13C Isotopic Ratios of HC3N on Titan Measured with ALMA

Abstract

We present the first determination of the abundance ratios of 13C substitutions of cyanoacetylene (HC3N), [H13CCCN]:[HC13CCN]:[HCC13CN], in Titan's atmosphere measured using millimeter-wave spectra obtained by the Atacama Large Millimeter/submillimeter Array. To compare the line intensities precisely, data sets that include multiple molecular lines were extracted to suppress effects of Titan's environmental conditions and observation settings. The [HC13CCN]:[HCC13CN] and [H13CCCN]:[HCC13CN] ratios were obtained from 12 and 1 selected data sets, respectively. As a result, nearly the uniform [H13CCCN]:[HC13CCN]:[HCC13CN] abundance ratios of 1.17 ( ±0.20) : 1.09 ( ±0.25) : 1 (1σ) were derived, whereas previously reported ratios for the interstellar medium (ISM) have shown large anomalies that may be caused by 13C concentrations in precursors. The result obtained here suggests that 13C concentration processes suggested in the ISM studies do not work effectively on precursors of HC3N and HC3N themselves owing to Titan's high atmospheric temperature and/or depletion of both 13C and 13C+ 2021.