Maunakea, Hawaiʻi – A team of astronomers led by Stockholm University has discovered an unusual Type Ia supernova – or thermonuclear supernova – called SN 2020eyj. Not only did they make the first detection of such a supernova in radio waves, follow-up observations from W. M. Keck Observatory on Maunakea, Hawaiʻi Island also showed strong emission lines of helium.
This marks the first confirmed Type Ia supernova triggered by a white dwarf star that pulled material from a companion star with an outer layer consisting primarily of helium; normally, in the rare cases where the material stripped from the outer layers of the donor star could be detected in spectra, this was mostly hydrogen.
Type Ia supernovae are important for astronomers since they are used to measure the expansion of the universe. However, the origin of these explosions has remained an open question. While it is established that the explosion is caused by a compact white dwarf star that somehow accretes too much matter from a companion star, the exact process and the nature of the progenitor is not known.
The new discovery of supernova SN 2020eyj is evidence the companion star was a helium star that had lost much of its material just prior to the explosion of the white dwarf.
The study, which includes data from Keck Observatory’s Low Resolution Imaging Spectrometer (LRIS), is published in today’s issue of the journal Nature.
Artist’s impression of SN2020eyj, a double star system with a compact white dwarf star accreting matter from a helium-rich donor companion, surrounded by dense and dusty circumstellar material. It was the interaction of the exploded star and the material left over from this companion that gave rise to the strong radio signal and the conspicuous helium lines in the optical spectra of SN 2020eyj. Credit: W. M. Keck Observatory/Adam Makarenko
“Once we saw the signatures of strong interaction with the material from the companion, we tried to also detect it in radio emission,” says Erik Kool, a postdoc at Stockholm University’s Department of Astronomy and lead author of the paper. “The detection in radio is the first one of a Type Ia supernova – something astronomers have tried to do for decades.”
Supernova 2020eyj was first spotted by the Zwicky Transient Facility at Palomar Observatory near San Diego where the Oskar Klein Centre at Stockholm University are members.
“The Nordic Optical Telescope on La Palma was fundamental for following up this supernova,” says Jesper Sollerman, a professor at Stockholm University’s Department of Astronomy and co-author of the paper. “As were spectra from the large Keck telescope in Hawaiʻi that immediately revealed the very unusual helium-dominated material around the exploded star.”
“This is clearly a very unusual Type Ia supernova, but still related to the ones we use to measure the expansion of the universe,” adds co-author Joel Johansson from the Department of Physics at Stockholm University. “While normal Type Ia supernovae appear to always explode with the same brightness, this supernova tells us that there are many different pathways to a white dwarf star explosion.”
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile and ultra-sensitive visible-wavelength imager and spectrograph built at the California Institute of Technology by a team led by Prof. Bev Oke and Prof. Judy Cohen and commissioned in 1993. Since then it has seen two major upgrades to further enhance its capabilities: the addition of a second, blue arm optimized for shorter wavelengths of light and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the universe. LRIS was used in observing distant supernovae by astronomers who received the Nobel Prize in Physics in 2011 for research determining that the universe was speeding up in its expansion.
ABOUT W. M. KECK OBSERVATORY
The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.