AAS 201st Meeting, January, 2003
Session 7. Globular Clusters and Their Contents
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[7.15] The Helium and Nitrogen-Rich UV-Bright Star UIT-644 in Omega Cen

W.B. Landsman (SSAI/NASA/GSFC), A.P.S. Crotts (Columbia U.), S. Heap (NASA/GSFC), T. Lanz (UMD/NASA/GSFC), A. Sweigart (NASA/GSFC)

We report on STIS medium-resolution echelle (E230M) observations of the UV-Bright star UIT-644, located near the center of the globular cluster Omega Centauri. The observations cover the wavelength region 1625 -- 2364 Å\ with a resolution R=30,000. Earlier low-resolution optical spectra with the CTIO-4m have been used to derive the temperature (20,000 K), gravity (log g = 3.5) and bolometric luminosity (log L/LSun = 2.9), while revealing the star to be helium-rich (Y=0.68). The STIS spectra are used to show that the star is also extremely nitrogen-rich ([N/H] ~ 10), while having a typical cluster iron abundance ([Fe/H] = -1.5). The large nitrogen abundance probably requires that the products of helium burning be mixed to the surface, with the carbon subsequently converted to nitrogen. We speculate on a scenario in which UIT-644 ejected its envelope subsequent to a helium flash on the AGB.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.