Systematic observations on Galactic Interstellar isotope ratios

We are performing systematic observation studies on the Galactic interstellar isotopic ratios, including ¹⁸O/¹⁷O, ¹²C/¹³C, ¹⁴N/¹⁵N and ³²S/³⁴S. Our strategy focuses on combination of multi-transition observation data toward large samples with different Galactocentric distances. Our preliminary results show positive Galactic radial gradients of ¹⁸O/¹⁷O and ¹²C/¹³C. In both cases, the ratio increases with the Galactocentric distance, which agrees with the inside-out scenario of our Galaxy. Observations of other isotopes such as ¹⁴N/¹⁵N and ³²S/³⁴S are on-going.