Observations of thermal processing in protoplanetary disks

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

The temperature distribution is not only a function of radius, but also depends on the stellar luminosity, the disk geometry, and may depend on the accretion rate (see Table 8.1 and Section 3.3) for example, at a given radius irradiated flared disks will be warmer than flat disks. Naturally, hotter stars will heat their disks to higher temperatures at a given radius thus, mid-infrared spectroscopy probes different radii in different disks. 

Sensitive observations enabled comparative surveys of the brightest disks around intermediate-mass and Sun-like stars. These observations revealed a wealth of different silicate emission features, many of them with sharp peaks of crystalline silicates. The motivation of these surveys was to use crystallinity as a proxy for the evolution of the dust component as well as for the overall disk. Indirectly, these are thought to be linked to the formation of rocky planets. 

Surprisingly, the early ground-based surveys could not find any correlation between the presence of crystalline silicates and the key stellar and disk parameters (e.g. van Boekel et al. 2003 Honda et al. 2006). An inherent difficulty of recognizing such correlations in the spectra of intermediate-mass stars is the ubiquitous presence of the sharp polycyclic aromatic hydrocarbon (PAH) peaks, often contaminating the crystalline silicate peaks. 

More sensitive space-based studies with Spitzer helped secure identification of crystalline silicate peaks in PAH-free spectra of Sun-like stars, resolving this problem (e.g. Sargent et al. 2006 Bouwman et al. 2008 Watson et al. 2009). These studies, however, could all but reinforce the lack of strong correlations with age, 

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Piecing together the observations of thermal processing in protoplanetary disks reveals several promising avenues for future studies. We identify the following topics as those most needed to understand the thermal processing of protoplanetary materials ... 

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