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Register a new userEmpirical assessment of cosmic ray propagation in magnetized molecular cloud complexes
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Molecular clouds are complex magnetized structures, with variations over a broad range of length scales. Ionization in dense, shielded clumps and cores of molecular clouds is thought to be caused by charged cosmic rays (CRs). These CRs can also contribute to heating the gas deep within molecular clouds, and their effect can be substantial in environments where CRs are abundant. CRs propagate predominantly by diffusion in media with disordered magnetic fields. The complex magnetic structures in molecular clouds therefore determine the propagation and spatial distribution of CRs within them, and hence regulate their local ionization and heating patterns. Optical and near-infrared (NIR) polarization of starlight through molecular clouds is often used to trace magnetic fields. The coefficients of CR diffusion in magnetized molecular cloud complexes can be inferred from the observed fluctuations in these optical/NIR starlight polarisations. Here, we present calculations of the expected CR heating patterns in the star-forming filaments of IC 5146, determined from optical/NIR observations. Our calculations show that local conditions give rise to substantial variation in CR propagation. This affects the local CR heating power. Such effects are expected to be severe in star-forming galaxies rich in CRs. The molecular clouds in these galaxies could evolve differently to those in galaxies where CRs are less abundant.
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Marco Padovani Laboratoire den Radioastronomie Millimetrique
, Ecole Normale Superieure et Observatoiren de Paris
2013
Cosmic-rays constitute the main ionising and heating agent in dense, starless, molecular cloud cores. We reexamine the physical quantities necessary to determine the cosmic-ray ionisation rate (especially the cosmic ray spectrum at E < 1 GeV and the ionisation cross sections), and calculate the ionisation rate as a function of the column density of molecular hydrogen. Available data support the existence of a low-energy component (below about 100 MeV) of cosmic-ray electrons or protons responsible for the ionisation of diffuse and dense clouds. We also compute the attenuation of the cosmic-ray flux rate in a cloud core taking into account magnetic focusing and magnetic mirroring, following the propagation of cosmic rays along flux tubes enclosing different amount of mass and mass-to-flux ratios. We find that mirroring always dominates over focusing, implying a reduction of the cosmic-ray ionisation rate by a factor of 3-4 depending on the position inside the core and the magnetisation of the core.
Cosmic rays pervade the Galaxy and are thought to be accelerated in supernova shocks. The interaction of cosmic rays with dense interstellar matter has two important effects: 1) high energy (>1 GeV) protons produce {gamma}-rays by {pi}0-meson decay; 2) low energy (< 1 GeV) cosmic rays (protons and electrons) ionize the gas. We present here new observations towards a molecular cloud close to the W51C supernova remnant and associated with a recently discovered TeV {gamma}-ray source. Our observations show that the cloud ionization degree is highly enhanced, implying a cosmic ray ionization rate ~ 10-15 s-1, i.e. 100 times larger than the standard value in molecular clouds. This is consistent with the idea that the cloud is irradiated by an enhanced flux of freshly accelerated low-energy cosmic rays. In addition, the observed high cosmic ray ionization rate leads to an instability in the chemistry of the cloud, which keeps the electron fraction high, ~ 10-5, in a large fraction (Av geq 6mag) of the cloud and low, ~ 10-7, in the interior. The two states have been predicted in the literature as high- and low-ionization phases (HIP and LIP). This is the observational evidence of their simultaneous presence in a cloud.
We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-$Lstar$ starburst, and $Lstar$ galaxies with different propagation models, including advection, isotropic and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two moment method. We forward-model and compare to observations of $gamma$-ray emission from nearby and starburst galaxies. We reproduce the $gamma$-ray observations of dwarf and $Lstar$ galaxies with constant isotropic diffusion coefficient $kappa sim 3times 10^{29},{rm cm^{2},s^{-1}}$. Advection-only and streaming-only models produce order-of-magnitude too large $gamma$-ray luminosities in dwarf and $Lstar$ galaxies. We show that in models that match the $gamma$-ray observations, most CRs escape low-gas-density galaxies (e.g. dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for $gamma$-ray emissivities. Models where CRs are ``trapped in the star-forming disk have lower star formation efficiency, but these models are ruled out by $gamma$-ray observations. For models with constant $kappa$ that match the $gamma$-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc.
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 the 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 $1~{rm K}$ 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.
The local cosmic-ray (CR) spectra are calculated for typical characteristic regions of a cold dense molecular cloud, to investigate two so far neglected mechanisms of dust charging: collection of suprathermal CR electrons and protons by grains, and photoelectric emission from grains due to the UV radiation generated by CRs. The two mechanisms add to the conventional charging by ambient plasma, produced in the cloud by CRs. We show that the CR-induced photoemission can dramatically modify the charge distribution function for submicron grains. We demonstrate the importance of the obtained results for dust coagulation: While the charging by ambient plasma alone leads to a strong Coulomb repulsion between grains and inhibits their further coagulation, the combination with the photoemission provides optimum conditions for the growth of large dust aggregates in a certain region of the cloud, corresponding to the densities $n(mathrm{H_2})$ between $sim10^4$ cm$^{-3}$ and $sim10^6$ cm$^{-3}$. The charging effect of CR is of generic nature, and therefore is expected to operate not only in dense molecular clouds but also in the upper layers and the outer parts of protoplanetary discs.
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