In a paper published in The Astrophysical Journal Letters, the team posits that this rebrightening event in SN 2018ivc provides a missing link between supernovae -- or SN -- that occur in binary star systems and those that involve solitary massive stars.

Figure 1: An illustration of a supernova before and after its explosion. (First image) The orange star in the center of the screen will eventually undergo a supernova explosion. The blue star is its companion. (Second image) The material surrounding the star is scattered due to gravitational interaction with its companion, about 1500 years before the explosion. (Third image) The moment of the supernova explosion. (Fourth image) The debris of the exploded star reaches the material surrounding it with a time delay, and is observed as a radio wave flare about a year later.

Credit: ALMA (ESO/NAOJ/NRAO), K. Maeda et al

Massive stars end their life with catastrophic SN explosions. How they evolve toward this demise is marked as a significant problem in astrophysics.

A key process that affects their evolution and its final outcome is the binary interaction since most massive stars are found in binary systems. The celestial envelope of one of the binary components can be stripped away by the gravitational pull from its companion star. A fraction of the stripped gas are ejected from the binary system, forming a CSM. This happens in a brief period of time as compared to the whole life of a star. Thus directly witnessing such a phenomenon is far too challenging.

Alternatively, one can investigate this effect by diagnosing its properties, such as the amount and distribution of CSM created by the binary interaction, if such measurements can be precisely performed.

Led by Keiichi Maeda, professor from the Graduate School of Science at Kyoto University, and Tomonari Michiyama, ALMA Grant Postdoctoral Fellow from the Graduate School of Science at Osaka University, the research team tackled this problem by studying radio emission from an SN.

Ejecta -- the debris from exploded stars -- expand into the surrounding space at velocities reaching 10% the speed of light. The ejecta crash into CSMs, creating radio synchrotron emission. By studying the properties of synchrotron emissions, including their intensity and time evolution, it is possible to measure CSM distribution. This ability helps derive the history of how much mass was being shed by the dying massive star until the moment of SN explosion.

In the reported work, the team spent a few years using ALMA to monitor SN 2018ivc, which took place in the famous spiral galaxy M77. The millimeter emission from the SN had initially decayed quickly until about 200 days after the explosion, but the team noticed that it was rebrightening in the ALMA data obtained at later epochs (>1000 days). Such rebrightening in the radio synchrotron emission from SNs is rare, previously known only for a handful of events and all at centimeter wavelengths.

In comparison, transparency of this CSM at mm wavelengths has allowed the team to get a much clearer picture at immediate environments carrying the footprints of progenitor stars than previous attempts with cm wavelengths. The reported work presents the first example in which rebrightening is caught in millimeter wavelengths, thanks to ALMA's sensitivity.

Figure 2: Image of the central region of M77, taken by Hubble space telescope (left), marked with position of SN 2018ivc. Right panels show expanded view around SN 2018ivc based on data taken by ALMA at about 200 days (upper right) and about 1000 days (lower right), clearly showing rebrightening about one year after SN explosion.

Credit: ALMA (ESO/NAOJ/NRAO), K. Maeda et al

A large amount of CSM surrounding the exploding star was detected at a distance of about 0.1 light-years. Ejecta eventually reached this dense CSM but had not reached it in 200 days. Observed by ALMA in later epochs, the outcome of the collision was further quantified by conducting numerical modeling of this phenomenon and comparing the observed synchrotron flux and time evolution with model predictions. The team concluded that large amounts of CSM were the outcome of a strong binary interaction that took place about 1500 years before the SN explosion.

The evolution of massive stars is categorized into two major channels: single star evolution and binary evolution. The single star channel is an outcome of a star without a binary companion, but a star in a binary can also evolve effectively as a single star if separation between two stars is too large. If the separation of a binary system is small enough, the binary interaction takes place far before the time of the SN explosion, typically on the order of a million years. The star that has lost a large amount of gas at this interaction will stay quiet up until the moment of SN explosion.

"This is standard outcome of binary evolution. We have been searching for a missing link where binary separations are not too large but not too small. SN 2018ivc matches this picture perfectly, filling missing gaps that confirm the importance of binary evolution in massive stars,” explains Keiichi Maeda.

ALMA has the ToO (Target-of-Opportunity) mode with which a new transient object, such as an SN, can be immediately observed by disrupting already scheduled queues. SN 2018ivc was observed by the ToO mode, and data analysis suggested that this was a highly interesting event. The team thus investigated its late evolution, covering the SN position in later epochs, and found signs of rebrightening.

"We were highly motivated to obtain additional data to further quantify this rare rebrightening event and conducted further follow-up observations through the DDT Director’s Discretionary Time. The present findings here thus relied on the capability of the multiple observing modes ALMA provides,” says Tomonari Michiyama, emphasizing the importance of ALMA's flexible observing scheme.

The Universe is filled with various transient and explosive phenomena, including binary neutron star mergers emitting not only electromagnetic radiation but also gravitational waves, stellar merger events, nova explosions, and surface explosions of massive stars, among a large variety of such explosive objects that forms the “dynamic” Universe.

"The present work is marked as a proof-of-concept that ALMA can provide unique data in investigating these transient events and therefore is a key component of time-domain astronomy,” concludes Keiichi Maeda.

Movie: An astronomer's visualization of the demise of a massive star.

Credit: ALMA (ESO/NAOJ/NRAO), K. Maeda et al

The article, “Resurrection of Type IIL Supernova 2018ivc: Implications for a Binary Evolution Sequence Connecting Hydrogen-rich and Hydrogen-poor Progenitors” will be published in The Astrophysical Journal Letters at DOI: https://doi.org/10.3847/2041-8213/acb25e