Constraining the cosmic-ray ionization rate and their spectrum with NIR spectroscopy of dense clouds -- A test-bed for JWST

Abstract

Low-energy cosmic-rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds, the sites of star-formation. However, current estimates of the low-energy CR spectrum ($E\lesssim 1$ GeV) and the associated CR ionization rate are highly uncertain. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H${}_2$ rovibrational lines from cold molecular clouds. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores, G150, G157, G163, G198, with column densities $N_{{\mathrm{H}}_2}\approx 10^{22}$ cm${}^{-2}$. We derive 3$\sigma$ upper limits on the H${}_2$ $(1-0)$S(0) line (2.22 $\mu$m) brightness in the range $I=5.9\times 10^{-8}$ to $1.2\times 10^{-7}$ 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, $\zeta =1.5$ to $3.6\times 10^{-16}$ s${}^{-1}$, and lower limits on the low-energy spectral slope of interstellar CR protons, $\alpha =-0.97$ to $-0.79$. We show that while MMT was unable to detect the H${}_2$ lines due to high atmospheric noise, JWST/NIRSpec will be able to efficiently detect the CR-excited H${}_2$ lines, making it the ideal method for constraining the otherwise elusive low-energy CRs, shedding light on the sources and propagation modes of CRs.

accepted for publication in A&A Letters

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• A new analytic approach to infer the cosmic-ray ionization rate in hot molecular cores from HCO${}^{+}$, N${}_2$H${}^{+}$, and CO observations
• Cosmic-ray-induced H${}_2$ line emission: Astrochemical modeling and implications for JWST observations
• The first detection of cosmic-ray excited H${}_2$ in interstellar space
• The evolution of CH in Planck Galactic Cold Clumps
• Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds
• Gauging the Impact of Cosmic Ray Feedback on the Stellar Initial Mass Function