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The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observationsWe present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosley's best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Garnett, D. R. (Univ. of Minnesota, Minneapolis, MN United States)
Skillman, E. D. (Univ. of Minnesota, Minneapolis, MN United States)
Dufour, R. J. (Rice Univ. Houston, TX, United States)
Peimbert, M. (UNAM Mexico, Mexico)
Torres-Peimbert, S. (UNAM Mexico, Mexico)
Terlevich, R. (Royal Greenwich Observatory, Cambridge, England United Kingdom)
Terlevich, E. (Royal Greenwich Observatory, Cambridge, England United Kingdom)
Shields, G. A. (Univ. of Texas, Austin, TX United States)
Date Acquired
August 16, 2013
Publication Date
October 4, 1995
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 443
Issue: 1
ISSN: 0004-637X
Subject Category
Funding Number(s)
Distribution Limits