Paper title: Searching for star-planet magnetic interaction in CoRoT observations
Authors: A. F. Lanza
First author’s affiliation: NAF-Osservatorio Astrofisico di Catania
Introduction: This paper investigates the interaction between close-in (semimajor axis a<0.15AU) massive planets (a.k.a. “hot Jupiters'') and their host (late-type) stars. Two possible mechanisms for interaction are tidal and magnetic, with the focus of this paper being the latter. The pioneering work on the topic of stellar activity enhancement (such as dark spots, faculae, etc) due to planet interaction is by Cuntz et al. (2000). You can see related contributions about stellar activity on previous astrobites posts.
Premise: Previous observational evidence for magnetic interaction between planets and their stars came from a sample of twelve hot Jupiter systems reported by Shkolnik et al. (2003, 2005). In some cases, the stellar activity was related to the planet’s period, rather than the star’s rotation period. When observing chromospheric emission (seen in CaII H&K), hot spots rotated in phase with orbiting planets, irradiated powers up to 1021 W, and appeared to turn “on” and “off” within lifetimes of about 300-400 days.
Observations: Shkolnik et al. (2008) called this effect “star-planet magnetic interaction” (SPMI), and Lanza provides a nice figure to envision this interaction (see the featured figure). SPMI signatures have been searched for in a range of wavelengths, from X-ray to radio. The three cases specifically investigated in this paper were observed photometrically with CoRoT. These three stars (named CoRoT-2, CoRoT-4, and CoRoT-6), are interesting test-cases because they have relatively different ratios of rotational period (Prot) to orbital period (Porb). For CoRoT-2, Prot > Porb; for CoRoT-4, Prot ∼ Porb; and for CoRoT-6, Prot < Porb.
For each star, observations (see figure to left) are compared with models, to investigate the distribution of active regions versus longitude, as well as the lifetimes of these active regions. From the figure it is obvious that the total spotted area is not constant in time. One explanation for the oscillations of the total spotted area is analogous to the oscillations of the total sunspot area related to the eleven-year cycles in the Sun. In this case, magnetohydrodynamic waves are excited at the interface of the convection zone and the radiative interior of the Sun.
Alternatively, the modulation of the spotted area could be a signature of the SPMI, and this may be the case if periodicity correlates with the period of the planet. This is explained if the passage of the planet over an active region triggers the emergence of magnetic flux. Spots occulted by the planet during transit are seen as characteristic light bumps (maxima) along the transit light curve.
Models: The hypothesis is that the periodicity of the total area of occulted spots, and thus activity, relates to the planet’s period. To quote Lanza, “the passage of the planet over the stellar active regions seems to be associated with a triggering of new magnetic flux emergence.”
How do the models explain what is observed? This is Lanza’s forte. The first proposed explanation for chromospheric hot spots rotating in phase with the planet is a scaled version of the so-called unipolar induction model proposed for the Jupiter-Io system — where bright UV spots are observed. However, there are several aspects to observations of the observed star-planet systems that the model has difficulty explaining: the planet lags the hot spot, and the emitted power is off by several orders of magnitude. These seem to be reconciled with twisted “linear force-free field” lines, explored more by Lanza (2008). Also, when this helicity or the boundary conditions change, then the SPMI features switch “on” or “off”, as was observed (and mentioned previously in this post).
Finally, Lanza goes one step further by considering the connection between the perturbation produced by the planet, which powers the stellar dynamo. The alpha effect (related to the mean helicity of the turbulent motions, discussed more by Reudiger et al. 2011), owing to the interaction with the planet, modulates the field of a late-type star. This modulation has a period related to the planet and allows the dynamo effect.
More to come: Now is an exciting time to for this type of study, because the Kepler space telescope is recording light curves for many, many stars (check out the “Bad Astronomy” blog post about Kepler). The paper by Lanza reports several preliminary Kepler results, and more are sure to come. Okay, so the planet affects the stellar dynamo, as seen by CoRoT and Kepler. What next? Theoretical models should account for the mechanism of the interaction, and for this, maps need to be made by observing the chromospheric hot spots and photospheric field to tell more about the coronal configuration. The author seems optimistic of observations to be made by current and upcoming telescopes.