Practical Limitations on Astrophysical Observations of Methanol to Investigate Variations in the Proton-to-Electron Mass Ratio

Abstract

The possibility of using astrophysical observations of rotational transitions in the methanol molecule to measure, or constrain temporal and spatial variations in the proton-to-electron mass ratio (µ) has recently been investigated by several groups. Here we outline some of the practical considerations of making such observations, including both the instrumental and astrophysical limitations which exist at present. This leads us to conclude that such observations are unlikely to be able to improve evidence either for, or against the presence of variations in the proton-to-electron mass ratio by more than an order of magnitude beyond current limits.PACS numbers: 06.20. Jr, 98.38.Er, 98.58.Ec The possibility of using astrophysical observations of rotational transitions in the methanol molecule to measure, or constrain temporal and spatial variations in the proton-to-electron mass ratio (µ) has recently been suggested by several groups [1,2]. The first attempts to use astronomical observations of methanol masers to constrain µ within the Milky Way show that |∆µ/µ| < 28 × 10 −9 [2].The hindered internal rotation exhibited by methanol produces the degeneracy which makes the different rotational transitions particularly sensitive to the protonto-electron mass ratio [1]. It also leads to the rich rotational and vibrational spectrum, which due to quantum mechanical selection rules produces a large number of centimetre and millimetre maser transitions in interstellar space. The strongest and most common of these is the 5 1 → 6 0 A + transition which has a rest frequency of approximately 6.7 GHz. It has been detected towards more than 900 sites of high-mass star formation in the Milky Way [see for example 3, 4]. The second strongest astrophysical transition of methanol is from the 2 0 → 3 −1 E transition which has a rest frequency of approximately 12.2 GHz, and is observed towards 43% of those sources showing 6.7 GHz emission [5]. In most methanol maser regions multiple spectral features are observed in the spectra of 6.7 or 12.2 GHz methanol masers at different Doppler shifted velocities [3,4]. The spectral features typically have near-Gaussian profiles (although spectral blending between spatially unresolved components affects most observations), and FWHM (full-width half maximum) of a few tenths of kilometers per second.The K µ coefficient measures the sensitivity of a transition to the proton-to-electron mass ratio and for the 6.7 and 12.2 GHz methanol transitions these have been calculated to be -42 and -33 respectively [1]. To improve constraints on |∆µ/µ|, beyond those already achieved [2], requires measurement of the relative Doppler shifted velocities of the different transitions to an accuracy of better than 100 ms −1 . When the 6.7 and 12.2 GHz methanol transitions exhibit peaks at the same velocity they have been demonstrated to arise from the same locations at the milliarcsecond level (corresponding to linear scales of a few AU at distances of a few kpc) [6,7]. The large number of 6.7 and 12.2 GHz methanol...

An upper limit to the variation in the fundamental constants at redshiftz= 5.2

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Methanol absorption in PKS B1830−211 at milliarcsecond scales

“…However, the 12.2-GHz data were likely to have been affected by systematic errors, thus Kanekar et al (2015) also completed an analysis with just the high-frequency transitions to obtain a constraint of ∆µ/µ ≤ 4 × 10 −7 (2σ) over a look back time of ∼ 7.5 Gyr. Ellingsen et al (2011) discuss the major sources of uncertainty which limit the ability of astrophysical observations of methanol to measure (or constrain) changes in µ. Spatial segregation of molecular species can mimic or hide any true changes in µ. The excitation conditions differ for each of the transitions, hence at some level the gas responsible for the absorption of each transition will not be identical and hence will not be entirely co-spatial.…”

Class I methanol masers in NGC 253: Alcohol at the end of the bar

“…To determine if the rest frequency of a line has changed compared to the expectations from the laboratory we have to compare two different spectral lines which have different dependencies on the fundamental constants of physics. However, in general different spectral lines will trace different physical conditions and hence environments, which introduces additional uncertainties in the relative line of sight velocities (see Ellingsen et al 2011, for more detailed discussion).…”