Retrograde Orbit Of HAT-P-18b: GAPS Programme With HARPS-N At TNG.pdf

This letter to the editor presents the study of the retrograde orbit of HAT-P-18b, a Saturn-mass exoplanet transiting a K2 dwarf star. The research is part of the Global Architecture of Planetary Systems (GAPS) program using the HARPS-N spectrograph at the TNG telescope.

Key Observations and Findings:

1. Radial Velocity Measurements: Spectra acquired over a full transit revealed precise radial velocity measurements, enabling the detection of the Rossiter-McLaughlin (RM) effect.

2. Retrograde Orbit: The RM effect analysis indicates that HAT-P-18b is on a counter-rotating orbit, with the sky-projected angle λ between the stellar spin axis and planet orbital axis measured as 132 ± 15 degrees.

3. Stellar and Planetary Parameters: By jointly modeling radial velocity and photometric data (from another full transit), researchers determined new parameters for the host star and planet:
- Star: M⋆ = 0.770 ± 0.027 M⊙, R⋆ = 0.717 ± 0.026 R⊙, V sin I⋆ = 1.58 ± 0.18 km s−1
- Planet: Mp = 0.196 ± 0.008 MJ, Rp = 0.947 ± 0.044 RJ

4. Stellar Characteristics: The star's effective temperature (Teff) is measured as 4870 ± 50 K, surface gravity (log g⋆) as 4.57 ± 0.07 cm s−2, and iron abundance ([Fe/H]) as 0.10 ± 0.06.

5. Significance: HAT-P-18b is notable for its low planet mass, long orbital period, and retrograde orbit, making it an ideal target to study the causes of orbital evolution in cool main-sequence stars.

Methodology:

- Data reduction using HARPS-N pipeline for radial velocity measurements.
- Stellar atmosphere characterization from 1D spectra.
- RM effect analysis to determine the misalignment angle Θ between stellar spin and planet orbital axes.