An improved method to measure 12C/13C and 14N/15N abundance ratios: revisiting CN isotopologues in the Galactic outer disc

The variations of elemental abundance and their ratios along the Galactocentric radius result from the chemical evolution of the Milky Way discs. The ¹²C/¹³C ratio in particular is often used as a proxy to determine other isotopic ratios, such as ¹⁶O/¹⁸O and ¹⁴N/¹⁵N. Measurements of ¹²CN and ¹³CN (or C¹⁵N) – with their optical depths corrected via their hyper-fine structure lines – have traditionally been exploited to constrain the Galactocentric gradients of the CNO isotopic ratios. Such methods typically make several simplifying assumptions (e.g. a filling factor of unity, the Rayleigh–Jeans approximation, and the neglect of the cosmic microwave background) while adopting a single average gas phase. However, these simplifications introduce significant biases to the measured ¹²C/¹³C and ¹⁴N/¹⁵N. We demonstrate that exploiting the optically thin satellite lines of 12CN constitutes a more reliable new method to derive ¹²C/¹³C and ¹⁴N/¹⁵N from CN isotopologues. We apply this satellite-line method to new IRAM 30-m observations of 12CN, 13CN, and C15N N = 1 → 0 towards 15 metal-poor molecular clouds in the Galactic outer disc (Rgc > 12 kpc), supplemented by data from the literature. After updating their Galactocentric distances, we find that ¹²C/¹³C and ¹⁴N/¹⁵N gradients are in good agreement with those derived using independent optically thin molecular tracers, even in regions with the lowest metallicities. We therefore recommend using optically thin tracers for Galactic and extragalactic CNO isotopic measurements, which avoids the biases associated with the traditional method.