Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies

We use spectroscopic observations of a sample of 82 HII regions in 76 blue compact galaxies to determine the primordial helium abundance Yp and the slope dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ measurement, we have included new spectrophotometric observations of 33 HII regions 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 HII regions, we derive the He mass fraction taking into account known systematic effects, including collisional and fluorescent enhancements of HeI emission lines, collisional excitation of hydrogen emission, underlying stellar HeI absorption and the difference between the temperatures Te(HeII) in the He^+ zone and Te(OIII) derived from the collisionally excited [OIII] lines. We find that the net result of all the systematic effects combined is small, changing the He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linear regression to O/H=0 for 7 HII regions of this subsample, we obtain Yp=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 the standard 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 recent deuterium abundance and microwave background radiation measurements. The linear regression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for the whole sample of 82 HII regions is similar to that derived for the subsample of 7 HII regions, although it has a considerably smaller uncertainty.