The Active Radio Star HD 36705

Abstract

The single G8V active chromosphere star HD36705 (AB Dor) was observed at 8.4 GHz with the Parkes 64 m telescope during three observing sessions involving a total of 21 days in the interval 1985 December to 1986 February. Subsequent photometric observations were made of the star with the 0.25 m and 0.45 m telescopes of the Monash Observatory in 1986 March-April. Two strong radio flares, each lasting three days, were detected; they yielded peak radio powers of / ) !.4«4xlO'WHz"', comparable with the microwave power emitted by the RS CVn binaries. Significant circular polarization of 13% left-hand was measured on only one of the six active days. The 8.4 GHz flux density showed smooth variation over an interval of several hours, consistent with the flare source being partly occulted by the stellar disk as the star rotated. When all the radio data was phase-binned using the known rotation period of 0.514 day we found two radio maxima corresponding to radio sources at stellar longitudes ~180° apart. The subsequent photometric data showed intensity variations that were consistent with the starspots at the same approximate longitudes. We thus interpret our radio curve as showing the presence of comparatively small (<0.5 D t ) radio sources in the corona above the star spots. The upper limit to source diameter gives a peak brightness temperature ^2xl0 1 0 K, which can be achieved by gyro-synchrotron emission only if the source is optically thick and the electrons, with average energy ~ 2 MeV, have a hard energy spectrum. The observed radiation can be due only to very high harmonics of the gyro-frequency, leading to an estimate for the magnetic field strength of -30G.

Spot activity cycles and flip‐flops on young solar analogs

“…The stellar corona produces strong and variable radio and Xray radiation (Slee et al 1986;Pakull 1981). The optical brightness of the star is highly variable because of rapidly evolving cool spots on the stellar surface.…”

Magnetic activity in the young solar analog AB Dor

“…The stellar corona produces strong and variable radio and X-ray radiation (Slee et al 1986;Pakull 1981). The radio emission of AB Dor varies periodically over many rotations, suggesting that the magnetic field structure(s) containing each radio source must persist at the same stellar longitude(s) for long time intervals.…”

Coordinated multiwavelength observations of the RS CVn system CF Tuc

“…The light curves show a number of appreciably sized up‐down features, generally with time‐scales of ∼/10. Previous authors have subdivided microwave emission from active stars into a shorter term (minutes to hours) impulsive component, associated more directly with flares, and a more long‐lasting `quiescent' component undulating over a time‐scale comparable to that of the stellar rotation (Lim et al 1992; see also, e.g., Doiron & Mutel 1984, Slee et al 1986 and Patterson, Caillault & Skillman 1993). The latter have sometimes been associated with the large 'halo' formations resolved by VLBI observations (cf.…”

“…The idea of a greatly scaled‐up solar active region is supported by the microwave emission tending to anticorrelate with broad‐band optical flux (Slee et al 1986). This anticorrelation was apparent for UX Ari (Elias et al 1995), although, in general, strict period correlation or clear pattern repeatability is often weak or absent (Chambliss et al 1978).…”