Analysis of Molecular Hydrogen Absorption toward QSO B0642–5038 for a Varying Proton-to-electron Mass Ratio

Rovibronic molecular hydrogen (H2) transitions at redshift z abs 2.659 toward the background quasar B0642–5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio μ. We utilize an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (UVES) with a signal-to-noise ratio of ~35 per 2.5 km s–1 pixel at the observed H2 wavelengths (335-410 nm). Some 111 H2 transitions in the Lyman and Werner bands have been identified in the damped Lyα system for which a kinetic gas temperature of ~84 K and a molecular fraction log f = –2.18 ± 0.08 are determined. The H2 absorption lines are included in a comprehensive fitting method, which allows us to extract a constraint on a variation of the proton-electron mass ratio Δμ/μ from all transitions at once. We obtain Δμ/μ = (17.1 ± 4.5stat ± 3.7sys) × 10–6. However, we find evidence that this measurement has been affected by wavelength miscalibration errors recently identified in UVES. A correction based on observations of objects with solar-like spectra gives a smaller Δμ/μ value and contributes to a larger systematic uncertainty: Δμ/μ = (12.7 ± 4.5stat ± 4.2sys) × 10–6.