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

M. VIEYTES; P. MAUAS; C. CACCIARI; L. ORIGLIA; E. PANCINO

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2010 vol. 526 p. 1 - 8

0004-6361

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 toaccount 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 luminousred 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 andCa 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 metalrichpopulation 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 correlationwith astrophysical parameters.Conclusions. This provides direct evidence that outward velocity fields and mass motions exist in RGB stars as much as 2.5 magfainter than the tip. On the assumption that the mass outflow may eventually lead to mass loss from the star, we estimate mass-lossrates of some 10−9−10−10 M yr−1 that are compatible with the stellar evolution requirements. These rates seem to be correlated withluminosity rather than metallicity.