Developed a method to account for spots and limb darkening to retrieve exoplanets atmospheric spectra. The study: “Correcting the effect of stellar spots on ARIEL transmission spectra – II. The limb-darkening effect” of G. Cracchiolo (INAF – OAPA/UNIPA) recently appeared on MNRAS
To date, exoplanetary transits have allowed astronomers to discovery more than 70% of known exoplanets. Besides, by analysing stellar spectra acquired during the transits it is possible to study the atmospheres of the transiting exoplanet. During the transit, in fact, the planet is crossing the stellar disc, causing a small and periodic dimming of stellar observed flux. If the exoplanet has an extended atmosphere, part of the stellar radiation goes through it before reaching Earth. Thus, the stellar spectrum observed during the transit will show features due to the chemical elements which populate the exoplanet atmosphere.
In the next future, ARIEL (Atmospheric Remote-Sensing Infrared Exoplanet Large-survey) will be one of the main telescopes designed to study of exoplanetary atmospheres. The telescope will be launched on 2029, and during its mission it will observe more than 1000 transiting exoplanets. During this years, astronomers are working relentlessly to define the scientific cases of the mission, to select the candidate targets, and to prepare tools necessary for data analysis and interpretation.
In a study published on 2020, the ph.D. student G. Cracchiolo (Astronomical Observatory of Palermo and University of Palermo), presented a new method aimed at retrieving the spectra of the atmosphere of transiting exoplanets accounting for the presence of spots on the stellar surface. The method has been recently developed with the aim of including the effects of limb darkening (which is the decrease of luminosity across stellar disk, going from the center toward the limb). It is quite intuitive that, since stellar luminosity is not constant across its disk due to limb darkening, the effects of spots on the transmission spectrum of the transiting exoplanet (which is the absorbed spectrum of the atmosphere of the planet obtained by subtracting from the observed spectrum the stellar contribution observed when the planet is not transiting) depend on their positions in the stellar disks. The new study has confirmed this expectation: the presence of spots in the stellar surface during the transit affects the observed transmission spectrum, with the effect being more important when the spots are close to the center of the stellar disk, when they cover more than 5% of stellar surface, and when the temperature contrast with the stellar photosphere is larger than 500 degrees. The new method developed by the team led by G. Cracchiolo, and described in the paper: “Correcting the effect of stellar spots on ARIEL transmission spectra – II. The limb-darkening effect“, recently appeared on the Monthly Notices of the Royal Astronomical Society, allows to extract the correct transmission spectrum, both in the case when the spots are and are not occulted by the planet. The team of researchers also counts the astronomers G. Micela and G. Morello of INAF – Astronomical Observatory of Palermo, and prof. G. Peres of the University of Palermo.
The figure (click here to visualize the entire figure) shows simulated transmission spectra of three known exoplanets as they will be observed by ARIEL. The dashed lines show the input spectra. The red lines shows the observed spectra not corrected by the presence of spots and limb darkening, while the solid line show the corrected spectra. In this particular case, spots are not occulted by the transiting planets and the spot-photosphere temperature contrast exceeds 1000 degrees.
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