[Paper Review] Transiting exoplanets from the CoRoT space mission IX. CoRoT-6b: a transiting `hot Jupiter' planet in an 8.9d orbit around a low-metallicity star

M. Fridlund, G. Hébrard+46 more|Scientific Electronic Library Online (São Paulo Research Foundation, Latin American and Caribbean Center on Health Sciences Information, Conselho Nacional de Desenvolvimento Científico e Tecnológico)|Jan 9, 2010

Stellar, planetary, and galactic studies73 references46 citations

TL;DR

This paper reports the discovery of CoRoT-6b, a transiting 'hot Jupiter' exoplanet with a 2.96 M_Jup mass and 1.166 R_Jup radius, in an 8.9-day orbit around a solar-type F9V star with low metallicity (−0.20 dex). The planet's high density (2.32 g cm⁻³) and lack of secondary eclipse in CoRoT data confirm its short-period, close-in orbit, demonstrating CoRoT's capability to detect long-period transiting planets despite stellar activity and low metallicity.

ABSTRACT

The CoRoT satellite exoplanetary team announces its sixth transiting planet in this paper. We describe and discuss the satellite observations as well as the complementary ground-based observations - photometric and spectroscopic - carried out to assess the planetary nature of the object and determine its specific physical parameters. The discovery reported here is a `hot Jupiter' planet in an 8.9d orbit, 18 stellar radii, or 0.08 AU, away from its primary star, which is a solar-type star (F9V) with an estimated age of 3.0 Gyr. The planet mass is close to 3 times that of Jupiter. The star has a metallicity of 0.2 dex lower than the Sun, and a relatively high $^7$Li abundance. While thelightcurveindicatesamuchhigherlevelof activity than, e.g., the Sun, there is no sign of activity spectroscopically in e.g., the [Ca ] H&K lines.

Motivation & Objective

• To identify and characterize a transiting exoplanet from CoRoT space mission data, focusing on planetary parameters and host star properties.
• To investigate the impact of low stellar metallicity on planetary system formation and evolution, particularly in the context of lithium abundance and age estimation.
• To assess the reliability of transit detection and radial velocity measurements in the presence of stellar activity and high rotation rates.
• To determine the physical parameters of CoRoT-6b, including mass, radius, orbital period, and density, using combined space-based and ground-based observations.
• To evaluate the role of lithium abundance as a stellar age indicator in low-metallicity, active stars, and its implications for planetary system evolution.

Proposed method

• Utilized CoRoT space telescope's high-precision, near-continuous photometry to detect transit events in the light curve of CoRoT-0106017681.
• Conducted ground-based photometric and spectroscopic follow-up observations using multiple telescopes to confirm planetary transit and measure radial velocity variations.
• Applied radial velocity measurements to determine the planet's mass, using the Doppler shift in stellar spectral lines.
• Employed stellar evolution models (STAREVOL and CESAM) combined with the M¹ᐟ³/R⋆ relation to estimate the star's age from its radius and mass.
• Measured lithium (⁷Li) equivalent width (75 mÅ) in the stellar spectrum to infer age and assess lithium depletion as a function of metallicity and activity.
• Analyzed the absence of secondary eclipse in CoRoT data to infer orbital distance and constrain the planet's albedo and thermal emission.

Experimental results

Research questions

• RQ1 What are the physical parameters (mass, radius, orbital period) of the transiting planet CoRoT-6b, and how do they compare to other known hot Jupiters?
• RQ2 How does the low metallicity (−0.20 dex) of the host star CoRoT-6 affect planetary system formation and lithium abundance?
• RQ3 Can the high lithium abundance (log N/N_tot = +1.91) and stellar activity be reconciled with the estimated age of ~3 Gyr, and what does this imply about stellar evolution models?
• RQ4 Why is no secondary eclipse detected in the CoRoT light curve, and how does this constrain the orbital inclination and distance from the star?
• RQ5 To what extent does the planet's gravitational influence affect lithium depletion in the host star, given its relatively large orbital separation (18 R⋆)?

Key findings

• CoRoT-6b is a massive hot Jupiter with a mass of 2.96 M_Jup and a radius of 1.166 R_Jup, resulting in a high density of 2.32 g cm⁻³.
• The planet orbits its host star in an 8.9-day period with a circular orbit (e ≤ 0.1), at a distance of 18 stellar radii (0.08 AU), consistent with a transiting configuration.
• The host star, CoRoT-6, is an F9V-type star with a mass of 1.05 M_⊙, radius of 1.025 R_⊙, and low metallicity of −0.20 dex, placing it below the typical metallicity of planet-hosting stars.
• The star exhibits a high ⁷Li abundance (log N/N_tot = +1.91), which, combined with low metallicity, suggests a relatively young age (2.5–3.3 Gyr) despite high activity and rapid rotation.
• No secondary eclipse was detected in the CoRoT light curve, consistent with the planet's large orbital distance and low thermal emission, supporting the absence of significant reflected light or thermal flux.
• Stellar evolution models and lithium abundance analysis jointly constrain the age of CoRoT-6 to 1–3.3 Gyr, with the higher end favored by lithium data, indicating that low-metallicity stars may preserve lithium longer due to shallower convection zones.