THE TRANSIENT MOLECULAR ENVELOPE IN THE OUTFLOW OF THE NOVA V2676 Oph DURING ITS EARLY PHASE

Abstract

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.

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

“…The classical nova V2676 Oph, which is the focus of this paper, is unique because it was the first nova to provide evidence of C 2 molecules, and is only the second example of a nova in which CN molecules have been detected (the first example was DQ Her in 1934) during its near-maximum phase, as reported by Nagashima et al (2014). The nova was discovered by Nishimura (2012) with a UT on 2012 March 25.8 (probably just after the explosion) and was considered an "Fe II-type" classical nova (according to the classification by Williams 1992) based on optical low-resolution spectroscopic observations (Arai & Isogai 2012).…”

“…The lightcurve evolution of V2676 Oph is slower than that of typical novae (Nagashima et al 2014), and it is believed that such slow novae occur on WDs that have relatively smaller masses (Hachisu & Kato 2006). The similarity between the lightcurves of V2676 Oph and DQ Her (in 1934) implies that the mass of the WD star hosting V2676 Oph was similar to that of DQ Her, ~0.6 M sun (Horne et al 1993;Hachisu & Kato 2015).…”

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

“…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].…”

“…They exhibit high effective temperatures (∼8000 K) even at their visual-brightness maxima, when the size of the nova photosphere is at a maximum (and the photospheric temperature is therefore a minimum). Nevertheless, some simple diatomic molecules have been detected in several classical novae with excitation temperatures in the range from ∼2500 K to ∼5000 K (e.g., [8,45,36,17] and references therein). Spectroscopic observations of molecular bands in novae provide information about isotopic abundances, and observations of the molecules (as intermediate products in the evolution from atomic species to dust grains) are essential for understanding the dust formation in nova outflows.…”

“…The classical nova V2676 Oph is unique in that C 2 , CN, and CO molecules -and larger molecules such as polycyclic aromatic hydrocarbons (PAHs) or hydrogenated amorphous carbon (HAC) -have all been detected in the same nova. In addition, significant dust formation began ∼90 days after the nova explosion [36,51,34,46]. Dust formation is recognized in about ∼20% of classical novae [59] and usually occurs in "CO novae" that involve CO 1 white dwarfs (WDs), whereas dust formation rarely occurs in "ONe novae" involving ONe WDs [20].…”