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Fig. 2

image

Steps involved in the modelling of the total cross-section around the Balmer limit. The Einstein B coefficients are first calculated, either explicitly or by interpolation (blue). The cross-section is then computed for λ < λ+ (upper green region) using either Eq. (11) or Eq. (12). For λ > λ+, the distance between lines is comparable to the line’s FWHM, so that the approximation of Eq. (10) is no longer valid, and lines have to be computed individually, considering both a finite truncated sum and its sum remainder (SR; red). The opacity of the lines is finally summed at wavelengths λ > λ+ (lower greenregion).

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