2017MNRAS.472.4434D

The dependence of the proton-to-electron mass ratio, µ, on the local matter density was investigated using methanol emission in the dense dark cloud core L1498. Towards two different positions in L1498, five methanol transitions were detected and an extra line was tentatively detected at a lower confidence level in one of the positions. The observed centroid frequencies were then compared with their rest-frame frequencies derived from least-squares fitting to a large data set. Systematic effects, as the underlying methanol hyperfine structure and the Doppler tracking of the telescope, were investigated and their effects were included in the total error budget. The comparison between the observations and the rest-frame frequencies constrains potential µ variation at the level of Δµ/µ < 6 x 10^–8, at a 3σ confidence level. For the dark cloud, we determine a total CH₃OH (A+E) beam averaged column density of ∼3-4 x 10¹² cm^–2 (within roughly a factor of two), an E- to A-type methanol column density ratio of N(A-CH₃OH)/N(E-CH₃OH) ∼1.00 ± 0.15, a density of n(H₂) = 3 x 10⁵ cm^–3 (again within a factor of two) and a kinetic temperature of T_kin = 6 ± 1 K. In a kinetic model including the line intensities observed for the methanol lines, the n(H₂) density is higher and the temperature is lower than that derived in previous studies based on different molecular species; the intensity of the 1₀ - 1_–1 E line strength is not well reproduced.