Imagine a material so dense that a teaspoon of it weighs as much as 900 Pyramids of Giza. Some objects in the universe really are this dense: neutron stars. Despite their confusing name, they are not stars; instead, they are instead one possible type of remnant from particularly massive stars that spectacularly explode in the final stages of their lifetime. Such explosions are given an appropriately glorious name: supernovae. When the stars explode, the gravitational pull of their inner cores is so intense that, within the atoms of the stellar material, the electrons are forced inwards towards the nuclei and combine with the protons, ultimately forming neutrons. Hence, the remaining core is what is known as a neutron star.
A neutron star binary is a system whereby two neutron stars rotate tightly about each other. But how do such systems come to arise in the first place? Although astronomers have proposed a theoretical model predicting how a binary star system, which are abundant in the universe, might eventually evolve into a binary neutron star system, no direct observations of the process in its entirety has been made so far, given the lengthy timescales and relative rarity of the process. Indeed, it seems that there would need to be a very particular succession of events in order to give rise to this relatively stable system, especially since neutron stars have such strong gravitational fields. Moreover, the later stages of the proposed model have never been observed directly.
A team of astrophysicists at the California Institute of Technology (Caltech), including graduate student Kishalay De, have provided us with the first direct piece of evidence of the formation of such a binary star system in one of its later stages. On October 14, 2014, one supernova, named iPTF 14gqr, was discovered by the intermediate Palomar Transient Factory (iPTF). The iPTF was a former astronomical survey whose goal was to observe a region of the night sky and detect new stars or other notable changes, now succeeded by the Zwicky Transient Facility (ZTF). The iPTF had found other supernovae before, but there was something extraordinary about this one in particular.
The supernova iPTF 14gqr was located in a spiral galaxy around 920 million light-years away from earth, at a distance 92,000 light years from the galaxy’s center. De found that iPTF 14gqr did not evolve according to the pace of a regular supernova; it quickly dimmed and faded within the span of a few days.
“I had initially started working on this object as a project to understand the population of supernovae that fade quickly in brightness with time,” De said. “Most supernovae exhibit relatively slow evolving light curves—they increase in brightness over about 20 days and then fade away in about 50 days. On the other hand, A small fraction of supernovae brighten and fade very rapidly—and this behavior generally suggests that the amount of mass ejected in those explosions is very small, say about less than 20% of a typical supernova.”
So from this principle, De could infer that iPTF 14gqr ejected around one fifth of the sun’s mass, which is very little when compared to a typical supernova. “Understanding this population [of supernovas] is important because they tell us about the unique end-points in the lives of stars that are relatively rare in the universe. iPTF 14gqr was a part of this family that faded quickly with time,” De said.
However, other measurements showed that the supernova was consistent with the mass of a star with a radius around 500 times that of the sun, which meant that its ejected mass should have been orders of magnitudes greater than the solar mass. So there was a mystery: how could such a massive star eject such little mass? Indeed, such objects seem to constitute a unique population of supernovas.
Now the task at hand was to debunk exactly where the missing mass went, and exactly what sort of characteristics this supernova possessed. There were several other clues that De used. For instance, coupled with the low ejected mass, the supernova also possessed a helium-rich envelope. Normally, before exploding as supernovae, stars do not have such an envelope because the helium is usually contained within the inner regions of the star, with mostly hydrogen on the outer regions. Therefore, this was an indication that somehow the star had undergone a “pre-explosion” of some sort. This was consistent with the fact that the observed supernova was so small. These lines of evidence converged to a possible explanation: perhaps there was a close but dense companion star nearby whose intense gravitational pull was strong enough to steal the other star’s mass. This companion star was also most likely the remnant of a supernova. This would also subsequently explain the extremely small ejecta mass of iPTF 14gqr—the majority of its mass has been stolen by the companion star.
This insight, along with comparisons of such inferences with mathematical models, led De and the research team to infer that iPTF 14gqr was an ultra-stripped supernova. This class of supernovae are very rare because they require just the right conditions to form. Furthermore, since the light they produce is very faint, they are very difficult to detect from earth. “The models behind ultra-stripped supernovae have been only recently been suggested and developed in detail. The key challenge was to link all of the data that we had to the right parameters for an ultra-stripped explosion and verify if the observations were consistent,” said De.
Hence, iPTF 14gqr is the first piece of direct evidence of an ultra-stripped supernova involved in the formation of a binary star neutron system. Such supernovas are remarkably important in our understanding of astrophysics. For example, it has long been understood that one mechanism whereby elements with heavy atomic mass—those heavier than iron—involves production under the intense internal pressure of a supernova. Furthermore, when two neutron stars in a binary neutron star system eventually collide, they can also produce precious elements like gold and platinum.
“Apart from providing evidence of the existence of these rare supernova explosions that might produce binary neutron star systems, which had so far remained only as a theoretical suggestion, this discovery opens up the possibility for finding a larger population of these explosions and trying to relate their properties to the properties of known BNS systems in the galaxy. We can then ask questions like how frequently do BNS systems form, where do they typically form and if these supernovae are the only way that these systems can form. In addition, as we find more mergers of these systems via gravitational wave observations, we can relate the properties of these mergers to these supernovae,” De says.
Indeed, this research has opened up many doors of further inquiry. “While a number of the basic properties of the explosion were explained with our simple modeling, there are a number of avenues for further theoretical research,” De said. “These include why the star was so large at the time of the explosion, how this is related to final stages in the lives of these massive stars and what they tell us about the inner nuclear processes in these objects.”