Observations of Chromospheres in Metal Deficient Field Giants

HST high resolution spectra of metal-deficient field giants more than double the stars in previous studies, span ∼ 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 (λ2800) 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 (MV = −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. Subject headings: stars:chromospheres; stars: winds, outflows; stars: Population II; ultraviolet: stars