Ejecta Wakes from Companion Interaction in Type Ia Supernova Remnants

Type Ia supernovae can be triggered by accretion onto one white dwarf from another in a tight binary. Here a shell of helium builds up on the primary until the shell detonates, subsequently detonating the carbon-oxygen core. This is known as the "double-detonation" model.

Because the donor must be Roche-lobe filling, a significant portion of the ejecta is guaranteed to collide with the donor. This carves a conical wake into the ejecta filled with low-density, shock-heated gas. Recent work has shown that it is possible for this collision to ignite nuclear burning in the donor, inducing its own double detonation. This ultimately results in the destruction of both objects and is known as the "quadruple-detonation" scenario.

Both the hydrodynamic interaction between the ejecta and donor as well as the possible detonation of the donor substantially modify the ejecta structure. Here we aim to study the effects of these modifications on the evolution of supernova remnants resulting from such events.

As initial conditions for our remnant evolution, we use the outcomes of detonation simulations performed by Boos et al. (2024). Here a suite of 8 models were investigated, varying the parameters of the progenitor binary. These include two models (1+0.4 Msun and 1.1+1 Msun) in which the donor is directly ignited upon contact with the ejecta, leading to a "triple detonation" scenario in which the time delay between the detonations of the primary and donor is markedly shorter. Also included are one case in which the donor survives and one in which the donor is nonexistent (this is unphysical and only for the purpose of comparison).

There are several notable features of the resulting ejecta structure:

1. If the donor detonates, it injects a shell of dense material at ~10,000 km/s. Ejecta at larger velocities originated from the primary detonation.
2. The fact that the primary was detonated off-center creates asymmetry in the ejecta regardless of the donor. In the 2-D calculations of Boos et al. (2024) this asymmetry is taken to be coaxial with that created by the interaction between the ejecta and donor, though this is not generally the case.
3. The wake resulting from the collision between the donor and ejecta leaves an imprint on the ejecta structure regardless of whether the donor detonates. This is due to the time delay between the destruction of the primary and donor.
4. The shock-heated gas within the wake is generally radiation-pressure dominated, in contrast to the majority of the ejecta.
5. The ejecta within the wake is accelerated by its interaction with the donor.

We homologously evolve the ejecta to 10 years after the supernova, at which point the ISM density becomes significant, and use the moving-mesh code Sprout to integrate the remnant evolution through 3000 years.

The differing ejecta structures across our suite of models produce significant variations in remnant morphology. The initially high-velocity (but low-density) gas in the wake is quickly slowed by the ISM, temporarily indenting the forward shock at times earlier than ~1000 yrs. The reverse shock closely traces the forward shock at early times, but after several centuries it encounters the dense shell of ejecta from the donor detonation. This tends to slow and sphericize the reverse shock, with the exception of the triple-detonation systems: here the asymmetrical donor shell causes the reverse shock to quickly traverse the wake, leading to a highly-asymmetrical reverse shock. In all models in which a donor was present, large Rayleigh-Taylor plumes form at the boundary of the wake, creating structures which persist for thousands of years.

As all high-velocity ejecta originated from the primary, the forward shock morphology is initially determined solely by the mass of the primary (with the exception of the wake). In the figure above, all models with a primary mass of 1 Msun are shown as solid lines, which are all nearly equal in the top panel. The bottom panel shows a much later time, demonstrating that a bifurcation has occurred: without fail, systems in which the donor detonated exhibit larger forward shocks than those in which it did not. This bifurcation occurs in both the forward and reverse shocks, and occurs when the shock reaches a velocity comparable to that of the dense inner shell (~10,000 km/s).

In short, the primary determines the early evolution of the remnant, whereas the late evolution is dominated by the donor.

Because supernova remnants are optically thin to X-rays, X-ray telescopes such as the recently-launched XRISM mission are ideal for studying remnants. Shocked ejecta emits X-rays due to both line emission and thermal bremsstrahlung, and XRISM has already proven useful in obtaining images and spectra for several remnants. Because X-ray spectroscopy can easily identify alpha elements, we track the distribution of several such elements through the remnant phase. This yields an estimate of the thermal emission, which is shown in the figure above. Asymmetries in the remnant composition have a substantial effect on the X-ray emission; notably, ISM even reaches the center of the remnant in some cases. We overlay the properties of the XRISM instruments, showing that for a galactic remnant at a distance of ~10 kpc multiple tilings of the Resolve instrument would be needed to obtain spectra across a 3000-year-old remnant.

The line emission offers a way to differentiate between models, shedding light on the physics of the original explosions. Below we show the elemental distributions within each model at 3000 yrs, demonstrating that several elements differ qualitatively by detonation mechanism. Helium in particular differs between models; although it is not a nucleosynthetic product of the supernovae, the unique remnant dynamics resulting from each detonation shape the helium into distinguishable morphologies.