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THE NITROGEN ISOTOPE RATIO IN DENSE MOLECULAR CLOUDS Gilles Adande Lucy M. Ziurys Department of Chemistry, Department of Astronomy, Steward Observatory and Arizona Radio Observatory University of Arizona
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Big Bang nucleosynthesis 1 H, 2 H, 3 He, 4 He Stellar nucleosynthesis 12 C, 14 N, …., 56 Fe Supernova nucleosynthesis ≥ 56 Fe Cosmic rays spallation 3 Li, 4 Be, 5 B Isotope ratios diagnostic tool for nucleosynthesis processes, distribution of star masses…
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14 N/ 15 N variations understanding solar systems evolution
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Arpigny et al, science, 2003 14 N/ 15 N across our Solar System
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14 N/ 15 N estimation Dust extinction at galactic center Millimeter wave astronomy primary method Challenges & Difficulties N 2 no dipole moment N + or N high energy spectra Possible fractionation for observable species: HCN, HNC, N 2 H +, CN, NH 3, HC 3 N… Main isotope large optical depth estimation of abundances problematic Existing studies methodology: double isotope ratio Rarer 15 N isotope, long integration time for good signal to noise 14 N 15 N 13 C 12 C ×= 13 C 15 N
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Few galactic studies Large uncertainties ( > 50 %) 14 N/ 15 N range 200-800 Dahmen et al, A&A, 1994 Milam et al, ApJ, 2005
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HNC probe molecule Comparisons with HCN studies Main isotope moderately thick Rarer isotope observable reasonable observing time Limited fractionation Terzieva & Herbst, MNRAS, 2000 Our study
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ARO 12 m and SMT telescope ALMA type Band 3 receiver (84-116 GHz) ALMA type Band 6 (211-275 GHz) 12 m at Kitt Peak SMT on Mt Graham
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SpeciesFrequency (MHz) Transition J ′ J cc HNC 90663.6 1 0 0.88 271981.0 3 2 0.75 HN 13 C 87090.8 1 0 0.88 174179.4 2 1 0.69 261263.3 3 2 0.75 H 15 NC 88865.7 1 0 0.88 266587.8 3 2 0.75 Source Distance to GC (kpc) Sgr B2(2N)0.1 W313.1 G34.35.1 M17-SW6.1 W51M6.1 DR-218.0 Orion Bar8.3 Orion-KL8.3 NGC75388.9 W3 (OH)9.6 S15610.9 DR-21
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Example spectra toward NGC 7538 HN 13 C H 15 NC = 5.4 ± 0.9 14 N 15 N = 302 ± 74 Using 12 C/ 13 C from CN:
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direct measurement for NGC 7538 with HCN Hyperfine lines follow LTE spin statistics Transition optically thin H 13 CN H 13 C 15 N = 215 ± 61 Existing studies: 14 N 15 N = 463 ± 102 14 N 15 N = 302 ± 74 Using 12 C/ 13 C from CN:
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14 N/ 15 N for 11 molecular clouds Conclusions: Galactic gradient Values smaller than previous estimations 14 N/ 15 N ~ 100-500 For HNC in local ISM 14 N/ 15 N ~ 150-300 Close to comets measurements 14 N/ 15 N = 18.0 (9.8) × D gc + 92.4 (72.4)
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Future Future directions Increase S/N Expand survey to more clouds Direct measurement with LVG modeling Evaluate fractionation with mapping observations of 14 N/ 15 N in Orion
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Acknowledgements Professor Lucy Ziurys Dr. Stefanie Milam Dr. DeWayne Halfen, Dr. Emmy Tenenbaum, Lindsay Zack, Jessica Dodd, Ming Sun, Matthew Bucchino, Jie Min, Robin Pulliam, Brent Harris Funding: NASA-NSF
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