Large Adaptive Optics Survey for Substellar Objects (LASSO) at Wide Separations Around Young, Nearby, Low-Mass Stars
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2021
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University of Hawaii at Manoa
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The occurrence rates and population trends of massive substellar companions (giant planets and brown dwarfs, $2-70$~M$_{Jup}$) at wide separations ($>$50~AU) from their host star are critical to furthering our understanding of their origins and evolution. However, these objects are very rare and building up a large sample for population-level studies has been challenging. Therefore, large surveys are necessary to search for these objects, which requires technology that can efficiently observe large numbers of stars. One of the two main components of my dissertation was instrumentation development on the laser guide star robotic adaptive optics instrument Robo-AO. I commissioned a new infrared camera that uses a high-speed low-noise SAPHIRA avalanche photodiode detector, allowing us to simultaneously image in the visible and near-IR. I also report on performance optimizations of the instrument. This made possible the second component of my dissertation work, conducting the Large Adaptive optics Survey for Substellar Objects (LASSO) at wide separations around young ($\lesssim$300 Myrs), nearby ($<$100 pc), low-mass ($\approx$0.1--0.8 M$_{\odot}$) stars. Low-mass stars are the most common type of stars and offer the most favorable contrasts for detecting substellar companions. I directly imaged nearly 900 stars, making this survey one of the largest adaptive optics surveys searching for substellar companions to date. I observed 427 of the stars with Robo-AO successively mounted on the Kitt Peak 2.1-m and the Maunakea UH 2.2-m telescopes. With the Subaru/IRCS and Keck/NIRC2 adaptive optics systems, I directly imaged an additional 470 young low-mass stars. The majority of confirmed and pending candidates are stellar companions, with $\sim$5 potentially new substellar companions that will require follow-up observations to confirm. I also report on systems with measured proper motion accelerations from the Hipparcos-Gaia catalog of accelerations. The significance of the acceleration increases with decreasing companion separation and these young accelerating low-mass stars with companions will eventually yield dynamical masses and orbital information. I also report the detection of higher-order stellar multiple systems, which provide important constraints on stellar evolution models. In order to assess the multiplicity statistics of stars across spectral types and populations in a volume-limited sample, I analyzed 1157 stars of all spectral types within 25~pc observed with Robo-AO. I combined Robo-AO detections of close-in companions with a search for wide co-moving companions using Gaia and corrected for unresolved companions to calculate multiplicity fractions across SpTs. These large survey results allow us to better understand the different formation pathways by studying the full spectrum of substellar to stellar companions on a population level.
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