Constraining the cosmic-ray ionization rate and spectrum with NIR spectroscopy of dense clouds. A testbed for JWST

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-2-1 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 NH2-1022 cm-2. Results. We derived 3ÏA? upper limits on the H2 (1-0)S(0) line (2.22 μm) brightness in the range I-=-5.9-ÃA - -10-8 to 1.2-ÃA - -10-7 erg cm-2 s-1 sr-1 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-ÃA - -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.