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Date and Time of the Query: 2019-05-20 T03:38:09 PDT
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For refcode 2006ApJ...637..269V:
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Copyright by American Astronomical Society. Reproduced by permission
2006ApJ...637..269V Oxygen and Nitrogen in Leo A and GR 8 Liese van Zee, Evan D. Skillman, and Martha P. Haynes Abstract. We present elemental abundances for multiple H II regions in Leo A and GR 8 obtained from long-slit optical spectroscopy of these two nearby low-luminosity dwarf irregular galaxies. As expected from their luminosities, and in agreement with previous observations, the derived oxygen abundances are extremely low in both galaxies. High signal-to-noise ratio (S/N) observations of a planetary nebula in Leo A yield 12+ log (O/H)=7.30+/-0.05; semiempirical calculations of the oxygen abundance in four H II regions in Leo A indicate 12+ log (O/H)=7.38+/-0.10. These results confirm that Leo A has one of the lowest ISM metal abundances of known nearby galaxies. Based on results from two H II regions with high S/N measurements of the weak [O III] {lambda}4363 line, the mean oxygen abundance of GR 8 is 12+ log (O/H)=7.65+/-0.06; using "empirical" and "semiempirical" methods, similar abundances are derived for six other GR 8 H II regions. Similar to previous results in other low-metallicity galaxies, the mean log (N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is no evidence of significant variations in either O/H or N/O in the H II regions. The metallicity-luminosity relation for nearby (D<5 Mpc) dwarf irregular galaxies with measured oxygen abundances has a mean correlation of 12+ log (O/H)=5.67M_B_ -- 0.151M_B_, with a dispersion in oxygen about the relationship of {sigma}=0.21. These observations confirm that gas-rich, low-luminosity galaxies have extremely low elemental abundances in the ionized gas phase of their interstellar media. Although Leo A has one of the lowest metal abundances of known nearby galaxies, detection of tracers of an older stellar population (RR Lyrae variable stars, horizontal branch stars, and a well-populated red giant branch) indicate that it is not a newly formed galaxy, as has been proposed for some other similar low-metallicity star-forming galaxies. Key words: Galaxies: Abundances, Galaxies: Dwarf, Galaxies: Evolution, Galaxies: Individual: Name: Leo A, Galaxies: Individual: GR 8, Galaxies: Irregular
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