Constraint on a cosmological variation in the proton-to-electron mass ratio from electronic co absorption

Carbon monoxide (CO) absorption in the sub-damped Lyα absorber at redshift $z_{abs}\simeq 2.69$ toward the background quasar SDSS J123714.60+064759.5 (J1237+0647) was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio, μ, over a cosmological timescale. The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 km s⁻¹ per pixel at ∼5000 Å. Thirteen CO vibrational bands in this absorber are detected: the $A^1\Pi$ − $X^1{\Sigma }^{+}$ (${\nu }^{\prime }$, 0) for ${\nu }^{\prime }=0\text{--}8$, $B^1{\Sigma }^{+}$ − $X^1{\Sigma }^{+}$ (0, 0), $C^1{\Sigma }^{+}$ − $X^1{\Sigma }^{+}$ (0, 0), and $E^1\Pi$ − $X^1{\Sigma }^{+}$ (0, 0) singlet–singlet bands and the $d^3\Delta$ − $X^1{\Sigma }^{+}$ (5, 0) singlet–triplet band. An updated database including the most precise molecular inputs needed for a μ-variation analysis is presented for rotational levels J = 0–5, consisting of transition wavelengths, oscillator strengths, natural lifetime damping parameters, and sensitivity coefficients to a variation of the proton-to-electron mass ratio. A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of $\Delta \mu /\mu =(0.7\pm {1.6}_{stat}\pm {0.5}_{syst})\times {10}^{-5}$ was derived from CO only. A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of $\Delta \mu /\mu =(-5.6\pm {5.6}_{stat}\pm {3.1}_{syst})\times {10}^{-6}$, which is consistent with no variation over a look-back time of ∼11.4 Gyr.