The building blocks of planets within the ‘terrestrial’ region of protoplanetary disks

Abstract

Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System.

Episodic formation of cometary material in the outburst of a young Sun-like star

“…Longward of this peak no other crystalline features are present in the spectrum. The observed crystalline features are similar to those present in comet spectra 3,14 (panel d) and in a number of protoplanetary disks 15,16 . The remaining differences between EX Lupi and Since the quiescent spectrum exhibits no crystalline silicate features, the appearance of crystalline features in the EX Lupi outburst strongly suggests that we witness on-going crystal formation; to our knowledge for the first time in a celestial object.…”

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets

“…Crovisier et al, 1997) and circumstellar disks (e.g. Malfait et al, 1998;Van Boekel et al, 2004;Kessler-Silacci et al, 2006) are indeed formed in the inner part of the accretion disk rather than being annealed interstellar grains (e.g. Bockelee-Morvan et al, 2002).…”

Circumstellar disks and planets

“…One finds that the surface of the inner disk has larger and more crystalline (crystallinity fraction of 40% to 100%) silicate grains than that of the outer part, being smaller and more amorphous. Remarkably enough, such a property holds for a large range of stellar masses, from the intermediate Herbig Ae/Be stars (van Boekel et al, 2004) to less massive T Tauri stars (Ratzka et al, 2007;Schegerer et al, 2008Schegerer et al, , 2009). …”

“…Particularly, such a technique has enabled to unveil its vertical structure as well as the composition of its surface layer. By observing a sample of Herbig Ae/Be stars with the MIDI/VLTI instrument, Leinert et al (2004) have measured the radius of the 10 µm emission region of the whole young stars and compared them with their mid-IR color. It was shown that the more red objects were displaying more extended emission than that of the more blue sources.…”

“…Given the typical distance of nearby star-forming regions of ∼140-200 pc and the spatial resolution achievable with the Very Large Telescope Interferometer (VLTI) in the 8-13 µm atmospheric window of 10-20 mas, these instruments are best suited to study the planet-forming region in circumstellar disks. One of the first exciting results achieved with MIDI was to show the difference in the dust grain evolution between the "inner" disk (represented by the correlated flux) and the outer disk (net flux), using the low-resolution spectroscopy observing mode (van Boekel et al, 2004). However, given the intrinsic limitations of the analysis of single-baseline observations (interpretation of visibilities, such as in the case of MIDI), one has only weak constraints on the structure and size of the emitting region: The interpretation is based on models, which in turn are weakly constrained in the inner disk region because of the lack of adequate observations since the main constraints are given at best by midinfrared SEDs in a few cases.…”