Observational Signatures of Cosmic-Ray Interactions in Molecular Clouds

Abstract

We investigate ionization and heating of gas in the dense, shielded clumps/cores of molecular clouds bathed by an influx of energetic, charged cosmic rays (CRs). These molecular clouds have complex structures, with substantial variation in their physical properties over a wide range of length scales. The propagation and distribution of CRs is thus regulated accordingly, in particular, by the magnetic fields threaded through the clouds and into the dense regions within. We have found that a specific heating rate reaching 10 −26 erg cm −3 s −1 can be sustained in the dense clumps/cores for Galactic environments, and this rate increases with CR energy density. The propagation of CRs and heating rates in some star-forming filaments identified in IC 5146 are calculated, with the CR diffusion coefficients in these structures determined from magnetic field fluctuations inferred from optical and near-infrared polarizations of starlight, which is presumably a magnetic field tracer. Our calculations indicate that CR heating can vary by nearly three orders of magnitude between different filaments within a cloud due to different levels of CR penetration. The CR ionization rate among these filaments is similar. The equilibrium temperature that could be maintained by CR heating alone is of order 1 K in a Galactic environment, but this value would be higher in strongly star-forming environments, thus causing an increase in the Jeans mass of their molecular clouds.

New Constraints on Cosmic Particle Populations at the Galactic Center using X-ray Observations of the Molecular Cloud Sagittarius B2

“…In this section, we probe 6.4 keV line emissions from different cloud regions, toward understanding LECR populations within Sgr B2 in the context of both ambient LECRs in the CMZ and LECR production in the cloud. The problem of CR transport into and within MCs is model dependent and has been the subject of several theoretical works with different predictions for the geometry of energy deposition by LECRs, all of which necessarily make simplifying assumptions about the gas distribution and magnetic field structure, for either a generic cloud or Sgr B2 specifically (Dogiel et al 2015;Morlino & Gabici 2015;Gabici 2013;Owen et al 2021).…”

CRAFT (Cosmic Ray Acceleration From Turbulence) in Molecular Clouds

“…Within the Milky Way, cosmic-ray transport studies and observations have indicated an average diffusion coefficient between D 0 = 10 28 -3 × 10 28 cm 2 s −1 (Evoli et al 2020). However, regarding the dense gas, studies have shown a spread over several orders of magnitude, from D 0 = 10 27 -10 30 cm 2 s −1 (Dogiel et al 2015;Owen et al 2021). As such, it is even more paramount to understand what is constraining the CR transport within molecular gas, and how the local environment changes the diffusion coefficient.…”

Empirical assessment of cosmic ray propagation in magnetized molecular cloud complexes

“…The choice of spectral model is discussed in detail in [6], however its exact form (if reasonable) does not substantially affect the results of this work. The diffusive term ∇ • [𝐷 (𝐸, 𝒔)∇𝑛] is governed by the coefficient 𝐷 (𝐸, 𝒔) which depends on the gyro-scattering radius (or frequency) of the CRs of energy 𝐸 in their local magnetic field, as well as turbulence and magnetohydrodynamical (MHD) perturbations along local magnetic field vectors.…”

“…Here 𝜆 1 = 𝜆 td (|𝜔 L |/𝜔 p,0 ) is the CR resonant scattering length scale parallel to the background magnetic field line. The turbulent decay length scale is 𝜆 td ≈ 𝑣 A 𝜏 td , (where we use 𝜏 td = 2 Myrfor discussion, see [6]). The CR gyro-frequency is 𝜔 L , with a sign convention set by the charge (in units of proton charge).…”

“…5]. In this work, we consider an empirical propagation and heating model of CRs in MC environments, which was first introduced in [6]. This accounts for CR cooling and interaction mechanisms relevant to CR species (protons and electrons) in a self-consistent manner.…”