Stringent bounds to spatial variations of the electron-to-proton mass ratio in the Milky Way

The ammonia method, recently proposed by Flambaum and Kozlov (2007) to probe variations of the electron-to-proton mass ratio, μ=me/mp, is applied for the first time to dense prestellar molecular clouds in the Milky Way, allowing to test Δμ/μ at different galactocentric distances. High quality radio-astronomical observations are used to check the presence of possible relative radial velocity offsets between the inversion transition of NH3(J,K)=(1,1), and the CCS JN=21−10 and N2H+J=1−0 rotational transitions. Carefully selected sample of 21 NH3/CCS pairs observed in the Perseus molecular cloud provide the offset . A similar offset of between NH3(J,K)=(1,1) and N2H+J=1−0 has been found in an isolated dense core L183 by Pagani et al. (2009).Overall these observations provide a safe bound of a maximum offset between ammonia and the other molecules at the level of . Being interpreted in terms of Δμ/μ, this bound corresponds to Δμ/μ⩽1×10−7, which is an order of magnitude more sensitive than available extragalactic constraints. Taken at face value the measured ΔV shows positive shifts between the line centers of NH3 and these two other molecules and suggest a real offset, which would imply a Δμ/μ∼4×10−8. If Δμ/μ follows the gradient of the local gravitational potential, then the obtained results are in conflict with laboratory atomic clock experiments in the solar system by ∼5 orders of magnitude, thus requiring a chameleon-type scalar field model. New measurements involving other molecules and a wider range of objects along with verification of molecular rest frequencies are currently planned to confirm these first indications.