Influence of Dust Grain Evolution on the Structure of Protoplanetary Disks.

Abstract

he formation and composition of planets is a direct consequence of the processing of solid dust particles in protoplanetary disks: e.g. grain growth, dust settling, crystallization, and segregation of different dust species. Understanding the connections between these effects and how they vary as a function of time is the first step to producing a map of how planet-forming materials are distributed in disks, providing initial conditions for planet formation and evolution models. These will be necessary to analyze the composition and migration history of increasingly large numbers of confirmed exoplanets.

Here I present near-, mid-, and far-infrared observations of young protoplanetary disks and their surroundings to identify when grain processing starts and how far it proceeds in the first 1-2Myr, by which time planet formation is observed. Using Spitzer IRS 5-40um spectra, I construct an extinction curve for molecular clouds, which I use to measure dust processing in IRS spectra of the youngest disks (<1Myr) in the Ophiuchus star-forming region. I then develop a method, using 1-5um (NASA IRTF SpeX), to extract the inner disk excess from these systems and determine the dust properties of that region, finding strong evidence for increased grain growth and settling in the inner disk relative the the outer disk. Fitting this excess using radiative transfer disk structure models suggests a grain-size limit of ~10um in the midplane due to accretion heating in the inner 0.5AU. Iron-rich dust was required to fit the inner disk excess.

In Herschel PACS spectra, I detect water ice originating in the disk upper layers, below the photodesorption layer. At half-solar abundance, these detections indicate settling of icy grains. There is evidence of radial structure in the snowline in some of these disks, which combined with the implications of the vertical iron gradient in the inner disk suggests strong spatially variable gas compositions. This has implications for the type of molecules expected to be detectable in the atmospheres of planets forming there. Further work, including interferometry and scattered light observations, are necessary to fully map the distribution of planetary building blocks in these disks at all ages.