THE PRIMORDIAL HELIUM ABUNDANCE FROM A NEW SAMPLE OF METAL-DEFICIENT BLUE COMPACT GALAXIES

We use high-quality spectrophotometric observations of 10 low-metallic ity blue compact galaxies (BCGs) with oxygen abundance ranging from 12 + log(O/H) = 7.37 to 8.04 to determine the primordial helium abundanc e. We take special care into investigating the physical effects which may affect such a determination. We find that underlying stellar absor ption, enhancement of helium lines due to self-absorption, and correct ions for neutral and doubly ionized helium to be negligible. The main physical mechanism changing the He I line intensities from their recom bination values is collisional excitation. To correct for it, we calcu late the electron number density in the He+ zone by a self-consistent procedure which constrains the He I lambda 5876/lambda 4471, lambda 66 78/lambda 4471 and lambda 7065/lambda 4471 line ratios to have their r ecombination values, after correction for collisional enhancement. The density sensitive He I lambda 7065 line plays an important role in th is respect. The usual practice of using the electron number density de rived from the [S II] lambda 6717/lambda 6731 ratio underestimates the collisional enhancement correction and hence overestimates the helium abundance by similar to 5%. We find that Brocklehurst's (1972) He I r ecombination emissivities give a smaller dispersion of the data points in the Y versus O/H and Y versus N/H planes and within the derived He abundances from different lines in a given galaxy than Smits's old (1 991a, b) emissivities. The dispersion is comparable when Smits's new ( 1994) emissivities are used. By taking the mean of the intercepts of b oth Y versus O/H and Y versus N/H linear regressions at O/H = N/H = 0, we determine a primordial helium mass fraction Y-p = 0.229 +/- 0.004, with Brocklehurst's emissivities, and Y-p = 0.240 +/- 0.005 with Smit s's new emissivities. Taking the mean Y of the four most metal-deficie nt BCGs (Z < Z(.)/15) in our sample gives Y-p = 0.233 +/- 0.003 with B rocklehurst's emissivities. Adding the four known BCGs with Z < Z(.)/1 5 from the literature yields: Y-p = 0.232 +/- 0.002. These determinati ons are consistent with the lower limit of Y-p = 0.236 required by sta ndard big bang nucleosynthesis theory. We obtain a slope dY/dZ = 5.8 /- 1.7 with Brocklehurst's emissivities and dY/dZ = 5.8 +/- 4.4 with S mits' new emissivities, larger than the values predicted by closed-box models, but consistent with those given by chemical evolution models with differential galactic winds.