On the Question of the Primordial Helium Abundance by the RRL Observations in Orion A

Radio recombination lines comprise a powerful tool for studying the interstellar medium. One of the important tasks is to measure the primordial abundance of helium formed during the primordial nucleosynthesis of the Universe, which, in turn, allows us to verify the conclusions of the Standard Cosmological Model. The Orion A nebula is an interesting object for studying this problem. Previously, we found that, in this HII region, the region of ionized helium is smaller than the region of ionized hydrogen. Therefore, the actual helium abundance, n(He)/n(H), is not less than the maximum measured value of y+ = n(He+)/n(H+). This makes it possible to obtain restrictions on the primordial helium abundance. This article presents new observations of radio recombination lines in Orion A at a wavelength of 13 mm. It was found that the maximum value of y+ is in the range 10.03–11.55%. Therefore, it can be expected that the primordial helium abundance (Yp, He/H mass ratio) can be not less than ≈24.93–29.40%, which allows deviations from the conclusions of the Standard Model; for instance, it allows the presence of unknown light particles during primordial nucleosynthesis. Observations of doubly ionized helium in Orion A and the planetary nebula NGC 7027 were also made. It was found that the contribution of doubly ionized helium, y++ = n(He++)/n(H+), is <7 × 10–4 in Orion A and 2.7(±1.3)% for NGC 7027. Estimates of the electron temperature are also made. In particular, for NGC7027, it was found that Тe ≈ 11 900–12 300 K, which is higher than for the HII regions.