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Tiago Campante

Filipe Pereira's PhD viva

My grad student Filipe Pereira has successfully defended his PhD thesis on the 17th of December. Titled 'Detection and characterization of planets orbiting oscillating red-giant stars with NASA's TESS mission', Filipe's thesis not only constitutes, in my view, a significant contribution to the field but should also be regarded for its legacy value. Before telling you a little bit more about Filipe's work, let me take this opportunity to congratulate him on his thesis and also to thank my colleagues Margarida Cunha and Nuno Santos for helping me co-supervise Filipe over the past few years.


The Kepler/K2 mission mainly targeted main-sequence stars, observing too few red giants to detect enough planets for robust statistics. The advent of NASA's Transiting Exoplanet Survey Satellite (TESS) has since lifted this restriction, as it enables the systematic search for transiting planets around giant stars. With the aim of assessing both the occurrence rate (notionally at the subpercent level) of close-in giant planets around red-giant branch (RGB) stars and the role of the incident stellar flux on the radius inflation of giant planets, we devised a multifaceted work plan that included:

  1. Improving the characterization of these planetary systems. This meant developing a Gaussian process regression framework to model granulation and oscillations in red-giant stars. The method is capable of describing stellar signals while simultaneously modeling an exoplanet's transit. The implementation of the method was described in Pereira, Campante et al. (2019) and the corresponding software tool (gptransits) made open source and freely available through GitHub (link here).

  2. Developing a pipeline to produce systematics-corrected light curves from the TESS full-frame images (FFIs). The pipeline was used to produce light curves for a magnitude-limited sample of ~40,000 low-luminosity RGB stars in the southern ecliptic hemisphere. This sample could be the starting point for a future occurrence rate study.

  3. Searching these light curves for transiting giant planets with short orbital periods (<20 days). The search was followed by a validation procedure during which statistical and astrophysical false-positive scenarios were evaluated. As an output, 4 planet candidates were retained (2 of which are newly found candidates, the other 2 being previously known hosts). A fainter fifth candidate (newly found) was subsequently added to the list as a result of an ongoing collaboration with Sam Grunblatt (American Museum of Natural History, USA). Two of the above newly found candidates are going to be presented in Pereira et al. (2022, in prep.), having in the meantime been confirmed through radial-velocity (RV) follow-up.

The first plot below shows the transit and RV light curves for Gimli, a massive and seemingly non-inflated close-in giant planet. This system could potentially provide further insight into the planet-mass dependence of the radius inflation mechanism at play. The second plot is an up-to-date planet mass vs. orbital period diagram showing RV (blue crosses) and transiting planets (all others) around red-giant stars (note that KELT-11 b and TOI-197 b orbit subgiants).




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