arX

iv:1

302.

6318

v1 [

astr

o-ph

.GA

] 2

6 Fe

b 20

13

The 15N-enrichment in dark clouds and Solar System objects

Hily-Blant P., Bonal L., Faure A., Quirico E.

Universite Joseph Fourier/CNRS, Institut de Planetologie et d’Astrophysique de Grenoble, France

Abstract

The line intensities of the fundamental rotational transitions of H13CN and HC15N wereobserved towards two prestellar cores, L183 and L1544, and lead to molecular isotopic ra-tios 140 ≤ 14N/15N ≤ 250 and 140 ≤ 14N/15N ≤ 360, respectively. The range of valuesreflect genuine spatial variations within the cores. A comprehensive analysis of the availablemeasurements of the nitrogen isotopic ratio in prestellar cores show that molecules carryingthe nitrile functional group appear to be systematically 15N-enriched compared to thosecarrying the amine functional group. A chemical origin for the differential 15N-enhancementbetween nitrile- and amine-bearing interstellar molecules is proposed. This sheds new lighton several observations of Solar System objects: (i) the similar N isotopic fractionationin Jupiter’s NH3 and solar wind N+; (ii) the 15N-enrichments in cometary HCN and CN(that might represent a direct interstellar inheritance); and (iii) 15N-enrichments observed inorganics in primitive cosmomaterials. The large variations in the isotopic composition of N-bearing molecules in Solar System objects might then simply reflect the different interstellarN reservoirs from which they are originating.

Keywords: astrochemistry, cosmochemistry, Solar Nebula, meteorites, Origin SolarSystem, radio observations, prestellar cores, objects: L1544, L183

1. Introduction

Nitrogen, the fifth most abundant ele-ment in the Universe, exists naturally as ahighly volatile gas (N2, N) and a mixtureof compounds of varying volatility (such asNH3, HCN, HNC, etc). The relative abun-dances and isotopic compositions of thesedifferent nitrogen occurrences in various as-tronomical sources can provide useful cluesto the origin and history of the Solar Sys-tem.

The Sun formed from a cold and densecore embedded in its parental interstellar

molecular cloud rich in gas and dust. Theso-called “protosolar nebula” (PSN) is theevolutionary stage issued from the collaps-ing prestellar core. The nitrogen volatileisotopologues in this nebula may have beenfractionated with respect to the originalinterstellar material, i.e. the isotopic ra-tio measured in these molecules may dif-fer from the elemental ratio. Such frac-tionation processes are invoked to explainthe large enhancements of the D/H ratiomeasured in several molecular species inprestellar cores (e.g. Caselli et al., 2003;Roueff et al., 2005). The efficiency of these

Preprint submitted to Icarus November 4, 2018

processes however depends on the physicalconditions in the core during its collapse(Flower et al., 2006). One of the currentchallenges in astrochemistry is to follow thechemical composition of a starless core dur-ing its evolution towards a planetary sys-tem. The related challenge in cosmochem-istry is to identify, in primitive objects ofthe Solar System, residual materials fromthe original cloud.The Sun is the largest reservoir of ni-

trogen in the Solar System. Isotopic mea-surements of solar wind trapped in lunarsoils (Hashizume et al., 2000), analysis ofJupiter’s atmosphere (Fouchet et al., 2000;Owen et al., 2001) and osbornite (TiN),considered as the first solid N-bearing phaseto form in the cooling protosolar nebula(Meibom et al., 2007), all independentlyshowed that nitrogen in the PSN was muchpoorer in 15N than the terrestrial atmo-sphere. The analysis of the present-day so-lar wind trapped on Genesis targets finallyconcluded on and confirmed these previousstudies. The solar wind is depleted in 15Nrelative to inner planets and meteorites, anddefine the following atomic composition forthe present-day Sun 14N/15N = 441 ± 5(Marty et al., 2010, 2011). The isotopiccomposition of nitrogen in the outer con-vective zone of the Sun has not changedthrough time and is considered as repre-sentative of the PSN. In the present pa-per, we only consider the original/primaryN isotopic fractionation, as opposed to sec-ondary 15N-enrichments acquired throughatmospheric process (e.g. Titan, Mars) forexample. In the remainder of the paperand for the sake of clarity, the elementalisotopic ratio is noted 14N/15N, whilst theisotopic ratio X15N/X14N measured in anyN-bearing species X is noted RX.

In our Solar System, any object (with

the exception of Jupiter) is actually en-riched in 15N compared to the PSN (seeFig. 1). Large excesses in 15N have beenfound in organic material of chondrites andinterplanetary dust particles (IDPs). En-richments in 15N are measured at differentscales of the material (bulk vs hotspots)and can be as high as R = 50 (Messenger,2000; Bonal et al., 2010). Molecules incometary coma also appear to be 15N-enriched, withR ratios varying between 139and 205 in HCN and CN (see the review byJehin et al., 2009).

The variation of the nitrogen isotopiccomposition in Solar System objects ismost likely caused by a variety of effects.These include : (i) nucleosynthetic origin(Audouze, 1985; Adande and Ziurys, 2012,and references therein); (ii) photochemicalself-shielding in the solar nebula (Clayton,2002; Lyons et al., 2009); (iii) spalla-tion reactions caused by the irradiationof the young sun (Kung and Clayton,1978; Chaussidon and Gounelle, 2006);(iv) low temperature isotope ex-changes (Terzieva and Herbst, 2000,hereafter TH00). The absence of large15N-enrichments accross the Galaxy(Adande and Ziurys, 2012, and referencestherein) and the small fractionation effectspredicted by standard gas-phase chemicalmodels (TH00) have weakened so far thehypothesis of a preserved (low tempera-ture) interstellar chemistry to explain the15N-enrichments observed in primitive solarcosmomaterials. However, the gas-grainchemical model of Charnley and Rodgers(2002) is able to reach a significant 15Nenrichment of ammonia which is eventuallylocked into ices. The absence of a directcorrelation between D and 15N-enrichmentsin organics from primitive cosmomate-rials has also been interpreted as the

2

lack of remnant interstellar chemistryfor N isotopologues (Briani et al., 2009;Marty et al., 2010; Aleon, 2010).In the present work, we analyze the

line intensities of the fundamental rota-tional transitions of H13C14N and H12C15N(H13CN and HC15N in the following) to-wards two starless dense cores, L1544 andL183 (Hily-Blant et al., 2010). The mainnovelty in our analysis stems from the re-cent availability of accurate collisional hy-perfine selective rate coefficients for HCNwith H2 (Ben Abdallah et al., 2012). Thepresent work (i) brings new observationalconstraints on nitrogen isotopic fractiona-tion in gas phase and (ii) puts a new per-spective on the actively debated and longquestioning issue of the origin of the 15N-enrichments observed in primitive cosmo-materials as compared to the protosolarnebula.

2. Material and methods

2.1. Observations

Observations of the pure rotational J =1 − 0 lines of H13CN and HC15N were car-ried out with the IRAM-30m telescope byHily-Blant et al. (2010). Spectra along per-pendicular directions towards the L183 andL1544 starless cores were obtained, with ex-tremely high spectral resolution (ν0/δν ≈4×106), such that the hyperfine structure ofthe H13CN(1-0) is resolved. The details ofthe observational setup and hardware per-formances are available in Hily-Blant et al.(2010). The H13CN and HC15N(1-0) spec-tra towards L183 and L1544 are shown inFig. A.4. The data are analyzed followinga more robust method than the one previ-ously adopted, where column densities werederived under the Local Thermal Equilib-rium (LTE) assumption at a temperature

of 8 K. In the present analysis, we make useof the hyperfine structure of the H13CN(1-0) line and of new collisional coefficients forHCN-H2 (Ben Abdallah et al., 2012) whichwere also adopted for H13CN and HC15N.

2.2. Data analysis

The analysis of the data makes use ofthe hyperfine structure of H13CN. The to-tal opacity and excitation temperature ofthe H13CN(1-0) transition are derived, as-suming equal excitation temperature withinthe hyperfine multiplet. This assumptionis justified as long as the opacity remainsof the order of unity, which as will be seenlater, holds for the lines towards L1544 andL183. The opacity and excitation temper-ature may then be used to derive the col-umn densities under the LTE assumption(see details in the Appendix). Alternatively,the opacity and line intensity may serveto compute the column density, H2 num-ber density, and kinetic temperature, fromnon-LTE calculations, under the so-calledLarge Velocity Gradient framework. In suchcase, we have used the RADEX public code(van der Tak et al., 2007). In these calcula-tions, the H13CN column density is searchedfor by varying the H2 density and the kinetictemperature in the range 1011 to 1014 cm−2,103 to 107 cm−3, and 5 to 15 K, respec-tively.In the case of L183, three methods have

been compared. 1/ The HFS method fromthe CLASS software was applied (see Ap-pendix) with the opacity and the excita-tion temperature as outputs, which in turnserve to compute a LTE column density. 2/Another fitting method was based on threeindependent Gaussians, yet constrained tohave the same linewidth, whose peak inten-sities were used to derive the opacity andthe excitation temperature. These two out-

3

Figure 1: Nitrogen isotopic composition of Solar System objects as compared to the composition of simplemolecules in interstellar clouds. The isotopic composition is expressed in term of 14N/15N ratios (left scale)and in δ15N notation (right scale, δ15N = [R⊕/R − 1] × 1000, where R⊕ = 272 is the nitrogen isotopiccomposition of the terrestrial atmosphere, see also Table 3). Square and circle symbols are for measurementsmade on molecules with amine and nitrile functional groups, respectively. IOM stands for Insoluble OrganicMatter, SOM for Soluble Organic Matter, and CAI for Calcium-, Aluminum-rich Inclusions. The range ofvalues reported towards L183 and L1544 reflect the spatial variations accross the sources.

4

Table 1: Column densities (×1012 cm−2) of H13CN(1-0) from three methods, and of HC15N (LVG calcula-tion) towards L183.

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)Offset FWHM τ0 Tex N13 τ0 Tex N13 N13 N15

N13

N15RHCN δ15N

arcsec km s−1 K cm−2 K cm−2 cm−2 cm−2

-40 0 0.36 2.3 2.9 1.8 3.1 2.8 2.3 2.3 –-20 0 0.42 1.6 3.2 1.7 1.8 3.0 1.7 1.7 –0 -40 0.34 3.7 3.0 2.9 3.4 2.9 2.5 2.7 –0 -20 0.33 3.2 3.1 2.5 3.1 2.9 2.2 2.6 0.9 3.1 208 3060 0 0.47 4.4 3.0 4.7 4.5 2.8 4.4 4.7 1.3 3.7 252 790 20 0.42 3.7 3.0 3.5 3.1 2.9 2.8 3.3 1.1 2.9 197 3820 40 0.36 3.5 3.1 3.0 2.6 2.9 2.0 2.3 1.1 2.0 136 99520 0 0.43 1.7 3.1 1.7 0.5 3.2 0.5 0.5 –40 0 0.46 0.3 3.8 0.4 0.8 3.0 0.8 0.9 –

Notes:

(1): spatial offsets with respect to (α, δ)J2000 = (15h54m08.80s,−02◦52′44.0′′).

(2): line width (assumed identical for the three components) from 3-components Gaussian fits, for the 3 hyperfinecomponents.

(3), (4), (5): total center line opacity, excitation temperature, and total H13CN column density ( 1012 cm−2) in LTE asdeduced from a HFS fit in CLASS (see text).

(6), (7), (8): same as above but as derived from the 3-components Gaussian fits.

(9): the column density is calculated in the LVG approximation, from a χ2-minimization against the opacity τ0 and theline intensity of the strongest hyperfine component. The column densities only weakly vary with the kinetic temperaturein the range 5 to 10 K. The values here correspond to Tkin = 8 K.

(10): column density of HC15N calculated under the LVG approximation for the density and kinetic temperaturecorresponding to the best solution from the H13CN LVG calculations.

(11), (12), (13): column density ratios and isotopic ratios assuming HCN/H13CN=68. δ15N = [R⊕/R−1]×1000, whereR⊕ = 272 is the nitrogen isotopic composition of the terrestrial atmosphere

5

Table 2: Column densities (×1012 cm−2) of H13CN and HC15N towards L1544.

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)Offset FWHM τi Tex nH2

N13 N15N13

N15RHCN δ15N

arcsec km s−1 K 104 cm−3 cm−2 cm−2

-40 40 0.53 2.50 3.1 6.3 5.3 1.7 3.2 215 266-20 -20 0.40 1.28 3.3 12.5 2.5 0.6 4.5 309 -119-20 20 0.59 1.86 3.2 10.0 4.7 1.1 4.4 296 -810 0 0.46 1.83 3.5 12.5 4.6 1.2 3.8 257 5820 -20 0.41 2.16 3.3 10.0 4.1 1.2 3.5 238 14220 20 0.42 1.52 3.1 10.0 2.6 0.9 3.0 207 31640 -40 0.30 1.68 3.2 10.0 2.4 1.2 2.0 136 993

Notes:

(1): spatial offsets with respect to (α, δ)J2000 = 05h04m16.90s, 25◦10′47′′).

(2): line width (assumed identical for the three hyperfine components) from independent Gaussian fits.

(3), (4): H13CN(1-0) center line opacity of the hyperfine component with relative intensity RI=0.5556, and excitationtemperature, derived from the relative integrated intensities of the three hyperfine components, assuming equal Tex forthe three hyperfine components.

(5), (6), (7): H2 density and total H13CN and HC15N column densities, derived through χ2-minimization accross LVGcalculations. Minimization is done in the nH2

, Tkin plane using the H13CN opacity and line intensity of the RI=0.5556component as constraints. The column density of HC15N derives from LVG calculations at the nH2

, Tkin given byH13CN. The values here correspond to Tkin = 8 K.

(8), (9), (10): column density ratios and isotopic ratios assuming HCN/H13CN=68.

6

puts give another LTE estimate of the totalcolumn density. 3) The opacity and line in-tensity of a given hyperfine component (e.g.the one with RI=0.5556) from the latter fit-ting method were used to derive the columndensity from LVG calculations. The resultsof these three methods are summarized inTable 1.

The case of L1544 was tackled in aslightly different fashion, to handle thedouble peak line profiles, which likely re-sult from two different velocity componentsalong the line of sight rather than infall, asthese double peaks are seen in both opti-cally thin and thick tracers. Each hyperfinecomponent was thus fitted as two indepen-dent Gaussian profiles, from which an in-tegrated intensity and equivalent linewdithare derived. The relative integrated inten-sites are used to estimate the opacity andexcitation temperature (see Eq. A.2). Fi-nally, the opacity and integrated intensityare χ2-minimized in the {nH2

, N(H13CN)}plane through LVG calculations, at variouskinetic temperatures.The HC15N column density was obtained

from LVG calculations using the HC15N(1-0) line intensity as a constraint. Solutionsin terms of the HC15N column density arethus obtained by matching the LVG predic-tions to the observed intensities. Becausethe hyperfine structure of HC15N is not re-solved out, we adopted the physical condi-tions derived from the LVG H13CN analysiswhile varying only the HC15N column den-sity. This assumes that the two moleculescoexist spatially, which is a reasonable as-sumption based on simple chemical consid-erations which show that both moleculesderive from the same chemical paths (e.g.TH00, Hily-Blant et al., 2010). The signal-to-noise ratio of the HC15N spectra towardsL183 was found to be good enough for only

4 positions. The results of these calcula-tions are given in Tables 1 and 2. The corre-sponding isotopic ratios are shown in Fig. 2.The typical statistical uncertainty on thederived column densities is 10%. TowardsL183, the comparison of the column densityresulting from the three methods provide amore reliable estimate of the uncertainty onthe column density determination, of the or-der of 20%. Towards L1544, we have usedseparately the RI=0.3333 and RI=0.5556line intensities as constraints, which resultsin a dispersion of 10 to 30%.

2.3. Results

The excitation temperatures are in therange 3–4 K, which is significantly lowerthan the value assumed by Hily-Blant et al.(2010), but very close to the values de-termined by Padovani et al. (2011) to-wards other starless cores. The associ-ated column densities lead to isotopic ra-tios H13CN/HC15N= 2 to 4.5. As is evi-dent from Fig. 2, the LVG column densi-ties of both H13CN and HC15N depend onlyslightly on the kinetic temperature. Withina given source, the range of values for theisotopic ratio reflects genuine spatial varia-tions accross the source. These variationsare up to a factor of 2 in L1544.To derive the isotopic ratio RHCN we as-

sumed that [HCN]/[H13CN] = [12C]/[13C],such that

[HCN]

[HC15N]=

[H13CN]

[HC15N]×

[12C]

[13C]. (1)

This amounts in assuming that HCN doesnot undergo significant carbon fractionationand that the HCN/H13CN ratio reflects theelemental ratio. Carbon fraction of HCNis unlikely for several reasons. First, mostof the carbon is locked into CO and 13CO,and little carbon ions are then available for

7

isotope exchange. In addition, Milam et al.(2005) concluded that CN is at most onlyweakly affected by chemical fractionation,and the chemical similarity between CN andHCN led Adande and Ziurys (2012) to ar-gue that carbon fractionation of HCN mustbe small. Last, it is to be noted thatchemical fractionation would increase theH13CN/HCN hence driving the molecularisotopic ratio RHCN towards lower values.We thus argue that the nitrogen fraction-ation observed in HCN is robust. For theelemental isotopic ratio 12C/13C of carbon,we adopt the value of 68 from Milam et al.(2005). The values for this ratio rangefrom 140 to 360 towards L1544, and from140 to 250 towards L183. These valuesare significantly lower than the isotopic ra-tios reported by Bizzocchi et al. (2010) to-wards L1544, using N2H

+ as a tracer. Theyare also well below the ratios determinedtowards other cores by Gerin et al. (2009)and Lis et al. (2010), who used NH2D andNH3 as nitrogen carriers, respectively (seeFig. 1). In contrast, these values encom-pass the low ratio RHCN = 150 determinedby Ikeda et al. (2002) towards L1521E.In the following, we compare these results

with isotopic ratios in Solar System objects,and propose a unified view of these measure-ments based on simple chemical arguments.

3. Discussion

3.1. Differential fractionation for nitrilesand amines

In prestellar cores, millimeter observa-tions show that in contrast to CO, nitrogen-bearing species such as CN and HCNmanage to remain in appreciable amountsin the gas phase (Hily-Blant et al., 2008;Padovani et al., 2011). In such environ-ments, isotope exchange reactions are the

only source of fractionation. These arecaused by a thermodynamic effect in whichthe exchange of isotopic atoms within a re-action has a preferred direction owing toexothermicity, which is caused by zero pointenergy differences. This process is efficientwhen the temperature is lower than theexothermicity, provided that the exchangereactions are competitive with other reac-tions. Rodgers and Charnley (2008) haveshown theoretically that significant 15N en-hancements can occur for various moleculesin N-rich prestellar cores depleted in COand OH. As recognized by these authors,however, their chemical model is hamperedby the lack of accurate rate coefficients forthe numerous isotopologue exchange reac-tions, which drive the fractionation.

Among the amines detected in prestel-lar cores, NH3 and its deuterated isotopo-logues, and N2H

+, present isotopic ratios ofthe order of 400 or larger (Gerin et al., 2009;Lis et al., 2010; Bizzocchi et al., 2010). Inconstrast, HCN shows significantly lowervalues such as RHCN = 150 toward L1521E(Ikeda et al., 2002) and 150 − 260 towardsL1544 (Milam and Charnley, 2012). Thesevalues are all consistent with our new mea-surements towards L183 and L1544, alsobased on HCN observations. Put all to-gether, these observations suggest a differ-ential behaviour of nitriles and amines withrespect to fractionation (see Fig. 1 and Ta-ble 3). This is indeed also visible in the gas-phase model of TH00, though at very lowlevels, and at a higher level in the gas-grainmodel of Rodgers and Charnley (2008).

A comprehensive analysis of nitrogen in-terstellar chemistry in dark clouds is sum-marized in Fig. 3. It appears that N-bearingmolecules can be divided into two almostdistinct chemical families: those carryingthe nitrile (-CN) functional group and those

8

Figure 2: Nitrogen isotopic ratio RHCN as measured towards L183 (upper panel) and L1544 (lower panel).Measurements for several positions are reported for each core (see also Tables 1 and 2). The isotopicenrichment in delta notation is indicated on the right scale. In each panel, the thick lines indicate theprotosolar nebula value of 441± 5 (Marty et al., 2011) and the terrestrial reference (14N/15N = 272). Theratios determined for kinetic temperatures ranging from 5 to 10 K are shown. At each position and foreach kinetic temperature, several values are displayed, which correspond to different analysis methods (seeAppendix for details).

9

carrying the amine (-NH) functional group.The former family derives from atomic ni-trogen while the latter are formed via N+,which is the product of N2 dissociative ion-ization. As a consequence, these two fami-lies are not expected to exchange their 15N.On the other hand, the 14N/15N exchangereactions among nitriles and amines mostlikely present different time scales and/orefficiency (TH00, Rodgers and Charnley,2008). Different 15N enhancements aretherefore expected between e.g. NH3 andHCN, even if rate coefficients are uncertain.The present work also shows that the ni-

trogen isotopic ratio varies inside a givenprestellar core. Although a variety ofphysical parameters (density, temperature),known to present spatial variations in theseobjects, could be invoked to explain theseinhomogeneities, source modelling includ-ing radiative transfer and chemistry is mostlikely needed to draw conclusions in this re-gard. Yet, in the context of this work, thesespatial variations may be related with thelarge range of values measured in the SolarSystem.

3.2. Potential N reservoirs sampled by So-lar System objects

Similarly to interstellar clouds, the PSNwas most likely composed of several ni-trogen reservoirs characterized by differentrelative abundances and isotopic composi-tions. In interstellar clouds, molecules car-rying the nitrile functional group appearto be systematically 15N-enriched comparedto molecules carrying the amine group (seeSection 1 and Fig. 1). Thus, we proposethat the highly variable 14N/15N ratios inobjects of the Solar System might simply re-flect the interstellar nitrogen reservoir fromwhich they are originating. The Sun and gi-ant planets, sampled atomic and/or molec-

ular nitrogen, considered as the major reser-voir in the PSN. Asteroids and comets, thatare N-depleted compared to the Sun, mayhave sampled minor, less volatile, and iso-topically fractionated N reservoirs of com-pounds such as HCN. These are found tobe systematically 15N-enriched compared tothe Sun, hence the PSN. In the followingparagraphs, we discuss in details each ofthese issues.

3.3. Variable fractionation in Solar Systemobjects

Nitrogen isotopic composition was deter-mined using ammonia in the atmosphere ofJupiter (Fouchet et al., 2000; Owen et al.,2001) leading to RNH

3≈ 440. It is now

largely interpreted as representative of theaverage value for nitrogen in the solar neb-ula. The similarity of the high Jovian andnebular RNH

3and RN+ ratios, respectively,

reinforces the ideas that molecules carryingthe amine functional group, deriving fromN2, are not fractionated.Comets may have better preserved than

asteroids the volatile molecules that werepresent in the protosolar cloud. The abun-dances of the simple molecules such as CO,CO2, CH3OH, H2CO and HCN suggest in-deed the partial preservation of an interstel-lar component (Irvine et al., 2000). Thesemolecules, present as ices in the nucleus,are detected in the coma after their sub-limation when the comets approach thesun. HCN is the most abundant N-bearingmolecule that has been detected so far (di-rectly or through the CN radical), and alsothe only one whose nitrogen isotopic com-position was measured. There has beensome debate whether the radical CN isproduced through the photodissociation ofHCN, or is a thermo-degradation prod-uct of refractory CHON grains (Fray et al.,

10

Table 3: Nitrogen isotopic ratios in Solar System objects and in the cold ISM.

Probe Source R† δ15N‡ References

NH2D Barnard 1 470± 150 −420± 180 Gerin et al. (2009)L1689B 810+600

−250 [−800 : −500] Gerin et al. (2009)NH3 Barnard 1 334±50 −180± 120 Lis et al. (2010)N2H

+ L1544 446± 71 −390± 100 Bizzocchi et al. (2010)HCN L1521E 150 815 Ikeda et al. (2002)

L183 [140: 250] [1000: 80] This workL1544 [140: 360] [1000:-245] This work

Amino Acids [263:230] [37:184] Sephton et al. (2002)IOM (bulk) < 195 400 Alexander et al. (2007)IOM (hotspots) < 65 3200 Busemann et al. (2006)Isheyevo - clasts 50 4450 Bonal et al. (2010)IDPs (bulk) [305:180] [-107: 514] Floss et al. (2006)IDPs (hotspots) up to 118 1300 Floss et al. (2006)

† Molecular isotopic ratio measured in a given N-bearing species.

‡ Deviation from the standard terrestrial value in parts per thousand defined as δ15N =[R⊕/R−1]×1000, whereR⊕ = 272 is the nitrogen isotopic composition of the terrestrialatmosphere.

11

Figure 3: Principal gas-phase reactions involved in the interstellar chemistry of nitrogen in dense cloudswhere UV photons can be ignored . Amines (left) and nitriles (right) have been clearly separated.

12

2005). However, a genetic link betweenHCN and CN is strengthened based intheir comparable nitrogen isotopic compo-sitions deduced from a careful data reanal-ysis (Bockelee-Morvan et al., 2008) and onconsistent production rates (Paganini et al.,2010). Therefore, a relevant compari-son between cometary molecules and ISMis possible through the same molecularspecies: HCN. The CN nitrogen isotopiccomposition was measured in a large num-ber of Oort Cloud comets (14N/15Nave =144 ± 6.5) and Jupiter family comets,(14N/15Nave = 156.8±12.2) revealing a largeand fairly constant nitrogen fractionation(130 < 14N/15N < 170) with no depen-dence on the origin and heliocentric dis-tance of the observed comets (see the re-views by Jehin et al., 2009; Manfroid et al.,2009, and references therein). These ratiosare similar to the lowest ones in dark cloudsL1544 and L183 (see Fig. 1) but do not re-flect the spatial heterogeneity seen in thesetwo dark clouds. However, these two cloudsdo not evidence the same levels of hetero-geneity, and one possible explanation mightbe that the PSN emerged from a more ho-mogeneous dark cloud than L183. In addi-tion, only a small mass fraction of the ma-terial from the dark cloud – that may bemore homogenous – is eventually incorpo-rated stars and planetary systems. Last, po-tential isotopic heterogeneity of a cometarynucleus could remain undetected, since ob-servations from the ground generally pro-vide an averaged measurement of the coma(e.g. Blake et al., 1999). The N isotopiccomposition of HCN in comets is there-fore consistent with an interstellar heritage.The preservation of cometary ices highly de-pends on the thermal history of the objects.The actual presence of the highly volatileHCN in comets attests of the absence of sig-

nificant heating. Moreover, only few pro-cesses are expected to modify the nitrogenisotopic composition of the HCN moleculeafter its formation. Indeed, nitrogen atomsare not easily exchangeable unlike protonsthat easily exchange with ice. Evidenceswere provided experimentally for protonsin methanol (Ratajczak et al., 2009) andthrough observations in HCN (Blake et al.,1999). In this regard, the R ratio may ap-pear as a more reliable proxy of the originof the molecule than the D/H ratio.Chondrites might not be considered as

a representative sampling of the nitrogenin the PSN. Indeed, asteroids, hence chon-drites, most likely did not accrete the highlyvolatile nitrogen reservoirs (N2 and N).Thus, the 15N-enriched organics in chon-drites might have originally sampled someof the minor reservoirs made of nitrogencompounds such as HCN and N-bearingmolecules of higher molecular weight.Carbonaceous chondrites contain up to

5% elemental carbon in a variety of forms,organic matter being the major one. Aminor fraction (less than 25%) of the or-ganic matter in carbonaceous chondrites ispresent as relatively low-molecular-weightcompounds, extractable with common or-ganic solvents, the so-called “soluble or-ganic matter” (SOM). SOM consists in acomplex mix of organic molecules bear-ing H, C, O, N, S, and P elements, withmasses up to 800 amu (Sephton et al.,2002; Gilmour, 2003; Schmitt-Koplin et al.,2010). The remaining fraction (75% or so) ispresent as a high-molecular-weight macro-molecular material, persisting after harshdemineralization of the chondrites, the so-called “insoluble organic matter” (IOM).Interplanetary Dust Particles (IDPs) andAntarctic micrometeorites (AMMs) are mi-crometric particles that have either an

13

asteroidal or a cometary origin. Theyalso contain organics that present similar-ities with those of carbonaceous chondrites(Dobrica et al., 2011).The soluble and insoluble organic frac-

tions both contain some nitrogen and arecharacterized by 15N-enrichments relativelyto the PSN. The nitrogen isotopic compo-sitions of amino acids have mostly beendetermined in the Murchison chondrite(Pizzarello et al., 1994; Engel and Macko,1997). The R ratios are typically be-tween 230 and 263. The isotopic frac-tionation is obviously reported on aminefunctional groups that are not fraction-ated in our model scheme (see Fig. 3 andSect. 3.1). The origin of amino acids isyet unknown. Multiple pathways of for-mation have been proposed in the liter-ature. Some recent experiments on in-terstellar ices analogs showed that a vi-able model of formation is based on ni-triles as amino acids precursor molecules(Elsila et al., 2007). Hence the nitrogen iso-topic composition of amino acids might re-flect that of the precursor HCN and not thatof NH3.Nitrogen is a minor element of IOM

(2% in weight in average Alexander et al.,2007). It is mostly present in heterocyclessuch as pyrroles (e.g. Sephton et al., 2003;Remusat et al., 2005; Derenne and Robert,2010). The contribution of N aspresent in nitriles appears to be rela-tively low (Npyrrole/Nnitrile = 5 in Murchi-son; Derenne and Robert, 2010). Themost primitive chondrites are character-ized by bulk 15N-enrichments up to R =195 (Alexander et al., 2007). Chondriticclasts in the unique Isheyevo meteoriteare characterized by the highest bulk 15N-enrichment at the present day, with R = 50(Bonal et al., 2010). Analytical techniques

with submicron-scale imaging abilities re-vealed very localized 15N-enrichments (com-monly referred to as 15N-hotspots), up toR = 65 (Busemann et al., 2006). Due tothe experimental challenges implied by theirmicron-scale size, SOM and IOM in IDPsare not isolated; only isotopic compositionsof bulk material are measured. High 15N-enrichments were revealed in IDPs; bulksuch as 180 < R < 305 - hotspots upto R = 118 (Floss et al., 2006). As asummary, similar 15N-enrichments are mea-sured in bulk IOM of cosmomaterials andin HCN in L1544 and L183. However, tobe meaningful the comparison between ISMand cosmomaterials must be based on sim-ilar molecules (e.g. HCN in comets) or onmolecules linked by determined chemicalpathways (e.g. 15N of amino acids inher-ited from nitriles precursors). The chemi-cal carriers of the isotopic anomalies (bulkand hotspots) in the IOM are not identi-fied yet. They may be located onto hetero-cycles, nitriles, and/or unidentified chemi-cal group or compound. The IOM as cur-rently observed in cosmomaterials was mostlikely synthesized through multistep pro-cesses that possibly involved recycling of in-terstellar species within the protosolar disk(Sephton et al., 2002; Dartois et al., 2004;Okumura and Mimura, 2011). As a conse-quence, it is impossible to draw a direct linkbetween the 15N-enrichments in the IOMof cosmomaterials to interstellar moleculesor to a series of chemical reactions as theyare expected to occur in ISM. Furthermore,physical processes like radiolysis or heat-ing could have modified IOM or even beinvolved in its synthesis (e.g. Huss et al.,2003). Little is known about the isotopicfractionation due to these processes, a sig-nificant role cannot be excluded. Hencea genetic link between ISM molecules and

14

IOM cannot be currently firmly established,but is at least suggested based on consistent15N-enrichments.

4. Conclusions and perspectives

Among the arguments against the idea ofinterstellar chemistry at the origin of 15N-enrichments in organics of primitive cosmo-materials are: (i) the assumption of nitro-gen isotopic ratios of the order of 400 orhigher in interstellar HCN and NH3; (ii) thefailure of classical gas-phase ion-moleculereactions in interstellar chemical models toproduce large 15N-enrichments TH00; (iii)the absence of spatial correlation betweenD- and 15N-enrichments in primitive organ-ics is interpreted as a proof of different pro-cesses at their origins (Briani et al., 2009;Aleon, 2010; Marty et al., 2010).The observations reported here irrevo-

cably show that considerable nitrogen iso-topic fractionation occurs at low temper-ature in the gas phase of prestellar cores.These new measurements provide strongconstraints to interstellar chemistry mod-els and are consistent with the early-timechemistry predicted by the gas-grain modelof Rodgers and Charnley (2008). More-over, even though fractionation of bothhydrogen and nitrogen might reflect low-temperature gas-phase chemistry, it is prob-ably not driven by the same molecular car-riers. Indeed, the isotopic composition ofa given species is determined by the com-plex interplay of a reaction network andthe isotopic compositions of the precur-sors. In addition, the typical exothermic-ities of reactions leading to D-enrichmentsand 15N-enrichments are different (≈ 230 Kand ≈30K, respectively) and leave room fora differential fractionation between hydro-gen and nitrogen, depending on the ther-

mal history of prestellar cores. Last, itwas recently shown that varying the ortho-to-para ratio of H2 in interstellar chem-istry can lead to D-enrichments and atthe same time inhibit nitrogen fractionation(Wirstrom et al., 2012). There is thus littlereason to expect correlated isotopic anoma-lies between these two elements.Even though the link between organics

in primitive cosmomaterials and interstellarmolecules cannot be directly determined,isotopic fractionation is a strong diagnosticfeature. The present study evidences thatthe large nitrogen fractionations observed incomets and chondrites are consistent with apresolar chemistry. Several arguments usedagainst such an idea are here clearly invali-dated.

Appendix A. Column density deter-

mination

For a resolved hyperfine structure spec-trum, such as H13CN(1-0), the assumptionof a common excitation temperature for allhyperfine components allows a derivation ofthe excitation temperature and of the opac-ity of each component. Radiative transferthrough gas with a constant excitation tem-perature leads to the following expressionfor the emergent intensity in ON-OFF observ-ing mode:

Tmb = [Jν(Tex)−Jν(2.73)] (1−e−τ ) = ∆Jν(Tex) (1−e−τ )

(A.1)

with Jν(T ) = T0/[1 − exp(−T0/T )] andT0 = hν/k. Noting rk the relative inten-sities of the various components of a hyper-fine multiplet, the ratio of the opacities oftwo components is τi/τj = ri/rj . Hence, as-suming a constant Tex for all hyperfine com-ponents of a given multiplet, one directly

15

obtains from Eq. A.1, that

Tmb,i

Tmb,j

=1− exp(−riτ0)

1− exp(−rjτ0)(A.2)

where we choose∑

i ri = 1 and we havenoted τ0 =

∑i τi. From the measured Tmb

and known ri, it is thus possible to deriveτ0, from which the excitation temperaturefollows by inverting Eq. A.1. For lines ofmoderate opacity (τ0 < 1), peak or inte-grated intensity ratios may be used with nodifference. The column density is then ob-tained directly from the integrated opacityof the hyperfine component k as:

Ntot =8πν3

c3Q(Tex)

Akgk

∫τk(v) dv

1− e−T0/Tex= Nk(Tex)

∫τk(v) dv

(A.3)

The HFS method of the CLASS software,used in the case of L183, fits simultaneouslythe hyperfine components with Gaussians,by fixing their relative positions and inten-sities. Fit results are shown in Fig. A.4.In the case of L1544, the double-peak na-ture of the emission spectrum made thisprocedure unfruitful. The adopted strat-egy therefore was to first determine the in-tegrated intensity of each hyperfine compo-nent as obtained from independent double-Gaussian fit (see Fig. A.5 and A.6). The rel-ative integrated intensities were then usedto derive the opacity and excitation tem-perature through Eq. A.2.

References

Adande, G.R., Ziurys, L.M., 2012. Millimeter-waveObservations of CN and HNC and Their 15N Iso-topologues: A New Evaluation of the 14N/15NRatio across the Galaxy. ApJ 744, 194.

Aleon, J., 2010. Multiple Origins of Nitrogen Iso-topic Anomalies in Meteorites and Comets. ApJ722, 1342–1351.

Figure A.5: Results of the Gaussian fitting pro-cedure applied to H13CN(1-0) towards L1544 (seeSect. 2). On each row, the three hyperfine compo-nents are shown in separate panels emphasize thedouble peak profile. The residuals are shown belowthe original spectrum. The fit result is overlaid ontop of the spectrum.

16

Figure A.4: Top: Results of the H13CN(1-0) hyperfine structure fitting procedure towards L183 for eachspatial position (offsets in arcsec are indicated). The residuals are shown below the original spectrum. Thefit result is overlaid on top of the spectrum. Bottom: HC15N(1-0) spectra. Results from single-componentGaussian fits are shown.

17

Figure A.6: HC15N(1-0) spectra towards L1544. Gaussian fits are overlaid on top of each spectrum, and theresiduals are shown below.

18

Alexander, C.M.O.., Fogel, M., Yabuta, H., Cody,G.D., 2007. The origin and evolution of chon-drites recorded in the elemental and isotopiccompositions of their macromolecular organicmatter. Geochim. Cosmochim. Ac. 71, 4380–4403.

Audouze, J., 1985. C, N and O isotopes and chem-ical evolution of our Galaxy, in: I. J. Danziger,F. Matteucci, & K. Kjar (Ed.), European South-ern Observatory Conference and Workshop Pro-ceedings, pp. 373–384.

Ben Abdallah, D., Najar, F., Jaidane, N., Du-mouchel, F., Lique, F., 2012. Hyperfine excita-tion of HCN by H2 at low temperature. MNRAS419, 2441–2447.

Bizzocchi, L., Caselli, P., Dore, L., 2010. Detectionof N15NH+ in L1544. A&A 510, L5. 1001.4903.

Blake, G.A., Qi, C., Hogerheijde, M.R., Gurwell,M.A., Muhleman, D.O., 1999. Sublimation fromicy jets as a probe of the interstellar volatile con-tent of comets. Nature 398, 213–216.

Bockelee-Morvan, D., Biver, N., Jehin, E.,Cochran, A.L., Wiesemeyer, H., Manfroid, J.,Hutsemekers, D., Arpigny, C., Boissier, J.,Cochran, W., Colom, P., Crovisier, J., Miluti-novic, N., Moreno, R., Prochaska, J.X., Ramirez,I., Schulz, R., Zucconi, J.M., 2008. Large Excessof Heavy Nitrogen in Both Hydrogen Cyanideand Cyanogen from Comet 17P/Holmes. ApJ679, L49–L52. 0804.1192.

Bonal, L., Huss, G.R., Krot, A.N., Nagashima, K.,Ishii, H.A., Bradley, J.P., 2010. Highly 15N-enriched chondritic clasts in the CB/CH-like me-teorite Isheyevo. Geochim. Cosmochim. Ac. 74,6590–6609.

Briani, G., Gounelle, M., Marrocchi, Y., Moste-faoui, S., Robert, F., Leroux, H., Meibom, A.,2009. Ultra-Pristine Extra-Terrestrial Materialwith Unprecedented Nitrogen Isotopic Variation,in: Lunar and Planetary Institute Science Con-ference Abstracts, p. 1642.

Busemann, H., Young, A.F., O’D. Alexander, C.M.,Hoppe, P., Mukhopadhyay, S., Nittler, L.R.,2006. Interstellar Chemistry Recorded in Or-ganic Matter from Primitive Meteorites. Science312, 727–730.

Caselli, P., van der Tak, F.F.S., Ceccarelli, C.,Bacmann, A., 2003. Abundant H2D

+ in thepre-stellar core L1544. A&A 403, L37–L41.arXiv:astro-ph/0304103.

Charnley, S.B., Rodgers, S.D., 2002. The End of

Interstellar Chemistry as the Origin of Nitrogenin Comets and Meteorites. ApJ 569, L133–L137.

Chaussidon, M., Gounelle, M., 2006. IrradiationProcesses in the Early Solar System. Universityof Arizona Press, Tucson. pp. 323–339.

Clayton, R.N., 2002. Solar System: Self-shieldingin the solar nebula. Nature 415, 860–861.

Dartois, E., Munoz Caro, G.M., Deboffle, D.,d’Hendecourt, L., 2004. Diffuse interstellarmedium organic polymers. Photoproduction ofthe 3.4, 6.85 and 7.25 µm features. A&A 423,L33–L36.

Derenne, S., Robert, F., 2010. Model of molecularstructure of the insoluble organic matter isolatedfromMurchison meteorite. Meteoritics and Plan-etary Science 45, 1461–1475.

Dobrica, E., Engrand, C., Quirico, E., Montagnac,G., Duprat, J., 2011. Raman characterization ofcarbonaceous matter in CONCORDIA Antarc-tic micrometeorites. Meteoritics and PlanetaryScience 46, 1363–1375.

Elsila, J.E., Dworkin, J.P., Bernstein, M.P., Mar-tin, M.P., Sandford, S.A., 2007. Mechanismsof Amino Acid Formation in Interstellar IceAnalogs. ApJ 660, 911–918.

Engel, M.H., Macko, S.A., 1997. Isotopic evidencefor extraterrestrial non- racemic amino acids inthe Murchison meteorite. Nature 389, 265–268.

Floss, C., Stadermann, F.J., Bradley, J.P., Dai,Z.R., Bajt, S., Graham, G., Lea, A.S., 2006.Identification of isotopically primitive interplan-etary dust particles: A NanoSIMS isotopic imag-ing study. Geochim. Cosmochim. Ac. 70, 2371–2399.

Flower, D.R., Pineau des Forets, G., Walmsley,C.M., 2006. The importance of the ortho:paraH2 ratio for the deuteration of molecules dur-ing pre-protostellar collapse. A&A 449, 621–629.arXiv:astro-ph/0601429.

Fouchet, T., Lellouch, E., Bezard, B., Encrenaz, T.,Drossart, P., Feuchtgruber, H., de Graauw, T.,2000. ISO-SWS Observations of Jupiter: Mea-surement of the Ammonia Tropospheric Profileand of the 15N/14N Isotopic Ratio. Icarus 143,223–243. arXiv:astro-ph/9911257.

Fray, N., Benilan, Y., Cottin, H., Gazeau, M.C.,Crovisier, J., 2005. The origin of the CN radi-cal in comets: A review from observations andmodels. Planetary Space Science 53, 1243–1262.

Gerin, M., Marcelino, N., Biver, N., Roueff, E.,Coudert, L.H., Elkeurti, M., Lis, D.C., Bockelee-

19

Morvan, D., 2009. Detection of 15NH{2}Din dense cores: a new tool for measuring the14N/15N ratio in the cold ISM. A&A 498, L9–L12. 0903.3155.

Gilmour, I., 2003. Structural and Isotopic Analysisof Organic Matter in Carbonaceous Chondrites.Treatise on Geochemistry 1, 269–290.

Hashizume, K., Chaussidon, M., Marty, B., Robert,F., 2000. Solar Wind Record on the Moon: De-ciphering Presolar from Planetary Nitrogen. Sci-ence 290, 1142–1145.

Hily-Blant, P., Walmsley, M., Pineau des Forets,G., Flower, D., 2008. CN in prestellar cores.A&A 480, L5–L8. arXiv:0801.2876.

Hily-Blant, P., Walmsley, M., Pineau des Forets,G., Flower, D., 2010. Nitrogen chemistry anddepletion in starless cores. A&A 513, A41+.1001.3930.

Huss, G.R., Meshik, A.P., Smith, J.B., Hohenberg,C.M., 2003. Presolar diamond, silicon carbide,and graphite in carbonaceous chondrites: impli-cations for thermal processing in the solar neb-ula. Geochim. Cosmochim. Ac. 67, 4823–4848.

Ikeda, M., Hirota, T., Yamamoto, S., 2002.The H13CN/HC15N Abundance Ratio in DenseCores: Possible Source-to-Source Variation ofIsotope Abundances? ApJ 575, 250–256.

Irvine, W.M., Schloerb, F.P., Crovisier, J., Fegley,Jr., B., Mumma, M.J., 2000. Comets: a Link Be-tween Interstellar and Nebular Chemistry. Pro-tostars and Planets IV , 1159.

Jehin, E., Manfroid, J., Hutsemekers, D., Arpigny,C., Zucconi, J.M., 2009. Isotopic Ratios inComets: Status and Perspectives. Earth Moonand Planets 105, 167–180.

Kung, C.C., Clayton, R.N., 1978. Nitrogen abun-dances and isotopic compositions in stony mete-orites. Earth and Planetary Science Letters 38,421–435.

Lis, D.C., Wootten, A., Gerin, M., Roueff, E., 2010.Nitrogen Isotopic Fractionation in InterstellarAmmonia. ApJ 710, L49–L52. 1001.0744.

Lyons, J.R., Bergin, E.A., Ciesla, F.J., Davis, A.M.,Desch, S.J., Hashizume, K., Lee, J.E., 2009.Timescales for the evolution of oxygen isotopecompositions in the solar nebula. Geochimica etCosmochimica Acta 73, 4998–5017.

Manfroid, J., Jehin, E., Hutsemekers, D., Cochran,A., Zucconi, J.M., Arpigny, C., Schulz, R.,Stuwe, J.A., Ilyin, I., 2009. The CN isotopic ra-tios in comets. A&A 503, 613–624. 0907.0311.

Marty, B., Chaussidon, M., Wiens, R.C., Jurewicz,A.J.G., Burnett, D.S., 2011. A 15N-Poor IsotopicComposition for the Solar System As Shown byGenesis Solar Wind Samples. Science 332, 1533–.

Marty, B., Zimmermann, L., Burnard, P.G.,Wieler, R., Heber, V.S., Burnett, D.L., Wiens,R.C., Bochsler, P., 2010. Nitrogen isotopes inthe recent solar wind from the analysis of Gene-sis targets: Evidence for large scale isotope het-erogeneity in the early solar system. Geochim.Cosmochim. Ac. 74, 340–355.

Meibom, A., Krot, A.N., Robert, F., Mostefaoui,S., Russell, S.S., Petaev, M.I., Gounelle, M.,2007. Nitrogen and Carbon Isotopic Com-position of the Sun Inferred from a High-Temperature Solar Nebular Condensate. ApJ656, L33–L36.

Messenger, S., 2000. Identification of molecular-cloud material in interplanetary dust particles.Nature 404, 968–971.

Milam, S.N., Charnley, S.B., 2012. Observations ofNitrogen Fractionation in Prestellar Cores: Ni-triles Tracing Interstellar Chemistry, in: Lunarand Planetary Institute Science Conference Ab-stracts, p. 2618.

Milam, S.N., Savage, C., Brewster, M.A., Ziurys,L.M., Wyckoff, S., 2005. The 12C/13C IsotopeGradient Derived from Millimeter Transitions ofCN: The Case for Galactic Chemical Evolution.ApJ 634, 1126–1132.

Okumura, F., Mimura, K., 2011. Gradual andstepwise pyrolyses of insoluble organic matterfrom the Murchison meteorite revealing chemi-cal structure and isotopic distribution. Geochim.Cosmochim. Ac. 75, 7063–7080.

Owen, T., Mahaffy, P.R., Niemann, H.B., Atreya,S., Wong, M., 2001. Protosolar Nitrogen. ApJ553, L77–L79.

Padovani, M., Walmsley, C.M., Tafalla, M., Hily-Blant, P., Pineau des Forets, G., 2011. Hydrogencyanide and isocyanide in prestellar cores. A&A534, A77.

Paganini, L., Villanueva, G.L., Lara, L.M., Lin,Z.Y., Kuppers, M., Hartogh, P., Faure, A., 2010.HCN Spectroscopy of Comet 73P/Schwassmann-Wachmann 3. A Study of Gas Evolution and itsLink to CN. ApJ 715, 1258–1269.

Pizzarello, S., Feng, X., Epstein, S., Cronin, J.R.,1994. Isotopic analyses of nitrogenous com-pounds from the Murchison meteorite: ammo-nia, amines, amino acids, and polar hydrocar-

20

bons. Geochim. Cosmochim. Ac. 58, 5579–5587.Ratajczak, A., Quirico, E., Faure, A., Schmitt, B.,

Ceccarelli, C., 2009. Hydrogen/deuterium ex-change in interstellar ice analogs. A&A 496, L21–L24.

Remusat, L., Derenne, S., Robert, F., Knicker, H.,2005. New pyrolytic and spectroscopic data onOrgueil and Murchison insoluble organic matter:A different origin than soluble? Geochim. Cos-mochim. Ac. 69, 3919–3932.

Rodgers, S.D., Charnley, S.B., 2008. Nitrogen Iso-topic Fractionation of Interstellar Nitriles. ApJ689, 1448–1455.

Roueff, E., Lis, D.C., van der Tak, F.F.S., Gerin,M., Goldsmith, P.F., 2005. Interstellar deuter-ated ammonia: from NH3 to ND3. A&A 438,585–598. arXiv:astro-ph/0504445.

Schmitt-Koplin, P., Gabelica, Z., Gougeon, R.D.,Fekete, A., Kanawatu, B., Harir, M., Gebefuegi,I., Eckel, G., Hertkorn, N., 2010. High molecu-lar diversity of extraterrestrial organic matter inmurchison meteorite revealed 40 years after itsfall. PNAS 107, 2763–2768.

Sephton, M.A., Verchovsky, A.B., Bland, P.A.,Gilmour, I., Grady, M.M., Wright, I.P., 2003. In-vestigating the variations in carbon and nitrogenisotopes in carbonaceous chondrites. Geochim.Cosmochim. Ac. 67, 2093–2108.

Sephton, M.A., Wright, I.P., Gilmour, I., de Leeuw,J.W., Grady, M.M., Pillinger, C.T., 2002. Highmolecular weight organic matter in martian me-teorites. Planetary Space Science 50, 711–716.

Terzieva, R., Herbst, E., 2000. The possibilityof nitrogen isotopic fractionation in interstellarclouds. MNRAS 317, 563–568.

van der Tak, F.F.S., Black, J.H., Schoier, F.L.,Jansen, D.J., van Dishoeck, E.F., 2007. A com-puter program for fast non-LTE analysis of inter-stellar line spectra. With diagnostic plots to in-terpret observed line intensity ratios. A&A 468,627–635.

Wirstrom, E.S., Charnley, S.B., Cordiner, M.A.,Milam, S.N., 2012. Isotopic Anomalies in Primi-tive Solar System Matter: Spin-state-dependentFractionation of Nitrogen and Deuterium in In-terstellar Clouds. ApJ 757, L11. 1208.0192.

21