A Multiwavelength Study of CC Eridani

Abstract

Radio and optical observations from December 2001 and January 2002 of the active RS CVn-like binary CC Eri are presented. The star was monitored at 4.80 and 8.64 GHz over 3 × 12 h allocations with the Australia Telescope Compact Array on 28 to 30 December 2001. The Anglo-Australian Telescope was used for simultaneous optical spectropolarimetry during a 0.5 h period on 30 December. Data from four nights of broadband photometry gathered around the same period are also included in this present multiwavelength study.

The low levels of radio emission were circularly polarised at ~20% with slightly positive spectral indices of ~0.26. Two flare-like increases were observed on successive nights with steep positive spectral indices and no detectable polarisation. Cross-correlation analysis of the 4.80 and 8.64 GHz intensities over the stronger flare showed that the higher frequency emission preceded that at the lower frequency by ~5 min, a result consistent with the propagation of a hydromagnetic disturbance outwards through the corona. On the same night, a significant cross-correlation in the ‘quiescent’ emission indicates the presence of micro-flaring, although its low intensity does not permit the evaluation of a time delay. The emission parameters on the three nights are compatible with a gyrosyncrotron mechanism, in which the radio source becomes optically thick during strong flaring. We develop a simple model, which is based on assuming that the number of radiating electrons is a given function of the magnetic field in the source region, and derive feasible values for the field, source radius, and number of emitting electrons, which are not strongly dependent on the field modelling function or the aspect ratio of the source. Spectropolarimetry demonstrates the presence of a strong surface magnetic field. Optical photometry, covering a sufficient amount of the orbit, indicates a maculation region of significant size (~14° radius).

The results help develop a three-dimensional picture of a large stellar magnetically active region and encourage more detailed follow-up multiwavelength studies of this and similar stars.