HST high resolution spectra of metal-deficient field giants more than double the stars in previous studies , span \sim 3 magnitudes on the red giant branch , and sample an abundance range [ Fe/H ] = - 1 to -3 .
These stars , in spite of their age and low metallicity , possess chromospheric fluxes of Mg ii ( \lambda 2800 ) that are within a factor of 4 of Population I stars , and give signs of a dependence on the metal abundance at the lowest metallicities .
The Mg ii k-line widths depend on luminosity and correlate with metallicity .
Line profile asymmetries reveal outflows that occur at lower luminosities ( M _ { V } = - 0.8 ) than detected in Ca K and H-alpha lines in metal-poor giants , suggesting mass outflow occurs over a larger span of the red giant branch than previously thought , and confirming that the Mg ii lines are good wind diagnostics .
These results do not support a magnetically dominated chromosphere , but appear more consistent with some sort of hydrodynamic , or acoustic heating of the outer atmospheres .