[Paper Review] The TRAPPIST survey of southern transiting planets. I. Thirty eclipses of the ultra-short period planet WASP-43 b

M. Gillon, A. H. M. J. Triaud+17 more|Archive ouverte UNIGE (University of Geneva)|Jan 13, 2012

Stellar, planetary, and galactic studiesPhysics and Astronomy56 references101 citations

TL;DR

This study presents 23 transit and 7 occultation light curves of the ultra-short period exoplanet WASP-43 b, combined with 8 new radial velocity measurements, to significantly refine the system's parameters. The high-precision data break the stellar mass degeneracy from the discovery paper, yielding a refined stellar mass of 0.717±0.025 M☉ and radius of 0.667±0.011 R☉, and confirm a high planetary density of 2.034±0.052 M_Jup with a 1.036±0.019 R_Jup, indicating a massive core and poor heat redistribution from dayside to nightside.

ABSTRACT

We present twenty-three transit light curves and seven occultation light curves for the ultra-short period planet WASP-43 b, in addition to eight new measurements of the radial velocity of the star. Thanks to this extensive data set, we improve significantly the parameters of the system. Notably, the largely improved precision on the stellar density (2.41+-0.08 rho_sun) combined with constraining the age to be younger than a Hubble time allows us to break the degeneracy of the stellar solution mentioned in the discovery paper. The resulting stellar mass and size are 0.717+-0.025 M_sun and 0.667+-0.011 R_sun. Our deduced physical parameters for the planet are 2.034+-0.052 M_jup and 1.036+-0.019 R_jup. Taking into account its level of irradiation, the high density of the planet favors an old age and a massive core. Our deduced orbital eccentricity, 0.0035(-0.0025,+0.0060), is consistent with a fully circularized orbit. We detect the emission of the planet at 2.09 microns at better than 11-sigma, the deduced occultation depth being 1560+-140 ppm. Our detection of the occultation at 1.19 microns is marginal (790+-320 ppm) and more observations are needed to confirm it. We place a 3-sigma upper limit of 850 ppm on the depth of the occultation at ~0.9 microns. Together, these results strongly favor a poor redistribution of the heat to the night-side of the planet, and marginally favor a model with no day-side temperature inversion.

Motivation & Objective

• To resolve the degeneracy in the stellar mass solution of WASP-43 b reported in the discovery paper.
• To improve the precision of the planetary and stellar parameters through extensive ground-based photometric monitoring of transits and occultations.
• To constrain the orbital eccentricity and detect thermal emission from the planet's dayside at multiple wavelengths.
• To assess the potential presence of additional companions via transit timing variations and radial velocity analysis.
• To investigate the thermal structure of the planet’s atmosphere, particularly heat redistribution efficiency and possible temperature inversions.

Proposed method

• High-precision photometry was obtained using the TRAPPIST and Euler telescopes at ESO La Silla, and VLT/HAWK-I at Paranal, to observe 23 transits and 7 occultations of WASP-43 b.
• Radial velocity measurements of the host star were collected to improve orbital and stellar mass constraints.
• Global modeling of the transit and occultation light curves was performed to derive joint constraints on planetary and stellar parameters, including orbital inclination, impact parameter, and limb darkening.
• Stellar density was tightly constrained to 2.41±0.08 ρ☉, and combined with age constraints (younger than a Hubble time), the stellar mass and radius were uniquely determined.
• Occultation depths were measured at 0.9, 1.19, and 2.09 μm, with statistical significance assessed via χ² and signal-to-noise ratios.
• A dynamical analysis using the Mercury n-body integrator was applied to simulate potential perturbers, assessing detectability thresholds for additional planets via transit timing variations and radial velocity amplitudes.

Experimental results

Research questions

• RQ1 What is the true stellar mass and radius of WASP-43 b's host star, resolving the degeneracy present in the discovery paper?
• RQ2 What is the precise orbital eccentricity of WASP-43 b, and is it consistent with tidal circularization?
• RQ3 What is the thermal emission spectrum of WASP-43 b, and what does it reveal about heat redistribution to the nightside?
• RQ4 Are there detectable transit timing variations that could indicate the presence of a second planet in the system?
• RQ5 Does the observed thermal emission favor a model with or without a day-side temperature inversion?

Key findings

• The stellar mass and radius were determined with high precision as 0.717±0.025 M☉ and 0.667±0.011 R☉, resolving the degeneracy in the original discovery paper.
• The planetary mass and radius were refined to 2.034±0.052 M_Jup and 1.036±0.019 R_Jup, indicating a high density consistent with a massive core.
• The orbital eccentricity was measured as 0.0035−0.0025+0.0060, consistent with a fully circularized orbit.
• A robust detection of thermal emission was achieved at 2.09 μm with a depth of 1560±140 ppm at >11σ significance.
• A marginal detection at 1.19 μm was found with a depth of 790±320 ppm, while a 3σ upper limit of 850 ppm was set at ~0.9 μm.
• The combined thermal emission data strongly favor inefficient heat redistribution from the dayside to the nightside and marginally support a model with no day-side temperature inversion.