Observations of “Gas-Rich” Disks

Abstract

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 the mystery of the formation of our Solar System. This chapter focuses on observational studies 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 typical star forming regions (e.g., ∼140pc for Taurus), a planetary system like ours (with diameter ≃ 50AU out to Pluto, but excluding the Kuiper belt) 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 I will attempt to give an overview of the structural and physical parameters of protoplanetary disks that can be estimated today from direct observations.