Place: Room FB52, AlbaNova University Centre, Roslagstullsbacken 21, Stockholm
Time: Friday, 2003 December 19, 13:00.
Opponent: Dr Charles Beichman, Chief Scientist, Jet Propulsion Laboratory

Summary. To understand star formation in detail has proven to be a challenge, largely due to the difficulties in observing the masses of young stars directly. In the first part of this thesis, observations with telescopes using adaptive optics are presented, a modern technique that counteracts the blurring normally caused to light passing the Earth atmosphere. By taking advantage of the new possibilities opened by sharper observations, the nearest environments around young stars are probed. Discoveries of several companions to young stars are reported. The significance in finding these close companions lies in the possibility of determining the dynamical mass of a young star, by following the orbital motion. A few higher-order multiple systems are found as well, giving important clues to the mechanisms of star formation.

The recent discoveries of planets around other stars than the Sun have revealed planets very different from those in the solar system. This has arisen the question of how and when these kind of planetary systems are formed, since they were not predicted by standard planet formation theory. It is known that planets are formed out of the circumstellar disks of gas and dust that surround young stars. In the second part of this thesis, observations of the most well-known circumstellar disk, around the nearby young star ß Pictoris, are reported and discussed. An image obtained in radio wavelengths, at 1.2 mm, shows the dust disk to be strongly asymmetric, with the (thermal) emission coming from the south-western part dominating over the north-eastern part. This emission asymmetry is opposite to that observed in scattered light at optical wavelengths, where the north-eastern side shows the strongest emission. Our conclusion is that the properties of the dust grains differ between the two sides; the small, bright grains of the north-eastern side scatter light at optical wavelengths efficiently, while the bigger (~1 mm), dark dust grains of the south-western side preferably emit thermal emission at radio wavelengths.

The discovery of extended emission from circumstellar gas around ß Pictoris is reported. To determine the properties of the gas in this disk, at the border between the gas-rich, planet-forming disks around T Tauri stars and the gas poor around mature stars at the main sequence, is particularly important for the understanding of the time scale of giant planet formation. The gas is observed with an echelle spectrograph with a long slit, featuring both high spatial and high spectral resolution. The velocity field of the disk, observed by the Doppler shift of the narrow emission lines from the gas, is consistent with a disk in Keplerian rotation. 88 lines from 13 metallic ions are seen in emission extending to the limits of our observations, corresponding to a projected radial distance from the star of 323 AU, and a height of 80 AU above the disk plane. Many of these ions are subject to a strong radiation pressure from ß Pictoris, and should quickly accelerate out of the system if no invisible force restrained their departure. Two different mechanisms for braking the gas are investigated in detail: braking caused by friction against an invisible dense surrounding gas (consisting of, e.g., Hydrogen or Oxygen), and braking by a magnetic field. Because the radiation pressure depends on the ion, a non-cosmic composition of the gas is predicted.