Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system (1407.1044v1)

Published 3 Jul 2014 in astro-ph.EP and astro-ph.SR

Abstract: Since the discovery of the transiting super-Earth CoRoT-7b, several investigations have yielded different results for the number and masses of planets present in the system, mainly owing to the star's high level of activity. We re-observed CoRoT-7 in January 2012 with both HARPS and CoRoT, so that we now have the benefit of simultaneous radial-velocity and photometric data. This allows us to use the off-transit variations in the star's light curve to estimate the radial-velocity variations induced by the suppression of convective blueshift and the flux blocked by starspots. To account for activity-related effects in the radial-velocities which do not have a photometric signature, we also include an additional activity term in the radial-velocity model, which we treat as a Gaussian process with the same covariance properties (and hence the same frequency structure) as the light curve. Our model was incorporated into a Monte Carlo Markov Chain in order to make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We measure the masses of planets b and c to be 4.73 +/- 0.95 Mearth and 13.56 +/- 1.08 Mearth, respectively. The density of CoRoT-7b is (6.61 +/- 1.72)(Rp/1.58 Rearth)^-3 g.cm^-3, which is compatible with a rocky composition. We search for evidence of an additional planet d, identified by previous authors with a period close to 9 days. We are not able to confirm the existence of a planet with this orbital period, which is close to the second harmonic of the stellar rotation at around 7.9 days. Using Bayesian model selection we find that a model with two planets plus activity-induced variations is most favoured.

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Summary

The paper distinguishes stellar activity from planetary signals in the CoRoT-7 system using simultaneous RV and photometric observations.
It determines the masses of CoRoT-7b (4.73 ± 0.95 M⊕) and CoRoT-7c (13.56 ± 1.08 M⊕), confirming CoRoT-7b’s rocky composition.
It demonstrates that advanced modeling with Gaussian processes can effectively isolate activity-induced variations, questioning the need for a third planet.

Analyzing Stellar Activity and Planetary Signals in the CoRoT-7 System

This paper investigates the planetary system around CoRoT-7, with a focus on distinguishing between stellar activity and planetary signals. Using data from the HARPS spectrograph and the CoRoT satellite, this paper provides insights into the complex interaction between exoplanetary detection methods and stellar noise.

Background

Since the discovery of CoRoT-7b, a transiting super-Earth, there have been conflicting results regarding the number and properties of planets in this system. These discrepancies are primarily due to challenges posed by the star’s high level of activity, which significantly affects radial-velocity (RV) measurements. Initially, the star was found to harbor at least two planets, CoRoT-7b and CoRoT-7c, but other possible planets have been debated.

Methodology

The authors employed simultaneous RV and photometric observations. They modeled activity-induced RV variations using data from the CoRoT light curve. The RV perturbations due to stellar activity were estimated by accounting for the suppression of convective blueshift and starspots' influence on the stellar surface. This model incorporated a Gaussian process to address activity-related RV signals without apparent photometric counterparts.

Key Results

Mass Determination: The masses of CoRoT-7b and CoRoT-7c were determined to be 4.73 ± 0.95 M_⊕ and 13.56 ± 1.08 M_⊕, respectively.
Density of CoRoT-7b: The density of CoRoT-7b suggests a rocky composition, inferred to be (6.61 ± 1.72) g/cm³.
Existence of Additional Planets: The authors explored evidence for a third planet, CoRoT-7d, previously suggested to have a ~9-day period. Bayesian analysis favored a model with only the two confirmed planets plus activity-induced variations, ruling out the need for a third planetary companion in the dataset analyzed.

Implications

This paper underscores the challenges of detecting small exoplanets around active stars. The emphasis on a rigorous approach to differentiating stellar activity from planetary signals reinforces the need for advanced techniques in exoplanet detection and characterization. By employing simultaneous photometric and RV observations, combined with sophisticated modeling techniques like Gaussian processes, the paper provides a methodological framework for analyzing other similarly challenging systems.

Future Directions

Speculation on future developments might involve improvements in detection algorithms to better handle stellar activity, and potential advancements in observational technologies that minimize stellar noise interference. Research in this domain is crucial for accurately identifying and confirming the existence of small exoplanets in habitable zones around active host stars.

This paper contributes significantly to the precise measurement and interpretation of planetary properties in systems with dynamic stellar environments, enhancing our understanding of the challenges inherent in exoplanetary science.

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