First snapshot of a magnetosphere around a Herbig Ae/Be star

Context. The Herbig Ae/Be star HD 190073 is one of the very few magnetic Herbig Ae/Be stars for which close low-mass companions have been reported. Previously published magnetic field measurements indicated an annual change in the field configuration. Aims. We aim to study in detail the spectral and magnetic variability of this star and characterise its magnetosphere for the first time. Methods. Newly acquired and archival spectropolarimetric observations are combined to determine a more precise magnetic period and to constrain the geometry of the magnetic field. The variability of hydrogen line profiles is studied using dynamical spectra. Archival X-shooter observations of the He I 10 830 Å triplet are used to characterise its variability over the rotation cycle. Further, we carry out 2D magnetohydrodynamical (MHD) simulations of the magnetosphere using the NIRVANA MHD code. Results. From the spectropolarimetric observations, we determine for HD 190073 a magnetic period P = 51.70 ± 0.06 d. We estimate a magnetic obliquity angle β = 82.9 ± 6.4° and a dipole strength Bd = 222 ± 66 G. Our dynamical spectra constructed for the hydrogen line profiles observed during 2011 clearly reveal a ringlike magnetospheric structure appearing at the rotation phase of best visibility of the positive magnetic pole. These spectra present the first snapshot of a magnetosphere around a Herbig Ae/Be star. 2D MHD simulations involving nonisothermal gas show that the magnetosphere is compact, with a radius of about 3 R*, and that the wind flow extends over tens of R*. With a reported radius of the accretion disk of 1.14 au around HD 190073, the distance between the star and the disk is about 25 R*. The detection of a magnetosphere around HD 190073, and the possible presence of lower-mass companions at different distances, make this system a valuable laboratory for studying the magnetic interaction between the host star, its companions, and the accretion disk.