Astrophysical and cosmological diffusions

Par Claire Chevalier

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 13:00 à 13:45

Diffusion phenomena play an important role in theoretical astrophysics and cosmology. For instance, neutron diffusions in the early universe have affected abundances of elements such as D, He or Li. Another example is the diffusion of neutrinos inside the core of a neutron star, which has an impact on the star evolution. Diffusions generated by quantum fluctuations of the gravitational field in the cosmological framework are also studied by several authors.

The simplest models of relativistic diffusions are relativistic stochastic processes. A very general class of relativistic stochastic processes will be introduced, which are based on generalizations of the Langevin equation. Particular processes of the classe will be displayed; models of cosmological diffusions will notably be presented, along with extended fluctuation-dissipation relations satisfied by some of these models.

The ionization fraction in ISM clouds

Par Javier R. Goicoechea

Salle D18 (2nd floor) at the ENS, 24 rue Lhomond, 13:00 à 13:45

In this Journal Club talk I will describe how chemistry and observations of molecular ions can be used to determine the ionization fraction* in molecular clouds. I will show some models and observations of the Horsehead Nebula.

* i.e., the abundance of electrons.

Exotic cold clouds in the diffuse ISM

Par Snezana Stanimirovic (Uni of Wisconsin-Madison)

Salle D18 (2e étage) à l'ENS, 24 rue Lhomond, 13:00 à 13:45

While on spatial scales >1 pc we can trace the entire hierarchy of structures in the diffuse interstellar medium (ISM), the extremely small-scale end of this spectrum (tens of AUs) is still not understood. Cold HI clouds on scales down to 10-100 AU have been sporadically detected for the last three decades, suggesting the existence of highly over-pressured and over-dense blobs of gas which are totally puzzling from the traditional ISM point of view.

I will discuss properties of these exotic cold HI clouds traced through time variability of HI absorption profiles against pulsars and very sensitive HI absorption observations of continuum sources. In particular, I will focus on two issues. First, what do we know about the ubiquitousness of these clouds and the conditions conducive to their formation and maintenance. Second, what are these (AU-scale) measurements telling us about the turbulent dissipation scales in the cold neutral ISM.