Probing the electron-to-proton mass ratio gradient in the Milky Way with Class I methanol masers

Abstract

We estimate limits on non-universal coupling of hypothetical hidden fields to standard matter by evaluating the fractional changes in the electron-to-proton mass ratio, μ = me/mp, based on observations of Class I methanol masers distributed in the Milky Way disc over the range of the Galactocentric distances 4 ≲ R ≲ 12 kpc. The velocity offsets ΔV = V44 - V95 measured between the 44- and 95-GHz methanol lines provide, so far, one of the most stringent constraints on the spatial gradient kμ d(Δμ/μ)/dR < 2 × 10-9 kpc-1 and the upper limit on Δμ/μ <2 × 10-8, where Δμ/μ = μobs-μlab)/μlab. We also find that the offsets ΔV are clustered into two groups which are separated by δΔV = 0.022 ± 0.003 km s-1 (1σ confidence level). The grouping is most probably due to the dominance of different hyperfine transitions in the 44- and 95-GHz methanol maser emission. Which transition becomes favoured is determined by an alignment (polarization) of the nuclear spins of the four hydrogen atoms in the methanol molecule. This result confirms that there are preferred hyperfine transitions involved in the methanol maser action.