2017ApJ...839..123M

We report the relative abundances of the three stable isotopes of silicon, ²⁸Si, ²⁹Si, and ³⁰Si, across the Galaxy using the v=0,J=1→0 transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii (R_GC) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Green Bank Telescope permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios ²⁹Si/²⁸Si and ³⁰Si/²⁸Si vary much less than predicted on the basis of other stable isotope ratio gradients across the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with R_GC. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with R_GC exhibited by published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for chemical evolution, suggest that the reported oxygen isotope ratio trends, and perhaps those for carbon as well, require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon, and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.