[Paper Review] Transiting exoplanets from the CoRoT space mission V. CoRoT-Exo-4b: Stellar and planetary parameters

C. Moutou, H. Bruntt+3 more|Bern Open Repository and Information System (University of Bern)|Jul 23, 2008

Stellar, planetary, and galactic studies26 references43 citations

TL;DR

This paper presents the characterization of CoRoT-Exo-4b, a transiting gas giant exoplanet with a 9.2-day orbital period, discovered by the CoRoT space mission. Using combined space photometry, ground-based radial velocity measurements from SOPHIE and HARPS, and high-resolution spectroscopy, the study determines the planet's mass (0.72 M_Jup), radius (1.19 R_Jup), and low mean density (0.525 g cm⁻³), confirming it as a typical hot Jupiter with a normal H/He-dominated interior structure, filling a previously empty region in the mass-period diagram of transiting exoplanets.

ABSTRACT

The CoRoT satellite has announced its fourth transiting planet (Aigrain et al. 2008) with space photometry. We describe and analyse complementary observations of this system performed to establish the planetary nature of the transiting body and to estimate the fundamental parameters of the planet and its parent star. We have analysed high precision radial-velocity data, ground-based photometry, and high signal-to-noise ratio spectroscopy. The parent star CoRoT-Exo-4 (2MASS 06484671-0040219) is a late F-type star of mass of 1.16 Msun and radius of 1.17 Rsun. The planet has a circular orbit with a period of 9.20205d. The planet radius is 1.19 Rjup and the mass is 0.72 Mjup. It is a gas-giant planet with a ''normal'' internal structure of mainly H and He. CoRoT-Exo-4b has the second longest period of the known transiting planets. It is an important discovery since it occupies an empty area in the mass-period diagram of transiting exoplanets.

Motivation & Objective

• To confirm the planetary nature of CoRoT-Exo-4b, a transiting object detected by the CoRoT space mission.
• To determine the fundamental parameters of the host star and the transiting planet using multi-wavelength follow-up observations.
• To characterize the orbital and physical properties of CoRoT-Exo-4b, particularly its mass, radius, and internal structure.
• To place the planet in the context of the mass-period diagram of transiting exoplanets, addressing the scarcity of long-period transiting giants.
• To assess the consistency of the planet's properties with theoretical models of planetary evolution and internal structure.

Proposed method

• High-precision radial velocity measurements were obtained using the SOPHIE spectrograph at the Observatoire de Haute-Provence and the HARPS spectrograph at ESO’s La Silla Observatory.
• Ground-based photometry was used to validate the transit signal and assess contamination from nearby stars.
• High signal-to-noise ratio spectroscopy was employed to derive the stellar effective temperature, surface gravity, metallicity, and v sin i of the host star.
• Stellar parameters were derived from spectroscopic analysis and combined with transit light curve modeling from CoRoT space photometry.
• Orbital parameters, including period, semi-amplitude, and eccentricity, were derived from radial velocity curve fitting.
• Planetary mass and radius were computed using the transit depth from CoRoT light curves and radial velocity semi-amplitude, with uncertainties propagated via Monte Carlo simulations.

Experimental results

Research questions

• RQ1 What are the precise mass and radius of CoRoT-Exo-4b, and how do they compare to theoretical expectations for gas giants?
• RQ2 Is CoRoT-Exo-4b a bona fide planet, and what evidence supports its planetary nature beyond the transit signal?
• RQ3 How does the planet’s location in the mass-period diagram compare to other known transiting exoplanets, and what does this imply for planet formation and evolution?
• RQ4 What is the internal structure of CoRoT-Exo-4b, and does it contain a significant fraction of heavy elements compared to other gas giants?
• RQ5 How do the derived parameters of the host star (mass, radius, age, metallicity) constrain the system’s formation history and planetary evolution?

Key findings

• The host star CoRoT-Exo-4 has a mass of 1.16 M_☉ and a radius of 1.17 R_☉, with an effective temperature of 6190 K and surface gravity log g = 4.41.
• CoRoT-Exo-4b has a circular orbit with a period of 9.20205 ± 0.00037 days and a semi-major axis of 0.090 ± 0.001 AU.
• The planet has a mass of 0.72 ± 0.08 M_Jup and a radius of 1.19_-0.05^+0.06 R_Jup, resulting in a mean density of 0.525 ± 0.15 g cm⁻³.
• The planet's low density and location in the mass-radius diagram are consistent with a gas-giant composition dominated by hydrogen and helium, with a heavy element content of 0 to ∼30 M_⊕ depending on the model.
• CoRoT-Exo-4b is the second-longest-period transiting planet known at the time of publication, occupying a previously unpopulated region in the mass-period diagram.
• The estimated age of the host star is approximately 1 Gyr, with a range of 0.7 to 2.0 Gyr, and the star shows signs of moderate activity consistent with its age and rotation rate.