A microwave survey of southern active stars

Abstract

We report the results of a survey of 153 active-chromosphere stars made with the Parkes 64-m telescope at 5.0/8.4 GHz from 1981 to 1987. Most of the stars were observed on at least 10 separate days in order to better establish flare rate and luminosity. Our sample included RS CVn and Algol-type binaries, Ca II emitters with as yet unknown stellar content and AM-Her-type cataclysmic variables. For some of the strongest stellar flares we measured the circular polarization at 8.4 GHz and obtained crude radio spectra involving frequencies in the range 0.843<ν<22 GHz. We detected microwave emission from 70 stars on at least one occasion, with the highest detection rate of 68 per cent from the RS CVn group. The Ca II stars and Algol-type binaries yielded detection rates of 44 and 30 per cent respectively. Two of the four observed AM-Her-type binaries were detected. The maximum powers emitted by the stars at 5.0/8.4 GHz ranged over five orders of magnitude, with a median of 2.5×1010W Hz−1. The power and distance distributions of the detected stars in the RS CVn, Ca II and Algol classes are not significantly different. The distance distributions of the detected and undetected stars are not significantly different for stars in the RS CVn/Ca II classes but are markedly different for the Algols. Thus RSCVn/Ca II stars are not detected because they emit less power or have a lower flare rate; the lower detection rate for Algols is probably due to their greater distances. The maximum brightness temperatures had a smaller range of three orders of magnitude, with a median of 3.6×109 K. The brightness temperature distributions of stars in the RS CVn/Ca II classes are not significantly different, but the Algols possess a markedly higher median brightness temperature of 1.1×1010 K. The AM Her binaries are still higher by at least one order of magnitude. Measurements of radio spectra during the brighter phases of flares on HD 22468, 32918, 36705 and 127535 yielded spectral indices in the range −0.1<α<1.2 (flux ∝ να), suggesting optically thick sources. We have measured circular polarization of 0 to 60 per cent in the 8.4-GHz emission from HD 22468,32918, 36705, 81410, 127535, 132742 and 196818. The moderate values of brightness temperature of most of the stars, combined with a positive spectral index and generally low polarization, suggest that optically thick gyro-synchrotron emission is the dominant mechanism. However, there are a number of stars, including most of the Algols and the AM Her binaries, with sufficiently high brightness temperatures (sometimes combined with high polarization) that may emit by a coherent process.