Resolving Red Giant Winds with the Hubble Space Telescope

TL;DR

This study uses Hubble Space Telescope spectroscopic data to spatially resolve and analyze the winds of red giant stars, revealing differences in wind strength and extent between K and M giants.

Contribution

First spatially resolved observations of red giant winds using HST, providing detailed measurements of wind properties and extent for different spectral types.

Findings

M giants have higher mass-loss rates than K giants.

The spatial extent of winds is larger for M giants, up to 193 stellar radii.

Detected wind signatures in multiple spectral lines, indicating complex wind structures.

Abstract

We describe recent spectroscopic observations of red giant stars made by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope, which have provided spatially resolved observations of the warm chromospheric winds that predominate for early K to mid-M giants. The H I Lyman-alpha lines of a set of 11 red giants observed with the STIS/E140M echelle grating are first analyzed to ascertain wind H I column densities and total wind mass-loss rates. The M giants have estimated mass-loss rates of Mdot=(14-86)e-11 Msun/yr, while the K giants with detected wind absorption have weaker winds with Mdot=(1.5-2.8)e-11 Msun/yr. We use long-slit spectra of H I Lyman-alpha for two particular red giants, Alpha Tau (K5 III) and Gamma Cru (M3.5 III), to study the spatial extent of the Lyman-alpha emission. From these data we estimate limits for the extent of detectable emission, which are r=193 Rstar for Gamma Cru and r=44 Rstar for Alpha Tau. Cross-dispersion emission profiles in the STIS echelle spectra of the larger sample of red giants also show evidence for spatial resolution, not only for H I Lyman-alpha but for other lines with visible wind absorption, such as Fe II, Mg II, Mg I, O I, and C II. We characterize the nature of these spatial signatures. The spatial extent is far more apparent for the M giants than for the K giants, consistent with the stronger winds found for the M giants from the Lyman-alpha analysis.