Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane⋆
Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029
2 Ioffe Physical-Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia
3 St. Petersburg Electrotechnical University “LETI”, Prof. Popov Str. 5, 197376 St. Petersburg, Russia
4 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
5 Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
6 INAF – Istituto di Radio Astronomia, via P. Gobetti 101, Bologna, Italy
7 INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
8 Centro de Astrofsica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
Received: 26 August 2013
Accepted: 7 October 2013
Aims. We aim to validate the Einstein equivalence principle (local position invariance) by limiting the fractional changes in the electron-to-proton mass ratio, μ = me/mp, measured in Galactic plane objects.
Methods. 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 = Vrot − Vinv, between the rotational and inversion transitions, which have different sensitivities to the value of μ.
Results. In our previous observations (2008–2010), a mean offset of ⟨ΔRV⟩ = 0.027 ± 0.010 km s-1 (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 ~0.02 km s-1 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 ± 0.018 km s-1 (3σ C.L.). This value, when interpreted in terms of Δμ/μ = (μobs − μlab)/μlab, constraints the μ-variation at the level of Δμ/μ < 2 × 10-8 (3σ C.L.), which is the most stringent limit on the fractional changes in μ based on astronomical observations.
Key words: line: profiles / ISM: molecules / radio lines: ISM / techniques: radial velocities / elementary particles
© ESO, 2013