First conclusive evidence for rotational modulation of stellar radio emission

Abstract

We present what we believe to be the first conclusive evidence for rotational modulation of stellar radio emission, observed on the young and rapidly rotating K1 dwarf star AB Doradus (HD 36705). The radio emission of this star peaked near two stellar longitudes where large starspots have been observed to form preferentially. One of the peaks in radio emission was coincident with the central meridian passage of a large starspot; the starspot associated with the other peak may have been overwhelmed by the visible starspot. The modulation in the radio emission was probably caused by a combination of two factors: (1) occultation of the starspot active regions by the stellar disk, and (2) high directivity of the radio emission from these regions. The radio emission of AB Dor is nonthermal, and requires semicontinuous acceleration of electrons to high energies. This observation provides the first evidence that the radio emission of late-type stars, like that of the Sun, originates from regions of strong coronal magnetic fields associated with starspots.