The Time Variability Of Spectral Line Asymmetries And Equivalent Widths For The G8 Dwarf Xi Boo A: Evidence For A starpatch

The chromospherically active G8 dwarf Xi Boo A has been monitored spectroscopically for four observing seasons. A total of 121 high resolution, high signal-to-noise Reticon exposures (representing {dollar}\sim{dollar}300 hours of observing) were obtained at the University of Western Ontario\u27s 1.2 m telescope.;The data were analyzed for variations in the mean line asymmetry, line strength, and line broadening, using a combination of line bisector and Fourier techniques. Systematic variations in all quantities were found, which repeat with a period of 6.43 {dollar}\pm{dollar} 0.01 days. The line broadening variations show no evidence of being magnetic in nature, and there is no evidence of a change in period or phase shift over the four seasons.;A self-consistent model is presented, which is found to be very successful at explaining the observations. The model consists of a single surface feature, carried across the stellar disk by rotation. The characteristics of the feature have been determined through extensive use of numerical experiment. The feature\u27s most distinguishing characteristics is a velocity dispersion which is enhanced by a factor of 1.5-2.0 relative to the surrounding photosphere. The feature is found to cover 10% {dollar}\pm{dollar} 5% of the visible disk (2.5% of the total surface), and is {dollar}\sim{dollar}3.7% cooler ({dollar}\sim{dollar}200 K), and 15% fainter than the rest of the star. The feature lies at a relatively high latitude (55{dollar}\sp\circ{dollar} {dollar}\pm{dollar} 10{dollar}\sp\circ{dollar}), and has remained essentially unchanged for at least the four years of this investigation.;The deduced characteristics of the feature cannot be reconciled with the classical sunspot or plage. Therefore, it is believed that this is an entirely new type of stellar feature, quite distinct from both the classical sunspot and plage. The name Starpatch is suggested in order to distinguish it from its solar cousins.;A note is added in warning for those who are attempting to use single exposures of solar-type stars to obtain information on their granulation properties. The existence of starpatches makes conclusions based on a small amount of data very uncertain, since one cannot determine from such data whether or not a starpatch is in view