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We use spectroscopic observations of a sample of 82 HII regions in 76 bluecompact galaxies to determine the primordial helium abundance Yp and the slopedY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZmeasurement, we have included new spectrophotometric observations of 33 HIIregions which span a large metallicity range, with oxygen abundance 12+log(O/H)varying between 7.43 and 8.30 (Zsun/30<Z<Zsun/4). For a subsample of 7 HIIregions, we derive the He mass fraction taking into account known systematiceffects, including collisional and fluorescent enhancements of HeI emissionlines, collisional excitation of hydrogen emission, underlying stellar HeIabsorption and the difference between the temperatures Te(HeII) in the He^+zone and Te(OIII) derived from the collisionally excited [OIII] lines. We findthat the net result of all the systematic effects combined is small, changingthe He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linearregression to O/H=0 for 7 HII regions of this subsample, we obtainYp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds to dY/dZ=3.7+/-1.2,assuming the oxygen mass fraction to be O=0.66Z. In the framework of thestandard Big Bang nucleosynthesis theory, this Yp corresponds to Omega_b h^2 =0.012^+0.003_-0.002, where h is the Hubble constant in units of 100 km/s/Mpc.This is smaller at the 2sigma level than the value obtained from recentdeuterium abundance and microwave background radiation measurements. The linearregression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for thewhole sample of 82 HII regions is similar to that derived for the subsample of7 HII regions, although it has a considerably smaller uncertainty.Comment: 53 pages, 3 Postscript figures, accepted for publication in the Astrophysical Journa

astrophysical journal/the astrophysical journal

Systematic Effects and a New Determination of the Primordial Abundance of4He anddY/dZfrom Observations of Blue Compact Galaxies