Indications of electron-to-proton mass ratio variations in the Galaxy. II. 3 mm methanol lines toward Sgr B2(N) and (M)

TL;DR

This study tests potential spatial variations of the electron-to-proton mass ratio $\mu$ in the Galactic Center by measuring $\Delta\mu/\mu$ with methanol transitions. It analyzes CH$_3$OH torsion-rotation lines in the 3 mm band (80–112 GHz) toward Sgr B2(N) and Sgr B2(M) using IRAM 30-m data, comparing observed line centers to both laboratory rest frequencies $f_{lab}$ and theoretically calculated frequencies $f_{cal}$ for lines with known sensitivity coefficients $Q$. The main finding is a negative $\langle \Delta\mu/\mu \rangle$ of about $(-2.1 \pm 0.6)\times10^{-7}$ toward Sgr B2(N)$, consistent with prior high-frequency results, while the Sgr B2(M) result is not statistically significant due to limited line availability. The work demonstrates the viability of using methanol transitions to probe fundamental constant stability in the Galactic Center and highlights the need for higher spectral resolution and more precise laboratory measurements to solidify conclusions.

Abstract

Differential measurements of the fundamental constant mu = m_e/m_p (the electron-to-proton mass ratio) for two sources near the Galactic Center - the Sgr B2(N) and B2(M) molecular clouds - suggest that mu is lower in these clouds than its laboratory value. Based on observations of methanol (CH3OH) emission lines in the 80-112 GHz range (data from the IRAM 30-m telescope), a weighted mean value (Delta mu/mu) = (mu_obs - mu_lab)/mu_lab = (-2.1 +/- 0.6)*10^(-7) (1 sigma) was obtained for Sgr B2(N) at the sample size n = 9. This value of (Delta mu/mu) has the same sign as the result of recent measurements of methanol lines in the higher frequency range of 542-543 GHz (data from the Herschel space telescope) for Sgr B2(N): (Delta mu/mu) = (-4.2 +/- 0.7)*10^(-7) (sample size n = 2).

Figures (2)

• Figure 1: Selected methanol (CH$_3$OH) lines toward Sgr B2(N) and (M) obtained with the IRAM 30-m telescope in a main-beam temperature scale Bel. The original spectra are displayed by black dots, whereas the fitting curves are shown in red. The horizontal green lines indicate the ranges in which $\chi^2$ was calculated. The vertical blue bars mark the identified blends in accord with Figs. 2 and 5 from Bel. A weaker methanol emission from the second hot core N2 (offset by $6^{\prime\prime}$) is seen at about 75 km s$^{-1}$ in the Sgr B2(N) spectra.
• Figure 2: Comparison of peak relative intensities of observed lines (black) with theoretical ones (colored lines). The numbers correspond to the line numbers from Table \ref{['T1']}. The upper panel shows the relative line intensities in Sgr B2(N), normalized to the 86.615 GHz line (No.3), and on the lower panel -- for Sgr B2(M) -- relative to the 95.169 GHz line (No.8). The shaded areas show the confidence zones $\pm 1\sigma$, calculated with a maximum flux calibration error of 20% Bel.