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The new "Atmospheric Rossiter-McLaughlin effect" discovered with HARPS-N

The researchers of the Global Architecture of Planetary Systems (GAPS) project discovered a new effect induced on the radial velocity measurements by the iron in the atmosphere of an ultra-hot Jupiter. The team of researchers analyzed four transits of KELT-9 observed with the high-resolution spectrograph HARPS-N at the Telescopio Nazionale Galileo. They found that the precise radial velocities were not matching the theoretical Rossiter-McLaughlin effect predictions (Fig. 1, top panel).

Fig. 1 – Top: Mean RVs of the four transits (filled circles). The theoretical RV solution (blue line) computed using Keplerian motion and the classical RML effect alone clearly does not match the observations. Bottom: The same measurements, after correction for the orbital motion, are now well fitted by the new Atmospheric Rossiter-McLaughlin effect. The vertical dashed lines show the ingress and egress of transit.

The team led by F. Borsa (INAF-OA Brera) had the idea to look at the radial velocities using the line-profile tomography technique. As the star KELT-9 is a fast rotator (vsini=112 km/s), with very broad lines, they applyied a custom extraction of the radial velocities developed for fast rotators. After a precise estimation of the stellar and planetary parameters, the researchers could confirm that the mismatch with theoretical predictions was real and provide a satisfactory model to explain it (Fig. 1, bottom panel). They interpret it as caused by the presence of iron in the planetary atmosphere. Basically, the mean line-profile of the star, used to calculate the radial velocity value and mostly based on iron lines, is modified by the presence of the iron present in the superimposed planetary spectrum visible during the transit since it is filtering the stellar light. It results in a rapidy moving feature in the stellar restframe (Fig. 2).

Fig. 2 - Mean line-profile residuals tomography of the four KELT-9 transits centred in the stellar restframe. The Doppler shadow of the planet (red) and the planetary atmosphere track (blue) are both evident. The depression during the transit (light blue) is caused by the photometric decrease of flux. The horizontal white lines show the ingress and egress of transit.

Iron was already known to be present in the KELT-9b’s atmosphere (see http://www.tng.iac.es/news/2018/08/16/kelt9b/"), but this is the first time that it is seen to change the apparent radial velocities of the parent star, causing an in-transit deviation (the “Atmospheric Rossiter-McLaughlin effect”,as called by F. Borsa). By modeling the radial velocities, the researchers could estimate for the first time that iron (both neutral and ionised) is present up to high altitudes in the atmosphere of KELT-9b, in particular up to a height of about 22% of the planetary radius. This is a very large value for a heavy atom like iron, since usually these heavy atoms tend to stay in the lower part of the atmosphere as they are more attracted by the force of gravity.

More information in the paper “The GAPS Programme with HARPS-N at TNG XIX. Atmospheric Rossiter-McLaughlin effect and improved parameters of KELT-9b”, https://arxiv.org/abs/1907.10078 di F. Borsa, M. Rainer, A. S. Bonomo, D. Barbato, L. Fossati, L. Malavolta, V. Nascimbeni, A. F. Lanza, M. Esposito, L. Affer, G. Andreuzzi, S. Benatti, K. Biazzo, A. Bignamini, M. Brogi, I. Carleo, R. Claudi, R. Cosentino, E. Covino, M. Damasso, S. Desidera, A. Garrido Rubio, P. Giacobbe, E. González-Álvarez, A. Harutyunyan, C. Knapic, G. Leto, R. Ligi, A. Maggio, J. Maldonado, L. Mancini, A. F. M. Fiorenzano, S. Masiero, G. Micela, E. Molinari, I. Pagano, M. Pedani, G. Piotto, L. Pino, E. Poretti, G. Scandariato, R. Smareglia, A. Sozzetti.