Accretion disks are pivotal elements in the formation and early
evolution of solar-like stars. On top of supplying the raw material,
their internal conditions also regulate the formation of planets.
Their study therefore holds the key to solve this long standing
mystery: how did our Solar System form? This chapter focuses on
observational studies of the circumstellar environment, and in
particular of circumstellar disks, associated with pre-main sequence
solar-like stars. The direct measurement of disk parameters poses an
obvious challenge: at the distance of the typical star forming regions
~140 pc for Taurus), a planetary system like ours (with
diameter ≅50 AU out to Pluto, but excluding the Kuiper belt
which could extend much farther out) subtends only 0.35''. Yet
its surface brightness is low in comparison to the bright central star
and high angular and high contrast imaging techniques are required if
one hopes to resolve and measure these protoplanetary disks.
Fortunately, capable instruments providing 0.1'' resolution or better
and high contrast have been available for just about 10 years
now. They are covering a large part of the electromagnetic spectrum,
from the UV/Optical with HST and the near-infrared from ground-based
adaptive optics systems, to the millimetric range with long-baseline
radio interferometers. It is therefore not surprising that our
knowledge of the structure of the disks surrounding low-mass stars has
made a gigantic leap forward in the last decade.
In the following pages we will attempt to describe, in a historical
perpective, the road that led to the idea that most solar-like stars
are surrounded by an accretion disk at one point in their early life
and how, nowadays, their structural and physical parameters can be
estimated from direct observations. We will follow by a short
discussion of a few of the constraints available regarding the
evolution and dissipation of these disks. This last topic is
particularly relevant today to understand the mechanism leading to the
formation of planets.