Effect of the continuum emission of R Leo on the vibrational excitation of CO₂. a: continuum emission of R Leo based on photometric observations available in several databases. This data set has been fitted with two black-bodies between 0.8 and 100 μm (solid black curve) to represent the star (“eff”) and the dusty component of the envelope (“dust”). The dashed black curve is an extrapolation of this fit below 0.8 μm. The T_X and θ_X are the temperature and the angular diameter of the X emitting region. The vertical gray bands indicate the spectral ranges covered by the strongest CO₂ bands. Neither dust opacity nor scattering were considered in our fits. b: low spectral resolution absorption spectrum of CO₂ calculated under LTE at a temperature of 1500 K and with a column density of 5 × 10¹⁸ cm⁻². The strongest bands are highlighted in gray. c: vibrational energy diagram of CO₂ with the most important vibrational states involved in the observed bands. The possible vibrational transitions are plotted as red lines, with thickness increasing as their A-Einstein coefficient grows (dashed lines for the lowest coefficients).