24 research outputs found
Diffusive Versus Free-Streaming Cosmic Ray Transport in Molecular Clouds
Understanding the cosmic ray (CR) ionization rate is crucial in order to
simulate the dynamics of, and interpret the chemical species observed in
molecular clouds. Calculating the CR ionization rate requires both accurate
knowledge of the spectrum of MeV to GeV protons at the edge of the cloud as
well as a model for the propagation of CRs into molecular clouds. Some models
for the propagation of CRs in molecular clouds assume the CRs to stream freely
along magnetic field lines, while in others they propagate diffusively due to
resonant scattering off of magnetic disturbances excited by MHD turbulence
present in the medium. We discuss the conditions under which CR diffusion can
operate in a molecular cloud, calculate the local CR spectrum and ionization
rate in both a free-streaming and diffusive propagation model, and highlight
the different results from the two models. We also apply these two models to
the propagation through the ISM to obtain the spectrum seen by Voyager 1, and
show that such a spectrum favors a diffusive propagation model.Comment: Submitted to Ap
Magnetic Mirroring and Focusing of Cosmic Rays
We study the combined impact of magnetic mirroring and focusing on the
ionization by cosmic rays (CRs) in dense molecular clouds and circumstellar
disks. We show that for effective column densities of up to
cm (where ionization is the main mechanism of energy losses by CRs) the
two effects practically cancel each other out, provided the magnetic field
strength has a single peak along field lines. In this case the ionization rate
at a given location is controlled solely by attenuation of interstellar CRs due
to energy losses. The situation is very different in the presence of magnetic
pockets -- local minima of the field strength, where the CR density and thus
ionization can be reduced drastically. We obtain simple analytical expressions
allowing accurate calculation of the ionization rate in these regions.Comment: Accepted to Ap
Rapid Elimination of Small Dust Grains in Molecular Clouds
We argue that impact velocities between dust grains with sizes less than
in molecular cloud cores are dominated by drift arising from
ambipolar diffusion. This effect is due to the size dependence of the dust
coupling to the magnetic field and the neutral gas. Assuming perfect sticking
in collisions up to m/s, we show that this effect causes rapid
depletion of small grains - consistent with starlight extinction and
IR/microwave emission measurements, both in the core center (
cm) and envelope ( cm). The upper end of the size
distribution does not change significantly if only velocities arising from this
effect are considered. We consider the impact of an evolved dust size
distribution on the gas temperature, and argue that if the depletion of small
dust grains occurs as would be expected from our model, then the cosmic ray
ionization rate must be well below s at a number density of
cm