Dr. Catherine C. Espaillat
Assistant Professor of Astronomy
Boston University


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Research

I am interested in understanding the origin of planets. We know that most, if not all, stars were once surrounded by protoplanetary disks. The details of how these disks evolve from initially well-mixed distributions of gas and dust to systems composed mostly of rocky planets and gas giants like our own solar system is a fundamental question in astronomy. It is widely accepted that dust grain growth and settling to the disk midplane are the first steps in creating the planetesimals that amalgamate into planets. Forming planets will then interact with the disk, clearing the material around themselves and leaving behind observable signatures in the disk in the form of gaps. I am pursuing a multi-wavelength analysis of full disks, pre-transitional disks, and transitional disks with the goal of expanding our knowledge of planet-forming disks.

Observational Evidence for Disk Clearing by Planets

Some of Spitzer's most notable observations were of disks which had large, inner holes, i.e., transitional disks (TDs). Using Spitzer, my team and I identified pre-transitional disks (PTDs), which have gaps rather than holes. This first clear identification of gaps in disks, pointed to planet formation and not to other inside-out clearing mechanisms, which have been proposed to explain TDs. This result established a connection between newborn planets and young disks, marking gapped disks as signposts for young planets.
Left: Observed (blue) and simulated (black) spectral energy distribution (SED) of the PTD LkCa 15 taken from Espaillat et al. (2011). The model consists of the stellar photosphere (magenta dotted), an inner disk (gray short-long-dash), and an outer disk (red dot-short-dash). Within the hole there is 0.0002 lunar masses of sub-micron-sized optically thin dust (green long-dash) that creates the strong 10 micron silicate emission feature. Right: Submillimeter continuum image of LkCa 15 from Andrews, Wilner, Espaillat et al. (2011) which confirms the presence of a clearing with a 50 AU radius. SED-inferred clearings have been confirmed with submillimeter imaging and are signatures of planet formation.


Observational Evidence for Variable Structure in Gapped Disks

More recently, my Spitzer IRS survey of disk variability revealed structural changes in PTDs that may be due to planet-induced perturbations. The flux in PTDs seesaws, meaning that the flux at shorter wavelengths varies inversely with the flux at longer wavelengths. SED modeling can explain this seesaw behavior by changing the height of the disk's inner edge (also known as the "wall") and hence, causing the shadowing on the outer disk to change as well. The detection of this seesaw-like variability is an independent confirmation of the gapped structure of PTDs.

Left top: Observed Spitzer IRS spectra (green and magenta) and models (solid and broken black) for the pre-transitional disk of UX Tau A from Espaillat et al. (2011). We can explain the variability between the two spectra by changing the height of the inner disk wall in our model by 17%.



Left bottom: Percentage change in flux between the two IRS spectra above. The observed variability cannot be explained by the observational uncertainties of IRS (error bars).





Schematic depicting the link between the observed variability and disk structure. Red corresponds to visible areas, light blue and dark blue areas are in the penumbra and umbra, respectively. The first and second cartoon apply to the green and magenta IRS spectra, respectively. Mid-infrared variability in PTDs can be explained by changes in the height of the inner disk wall, possibly due to planet-induced warps.

For press coverage see:
Planet Formation is Child's Play
Astronomers Discover Youngest Solar Systems Ever
Youthful Star Sprouts Planets Early
Cosmic Neon Lights the Way