Star formation history, double degenerates and type Ia supernovae in the thin disc

Shenghua Yu and C. Simon Jeffery


Fig. 16. The rates of type Ia supernovae from double degenerate mergers channel in different star formation models (left panel), and the rate contribution function to the present-day (tdisc = 10Gyr) supernovae rates in the thin disc (right panel).

Abstract We investigate the relation between the star formation history and the evolution of the double-degenerate (DD) population in the thin disc of the Galaxy, which we assume to have formed 10 Gyr before the present. We introduce the use of star-formation contribution functions as a device for evaluating the birth rates, total number and merger rates of DDs. These contribution functions help to demonstrate the relation between star-formation history and the current DD population and, in particular, show how the numbers of different types of DD are sensitive to different epochs of star formation.

Analysis of the contribution functions given by a quasi-exponentially decline in the star-formation rate shows that star formation from 0 to 8 Gyr after thin-disc formation dominates the present rates and total numbers of He+He DDs and CO+He DDs. Similarly, the current numbers of CO+CO and ONeMg+X DDs come mainly from early star formation (< 6 Gyr) , although star formation from 4 to 8 Gyr continued to contribute more CO+CO than He+He DDs. The present birth-rates for CO+CO and ONeMg+X DDs are strongly governed by recent star formation (i.e. 8 − 9.95 Gyr). Star formation from < 7.5 Gyr does not contribute to the present birth rates of CO+CO and ONeMg+X DDs, but it has a distinct contribution to their merger rates.

We have compared the impact of different star-formation models on the rates and numbers of DDs and on the rates of type Ia (SNIa) and core-collapse supernovae (ccSN). In addition to a quasi-exponential decline model, we considered an instantaneous (or initial starburst) model, a constant-rate model, and an enhanced-rate model. All were normalised to produce the present observed star density in the local thin disc. The evolution of the rates and numbers of both DDs and SNIa are different in all four models, but are most markedly different in the instantaneous star-formation model, which produces a much higher rate than the other three models in the past, primarily as a consequence of the normalisation.

Predictions of the current SNIa rate range from ≈ 2 to 5 × 10-4yr-1 in the four models, and are slightly below the observed rate because we only consider the DD merger channel. The predicted ccSN rate ranges from 1.5 to 3 century-1, and is consistent with observations.

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Last Revised: 2011 August 17th