Infrared spectroscopy of Nova Cassiopeiae 1993 — I. The pre-dust phase

We present infrared spectroscopy of Nova Cassiopeiae 1993 (V705 Cas) in the period before dust formation. We find strong emission in the first-overtone CO bands before the nova reached maximum light — the earliest detection of a molecule in a nova. Our observations suggest the presence of a relatively cool ( ≲ 4500 K), neutral zone in the ejecta, as required by chemical modelling of nova winds. CO formation is likely to have been dominated by the H2 route rather than by direct radiative association of C and O. We suggest that radiative cooling of the ejected material by CO could play a major role in the grain formation process.