A comparative study of the discs of young objects: towards an understanding of disc evolution and the first stages of planet formation
Final Report Abstract
In this project, we studied the dust and disc properties around young stars, in order to better understand the evolution of a disc as a natural by-product of the star formation process towards a protoplanetary disc and eventually a solar system analogue. As the dusty discs radiate thermally in the infrared, most of our analysis is based on infrared observations. In particular, we made use of the high sensitivity of the space observatory Spitzer to obtain IR spectra and photometry for a large sample of objects with a wide range in masses and ages. The advantage of such a large sample is that the derived properties of the dust and the discs can be related with the main parameters of the central stars, such as age, temperature or luminosity, and we can look for trends between the different properties. We found that the temperature of the central object is not crucial in determining the evolution of the dust in the disc, nor is its age. We do, however, see a trend, in which the dust grains that are located in the disc atmosphere grow in size and settle towards the disc mid-plane, herewith reducing the amount of flaring. We also discovered a spatial gradient of the dust composition in the discs of young stars, which points to the absence or substantial radial mixing. Finally, we found that the later the spectral type (the cooler the star), the larger the derived average grain size. We related this to the different areas that are seen in our mid-IR observations: for a cooler object, this area needs to be located closer to the star to reach the required temperature, than in a warmer object. If we then also assume that grains grow much faster in the inner regions (as they are denser), then it follows naturally that cooler objects show spectral features of larger grains. On the other hand, we made use of the unprecedented high spatial resolution obtained by combining the light of several telescopes, a technique called interferometry. With MIDI on the VLTI at ESO, it was possible for the first time to have a look at the innermost regions of circumstellar discs. We were able to put constraints on the shape and size of the discs around several Herbig Ae stars, these are young objects of a few solar masses. However, since this technique is still quite new, and only a few observational points are available, our results are quite limited, what we expect to change drastically in the future, when more data points will become available. In another part of our study, we concentrated on very lowmass objects, the so-called brown dwarfs, which are not massive enough to start hydrogen burning and are also considered as 'failed stars'. We discovered 2 brown dwarfs in the 625 Myr-old Hyades cluster, and were able to characterise them. We also derived spectral types for brown dwarfs in the young Orion Nebula Cluster. Furthermore, we could show that young brown dwarfs have discs with properties similar to the disc of young solarmass stars, and also with similar evolutionary patterns. This observation argues for a common formation mechanism of brown dwarfs and stars, and the possibility that even brown dwarfs can form planets in their discs.
Publications
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Getman, K.V., Flaccomio E., Broos P.S., et al., 2005, ApJS 160, 319: Chandra Orion Ultradeep Project: Observations and Source Lists
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Tamanai, A., Mutschke, H., Blum, J. and Meeus G., 2006, ApJ 648, 147: The 10 µm infrared band of silicate dust: A laboratory study comparing the aerosol and KBr pellet techniques
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Bouvier, J,, Kendall, T, T., Meeus, G. et al. 2008, A&A 481. 661: Brown dwarfs and very low mass stars in the Hyades cluster : a dynamically evolved mass function
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Correia, S., Köhler, R., Meeus, G., Zinnecker, H., 2008, POII conference, 175: First Evidence for a Spatially Resolved Disk Structure around the Herbig Ae Star R CrA
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de Wit W.J., Beaulieu J.P., Lamers H.J.G.L.M., Coutures C. and Meeus G., 2005, A&A, 432, 619: On the nature of pre-main sequence candidate stars in the Large Magellanic Cloud
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Meeus G., McCaughrean, M.J., 2005, AN, 326, 977: Using near-IR spectroscopy to classify substellar candidates in the Trapezium Cluster
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Preibisch, T., McCaughrean, M.J., Grosso, N. et al, 2005, ApJS 160, 582: X-Ray Emission from Young Brown Dwarfs in the Orion Nebula Cluster
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Scholz, A., Jayawardhana, R., Wood, K., Meeus G., et al., 2007, ApJ 660, 1517: Evolution of brown dwarf disks: A Spitzer survey in Upper Scorpius
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Schütz 0., Meeus G. and Sterzik M, 2005(b), A&A 431, 175: Mid-IR observations of circumstellar disks. Part II: Vega-type stars and a post-main sequence object.
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Schütz 0., Meeus G. and Sterzik M., 2005(a), A&A 431, 165: Mid-IR observations of circumstellar disks. Part I: Pre-main sequence objects.
