Evolution of Circumstellar Disks: Lessons from the VLT and ISO

摘要

There is strong evidence that the planets in the solar system evolved from a disk-shaped solar nebula 4.56 Gyr ago. By studying young stars in various evolutionary stages, one aims at tracing back the early history of the solar system, in particular the timescales for disk survival and for the formation of planetary systems. We used the VLT & ISAAC and ESA's Infrared Space Observatory & ISOCAM to study the circumstellar environment of young low-mass stars. ISOCAM observations show a steady decrease in the amount of infrared excess from <2 Myr old T Tauri stars to post-T Tauri stars with ages around 10 Myr. The diminished infrared excess can be explained by changes in the global dust opacities due to, e.g., grain growth on timescales of 5 to 15 Myr. Follow-up studies in the infrared with high-spectral resolution using, e.g., CRIRES, could probe and (weigh in) the gaseous disks around the post-T Tauri stars by searching for H2 features seen in absorption against the stellar photosphere. In a second program we employed VLT & ISAAC to search for spatially resolved circumstellar disks around young stellar objects (YSOs) in southern star forming regions. The project aimed at establishing a sample suitable for follow-up studies with even higher angular resolution using adaptive optics at the VLT (e.g., NAOS & CONICA). Two disks seen close to edge-on could be identified. A comparison to HH 30 confirms theoretical predictions that a slight change in the viewing angle of a disk leads to a dramatic difference in the spectral energy distribution of YSOs. This highlights the importance of spatially resolved observations. Detailed images of the distribution of scattered light and polarization maps combined with theoretical models will enable us to determine physical properties of young disks, such as disk geometry, density structure, or dust properties. Finally, we used VLT & ISAAC for a near infrared study of the low-mass population in the starburst cluster NGC 3603. The absence of intrinsic infrared excess in the majority of star indicates a paucity of circumstellar disks. Hence extreme environments like starburst clusters might not be fostering the formation of planetary systems.