Determination of the primordial helium abundance from radio recombination line observations: New data. The source W51

Abstract

Observations of H and He radio recombination lines in the source W51 have been performed with the RT-22 radio telescope (Pushchino) in two transitions: 56α (8 mm) and 65α (13 mm). We have estimated the spectral line parameters and determined the relative abundance of ionized helium, y+ = (9.3 ± 0.35)%. We have carried out a model study of the correction (R) for the ionization structure of HII regions (when passing from the observed y+ = N(He+)/N(H+) to the actual y = N(He)/N(H)) as a function of the spectral type of the ionizing star. Hence it follows that it is desirable to choose the sources excited by hot stars of spectral types no later than O6 V to estimate the helium abundance. In this case, the correction is expected to be small and essentially constant, R in the range 1.0–1.05. We have analyzed the correction for the ionization structure of W51, obtained an actual abundance of helium in the range y = (8.9–9.7)%, and determined its primordial abundance Yp (produced during primordial nucleosynthesis in the Universe) in this source. We have made a new estimate of the primordial helium abundance from six Galactic HII regions, where we observed H and He radio recombination lines at different times. The weighted mean Yp = 25.64(±0.70)% has been obtained. On the one hand, this value of Yp does not yet disagree strongly with the conclusions of the standard cosmologicalmodel, but, on the other hand, it admits the existence of at least one unknown light particle in the period of primordial nucleosynthesis outside the scope of the standard cosmological model. One should continue to refine Yp for more reliable conclusions to be reached.