HST Observations of Chromospheres in Metal Deficient Field Giants

TL;DR

This study uses HST spectra to analyze chromospheres in metal-deficient giants, revealing that they have significant chromospheric activity and outflows, with implications for understanding stellar atmospheres at low metallicity.

Contribution

It provides a larger sample of metal-poor giants with detailed chromospheric measurements, showing outflows occur at lower luminosities and challenging magnetic heating models.

Findings

Chromospheric Mg II fluxes are within a factor of 4 of Population I stars.

Outflows are detected at lower luminosities than previously observed.

Mg II lines are effective diagnostics for stellar winds.

Abstract

HST high resolution spectra of metal-deficient field giants more than double the stars in previous studies, span about 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 (2800 Angstrom) 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.