Time evolution and rotation of starspots on CoRoT-2 from the modelling of transit photometry

Abstract

Context. CoRoT-2, the second planet-hosting star discovered by the CoRoT satellite, is a young and active star. A total of 77 transits were observed for this system over a period of 135 days. Aims. Small modulations detected in the optical light curve of the planetary transits are used to study the position, size, intensity, and temporal evolution of the photospheric spots on the surface of the star that are occulted by the planetary disk. Methods. We apply a spot model to these variations and create a spot map of the stellar surface of CoRoT-2 within the transit band for every transit. From these maps, we estimate the stellar rotation period and obtain the longitudes of the spots in a reference frame rotating with the star. Moreover, the spots temporal evolution is determined. This model achieves a spatial resolution of 2 • . Results. Mapping of 392 spots vs. longitude indicates the presence of a region free of spots, close to the equator, which is reminiscent of the coronal holes observed on the Sun during periods of maximum activity. With this interpretation, the stellar rotation period within the transit latitudes of −14.• 6 ± 10 • is obtained from the auto-correlation function of the time-integrated spot flux deficit, which yields a rotation period of 4.48 days. With this period, the temporal evolution of the spot surface coverage in individual 20 • longitude bins has periodicities ranging from 9 to 53 days with an average value of 31 ± 15 days. On the other hand, the longitude integrated spot flux, which is independent of the stellar rotation period, oscillates with a periodicity of 17.7 days, and its false-alarm probability is ∼3%. Conclusions. The rotation period of 4.48 days obtained here is shorter than the 4.54 days derived from the out-of-transit light modulation. Because the transit data sample a region close to the stellar equator while the period determined from out-of-transit data reflects the average rotation of the star, this is taken as an indication of a latitudinal differential rotation of about 3% or 0.042 rad/d.

Starspot activity and rotation of the planet-hosting star Kepler-17

“…For starspot C we find almost no coincidence, as expected owing to its smaller spotted area. The resolution of the transit-mapping method is of the order of some degrees (cf., e.g., Silva-Valio et al 2010;Silva-Valio & Lanza 2011, and references therein) while that of our mapping based on the out-of-transit light curve is approximately 40 • −50 • . Therefore, we are not able to resolve the occulted spots with the same detail as D11, especially when there are several small spots close in longitude.…”

“…For a different modelling approach, based on discrete starspots, see, e.g., Mosser et al (2009). A comparison with other modelling approaches is given in, e.g., Fröhlich et al (2009);Huber et al (2010);Silva-Valio & Lanza (2011).…”

“…Therefore, we are not able to resolve the occulted spots with the same detail as D11, especially when there are several small spots close in longitude. Note that a detailed comparison of the spot maps based on the in-transit and out-of-transit photometry is outside the scope of the present work because it requires a detailed modelling of the transit profile distortions induced by occulted starspots (see Silva-Valio & Lanza 2011, for such a detailed comparison for CoRoT-2). Here we limit ourselves to exploiting the preliminary modelling of D11 to find that our trails of spots with a positive migration rate, i.e., rotating faster than our reference frame, are located inside the belt that is occulted by the planet.…”

“…In addition, the planet occults starspots during transits, giving rise to the typical bumps observed in the bottom of the transit profiles (e.g., Silva-Valio et al 2010;Silva-Valio & Lanza 2011). The integer ratio between P rot and P allowed D11 to see a "stroboscopic" effect with the short-cadence Kepler data: the spots are "mapped" by the planet each 45 • in longitude.…”

Photospheric activity, rotation, and magnetic interaction in LHS 6343 A

“…The occultation of the stellar disc by the BD companion during the transits can be used to resolve the fine structure of the spot pattern and study its evolution (e.g., Silva-Valio et al 2010;Silva-Valio & Lanza 2011). From the phase-folded data shown in Fig.…”