HARPS-N revealed the atmospheric dynamics of KELT-20b

Transiting ultra-hot Jupiters are ideal candidates for studying the exoplanet atmospheres and their dynamics, particularly by means of high-resolution spectra with high signal-to-noise ratios. One such object is KELT-20b, orbiting the fast-rotating A-type star KELT-20.

In the framework of the Global Architecture of Planetary Systems (GAPS) project, two transits of KELT-20b were observed with GIARPS, with the purpose of studying the extension of the planetary atmosphere and any variation of the planetary atmospheric signal using the atmospheric Rossiter-McLaughlin effect and the line profile tomography.

Three additional transits were present in the TNG HARPS-N archive as public data, and they were added to the dataset.

The mean line profiles of the spectra were computed with a least-squares deconvolution using a stellar mask obtained from the Vienna Atomic Line Database (Teff =10 000 K, log g=4.3), and then the stellar radial velocities were obtained by fitting the line profiles with a rotational broadening function. The mean line profile residuals tomography was used to analyse the planetary atmospheric signal and its variations. As an additional analysis, the cross-correlation method was used to study a previously reported double-peak feature in the FeI planetary signal.

Both the classical and the atmospheric Rossiter-McLaughlin effect were observed in the radial velocity time-series. The latter yielded an estimate of the radius of the planetary atmosphere that correlates with the stellar mask used in this work (Rp+atmo/Rp =1.13±0.02).

The planetary atmospheric trace was isolated in the tomography, and radial velocity variations of the planetary atmospheric signal were found during transit with an overall blueshift of ≈ 10 km s−1, along with small variations in the signal depth, and less significant, in the full width at half maximum (FWHM). A possible variation in the structure and position of the FeI signal was found in different transits.

The FWHM variations of the atmospheric signal, if confirmed, may be caused by more turbulent condition at the beginning of the transit, by a variable contribution of the elements present in the stellar mask to the overall planetary atmospheric signal, or by iron condensation.

This study was lead by Monica Rainer, as a part of the GAPS project.

Link to the paper published in Astronomy & Astrophysics.

Mean line profile tomography

Mean line profile tomography. The average stellar line has been removed from all the mean line profiles. The residuals have been phase-folded with the planetary orbital period. The Doppler shadow (red excess) and the atmospheric trace (blue absorption, plotted by the dotted blue line) are visible in the residuals. The horizontal dashed lines show the transit ingress and egress.