Abstracts Earth and Environmental Sciences

The winds of young Suns

by Dag Evensberget

Stars produce stellar wind—a tenuous flow of magnetised plasma extending deep into circumstellar space. It is thought that forceful stellar wind can erode the atmospheres of planets lacking strong magnetic fields, and thus reduce the planets' potential habitability. The slowing rotation ofSolar-type stars with increasing age is also caused by the stellar wind. In this Ph. D. project we have mapped the stellar winds of a selection of young, Sunlike stars with well characterised ages. These stars have had their surface magnetic field mapped using Zeeman-Doppler imaging. By driving a state of the art Solar wind code with the stellar surface magnetic fields we have obtained three-dimensional steady-state wind maps of 25 stars aged between 0.1 and 0.6 billion years. Unlike later epochs, this age range is characterised by the absence of single valued relations between stellar age, stellar rate of rotation, and measures of stellar magnetic activity. The resulting collection of wind maps is the largest created to date, and permits the application of robust statistics. We find that the Earth's magnetic field has protected the Earth from atmospheric erosion by strong Solar winds for at least 4 billion years in spite of the more powerful winds of the young Sun. For Solar-type stars younger than about 1/4 billion years, we find that the stellar spin-down resulting from our wind models is insufficient to explain the observed spin-down of stars with increasing age. This suggests that transient phenomena such as coronal mass ejections may account for a large part of these stars' spin-down.