS 16293$-$2422 B
• Seeds of Life in Space (SOLIS). XIII. Nitrogen fractionation towards the protocluster OMC-2 FIR4
• Gas-phase Elemental abundances in Molecular cloudS (GEMS) XI. The evolution of HCN, HNC, and N2H+ isotopic ratios in starless cores
• Quantifying the temperature-dependent yields of N${}_2$ and N${}_2$H${}_4$ formation in vacuum-ultraviolet-irradiated NH${}_3$ ice
• Seed of Life in Space (SOLIS) XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow