ON THE ORIGIN OF THE 1.2-MU-M FEATURE IN TYPE IA SUPERNOVA SPECTRA

The infrared colour evolution of type Ia supernovae is very uniform, e xhibiting a dip in the J-H light curve approximately 30 d after maximu m light. Spectroscopically, this dip has been identified with what app ears as a broad absorption feature (5 x 10(4) km s(-1)) at 1.2 mu m. A lthough many candidate transitions have been investigated in the past, no plausible species of ion or molecule has been found that can accou nt for an absorption at this wavelength. We suggest that this 'absorpt ion' is in fact a lack of emission caused by the absence of any signif icant opacity in this wavelength interval. At this epoch the supernova is merely a broad-line nebula with no lines near 1.2 mu m.