Cosmic rays in molecular clouds probed by H${}_2$ rovibrational lines -- Perspectives for the James Webb Space Telescope

Abstract

Cosmic rays (CRs) at sub-TeV energies play a fundamental role in the chemical and dynamical evolution of molecular clouds, as they control the ionisation, dissociation, and excitation of H${}_2$. Their characterisation is important both for the interpretation of observations and for the development of theoretical models. The methods used so far for estimating the CR ionisation rate ($\zeta$) in molecular clouds have several limitations due to uncertainties in the adopted chemical networks. We refine and extend the method proposed by Bialy (2020) to estimate $\zeta$ by observing rovibrational transitions of H${}_2$ at near-infrared wavelengths, which are mainly excited by secondary CR electrons. Combining models of interstellar CR propagation and attenuation with the calculation of the expected secondary electron spectrum and updated H${}_2$ excitation cross sections by electron collisions, we derive the intensity of the four H${}_2$ rovibrational transitions observable in dense, cold gas: (1-0)O(2), (1-0)Q(2), (1-0)S(0), and (1-0)O(4). The proposed method allows the estimation of $\zeta$ for a given observed line intensity and H${}_2$ column density. We are also able to deduce the shape of the low-energy CR proton spectrum impinging upon the molecular cloud. We present a look-up plot and a web-based application that can be used to constrain the low-energy spectral slope of the interstellar CR proton spectrum. We comment on the capability of the James Webb Space Telescope to detect these near-infrared H${}_2$ lines, making it possible to derive for the first time spatial variation of $\zeta$ in dense gas. Besides the implications for the interpretation of the chemical-dynamic evolution of a molecular cloud, it will be possible to test competing models of CR propagation and attenuation in the interstellar medium, as well as compare CR spectra in different Galactic regions.

arXiv comment14 pages, 11 figures, accepted by A&A

References in corpus (54)

• The Interstellar Environment of our Galaxy
• Structure of Stationary Photodissociation Fronts
• Cosmic-ray ionization of molecular clouds
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• The cosmic-ray electron flux measured by the PAMELA experiment between 1 and 625 GeV
• Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
• Structure and Evolution of the Envelopes of Deeply Embedded Massive Young Stars
• The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation
• Cosmic-ray ionisation in circumstellar discs
• Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• Protostars: Forges of cosmic rays?
• Impact of low-energy cosmic rays on star formation
• Interstellar dust charging in dense molecular clouds: cosmic ray effects
• Cosmic-ray ionisation in collapsing clouds
• Imprints of Cosmic Rays in Multifrequency Observations of the Interstellar Emission
• Gravity Driven Magnetic Field at ~1000 au Scales in High-mass Star Formation
• SOLIS II. Carbon-chain growth in the Solar-type protocluster OMC2-FIR4
• Cosmic-ray acceleration in young protostars
• Effects of magnetic fields on the cosmic-ray ionization of molecular cloud cores
• Three-fluid plasmas in star formation II. Momentum transfer rate coefficients
• Analytic HI-to-H2 Photodissociation Transition Profiles
• CO/H2, C/CO, OH/CO, and OH/O2 in Dense Interstellar Gas: From High Ionization to Low Metallicity
• Magnetic field in a young circumbinary disk
• Ionization fraction and the enhanced sulfur chemistry in Barnard 1
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation
• The Herschel-PACS legacy of low-mass protostars: Properties of warm and hot gas and its origin in far-UV illuminated shocks
• The Interaction of Cosmic Rays with Diffuse Clouds
• ALMA resolves the hourglass magnetic field in G31.41+0.31
• Exploration of Cosmic Ray Acceleration in Protostellar Accretion Shocks and A Model for Ionization Rates in Embedded Protoclusters
• Cosmic ray penetration in diffuse clouds
• SOLIS IV. Hydrocarbons in the OMC-2 FIR 4 region, a probe of energetic particle irradiation of the region
• Production of atomic hydrogen by cosmic rays in dark clouds
• Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption
• Gamma rays as probes of cosmic-ray propagation and interactions in galaxies
• On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
• Diffusive Versus Free-Streaming Cosmic Ray Transport in Molecular Clouds
• Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)
• Stochastic Fluctuations of Low-Energy Cosmic Rays and the Interpretation of Voyager Data
• Survey of ortho-H${}_2$D${}^{+}$ in high-mass star-forming regions
• Magnetic Mirroring and Focusing of Cosmic Rays
• A new proxy to estimate the cosmic-ray ionisation rate in dense cores
• Chemical Abundances in a Turbulent Medium -- H${}_2$, OH${}^{+}$, H${}_2$O${}^{+}$, ArH${}^{+}$
• Cold Clouds as Cosmic-Ray Detectors
• Rigorous theory for secondary cosmic-ray ionization
• Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium
• Synchrotron emission in molecular cloud cores: the SKA view
• Formation Pumping of Molecular Hydrogen in Dark Clouds
• Constraining the cosmic-ray ionization rate and their spectrum with NIR spectroscopy of dense clouds -- A test-bed for JWST
• H${}_2$ ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?
• Ionization toward the high-mass star-forming region NGC 6334 I
• The double signature of local cosmic-ray acceleration in star-forming regions
• CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

Cited by in corpus (32)

• Low energy cosmic rays
• Magnetically regulated collapse in the B335 protostar? II. Observational constraints on gas ionization and magnetic field coupling
• First ALMA maps of cosmic ray ionisation rate in high-mass star-forming regions
• Probing the physics of star formation (ProPStar): I. First resolved maps of the electron fraction and cosmic-ray ionization rate in NGC 1333
• Constraining the cosmic-ray ionization rate and their spectrum with NIR spectroscopy of dense clouds -- A test-bed for JWST
• Constraints on Dark Matter-Electron Scattering from Molecular Cloud Ionization
• Dependence of Chemical Abundance on the Cosmic Ray Ionization Rate in IC 348
• Astrochemical models of interstellar ices: History matters
• Abundance ratios of OH/CO and HCO+/CO as probes of the cosmic ray ionization rate in diffuse clouds
• Parsec-scale cosmic-ray ionisation rate in Orion
• Cosmic-ray-induced H${}_2$ line emission: Astrochemical modeling and implications for JWST observations
• A new analytic approach to infer the cosmic-ray ionization rate in hot molecular cores from HCO${}^{+}$, N${}_2$H${}^{+}$, and CO observations
• Dust evolution during the protostellar collapse: influence on the coupling between the neutral gas and the magnetic field
• Constraints on the non-thermal desorption of methanol in the cold core LDN 429-C
• Cosmic-ray ionisation rate in low-mass cores: the role of the environment
• The impact of cosmic ray heating on the cooling of the low-metallicity interstellar medium
• The first detection of cosmic-ray excited H${}_2$ in interstellar space
• Formation of ${\text{H}}_2$ on polycyclic aromatic hydrocarbons under conditions of the ISM: an ab initio molecular dynamics study
• The dynamical impact of cosmic rays in the Rhea magnetohydrodynamics simulations
• The evolution of CH in Planck Galactic Cold Clumps
• Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds
• Predicting HCN, HCO${}^{+}$, multi-transition CO, and dust emission of star-forming galaxies -- Extension to luminous infrared galaxies and the role of cosmic ray ionization
• Shocked, Heated, and Now Resolved: H${}_2$ excitation in the low-luminosity AGN at M58 core with JWST
• Chemical templates of the Central Molecular Zone. Shock and protostellar object signatures under Galactic Center conditions
• The fragmentation of molecular clouds in starburst environments
• The impact of attenuation on cosmic-ray chemistry: I. Abundances and chemical calibrators in molecular clouds
• SILCC -- IX. The multi-phase interstellar medium at low metallicity
• Numerical model of fast electron energy deposition in interstellar molecular gas
• Imladris: a detailed and flexible model for galaxy simulations with individual stars
• Are Stellar Embryos in Perseus Radio-Synchrotron Emitters? Statistical data analysis with Herschel and LOFAR paving the way for the SKA
• The contribution of neutral gas to Faraday tomographic data at low frequencies. A first extensive comparison between real and synthetic data
• Magnetic clumping of charged dust in the dense interstellar medium