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High-Precision Calibration and Reduction Techniques for Molecular Line Emission in Radio Astronomy & Their Application to the Galactic Chemical Evolution of Silicon

Abstract

A report on 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 (RGC) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Robert C. Byrd Green Bank Telescope (GBT) permit isotope ratios to be corrected for optical depth, using a novel method developed as part of this study. 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 ratios in the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with RGC. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with RGC in published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for galactic chemical evolution, suggest that the reported isotope ratio trends in oxygen, and likely carbon as well, may be in error and 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.

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