Some Constraints on the Lower Mass Limit for Double-degenerate Progenitors of Type Ia Supernovae

Chen, X., Jeffery, C. S., Zhang, X., Han, Z.

Figure 1: Left panel: constraints on the progenitors of SNe Ia of double CO WDs in the M₁–M₂ plane. The black lines are contours of the ejected mass fraction 1 òÈÒ β with values of, from left to right, 0.0, 0.1, … and 0.9, respectively. The red lines mark contours of merger mass M₁ + M₂ = M_ch (conservative case, solid line), M₁ + βM₂ = M_ch (non-conservative, dotted line), and M₁ + β(t)M₂(t)dt = M_ch (non-conservative, dashed line). The vertical lines give the boundaries for M₁ = 0.9 and 1.0 M_☉. The areas to the right of each color line represent M ≥ M_ch and hence DD SN Ia progenitor candidates under the corresponding constraints. Right panel: CO DDs from our population synthesis with M₁ + M₂ > M_ch which can merge within a Hubble time; the blue dots have mass ratios q = M₂ /M₁ ≥ 2/3. The black dash-dot-dot-dotted line shows the boundary of q = 2/3, and the red lines are the same as those in the left-hand panel.

Abstract

Recent theoretical and observational studies both argue that the merging of double carbon-oxygen white dwarfs (WDs) is responsible for at least some Type Ia supernovae (SNe Ia). Previous (standard) studies of the anticipated SN birthrate from this channel have assumed that the merger process is conservative and that the primary criterion for explosion is that the merged mass exceeds the Chandrasekhar mass. Han & Webbink demonstrated that mass transfer and merger in close double WDs will in many cases be non-conservative. Pakmor et al. further suggested that the merger process should be violent in order to initiate an explosion. We have therefore investigated how the SN Ia birthrate from the double-degenerate (DD) channel is affected by these constraints. Using the binary-star population-synthesis method, we have calculated the DD SN Ia birthrate under conservative and non-conservative approximations, and including lower mass and mass-ratio limits indicated by recent smoothed-particle-hydrodynamic calculations. The predicted DD SN Ia rate is significantly reduced by all of these constraints. With dynamical mass loss alone (violent merger) the birthrate is reduced to 56% of the conservative rate. Requiring the mass ratio q > 2/3 further reduces the birthrate to 18% that of the standard assumption. An upper limit of 0.0061 SNuM, or a Galactic rate of 4.6 †× 10^òÈÒ4 yr^òÈÒ1, might be realistic.

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Last Revised: 2012 September 26th