The underlying theme of my research is to understand the physical evolution of interstellar metals and the life cycle of dust: from its nuclear reactive origins to planets. Explaining the prevalence of dust in the Universe as well as its influence on astrophysical processes requires understanding the micro-properties of dust – its mineralogical composition, size, and shape distribution – which informs its origin and fate.
X-ray absorption spectroscopy and X-ray imaging of dust scattering halos provides a treasure trove of information about dust mineralogy, size, and position in space. This wavelength regime is particularly useful for answering astromineralogy because one can distinguish between the gas and solid phases of interstellar metals with X-ray absorption spectroscopy, making direct measurement of abundances feasible. Astromineralogy is a current frontier of X-ray astrophysics, and major breakthroughs are expected with the advent of X-ray IFU spectroscopy that will come available on XRISM and Athena.
With high resolution X-ray spectroscopy of current and future telescopes, we can answer the following questions:
- What is the X-ray signature of solid-phase Oxygen? The most advanced theoretical cross-sections for atomic oxygen are capable of fitting X-ray absorption spectra with no residual features.
- What fraction of diffuse interstellar dust is crystalline? How does this fit with the picture of infrared emission from crystalline and amorphous silicates?
- How does interstellar iron get incorporated into dust? Determining the iron fraction taken up in silicates or nanoparticles is important for understanding dust processing by supernovae and the ISM.
- What is the absolute abundance of interstellar metals? By differentiating between the solid and gas phase, X-ray observations are capable of measuring depletion in the ISM without assuming and abundance table.
- Answering these questions requires fully accounting for the effects of scattering, absorption, and attenuation of X-ray light by interstellar dust (upper right, from Corrales et al., 2016).
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X-ray datasets are information-rich, a fact that has guided my interest in applying cutting edge statistical techniques to modern problems in observational astrophysics. My research on dust scattering requires attention to calibration details and the statistics of X-ray photon counting.
I apply my Chandra data expertise to study Sgr A*, the supermassive black hole at the center of our Milky Way. The quiescent emission from Sgr A* is surprisingly dim, and is easily outshined by nearby objects. I developed observational techniques to extract and model the low signal-to-noise quiescent spectrum of Sgr A*, made possible by the Chandra Galactic Center X-ray Visionary Project.
Earth’s position near the outskirts of the Milky Way means that the GC is obscured by a particularly large amount of dust and gas. The dust scattering halos around compact objects in the dense GC environment cause significant blurring, casting a fog over the heart of our Galaxy. By using X-ray flares from GC compact objects to study the foreground ISM, I seek to measure and correct images of Sgr A* for the effects of dust blurring.
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High energy stellar phenomena, such as stellar flares and UV irradiation, are crucial conditions for planet habitability and suspected to play a role in the atmospheric evolution of rocky planets. UV and X-ray transit measurements can also provide information about atmospheric evaporation and composition. The UM SPICEs group takes a multi-faceted approach to characterizing exoplanets with X-ray and UV observational facilities.
How do the high energy properties of the host star affect the evolution of an exoplanet’s atmosphere? Utilizing the X-ray and UV capabilities of the Neil Gehrels Swift Observatory and XMM-Newton, we fill gaps in the available databases of UV and X-ray properties of exoplanet hosting stars and use planet evolutionary codes to characterize the past and future mass loss of these planets (King et al. 2019, 2021).
What can we learn from measuring exoplanet transits in the near UV? Measuring UV transits might provide insights into key physical processes shaping exoplanet atmospheres: evaporation, photochemistry in the upper atmosphere, and haze formation. Working to extend the current capabilities of UV enabled telescopes, Swift and XMM-Newton, we have measured NUV transits from HD 189733b, XO-3b, and KELT-3b (King et al. 2021, Corrales et al. 2021). Searching for trends in NUV transit depths provides insight into the physical mechanisms for NUV absorption and their influence on exoplanet demographics. Our group continues to work on statistical and instrument techniques to increase the utility of Swift and XMM-Newton for measuring NUV transits.
What is the composition of aerosols in exoplanet atmospheres? Understanding the composition and sizes of photochemically produced hazes and condensates (clouds) provides one of the most difficult challenges for interpreting observations of exoplanet atmospheres. Our latest projects will provide optical properties of new potential aerosol species to the exoplanet community, examine how aerosol signatures in exoplanet transmission spectra can provide insights into atmospheric chemistry and formation scenarios, and determine whether aerosols are responsible for the tentative NUV transit trends observed in Corrales et al. (2021).
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