Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters II. M13 and M92

TL;DR

This study analyzes high-resolution spectra of red giant stars in globular clusters M13 and M92, revealing how chromospheric activity and outflows vary with stellar luminosity, temperature, and metallicity, and comparing these features across different clusters.

Contribution

It provides new insights into the chromospheric outflows and activity in red giants, highlighting differences related to metallicity and evolutionary stage, and compares these phenomena across multiple globular clusters.

Findings

Chromospheric outflows increase with stellar luminosity.

Cool giants in metal-rich M13 show reduced outflows.

AGB stars exhibit faster outflows than RGB stars.

Abstract

High resolution spectra of 123 red giant stars in the globular cluster M13 and 64 red giant stars in M92 were obtained with Hectochelle at the MMT telescope. Emission and line asymmetries in Halpha, and Ca K are identified, characterizing motions in the extended atmospheres and seeking differences attributable to metallicity in these clusters and M15. On the red giant branch, emission in Halpha generally appears in stars with T_eff < 4500 K and log L/L_sun > 2.75. Fainter stars showing emission are asymptotic giant branch (AGB) stars or perhaps binary stars. The line-bisector for Halpha reveals the onset of chromospheric expansion in stars more luminous than log L/L_sun ~ 2.5 in all clusters, and this outflow velocity increases with stellar luminosity. However, the coolest giants in the metal-rich M13 show greatly reduced outflow in Halpha most probably due to decreased T_eff and changing atmospheric structure. The Ca K_3 outflow velocities are larger than shown by Halpha at the same luminosity and signal accelerating outflows in the chromospheres. Stars clearly on the AGB show faster chromospheric outflows in Halpha than RGB objects. While the Halpha velocities on the RGB are similar for all metallicities, the AGB stars in the metal-poor M15 and M92 have higher outflow velocities than in the metal-rich M13. Comparison of these chromospheric line profiles in the paired metal-poor clusters, M15 and M92 shows remarkable similarities in the presence of emission and dynamical signatures, and does not reveal a source of the `second-parameter' effect.