Observations of Debris Disks

The focus of my previous research projects have been optical- and submillimeter observations of debris disks. In one study we observed optical light scattered from dust grains in nearby debris disks (chosen from IRAS, ISO, and Spitzer IR-excess samples) using the polarimetric coronagraph PolCor-2, developed at Stockholm Observatory. The instrument used a high-speed CCD in conjunction with a coronagraphic technique, where the direct light from the star is blocked out and the stellar point-spread-function wings are suppressed, making the faint scattered optical light from the disk detectable. By also inserting a polarizing filter in different orientations, the angle and degree of polarization can be calculated and used to further enhance the contrast. Our high-contrast imaging of low surface brightness features successfully captured the AU Mic debris disk and produced interesting results for the controversial circumbinary disk around BD+31°643 (Olofsson et al. 2012).

In another project we performed a submillimeter survey of a sample of IR-excess main-sequence stars with the LABOCA bolometer array, observing at 870 μm on the APEX telescope, to determine the existence, extent, mass, and evolution of Kuiper Belt-like disk structures. Studies at mid- to far infrared wavelengths have shown excess emission in the spectral energy distribution (SED) of many young main-sequence stars, indicating warm circumstellar dust extending out to some tens of astronomical units (AU). Probing more extended, cold dust components, in Kuiper-Belt analogues reaching out to some hundreds of AU, requires sensitive submillimeter observations. Results from a precursor study of the β Pictoris Moving Group were presented in Nilsson et al. (2009). Out of the observed sample of stars in the larger survey, we detected 10 submillimeter disks with at least a 3σ significance, increasing the number of currently known exo-Kuiper-Belts (inferred from cold extended dust disks detected in the submillimeter) from 27 to 32 (Nilsson et al. 2010).

A third project involved spectroscopic observations of the disk around the young (∼12 Myr old) star β Pictoris. We imaged the disk in three emission lines using integral-field spectroscopy at the Very Large Telescope, and obtained the first complete image of Fe Ⅰ emission (probing the neutral gas in the disk plane) and Ca Ⅱ (probing vertically more extended gas) (Nilsson et al. 2012). We concluded that the paucity of mid-plane Ca Ⅱ emission can be explained by optical thickness, and does not require the Ca gas to be spatially separated from the Fe gas, as previously suggested.

Much of this work was summarized in my PhD thesis.