2005A&A...434..827L

We propose a new methodology for probing the cosmological variability of α from pairs of FeII lines (SIDAM, single ion differential α measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to (i) ionization inhomogeneities in the absorbers; and (ii) non-zero offsets between different exposures. Applied to the FeII lines of the metal absorption line system at z_abs=1.839 in the spectrum of Q 1101-264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides Δα/α=(2.4±3.8_stat)x10^–6. The z_abs=1.15 FeII system toward HE 0515-4414 has been re-analyzed by this method thus obtaining for the combined sample Δα/α=(0.4±1.5_stat)x10^–6. These values are shifted with respect to the Keck/HIRES mean Δα/α=(-5.7±1.1_stat)x10^–6 (Murphy et al., 2004, in Astrophysics, Clocks and Fundamental Constants, ed. S.G. Karshenboim, & E. Peik (Berlin, Heidelberg: Springer-Verlag), 131) at very high confidence level (95%). The fundamental photon noise limitation in the Δα/α measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of ∼10 times the UVES dispersion coupled to a 100 m class telescope the present Oklo level (Δα/α≥4.5x10^–8) can be achieved along cosmological distances with differential measurements of Δα/α.