2013A&A...559A..91L

We aim to validate the Einstein equivalence principle (local position invariance) by limiting the fractional changes in the electron-to-proton mass ratio, µ=m_e/m_p, measured in Galactic plane objects. High-resolution spectral observations of dark clouds in the inversion line of NH₃(1, 1) and pure rotational lines of other molecules (the so-called ammonia method) were performed at the Medicina 32-m and the Effelsberg 100-m radio telescopes to measure the radial velocity offsets, ΔRV=V_rot-V_inv, between the rotational and inversion transitions, which have different sensitivities to the value of µ. In our previous observations (2008-2010), a mean offset of <ΔRV≥0.027±0.010km/s (3σ confidence level (C.L.)) was measured. To test for possible hidden errors, we carried out additional observations of a sample of molecular cores in 2010-2013. As a result, a systematic error with an amplitude ∼0.02km/s in the radial velocities was revealed. The averaged offset between the radial velocities of the rotational transitions of HC₃N(2-1), HC₅N(9-8), HC₇N(16-15), HC₇N(21-20), and HC₇N(23-22), and the inversion transition of NH₃(1, 1) is <ΔRV≥0.003±0.018km/s (3σ C.L.). This value, when interpreted in terms of Δµ/µ=(µ_obs-µ_lab)/µ_lab, constraints the µ-variation at the level of Δµ/µ<2x10^–8 (3σ C.L.), which is the most stringent limit on the fractional changes in µ based on astronomical observations.