H₃⁺ absorption in IRAS 07251−0248 and comparison with models. a) Observed 3.4 − 4.4 μm spectrum in IRAS 07251−0248 and adopted spline-interpolated baseline (in blue). The magenta and green lines show the modeled spectra, with the temperatures of the central source T_c and the hot-dust optical depth (τ in the inset) also indicated with colors. b) The continuum-subtracted spectrum (yellow histogram) is compared with the predictions by the two models. The spectra and models for individual lines are plotted in units of velocity in panel c. The vertical dashed line indicates the rest-frame zero velocity determined from the H₂ and H I lines. The position of the H₂ 1 − 1 S(13) 4.068 μm line, which is blended with the P(1,1) 4.070 μm line, is indicated by an arrow.