Formation of C2 and CN in nova V2676 Oph around its visual brightness maximum

Abstract

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.

Mid-Infrared Spectroscopic Observations of the Dust-Forming Classical Nova V2676 Oph*

“…However, the observed isotopic ratios of carbon and nitrogen in V2676 Oph can be reproduced by the nucleosynthesis models for WDs with a moderate mass (~1.1 M sun ) close to the high-end of the mass range for CO-rich WDs (Denissenkov et al 2014). The carbon-rich chemistry of the ejecta (C/O>1) as well as the low isotopic ratios ( 12 C/ 13 C and 14 N/ 15 N) observed in V2676 Oph (Kawakita et al 2015(Kawakita et al , 2016 challenge the traditional TNR models for low-mass CO-rich WDs (José & Hernanz 1998José et al 2006José et al , 2016Denissenkov et al 2014).…”

“…As regards the isotopic ratios found in V2676 Oph ( 12 C/ 13 C ~4 and 14 N/ 15 N ~2; Kawakita et al 2015), the calculations by José & Hernanz (2007) were not able to reproduce its observed isotopic ratios. On the other hand, recent calculations by Denissenkov et al (2014) reproduced similar values to those observed in V2676 Oph in the cased of a CO-rich WD with a mass of ~1.15 M sun (Fig.…”

A Review of the Evolution of Classical Nova V2676 Oph: Formation of Molecules and Dust Grains

“…The C 2 Swan-band absorption and the CN "red system" band absorption were detected simultaneously at days t = 13 and 14 [36,32]. The C 2 molecule has never been detected previously in other classical novae, whereas the CN molecule was detected clearly in DQ Her just after its visualbrightness maximum, and absorption due to both the CN violet-and red-system bands lasted about one week [69,54,58,6,2,56].…”

“…The energy source for this brightening may have been supplied by a transition in the internal structure of the nova from a static configuration to a "nova wind" configuration, as noted by Kato & Hachisu in 2009 [31]. At the same time, there were a few weak emission lines, and the expansion velocity was still as slow as ∼450 km s −1 [32], indicative of slowly expanding gas around the nova photosphere. If the optically thin wind had already ceased by that time, the observed emission lines may have arisen from remnants of gas ejected as an optically thin wind during an earlier phase.…”

“…Low-resolution optical spectroscopic observations were performed by several observers around the visual-brightness maximum of V2676 Oph [36,32,46]. Just after its discovery, the emission lines were narrow, and the P-Cygni profile of Hα emission indicated a decrease in the expansion velocity of the nova outflow, from ∼750 km s −1 to ∼400 km s −1 , during days t = 2 -13 [32]. The P-Cygni absorption profile of Na I indicated an outflow velocity of 600 km s −1 at day t = 10 [46].…”