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

Context. Mass loss of similar to 0.1-0.3 M-circle dot 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 omega Cen. Methods. We compare synthetic line profiles computed using relevant model chromospheres to observed profiles of the H alpha 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 metal-rich 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-circle dot yr(-1) that are compatible with the stellar evolution requirements. These rates seem to be correlated with luminosity rather than metallicity.