Massive star formation in Wolf-Rayet galaxies: IV b. Using empirical
  calibrations to compute the oxygen abundance
  We have performed a comprehensive multiwavelength analysis of a sample of 20
starburst galaxies that show a substantial population of very young massive
stars, most of them classified as Wolf-Rayet (WR) galaxies. We have analysed
optical/NIR colours, physical and chemical properties of the ionized gas,
stellar, gas and dust content, star-formation rate and interaction degree
(among many other galaxy properties) of our galaxy sample using
multi-wavelength data. We compile 41 independent star-forming regions --with
oxygen abundances between 12+log(O/H) = 7.58 and 8.75--, of which 31 have a
direct estimate of the electron temperature of the ionized gas. This paper,
only submitted to astro-ph, compiles the most common empirical calibrations to
the oxygen abundance, and presents the comparison between the chemical
abundances derived in these galaxies using the direct method with those
obtained through empirical calibrations, as it is published in Lopez-Sanchez &
Esteban (2010b). We find that (i) the Pilyugin method (Pilyugin 2001a,b;
Pilyugin & Thuan 2005) which considers the R23 and the P parameters, is the
best suited empirical calibration for these star-forming galaxies, (ii) the
relations between the oxygen abundance and the N2 or the O3N2 parameters
provided by Pettini & Pagel (2004) give acceptable results for objects with
12+log(O/H)>8.0, and (iii) the results provided by empirical calibrations based
on photoionization models (McGaugh, 1991; Kewley & Dopita, 2002; Kobulnicky &
Kewley, 2004) are systematically 0.2 -- 0.3 dex higher than the values derived
from the direct method. These differences are of the same order that the
abundance discrepancy found between recombination and collisionally excited
lines. This may suggest the existence of temperature fluctuations in the
ionized gas, as exists in Galactic and other extragalactic HII regions.
