My Research

Here I highlight my recent research projects.

See the full inventory of my research on ADS or arXiv.org

Warm debris disks likely arise from exo-asteroid belts

Warm debris disks have been identified around a number of main sequence stars, but their origin has remained a mystery. Does this dust arise from collisions among larger objects in an exo-asteroid belt? Or is this warm material delivered inward from an cold outer reservoir of material (an exo-Kuiper belt)? Fortunately, we can discern between these two hypothesis by examining the relation between the location of the warm debris and the mass of the star it orbits. The locations of exo-asteroid belts are set by the snow-line (where water transitions from vapor to ice) in the primordial protoplanetary disk, and this location obeys a shallow relation with stellar mass. On the other hand, the location of material delivered inward is set by the present-day snow line, which follows a steeper relation with stellar mass. I found that warm disks follow the trend predicted for the primordial snow-line, and thus they most likely indicate the presence of exo-asteroid belts.

The beta Pic debris disk is composed of silicates plus organic refractory material

Debris disk dust can be measured in two ways: from the thermal radiation it emits and by the starlight it scatters. The inability to match both thermal and scattered light observations simultaneously has been a persistent problem in debris disk modeling. I investigated whether this problem could be solved by varying the composition of the dust, which is often simply assumed to be astronomical silicates. I applied this investigation to the bright, edge-on debris disk around beta Pictoris. I fit model images to the observations at five wavelengths: two in scattered light from the Hubble Space Telescope and three in thermal emission from the Spitzer Space Telescope, the Herschel Space Observatory, and ALMA. I found that all five images could be fit with dust consisting of a nearly-equal mixture of silicates and refractory organic material, whereas icy or porous grains were not favored.

 

See all the details of this project in our ApJ paper.

Probing exozodiacal dust via sililcate emission features

Exozodiacal dust (exozodi) is the dust in the terrestrial (and habitable) zones around other stars. Exozodi is useful for studying rocky exoplanet formation, but it is also a source of noise for future missions to image Earth-like exoplanets. At a temperature of ~300 K, exozodi emits at ~10 microns. However, a detection of an infrared excess at this wavelength does not necessarily mean exozodi is present, because the location and temperature of dust are degenerate with the size of the grains. Warm, small, silicate grains exhibit distinctive solid state emission features in the mid-IR. I searched the Spitzer/IRS spectra of stars with debris disks and discovered previously unknown silicate features in 22 systems. With detailed fitting to the shapes of these features, I was able to break the degeneracy and accurately locate the dust. I found that the dust resided in the terrestrial zones of these systems, confirming the presence of exozodi. There may be undetected silicate features in the spectra of additional disks that could be observed with the improved sensitivity of JWST, providing a means to measure low-levels of exozodi in the near future.

See all the details of this project in our ApJ paper.

What do cold debris disk reveal about the outer regions of planetary systems?

I analyzed the Spitzer/IRS spectra of 546 main-sequence stars in search of excess infrared emission arising from circumstellar debris disks. This was the largest sample of debris disks studied with IRS at the time. I identified 174 disks with cold components (analogous to the Kuiper belt in the solar system). With the unprecedented size of the sample, I could search for new correlations between the debris disk properties and the properties of their host stars. I discovered a trend in the temperatures of the cold components with the temperatures of their stars; that is, hotter, earlier-type stars had warmer cold debris components. The temperature of a debris disk is an indicator of its orbital location. The locations of cold debris disks may be set by the presence of planets, as suggested by systems with planets imaged between warm and cold debris components. Do cold debris disks mark the outer boundaries of giant planet formation? This hypothesis predicts a correlation in the temperature of cold debris disks with stellar type, thus, the trend I discovered lends support to this hypothesis.

 

See our ApJ paper for the full details of this project.

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