The Starspots and Magnetic Fields of Young Late-F Stars

Observations of the magnetic activity of young solar-type stars offer insights into the early history of our Sun, its dynamo activity and planetary impacts. The warmest of solar-type stars, of F spectral type, have been shown to represent a transition from dynamo driven cool stars to fossil field dominated hot stars due to their thin convective zones, however, there are few detailed observations of the surface magnetic fields and cycles of these stars. This study thus presents multiple-epoch star spot and surface magnetic field maps of three young late-F solar-type stars, namely VXR 77A (V380 Vel, F8V, ∼27 Myr), HD 43989 (V1358 Ori, F9V, ∼26 Myr) and HIP 71933 (HD 128181, F8V, ∼15 Myr). Those images were produced using the technique of Doppler and Zeeman-Doppler Imaging using spectroscopic and spectropolarimetric data, and derived from observations made across fourteen different epochs and multiple telescopes between 2003 and 2019. The resulting star spot brightness maps and the implied underlying dynamos for two of the stars (HD 43989 and VXR 77A) are consistent with those of other solar-type stars including polar spots, but there is a persistent lack of a polar spot on the third star, HIP 71933. Differential rotation measurements performed using these images for all three stars, were found to be slightly higher than that for young G-type stars, a stellar mass dependence consistent with dynamo theory and previous observations. However, in terms of the appearance and temporal evolution of the magnetic fields observed for two of the stars, HD 43989 shows complex magnetic fields apparently evolving over a timescale of two years, but in contrast, HIP 71933 shows a more stable magnetic field configuration, with a dominant and apparently unchanging poloidal field topology over a similar timescale, that warrants further investigation to look for any potential evolution. The radial velocities measured for two of the stars (VXR 77A and HIP 71933) indicate the possibility of a binary, but this is not expected to affect the star spot and magnetic fields results due to the wide separation of any companion, if it exists. In overall terms, this study provides evidence for magnetic dynamos in warm young solar-type showing similarities to those of other young solar-type stars, but with some unusual but not unique features on HIP 71933.