Measuring the fine structure constant on white dwarf surfaces; uncertainties from continuum placement variations

TL;DR

This study develops an automated method for continuum placement in white dwarf spectra to improve the accuracy of measuring the fine structure constant, highlighting the impact of continuum uncertainties on such measurements.

Contribution

It introduces a reproducible cubic spline-based continuum estimation technique and assesses its effect on fine structure constant measurements in white dwarf spectra.

Findings

Continuum placement variations cause systematic shifts in absorption line centroids.

These shifts significantly affect fine structure constant measurements.

The method can be applied to other astrophysical contexts involving spectral analysis.

Abstract

Searches for variations of fundamental constants require accurate measurement errors. There are several potential sources of errors and quantifying each one accurately is essential. This paper addresses one source of uncertainty relating to measuring the fine structure constant on white dwarf surfaces. Detailed modelling of photospheric absorption lines requires knowing the underlying spectral continuum level. Here we describe the development of a fully automated, objective, and reproducible continuum estimation method, based on fitting cubic splines to carefully selected data regions. Example fits to the Hubble Space Telescope spectrum of the white dwarf G191-B2B are given. We carry out measurements of the fine structure constant using two continuum models. The results show that continuum placement variations result in small systematic shifts in the centroids of narrow photospheric absorption lines which impact significantly on fine structure constant measurements. This effect must therefore be included in the overall error budget of future measurements. Our results also suggest that continuum placement variations should be investigated in other contexts, including fine structure constant measurements in stars other than white dwarfs, quasar absorption line measurements of the fine structure constant, and quasar measurements of cosmological redshift drift.