Essentially all classes of stars possessing a soft X-ray emitting corona also have members which show transient energetic activity. The activity manifests itself in powerful flares, with individual events releasing times the energy of a strong solar flare. These include the classical flare stars (the UV Cet or dMe stars), the RS CVn binaries, and the pre-main sequence weak-line T Tauri (WTT) stars.
The flares are accompanied by radio bursts. At frequencies GHz the dMe stars emit bursts of highly circularly polarized coherent radiation with a brightness temperature exceeding K at times. The emission shows a rich variety of structure in both the time and frequency domains. The emission mechanisms have not yet been identified. The RS CVn stars also display broadband, highly circularly polarized, and extremely bright radio bursts at times. Observations to date have been hampered by insufficient spectral coverage and insufficient temporal resolution (which requires high sensitivity).
An exciting possibility is to exploit the broadband spectral processing capabilities of the new correlator to perform spectroscopic observations of coherent bursts on stars. The motivations for doing so are several: i) to date, spectral observations of stellar radio bursts have only been possible over relative bandwidths few percent. The possibility of processing 500 MHz of IF bandwidth instantaneously will allow access to relative bandwidths of several tens of percent, sufficient to resolve structures drifting in the frequency domain (electron beams and MHD shocks); ii) elucidation of the spectral properties of these bursts, of which there are no analogs on the Sun, may allow identification of the relevant emission mechanisms; iii) the extremely high brightness temperatures of these bursts may drive phenomena which are inaccessible to study in other astrophysical objects: e.g., stimulated Compton or Raman scattering.
It should also be pointed out that the incoherent synchrotron and gyrosynchrotron emission from the dMe stars offers a direct diagnostic of physical parameters in the flaring source. Because it is relatively weak (few to a few tens of mJy) the study of incoherent flare emissions would benefit enormously from the high frequency receiver upgrades.
The study of all active stars would benefit from the enhanced frequency coverage and agility proposed in the VLA Development Plan, allowing observers to obtain broad-band spectra of the evolving source over relatively short timescales.