Mg II emission from hybrid-chromosphere stars: 1.5 decades of chromospheric variability monitoring

We present an analysis of the available long-wavelength, high-dispersion spectra of seven hybridchromosphere stars obtained with the International Ultraviolet Explorer between 1978 and 1993. Our investigation of the variability of the Mg II h and k resonance doublet demonstrates that the emission-line fluxes are not rotationally modulated with the periods previously suggested by Brosius, Mullan, & Stencel. Furthermore, we find no evidence in the Mg II data to corroborate the multiple periodicities reported in the Ca II emissions of hybrid stars by Rao et al. Examination of 40 pairs of closely spaced Mg II observations failed to reveal the presence of any strong chromospheric flaring on the sample stars. Significant (20%-40%) nonperiodic Mg II flux variability, on timescales of days to years, is observed in six of the seven stars. The flux variations occasionally are accompanied by dramatic changes in the morphology of the Mg II profiles, indicating variable stellar-wind absorption. We argue that the variability observed is consistent with stochastic changes associated with the slow growth and decay of chromospheric active regions and the gradual evolution of the physical conditions in the winds. It is our opinion that no compelling argument has yet been made for periodic variability in the chromospheric diagnostics of hybrid stars.