9 mins read 12 Aug 2021

Australian Scientists help Catch The First Moments of A Supernova

Astronomers from the Australian National University, have led an international team of researchers to observe, for the first time, the early light curve from a supernova event and modelled the type of progenitor star that caused it.

The initial shock breakout of light just as the supernova occurs on a star. Credit: NASA.

Astronomers from the Australian National University (ANU) have led an international team of researchers to make the first observations of the light emitted just as a supernova explosion detonates in space. The data has also provided an opportunity to test different models in which the progenitor star can be inferred through measuring the supernova’s light curve.

Until now, most of the light received from supernovae events was the result of astronomers making detections after the initial blast and produced by the decay of radioactive elements in the expanding debris shell from the blast, which usually occurs sometime after the original event.

But this new research, published in the journal Monthly Notices of the Royal Astronomical Society, outlines the detection of a light curve peak (known as a ‘shock cooling light-curve’)  which presents the burst of emissions immediately after the explosion occurs – a rare observation, as these events fade quickly. The new discovery has been named SN2017jgh.

"This is the first time anyone has had such a detailed look at a complete shock cooling curve in any supernova," said PhD scholar and lead author, Mr Patrick Armstrong.

"Because the initial stage of a supernova happens so quickly, it is very hard for most telescopes to record this phenomenon.”

"Until now, the data we had was incomplete and only included the dimming of the shock cooling curve and the subsequent explosion, but never the bright burst of light at the very start of the supernova.

"This major discovery will give us the data we need to identify other stars that became supernovae, even after they have exploded," he said.

Supernovae events can be triggered by a number of different factors, including merging compact stars like white dwarfs or the collapse of more massive stars. So violent and powerful are these events, that they forge a large number of elements that we see around us including many of the heavier elements in the periodic table.

By studying the lights from these events, effectively astronomers are given a tool to delve into the formation of elements across the Universe. Additionally, by studying the shock cooling light-curves that is produced during supernovae, astronomers can now also start to answer some of the ongoing questions around the dynamics of collapsing stellar objects once they reach the end of their main sequence lives.

The Star that Was…

Artist illustration of a yellow supergiant star. Credit: M. Jadraef.

Based on the observations made of the shock cooling light-curves, and modelling completed, the research team were able to determine that the progenitor star to SN2017jgh was a yellow supergiant star. These types of stars are usually evolved F or G class stars that are no longer burning hydrogen in their cores, so have since expanded to enormous sizes – which in turn increases their luminosity.

Yellow supergiants usually have a temperature range of around 4,000 – 7,000 Kelvin, with their luminosities shining from about 1,000 times that of our Sun, but in the most extreme cases, this can also get up to 100,000 times the solar luminosity.

SN2017jgh progenitor star was also identified in imagery, prior to the explosion, and determined to have had an effective temperature somewhere between 4000 – 5000 Kelvin and contain an original mass of 17 times that of our Sun.

However, yellow supergiants are less common than the regular red supergiants that are dominant in the night sky, like Antares and Betelgeuse, and are usually smaller in size. The northern pole star, Polaris, is catalogued as a yellow supergiant.

SN2017jgh also features a surrounding envelope of hydrogen gas whose mass is expected to be ranged between 0.5 - 1.7 times that of the Sun. These envelopes form as the star ages and throw off enormous volumes of hydrogen into their local surrounding regions through expected mechanisms like strong stellar winds, stellar rotation, binary interactions and nuclear instabilities.

The size of the hydrogen envelope from SN2017jgh is reported in the paper to have a radius of approximately 130 solar radii, which equates to about 180 million kilometres – so if you placed it where our Sun was, its surface would lie beyond the Earth’s orbit. 

The supernovae event was located 0.157 arcseconds away from its host galaxy centre, which resides at a distance of just over one billion light-years from Earth.

Supernovae in a Range of Flavours

Classifying supernovae events into categories, based on how they exhibit hydrogen, silicon and helium features. Credit: H. Stevance.

Supernovae can be triggered by a number of different progenitor objects and events. The taxonomy of these events is divided into two main classes – those that feature hydrogen in the light curve’s spectrum, and those that don’t. From these two classes, further sub-classes are also established.

The first type, Type I supernovae, is considered thermal runaway events, and usually associated with massive compact objects like white dwarfs. These supernovae are usually triggered when accretion of material builds up on one star from a companion; an accretion of materials create enough pressure to trigger a core ignition, or when two compact objects merge (though in the case of Neutron Star mergers, these are known as a Kilonova).

But the other category, called Type II supernovae, is a much more powerful and destructive event. These are triggered when a massive star (usually 8 – 25 solar masses) can no longer produce core energy to sustain the outwards pushing radiation pressure and thus succumbs to the inward gravitational force.

This causes the star to collapse inwards, crushing the core before rebounding in the supernova explosion – this is also how exotic objects like neutron stars, pulsars and black holes are born.

Type II Supernovae can be further sub-categorised depending again on a number of different factors presented in the light curve observed (esp. if it does or doesn’t feature silicon, helium, narrow lines, or an evolving spectrum).

One indicator that the progenitor star had a large hydrogen envelope surrounding it is the fading of hydrogen lines weeks after the initial explosion, giving way to the rise of dominant helium lines, suggesting that a lot of the hydrogen layer of the star had become stripped during the envelope shedding.

Reading the Light curve of an Exploding Star

When it comes to the Type II supernovae (core-collapse models), there are generally two prominent peaks that appear in the light curve signature. The first is created when photons that have been trapped inside the star rush outwards in the early onset of the violent explosion, and only last a few days. These emissions can provide astronomers with lots of information about the progenitor star, and the shockwave generated from the explosion.

The second, are caused by the nuclear-powered emissions from the radioactive decay of 56Ni into cobalt, then iron over the course of some time, usually coming in a few days and weeks after the event. This is the source of the second peak in the light curve, but also eventually reduces in luminosity over the period. It is during these times in which new elements are forged through nucleosynthesis associated with the stellar explosion.

Historically, a number of shock cooling light-curves have been observed as reported for other supernovae events, but these new findings have allowed astronomers to capture the complete evolution of the initial peak associated with a supernova for the first time.

Whilst many supernovae occur and can be studied, catching them in their early onset is the only time where these shock cooling light-curves can be observed, and so having telescopes pointed at the right place and the right time is a rare chance.

Infographic on what the different light curves of supernovae look like - note the difference in the shape of the curves, as well as the elementary composition. The two main categories of supernovae events can be classed under thermonuclear and core-collapse models. Credit: H. Stevance.

Analysis and modelling of the light from 2017jgh

The Kepler Space Telescope, which assisted in obtaining data from this discovery. Credit: NASA.

A number of different observations were made to come to establish the results outlined in this latest paper, relating to SN2017jgh. Originally, the supernova was discovered by Pan-STARRS1 – a 1.8-metre telescope located in Maui, Hawaii. Using the 1.4 Gigapixel camera, it identified the supernova which presented at roughly magnitude 20.

Photometry (measuring light in different bands in similar wavelengths that the humane eye observes in) was also produced using Pan-STARRS1 filtering system (grizy), and the Swope Supernova Survey (SSS) – which uses a 1-metre aperture telescope, located in Las Campanas in Chile also complemented with its own observations. The SSS telescope’s filtering system (gri) observed the supernova between December 2017 and February 2018.

As well as ground-based observations, the Kepler/K2 spacecraft took observations of the event from orbit, avoiding any disturbances produced by our atmosphere. It observed the event at 30 minutes cadence over an 80-day campaign, which really highlighted the rise in the light curve’s first peak in lots of detail.

For the optical spectroscopy component, the Gemini Multi-Object Spectrograph on the Gemini South Telescope (also located in Chile) was used and took in observations of the spectrum in early January 2018, two days prior to the radioactive maximum peak which occurs roughly 14 days after discovery.

Overall, the light curves that were analysed across all observations point to a similar supernova to another observed in the early 1990s, known as SN1993J, which also featured a yellow supergiant star.

A number of shock cooling light-curve models were then used to test the results, with the SW 17 model fitting the most accurate to the data observed.

"We've proven one model works better than the rest at identifying different supernovae stars and there is no longer a need to test multiple other models, which has traditionally been the case," said Astrophysicist and ANU researcher Dr Brad Tucker, also a co-author of the paper.

"Astronomers across the world will be able to use SW 17 and be confident it is the best model to identify stars that turn into supernovas."

As well as providing global researchers with a well-fitted model for these early peaks in supernovae events, these new findings now showcase a little bit more of the detail around those first, early moments during one of the most violent and destructive incidents in our Universe, which in turn gives birth to new materials.

"This will provide us with further opportunities to improve our models and build our understanding of supernovae and where the elements that make up the world around us come from," said Mr Armstrong.

The pre-print is now available in the journal, MNRAS.