PROTOPLANETARY DISK DEMOGRAPHICS WITH ALMA
My thesis with Jonathan Williams focused on conducting the first large-scale, high-sensitivity (sub-)mm surveys of protoplanetary disks that are capable of placing statistical constraints on the evolution of fundamental disk properties and planet-formation timescales. While the Kepler mission opened the field of exoplanet statistics, similar demographic surveys of the preceding protoplanetary disks have been limited by the sensitivity and resolution of (sub-)mm arrays, our best tools for observing these cold and faint objects. Fortunately, the recently commissioned Atacama Large Millimeter Array (ALMA) is overcoming these observational barriers. I used ALMA to observe the protoplanetary disk populations in Lupus and σ Orionis, two star-forming regions at distinct stages of disk evolution. I measured total dust and gas masses, which are fundamental disk properties that strongly influence the subsequent planetary outcomes, yet remain poorly understood on a population level, until now.
Our first ALMA survey (Ansdell et al. 2016c) observed ~100 protoplanetary disks in Lupus, a star-forming region that is only 1-3 Myr old, thereby serving as a benchmark for disk properties at early times. We found that only ~30% of disks at this early stage have sufficient dust masses to form giant planet cores, suggesting that giant planet formation is either rare or rapid -- the former being more consistent with exoplanet statistics. In our follow-up ALMA survey of Lupus (Ansdell et al. 2018b), we focused on disk size, showing for the first time that disk gas radii are universally larger than disk dust radii at (sub-)mm wavelengths, suggesting that the inward radial drift of dust is a common process in protoplanetary disks.
Our second ALMA survey (Ansdell et al. 2017) observed another ~100 protoplanetary disks, this time in the more evolved (3-5 Myr) σ Orionis cluster. A key finding from this work was the impact of external photoevaporation driven by the central O9V star, σ Ori: disk dust masses significantly decline with proximity to the σ Ori, and CO gas detections are found only in the outer (> 1.5 pc) regions.
The figure illustrates these effects of external photoevaporation. Orange points are ALMA continuum detections and gray triangles are 3-sigma upper limits; blue outlines indicate ALMA CO gas detections. Dust mass (M_dust) clearly declines with smaller separation from σ Ori, and massive disks (> 3 M_earth) are missing within ~0.5 pc. CO detections are rare and only exist > 1.5 pc from σ Ori. The orange points are scaled according to M_dust/M_star to show that the declining trend is not due to stellar mass segregation in the cluster (a potential concern as M_dust scales with M_star; e.g., Andrews et al. 2013).
INNER DISK DYNAMICS WITH YOUNG "DIPPER" STARS
While ALMA primarily probes the outer disk, I am also interested in studying the inner regions (< 1 AU) of protoplanetary disks and their relation to the outer disk. I therefore research young (< 10 Myr) "dipper" stars that are thought to probe the dynamics of the inner disk and may provide insight into terrestrial planet formation at < 1 AU scales --- a region that is otherwise difficult to observe.
Dippers are late-type stars whose high-precision light curves show dips in flux lasting ~0.5-2 days with depths of up to ~50% (see Figure below using K2 data; Ansdell et al. 2016a). Such signals are inconsistent with planet or comet transits; rather, we believe the dips are due to occultations of the star by dusty structures orbiting in the surrounding disk. Dippers appear to be common, comprising 20-30% of accreting systems in young regions (e.g., NGC 2264 at 3 Myr; Alencar et al. 2010).
We have published multi-wavelength follow-up observations of a selection of 10 dippers in Upper Sco and ρ Oph. In this work, we considered three mechanisms to explain the dipper phenomenon based on our observations: 1) inner disk warps near the co-rotation radius related to accretion; 2) vortices at the inner disk edge produced by the Rossby Wave Instability; and 3) clumps of circumstellar material related to planetesimal formation. However, much work is left to be done in order to understand the mechanism(s) driving the dipper phenomenon and therefore what the dippers can tell us about terrestrial planet formation.
Dippers were originally thought to be nearly edge-on systems, which would allow for transits of the circumstellar dust to produce the dimming events. However, using resolved sub-mm images from the public ALMA archive, we showed that dipper disk inclinations can range from nearly edge-on to completely face-on (see Figure below; Ansdell et al. 2016b). These findings challenged some of the proposed dipper mechanisms described above, and point to unexplored inner disk dynamics that appear to be common in planet-forming systems.