Photospheric activity, rotation, and radial velocity variations of the planet-hosting star CoRoT-7

Abstract

Context. The CoRoT satellite has recently discovered the transits of an Earth-like planet across the disc of a late-type magnetically active star dubbed CoRoT-7, while a second planet was detected after filtering out the radial velocity (hereafter RV) variations due to stellar activity. Aims. We investigate the magnetic activity of CoRoT-7 and use the results for a better understanding of the impact of magnetic activity on stellar RV variations. Methods. We derived the longitudinal distribution of active regions on CoRoT-7 from a maximum entropy spot model of the CoRoT lightcurve. Assuming that each active region consists of dark spots and bright faculae in a fixed proportion, we synthesized the expected RV variations. Results. Active regions are mainly located at three active longitudes that appear to migrate at different rates, probably as a consequence of surface differential rotation, for which a lower limit of ΔΩ/Ω = 0.058 ± 0.017 is found. The synthesized activity-induced RV variations reproduce the amplitude of the observed RV curve and are used to study the impact of stellar activity on planetary detection. Conclusions. In spite of the non-simultaneous CoRoT and HARPS observations, our study confirms the validity of the method previously adopted to filter out RV variations induced by stellar activity. We find a false-alarm probability <10 −4 that the RV oscillations attributed to CoRoT-7b and CoRoT-7c are spurious effects of noise and activity. Additionally, our model suggests that other periodicities found in the observed RV curve of CoRoT-7 could be explained by active regions whose visibility is modulated by a differential stellar rotation with periods ranging from 23.6 to 27.6 days.

Transiting exoplanets from the CoRoT space mission

“…We have discussed the consequences of this characterization and we presented evidence that this particular target provides important information about the knowledge of the interactions between the structure of stars, the formation of planets, and their joint evolution (Ikoma et al 2006;Ammler-von Eiff et al 2009;Meléndez et al 2009a;Nordlund 2009;Ramírez et al 2009;Lanza 2010).…”

“…We searched for signs of stellar rotation in the light curve (LC) observed by CoRoT to compare with the clear signs of Table 5. spot modulation found in the cases of CoRoT-2b (Lanza et al 2009b), CoRoT-4b (Lanza et al 2009a), CoRoT-6b (Fridlund et al 2010) or CoRoT-7b (Lanza et al 2010). The Lomb-Scargle periodogram of the LC, once the planetary transits have been removed and the hot-pixel events treated, indeed exhibits a significant broad peak around 77 days, but no significant peak at the expected rotational frequencies.…”

“…We therefore conclude that the modulation measured in the LC of CoRoT-13 is due to a background contaminant and not the main target. Lanza (2010) found a relation between the presence of giant planets and the angular momentum loss of their host stars, such that giant planet hosting stars with T eff 6000 K tend to have an n/Ω ≈ 1, 2 orbital period/rotational period synchronization with their respective planets. The effective temperature of CoRoT-13 is immediately below this lower limit and the rotational period obtained from the spectroscopic analysis (around 13 days) is indeed longer than the expected 2:1 resonance or the synchronization.…”

Multiwavelength flux variations induced by stellar magnetic activity: effects on planetary transits

“…4 shows ΔR p /R p vs. wavelength for CoRoT-7. The spot filling factor value is f 0 = 0.04 by deducing it from the optical flux variability reported in Lanza et al (2010).…”

Astrophysical false positives in direct imaging for exoplanets: a white dwarf close to a rejuvenated star

“…Simultaneous monitoring of activity indicators, spectral line profile changes, and photometric variations may allow one to recognize the true origin of the radial velocity variations and to at least partially account for them (see e.g. Boisse et al 2011;Lanza et al 2010;Dumusque et al 2012).…”