The evolution of photospheric temperature in nova V2676 Oph toward the formation of C2 and CN during its near-maximum phase

Abstract

The molecular formation of C2 and CN in the dust-forming classical nova V2676 Oph occurs during its near-maximum phase. We investigated the temporal evolution of the photospheric temperature of the nova as it approached molecular formation during its early phase. The effective temperature of the nova around the maximum decreased from ∼7000 K to ∼5000 K over the course of ∼3 d. The molecules formed at temperatures of <∼5000 K. Such low temperatures of nova photosphere and low expansion velocities along with carbon-rich atmosphere might play an important role in achieving conditions favorable to the molecular formation of C2 and CN in V2676 Oph.

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).…”

“…We assumed astronomical silicate grains of a 0.1 µm radius to reproduce the shape of the extinction curve in the N-band. The optical depth at 9.7 µm for interstellar extinction (τ 9.7 ) was calibrated by the estimated value of τ 9.7 based on extinction in the V-band (A V ) towards the direction of V2676 Oph, A V = 2.65 ± 0.15 (Kawakita et al 2016). We converted the value of A V to τ 9.7 based on the formula: A V /τ 9.7 = 18.5 ± 1.5 (Roche & Aitken 1984).…”

“…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). Kawakita et al (2016) revealed that the photospheric temperature of V2676 Oph had cooled to ~4500 K around its visual brightness maximum; C 2 and CN molecules form while the estimated typical photospheric temperature of a nova is ~8000 K (Evans et al 2005), corresponding to F-type supergiant stars. During the early decline phase of the nova, strong emission from CO molecules was also detected (Rudy et al 2012).…”

“…During the early decline phase of the nova, strong emission from CO molecules was also detected (Rudy et al 2012). The existence of both C 2 and CN radicals in the photosphere indicates that the nova envelope was C-rich, with C/O>1 (Kawakita et al 2016).…”

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

“…Nagashima et al in 2015 [37] had obtained A V = 2.35 ± 0.11, from their spectroscopic observations, using the same method. Another determination based on a different method, by Kawakita et al in 2016 [33], found A V = 2.65 ± 0.15 by comparing the optical spectrum just after the visual-brightness maximum (which is dominated by absorption) with the synthesized spectrum of a supergiant.…”