Directivity of the radio emission from the K1 dwarf star AB Doradus

Abstract

We present measurements of the spectrum and polarization of the flaring radio emission from the K1 dwarf star AB Doradus, together with previously reported single frequency measurements (with no polarization information) on 3 other days. On all 4 days spanning a 6 month period, the emission was strong and, when folded with the stellar rotation period, showed similar time variations with two prominent peaks at phase 0.35 and 0.75. These peaks coincide in longitude with two large starspots identified from the stellar optical light curve and have half-power widths as small as 0.1 rotations and no larger than 0.2 rotations. The modulated emission shows no measurable circular polarization, and its two peaks have different turnover frequencies. We discuss four contrasting models that can reproduce the observed properties of the modulated emission. Each model places a stringent upper limit on the source size and therefore a lower limit on its brightness temperature. In the first model the modulation is produced purely by the geometrical effects of occultation and requires sources with brightness temperature up to 10 13 K. In the second model we include the effects of limb darkening but find that neither the required source dimension nor its brightness temperature is significantly changed. In the third model the emission has a high directivity imposed extrinsically by coronal structures that absorb the radiation along all but the radial direction (to the stellar surface) and requires sources with brightness temperatures up to 10 11 K. In the fourth model the directivity is intrinsic to the emission process itself combined with a magnetic structure of a particular shape and requires sources with brightness temperatures up to 6 × 10 10 K. The high brightness temperatures, the broad-band and unpolarized nature of the radiation, and the coincidence of the radio peaks with large starspots suggest that the modulated emission of AB Dor is produced by gyroemission from ultrarelativistic electrons, that is synchrotron emission. This is in contrast to the Sun, which even in flares produces relatively few such energetic electrons. If as Readhead (1994) suggests the maximum (intrinsic) brightness temperature attainable by synchrotron emission is ∼10 11 K, then only models incorporating emission with high directivity - imposed extrinsically or, more likely, intrinsic to the source - can explain all the observed properties of the modulated emission. This then is the first indirect evidence that the incoherent radio emission of active stars can be highly directive.