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 May, 2006 May, and 2006 October. Echelle orders containing Hα and Ca ii H & K are used to identify emission and line asymmetries characterizing motions in the extended atmospheres. Emission in Hα is detected to a luminosity of log(L/L ) = 2.36, in this very metal-deficient cluster, comparable to other studies, suggesting that the appearance of emission wings is independent of stellar metallicity. The faintest stars showing Hα emission appear to lie on the asymptotic giant branch (AGB) in M15. A line-bisector technique for Hα reveals outflowing velocities in all stars brighter than log(L/L ) = 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 Hα 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.

Fast Winds and Mass Loss From Metal-Poor Field Giants

“…7 Thus it is not surprising to find differing dynamical signatures in these line diagnostics, in addition to possible time variations. There may be a similarity here between the well-known changing asymmetries of Hα emission wings in metal-deficient giants (Smith & Dupree 1988;Cacciari et al 2004;Mészáros et al 2008Mészáros et al , 2009b) and the Mg ii emission.…”

“…Mass flows can produce asymmetric profiles of Hα and Na D, as well as emission asymmetries and velocity shifts in the reversed absorption core of Ca ii and Mg ii. These features have been measured in red giants in many globular clusters (Cacciari et al 2004;McDonald & van Loon 2007;Mészáros et al 2008Mészáros et al , 2009b and in metal-poor field stars (Smith & Dupree 1988;Dupree & Smith 1995;Dupree et al 2007). However, the velocities inferred from the optical profiles are generally less than the escape velocity from the star, and cannot truly be identified as stellar winds.…”

“…The helium line profiles reported here overwhelmingly display a signature of outflowing gas in stars brighter than M V ∼ 0. Hα line cores show generally increasing outflowing velocities with luminosity (Mészáros et al 2008(Mészáros et al , 2009b. Even though some measured outflow velocities are less than the escape velocity, conservation of mass suggests that the velocities will yield meaningful mass loss rates.…”

“…We emphasize the difference between the presence of mass loss from red giants determined from diagnostics of the gas (from He i 10830 Å and the Hα line) and that derived from infrared detections of circumstellar dust. Evidence suggests that dust formation is an episodic process (Origlia et al 2007;Mészáros et al 2008), whereas the velocity measurements of the gas indicate continuous outflow of material.…”

“…Since not all giants display excess infrared emission, mass loss associated with dust appears to be episodic (Origlia et al 2007;Mészáros et al 2008). Caloi & D'Antona (2008) further suggest that the mass loss rate might be "sharply" peaked at one value along the RGB.…”