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

Abstract

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 H2rovibrational 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 ofNH2 ≈ 1022cm−2.Results.We derived 3σupper limits on the H2(1 − 0)S(0) line (2.22 μm) brightness in the rangeI = 5.9 × 10−8to 1.2 × 10−7erg cm−2s−1sr−1for 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 × 10−16s−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 H2lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H2lines, making it the ideal method for constraining the otherwise elusive low-energy CRs and shedding light on the sources and propagation modes of CRs.