Infrarot-Interferometrische Untersuchungen der zirkumstellaren Materie und Multiplizität junger Sterne
Final Report Abstract
Many important processes related to the formation of stars and the evolution of their circumstellar disks, in which planets may form, occur on small spatial scales of just a few astronomical units (AUs) or less. Since almost all young stars are located as distances of more than 100 parsecs, the angular size of one AU is less than 100 milli-arcseconds, much less than the seeing-limit of about 1 arcsecond for conventional astronomical optical/infrared images and also less than the best possible, diffraction limited angular resolution of current telescopes (about 50 milli-arcseconds for an 8-m telescope observing at a wavelength of 2 micrometer). Therefore, the processes in the innermost regions of circumstellar disks can usually not be directly observed with single telescopes. The technique of Long-baseline Infrared Interferometry can overcome these problems and provide much higher angular resolution by combining the light of several telescopes coherently. The angular resolution that can be achieved does no longer depend on the size of the individual telescopes, but only on the distance between the coupled telescopes, the so-called baseline. Modern interferometers such as the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO) in Chile provide baselines up to 200 m, and can thus yield an angular resolution of as low as 2 milli-arcseconds (in the near-infrared H -band [1.6 µm]). This corresponds to physical scales of about 0.8 AU at the distance of the famous Orion Nebula Cluster (400 pc). Long-baseline infrared interferometric observations thus allow, for the first time, to directly study the formation region of terrestrial planets in proto-planetary disks, the way in which protostellar jets and outflows are launched and collimated, and also to resolve close multiple stellar systems. In this project, we have used the VLTI to observe the inner regions of the circumstellar disks around young stars in the near-infrared with the instrument AMBER and in the mid-infrared with the instrument MIDI. We also used AMBER to perform a survey for yet undetected very close companions to highmass stars in two very different star forming regions. In one of the sub-projects, we observed the embedded infrared source NGC 2264 IRS 1, a young stellar object with a high mass (≈ 10 M⊙ ). The formation mechanism of high-mass stars is not yet well understood; it is not clear whether they form similar as lower-mass stars, i.e. by accreting matter from a surrounding circumstellar disk, or whether fundamentally different mechanisms, such as protostellar collisions or interaction in close binary systems are involved. Our MIDI observations of NGC 2264 IRS 1 showed that the central star is most likely surrounded by a flat circumstellar disk similar to the disks found around lower mass young stellar objects. This result supports the assumption that massive young stellar objects can form via accretion from circumstellar disks. We also observed the young stellar object MWC 147, for which previous interferometric observations had shown that the bulk of the near-infrared emission does not come (as usually assumed) from the inner edge of the dusty disk around the star, but from some unknown source closer to the star. Until recently, the favored explanation for this discrepancy was to assume that hot gas close to the central star would produce this very compact emission. In an attempt to reveal the origin of this emission, we performed spectro-interferometric observations in order to search for the spectral signatures of hot molecular or atomic gas close to the star. Our data, however, did not show such signatures. This demonstrates that our understanding of the disk structure and accretion processes in young stellar objects is still incomplete, and needs refinement by further observational studies. In another part of the project we searched for very close, yet unresolved companions around complete samples of O- and B-type stars in the Orion Nebula Cluster and the Upper Scorpius OB association. Previous studies of these stars had already revealed a considerable number of companions at either very close (≤ 2 AU) or rather wide (≥ 20 AU) separations, but the intermediate separation region was unexplored until now. The VLTI is ideally suited to close this observational gap. Unfortunately, a large fraction of the VLTI observing time granted to us for this project was lost due to technical and weather problems, and therefore just a few new tentative companions could be found. Nevertheless, our data show that each of these high-mass stars has, on average, at least 2 companions. This is clearly much more than the average number of 0.5 companions per low-mass star.
Publications
-
2011, Mid-infrared interferometry of the massive young stellar object NGC 2264 IRS 1, Astronomy & Astrophysics, Volume 532, A109
R. Grellmann, T. Ratzka, S. Kraus, H. Linz, T. Preibisch, G. Weigelt
