Every supernova observed so far has been considered the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining.
Now, astrophysicists at UC Santa Barbara and astronomers at Las Cumbres Observatory (LCO) have reported a
remarkable exception: a star that exploded multiple times over a period of more than 50 years. Their observations, published in the journal Nature, are challenging existing theories on these cosmic catastrophes.
“This supernova breaks everything we thought we knew about how they work,” Iair Arcavi, lead author and a NASA Einstein postdoctoral fellow in UC Santa Barbara’s Department of Physics and at LCO, said. “It’s the biggest puzzle I’ve encountered in almost a decade of studying stellar explosions.”
Discovered in Sept. 2014, iPTF14hls, although identical to type II-P supernovae in its spectroscopic features, has several properties never before seen in a supernova. Instead of a 100-day plateau, the light curve of iPTF14hls lasted over 600 days and has at least five distinct peaks during which the luminosity varies by as much as 50 percent.
Scientists examined archival data and were astonished to find evidence of an explosion in 1954 at the same location. Somehow this star survived that explosion and then exploded again in 2014. In the study, the authors calculated that the exploding star was at least 50 times more massive than the sun and probably much larger.
“Supernova iPTF14hls may be the most massive stellar explosion ever seen,” co-author Lars Bildsten, director of UCSB’s Kavli Institute for Theoretical Physics, said. “For me, the most remarkable aspect of this supernova was its long duration, something we have never seen before. It certainly puzzled all of us as it just continued shining.”
The progenitor of iPTF14hls probably experienced multiple energetic eruptions over the last decades of its life. Energetic eruptions are expected in stars with initial masses of about 95–130 solar masses. Interactions between the different shells and the supernova ejecta and the shells can produce a variety of luminous long-lived transients with highly structured light curves similar to that of iPTF14hls.
Such pulsational pair-instability supernovae are expected to occur in low-metallicity environments. iPTF14hls
occurred on the outskirts of a low-mass, star-forming galaxy, possibly of low metal content. However, models of stars undergoing the pulsational pair-instability eject most of the hydrogen envelope in the first eruption, whereas for iPTF14hls a few tens of solar masses of hydrogen were retained in the envelope after the 1954 outburst.
iPTF14hls demonstrates that stars in the local universe can undergo very massive eruptions in the decades leading to their collapse and yet, surprisingly, maintain a massive hydrogen-rich envelope for most of this period. Current models of massive star evolution and explosion need to be modified, or a completely new picture needs to be put forward, to account for the energetics of iPTF14hls, its lack of strong interaction signatures and the inferred amount of hydrogen it retains toward the end of its life.
LCO’s supernova group continues to monitor iPTF14hls, which remains bright three years after it was discovered. Their global telescope network is uniquely designed for this type of sustained observation, which has allowed researchers to observe iPTF14hls every few days for several years. Such long-term consistent monitoring is essential
for the study of this very unusual event.