Analysis of molecular hydrogen absorption toward QSO B0642-5038 for a varying proton-to-electron mass ratio

Rovibronic molecular hydrogen (H-2) transitions at redshift z(abs) similar or equal to 2.659 toward the background quasar B0642-5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio mu. We utilize an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph (UVES) with a signal-to-noise ratio of similar to 35 per 2.5 km s(-1) pixel at the observed H-2 wavelengths (335-410 nm). Some 111 H-2 transitions in the Lyman and Werner bands have been identified in the damped Ly alpha system for which a kinetic gas temperature of similar to 84 K and a molecular fraction log f = -2.18 +/- 0.08 are determined. The H-2 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 Delta mu/mu from all transitions at once. We obtain Delta mu/mu = (17.1 +/- 4.5(stat) +/- 3.7(sys)) x 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 Delta mu/mu value and contributes to a larger systematic uncertainty: Delta mu/mu = (12.7 +/- 4.5(stat) +/- 4.2(sys)) x 10(-6).