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1992ApJ...401..157C ON DETERMINING THE PRIMORDIAL HELIUM ABUNDANCE FROM THE SPECTRA OF H II GALAXIES ALISON CAMPBELL Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 Received 1992 February 18; accepted 1992 June 12 ABSTRACT A data set of 53 H II galaxies and giant H II regions with reliable observational uncertainties reveals a highly significant physical dispersion in the oxygen abundance-helium abundance relation commonly used to infer the primordial helium abundance (He/H)_p_. Consideration of possible sources of scatter indicates that it is almost certainly one- sided, i.e., upward from a minimum value of He/H at each O/H. The presence of an asymmetric physical dispersion violates the assumptions of conventional least-squares fitting, and hence casts doubt on values of (He/H)_p_ obtained by extrapolation of ridge-line fits to the O/H-He/H relation. The loss of oxygen-but not helium-in supernova explosions will lead naturally to a positive dispersion in He/H. Such galactic winds must be present in H II galaxies because their masses are significantly below the critical mass needed for prevention. The observed dispersion can be explained by chemical evolution models in which between 0% and 95% of the newly synthesized oxygen is lost. Variations in the time between successive bursts of star formation and incomplete mixing of newly synthesized helium will also introduce upward scatter in He/H. We therefore conclude that (He/H)_p_ can be reliably determined only by fitting to the lower envelope of the O/H-He/H relation. The paucity of galaxies near the envelope makes its position difficult to determine at present, but existing data give (He/H)_p_ = 0.0759 +/- 0.0014, somewhat higher than the recent, very low estimate of 0.0738 +/- 0.0016, obtained via ridge-line fitting. More high-quality H II galaxy data with reliable formal uncertainties are required to establish the position of the lower envelope of the O/H-He/H relation and produce an accurate value for (He/H)_p_. Subject headings: galaxies: abundances - galaxies: ISM - H II regions - ISM: abundances - nuclear reactions, nucleosynthesis, abundances ========================================================================== 1993ApJ...405..803C ERRATUM In the paper "On Determining the primordial Helium Abundance from the Spectra of H II Galaxies" by Alison Campbell (ApJ,401,157(1992]) a correction should be made to one of the values given for (He/HN)_p_. The value (He/N)_p_ = 0.0738 +/- 0.0016 which was quoted from the work of B. E. J. Pagel, E. A. Simonson, R. J. Terlevich, & M. Edmunds (MNRAS 255,325(1992]) is their standard least-squares fit to the 19 object O/H-He/H data set (Table 16). They prefer a value (He/N)_p_ = 0.0738+/-0.0021, based on the average of O/H-He/H and N/H-He/HN regressions and obtained by a maximum-likelihood technique which B. Pagel (private communication) has informed the author correctly accounts for errors in both y and x. This value for (He/N)_p_ is 2.0 {sigma}, rather than 2.7 {sigma}, below the theoretical lower limit of 0.078+/-0.0004. (The uncertainty in the theoretical (He/H)_p_ reflects the 1 {sigma} experimental uncertainty in the neutron lifetime; T. P. Walker, G. Steigman, D. N. Schramm, K. A. Olive, & H. Kang, ApJ, 376, 5I (1991), and references therein.] Further, the ~2% overestimate of the true value of (He/H)_p_ which arises from the neglect of x uncertainties (Sections III and VI of the author's paper) does not apply to Pagel et al.'s result. The presence of an intrinsic dispersion in helium abundance at a given oxygen abundance was noted by B. E. J. Pagel, R. J. Terlevich, & J. Melnick (PASP, 98,1005(1986]) and was associated by them with winds from WolfRayet (WR) stars. Pagel et al. (1992) exclude from their regression analysis all H II galaxies exhibiting WR emission features and find that the remaining objects in their sample show no physical scatter.
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