Research
Overview of my research
I am interested in characterizing the atmospheres of exoplanets, which means identifying the physical conditions that exist on these planets. I have primarily pursued this through the use of a three-dimensional atmospheric circulation model, with which I can predict the detailed temperature structure and wind pattern in a planet's atmosphere, from the global properties of the planet. I am particularly interested in studying the planets that are now being characterized, as well as preparing for the ones that soon will be. As a consequence, most of my work so far has been about hot Jupiter planets (Jupiter-sized planets that orbit several stellar radii away from their host stars), but I have also worked on planets with longer orbital periods and/or smaller sizes.
A specialty of mine is determining what sort of characterization can be done with different types of observations. Part of this includes understanding the limits of what information we can retrieve from any particular data set, since there are often degeneracies. I also work to identify new observational techniques that can be used to measure additional properties. Finally, I like to try to determine how to combine multiple types of measurements in order to arrive at a more robust and cohesive understanding of a planet's atmosphere and its global state.
A list of movies that you are welcome to download are here.
Some examples of my research are given below and all of my papers are listed on my CV or via my ORCID.
Three-dimensional atmospheric circulation model
Our code solves the "primitive equations of meteorology" (a standard simplification of the full fluid equations, applied to a thin atmosphere on a rotating sphere) and originates from the Intermediate General Circulation Model built by the University of Reading. Our version has been adapted to the context of hot Jupiters (Menou & Rauscher 2009; Rauscher & Menou 2010) and updated to include a "double-gray" radiative transfer scheme (Rauscher & Menou 2012), more recently replaced by a new double-gray scheme in Roman & Rauscher (2017).
The robust flexibility of this code has allowed us to include additional features, such as a simple treatment for the effects of magnetic drag on the winds (Rauscher & Menou 2013), a time-variable incident flux on the atmosphere (as when orbiting a pair of binary stars, May & Rauscher 2016, or for planets with non-zero obliquities, Rauscher 2017), radiatively active clouds (Roman & Rauscher 2019), and the influence of a solid surface (May & Rauscher 2020).
The movie on the left shows the temperature and wind structure throughout the atmosphere. Each successive frame shows a deeper level in the atmosphere. We are viewing the planet along the eastern terminator, with the star off to the left.
Observational techniques
My first exoplanet paper proposed that the method of eclipse mapping could be used to resolve a two-dimensional image of the day side of an exoplanet (Rauscher et al. 2007b; successfully done in 2012 by de Wit et al. 2012 and Majeau et al. 2012). More recently, we have presented a new, optimized method for creating eclipse maps, which can remove sources of uncertainty (Rauscher, Suri, & Cowan 2018).
An example of combining different sorts of measurements to build a cohesive understanding of a planet can be found when we consider the influence of magnetic effects. Currents induced in the partially ionized atmospheres of the hottest hot Jupiters can dissipate in the deep atmosphere/interior. This could potentially provide enough heating to inflate the planet's radius and should alter the planet's circulation pattern (Perna, Menou, & Rauscher 2010a,b), with observable consequences (Rauscher & Menou 2013).
The image on the right is from Rauscher et al. (2007b) and shows a partially eclipsed image of a planet.
Some of the more recent research in my group includes working with Eliza Kempton (Maryland) on using the characterization method of high-resolution spectroscopy to constrain planets' 3-D structure, including rotation rates and wind speeds. We determined the observational precision that would be needed to determine whether magnetic effects were slowing the wind speeds of hot Jupiters (Miller-Ricci Kempton & Rauscher 2012) and compared the Doppler signatures and thermal phase curves of non-synchronously rotating hot Jupiters to determine whether we might be able to empirically constrain the rotation rates of these planets (Rauscher & Kempton 2014).
Our first two papers considered Doppler signatures in absorption spectra during transit, but in Zhang, Kempton, & Rauscher (2017) we presented a study of these diagnostics in high-resolution emission spectra, which are sensitive to both the wind and temperature structure of the atmosphere, with Harada et al. (2021) presenting an analysis of these signatures in partly cloudy hot Jupiter atmospheres.
The movie on the left shows the temperature and wind field at 30 mbar within a planet's atmosphere. As the planet rotates through its orbit we view different longitudes. The blue and red lines show the light-of-sight velocity towards and away from the observer, respectively, due to both winds and rotation.
In a first-of-its kind analysis, we used simulated spectra from our GCM results as the cross-correlation template spectra for retrieving the planet signal in high-resolution spectra of a hot Jupiter (Flowers et al. 2019) and identified the empirical signature of winds. In complementary work, Beltz et al. (2021) performed a similar analysis for the emission spectra of a hot Jupiter and we found that the use of a 3-D model in the analysis detected the planet's signal at much higher significance than the best fit from a large suite of 1-D models (almost 7 sigma, compared to about 5 sigma).
With generous support from the Heising-Simons Foundation and in collaboration with Eliza Kempton's group, we have embarked on a comprehensive program for high-resolution spectroscopy of “hot Jupiter” exoplanets. Specifically, we are measuring emission spectra at multiple phases throughout each planet’s orbit and using three-dimensional atmospheric models to combine this information into a global understanding of each planet’s 3-D temperature and chemical structure.
Recent highlights
These are the abstracts to some of the more recent papers to come out of our group, with links to where each paper can be found. Enjoy!
ajac897bf5_video.mp4Magnetic Drag and 3-D Effects in Theoretical High-Resolution Emission Spectra of Ultrahot Jupiters: the case of WASP-76b
Beltz, Rauscher, Kempton, Malsky, Ochs, Arora, & Savel (2022)
Ultrahot Jupiters are ideal candidates to explore with high-resolution emission spectra. Detailed theoretical studies are necessary to investigate the range of spectra that we can expect to see from these objects throughout their orbit, because of the extreme temperature and chemical longitudinal gradients that exist across their dayside and nightside regions. Using previously published 3D general circulation models of WASP-76b with different treatments of magnetic drag, we postprocess the 3D atmospheres to generate high-resolution emission spectra for two wavelength ranges, throughout the planet's orbit. We find that the high-resolution emission spectra vary strongly as a function of phase, at times showing emission features, absorption features, or both, which are a direct result of the 3D structure of the planet. At phases exhibiting both emission and absorption features, the Doppler shift differs in direction between the two spectral features, making them differentiable, instead of canceling each other out. Through the use of cross correlation, we find different patterns in net Doppler shift for models with different treatments of drag: the nightside spectra show opposite signs in their Doppler shift, while the dayside phases display a reversal in the trend of net shift with phase. Finally, we caution researchers against using a single spectral template throughout the planet's orbit; this can bias the corresponding net Doppler shift returned, as it can pick up on a bright region on the edge of the planet disk that is highly redshifted or blueshifted.
ThERESA: Three-dimensional Eclipse Mapping with Application to Synthetic JWST Data
Spectroscopic eclipse observations, like those possible with the James Webb Space Telescope, should enable 3D mapping of exoplanet day sides. However, fully flexible 3D planet models are overly complex for the data and computationally infeasible for data-fitting purposes. Here, we present ThERESA, a method to retrieve the 3D thermal structure of an exoplanet from eclipse observations by first retrieving 2D thermal maps at each wavelength and then placing them vertically in the atmosphere. This approach allows the 3D model to include complex thermal structures with a manageable number of parameters, hastening fit convergence and limiting overfitting. An analysis runs in a matter of days. We enforce consistency of the 3D model by comparing the vertical placement of the 2D maps with their corresponding contribution functions. To test this approach, we generated a synthetic JWST NIRISS-like observation of a single hot-Jupiter eclipse using a global circulation model of WASP-76b and retrieved its 3D thermal structure. We find that a model that places the 2D maps at different depths depending on latitude and longitude is preferred over a model with a single pressure for each 2D map, indicating that ThERESA is able to retrieve 3D atmospheric structure from JWST observations. We successfully recover the temperatures of the planet's day side, the eastward shift of its hot spot, and the thermal inversion. ThERESA is open source and publicly available as a tool for the community.
Modeling the High-resolution Emission Spectra of Clear and Cloudy Nontransiting Hot Jupiters
Malsky, Rauscher, Kempton, Roman, Long, & Harada (2021)
The advent of high-resolution spectroscopy (R ≳ 25,000) as a method for characterization of exoplanet atmospheres has expanded our capability to study nontransiting planets, vastly increasing the number of planets accessible for observation. Many of the most favorable targets for atmospheric characterization are hot Jupiters, where we expect large spatial variation in physical conditions such as temperature, wind speed, and cloud coverage, making viewing geometry important. Three-dimensional models have generally simulated observational properties of hot Jupiters assuming edge-on viewing, which can be compared to observations of transiting planets, but neglected the large fraction of planets without nearly edge-on orbits. As the first investigation of how orbital inclination manifests in high-resolution emission spectra from three-dimensional models, we use a general circulation model to simulate the atmospheric structure of Upsilon Andromedae b, a typical nontransiting hot Jupiter with high observational interest, due the brightness of its host star. We compare models with and without clouds, and find that cloud coverage intensifies spatial variations by making colder regions dimmer and relatedly enhancing emission from the clear, hotter regions. This increases both the net Doppler shifts and the variation of the continuum flux amplitude over the course of the planet's orbit. In order to accurately capture scattering from clouds, we implement a generalized two-stream radiative transfer routine for inhomogeneous multiple scattering atmospheres. As orbital inclination decreases, four key features of the high-resolution emission spectra also decrease in both the clear and cloudy models: (1) the average continuum flux level, (2) the amplitude of the variation in continuum with orbital phase, (3) net Doppler shifts of spectral lines, and (4) Doppler broadening in the spectra. Models capable of treating inhomogeneous cloud coverage and different viewing geometries are critical in understanding results from high-resolution emission spectra, enabling an additional avenue to investigate these extreme atmospheres.
