Observations in the Bright Star in the globular cluster 47 Tucanae (NGC 104)

DIXON, WILLIAM V.; CHAYER, PIERRE; M. M. MILLER BERTOLAMI; SOSA FISCELLA, VALENTINA

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Londres; Año: 2021

0004-637X

The Bright Star in the globular cluster 47 Tucanae (NGC 104) is a post-AGB star of spectral typeB8 III. The ultraviolet spectra of late-B stars exhibit a myriad of absorption features, many due tospecies that are unobservable from the ground. The Bright Star thus represents a unique window intothe chemistry of 47 Tuc. To take advantage of it, we have analyzed archival observations obtained withthe Far Ultraviolet Spectroscopic Explorer (FUSE), the Cosmic Origins Spectrograph (COS) aboardthe Hubble Space Telescope, and the MIKE Spectrograph on the Magellan Telescope. By fitting thesedata with synthetic spectra, we determine various stellar parameters (Teff = 10, 850 ± 250 K andlog g = 2.20 ± 0.13) and the photospheric abundances of 26 elements, including Ne, P, Cl, Ga, Pd,In, Sn, Hg, and Pb, which have not previously been published for this cluster. The star?s low C/Nratio places it among the cluster?s second-generation stars. Abundances of intermediate-mass elements(Mg through Ga) generally scale with Fe, while the heaviest elements (Pd through Pb) have roughlysolar abundances. These elements are generally associated with s-process nucleosynthesis on the AGB;however, its low C abundance indicates that the star did not undergo third dredge-up and suggeststhese elements were made by a previous generation of stars. If so, then we would expect this patternto be present throughout the cluster, not just in this star. Stellar-evolution models that reproduce thecluster?s ZAHB have post-AGB masses consistent with that of the Bright Star (0.54 ± 0.16 M ). Thestar?s relatively low luminosity (log L∗ /L of 3.06 ± 0.04) suggests that it is powered by a He-burningshell, having left the AGB during or immediately after a thermal pulse.