Cosmic dust is similar to dust on Earth: groupings of molecules that have condensed and stuck together in a grain. An international team of astronomers, including researchers from the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC), has discovered a previously unknown source of dust in the universe: a Type Ia supernova interacting with gas from its surroundings.
The results are significant because the exact nature of dust creation in the universe has been a mystery for a long time. The study has been published today in Nature Astronomy, led by Dr. Lingzhi Wang, an associate researcher at the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), together with astronomers from China, the United States of America, Chile, United Kingdom and Spain. Among them, Lluís Galbany and Tomás Müller Bravo, ICE-CSIC and IEEC researchers, who contributed data to the study and participated in the analysis.
Supernovae have been known to play a role in dust formation, and to date, dust formation has only been seen in core-collapse supernovae (or type II), which are the explosions of massive stars. Since core-collapse supernovae do not occur in elliptical galaxies, the nature of dust creation there has remained elusive. These galaxies aren’t organized into a spiral pattern like our Milky Way galaxy but are giant swarms of stars. The new study finds that thermonuclear Type Ia supernovae, the explosion of a white dwarf star in a binary system with another star, may account for a significant amount of dust in these galaxies.
“This supernova initially didn't capture our attention, and we momentarily lost interest a few days into our observation campaign when it temporarily disappeared behind the Sun. However, to our surprise, when it reappeared a few months later, it not only remained detectable but was significantly brighter than anticipated. It was at that moment that we realized something truly extraordinary was going on”, says ICE-CSIC and IEEC researcher Lluís Galbany.
The researchers monitored the supernova SN 2018evt for over three years using space-based facilities like NASA’s Spitzer Space Telescope and NEOWISE missions, and ground-based facilities like the Las Cumbres Observatory’s global network of telescopes, and other facilities in China, South America, and Australia. ICE-CSIC and IEEC researchers collaborated monitoring data from the New Technology Telescope (NTT) in La Silla (Chile) as part of the ePESSTO+ collaboration. They also monitored the supernova using the camera ANDICAM (A Novel Dual Imaging CAMera), which was mounted on the 1.3-m telescope SMARTS in Cerro Tololo (Chile).
They found that the supernova was running into material previously cast off by one or both stars in the binary system before the white dwarf star exploded. The supernova sent a shock wave into this pre-existing gas. As the researchers monitored the supernova for more than 1,000 days, they noticed that its light began to dim precipitously in the optical wavelengths that our eyes can see, and then started glowing brighter in infrared light. This is a telltale sign that dust was being created in the circumstellar gas after it cooled following the supernova shock wave passing through it.
“As gas and dust emit infrared light, observations at these wavelengths are essential for this type of studies”, said Tomás E. Müller Bravo, ICE-CSIC and IEEC researcher. “However, despite the infrared monitoring of other thermonuclear supernovae, the detection of dust formation in these phenomena remained elusive. This is why we were surprised with this discovery”, he added.
“The origins of cosmic dust have long been a mystery. This research marks the first detection of a significant and rapid dust formation process in the thermonuclear supernova interacting with circumstellar gas,” said Lingzhi Wang, the first author of this study.
The study estimated that a large amount of dust must have been created by this one supernova event, more than 1% of the Sun's mass. As the supernova cools down, the amount of dust created should increase, perhaps, tenfold. While these dust factories aren’t as numerous or efficient as core-collapse supernovae, there may be enough of these thermonuclear supernovae interacting with their surroundings to be a significant or even dominant source of dust in elliptical galaxies.