Constraining the cosmic-ray ionization rate and spectrum with NIR spectroscopy of dense clouds

Context. Low-energy cosmic rays (CRs) control the thermo-chemical state and coupling between gas magnetic fields in dense molecular clouds. However, current estimates of low-energy CR spectrum ( E ≲ 1 GeV) associated ionization rate are highly uncertain. Aims. We apply, for first time, a new method constraining spectral shape using H 2 rovibrational lines from cold Methods. Using MMIRS instrument on MMT, we obtained deep near-infrared (NIR) spectra six positions within four cores, namely, G150, G157, G163, G198, with column densities N ≈ 10 22 cm −2 . Results. derived 3 σ upper limits (1 − 0)S(0) line (2.22 μm) brightness range I = 5.9 × −8 to 1.2 −7 erg s −1 sr different targets. both an analytic model numerical propagation, convert these into clouds’ interior, ζ 1.5 3.6 −16 , lower slope interstellar protons, α −0.97 −0.79. show that while MMT was unable detect due high atmospheric noise, JWST/NIRSpec will be able efficiently CR-excited lines, making it ideal otherwise elusive CRs shedding light sources propagation modes CRs.