Talare: Heather Cegla

Titel: Probing the surfaces of Sun-like stars using and 3D magnetohydrodynamical simulations and transiting planets

Sammanfattning: Inhomogeneities on stellar surfaces pose the fundamental stumbling block on the pathway to true Earth analogues. This is especially pertinent as we enter the era of 10 cm/s radial velocity (RV) precision, with spectrographs like ESPRESSO continuing to come online. From a spectroscopic point of view, manifestations of stellar activity (such as star-spots, plage/faculae, convective flows, and oscillations) alter the observed stellar line profiles. In turn, these time-variable line asymmetries can be mistakenly interpreted as whole-sale Doppler shifts that mask or mimic planetary signals. Here, I will focus on the impact of solar surface oscillations and magnetoconvection, as these ‘noise’ sources are present on even the (magnetically) quietest exoplanet host stars. I will demonstrate that we can bin down the pressure-mode oscillations to ~10 cm/s with an exposure time of just 5.4 minutes. Moreover, I will show how exposure times slightly larger than this can actually increase the noise level, and how even doubling the exposure time has little impact. In addition, I will show how magnetoconvection does not average out well over such timescales, and how its centre-to-limb dependence can impact exoplanet measurements. Using 3D solar MHD simulations as a backbone, I will explore both the oscillation and convective induced line shape changes, and demonstrate how these changes can be used to track the remaining convective noise. Hence, in the era of 10 cm/s RV precision, I will show that we should we be fine-tuning exposure times to our host star parameters, as well as exploiting the line profile characteristics to mitigate the astrophysical noise emanating from stellar convective envelopes. Alongside this, I will show how we can use transiting planets to probe and spatially resolve stellar surfaces, which in turn helps us to validate MHD simulations and determine 3D star-planet trajectories — that ultimately feed into planet formation, migration and evolution theories. We have successfully applied this new technique to HD 189733, as well as for Wasp-8, where we found previous results may have been biased. We have also shown this is an effective tool even for the coolest and slowest rotating stars, by determining the first obliquity for a (Neptune-mass) planet around a M dwarf (GJ 436).