Project duration: 1 year rotation. Potential to grow into thesis project.
PI: Néstor Espinoza (nespinoza@stsci.edu, www.nestor-espinoza.com)
Project motivation
Transiting exoplanets have been, to date, one of the most successful in terms of the atmospheric characterization of exoplanets. The technique of transmission spectroscopy --- the wavelength-dependent change in the planetary radii due to opacity sources in its atmosphere --- in particular, has been one of the main workhorses of the field in terms of providing constraints on the atmospheric elemental abundances in gas giant exoplanets. These, in turn, hold the promise to provide key information about the formation and evolution of the planetary systems under study.
To date, the technique of transmission spectroscopy relies on one simple, key assumption: the terminator region we observe during transit is homogeneous. Hot, highly irradiated gas giant exoplanets (T > 1500 K) --- most of which are thought to be tidally locked --- have been observed however to have large (> 500 K; Keating et al., 2019) temperature differences between their day and night sides, which implies not only energy but material should be transported between them by some mechanism. In reality, thus, the leading and trailing limbs of an exoplanet (see Figure 1a) might have distinct temperature, pressure and thus compositional profiles due to the inherent 3-D nature of the planet which would, in turn, give rise to different spectra on each side of it. Constraining them might give precious insights into circulation patterns and compositional stratification which might probe to be fundamental for our understanding of the weather patterns in distant worlds. For example, hazes are expected to be photochemically produced and thus they would most likely be able to form in the dayside. These could, in turn, be transported to the trailing limb, while clouds could be transported from the nightside (where they are expected to form due to the lower temperatures) into the leading limb, thus resulting in a drastically different transmission spectrum between them, and thus effective sizes of the radii of each limb. Detecting this effect would not only directly impact the fundamental assumption of transmission spectroscopy studies to date, but would imply that there is not one set of properties (e.g., abundances) to extract from transmission spectra.
Student projects
In this project, we propose to use a wide range of existing and to-be-obtained photometric and spectrophotometric data from both ground (from the Magellan/IMACS and LBT/MODS multi-object spectrographs) and space-based facilities (TESS & HST) to constrain the effect asymmetric limbs imprint on real transit lightcurves (see Figure 1b). This would be the first time this is performed with actual data in such a wide array of observations and will pave the way towards future observations with the upcoming James Webb Space Telescope (JWST).
The project involves the study of data from one or several facilities depending on the interest of the student and the time they are interested in investing in the project. The HST and ground-based data are already at hand, and thus if deciding to work on any of these datasets, the student will have to work through the data reduction with the guidance of the PI in order to obtain lightcurves and fit them with state-of-the-art software that is already in place for this. This is expected to lead to a 1-year project, which will result in a publication of the results in a refereed journal. As for the TESS data, this is to be obtained during the 2020-2021 periods during TESS' extended mission; however, work on improving the current photometric precision achieved by the TESS pipeline is needed to increase the chances of detecting the effect. To this end, the interested student will work with the PI trying different targeted data analysis tools in order to achieve the highest possible precision from the TESS data, training the methods on already known exoplanetary systems. This will in turn lead to (a) a set of best-practices for optimal photometric precision with TESS, (b) updated system parameters for known exoplanetary systems and (c) a study on constraints the TESS data is able to put on the impact of asymmetric limbs on transit lightcurves. This is expected to also lead to a refereed publication. The duration of the project is expected to be one year as well.
Figure 1: Detection of exoplanetary limbs with photometric observations. (a) Schematic exagerating the imprint of different exoplanetary limbs on transit lightcurves. Note how the main difference occurs during ingress/egress, which lasts, for a typical hot-Jupiter, around ~20 minutes. (b) Simulated, phased transit lightcurve showing the differences between symmetric and asymmetric lightcurves fits to simulated data. Both limb-darkening, the time-of-transit center and the radii were left as free parameters. Note how in this case the asymmetry of the lightcurve is imprinted in the residuals of the symmetric model fit, which is detectable by eye.
