Radio observations made with the Arcminute Microkelvin Imager (AMI) Large Array as part of the 4 PI SKY project have demonstrated that the massive stellar progenitor of the supernova SN 2014C experienced two very different mass-loss episodes before it finally exploded, These results have been presented in the recent paper Anderson et al. (2017, link below).

X-rays from SN 2014C in nearby galaxy NGC 7331. The insert shows images taken with the Chandra X-ray Observatory, showing the position of SN 2014C before and after the supernova explosion.   Image credit: X-ray images: NASA/CXC/CIERA/R.Margutti et al; Optical image: SDSS

The inset images are from NASA’s Chandra X-ray Observatory, showing a small region of the galaxy before the supernova explosion (left) and after it (right). Red, green and blue colors are used for low, medium and high-energy X-rays, respectively.

Mass-loss is an important ingredient in the evolution of massive stars (which are at least 8 times as massive as our Sun), and has a significant impact on their final stellar death known as supernovae. A star looses its mass through strong stellar winds with speeds between 10s to 1000s km/s. However, other factors such as the interaction with a binary companion star, or the rapid ejection of a large amount of stellar material, are likely the biggest contributors to a massive star shedding its mass.

The expanding shock-wave produced by a supernova, likely travelling at ~10% of the speed of light, impacts the surrounding gas that was lost from the massive stellar progenitor during its lifetime. This interaction produces radio radiation, and the denser the surrounding environment, the brighter the radio emission will be. Radio observations of supernovae can therefore directly track the mass-loss history of its progenitor, illuminating past eras of strong stellar winds or eruptive events just prior to explosion.

Figure 1: The radio emission from SN 2014C monitored for nearly 600 days following the explosion. 

A steady brightening and fading in the radio emission over time demonstrates that most supernovae are surrounding by environments with densities that drop off steadily with distance, thus illustrating that the progenitor had an uneventful past. However, this was not the case for the supernova SN 2014C, discovered on 5 January 2014 in the nearby galaxy NGC 7331, which lies nearly 50 million light years away. Shortly following its discovery, AMI detected the radio emission from SN 2014C. AMI monitored its radio emission, watching it brighten to a peak at 80 days post-burst, before it began to fade. However, around 200 days post-explosion the radio emission unexpectedly began to re-brighten, peaking a second time at 400 days with a luminosity 4 times brighter than the first peak. This double bump morphology is shown in Figure 1. Such behaviour is extremely unusual and has only been seen from a small number of supernovae.

The radio re-brightening that AMI detected 200 days post-explosion was produced by the supernova shock-wave encountering a dense shell of Hydrogen gas (see Figure 2), which was thrown off by the massive stellar progenitor at an earlier point during its evolution. This Hydrogen shell was likely lost during an extreme eruptive event or through interaction with a binary stellar companion. The progenitor of SN 2014C therefore experienced at least two very different episodes of mass-loss during its lifetime, which was illuminated through radio observations.

Figure 2: A schematic of the environment surrounding the supernova likely produced by the massive stellar progenitor before it exploded. The darker areas indicate regions of higher gas density surrounding the supernova site.

4 PI SKY team members Gemma Anderson, Kunal Mooley, Rob Fender, and Tim Staley are all co-authors on the paper.

Link to paper: https://arxiv.org/abs/1612.06059