2022A&A...658L..13B

Context. Low-energy cosmic rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds. However, current estimates of the low-energy CR spectrum (E ≤ 1 GeV) and the associated CR ionization rate are highly uncertain.
Aims. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H₂ rovibrational lines from cold molecular clouds.
Methods. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores, namely, G150, G157, G163, G198, with column densities of N_H2 ≃ 10²² cm^–2.
Results. We derived 3σ upper limits on the H₂ (1 - 0)S(0) line (2.22 µm) brightness in the range I = 5.9 x 10^–8 to 1.2 x 10^–7 erg/cm²/s/sr for the different targets. Using both an analytic model and a numerical model of CR propagation, we convert these into upper limits on the CR ionization rate in the clouds' interior, ζ = 1.5 to 3.6 x 10^–16 s^–1, and lower limits on the low-energy spectral slope of interstellar CR protons, α = -0.97 to -0.79. We show that while MMT was unable to detect the H₂ lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H₂ lines, making it the ideal method for constraining the otherwise elusive low-energy CRs and shedding light on the sources and propagation modes of CRs.