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Title: Supernova Ia Remnants with M-Dwarf Surviving Companions
Authors: Kuo-Chuan Pan (潘國全), Pilar Ruiz-Lapuente, and Jonay I. González Hernández
First Author’s Institution: National Tsing Hua University
Status: Published in ApJ
Type Ia supernovae are important tools astronomers use to measure distances in space. Despite their important role as “standardizable candles,” their origins remain hotly debated. It’s long been theorized that a Type Ia supernova can develop from one of two pathways: the single-degenerate channel or the double-degenerate channel. In the single-degenerate channel, a white dwarf and a non-degenerate star (such as a red giant or main-sequence star like our Sun) orbit each other in a close binary system. The white dwarf accretes material from the outer surface of the non-degenerate star until it reaches a critical mass and explodes. The double-degenerate channel begins instead with two white dwarfs orbiting each other. Most explosion mechanisms from this channel result in the complete obliteration of the secondary, less-massive white dwarf. However, one theoretical explosion mechanism, called the dynamically driven double-degenerate double-detonation (D6) channel, might allow the secondary companion star to survive an explosion from a double-degenerate Type Ia supernova.
Generally, due to the immense energy from a supernova explosion, we expect these surviving companion stars to be “kicked” away from the blasts at high velocities. Fortunately, astronomers have found evidence of these surviving companion stars. As covered in a previous Astrobite, three hypervelocity white dwarfs have been found in Gaia data and traced back to the locations of double-degenerate Type Ia supernovae. Given the prevalence of Type Ia supernovae, however, astronomers are led to question why we haven’t seen many more of these candidates. It’s additionally important to wonder what similar “runaway” companions might look like for the single-degenerate channel.
Recent observations by the Gaia spacecraft near the supernova remnant G272.2-3.2 show an M-dwarf star, MV-G272, with an unusually high velocity around 240 km/s and a trajectory tracing back to the center of the supernova remnant. Is it possible for this M dwarf to be the runaway companion to the remnant’s original supernova? The authors of today’s article look at simulations of potential companion stars from single-degenerate Type Ia supernovae and compare their results to this observed potential companion candidate.
Because this work focuses on the potential for M-dwarf companions, the authors explored a range of simulated companion-star masses and radii characteristic of M dwarfs: 0.5, 0.6, and 0.7 solar mass and 0.396, 0.454, and 0.483 solar radius, respectively. To make these models, they used the stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA) to construct and evolve a number of simulated stars with these specifications.
After setting up the models in MESA, they used the magnetohydrodynamics code FLASH to perform 2D and 3D simulations of the supernova explosion and the subsequent impact of the exploding star’s ejecta (or hot material) on its M-dwarf companion. After the explosion and impact were modeled, the companion star’s evolution was further followed in MESA.
Figure 1 shows the density of the supernova’s ejecta, or exploding material, as it impacts the companion star. (You can think of this figure as if you were looking at the companion star out in space.) After roughly 100 seconds, you can see a bow shock develop around the top of the companion (or the “front” of the star) as it wraps around the companion. This impact causes significant compression of the companion star and rips away upwards of 50% the companion’s mass. The impact of this shock is what gives the companion star its “kick,” sending it hurtling through space.
Figure 1: Visual of the density of the companion star (center of each plot) and the supernova ejecta over time. In the top row of plots and the bottom-left plot, you can see the higher-density bow shock bent around the top (or front) of the companion star. In the bottom-right plot, you can see some of the supernova ejecta and unbound mass from the companion star settling around the companion star. [Pan et al. 2025]
Figure 2: The different models shown in both plots indicate different initial masses and separation distances between the companion and the exploding star. Left: how the companion’s gravitationally bound mass evolves over time. The plot can be read left to right. The dip around 102 seconds indicates the impact of the supernova shock, which unbinds layers of the companion’s mass. Some of that mass becomes bound again to the companion after the shock passes. Right: how the companion star’s kick velocity changes based on initial mass and separation distance between companion and exploding star. The peak in values between 102 and 103 seconds indicates the impact, or “kick,” from the supernova. [Pan et al. 2025]
Notably, runaway companion stars of these Type Ia supernovae seem to retain information from the initial explosion. Given the final velocities of these companions, astronomers might be able to figure out the companion’s initial mass and trace back its trajectory to the system it originated from. Overall, the fact that MV-G272 has a trajectory that tracks back to the center of a supernova remnant makes it a good companion candidate. These results support the feasibility of M-dwarf runaway companions from single-degenerate Type Ia supernova systems. Now that we know what these companions might look like — and how fast they can be going — astronomers can hopefully better search for these high-velocity stars in the future.
Original astrobite edited by Catherine Slaughter.
About the author, Mckenzie Ferrari:
I’m currently a PhD student in the geophysical sciences program at the University of Chicago. While I now study the atmosphere and oceans of Earth, most of my previous research focused on simulations of Type Ia supernovae and galaxy formation and evolution. In my free time, I foster cats for a local organization, enjoy cooking, and can often be found running along Lake Michigan.