The neon nova. III. The infrared light curves of nova QU Vulpeculae (Nova Vul 1984 #2)

We report 1.25 to 19.5 μm broadband infrared (IR) photometric measurements acquired during an eight year period on the prototypical ONeMg "neon nova" QU Vulpeculae (Nova Vul 1984 #2). The energy distribution of the ejecta evolved through several phases. An early free-free emission phase was followed by an IR coronal phase characterized by the appearance of strong emission lines from forbidden atomic transitions. The lines of [Ne VI] at 7.6 μm, and [Ne II] at 12.8 μm were especially strong during the coronal phase. A small amount of silicate dust condensed in the ejecta after about a year. The evidence provided by our IR observations for high abundances of metals in the ejecta of QU Vul is reviewed. We present the IR light curves of QU Vul, and show that the temporal development of its persistent IR coronal emission phase was evident in the broadband K (2.3 μm) and L (3.6 μm) photometry. Using data from our previous studies of classical novae, we suggest that K and L photometry can distinguish between the slower ONeMg novae with persistent IR coronal activity and CO novae that produce copious quantities of circumstellar dust. The most striking signature is produced in the L band, which contains emission from [Mg VIII] at 3.02 μm, [Al VI] at 3.66 μm, and [Si IX] at 3.92 μm. We comment on the peculiar tendency of the IR light curves of novae to decay exponentially.