The Calibration of Pagel's Method to Derive the O/H Abundances of Extragalactic HRegions Based on O Recombination Lines

Details The Calibration of Pagel's Method to Derive the O/H Abundances of Extragalactic HRegions Based on O Recombination Lines The Calibration of Pagel's Method to Derive the O/H Abundances of Extragalactic HRegions Based on O Recombination Lines

Jun 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006rmxac..26r.163p&link_type=abstract

XI IAU Regional Latin American Meeting of Astronomy (Eds. L. Infante & M. Rubio) Revista Mexicana de Astronomía y Astrofísica (S

Astronomy and Astrophysics

Astronomy

1

Scientific paper

The difficulty of measuring I(4363) (or any other direct temperature indicator) led Pagel et al.(1979, MNRAS, 189, 95) to propose an empirical method based on the ratio of the nebular oxygen lines to I(H β ), R_{23} = [I(3727) + I(4959) + I(5007)]/I(Hβ), to determine the O/H ratio in giant extragalactic Hregions. There are three different ways to calibrate Pagel's Method: a) by determining the oxygen abundance assuming uniform temperature and using T_e(4363/5007); b) from photoionization models; and c) using the abundances derived from Orecombination lines. This third method uses abundances based on the much fainter recombination lines; these lines have become accessible with the new generation of 8m class telescopes.
We consider the option to calibrate Pagel's Method based on the O recombination lines to be superior to the one based on photoionization models, because even the best available models are not yet able to reproduce all the observed emission line ratios. The O recombination method is also better than the option based on the observationally determined T_e(4363/5007), because the O/H values derived from the nebular lines and T_e(4363/5007) are very sensitive to the t2 value while the O/H values derived from recombination lines are independent from it.
The O recombination lines are only available to calibrate the upper branch of Pagel's method; from 14 objects the average increase with respect to the T_e(4363/5007) method amounts to 0.21 dex. To calibrate the lower branch a different approach is needed, we propose the use of the t2 formalism defined by Peimbert (1967, ApJ, 150, 825); from 5 objects the average increase with respect to the T_e(4363/5007) method amounts to 0.11 dex.

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