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Observations of the Bright Star in the Globular Cluster 47 Tucanae (NGC 104)

Presentation #116.06 in the session “Stellar Atmospheres and Winds”.

Published onJan 11, 2021
Observations of the Bright Star in the Globular Cluster 47 Tucanae (NGC 104)

The Bright Star in the globular cluster 47 Tucanae (NGC 104) is the cluster’s brightest member at both ultraviolet and optical wavelengths. This blue giant (B8 III) is a post-asymptotic giant-branch (post-AGB) star. Having ascended the AGB, it is moving across the H-R diagram toward the tip of the white-dwarf cooling sequence. The spectra of late B-type stars exhibit a myriad of absorption features, particularly at ultraviolet wavelengths. The Bright Star thus represents a unique window into the chemistry of 47 Tuc. To take advantage of this opportunity, we have analyzed archival observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope, and the MIKE Spectrograph on the Magellan Telescope. By fitting these data with synthetic spectra, we have determined the photospheric abundances of He, C, N, O, Ne, Mg, Al, Si, P, S, Cl, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ga, Pd, In, Sn, Hg, and Pb. The He abundance is slightly enhanced relative to the sun. The CNO abundance pattern (depleted in C, enhanced in N) reflects CNO processing, perhaps the result of nonconvective mixing on the RGB. Elements from the third and fourth rows of the periodic table (Mg through Ga) generally scale with Fe. The heaviest elements (Pd through Pb) have roughly solar abundances. These elements are generally associated with s-process nucleosynthesis on the AGB; however, the low carbon abundance indicates that the star did not undergo third dredge-up and suggests they were made by a previous generation of stars. If so, then we would expect this pattern to be present throughout the cluster, not just in this star.

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