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Session 10 - Spiral Galaxies.
Display session, Monday, June 10
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.
Program listing for Monday