[Paper Review] The GAPS Programme with HARPS-N at TNG. XIII. The orbital obliquity of three close-in massive planets hosted by dwarf K-type stars: WASP-43, HAT-P-20 and Qatar-2

M. Esposito, E. Covino+46 more|arXiv (Cornell University)|Feb 10, 2017

Stellar, planetary, and galactic studies100 references16 citations

TL;DR

This study measures the orbital obliquity of three massive close-in exoplanets—WASP-43b, HAT-P-20b, and Qatar-2b—orbiting cool K-type dwarf stars using high-precision radial velocity measurements from HARPS-N at the Telescopio Nazionale Galileo. It finds that WASP-43b is aligned with its star’s spin (λ = 3.5° ± 6.8°), HAT-P-20b has a small but significant obliquity (Ψ = 36°⁻¹²⁺¹⁰), and Qatar-2b is marginally consistent with alignment, with evidence of enhanced stellar activity possibly due to star-planet interactions.

ABSTRACT

In the framework of the GAPS project, we are conducting an observational programme aimed at the determination of the orbital obliquity of known transiting exoplanets. The targets are selected to probe the obliquity against a wide range of stellar and planetary physical parameters. We exploit high-precision radial velocity (RV) measurements, delivered by the HARPS-N spectrograph at the 3.6m Telescopio Nazionale Galileo, to measure the Rossiter-McLaughlin (RM) effect in RV time-series bracketing planet transits, and to refine the orbital parameters determinations with out-of-transit RV data. We also analyse new transit light curves obtained with several 1-2m class telescopes to better constrain the physical fundamental parameters of the planets and parent stars. We report here on new transit spectroscopic observations for three very massive close-in giant planets: WASP43b, HATP20b and Qatar2b orbiting dwarf K-type stars with effective temperature well below 5000K. We find lambda = 3.5pm6.8 deg for WASP43b and lambda = -8.0pm6.9 deg for HATP20b, while for Qatar2, our faintest target, the RM effect is only marginally detected, though our best-fit value lambda = 15pm20 deg is in agreement with a previous determination. In combination with stellar rotational periods derived photometrically, we estimate the true spin-orbit angle, finding that WASP43b is aligned while the orbit of HATP20b presents a small but significant obliquity (Psi=36 _{-12}^{+10} deg). By analyzing the CaII H&K chromospheric emission lines for HATP20 and WASP43, we find evidence for an enhanced level of stellar activity which is possibly induced by star-planet interactions.

Motivation & Objective

• To determine the orbital obliquity of three massive close-in exoplanets orbiting cool K-type stars.
• To investigate whether tidal interactions or stellar companions influence the spin-orbit alignment of these systems.
• To assess the role of star-planet interactions through chromospheric activity indicators such as the R′HK index.
• To refine planetary and stellar parameters using transit light curves and radial velocity data.

Proposed method

• High-precision radial velocity measurements from the HARPS-N spectrograph at the TNG were used to detect the Rossiter-McLaughlin effect during planetary transits.
• Out-of-transit radial velocity data were used to refine orbital parameters and improve system ephemerides.
• Transit light curves from 1–2 m telescopes were analyzed to constrain planetary and stellar physical parameters.
• Stellar rotational periods were derived photometrically to estimate the true spin-orbit angle Ψ from the projected obliquity λ.
• Chromospheric activity was assessed via CaII H&K emission lines to detect potential star-planet interaction effects.
• Tidal decay timescales for obliquity and eccentricity were modeled to evaluate the likelihood of orbital circularization and alignment over time.

Experimental results

Research questions

• RQ1 What is the true spin-orbit angle of WASP-43b, HAT-P-20b, and Qatar-2b, and is their orbit aligned with the stellar spin axis?
• RQ2 Does the presence of a distant stellar companion influence the orbital architecture of HAT-P-20b, particularly its small obliquity and non-zero eccentricity?
• RQ3 Is there evidence of enhanced stellar activity in WASP-43 and HAT-P-20 due to tidal or magnetic star-planet interactions?
• RQ4 How do tidal dissipation timescales for obliquity and eccentricity compare to the estimated ages of the systems?
• RQ5 Do cooler K-type stars hosting massive close-in planets exhibit similar alignment trends as hotter stars?

Key findings

• The true spin-orbit angle for WASP-43b is consistent with zero (Ψ = 0°), indicating a well-aligned system.
• HAT-P-20b exhibits a small but significant obliquity of Ψ = 36°⁻¹²⁺¹⁰, suggesting a non-coplanar orbit.
• Qatar-2b shows only a marginal detection of the Rossiter-McLaughlin effect, but the best-fit λ = 15° ± 20° is consistent with alignment.
• Both WASP-43 and HAT-P-20 show enhanced chromospheric activity (R′HK) beyond what is expected for their rotation periods, indicating possible star-planet interactions.
• Tidal decay timescales for obliquity are shorter than the estimated ages of the systems, suggesting that tides could have realigned the orbits over time.
• The eccentricity of HAT-P-20b (e = 0.0172 ± 0.0016) is likely primordial, as its tidal decay timescale (~4 Gyr) exceeds the system’s age, while WASP-43’s eccentricity is damped in less than 10 Myr.