Radiative transfer effects in He I emission lines

We consider the effect of optical depth of the 2 3S level on the nebular recombination spectrum of He I for a spherically symmetric nebula with no systematic velocity gradients. These calculations, using many improvements in atomic data, can be used in place of the earlier calculations of Robbins. We give representative Case B line fluxes for UV, optical, and IR emission lines over a range of physical conditions: T = 5000-20, 000 K, n e = 1-108 cm-3, and τ3889 = 0-100. A FORTRAN program for calculating emissivities for all lines arising from quantum levels with n ≤ 10 is also available from the authors. We present a special set of fitting formulae for the physical conditions relevant to low-metallicity extragalactic H II regions: T = 12, 000-20, 000 K, n e = 1-300 cm-3, and τ3889 < 2.0). For this range of physical conditions, the Case B line fluxes of the bright optical lines 4471 Å, 5876 Å, and 6678 Å, are changed less than 1%, in agreement with previous studies. However, the 7065 Å corrections are much smaller than those calculated by Izotov & Thuan based on the earlier calculations by Robbins. This means that the 7065 Å line is a better density diagnostic than previously thought. Two corrections to the fitting functions calculated in our previous work are also given.