The Clearing of Discs around Young Stars


Circumstellar discs in young stars provide the raw material - gas and
dust - for planet formation; discs also effect the radial
re-distribution of planets and thus play a major role in shaping
planetary architecture. Understanding the interplay between discs and
planet formation and migration therefore requires a good understanding
of how discs evolve and how they are dispersed. Currently, however, it
is still debated whether discs are self-consumed through planet
formation or whether planet formation occurs in competition with some
other agent for disc dispersal. Much of this talk is devoted to new
work that shows how the photoevaporation of gas from such discs by
Xrays from the central star is an important agent for disc dispersal.

The talk gives an overview of the disc properties that can be deduced
from multi-wavelength data, increasingly supplemented by imaging,
showing that our now good statistical knowledge of various stages of
disc clearing is largely thanks to large scale surveys conducted in
recent years by the Spitzer Space Telescope. I will briefly discuss
the several candidate disc clearance mechanisms and then describe the
theory of Xray photoevaporation in more detail, including also some
observational signatures and possible discriminants. I conclude with a
description of the bewildering zoo of "transition discs" (i.e. discs
with cleared inner regions) and discuss the extent to which they can
be understood in terms of clearing by photoevaporation / planet
formation.