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Session 10 - Spiral Galaxies.
Display session, Monday, June 10
Great Hall,

[10.08] Localized Chemical Enrichment in NGC 5253 from Hubble Space Telescope FOS Spectroscopy

C. Kobulnicky, E. Skillman (Univ. of Minnesota), J. -R. Roy (U. Laval), M. Rosa, J. R. Walsh (HST ECF)

The central HII region in the amorphous galaxy NGC 5253 represents the best evidence that massive star clusters contribute significantly to the short-term, localized chemical enrichment of the surrounding interstellar medium. Ground based observations (Welch, 1970, ApJ, 161, 821; Walsh amp; Roy, 1989, MNRAS, 239, 297) revealed that in an 80 pc diameter region coinciding with strong Wolf-Rayet star features, the nebular abundance of N is elevated by 400% compared to galaxies of similar metallicity (12+log(O/H) = 8.15, Z=0.20 Z_ødot). From Hubble Space Telescope FOS spectroscopy, we confirm the elevated N abundances (log(N/O)= -0.90) at two locations in the central HII region. Interestingly, we find that He abundances are consistent with (He/H)\sim0.08, typical of low-metallicity galaxies. Measurements of the C III] \lambda1909Å\ emission line, in conjunction with [O III] \lambda5007Åyield log(C/O) = -0.68, typical of low-metallicity galaxies (Garnett et al. 1995, ApJ, 443, 64) and consistent with no C enrichment. As N and C production are thought to be produced mostly in intermediate and low-mass stars respectively, the lack of C enrichment in NGC 5253 suggests two possibilities. Either 1) massive stars are a significant source of primary N, and the N--rich and He--rich Wolf-Rayet star winds are responsible for the observed elevated abundances, or, 2) the elevated N is due to secondary production in intermediate mass stars, and N production is effectively de-coupled from C production. This latter possibility also requires an overproduction of N relative to O which must be explained in terms of an unusual star formation history or IMF in NGC 5253.

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