The magnetic fields of the late-type stars

Abstract

A powerful stellar magnetometer capable of measuring longitudinal magnetic fields as small as a few gauss has been assembled. Observations of known magnetic stars show that the instrument yields the expected values. A detailed study of the formal errors shows that they are realistic, although it cannot be excluded that the most precise measurements may have errors that are twice as large as those claimed. A large number of late-type stars have been observed, and mostly nulls or detections that are suspect have been obtained. However, definite detections for the Ca II emission dwarf Xi Boo A and probable detections in the RS CVn star UX Ari have been found. Observations of the transverse Zeeman effect do not show evidence of strong transverse fields, but the uncertainties are large (a few hundred gauss). An approximate estimate of the magnetic geometries of active dwarfs is made concluding that the most likely geometry is complex and composed of several hundred patches of opposite polarities.