Hubble Space Telescope Observations of Chromospheres in Metal-Deficient Field Giants

Abstract

Hubble Space Telescope high-resolution spectra of metal-deficient field giants more than double the stars in previous studies, span ~3 mag 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 they 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 Hα 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 they appear more consistent with some sort of hydrodynamic or acoustic heating of the outer atmospheres.