The temporal evolution of the near-infrared light curves of V1974 cygni (Nova Cygni 1992)

We report 1.25-18.5 μm infrared (IR) photometric measurements acquired during an 850 day period following outburst of the ONeMg "neon nova" V1974 Cygni (Nova Cygni 1992). The decline rates of the 0.55 V, 1.25 J, 1.6 H, and 2.3 μm K light curves all showed an abrupt transition from a t^-3/2 slope to a t^-3 slope at ≥170 days after outburst. The 3.5 μm (L) light curve exhibited a steady t^-3/2 decline rate from days 10 to ≈600, and the 4.5 μm (M) intensity maintained a t^-3/2 decline rate until the last observation on day ≈250. Examination of criteria for identifying 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 extreme case than QU Vul. We propose that the break in the slope of the 0.55-2.3 μm light curves is produced by a change in excitation conditions within a freely expanding optically thin shell that causes the cooling to shift rather abruptly from hydrogen recombination lines to near-IR coronal emission lines of highly ionized metals. The behavior of the visible light and IR light curves provides no evidence for dust formation in the ejecta of V1974 Cyg during the first ≈240 days following the outburst. We derive a value for the mass of gas in the ejecta of M_gas ≈ 2-5 × 10^-4 M_⊙.