Chromospheric Activity in Metal-Poor Dwarfs

We have obtained echelle spectra with the Hubble Space Telescope (HST) of the MgII 2800 Å region of ten stars whose metallicities range from 1/300 to 1/3 that of the Sun, and whose space velocities suggest a halo or old thick-disk origin. Spectra of all ten show doublepeaked emission in the MgII core, very much like the quiet Sun. A halfdozen apparently non-rotating stars were observed more than once, and show at most a low level of variability in the emission flux, comparable to that of quiet stars of solar metallicity. For four stars, we have obtained Lymanα spectra at 0.2 Å resolution; all four show emission. The data thus strongly suggest that chromospheric activity at a minimum level is present in all stars of near-solar temperature, regardless of age or metallicity. While this points to non-magnetic sources such as acoustic waves, a contribution from globally-organized magnetic fields is possible. A longer series of MgII and Lymanα observations is needed to constrain this. Background: quiet chromospheres in old stars. Whether chromospheric activity in quiet stars of solar metallicity and temperature is due to a globallyorganized magnetic field generated by internal stellar rotation remains an open question. Among active stars, such a “dynamo” is implicated as the dominant source of a chromosphere by two separate observations: by the fact that the brightest chromospheric emission on the surface of the Sun appears in regions near sunspots (Lemaire & Skumanich 1973; Schrijver et al. 1989; Brekke et al. 1991), and by the positive correlation between emission and surface rotation found among cool stars generally (Noyes et al. 1984). However, both observation and theory suggest that weak chromospheric emission may still produced in the absence of a dynamo. If generated by convection, then chromospheric emission at some low, roughly constant level should appear in all stars with convection zones. We test these ideas here by examining chromospheric MgII emission over a three-year period for a half-dozen apparently non-rotating solar-type stars believed to be older than the Sun, for which the rotation generating a magnetic dynamo has had the longest chance to spin down. As a more sensitive diagnostic of dynamo-related activity, we obtained a single Lymanα spectrum for four stars. On the Sun, most chromospheric emission is confined to regions of strong magnetic fields, in sunspot-related plages and in the magnetic network covering the entire surface. Most of that field changes in strength with the solar magnetic