Chromospheric Activity in Metal-Poor Dwarfs
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We have obtained echelle spectra with the Hubble Space Telescope (HST) of the MgII 2800A 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 double-peaked emission in the MgII core, very much like the quiet Sun. A half-dozen 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 Lyman-alpha spectra at 0.2A 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 at a low level. We plan a longer series of MgII and Lyman alpha observations to constrain this.
8 pages including 2 figures. To appear in "11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R. J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osorio
8 pages including 2 figures. To appear in "11th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun," ed. R. J. Garcia Lopez, R. Rebolo, and M. R. Zapatero Osorio