Cosmic Ray Propagation in Molecular Clouds

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• Kedron Silsbee (Max-Planck-Institut für extraterrestrische Physik)
• Monday 26 November 2018, 14:00-15:00
• MR14, Centre for Mathematical Sciences, Wilberforce Road, Cambridge.

If you have a question about this talk, please contact Cleo Loi.

I will give a brief overview of the acceleration and propagation of cosmic rays in our Galaxy, and their effects on molecular clouds. I will then focus on the interaction of low energy cosmic rays with molecular clouds. Cosmic rays are the dominant source of ionisation in dense molecular clouds. Understanding the cosmic ray ionisation rate is therefore a necessary input to simulations of the gas dynamics (as the strength of non-ideal MHD effects depends on the ionisation), and to interpreting observations of molecules in these clouds. Unfortunately there is uncertainty on the factor of 10 level in the cosmic ray ionisation rate in these regions. This arises from both uncertainty in the abundance of the sub-relativistic cosmic rays responsible for the majority of the ionisation, as well as uncertainty in the propagation. I will then discuss two areas I have been working on. First, is the role of magnetic mirroring and focussing in the propagation of cosmic rays in molecular clouds. I will show that these typically almost exactly cancel out, but if there are regions of low magnetic field surrounded by higher field, then the ionisation rate can be significantly reduced. Finally, there is increasing observational evidence to suggest that the decrease in the ionisation rate with increasing column density is steeper than would be expected given our current understanding. I will discuss a couple of ideas to explain this apparent steeper-than-expected attenuation of cosmic rays.

This talk is part of the DAMTP Astrophysics Seminars series.

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