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Tuesday, September 28, 2021

Supernova 2018zd observed by international team of researchers approximately three hours after star explosion

UC Davis assistant professor and graduate students help identify a new type of supernova 

By AARYA GUPTA — science@theaggie.org  

UC Davis assistant professor of physics and astronomy Stefano Valenti and graduate students from the Department of Physics, Azalee Bostroem and Yize Dong, were part of a team of international astronomers that recently discovered supernova 2018zd, or SN2018zd. Their research was recently published in Nature Astronomy on June 28. 

“This supernova was discovered just after its explosion, so we collected a complete set of data, which allowed us to do a comprehensive study on this object,” Dong said via email. 

The discovery of supernova 2018zd was part of the Global Supernova Project, a key program spearheaded by the Las Cumbres Observatory and Physics Professor at UC Santa Barbara Andy Howell. Through this project, 200 scientists across the globe are currently studying 500 supernovae using 40 telescopes, over the course of three years, according to the Las Cumbres Observatory website

According to NASA’s website, a supernova is the “largest explosion that takes place in space” and “is the explosion of a star.” 

Bostroem stated that the two main types of supernovae are thermonuclear and core-collapse supernovae, which can be differentiated by identifying the type of star and the way that star exploded. 

“Low mass stars, like the Sun, form really dense stars called white dwarfs at the end of their lives,” Bostroem said via email. “If a white dwarf accretes gas from another nearby star, it can explode as a thermonuclear supernova. On the other hand, stars that are more than 10 times the mass of our Sun fuse iron in their cores. It is the collapse of this core into a neutron star that produces iron core-collapse supernovae.” 

However, Bostroem explained that there are stars that lay amid these two extremes. Some stars are simply too large to form thermonuclear supernovae but also too small to form core-collapse supernovae. According to Dong, Ken’ichi Nomoto, a professor from the University of Tokyo, predicted in 1980 that these stars form electron capture supernovae, a third type of supernova, when they explode. 

“In this work, we confirm SN2018zd is an electron capture supernova,” Dong said. “The progenitor of this supernova is identified as a super-asymptotic giant branch star, and [these] types of stars are thought to be ended with electron capture supernovae.” 

Before the discovery of supernova 2018zd, Messier 1—more commonly known as the Crab Nebula—was the primary candidate for confirming the existence of electron capture supernovae. According to NASA’s website, the Crab Nebula, located 6,500 light-years away from Earth, was observed by Chinese astronomers in 1054. 

“We thought that the supernova that produced Crab Nebula was an electron capture supernova, but we didn’t have enough evidence to confirm that since that supernova exploded one thousand years ago,” Dong said. “Now, with the discovery of 2018zd, we have more confidence to say that electron capture supernova[e] exist in nature, and Crab Nebula may be the remnant of [an] electron capture supernova.” 

Bostroem added that for supernova 2018zd, their team of researchers were able to record observations tied to all six characteristics of an electron capture supernova: “progenitor identification, circumstellar material, chemical composition, explosion energy, light curve and nucleosynthesis,” according to the abstract of the paper.

Supernova 2018zd have “extremely low luminosity and special line features at late phase,” Dong said. Specifically, weak carbon, oxygen, magnesium and iron lines are apparent in the late-time spectra of supernova 2018zd. 

“I collected the late-time Keck spectra for this object when it was very faint,” Dong said.

These specific observations were taken via twin telescopes at the W. M. Keck Observatory in Waimea, Hawaii—among some of the largest telescopes in the world according to an article published by The Guardian—about 600 days following the explosion. 

 “The Keck spectra provide[s] strong evidence that this object is an electron capture supernova. In order to confirm that 2018zd is an electron capture supernova, we need to identify six features, and Keck data were used to confirm one of those features.” 

Bostroem explained that their data revealed there was more nickel than what is normally observed in a core-collapse supernova. 

“What makes supernova 2018zd unique among other electron capture supernova candidates is that we had observations of the star before it exploded, we discovered it soon after explosion and we collected observations of the full evolution until almost 2 years after it was discovered,” Bostroem said.  

Written by: Aarya Gupta — science@theaggie.org 

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