The origin of disks surrounding young stars has direct implications for our understanding of the formation of planetary systems. In the interstellar clouds from which star form, angular momentum is regulated by magnetic fields, preventing the spin up of contracting cores. When ∼ 0.03 pc-sized dense cores decouple from the magnetic field and collapse dynamically, ∼ 10−3 km s−1 pc of specific angular momentum is locked into the system. A viscous accretion disk is one of two possible mechanisms available for the necessary redistribution of angular momentum; the other one is the formation of a multiple stellar system. Recent observational results involving high-angular resolution observations are reviewed: the presence of disks deep inside collapsing envelopes; an accretion shock surrounding a disk; the velocity field in collapsing and slowly rotating envelopes; a possible transitional object, characterized as a large, contracting disk; and the velocity field in disks around T Tauri stars. Observational facilities becoming available over the next several years promise to offer significant progress in the study of the origin of protoplanetary disks.

As the number of detected extrasolar planetary systems has steadily grown over the past five years, so too has the number of circumstellar disks with resolved images. In this contribution, we take stock of the current inventory of disk images in the optical and infrared; summarize the results of a new disk imaging survey conducted with the Hubble Space Telescope; review the major inferences that can be drawn about disk structure from the extant images; and suggest areas for future progress.

I briefly review progress in high angular resolution observations of disks at millimetre wavelengths, with a focus on dust continuum observations of the disks surrounding low mass pre-main-sequence stars. I summarize the utility of observations at millimetre wavelengths, where dust emission is largely optically thin, and I touch on several recent directions, including imaging surveys using interferometers, testing physical models of disks with resolved observations, and the complexities introduced by stellar multiplicity.

We show that there exists a simple geometric picture for the geometries of protoplanetary disks around Herbig Ae/Be stars that explains the two main kinds of spectral energy distributions found for these objects, and that makes predictions that are qualitatively in agreement with currently available spatially resolved images and/or interferometric measurements. Also it qualitatively explains the phenomenon of UX Orionis variability.

A variety of processes play a role in the evolution of protostellar disks. Here I focus on the uncertain issue of magnetic field-disk coupling and its implications for magnetically-driven turbulence and disk-driven winds. At present it is clear that the magnetic field plays a crucial role in disk evolution, but detailed conclusions cannot be drawn because the complicated interplay between dynamics and the evolution of the grain population remains to be explored.

We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.

Protoplanetary disks are now intensively observed by mm arrays. While resolved images of CO rotation lines permit a better understanding of their physical structure, molecular surveys provided by current mm telescopes are currently sensitivity limited and do not allow a quantitative analysis of the chemical properties of disks. In this paper, I review the actual observational knowledge of the chemistry in the outer disks surrounding low and intermediate PMS stars and traced by mm data.

The circumstellar environment within 10 AU of young stars are of particular interest for star and planet formation. Unfortunately, present imaging facilities such as the Hubble Space Telescope or adaptive optics on 10-m telescopes cannot resolve this region. We have proved that “spectro-astrometry” is a powerful technique for discovering pre-main-sequence binaries, determining kinematics of outflows and providing evidence for gaps in circumstellar disks — all down to AU scales. In this paper, we summarise our progress to date.

We present subarcsecond observations at 2.7 and 1.4 mm of a sample of massive young stellar objects made with the BIMA millimetre array. For most sources the continuum emission on the smallest scales at 2.7 mm is dominated by free-free emission from the stellar wind or jet. Strong emission at 1.4 mm shows the presence of significant dust associated with Cep A and GL 490 but our resolution is not sufficient to resolve any structure. The 2.7-mm emission from GL 490 is resolved but it is not clear whether we are seeing a single circumstellar disk or a secondary companion, although near-infrared data support the disk hypothesis. Estimates of the dust mass yield values of ∼1–4 M⊙ within radii of 150 to 1000 AU.

Intermediate to late stages of star formation are characterized by rising infrared and visible luminosities, with much of the most interesting physics concentrated in the inner regions within the immediate vicinity of the central protostar(s). Long-baseline optical and infrared interferometry is ideally suited to study of such bright, compact structures. Contributions from presently operational arrays, which are capable of resolving the brightest few dozen, will be reviewed. These results presage the potential for further dramatic advances in our understanding of star formation with observations from more advanced instruments now being built.

Possible ways to detect extra-solar planets in circumstellar disks using high-angular resolution measurements (imaging) are discussed. Since dust is expected to outshine the planet by scattering of stellar radiation and thermal emission, the most promising approach is to look for characteristic signatures in the circumstellar disk caused by the interaction with the planet.

The Advanced Camera for Surveys (ACS) offers a coronagraphic imaging mode with angular resolution of 0.026″pixel−1. In combination with with the appropriate subtraction of reference star point spread functions (PSF) the coronagraph is capable of achieving contrast ratios of ∼1000. We present some of the first ACS observations of the optically thin debris disk HD141569A and discuss new results from these observations.

The Low Frequency Array (LOFAR), the Atacama Large Millimetre Array (ALMA), and the Square Kilometre Array (SKA) will be the largest and most capable aperture synthesis facilities of the next few decades. The advances in our knowledge of star formation that ALMA will permit are well known to the star formation community, but the potential of the SKA has so far attracted less attention. I will outline the star formation science cases of these upcoming facilities, with an emphasis on the SKA.

This presentation summarizes how some of the most pressing questions in the field of star and planet formation can be addressed by high angular resolution optical/infrared capabilities, and how many of these capabilities will in fact be available with realization of the space and ground facilities currently being planned for the 2005-2020 time frame.

I give a personal summary of IAU-Symposium 221 “Star Formation at High Angular Resolution”, including general impressions and scientific highlights (both observations and theory). Some future directions, like infrared interferometry and advanced SPH numerical simulations, are described. In passing, I also make some critical comments about information overload and the dangers of decreasing information transfer efficiency in the age of power-point.

Listed below are the titles and authors for the poster presentations at the Symposium. Abstracts for these papers, and in some cases the posters themselves, can be obtained from the Symposium website, www.phys.unsw.edu.au/iau221. For each entry, the first number is the display order of the posters, as listed in the Abstract book for the 25th IAU General Assembly. The second number listed on each line is the IAU GA Reference number, assigned on submission of the abstract, and as listed on the above website.

The complete program of oral presentations delivered at IAU Symposium 221 is given below, including additional events outside the conference sessions. Session titles, together with their chair(s) are listed, followed by the title of each talk in the session, and its presenter. The codes referring to the different categories of talks are described at the end of the program. Abstracts for all talks can be found on the Symposium website, http://www.phys.unsw.edu.au/iau221.