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 Delta mu/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 similar to 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) similar to 1.00 +/- 0.15, a density of n(H-2) = 3 x 10(5) 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(H-2) 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.