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: Department of Physics, National Tsing Hua University, Taiwan
Status: Published in the Astrophysical Journal [open access]
Type Ia supernovae (SNe Ia) are important tools for astronomers in measuring distances in space. Despite their important role as these “standardizable candles ,” their origins remain hotly debated. It’s long been theorized that an SN Ia 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 SN Ia.
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 in Gaia data near the supernova remnant (SNR) G272.2-3.2 have identified an M-dwarf, MV-G272, with an unusually high velocity around 240 km/s and a trajectory tracing back to the SNR center. Is it possible for this M-dwarf to be the runaway companion to the SNR’s original supernova? The authors of today’s paper 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 masses and 0.396, 0.454, and 0.483 solar radii, respectively. To make these models, they used the stellar evolution code MESA to construct and evolve a number of simulated stars with these specifications.
After setting up the models in MESA, they used the magnetohydrodynamical 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) and 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 ~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.
Because the authors explored a range of initial companion masses and distances between the primary and companion star, their results show some range. Figure 2 shows two important results: the evolution of the companion’s mass before and after the explosion (left plot), and the evolution of the companion’s kick velocity (right plot). You can read the leftmost plot left to right. The dip in mass around 100 seconds (102 s) indicates the impact of the bow shock. Even though the simulations model companions with a relatively narrow mass range (0.3 to 0.7 solar masses), there is considerable variation in the final masses after explosion. For example, the initial 0.5 solar mass model ends up with anywhere between 0.07 solar masses (just 14% of its initial mass) or almost its full initial mass of 0.5 solar masses, depending on the distance between the companion and its exploding star.
The results from this work generally show, predictably, that a companion closer to its primary star gets a bigger kick in velocity. These “kicks” range from ~50 km/s to ~150 km/s. When added to the system’s initial velocity in space, it brings the total velocity of these companions into the 150-200 km/s range. This result is generally well aligned with the observed velocity of MV-G272, which was ~240 km/s.
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 and velocity that tracks back to the center of an SNR makes it a good companion candidate. These results support the feasibility of M-dwarf runaway companions from single-degenerate type Ia supernovae 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.
Astrobite edited by Catherine Slaughter
Featured image credit: Mckenzie Ferrari (made in Canva)
