The infrared colour evolution of type Ia supernovae is very uniform, exhibiting a dip in the J–H light curve approximately 30 d after maximum light. Spectroscopically, this dip has been identified with what appears as a broad absorption feature |(5 × 10⁴ \ km \ s⁻¹)| at 1.2 |μ m|. Although many candidate transitions have been investigated in the past, no plausible species of ion or molecule has been found that can account for an absorption at this wavelength. We suggest that this ‘absorption’ is in fact a lack of emission caused by the absence of any significant opacity in this wavelength interval. At this epoch the supernova is merely a broad-line nebula with no lines near 1.2 |μ m|.
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Spyromilio et al. (1994) studied this question.