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The primordial abundance of 4 He : a self-consistent empirical analysis of systematic effects in a large sample of low-metallicity H ii regions

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic H ii regions. This sample constitutes the largest and most homogeneous high-quality data sets in existence for the determination of Yp. For comparison and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity H ii regions selected from the Data Release 5 of the Sloan Digital Sky Survey. Although this larger sample shows more scatter, it gives results that are consistent at the 2σ level with our original sample. We have considered known systematic effects which may affect the He abundance determination. They include different sets of He i line emissivities and reddening laws, collisional and fluorescent enhancements of He i recombination lines, underlying He i stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H ii region, and deviation of He i and H emission line intensities from case B. However, the most likely value of Yp depends on the