Helium line emissivities for nebular astrophysics

TL;DR

This paper provides detailed collisional-radiative models for helium line emissivities in nebulae, highlighting differences with previous models and offering data and tools for precise abundance measurements.

Contribution

It introduces improved helium emissivity models with updated atomic rates, compares them to prior models, and supplies comprehensive tables and interpolation code for astrophysical applications.

Findings

Significant differences found compared to previous models.

Identification of transitions least sensitive to atomic rate variations.

Provision of emissivity tables and FORTRAN code for practical use.

Abstract

We present the results of several collisional-radiative models describing optically-thin emissivities of the main lines in neutral helium formed by recombination, for a grid of electron temperatures and densities, typical of H II regions and Planetary Nebulae. Accurate emissivities are required for example to measure the helium abundance in nebulae and as a consequence its primordial value. We compare our results with those obtained by previous models, finding significant differences, well above the target accuracy of one percent. We discuss in some detail our chosen set of atomic rates and the differences with those adopted by previous models. The main differences lie in the treatment of electron and proton collision rates and we discuss which transitions are least sensitive to the choice of these rates and therefore best suited to high precision abundance determinations. We have focused our comparisons on the case B approximation where only He and He$^+$ are considered, but also present results of full models including the bare nuclei, photo-excitation and photo-ionisation and either black-body or observed illuminating spectrum in the case of the Orion nebula, to indicate which spectral lines are affected by opacity. For those transitions, accurate radiative transfer calculations should be performed. We provide tables of emissivities for all transitions within $n \le 5$ and all those between the $n \le 5$ and $n' \le 25$ states, in the log $T_{\rm e}$ [K]=10$^{3.0(0.1)4.6}$ and log $N_{\rm e}$ [cm$^{-3}$]=10$^{2(0.5)6}$ ranges, and a FORTRAN code to interpolate to any $T_{\rm e}, N_{\rm e}$ within these ranges.