Synthesis of C-rich dust in CO nova outbursts

Abstract

Context. Classical novae are thermonuclear explosions that take place in the envelopes of accreting white dwarfs in stellar binary systems. The material transferred onto the white dwarf piles up under degenerate conditions, driving a thermonuclear runaway. In those outbursts, about 10 −7 − 10 −3 M ⊙ , enriched in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, or Al, for ONe novae) are ejected into the interstellar medium. The large concentrations of metals spectroscopically inferred in the nova ejecta reveal that the (solar-like) material transferred from the secondary mixes with the outermost layers of the underlying white dwarf. Aims. Most theoretical models of nova outbursts reported to date yield, on average, outflows characterized by O > C, from which only oxidized condensates (e.g, O-rich grains) would be expected, in principle. Methods. To specifically address whether CO novae can actually produce C-rich dust, six different hydrodynamic nova models have been evolved, from accretion to the expansion and ejection stages, with different choices for the composition of the substrate with which the solar-like accreted material mixes. Updated chemical profiles inside the H-exhausted core have been used, based on stellar evolution calculations for a progenitor of 8 M ⊙ through H and He-burning phases. Results. We show that these profiles lead to C-rich ejecta after the nova outburst. This extends the possible contribution of novae to the inventory of presolar grains identified in meteorites, particularly in a number of carbonaceous phases (i.e., nanodiamonds, silicon carbides and graphites).

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).…”

“…Only some parts of the shell may have C/O>1. Furthermore, it is worth noting that nova models have traditionally assumed that the composition of C/O is ~1, as noted by José et al (2016). Recent calculations for the interior structure of CO-rich WDs predict C/O>1 for the outer layer of the WD (Fields et al 2016;José et al 2016).…”

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

“…These discrepancies may be due to incorrect assumptions employed in the simulations of nova explosions. As pointed out by Starrfield in 2008 [57], the outer regions of the cores of CO WDs have C/O ratios that differ strongly from unity, in contrast to the usual assumptions employed in traditional simulations [5,22,10,30]. Recent stellar-evolution models predict C/O > 1 for the outer layers of CO WDs [30].…”

Mixing fraction in classical novae

“…It has been suggested that the temperature at the envelope base (T base ) is about 10 8 K when the convection has already extended throughout the whole envelope (e.g. José et al 2016). Meanwhile, in the multidimensional simulations, Casanova et al (2011) mapped the 1D nova model on the 2D or 3D Cartesian grid when T base reached 10 8 K by considering the multidimensional mixing (see also Casanova et al 2018).…”