Radio Wavelength Observations of Magnetic Fields on Active Dwarf-M, RS CVN and Magnetics Stars

摘要

The dwarf M stars YZ Canis Minoris and AD Leonis exhibit narrow band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars AD Leonis and Wolf 424 emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process which could result from coherent mechanisms such as an electron cyclotron maser or coherent plasma radiation. If the electron cyclotron maser emits at the second or third harmonic of the gyrofrequency, the coronal magnetic field strength H = 250 C or 167 G and constraints on the plasma frequency imply an electron density of N sub e = 6 x 10 to the 9th power/cc. Coherent plasma radiation requires similar values of electron density but much weaker magnetic fields. Radio spikes from AD Leonis and Wolf 424 have rise times tau sub R < 5 ms, indicating a linear size of L < 1.5 x 10 to the 8th power cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour. The shortest variation implies a linear size much less than that of the halo observed by VLBI techniques, and most probably sizes smaller than those of the component stars. The observed variations might be due to absorption by a thermal plasma located between the stars.