Measuring Galactic CNO isotopic ratio gradients

The abundance ratios of carbon, nitrogen, oxygen, and their isotopes provide key constraints on the chemical evolution history and stellar nucleosynthesis models in star-forming regions. In the framework of the inside-out formation scenario of the Milky Way, the isotopic ratios of ¹²C/¹³C, ¹⁶O/¹⁸O/¹⁷O, and ¹⁴N/¹⁵N are expected to show gradients with Galactocentric radius, as supported by numerous observations.

However, the observational CNO isotopic ratios are poorly constrained in the Galactic metal-poor outer disk, particularly in regions with Galactocentric distances > 12 kpc. We are doing a series of studies to derive new CNO isotopic ratios in this region using sub-mm observations towards Galactic molecular clouds with the IRAM 30-m telescope, NOEMA, and ALMA data.

In addition, We are re-examining the assumptions behind current methods for deriving CNO isotopic ratios. We have found that the previous method to use CN isotopologues have several issues, and providing new ¹²CN/¹³CN results with an updated method (Sun et al. 2024).

My on-going follow-up works includes:
a. Reassessing the LTE assumption in current ¹²CN/¹³CN studies (to be submitted), also connected to project 3.
b. The ¹²C/¹³C and ¹⁶O/¹⁸O/¹⁷O results from ALMA data in Galactic outer disk (in preparation)
c. The ¹⁸O/¹⁷O results toward far-outer disk with IRAM 30-m and NOMEA data (in preparation)