Probing Galactic variations in the fine-structure constant using solar twin stars: methodology and results

TL;DR

This study introduces a differential method using solar twin stars to precisely probe potential variations in the fine-structure constant, achieving high accuracy and setting stringent local limits on its variability.

Contribution

The paper presents a new differential approach with solar twins to measure $eta$ variations, reducing systematic errors and improving sensitivity over previous methods.

Findings

No evidence for velocity separation differences in 17 transition pairs.

Limits on local $eta$ variations are approximately 50 parts per billion.

Method achieves velocity measurement precision of about 10 m/s.

Abstract

The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-structure constant, $\alpha$, which gauges the strength of electromagnetism. While individual line wavelengths are sensitive to $\alpha$, they are also sensitive to physical processes in the stellar atmospheres, which has precluded their use so far. Here we demonstrate a new, differential approach using solar twins: velocity separations between close pairs of transitions are compared across stars with very similar physical properties, strongly suppressing astrophysical and instrumental systematic errors. We utilise 423 archival exposures of 18 solar twins from the High-Accuracy Radial velocity Planetary Searcher (HARPS), in which calibration errors can be reduced to $\lesssim$3 m/s. For stars with $\approx$10 high signal-to-noise ratio spectra ($\ge$200 per pixel), velocity separations between pairs are measured with $\approx$10 m/s statistical precision. A companion paper assesses a range of systematic error sources using 130 stars, with a greater range of stellar parameters, providing accurate corrections for astrophysical effects and a residual, intrinsic star-to-star scatter of 0-13 m/s. Within these uncertainties, we find no evidence for velocity separation differences in 17 transition pairs between solar twins. In a second companion paper, this is found to limit local ($\lesssim$50 pc) variations in $\alpha$ to $\approx$50 parts per billion, $\sim$2 orders of magnitude less than other Galactic constraints.