Testing the variability of the proton-to-electron mass ratio from observations of methanol in the dark cloud core L1498

TL;DR

This study uses methanol emission observations in dark cloud L1498 to test for potential variations in the proton-to-electron mass ratio, constraining its change at a very precise level and providing detailed physical conditions of the cloud.

Contribution

It presents the first constraint on mu variation from methanol in a dark cloud, including systematic effects, and refines physical parameters of L1498 using methanol line data.

Findings

Mu variation constrained to Dmu/mu < 6 x 10^(-8) at 3 sigma

Determined methanol column density and ratio with uncertainties

Derived physical conditions of L1498, including density and temperature

Abstract

The dependence of the proton-to-electron mass ratio, mu, 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 mu variation at the level of Dmu/mu < 6 x 10^(-8), at a 3 sigma confidence level. For the dark cloud we determine a total CH3OH (A+E) beam averaged column density of 3-4 x 10^(12) 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 10^5 cm^(-3) (again within a factor of two), and a kinetic temperature of Tkin = 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 1_0 --> 1_-1 E line strength is not well reproduced.