Tuesday, 26 September 2017

Compositions of Circumstellar Disks and Delivery of Volatiles to Terrestrial Planets

Background: Dynamical modeling suggests that the water content of Earth-like planets in the habitable zone depends strongly on the architecture of the planetary system and the disk's volatile content as a function of radius (Chambers 2003). The planetesimals and smaller bodies in exosolar disks provide the raw material for delivery of volatiles to any terrestrial planets also encircling those stars. Surfaces of planetesimals are also processed by ultraviolet radiation from the central star. When these processed planetesimals collide, their surfaces may be stripped (Asphaug et al. 2006) and secondary collisions will grind the stripped material into small grains. These debris, i.e. dust grains, can be detected by telescopes in tenuous disks around other stars.

We measure the composition of circumstellar dust by spectroscopy of scattering or emission from the grains. Mid-infrared spectra of small grains in protoplanetary and transitional disks reveal various silicates and/or polycyclic aromatic hydrocarbons and can sometimes be spatially resolved (Weinberger et al. 2003). Dust composition, grain size distribution, and grain shape also affect the spectrum of scattered light. For example, KBOs show strong evidence for micron-sized water or methane ices and organics as part of dusty regoliths (e.g. Cruikshank et al. 1998, see also Section 5.2.1). Scattered light observations can detect materials, such as organics, not emitting at longer wavelengths as well as confirm the presence silicates or ices seen in the mid-IR.

Research to Date: Co-I Weinberger has led the efforts to produce the first visible light spectrum of a protoplanetary disk (Roberge et al. 2005) and the first multi-wavelength spectrophotometry of a debris disk (Figure 4; Debes et al. 2008). In the case of the protoplanetary disk around TW Hydrae, the scattered light spectrum was blue to neutral. The disk appears to have large grains and high optical depth all the way out to 120 AU. Close to the star, the scattering is somewhat blue, perhaps indicating Rayleigh scattering from small grains or the presence of an olivine absorption band. In the case of the debris disk around HR 4796A, 0.5 - 2.2 micron photometry showed a very red reflection spectrum similar to the Solar System Centaur 5145 Pholus (Figure 1). Complex organics, modeled as tholins, a mixture of polymers that share properties with the atmospheric haze of Titan, are necessary to reproduce such a very red slope.

Spitzer spectroscopy of dust emission has shown that protoplanetary disks are rich in processed silicates and ices, but gas-free debris disks consist of large grains that do not provide useful compositional information (Chen et al. 2006). A few extreme debris disks appear to result from late collisions, perhaps akin to the Late Heavy Bombardment, and produce many small dust grains that are tightly constrained in location by planetary systems (Beichman et al. 2005, Weinberger et al. 2008).

Proposed Work: Co-I Weinberger will lead an observational program to determine disk compositions. She will use the Hubble Space Telescope for spectroscopy and multi-color imaging to measure disk reflectivity over broad wavelength regions. The longer the wavelength region covered by the observations, the more diagnostic of the grain composition and size they will be. We can discriminate the competing effects of grain size and mineralogy (silicates versus ices versus organics) using measurements of scattering efficiency as a function of wavelength. Most disks have been imaged at only one or two wavelengths (see the review in Meyer et al. 2007 and references therein). Broader wavelength coverage demonstrates that disks with the same, e.g. V-J colors, have different spectra and hence different compositions. These tantalizing results became possible only when we greatly extended the number of wavelengths at which disks had been imaged. We can at least double the number of resolved disks with detailed spectral information.

The James Webb Space Telescope should launch at the end of the time period covered by this CAN. We will make predictions for the 3-5 micron fluxes from the disks in preparation for their imaging and spatially resolved spectroscopy with JWST; this region should be very diagnostic of both water ice and organics.

Our compositional modeling is hampered by the limited availability of measured absorption and scattering efficiencies (indices of refraction). For example, tholins are the only organic material for which such data is available, and they are a highly complex mixture. Weinberger will work with Co-I Conel Alexander to enlarge the number of materials found in the Solar System that would be good proxies for those detected remotely (see Section 5.2.2).