Uniform Silicon Isotope Ratios Across the Milky Way Galaxy

TL;DR

This study finds that silicon isotope ratios are remarkably uniform across the Milky Way, contrasting with other isotope ratios that show clear gradients, challenging existing models of galactic chemical evolution.

Contribution

It provides the first comprehensive measurement of silicon isotope ratios across the Galaxy, revealing a surprising uniformity and introducing methods applicable to other isotope systems.

Findings

Silicon isotope ratios show no significant variation with Galactocentric radius.

The results contrast with the decreasing trend of oxygen isotope ratios across the Galaxy.

The developed methods can be applied to future isotope ratio studies.

Abstract

We report the relative abundances of the three stable isotopes of silicon, $^{28}$Si, $^{29}$Si and $^{30}$Si, across the Galaxy using the $v = 0, J = 1 \to 0$ transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii ($R_{\rm GC}$) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the GBT permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios $^{29}{\rm Si}/^{28}{\rm Si}$ and $^{30}{\rm Si}/^{28}{\rm Si}$ 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 $R_{\rm GC}$. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with $R_{\rm GC}$ 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 that 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.