Is the activity level of HD 80606 influenced by its eccentric planet?

Abstract

Aims. Several studies suggest that the activity level of a planet-host star can be influenced by the presence of a close-by orbiting planet. Moreover, the interaction mechanisms that have been proposed, magnetic interaction and tidal interaction, exhibit a very different dependence on the orbital separation between the star and the planet. A detection of activity enhancement and characterization of its dependence on planetary orbital distance can, in principle, allow us to characterize the physical mechanism behind the activity enhancement. Methods. We used the HARPS-N spectrograph to measure the stellar activity level of HD 80606 during the planetary periastron passage and compared the activity measured to that close to apastron. Being characterized by an eccentricity of 0.93 and an orbital period of 111 days, the system's extreme variation in orbital separation makes it a perfect target to test our hypothesis. Results. We find no evidence for a variation in the activity level of the star as a function of planetary orbital distance, as measured by all activity indicators employed: log(R HK ), H α , NaI, and HeI. None of the models employed, whether magnetic interaction or tidal interaction, provides a good description of the data. The photometry revealed no variation either, but it was strongly affected by poor weather conditions. Conclusions. We find no evidence for star-planet interaction in HD 80606 at the moment of the periastron passage of its very eccentric planet. The straightforward explanation for the non-detection is the absence of interaction as a result of a low magnetic field strength on either the planet or the star and of the low level of tidal interaction between the two. However, we cannot exclude two scenarios: i) the interaction can be instantaneous and of magnetic origin, being concentrated on the substellar point and its surrounding area; and ii) the interaction can lead to a delayed activity enhancement. In either scenario, a star-planet interaction would not be detectable with the dataset described in this paper.

Close-by planets and flares in their host stars

“…The energy is released when the planet moves across a radial structure of the inhomogeneous stellar magnetic field such as a tall coronal streamer. In the case of HD 80606, the predicted energy release is about 4 times that of HD 17156, if its surface field is B 0 ∼ 10 G, confirming that this system is worth of a systematic monitoring for flaring activity close to periastron as suggested by Figueira et al (2016). A measurement of its coronal activity level or photospheric magnetic field would be welcome to exclude a pathologically weak surface field as in the case of WASP-18 (Pillitteri et al 2014b) that could make its flares extremely weak, sporadic, and undetectable.…”

“…A measurement of its coronal activity level or photospheric magnetic field would be welcome to exclude a pathologically weak surface field as in the case of WASP-18 (Pillitteri et al 2014b) that could make its flares extremely weak, sporadic, and undetectable. Indeed, the low level of its chromospheric activity (log R HK = −5.06, Figueira et al 2016), suggests that it is a rather inactive star. Table 4.…”

Detecting the general relativistic orbital precession of the exoplanet HD 80606b

“…HD 80606b is the only planetary companion detected today in that system. More than 15 years of radial-velocity monitoring 7 did not provide a hint of any additional companion (Hébrard et al 2011;Figueira et al 2016). With the available data, one can put a 3-σ upper limit of 0.4 m/s/yr of a possible linear drift in addition to the radial velocity signature of HD 80606b; this allows any additional planetary companion with a sky-projected mass larger than 0.5 Jupiter mass to be excluded with an orbital period shorter than 40 years around HD 80606.…”

Stellar magnetic activity and exoplanets

“…Repeated observations (at different epochs), however, failed to detect such modulations [40], leading the authors to suggest that activity enhancement is intermittent and could depend, among others, on the configuration of the star's magnetic field. Searching for chromospheric/transition region signatures of planet-star interaction has developed into an active field of research, yielding positive (or tentatively positive) [41][42][43] and negative [44][45][46][47][48] detections.…”

“…This emission, happening near the closest approach of the planet where planet-star interactions are expected to be enhanced, could have been mediated by magnetic reconnection or accretion of planetary material towards the star [e.g., 49]. Following the same idea, i.e., that eccentric planets can trigger stellar activity at orbital phases near the periastron passage, [48] investigated if the activity level of HD 80606 could have been influenced by its eccentric planet. For that, they used optical spectroscopy and photometric monitoring of the star during the periastron passage and close to the apastron passage.…”