Modelling chromospheric line profiles as diagnostics of velocity fields inω Centauri red giant stars

Abstract

Context. Mass loss of ∼0.1−0.3 M from Population II red giant stars (RGB) is a requirement of stellar evolution theory in order to account for several observational evidences in globular clusters. Aims. The aim of this study is to detect the presence of outward velocity fields, which are indicative of mass outflow, in six luminous red giant stars of the stellar cluster ω Cen. Methods. We compare synthetic line profiles computed using relevant model chromospheres to observed profiles of the Hα and Ca II K lines. The spectra were taken with UVES (R = 45 000) and the stars were selected so that three of them belong to the metalrich population and three to the metal-poor population, and sample as far down as 1 to 2.5 mag fainter than the respective RGB tips. Results. We do indeed reveal the presence of low-velocity outward motions in four of our six targets, without any apparent correlation with astrophysical parameters.Conclusions. This provides direct evidence that outward velocity fields and mass motions exist in RGB stars as much as 2.5 mag fainter than the tip. On the assumption that the mass outflow may eventually lead to mass loss from the star, we estimate mass-loss rates of some 10 −9 −10 −10 M yr −1 that are compatible with the stellar evolution requirements. These rates seem to be correlated with luminosity rather than metallicity.

Empirical determination of the integrated red giant and horizontal branch stellar mass-loss in ω Centauri

“…It is towards the higher end of the observed range of mass‐loss rates estimated from chromospheric line profiles (e.g. Dupree et al 2009; Vieytes et al 2011), suggesting that stronger mass‐loss at the RGB tip is important.…”

Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars

“…However, a varying metallicity should also change other properties of the convection-including the effective mixing length α parameter-thus possibly making this effect more important. To continue testing and refining these models, it is also important to utilize the newest and most accurate measurements of stellar mass loss rates (see, e.g., Schröder & Cuntz 2007;Willson 2009;Catelan 2009;Mauron & Josselin 2011;Vieytes et al 2011) and magnetic fields (Donati & Landstreet 2009;Vlemmings et al 2011). Finally, we emphasize that a complete description of latetype stellar winds requires the incorporation of other physical processes besides Alfvén waves and turbulence.…”

Dust Production and Mass Loss in the Galactic Globular Cluster 47 Tucanae

“…such low-metallicity, dust-producing giants, inferences from chromospheric line modelling (e.g. Judge & Stencel 1991;Vieytes et al 2010) appears to show that wind velocities do not decrease substantially from ∼10 km s −1 in the stages shortly prior to dust formation. We point out, however, that the highest mass-loss rates (and by inference the highest wind velocities) may be ruled out on evolutionary grounds ( §3.5).…”