2020ApJ...890...95I

The nitriles present in the atmosphere of Titan can be expected to exhibit different ¹⁴N/¹⁵N values depending on their production processes, primarily because of the various N₂ dissociation processes induced by different sources such as ultraviolet radiation, magnetospheric electrons, and Galactic cosmic rays. For CH₃CN, one photochemical model predicted a ¹⁴N/¹⁵N value of 120-130 in the lower stratosphere. This is much higher than that for HCN and HC₃N, ∼67-94. By analyzing archival data obtained by the Atacama Large Millimeter/submillimeter Array, we successfully detected submillimeter rotational transitions of CH₃C¹⁵N (J = 19-18) located in the 338 GHz band in Titan's atmospheric spectra. By comparing those observations with the simultaneously observed CH₃CN (J = 19-18) lines in the 349 GHz band, which probe from 160 to ∼400 km altitude, we then derived ¹⁴N/¹⁵N in CH₃CN as 125_–44⁺¹⁴⁵. Although the range of the derived value shows insufficient accuracy due to limitations on data quality, the best-fit value suggests that ¹⁴N/¹⁵N for CH₃CN is higher than values that have previously been observed and theoretically predicted for HCN and HC₃N. This may be explained by the different N₂ dissociation sources according to altitude, as suggested by a recent photochemical model.