Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2024)

The goal of this thesis is to characterize circumstellar disks and substellar objects, from moons to brown dwarfs, by improving and applying techniques from high-contrast imaging. To do so, I led four projects, two of which expand the range of targets accessible by high-contrast imaging. In Project 1, I develop an improved algorithm for high-contrast imaging data processing, enabling detection of fainter, smaller planets; this algorithm uses both spatial and temporal information on the speckle noise present in high-contrast images, and will be especially useful for next-generation photon-counting detectors such as Microwave Kinetic Inductance Detectors (MKIDs). In Project 2, I demonstrate a new technique to detect and monitor ejections of ice particles from subsurface oceans on outer solar system bodies, such as Jupiter’s icy moon Europa, using ground-based polarimetric imaging. Such plumes have not been confirmed on Europa, but are of significant interest for astrobiology and planning for the upcoming Europa Clipper mission.

The remaining two projects use existing analysis techniques to investigate individual systems of interest that add to our understanding of planet formation. In Project 3, I characterize the atmosphere of a brown dwarf around HIP 93398, a recently discovered companion identified via the Hipparcos Gaia Catalog of Accelerations, using high-resolution spectra from SCExAO/CHARIS. With both a dynamical mass measurement and high-resolution spectroscopy, HIP 93398 B provides substantial constraints on models of brown dwarf evolution. In Project 4, I constrain the morphology and scattering properties of the debris disk around the bright star HD 155623, a unique “hybrid” debris disk that shows signs of youth despite its age, using polarimetric data from the Gemini Planet Imager. With the information from my investigation, the HD 156623 system can now serve as a benchmark for models aiming to explain how gas and dust interact during the later stages of planet formation.

These investigations both further innovation in exoplanet imaging techniques and, respectively, provide insight into detections of extrasolar planets, the nature of oceans beyond Earth, the mechanisms of planet formation, and the atmospheres of giant planets and brown dwarfs.

    Cover page: Applications of High-Contrast Imaging Techniques and Polarimetry to (Exo-)Planetary Science
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2023)
    We introduce a new framework for point-spread function subtraction based on the spatiotemporal variation of speckle noise in high-contrast imaging data where the sampling timescale is faster than the speckle evolution timescale. One way that space-time covariance arises in the pupil is as atmospheric layers translate across the telescope aperture and create small, time-varying perturbations in the phase of the incoming wavefront. The propagation of this field to the focal plane preserves some of that space-time covariance. To utilize this covariance, our new approach uses a Karhunen-Loève transform on an image sequence, as opposed to a set of single reference images as in previous applications of Karhunen-Loève Image Processing (KLIP) for high-contrast imaging. With the recent development of photon-counting detectors, such as microwave kinetic inductance detectors, this technique now has the potential to improve contrast when used as a post-processing step. Preliminary testing on simulated data shows this technique can improve contrast by at least 10%-20% from the original image, with significant potential for further improvement. For certain choices of parameters, this algorithm may provide larger contrast gains than spatial-only KLIP.
      Cover page: Speckle Space–Time Covariance in High-contrast Imaging
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2021)
      Pluto's surface is geologically complex because of volatile ices that are mobile on seasonal and longer time scales. Here we analyzed New Horizons LEISA spectral data to globally map the nitrogen ice, including nitrogen with methane diluted in it. Our goal was to learn about the seasonal processes influencing ice redistribution, to calculate the globally averaged energy balance, and to place a lower limit on Pluto's N2 inventory. We present the average latitudinal distribution of nitrogen and investigate the relationship between its distribution and topography on Pluto by using maps that include the shifted bands of methane in solid solution with nitrogen (which are much stronger than the 2.15-μm nitrogen band) to more completely map the distribution of the nitrogen ice. We find that the global average bolometric albedo is 0.83±0.11, similar to that inferred for Triton, and that a significant fraction of Pluto's N2 is stored in Sputnik Planitia. We also used the encounter-hemisphere distribution of nitrogen ice to infer the latitudinal distribution of nitrogen over the rest of Pluto, allowing us to calculate the global energy balance. Under the assumption that Pluto's nitrogen-dominated 11.5μbar atmosphere is in vapor pressure equilibrium with the nitrogen ice, the ice temperature is 36.93±0.10 K, as measured by New Horizons’ REX instrument. Combined with our global energy balance calculation, this implies that the average bolometric emissivity of Pluto's nitrogen ice is probably in the range 0.47–0.72. This is consistent with the low emissivities estimated for Triton based on Voyager results, and may have implications for Pluto's atmospheric seasonal variations, as discussed below. The global pattern of volatile transport at the time of the encounter was from north to south, and the transition between condensation and sublimation within Sputnik Planitia is correlated with changes in the grain size and CH4 concentration derived from the spectral maps. The low emissivity of Pluto's N2 ice suggests that Pluto's atmosphere may undergo an extended period of constant pressure even as Pluto recedes from the Sun in its orbit.
        Cover page: Distribution and energy balance of Pluto’s nitrogen ice, as seen by New Horizons in 2015
        • Article
        • Peer Reviewed
        LBL Publications (2019)
        Support for early career astronomers who are just beginning to explore astronomy research is imperative to increase retention of diverse practitioners in the field. Since 2010, Astrobites has played an instrumental role in engaging members of the community -- particularly undergraduate and graduate students -- in research. In this white paper, the Astrobites collaboration outlines our multi-faceted online education platform that both eases the transition into astronomy research and promotes inclusive professional development opportunities. We additionally offer recommendations for how the astronomy community can reduce barriers to entry to astronomy research in the coming decade.
          Cover page: Astrobites as a Community-led Model for Education, Science Communication, and Accessibility in Astrophysics
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2021)
          We conducted a search for technosignatures in 2018 and 2019 April with the L-band receiver (1.15-1.73 GHz) of the 100 m diameter Green Bank Telescope. These observations focused on regions surrounding 31 Sun-like stars near the plane of the Galaxy. We present the results of our search for narrowband signals in this data set, as well as improvements to our data processing pipeline. Specifically, we applied an improved candidate signal detection procedure that relies on the topographic prominence of the signal power, which nearly doubles the signal detection count of some previously analyzed data sets. We also improved the direction-of-origin filters that remove most radio frequency interference (RFI) to ensure that they uniquely link signals observed in separate scans. We performed a preliminary signal injection and recovery analysis to test the performance of our pipeline. We found that our pipeline recovers 93% of the injected signals over the usable frequency range of the receiver and 98% if we exclude regions with dense RFI. In this analysis, 99.73% of the recovered signals were correctly classified as technosignature candidates. Our improved data processing pipeline classified over 99.84% of the ∼26 million signals detected in our data as RFI. Of the remaining candidates, 4539 were detected outside of known RFI frequency regions. The remaining candidates were visually inspected and verified to be of anthropogenic nature. Our search compares favorably to other recent searches in terms of end-to-end sensitivity, frequency drift rate coverage, and signal detection count per unit bandwidth per unit integration time.
            Cover page: A Search for Technosignatures around 31 Sun-like Stars with the Green Bank Telescope at 1.15–1.73 GHz
            Top