Mass motions and chromospheres of RGB stars in the globular cluster NGC 2808

Abstract

We present the results of the first observations, taken with FLAMES during Science Verification, of red giant branch (RGB) stars in the globular cluster NGC 2808. A total of 137 stars was observed, of which 20 at high resolution ( 47 000) with UVES and the others at lower resolution ( 19 000–29 000) with GIRAFFE in MEDUSA mode, monitoring ~3 mag down from the RGB tip. Spectra were taken of the Hα, Na i D and Ca ii H and K lines. This is by far the largest and most complete collection of such data in globular cluster giants, both for the number of stars observed within one cluster, and for monitoring all the most important optical diagnostics of chromospheric activity/mass motions. Evidence of mass motions in the atmospheres was searched from asymmetry in the profiles and coreshifts of the Hα, Na i D and Ca ii K lines, as well as from Hα emission wings. We have set the detection thresholds for the onset of Hα emission, negative Na D2 coreshifts and negative K3 coreshifts at 2.5, 2.9 and 2.8, respectively. These limits are significantly fainter than the results found by nearly all previous studies. Also the fraction of stars where these features have been detected has been increased significantly with respect to the previous studies. Our observations confirm the widespread presence of chromospheres among globular cluster giants, as it was found among Population I red giants. Some of the above diagnostics suggest clearly the presence of outward mass motions in the atmosphere of several stars.