Merging of Components in Close Binaries: Type Ia Supernovae, Massive White Dwarfs, and Ap stars

The “Scenario Machine” (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes.We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance should be taken into account in estimations of parameters of acceleration of the Universe. According to theoretical estimates, the most massive - as a rule, magnetic - white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main sequence stars (<1.5 mass of the Sun) with convective envelopes.

💡 Research Summary

The paper presents a comprehensive population‑synthesis study of merging compact and non‑compact objects in close binaries using the “Scenario Machine” code. By evolving ten million binary systems with realistic initial mass functions, binary fractions, orbital‑separation distributions, and parametrized common‑envelope efficiency (α_CE) and supernova kick velocity distributions, the authors calculate event rates and mass spectra for five classes of mergers: (1) main‑sequence (MS)–MS, (2) white‑dwarf (WD)–WD, (3) WD–neutron‑star (NS) or WD–black‑hole (BH), (4) NS–NS, NS–BH, and (5) BH–BH.

A key focus is the implication of WD–WD mergers for Type Ia supernovae (SNe Ia). The simulations show that before a galaxy reaches an age of roughly 1 Gyr, the number of close double‑WD systems capable of merging is negligible, so the SNe Ia rate is extremely low. After ≈1 Gyr the rate rises sharply as more massive progenitors have had time to evolve into WDs and orbital decay brings them together. Importantly, the average explosion energy of SNe Ia declines by about 10 % as the stellar population ages, while the spread between the most energetic and the least energetic events remains at least a factor of 1.5. This systematic evolution of luminosity must be taken into account when SNe Ia are used as standard candles for cosmology; otherwise distance estimates and the inferred acceleration of the Universe could be biased.

The mass distribution of merging WDs peaks between 0.6 M⊙ and 1.2 M⊙, with the most massive merged remnants (≈1.3 M⊙) arising from the coalescence of two ≈0.6 M⊙ WDs. The authors argue that such massive, highly magnetic WDs (B ≳ 10⁶ G) are natural products of these mergers: angular momentum conservation during coalescence spins up the remnant, amplifying any pre‑existing seed field into a strong dipole. This scenario aligns with observational surveys that find the most massive WDs to be magnetic, supporting a merger origin.

For MS–MS mergers, the study finds that when at least one component has a convective envelope and a total mass below about 1.5 M⊙, the merger product can acquire rapid rotation and deep mixing. The resulting chemical peculiarities and strong surface magnetic fields resemble those observed in Ap and Bp stars. The authors estimate that such low‑mass MS mergers constitute roughly 0.3 % of all binaries, suggesting that a non‑negligible fraction of magnetic chemically peculiar stars may have a merger‑driven origin, complementing the traditional “fossil field” hypothesis.

Other merger channels are also quantified. WD–NS and WD–BH coalescences can produce X‑ray binaries, sub‑luminous supernovae, or, in extreme cases, short gamma‑ray bursts. NS–NS and NS–BH mergers occur predominantly in older stellar populations (≥3 Gyr) and are identified as primary sites of r‑process nucleosynthesis and gravitational‑wave sources, consistent with events like GW170817. BH–BH mergers appear at a rate of ≈10⁻⁶ yr⁻¹ per galaxy, matching current LIGO/Virgo detection statistics.

The paper emphasizes the sensitivity of all these results to two poorly constrained parameters: the common‑envelope efficiency (α_CE) and the distribution of supernova kick velocities. Variations in α_CE by a factor of two can change merger rates by an order of magnitude, while different kick prescriptions shift the spatial distribution of post‑supernova binaries. Consequently, the authors call for high‑resolution three‑dimensional hydrodynamic simulations of common‑envelope evolution and for larger observational samples (e.g., Gaia‑identified double WDs, upcoming LSST supernova surveys, and next‑generation gravitational‑wave detectors) to refine these parameters.

In summary, the study provides a unified framework linking binary merger demographics to three distinct astrophysical phenomena: the time‑dependent luminosity of SNe Ia, the origin of massive magnetic white dwarfs, and the formation of magnetic Ap/Bp stars. By quantifying how merger rates evolve with galactic age and by highlighting the systematic energy drift of SNe Ia, the work offers crucial corrections for cosmological distance ladders and deepens our understanding of how binary interactions sculpt the observed stellar population.