The Forerunner Star Pair of Gold Producing Kilonova

Astronomers found a pair of stars that became the forerunner of the event kilonova which produces gold and silver.

Kilonova. A huge explosion that produces not only a burst of gamma rays but also generates gravitational waves. The explosion occurred when a pair bintang neutron or pairs of neutron stars and black holes collide and eventually merge. Not only that. Within the neutron-rich remnants of the explosion, there are vast amounts of the universe’s greatest elements forged. Among them are gold and platinum.

We know from the life history of stars that only very large-mass stars end in supernova explosions that eventually produce neutron stars and black holes. However, the life span of a massive star, which is short but long on the human time scale, makes it impossible for us to observe and study the original star that eventually exploded. For the record, massive stars have a shorter life span than stars like the Sun. It only took 10 million years.

This time, astronomers found a pair of stars that seem to be the forerunners of kilonovas. A rare find, as there are only about 10 such systems in the Milky Way.

Ultra-stripped Supernova

CPD-29 2176 system. This system was first discovered by NASA’s Neil Gehrels Swift Observatory and then observed by the 1.5-meter SMARTS telescope at Cerro Tololo Inter-American Observatory, Chile. Further observations were made to find out the characteristics of the orbits of the star pairs in the system.

The CPD-29 2176 system is 11,400 light years from Earth and consists of a neutron star and a massive star. Interestingly, the neutron star in CPD-29 2176 was formed from a supernova explosion in which the star was largely stripped of its shell. Astronomers give the term ultra-stripped supernova.

Meanwhile, astronomers suspect that a massive partner star is on its way supernova explosion ultra-stripped.

If a massive star ends up in a supernova explosion, of course, some of the outer shells are ejected at high speed and cause the companion star to be ejected from the system as well.

Interestingly, the CPD-29 2176 system still consists of a pair of stars in fairly close orbits. That means, before exploding as a neutron star, the interaction of this pair of massive stars produces a large amount of stellar matter so that when the star explodes, the explosive power is no longer as large as supernova in general. Thus, the massive star pair still exists in the system.

After a massive star explodes, what remains is the center of the star evolving into a neutron star. After that, the interaction of the pair of neutron stars and their partner massive stars results in mass transfer (matter stripping). As a result, the massive star will lose some of its envelope and eventually explode as an ultra-stripped supernova leaving a center of mass that ends up as a neutron star.

After becoming a neutron star, their interaction causes the pair of neutron stars to move closer to each other and eventually collide in a kilonova explosion. Eventually the two neutron stars merge.

Illustration of the stages of star pair evolution in the CPD-29-2176 system. Credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld

Rare Discovery

It is not easy to find the forerunner of the kilonova. You could even say this is a rare find. What is certain is that the discovery of the forerunner of this kilonova is very important for unraveling the mystery of the formation of kilonovas, as well as the origin of the heaviest elements in the Universe.

At least, through this discovery astronomers can figure out the scenario of the formation of a pair of neutron stars through the explosion of an ultra-stripped supernova. In this way, the two neutron stars remain safely in the system until they eventually merge.

Of course, this process is very long and not easy.

As we know today, the Milky Way is made up of hundreds of billions of stars. But the forerunner of the kilonova system is only one in ten billion systems. The results of previous research show that spiral galaxies like the Milky Way only have one or two systems that can become the forerunners of kilonovas.

In addition, it would take at least a million years for the massive star companion to go supernova and leave behind a neutron star. After that, the two stars still need time to interact and approach each other in a cosmic dance and slowly lose their orbital energy as gravitational radiation.

When the two finally combine to produce a kilonova, the resulting explosion will create a strong gravitational wave and leave large amounts of heavy elements, including gold and silver.

But future astronomers will be able to observe these events.

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