ALMA Observations of HCN and its Isotopologues on Titan

Edward M. Molter, C. A. Nixon, M. A. Cordiner, J. Serigano, P. G J Irwin, N. A. Teanby, S. B. Charnley, J. E. Lindberg

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Abstract

We present sub-millimeter spectra of HCN isotopologues on Titan, derived from publicly available ALMA flux calibration observations of Titan taken in early 2014. We report the detection of a new HCN isotopologue on Titan, H13C15N, and confirm an earlier report of detection of DCN. We model high signal-to-noise observations of HCN, H13CN, HC15N, DCN, and H13C15N to derive abundances and infer the following isotopic ratios: 12C/13C = 89.8 ±2.8, 14N/15N = 72.3 ±2.2, D/H = (2.5 ± 0.2) ×10-4, and HCN/H13C15N = 5800 ±270 (1σ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of ∼2.3 elevation in 14N/15N in HCN compared to N2 and a lack of fractionation in 12C/13C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of ∼2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.

Original languageEnglish
Article number42
Number of pages7
JournalAstronomical Journal
Volume152
Issue number2
DOIs
Publication statusPublished - 4 Aug 2016

Keywords

  • planets and satellites: atmospheres
  • planets and satellites: individual (Titan)

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