Observations of the Ultraviolet-Bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)

DIXON, WILLIAM V.; CHAYER, PIERRE; REID, I. N.; M. M. MILLER BERTOLAMI

Viena

Conferencia; General Assembly of the IAU; 2018

IAU

We have analyzed FUSE, COS, GHRS, and Keck/HIRES spectra of the UV-bright starBarnard 29 in M13. Fits to the star?s optical spectrum yield Teff = 20, 000 ± 100 K and log g =3.00 ± 0.01. Using modern stellar-atmosphere models, we are able to reproduce the complexshape of the Balmer Hα feature. We derive photospheric abundances of He, C, N, O, Mg, Al,Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits an abundance pattern typical ofthe first-generation stars in M13, enhanced in oxygen and depleted in aluminum. We see noevidence of significant chemical evolution since the star left the RGB; in particular, it did notundergo third dredge-up. Previous workers found that the star?s FUV spectra yield an ironabundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived fromall of our spectra are consistent with one another and with the cluster value. We attribute thisdifference to our use of model atmospheres without microturbulence. By comparing our best-fitmodel with the star?s optical magnitudes, we derive a mass M∗ /M = 0.40 − 0.49 and luminositylog L∗ /L = 3.20 − 3.29, depending on the cluster distance. Comparison with stellar-evolutionmodels suggests that Barnard 29 evolved from a ZAHB star of mass M∗ /M ∼ 0.50, placing itnear the boundary between the extreme and blue horizontal branches.