Isotopic Fractionation of Carbon, Deuterium, and Nitrogen: a Full Chemical Study?
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A&A 576, A99 (2015) Astronomy DOI: 10.1051/0004-6361/201425113 & c ESO 2015 Astrophysics Isotopic fractionation of carbon, deuterium, and nitrogen: a full chemical study? E. Roueff1;2, J. C. Loison3, and K. M. Hickson3 1 LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR8112, Place Janssen, 92190 Meudon Cedex, France e-mail: [email protected] 2 Sorbonne Universités, UPMC Univ. Paris 6, 4 Place Jussieu, 75005 Paris, France 3 ISM, Université de Bordeaux – CNRS, UMR 5255, 351 cours de la Libération, 33405 Talence Cedex, France e-mail: [email protected] Received 6 October 2014 / Accepted 5 January 2015 ABSTRACT Context. The increased sensitivity and high spectral resolution of millimeter telescopes allow the detection of an increasing number of isotopically substituted molecules in the interstellar medium. The 14N/15N ratio is difficult to measure directly for molecules con- taining carbon. Aims. Using a time-dependent gas-phase chemical model, we check the underlying hypothesis that the 13C/12C ratio of nitriles and isonitriles is equal to the elemental value. Methods. We built a chemical network that contains D, 13C, and 15N molecular species after a careful check of the possible fraction- ation reactions at work in the gas phase. Results. Model results obtained for two different physical conditions that correspond to a moderately dense cloud in an early evolu- tionary stage and a dense, depleted prestellar core tend to show that ammonia and its singly deuterated form are somewhat enriched 15 14 15 + in N, which agrees with observations. The N/ N ratio in N2H is found to be close to the elemental value, in contrast to previous 15 + models that obtain a significant enrichment, because we found that the fractionation reaction between N and N2H has a barrier in + 15 + + 15 + the entrance channel. The high values of the N2H / NNH and N2H /N NH ratios derived in L1544 cannot be reproduced in our model. Finally, we find that nitriles and isonitriles are in fact significantly depleted in 13C, thereby challenging previous interpretations of observed C15N, HC15N, and H15NC abundances from 13C containing isotopologues. Key words. astrochemistry – molecular processes – ISM: molecules – ISM: clouds 1. Introduction function of the galactic distance. The actual value of the 12C/13C isotopic ratio in the local interstellar medium (ISM) is assumed Understanding isotopic abundances on a large scale is a major to be 68 (Milam et al. 2005). goal that has received a great deal of attention for its application to terrestrial environments (ocean, meteorites), the solar system The possibility of nitrogen isotopic fractionation in interstel- (planets, comets), and galactic interstellar space. The variation lar clouds has been investigated by Terzieva & Herbst(2000), 15 in isotopic ratios may give us some information about the link who suggest various N isotopic exchange reactions. Rodgers between solar system objects and galactic interstellar environ- & Charnley(2004, 2008a,b) used these suggested reaction-rate ments as discussed by Aléon(2010). We mainly focus our study constants to predict nitrogen isotopic fractionation in chemical on interstellar environments where low-temperature conditions models of dense interstellar molecular cores. They specifically may significantly affect the isotopic ratios of the molecular con- discussed the role of the atomic-to-molecular nitrogen ratio in tent. Isotopic molecules are detected in a variety of environments the fractionation process and the possible link between nitro- and offer an additional tool for determining physical conditions gen hydrides and CN containing molecules (nitriles). The corre- because they usually do not suffer from opacity problems. sponding observations are sparse, however, and difficult because 14 15 Early modeling studies of 13C and 18O isotopic enrichment the elemental N/ N ratio is high (441 ± 5), as determined were performed by Langer et al.(1984), who introduced di ffer- by the recent Genesis solar wind sampling measurement (Marty ent isotopic exchange reactions, relying on previous theoretical et al. 2011), and is assumed to hold in the local ISM. and experimental studies by Watson et al.(1976) and Smith & In addition, the zero point energy (ZPE) differences involved Adams(1980). The possible e ffects of selective photodissocia- in nitrogen fractionation reactions are small, and the predicted tion of CO have subsequently been emphasized (Glassgold et al. corresponding chemical enrichment is moderate. Lucas & Liszt 1985; Le Bourlot et al. 1993; Visser et al. 2009), which tend to (1998) report HC15N absorption in diffuse clouds located in front increase the CO/13CO ratio. The use of the CN radical as a tracer of distant quasars with a HCN/HC15N ratio of 270 ± 27, close of 12C/13C isotopic ratio has been raised by Savage et al.(2002), to value reported for the Earth. However, various new observa- Milam et al.(2005), who studied the corresponding gradient as a tions of isotopic nitrogen containing molecules have been re- ported, including 15N substituted ammonia and deuterated am- ? Appendices are available in electronic form at monia (Gerin et al. 2009; Lis et al. 2010), the diazenylium ion + http://www.aanda.org (N2H ; Bizzocchi et al. 2010, 2013; Daniel et al. 2014), CN and Article published by EDP Sciences A99, page 1 of 18 A&A 576, A99 (2015) HCN (Ikeda et al. 2002; Pillai et al. 2007; Adande & Ziurys kf + kr = kcapture. From the equilibrium constant expres- 2012; Hily-Blant et al. 2013b; Daniel et al. 2014), HCN and sion, we then derive k = k × f (B;M) and 15 f capture [ f (B;M)+exp(−∆E=kT)] HNC (Wampfler et al. 2014). The strong depletion found in N exp(−∆E=kT) + kr = kcapture × . variants of the N2H isotopologue strongly contradicts model [ f (B;M)+exp(−∆E=kT)] predictions (Gerin et al. 2009), which motivates a reinvestiga- – C: reactions involving adduct formation with isomerization tion of the chemical processes at work. With this in mind, the pathways. Such a case holds for 13C+HCN ! 12C+H13CN. link between deuterated chemistry and the possible role of or- We again assume that the high pressure rate constant is given tho/para molecular hydrogen has been studied by Wirström et al. by capture theory (for reactions without a barrier). The iso- (2012). topic isomerization reaction competes with the dissociation We analyze in Sect.2 the various possible isotopic exchange of the adduct. The rate constant depends on the location of reactions that are involved for molecules containing carbon and the transition state, and statistical calculations are generally nitrogen. Indeed, most nitrogen-fractionation observational re- required to estimate the isomerization reaction rate constant. sults for CN-containing molecules involve only 13C and 15N – D: other reactive exothermic channels. The exchange reac- species so that the measure of the nitrogen isotopic ratio as- tion is generally discarded. (The possibility of N atom ex- 15 15 sumes a fixed 12C/13C fraction. We examine and extend the pi- change in the N + CN and in N + C2N reactions is oneering study of Terzieva & Herbst(2000) and check for the discussed.) possible presence of barriers in the entrance channels of iso- topic exchange reactions through theoretical calculations. We The knowledge of the exoergicity values ∆E is also a major con- also update the ZPE values involved and derive the correspond- cern. They are obtained from the differences of the ZPEs be- ing exothermicity values. We present our new chemical model tween products and reactants. We recall the corresponding ex- in Sect.3 and compare it with available observations and other pressions in Appendix B and derive their values by using the models. Our conclusions are presented in Sect.5. most recent determinations of spectroscopic constants reported in Tables B.2–B.4. We summarize in Table1 the di fferent isotopic exchange re- 2. Chemical reactions involving isotopic actions considered and display the corresponding reaction rate substitutes of 12C and 14N constants. Detailed information is provided in Appendix A on the theoretical methods used for the different systems. The re- 2.1. 13 C and 15 N exchange reactions actions involving 15N are displayed in the upper part of the ta- ble. We also consider 13C isotopic exchange reactions in the At very low temperatures, isotopic exchange reactions may only lower part of Table1. Table1 shows two main discrepancies occur if no barrier is present between the interacting atoms, compared to previous studies by Terzieva & Herbst(2000). The ions, and molecules or if tunnelling plays an important role. 15 + exchange reactions between atomic N and N2H as well as Experimental information is crucial, and we constrain the evalu- with HCNH+are found to be unlikely to occur because signif- ation of rate constants using that information. If no experimental icant barriers arise in the complex formation step. A similar data are available, we apply theoretical methods to determine result is obtained for 15N+ exchange with NO, whereas these the presence of a barrier: A first technique uses density func- reactions had been included in Terzieva & Herbst(2000). The tional theory (DFT) calculations with the hybrid M06-2X func- exchange reaction between atomic 15N and CN, which was sug- tional developed by Zhao & Truhlar(2008), which is well suited gested by Rodgers & Charnley(2008a), is found to be plau- to thermochemical calculations, associated to the cc-pVTZ ba- sible. Additional exchange possibilities have also been consid- sis set using GAUSSIAN09 software.The alternative is provided 15 13 ered, such as the reaction between N and C2N. As far as C by the MRCI+Q method (with the aug-cc-pVTZ basis set). For possible fractionation is concerned, we find that CN could be barrierless cases, we derive the reaction rate constants by using enriched in 13C through the exchange reactions of CN with 13C a simple capture theory (Georgievskii & Klippenstein 2005) for and 13C+.