Photospheric activity and rotation of the planet-hosting star CoRoT-Exo-4a

The space experiment CoRoT has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date. We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation. The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between about 30 and 60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 \pm 0.015 is derived for its relative amplitude. The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions.

💡 Research Summary

The CoRoT space mission discovered a transiting hot‑Jupiter orbiting the moderately active F‑type dwarf CoRoT‑Exo‑4a with an orbital period of 9.202 days, the second longest among known transiting planets. This paper exploits the uninterrupted 58‑day white‑light photometric time series obtained by CoRoT to investigate the host star’s surface rotation and magnetic activity. The authors adopt a maximum‑entropy (ME) spot‑modelling approach that has been extensively validated on solar total irradiance data. In this framework the stellar surface is divided into many small elements, each of which can host a cool spot and a bright facular component. The model parameters – spot and facular filling factors, latitude, longitude, and the spot‑to‑facula area ratio Q – are adjusted to reproduce the observed light curve while maximizing the entropy of the surface map, thereby avoiding over‑interpretation of noise.

Applying the ME model to the CoRoT‑Exo‑4a light curve reveals three dominant active longitudes (ALs) that persist for 30–60 days. Their longitudinal drift indicates that they rotate almost synchronously with the planet’s orbital motion. The measured rotation periods of the three ALs are 9.18 d, 9.24 d and 9.21 d, respectively, giving an average rotation period of 9.20 d, essentially identical to the planetary orbital period. The slight differences among the AL rotation rates are interpreted as a signature of surface differential rotation. By comparing the longest and shortest periods the authors derive a lower limit for the relative differential rotation amplitude ΔΩ/Ω = 0.057 ± 0.015. Although smaller than the solar value (≈0.20), this result demonstrates that even relatively hot F‑type stars can sustain measurable latitudinal shear.

The latitude distribution of the active regions is concentrated near the equator (±30°), reminiscent of the solar activity belt. The best‑fit spot‑to‑facula ratio Q lies in the range 0.2–0.3, indicating that bright facular areas contribute roughly 20 % of the total photometric modulation, a factor that must be included when modelling F‑type stars.

A particularly intriguing observation is a modest but systematic increase in spot coverage when the sub‑planetary longitude (the meridian directly beneath the planet) passes over a given stellar surface sector. This enhancement appears at the same orbital phase in successive rotations, suggesting a possible magnetic star‑planet interaction (SPI). In such a scenario the planet’s magnetic field or its induced magnetospheric currents could locally amplify the stellar magnetic field, triggering spot formation or facular brightening. However, the 58‑day baseline limits the statistical significance of this pattern; longer monitoring would be required to confirm a persistent SPI signal.

In summary, the paper provides three major contributions: (1) a robust detection of quasi‑synchronous rotation between CoRoT‑Exo‑4a’s photosphere and its hot‑Jupiter, (2) the first quantitative estimate of differential rotation for this star, establishing a lower bound of ΔΩ/Ω ≈ 0.06, and (3) tentative evidence for magnetic interaction between the planet and the stellar surface, manifested as enhanced activity at the sub‑planetary longitude. The authors advocate for extended photometric campaigns and high‑resolution spectroscopic follow‑up to refine the differential rotation profile, to map the evolution of active longitudes over longer timescales, and to test the SPI hypothesis with greater confidence.