1 Introduction

The presence of molecular emission in the spectra of supernovae while not new remains rarely detected. First detection of carbon monoxide in a supernova was in the spectra of SN1987A (Catchpole & Glass 1987; McGregor & Hyland 1987; Ames Research Center 1987; Oliva et al. 1987; Spyromilio et al. 1988). In SN1987A observations of the first overtone of CO at 2.3-$\mu$m were complemented by observations at 4.6-$\mu$m of the fundamental bands. In addition to carbon monoxide, the spectra of SN1987A revealed bands of SiO (Meikle et al. 1993; Roche et al. 1991) and CS (Meikle et al. 1993). The presence of H₃⁺ is also claimed in the spectra of SN1987A (Miller et al. 1992) although alternative atomic species can also explain the identified features. Spyromilio & Leibundgut (1996) reported the detection of CO first overtone emission in the spectra of the type II supernova 1995ad while Gerardy et al. (2000) and Fassia et al. (2001) reported the presence of CO in SN1998S.

The formation of molecules in the ejecta of supernovae, even CO - the most stable of diatomic molecules with a dissociation energy of 11.09 eV (Douglas & Møller 1955), is not trivial. The high UV field due to recombinations and the energetic electrons from the radioactive decays of ⁵⁶Ni and its daughter ⁵⁶Co create an inhospitable environment with multiple dissociation paths (see Lepp et al. 1990). The presence therefore of molecules in the ejecta imply a particular distribution of the material within the ejecta and place constraints on the degree of mixing possible. These effects have only been analyzed in detail in the well observed SN1987A. The degree to which that object is representative of the entire class of type II SNe, which will almost by definition not be observed with the same accuracy, is of some importance.

Molecules also affect the degree of cooling of the ejecta. The partition function of a diatomic molecule such as CO is enormous compared to that of even a heavy ion such as Fe⁺. Moreover, the lower rotational levels of the fundamental, first and second overtones of CO require low energies for their excitations allowing them to continue cooling the ejecta even as the temperature drops.

Gerardy et al. (2000) and Fassia et al. (2001) also report the detection of emission from dust in SN1998S and Fassia et al. argue that the cooling by CO may lead to conditions that favour the formation of dust.

Here we report on observations of two more type II SNe in the near infrared which exhibit emission by carbon monoxide. In Sect. 2 we report on the observations. The data are discussed and compared with those of SN1987A and theoretical models in Sect. 3.