Physical Conditions of the Small Magellanic Cloud HII Region NGC 456 and the Underestimated Heavy Element Abundances of the Universe

Details Physical Conditions of the Small Magellanic Cloud HII Region NGC 456 and the Underestimated Heavy Element Abundances of the Universe Physical Conditions of the Small Magellanic Cloud HII Region NGC 456 and the Underestimated Heavy Element Abundances of the Universe

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21743435p&link_type=abstract

American Astronomical Society, AAS Meeting #217, #434.35; Bulletin of the American Astronomical Society, Vol. 43, 2011

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We study the O/H abundance determinations in HII regions in the presence of temperature inhomogeneities (as proposed by Peimbert in 1967), t2. The first object of our study is NGC 456, which the second brightest HII region of the Small Magellanic Clouds (SMC); we find it has an abundance 1.75 times higher than previously determined. Traditionally it is assumed that these objects have the same temperature throughout the whole volume; this temperature is then used to calculate abundances. Our determination was done using Collisionally Excited Lines (CEL) and the t2 obtained from HeI lines; it is consistent with abundances determined with Recombination Lines (RL). The most well known strong-line ratio or metallicity indicator used to determine the O/H ratio is O23= ([OII]3727Å + [OIII]4959,5007Å) / Hβ introduced by Pagel et al. (1979). The log(O23) vs. 12+log(O/H) diagram constitutes an important tool in the study of objects with low intrinsecal brightness or high redshift. To account for the presence of thermal inhomogeneities, the upper branch of the O23-O/H diagram can be recalibrated using RLs whereas in the lower branch of the diagram RLs cannot be used due to their faintness. For low metallicity objects the formalism of t2 is applied to CELs to determine abundances and recalibrate the O23-O/H diagram.
We find that the point corresponding to NGC 456 in the diagram shifts up by 0.24 dex in the 12+log(O/H) axis. This is consistent with preliminary results for the next 3 objects of our sample and with the behavior of other HII regions from the literature were t2 has been measured. The systematic shift in the curve of the O23-O/H diagram implies that abundances in the Universe need to be corrected by a factor of approximately 2.

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