לוח שנה משולב

I will describe new analytic theory for the atomic to molecular (HI-to-H_2) transitions, and the build-up of atomic-hydrogen (HI) gas columns in star-forming clouds, based on fundamental physics principles. I will discuss my general-purpose formula for the total HI columns produced by photodissociation in optically thick media, valid for beamed or isotropic radiation fields, the weak- to strong-field limits, gradual to sharp HI-to-H_2 transitions, and for arbitrary metallicity. The analytic theory is validated with detailed numerical radiative transfer computations. The general theory may be broadly used for interpreting 21 cm studies of individual Galactic sources and global galaxy properties, and may also be incorporated into hydrodynamics simulations.

The origin of Solar System satellites is actively debated. We know understand that, despite the morphological analogy between a satellite system and a planetary system, the formation processes of satellites may be significantly different from planetary formation processes. in addition, satellites evolve quickly under the effects of tides. Different scenarios seem to be required for different types of planets (terrestrial, giant or ice giant). In this talk I will quickly review our current understanding of satellite formation and the different constrains. Based on Cassini images and numerical simulation, I will show that there is today on-going accretion processes at the edge of Saturn's rings, pointing to a new satellite formation process. I will show that satellite formation may be deeply linked to the evolution of planetary rings, to the point that it is very probable that most of Solar System’s regular satellites may have born inside rings, either massive, like the protolunar disk,or light, like giant planet’s rings. I will illustrate this vividly in the case of Saturn using numerical simulations and CASSINI images. Case of Ice giants, Mars and Jupiter will be also discussed. Some extension to the case of exoplanets’ regular satellites will be attempted.