THE TEMPORAL EVOLUTION OF THE NEAR-INFRARED LIGHT CURVES OF V1974-CYGNI (NOVA-CYGNI-1992)

We report 1.25-18.5 mu m infrared (IR) photometric measurements acquir ed during an 850 day period following outburst of the ONeMg ''neon nov a'' V1974 Cygni (Nova Cygni 1992). The decline rates of the 0.55 V, 1. 25 J, 1.6 H, and 2.3 mu m K light curves all showed an abrupt transiti on from a t(-3/2) Slope to a t(-3) Slope at greater than or equal to 1 70 days after outburst. The 3.5 mu m (L) light curve exhibited a stead y t(-3/2) decline rate from days 10 to approximate to 600, and the 4.5 mu m (M) intensity maintained a t(-3/2) decline rate until the last o bservation on day approximate to 250. Examination of criteria for iden tifying CO and ONeMg novae from their near-IR light curves shows that while V1974 Cyg is identifiable as an ONeMg nova, it is a much less ex treme case than QU Vu1. We propose that the break in the slope of the 0.55-2.3 mu m light curves is produced by a change in excitation condi tions within a freely expanding optically thin shell that causes the c ooling to shift rather abruptly from hydrogen recombination lines to n ear-IR coronal emission lines of highly ionized metals. The behavior o f the visible light and IR light curves provides no evidence for dust formation in the ejecta of V1974 Cyg during the first approximate to 2 40 days following the outburst. We derive a value for the mass of gas in the ejecta of M(gas)approximate to 2-5x10(-4) M..