Hydrodynamical Evolution of Merging Carbon–oxygen White Dwarfs: Their Pre-Supernova Structure and Observational Counterparts

Abstract

We perform smoothed particle hydrodynamics (SPH) simulations for merging binary carbon-oxygen (CO) white dwarfs (WDs) with masses of 1.1 and 1.0 M ⊙ , until the merger remnant reaches a dynamically steady state. Using these results, we assess whether the binary could induce a thermonuclear explosion, and whether the explosion could be observed as a type Ia supernova (SN Ia). We investigate three explosion mechanisms: a helium-ignition following the dynamical merger ('helium-ignited violent merger model'), a carbon-ignition ('carbon-ignited violent merger model'), and an explosion following the formation of the Chandrasekhar mass WD ('Chandrasekhar mass model'). An explosion of the helium-ignited violent merger model is possible, while we predict that the resulting SN ejecta are highly asymmetric since its companion star is fully intact at the time of the explosion. The carbonignited violent merger model can also lead to an explosion. However, the envelope of the exploding WD spreads out to ∼ 0.1R ⊙ ; it is much larger than that inferred for SN 2011fe (< 0.1R ⊙ ) while much smaller than that for SN 2014J (∼ 1R ⊙ ). For the particular combination of the WD masses studied in this work, the Chandrasekhar mass model is not successful to lead to an SN Ia explosion. Besides these assessments, we investigate the evolution of unbound materials ejected through the merging process ('merger ejecta'), assuming a case where the SN Ia explosion is not triggered by the heliumor carbon-ignition during the merger. The merger ejecta interact with the surrounding interstellar medium, and form a shell. The shell has a bolometric luminosity of more than 2 × 10 35 erg s −1 lasting for ∼ 2 × 10 4 yr. If this is the case, Milky Way should harbor about 10 such shells at any given time. The detection of the shell(s) therefore can rule out the helium-ignited and carbon-ignited violent merger models as major paths to SN Ia explosions.

A SYSTEMATIC STUDY OF CARBON–OXYGEN WHITE DWARF MERGERS: MASS COMBINATIONS FOR TYPE Ia SUPERNOVAE

“…By contrast, we find that the highest temperature in the remnant phase is barely affected by the initial condition (see Figure 7). A similar discussion appears in Tanikawa et al (2015).…”

“…The details of the numerical method have already been described in Nakasato et al (2012) and Tanikawa et al (2015).…”

Constraining the progenitor of the Type Ia Supernova SN 2012cg

“…5, to account for the earlytime excess flux in SN 2012cg, a progenitor radius of ≈ 2.0 R ⊙ is required. This is much larger than the typical progenitor radius which is an isolated WD, in both the SD and DD scenarios, although some recent hydrodynamical studies suggested that the envelope of the exploding WD could spread out to a radius of ≈ 0.1 R ⊙ in the violent merger model (Tanikawa et al 2015). Therefore, one can conclude that the diffusion of radiation from the shock-heated ejecta in the shock breakout model of RLW12 is unlikely to be the contributor of the early excess luminosity seen in SN 2012cg.…”

Does Explosive Nuclear Burning Occur in Tidal Disruption Events of White Dwarfs by Intermediate-mass Black Holes?

“…On the curve, the 12 C + 12 C reaction timescale is equal to the local dynamical timescale. Both the timescale are defined in the same way as Tanikawa et al (2015). If a particle is on the upper side from the curve, the particle experiences the explosive nuclear burning.…”