Planets and stellar activity: hide and seek in the CoRoT-7 system★

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 offtransit 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 M ⊕ and 13.56 ± 1.08 M ⊕ , respectively. The density of CoRoT-7b is (6.61 ± 1.72)(R p /1.58 R ⊕ ) −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 ∼ 7.9 days. Using Bayesian model selection we find that a model with two planets plus activity-induced variations is most favoured.

SECURE MASS MEASUREMENTS FROM TRANSIT TIMING: 10 KEPLER EXOPLANETS BETWEEN 3 AND 8 M ⊕ WITH DIVERSE DENSITIES AND INCIDENT FLUXES

“…Radial velocity (RV)spectroscopy has provided the bulk of mass measurements of transiting exoplanets (Batalha et al 2011;Winn et al 2011;Fressin et al 2012;Gillon et al 2012;Gautier et al 2012;Howard et al 2013;Pepe et al 2013;Weiss et al 2013;Haywood et al 2014;Marcy et al 2014;Dressing et al 2015). However, among subNeptune-mass planets, RV mass detections are limited to planets on short orbital periods, because the RV signal depends on the strength of planet-star interactions and declines with orbital distance.…”

Modelling the RV jitter of early-M dwarfs using tomographic imaging

“…Moreover, adapting our method to stars of other spectral types (G and K), for which the activity jitter is no longer dominated by the spot brightness contrast but by plages and the suppression of convective blueshift (e.g., Haywood et al 2014), is another obvious avenue worth exploring. Finally, a complementary study is in preparation to present the performances of this technique for M-dwarfs hosting a planet.…”

“…The accuracy to which the RV jitter can be corrected with this method largely depends on various parameters, e.g., the distribution of spots, the stellar inclination, the rotational broadening of line profiles. An alternative method is based on exploiting complementary information from velocimetric and high-cadence photometric simultaneous/contemporaneous observations, and make use of the predicted relationship between the photometric and RV signatures of spots (Aigrain et al 2012;Haywood et al 2014). These studies found that RV modulation caused by spots can be reliably modeled using the photometric flux F and its first derivative F .…”

The HARPS search for southern extra-solar planets

“…We modeled the short-term part of the activity as well as additional instrumental noise (not included in the measurement uncertainties provided by the pipeline, which only considers the photon noise contribution) using a Gaussian process (e.g., Baluev 2011;Haywood et al 2014). We assumed the noise ( k in Eq.…”