Astronomy Research

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 am using ALMA to observe the protoplanetary disk populations in Lupus and σ Orionis, two star-forming regions at distinct stages of disk evolution. I am measuring 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.

Figure: ALMA 890 μm continuum images of a selection of protoplanetary disks in the very young (1-3 Myr) Lupus star-forming region. The diversity of disk morphologies at this very young age is striking. Additionally, the "transitional disks" (i.e., disks with large inner cavities) are among the brightest disks, a trend that is also seen in other young star-forming regions. The full figure can be found in Ansdell et al. 2016.

Our first ALMA survey 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. Our second ALMA survey 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: disk dust masses significantly decline with proximity to the central O9V star, and CO gas detections are found only in the outer (> 1.5 pc) regions.

Figure: Effects of external photoevaporation in σ Orionis, as shown by disk dust mass (M_dust) as a function of projected separation from the central O9V star, σ Ori. Orange points are ALMA continuum detections and gray triangles are 3σ upper limits; blue outlines indicate ALMA CO gas detections. M_dust clearly declines with smaller separation from σ Ori, and massive disks (Mdust > 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). See Ansdell et al. 2017 for the full figure.

Inner Disk Dynamics With Young "Dipper" Stars

I also 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%. 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).

Figure: K2 light curves of a selection of young "dipper" stars in the Upper Sco and ρ Oph star-forming regions. The depth, duration, and cadence of the dips indicate that they likely originate from occultations of the star by dusty material orbiting at < 1 AU scales. For more, see Ansdell et al. 2016a and Ansdell et al. 2016b.

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 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.

For fun: here are some Google Tilt Brush 3D drawings of dippers. I asked my friend George Peaslee to take a shot at this when I was finding it difficult to describe to people what the dippers might look like.

Habitability Around Young M Dwarfs

I am interested in the UV evolution of young M dwarfs because planets in the close-in habitable zones around these late-type stars are exposed to heightened irradiation that can erode and alter the chemistry of planetary atmospheres. My work on young M dwarfs with Eric Gaidos and Andrew Mann has focused on the near-UV luminosity evolution of young M dwarfs; in Ansdell et al. 2015, we derived a near-UV luminosity function, which we found to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation.

This work included a published list of ~700 candidate nearby, young M dwarfs based on a GALEX near-UV luminosity selection criteria. We vetted these targets using optical spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated near-UV emission is due to unresolved background sources or stellar companions, rather than stellar youth; we estimated the overall occurrence of these false positives at ~16%.

Figure: GALEX-selected young stars (blue points) identified by their heightened near-UV luminosities relative to field M dwarfs (gray points). See Ansdell et al. 2015a for details on how the sample was selected and then vetted for false-positives.