HeiEmission in the Orion Nebula and Implications for Primordial Helium Abundance

Abstract

We apply a recently developed theoretical model of helium emission to observations of both the Orion Nebula and a sample of extragalactic H II regions. In the Orion analysis, we eliminate some weak and blended lines and compare theory and observation for our reduced line list. With our best theoretical model we find an average difference between theoretical and observed intensities I predicted /I observed − 1 = 6.5%. We argue that both the red and blue ends of the spectrum may have been inadequately corrected for reddening. For the 22 highest quality lines, with 3499Å ≤ λ ≤ 6678Å, our best model predicts observations to an average of 3.8%. We also perform an analysis of the reported observational errors and conclude they have been underestimated. In the extragalactic analysis, we demonstrate the likelihood of a large systematic error in the reported data and discuss possible causes. This systematic error is at least as large as the errors associated with nearly all attempts to calculate the primordial helium abundance from such observations. Our Orion analysis suggests that the problem does not lie in the theoretical models. We demonstrate a correlation between equivalent width and apparent helium abundance of lines from extragalactic sources that is most likely due to underlying stellar absorption. Finally, we present fits to collisionless case-B He I emissivities as well as the relative contributions due to collisional excitations out of the metastable 2s 3 S term.

The effects of He I λ10830 on helium abundance determinations

“…Second, as discussed in Porter et al [44] and updated in Porter, Ferland, Storey, & Detisch [32], the helium emissivities, the primary driver in determining the helium abundance from the observed fluxes, carry estimated uncertainties of ∼0.2-0.7%. Furthermore, significant, systematic improvements in the helium emissivities have been made in recent years [32,45]. Any further shifts in these emissivities will have a significant effect on the determination of Y p .…”

Spectroscopic mapping of the planetary nebula NGC 6302

“…We also mapped the He + and He 2+ abundances. The He + abundances were determined from the intensities of the He i λ5876 line, using the effective recombination coefficients for Hβ taken from Storey & Hummer (1995) and for the helium line calculated from the interpolation formula given by Porter et al (2007).…”

Reduce, reuse, recycle: Planetary nebulae as green galactic citizens

“…Both Clouds are deficient in He compared with the MW disk, the LMC by ∼0.05 dex, and the SMC by ∼0.1 dex. The SMC He abundance from Shaw et al (2010) is 0.15 dex higher than that of Idiart et al (2007), arising from the use of a new He + recombination rate (Porter et al 2007). This is a perfect example of why it is imperative to understand how results are obtained by different groups before forming conclusions about any discrepancies.…”