My current research interests range from the interactions of young stars with their circumstellar disks to exoplanet atmospheres and the interaction of a planet’s magnetic field with the wind of it’s parent star.
I do not currently have any of my data uploaded to the website. If you would like spectra from any of my papers, please contact me and I will send them to you.
The number of confirmed exoplanets currently stands near 4000 (NASA Exoplanet Archive; see exoplanets.org for a different count). While we have reliable radius and mass estimates for many of these objects, the atmospheres of most exoplanets are unexplored. Atmospheric content, temperature, and density are all fundamental parameters that must be know in order to fully characterize the planet. I use high resolution optical and UV spectra to search for signatures of atomic absorption in some of the hottest (i.e., shortest orbital period) known exoplanets around nearby stars. One of the strongest signatures in a class of exoplanets called hot Jupiters is due to atomic hydrogen, a product of hydrogen’s large abundance. Excited hydrogen has strong lines at optical wavelengths (the Balmer lines) and the shape and strength of these lines can be measured using spectrographs such as HiRes on the Keck telescopes and HRS on the Hobby Eberly Telescope at McDonald Observatory (Jensen et al. 2012, Cauley et al. 2015). Atomic sodium is also a strong absorber in Hot Jupiter atmospheres (see Redfield et al. 2008 and Jensen et al. 2011). We currently have ongoing observational programs to measure these lines in an ever increasing number of short period exoplanets.
Exoplanet magnetic fields
Magnetic fields probably play an important role in protecting planetary atmospheres from the stripping effects of incoming particles from the stellar wind. For habitable zone planets, this shielding effect could determine whether or not the atmosphere is around to help sustain life. We currently have not measured the magnetic field strength of an exoplanet, although many people are trying. I am interested in looking for signatures of the planet’s magnetic field interacting the stellar wind, compressing the stellar wind material and potentially making it observable. Hints of these interactions have been observed in a couple of objects and offer tantalizing clues to the strength hot planet magnetic fields (see Cauley et al. 2015 and Llama et al. 2013). The animation below demonstrates what such a signal might look like and how it is similar to observations we made of the hot Jupiter HD 189733 b.
The contrast effect for transiting exoplanets
When we observe a planet in transit, we are measuring light coming from a smaller fraction of the stellar surface than we observe when the planet is out-of-transit. Based on differences between the measured in-transit light and out-of-transit light, we can deduce things not only about the planet but also about the portion of the star that the planet is occulting. This is important for understanding how much of an observed signal is due to different pieces of the star and how much is actually forming in the planet’s atmosphere. I have developed simulations to test this phenomena, called the contrast effect, in order to better understand our observations. The animations below show some examples of the contrast effect for a hot Jupiter-type exoplanet transiting a stellar surface with various active regions.
Accretion and outflows around young stars
When our Sun was very young (a few million years old) it would have belonged to a class of objects we call T Tauri stars (TTSs) that live in star forming regions like the Orion Nebula which is featured as the background picture on this site. T Tauri stars are young, low mass stars that have not yet reached their main sequence hydrogen burning states, i.e., they are pre-main sequence stars and are still gravitationally contracting. The subclass of these young objects called classical T Tauri stars (CTTSs) are surrounded by massive disks of dust and gas which are the nurseries for planet formation. CTTSs are also strongly magnetic, hosting ordered magnetic fields that are many times stronger than our Sun. My thesis work focused on understanding how the intermediate-mass counterparts of CTTSs, the Herbig Ae/Be stars, interact with their surrounding disks. We found that, in general, Herbig Ae/Be stars show very different properties than CTTSs and that the geometry of their magnetic fields are different as well.
White dwarfs and leftover planetary systems
White dwarfs are old stars similar in mass to the Sun that have passed through their final stages of evolution. The only thing left is the hot, dense core of the star which is about the same size as planet Earth, although still of about the same mass as the Sun. Many of these white dwarf stars have lots of metallic elements in their atmospheres that could only have come from stuff external to the star. In other words, these metals, such as iron, nickel, and sodium, must have fallen onto the star at some point after the it evolved into a white dwarf. Astronomers have strong evidence that these metals originated in small rocky bodies, like an asteroid or planets like Mercury and Earth, and that these bodies are likely leftovers from the star’s original solar system. I have worked with collaborators to explain observations of a particular white dwarf, WD1145+017, showing the evolution of a gaseous disk around the star. We argue that the changes seen in the gas are due to the precession of the disk which is being caused by the strong gravitational well of the central star. This might have implications for how this gaseous material is produced and subsequently falls onto the white dwarf. The animation below shows how the gas disk moves over time to produce the spectral lines we observe.