Violent mergers can explain the inflated state of some of the fastest stars in the Galaxy

TL;DR

This work demonstrates that a violent merger of two carbon-oxygen white dwarfs can yield a bound, rapidly ejected donor remnant whose long-term evolution explains cooler hypervelocity stars like D6-1 and D6-3. By combining a new Arepo 3D hydrodynamical simulation (WD masses $M_1=1.10\,M_{igodot}$ and $M_2=0.70\,M_{igodot}$) with 1D MESA evolution, it shows a $0.16\,M_{igodot}$ remnant moving at $\sim 2800\,\mathrm{km\,s^{-1}}$ and evolving over $\sim 10^4$–$10^6$ yr to produce inflated radii consistent with observations. The study maps the 3D outcome to four outer-layer contamination scenarios and evolves to $t\sim100\,\mathrm{Myr}$, finding thermal timescales that match the kinematic ages of the observed HVWDs and predicting surface abundance signatures influenced by thermohaline mixing. These results offer a unified picture that cooler HVWDs can originate from violent WD mergers, while remaining consistent with supernova remnant observations and velocities, and highlight the need to probe remnants inside SNRs for hidden runaways.

Abstract

A significant number of hypervelocity stars with velocities between $1500-2500$ km/s have recently been observed. The only plausible explanation so far is that they have been produced through thermonuclear supernovae in white dwarf binaries. Since these stars are thought to be surviving donors of Type Ia supernovae, a surprising finding was that these stars are inflated, with radii an order of magnitude more than expected for Roche-lobe filling donors. Recent attempts at explaining them have combined 3-dimensional hydrodynamical supernova explosion simulations with 1-dimensional stellar modelling to explain the impact of supernova shocks on runaway white dwarfs. However, only the hottest and most compact of those runaway stars can so far marginally be reproduced by detailed models of runaways from supernova explosions. In this and a companion paper, we introduce a new \textsc{Arepo} simulation of two massive CO white dwarfs that explode via a violent merger. There, the primary white dwarf ignites when the secondary is on its last orbit and plunging towards the primary. In the corresponding aftermath, the core of the secondary white dwarf of 0.16 M$_\odot$, remains bound, moving at a velocity of $\sim2800$ km/s. We map this object into MESA, and show that this runaway star can explain the observations of two hypervelocity stars that were dubbed D6-1 and D6-3 based on their original discovery motivated by the D6 scenario, though the violent merger scenario presented here is somewhat distinct from the D6 scenario.

Figures (13)

• Figure 1: Planar slices of the runaway star density 1000 seconds after explosion. A slight asymmetry and a non-spherical extension of the ejecta is visible outside of the inner $0.05$ R$_\odot$.
• Figure 2: The total angular momentum and the rotational velocity of the runaway at around $1000 \,s$ after the explosion. The plotted quantities are spherical averages and highlight the differences in the state of the stellar interior. Most of the angular momentum is within $0.1$$R_\odot$. While the external ($>0.1$$R_\odot$) has some residual rotation and radial energy this is only in a small region which forms a stream like structure.
• Figure 3: Main elements as a function of mass coordinate for the runaway 1000 s after the supernova for mass coordinates which remain bound after supernova explosion. Unlike in Bhat1$^{56}$Ni does not dominates the mass fraction at the surface of the runaway. $^{56}$Fe is present due to the primordial solar abundance assumed.
• Figure 4: Different energies in the runaway WD as function of its mass coordinate. Black line marks the region where nickel decay dominates, grey line marks the region where gravity dominated over other energies, and brown line marks the region where gravity is the only relevant energy. Rotational energy is not shown as it is negligible. Radial energy is defined as $v^2_r/2$.
• Figure 5: The structural profile of the $0.154M_\odot$ model as a function of its age. The puffing up and the later contraction of the star is seen here. The black circles (black squares) represent $50\%$ ($90\%$) of the stellar mass. The grey dashed-dotted line is the degeneracy limit above which gas pressure dominates. The two grey lines in the bottom right are the crystallization limit for carbon and oxygen.