[Paper Review] Searching for chameleon-like scalar fields with the ammonia method. II. Mapping of cold molecular cores in NH3 and HC3N lines

S. A. Levshakov, А. В. Лапинов+5 more|arXiv (Cornell University)|Aug 6, 2010

Astrophysics and Star Formation Studies56 references22 citations

TL;DR

This study maps cold molecular cores using high-resolution radio observations of NH₃ (1,1) and HC₃N (2–1) lines to test for variations in the electron-to-proton mass ratio (μ). It finds a persistent velocity offset of 26.9 ± 3.2 m s⁻¹ in L1498 and L1512, corresponding to Δμ/μ = (26 ± 3) × 10⁻⁹, suggesting a possible violation of local position invariance linked to chameleon-like scalar fields.

ABSTRACT

(Abridged) In our previous work we found a statistically significant offset Delta V = 27 m/s between the radial velocities of the HC3N(2-1) and NH3(1,1) transitions observed in molecular cores from the Milky Way. This may indicate that the electron-to-proton mass ratio, mu = m_e/m_p, increases by 3x10^{-8} when measured under interstellar conditions with matter densities of more than 10 orders of magnitude lower as compared with laboratory (terrestrial) environments. We now map four molecular cores L1498, L1512, L1517, and L1400K selected from our previous sample in order to estimate systematic effects in Delta V due to possible velocity gradients. We find that in two cores L1498 and L1512 the NH3(1,1) and HC3N(2-1) transitions closely trace the same material and show an offset of Delta V = 26.9 +/- 1.2_stat +/- 3.0_sys m/s throughout the entire clouds. The measured velocity offset, being expressed in terms of Delta mu = (mu_obs - mu_lab)/mu_lab, gives Delta mu = (26 +/- 1_stat +/- 3_sys)x10^{-9}.

Motivation & Objective

• To test the local position invariance principle by probing potential variation in the electron-to-proton mass ratio (μ) under low-density interstellar conditions.
• To investigate whether the previously observed velocity offset between HC₃N and NH₃ lines in molecular cores could be due to μ variation rather than systematic effects.
• To map four cold molecular cores (L1498, L1512, L1517B, L1400K) to assess spatial consistency and reproducibility of the velocity offset over time.
• To rule out astrophysical and instrumental systematic effects such as magnetic fields, electric fields, and cosmic microwave background radiation effects.
• To evaluate the sensitivity of the ammonia method for detecting variations in fundamental constants using high-resolution radio spectroscopy.

Proposed method

• Conducted high-spectral-resolution observations (FWHM ~30–40 m s⁻¹) of NH₃ (1,1) inversion and HC₃N (2–1) rotational transitions at the 100-m Effelsberg telescope.
• Used hyperfine structure components of NH₃ (1,1) to determine line centers with ~1 m s⁻¹ precision, matching laboratory uncertainty.
• Calculated velocity offset ΔV = V_{lsr}(HC₃N) – V_{lsr}(NH₃) to infer μ variation, assuming the inversion transition is more sensitive to μ than rotational transitions.
• Modeled and quantified systematic effects from external electric and magnetic fields, and cosmic blackbody radiation-induced Stark shifts.
• Applied error budgeting including statistical and systematic uncertainties (±1.2 stat ±3.0 sys m s⁻¹) to derive Δμ/μ with confidence intervals.
• Compared results across multiple epochs to test reproducibility of the velocity offset.

Experimental results

Research questions

• RQ1 Is the observed velocity offset between HC₃N and NH₃ lines in molecular cores consistent across different regions of the same core?
• RQ2 Can the observed velocity offset be explained by systematic effects such as magnetic fields, electric fields, or blackbody radiation?
• RQ3 Is the velocity offset reproducible over a year-to-year timescale at the same observatory?
• RQ4 Does the velocity offset imply a variation in the electron-to-proton mass ratio (μ) under low-density interstellar conditions?
• RQ5 Can the ammonia method detect μ variation with sufficient sensitivity to test chameleon-like scalar field models?

Key findings

• In L1498 and L1512, the velocity offset between HC₃N (2–1) and NH₃ (1,1) lines is ΔV = 26.9 ± 1.2_{stat} ± 3.0_{sys} m s⁻¹, with high reproducibility over time.
• The offset corresponds to Δμ/μ = (26 ± 1_{stat} ± 3_{sys}) × 10⁻⁹, indicating a possible increase in the electron-to-proton mass ratio in low-density interstellar environments.
• In L1517B and L1400K, the offsets are 46.9 ± 3.3_{stat} ± 3.0_{sys} m s⁻¹ and 8.5 ± 3.4_{stat} ± 3.0_{sys} m s⁻¹, respectively, likely due to kinematic structure or spatial segregation of molecules.
• Systematic effects from static electric and magnetic fields and cosmic blackbody radiation-induced Stark shifts are estimated to be less than 1 m s⁻¹, negligible in the error budget.
• The observed velocity offset is not attributable to instrumental or environmental effects, supporting its potential origin in μ variation.
• The results tentatively support the existence of chameleon-like scalar fields that could mediate μ variation in low-density regions, though further data are needed for confirmation.