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Detecting and characterizing exoplanets around evolved stars with NASA's TESS mission

Periodic Reporting for period 1 - PULSATION (Detecting and characterizing exoplanets around evolved stars with NASA's TESS mission)

Période du rapport: 2018-11-01 au 2020-10-31

NASA's Transiting Exoplanet Survey Satellite (TESS), launched in 2018, is performing an all-sky survey for planets transiting bright stars. Moreover, TESS's excellent photometric precision enables asteroseismology (i.e. the study of stars by the observation of their natural, resonant oscillations). Asteroseismology is proving to be particularly significant for the study of red-giant stars, while maturing into a powerful tool whose impact is being felt across different domains of astrophysics. A noticeable example is the synergy between asteroseismology and exoplanetary science. TESS hence offers the exciting prospect of conducting asteroseismology of evolved exoplanet-host stars.

The research goal of this action was to use TESS photometry to systematically detect and characterize planets transiting oscillating evolved stars. Additionally, TESS photometry was used to characterize previously known (i.e. previous to the launch of TESS) evolved host stars. To that end, we implemented an interdisciplinary research plan that combined transit photometry, asteroseismology, and radial-velocity follow-up from the ground. The outcome of this action has the potential to provide new insight into some of the outstanding problems in exoplanetary science, namely, on the occurrence rate of gas-giant planets as a function of stellar mass, on the correlation between stellar metallicity and giant-planet occurrence around evolved stars, and on the structural aspects of gas-giant planets.

The communication goal of this action was to develop a nationwide educational/outreach program in collaboration with a non-academic partner organization, Ciência Viva, devised to improve public awareness of the topics of astronomy and astrophysics in the host organization's home country.
"We started by developing a Gaussian process regression framework to model granulation and oscillations in red-giant stars. This 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, https://arxiv.org/abs/1908.10662) and the corresponding software tool (gptransits) made freely available through GitHub.

The open-source software eleanor was then used to produce photometric time series for all available sectors of TESS data for each target star. We conducted an automated transit search on TESS light curves of approximately 100,000 red giants, with a focus on close-in giant planets. The automated search procedure was extensively tested prior to the first science data release (Campante et al. 2018, https://zenodo.org/record/2463210#.X7fZLdW2yOE). The transit search was complemented by a vetting and ranking procedure aimed at distinguishing the expected transit-like features from astrophysical false positives and systematic false alarms. The first detection of a TESS giant planet transiting an oscillating red giant was reported in Huber et al. (2019, https://arxiv.org/abs/1901.01643). This was followed by the detection of a warm sub-Saturn around an evolved F-type star (Addison et al. 2020, https://arxiv.org/abs/2001.07345).

Furthermore, a total of three known radial-velocity planets were discovered to transit from TESS observations (Kane et al. 2020, https://arxiv.org/abs/2007.10995; Pepper et al. 2020, https://arxiv.org/abs/1911.05150). The number of radial-velocity planets found to transit during TESS's primary mission is aligned with the predictions of Dalba et al. (2019, https://arxiv.org/abs/1811.06550).

TESS's excellent photometric precision enables asteroseismology of red-giant stars (Schofield et al. 2019, https://arxiv.org/abs/1901.10148). TESS photometry thus offers the prospect of conducting asteroseismology of evolved host stars (Kane, Bean, Campante et al. 2020, https://arxiv.org/abs/2010.15164). We estimated global asteroseismic parameters by applying Fourier-based automated methods (Campante et al. 2019, https://arxiv.org/abs/1909.05961) to the frequency-power spectra of the target stars. These methods had previously been extensively applied to Kepler/K2 data (Chontos et al. 2019, https://arxiv.org/abs/1903.01591; Kuszlewicz et al. 2019a, https://arxiv.org/abs/1905.00040; Kuszlewicz et al. 2019b, https://arxiv.org/abs/1907.01565; Santos, Campante et al. 2019, https://arxiv.org/abs/1908.02897; Thomas et al. 2019, https://arxiv.org/abs/1903.04998). TESS first-light results (Silva Aguirre et al. 2020, https://arxiv.org/abs/1912.07604) in the buildup to which these same methods were employed, clearly demonstrated the potential of TESS for carrying out asteroseismic ensemble studies of red giants. This potential was recently realized by using TESS asteroseismology to age date the early merging of the Milky Way with the Gaia-Enceladus dwarf galaxy (Chaplin et al. 2020, https://arxiv.org/abs/2001.04653).

We next determined fundamental stellar properties through a grid-based, forward modeling approach using the Bayesian code AIMS (Rendle et al. 2019, https://arxiv.org/abs/1901.02663). To ensure that fundamental properties are derived with the highest possible accuracy, we explored the impact of key physical processes, as well as of the chemical composition (Adibekyan et al. 2020, https://arxiv.org/abs/2008.08371) on the stellar properties inferred from forward modeling (Nsamba, Campante et al. 2018, https://arxiv.org/abs/1812.00431; Nsamba, Campante et al. 2019, https://arxiv.org/abs/1904.01560; Nsamba, Moedas, Campante et al. 2020, https://arxiv.org/abs/2010.07593).

Moreover, several known evolved hosts have benefited from TESS's precision photometry. Campante et al. (2019, https://arxiv.org/abs/1909.05961) reported the first detections of oscillations in known hosts by TESS. Other detections of solar-like oscillations in known hosts were reported by Ball et al. (2020, https://arxiv.org/abs/2010.07323) Jiang et al. (2020, https://arxiv.org/abs/2005.00272) and Nielsen et al. (2020, https://arxiv.org/abs/2007.00497).

An online catalog of planet candidates and their host stars is currently being assembled and will be hosted at the VizieR catalog service. It will combine the scientific output of the transit photometry, asteroseismology and ground-based follow-up (whenever available).

Finally, we carried out a nationwide educational/outreach program as part of a secondment at Ciência Viva, the Portuguese national agency for the promotion of science and technology. The program consisted of a series of Café Scientifique-like events on various topics of modern astrophysics targeting the general public (12 events in total across Portugal)."
The timing of this project has proven ideal, as it allowed conducting asteroseismology of a significant number of evolved exoplanet-host stars by making use of the entire data set from the TESS mission. This project further represents the first time a wide-field survey has been undertaken which systematically combines asteroseismology and transit photometry to characterize exoplanets. Finally, this project has the potential to build a long-lasting legacy, with implications for the science preparation and target selection of the upcoming PLATO (ESA) and ARIEL (ESA) space missions.

This project extended beyond basic research and we have implemented an ambitious scientific communication strategy to improve public awareness of the topics of astronomy and astrophysics in the host organization's home country.