Limits on the spatial variations of the electron-to-proton mass ratio in the Galactic plane^⋆

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: lev@astro.ioffe.rssi.ru
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

Abstract

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 = m_e/m_p, 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 = V_rot − V_inv, 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 = (μ_obs − μ_lab)/μ_lab, 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.