Planet Formation - Radial Velocity

I worked with Heather Knutson on a Doppler survey at Keck combined with NIRC2 K-band AO imaging to search for massive, long-period companions to a sample of 123 known exoplanet systems detected using the radial velocity method. In this large survey with unprecedented sensitivities (we were sensitive to 1 M_Jup planets out to 20 AU for the majority of stars in our sample), I tested whether close-in gas giant planets are more likely to have outer companions than their long-period counterparts, and whether planets in multi-planet systems are more likely to have higher eccentricities than single planet systems. I found that hot gas giants seem to be more likely to have an outer companion than cold gas giants, and that planets in multi-body systems have higher average eccentricities than do single-planet systems. These both indicate that dynamical interactions between planets play an important role in the evolution of these systems.

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Radial velocity measurements and best fit model for HD 180902. The trend in the data indicates the presence of a massive, long-period companion in the system.

Planet Formation - Direct Imaging

Over the past decade, direct imaging searches for young gas giant planets have revealed a new population of companions with wide orbital separations and masses near or at the deuterium burning limit. These planetary-mass companions pose significant challenges to possible formation mechanisms, including core accretion, disk instability, and turbulent fragmentation.

I led an angular differential imaging (ADI) survey with NIRC2 at Keck to search for close-in substellar companions to a sample of seven (out of 15) systems with confirmed planetary mass companions on wide orbits. In this study I tested the scattering mechanism as a potential avenue of formation, that is whether these companions formed closer in to their host stars and were subsequently scattered out to their present-day locations, by searching for other massive bodies at smaller separations. In addition to finding no potential scatterers down to separations of 15-50 AU depending on the system, we were able to put constraints on the eccentricities of two of the confirmed companions and found that these were inconsistent with predications from scattering simulations. These results suggest that in situ formation is more likely.

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This reduced Keck NIRC2 image shows a confirmed wide-separation planetary mass companion (labeled b), and an inner candidate companion in that system (labeled cc1).

Planet Formation - High Resolution Spectroscopy

To further explore the origins of super-massive gas giant planets at wide separations I led a project to measure rotation rates of young (2-300 Myr) wide-separation planetary-mass companions using near-IR high-resolution spectra from NIRSPEC/NIRSPAO at Keck to provide a first look at the spin distribution of these objects. I found that this distribution is indistinguishable from the spin distribution of free-floating planetary-mass brown dwarfs, indicating either that these two populations formed via the same mechanism, or that the processes that regulate rotation rates in the planetary-mass regime are independent of formation mechanism. We also placed the first constraints on angular momentum evolution in the planetary-mass regime. I found that rotation rates for both populations were well below their break-up velocities and did not evolve significantly over the first few hundred million years, suggesting that rotation rates are set early in their lifetimes, possibly via interactions with a circumplanetary disk.

For more information, please go to ADS


This plot shows rotational broadening in the ROXs 42B b spectrum. Cross correlation between the ROXs 42B b spectrum and a model atmosphere broadened to the instrumental resolution (black points) with 1 sigma uncertainties from a jackknife resampling technique. The cross correlation functions between a model atmosphere broadened to the instrumental resolution and that same model additionally broadened by a range of rotation rates are overplotted in color.