[Paper Review] Transiting exoplanets from the CoRoT space mission. XV. CoRoT-15b: a brown dwarf transiting companion

F. Bouchy, M. Deleuil+4 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)|Oct 1, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy35 references83 citations

TL;DR

This paper reports the discovery of CoRoT-15b, a transiting brown dwarf with a mass of 63.3 ± 4.1 M_Jup and radius of 1.12⁺⁰.³⁰₋₀.¹⁵ R_Jup, orbiting an F7V star every 3.06 days. The system shows evidence of double-synchronization, and the brown dwarf's inflated radius suggests it may be young or subject to enhanced atmospheric opacity, challenging standard evolution models.

ABSTRACT

We report the discovery by the CoRoT space mission of a transiting brown dwarf orbiting a F7V star with an orbital period of 3.06 days. CoRoT-15b has a radius of 1.12 +0.30 -0.15 Rjup, a mass of 63.3 +- 4.1 Mjup, and is thus the second transiting companion lying in the theoretical mass domain of brown dwarfs. CoRoT-15b is either very young or inflated compared to standard evolution models, a situation similar to that of M-dwarfs stars orbiting close to solar-type stars. Spectroscopic constraints and an analysis of the lightcurve favors a spin period between 2.9 and 3.1 days for the central star, compatible with a double-synchronisation of the system.

Motivation & Objective

• To identify and characterize a transiting brown dwarf in the CoRoT space mission's SRa02 field.
• To determine the physical properties of CoRoT-15b, including mass, radius, and orbital parameters, through photometric and spectroscopic follow-up.
• To investigate the dynamical state of the system, particularly the spin-orbit synchronization of the host star and brown dwarf.
• To explore the cause of the brown dwarf's inflated radius, comparing it to theoretical evolution models and other known inflated companions.
• To assess the role of irradiation, atmospheric opacity, and spot coverage in explaining the observed radius anomaly.

Proposed method

• CoRoT space telescope performed high-precision photometry over 31.7 days in the SRa02 field, detecting transit signals in target SRa02_E1_4106.
• Radial velocity measurements were obtained using the HARPS spectrograph at ESO La Silla Observatory to determine the mass of the companion.
• Stellar light curves and radial velocity curves were jointly modeled to derive orbital and physical parameters, including the stellar spin period.
• Evolutionary models were computed using CEPAM with pp-chain nuclear reactions and adjusted to Baraffe et al. (2003) tracks, incorporating thermal and visible opacities.
• The effects of irradiation, atmospheric opacity, and spot coverage on radius inflation were tested by varying model parameters such as metallicity and spot coverage fraction.
• Secondary transit observations were considered as a future method to probe the atmospheric properties of the highly irradiated brown dwarf.

Experimental results

Research questions

• RQ1 What is the mass and radius of CoRoT-15b, and does it fall within the theoretical brown dwarf mass range?
• RQ2 Is the host star's spin period synchronized with the orbital period, indicating double-synchronization in the system?
• RQ3 Why is CoRoT-15b's radius significantly larger than predicted by standard evolution models?
• RQ4 Can the radius inflation be explained by youth, atmospheric opacity, or spot coverage, and what constraints do the data place on these mechanisms?
• RQ5 How does the system's dynamical state inform tidal dissipation in F-type stars?

Key findings

• CoRoT-15b has a mass of 63.3 ± 4.1 M_Jup and a radius of 1.12⁺⁰.³⁰₋₀.¹⁵ R_Jup, placing it firmly in the brown dwarf mass domain.
• The orbital period is 3.06 days, and the host star's spin period is constrained to 2.9–3.1 days, indicating near double-synchronization.
• The brown dwarf's radius is larger than standard evolution models predict, suggesting it may be either very young or subject to enhanced atmospheric opacity.
• Modeling shows that irradiation effects are negligible in the brown dwarf regime, and the radius inflation cannot be explained by standard opacity assumptions.
• A combination of young age and extensive spot coverage (up to ~50% of the photosphere) could explain the inflation, though this remains speculative.
• The system is a key target for future infrared secondary transit observations to probe the atmosphere of this highly irradiated brown dwarf.