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The dust-forming nova V2676 Oph is unique in that it was the first nova to provide evidence of C 2 and CN molecules during its near-maximum phase and evidence of CO molecules during its early decline phase. Observations of this nova have revealed the slow evolution of its lightcurves and have also shown low isotopic ratios of carbon ( 12 C/ 13 C) and nitrogen ( 14 N/ 15 N) in its nova envelope. These behaviors indicate that the white dwarf (WD) star hosting V2676 Oph is a CO-rich WD rather than an ONe-rich WD (typically larger in mass than the former). We performed mid-infrared spectroscopic and photometric observations of V2676 Oph in 2013 and 2014 (respectively 452 and 782 days after its discovery). No significant [Ne II] emission at 12.8 µm was detected at either epoch. These provided evidence for a CO-rich WD star hosting V2676 Oph. Both carbon-rich and oxygen-rich grains were detected in addition to an unidentified infrared feature at 11.4 µm originating from polycyclic aromatic hydrocarbon molecules or hydrogenated amorphous carbon grains in the envelope of V2676 Oph. KEYWORDS

The slow nova V2676 Oph was unique in that both C2 and CN molecules were observed during its early phase around the visual brightness maximum. The absorption bands of these molecules were detected using spectroscopic techniques and disappeared within several days. The formation of CO was followed by dust formation, which was confirmed via observations in the latter phase of the nova. The formation of molecules is critically important in understanding the origin of dust grains in novae; however, the formation mechanism of C2 and CN molecules in the early phase of novae remain unclear. Here we report optical spectroscopic observations of the dust-forming nova V2676 Oph in the early phase, and we use these data in addition to spectroscopic observations reported previously to discuss the origin of diatomic molecules in the nova. We conclude that C2 and CN molecules formed in the nova outflow when the outer gas envelope cooled sufficiently and that they survived for more than 2 days but less than 7 days in the nova outflow.

Novae are generally considered to be "hot" astronomical objects which show effective temperatures of 10,000 K or higher at their visual maximum. However, theoretical predictions suggest that the outer envelope of the nova outflow can become cool enough to form molecules which would be dissociated at such high temperatures. We detected strong C 2 and CN absorption bands in the optical spectrum of the nova V2676 Oph, a very slow nova with dust formation. This is the first report of the detection of C 2, and the second of CN, in novae during an outburst. Although, based on previous studies, such simple molecules are predicted form in the envelope of the outflow, there are few reports of their detection thus far. The presence of the molecular envelope is considered to be very transient, with a duration of only a few days, in the case of V2676 Oph.

Novae are generally considered to be hot astronomical objects and show effective temperatures up to 10,000 K or higher at their visual maximum. But, it is theoretically predicted that the outer envelope of the nova outflow can become cool enough to form molecules that would be dissociated at high temperatures. We detected strong absorption bands of C 2 and CN radicals in the optical spectrum of Nova V2676 Oph, a very slow nova with dust formation. This is the first report of the detection of C 2 and the second one of CN in novae during outburst. Although such simple molecules are predicted to form in the envelope of the outflow based on previous studies, there are few reports of their detection. In the case of V2676 Oph, the presence of the molecular envelope is considered to be very transient, lasting several days only.