Mass Outflow and Chromospheric Activity of Red Giant Stars in Globular Clusters I: M15

Abstract

High resolution spectra of 110 selected red giant stars in the globular cluster M15 (NGC 7078) were obtained with Hectochelle at the MMT telescope in 2005 May, 2006 May, and 2006 October. Echelle orders containing Halpha and Ca H & K are used to identify emission and line asymmetries characterizing motions in the extended atmospheres. Emission in Halpha is detected to a luminosity of log (L/L_sun)=2.36, in this very metal deficient cluster, comparable to other studies, suggesting that appearance of emission wings is independent of stellar metallicity. The faintest stars showing Halpha emission appear to lie on the asymptotic giant branch (AGB) in M15. A line-bisector technique for Halpha reveals outflowing velocities in all stars brighter than log (L/L_sun)=2.5, and this outflow velocity increases with stellar luminosity, indicating the mass outflow increases smoothly with luminosity. Many stars lying low on the AGB show exceptionally high outflow velocities (up to 10-15 km s^{-1}) and more velocity variability (up to 6-8 km s^{-1}), than red giant branch (RGB) stars of similar apparent magnitude. High velocities in M15 may be related to the low cluster metallicity. Dusty stars identified from Spitzer Space Telescope infrared photometry as AGB stars are confirmed as cluster members by radial velocity measurements, yet their Halpha profiles are similar to those of RGB stars without dust. If substantial mass loss creates the circumstellar shell responsible for infrared emission, such mass loss must be episodic.