Radiative Transfer Effects in HeiEmission Lines

We consider the effect of optical depth of the 2 ^{3}S level on the nebularrecombination spectrum of He I for a spherically symmetric nebula with nosystematic velocity gradients. These calculations, using many improvements inatomic data, can be used in place of the earlier calculations of Robbins. Wegive representative Case B line fluxes for UV, optical, and IR emission linesover a range of physical conditions: T=5000-20000 K, n_{e}=1-10^{8} cm^{-3},and tau_{3889}=0-100. A FORTRAN program for calculating emissivities for alllines arising from quantum levels with n < 11 is also available from theauthors. We present a special set of fitting formulae for the physical conditionsrelevant to low metallicity extragalactic H II regions: T=12,000-20,000 K,n_{e}=1-300 cm^{-3}, and tau_{3889} < 2.0. For this range of physicalconditions, the Case B line fluxes of the bright optical lines 4471 A, 5876 A,and 6678 A, are changed less than 1%, in agreement with previous studies.However, the 7065 A corrections are much smaller than those calculated byIzotov & Thuan based on the earlier calculations by Robbins. This means thatthe 7065 A line is a better density diagnostic than previously thought. Twocorrections to the fitting functions calculated in our previous work are alsogiven.Comment: To be published in 10 April 2002 ApJ; relevant code available at ftp://wisp.physics.wisc.edu/pub/benjamin/Heliu