2018MNRAS.481.5286F

HD 219134 hosts several planets, with seven candidates reported, and the two shortest period planets are rocky (4-5 M_⊕) and transit the star. Here, we present contemporaneous multiwavelength observations of the star HD 219134. We observed HD 219134 with the Narval spectropolarimeter at the Observatoire du Pic du Midi and used Zeeman-Doppler imaging to characterize its large-scale stellar magnetic field. We found a weak poloidal magnetic field with an average unsigned strength of 2.5 G. From these data, we confidently confirm the rotation period of 42 d, measure a stellar inclination of 77 ± 8°, and find evidence for differential rotation. The projected obliquity of the two transiting super-Earths is therefore between 0 and 20°. We employed HST STIS observations of the Ly α line to derive a stellar wind mass-loss rate of half the solar value (10^–14 M_☉ yr^–1). We further collected photometric transit observations of the closest planet at near-UV wavelengths centred on the Mg II H&K lines with AstroSat. We found no detectable absorption, setting an upper limit on the transit depth of about 3 per cent, which rules out the presence of a giant magnesium cloud larger than 9R_planet. Finally, we estimated the high-energy flux distribution of HD 219134 as seen by planets b and c. These results present a detailed contemporaneous characterization of HD 219134 and provide the ingredients necessary for accurately modelling the high-energy stellar flux, the stellar wind, and their impact on the two shortest period planets, which will be presented in the second paper of this series.