Type Ia supernova explosions can be considered as some of the Universe’s most spectacular and inviting attractions, signifying the death of very massive stars. With peak luminosities that can briefly outshine an entire galaxy, these cataclysmic events expel most of the progenitor stars’ mass violently into space and play a crucial role in the recycling of material in the interstellar medium. Despite their importance, many of the underlying mechanisms of these cosmic phenomena have remained enigmatic to this day. A couple of new, recently published studies based on observations with NASA’s Kepler and Swift space telescopes have provided astronomers with a greater understanding to the causes that ignite these fascinating cosmic fireworks.
Supernova explosions, and the remnants they leave behind, have been studied extensively within the Milky Way as well as in other galaxies, with a variety of many ground- and space-based observatories across the entire electromagnetic spectrum during the last half century. During this time, two main theoretical models have arisen within the scientific community regarding the mechanisms by which type Ia supernovae form and evolve. The first posits that these types of supernovae result from the accretion of matter around carbon- and oxygen-rich white dwarfs in binary star systems from a less massive companion star (what is known as the single degenerate progenitor scenario). In this scenario, the infalling matter from the latter increases the mass of the white dwarf in the binary system past the Chandrasekhar limit of 1.4 solar masses above which a white dwarf star becomes unstable. Once it has passed this critical threshold, the star undergoes a runaway re-ignition of carbon nuclear fusion reactions in its core, which leads to its violent destruction in a supernova explosion. A second competing theoretical model argues that supernova explosions are the result of the complete merger of two white dwarfs in a binary system instead, through the loss of angular momentum which causes their combined mass to exceed that of the Chandrasekhar limit and lead to a supernova explosion (the double degenerate progenitor scenario).
The two leading theoretical models regarding the progenitor stars of Type Ia supernova explosions: the single degenerate progenitor scenario (left) and the double degenerate progenitor scenario (right). Image Credit: NASA/CXC/SAO
Nevertheless, a series of observations during the last couple of decades have revealed a greater variety to type Ia supernovae. For instance, the supernova SN 1991bg, which was observed within the galaxy NGC 4374 in 1991, at a distance of approximately 60 million light-years away, exhibited several strange peculiarities which countered theoretical predictions, including a fainter-than-expected peak luminosity and a faster decline in brightness afterwards. Twenty years later, in 2011, scientists uncovered an even more surprising type of extremely luminous type Ia supernova, called “Super-Chandrasekhar Supernovae,” which were found to be brighter due to the fact that their progenitor white dwarf stars would remain stable above the classical Chandrasekhar limit and would reach an upper limit of 2.58 solar masses before eventually exploding. These findings, as well as additional similar ones, indicated there were many more underlying processes driving these cosmic powerhouses than previously thought.
Using archival data from NASA’s Kepler space telescope, a research team led by Robert Olling, an astronomer at the University of Maryland, conducted a new comprehensive survey of supernova explosions, at a total of 400 galaxies. While it is better known as a planet-hunting observatory which has uncovered thousands of exoplanets in our galaxy to date, Kepler was given a new mission in 2014, called K2, which included a series of long observing sessions on behalf of the astrophysical community, including the fields of stellar formation and evolution among others. With the help of Kepler, Olling’s team discovered three supernovas in distant galaxies and was able to trace their evolution backwards in time by analysing more than two years’ worth of Kepler data. This allowed the researchers to study in detail the initial phases of these supernova explosions and the progression of their ejecting material through time, while being aided by Kepler’s temporal resolution of frequent observations of 30-minute intervals. The results of the team’s study, which was published in the 21 May edition of the journal Nature, showed that the ejected material from the three examined supernovae—KSN 2011b, KSN 2011c, and KSN 2012a—progressed through space without being disrupted by shock waves from collisions with any surrounding matter, which was a strong indicator that during the explosion, the supernovae weren’t orbited by any companion stars. This could only mean, according to the researchers, that these supernovae have most likely resulted from the merging of two white dwarf progenitor stars, giving more credence to the double degenerate progenitor scenario. “Models predict that the interaction of supernova ejecta with a companion star or circumstellar debris leads to a sudden brightening lasting from hours to days,” write the researchers in their study. “We present data for three supernovae that are likely to be type Ia observed during the Kepler mission, with a time resolution of 30 minutes. We find no signatures of the supernova ejecta interacting with nearby companions. The lack of observable interaction signatures is consistent with the idea that these three supernovae resulted from the merger of binary white dwarfs or other compact stars such as helium stars.”
During the same time, an independent research team, led by Yi Cao, a graduate student at the Astronomy Department of the California Insitute of Technology, was studying the supernova iPTF14atg inside the galaxy IC831, which is located approximately 300 million light-years away at the direction of the constellation Coma Berenices. The supernova was first spotted when it had just gone off, on May 3, 2014, by the intermediate Palomar Transient Factory, or iPTF for short, an automated wide-field robotic observing system at the Palomar Observatory in California. “We saw no evidence of this explosion in images taken the previous night, so we found iPTF14atg when it was only about one day old,” says Cao. “Better yet, we confirmed it was a young Type Ia supernova, something we’ve worked hard designing our system to find.”
Following the initial detection, the researchers mobilised a network of ground-based telescopes around the world as well as NASA’s Swift space telescope, in order to examine the immediate aftermath of the supernova explosion in multiple wavelengths. The timing proved to be of utmost importance. Even though supernova iPTF14atg lacked an X-ray afterglow, the Swift space telescope nevertheless detected a fading ultraviolet flash in the days following the explosion. “If Swift had looked just a day or two later, we would have missed the prompt UV flash entirely,” explains Brad Cenko, a member of the science team for the Swift space telescope at NASA’s Goddard Space Flight Center in Greenbelt, Md. “Thanks to Swift’s wavelength coverage and rapid scheduling capability, it is currently the only spacecraft that can regularly make these observations.”
Contrary to the results by Olling’s team, the UV flash that was seen in the aftermath of the iPTF14atg supernova explosion, indicated the presence of a nearby stellar companion onto which the ejecta from the supernova collided. “A reasonable physical model is ultraviolet emission arising in the ejecta, as the ejecta encounters a companion,” writes Cao’s team in their study, which was published on Nature, at the same day with Olling’s. “When the rapidly moving ejecta slams into the companion, a strong reverse shock is generated in the ejecta, that heats up the surrounding material. Thermal radiation from the hot material, which peaks in the ultraviolet part of the spectrum, can be seen for a few days until the fast-moving ejecta engulfs the companion and hides the reverse shock region.”
These findings by the Swift space telescope showed that type Ia supernova explosions indeed come in many flavors, from a variety of progenitor stellar sources. “This discovery provides direct evidence for the existence of a companion star in a Type Ia supernova, and demonstrates that at least some Type Ia supernovae originate from the single-degenerate channel,” comments Dr. Shrinivas Kulkarni, a professor of astrophysics at Caltech and member of Cao’s team.
The new results by both teams also underscore the need for more detailed follow-up observations, for better refining theoretical models about the mysterious cosmic dark energy. One key defining characteristic of Type Ia supernovae explosions is that they have a well-determined and predictable light curve and a uniform absolute magnitude. Because of these properties, they are used as standard cosmic candles by astronomers for accurately measuring distances in the Cosmos, and they have been instrumental in the discovery of dark energy and the accelerating expansion of the Universe in the late 1990s. Even though these varying new results may at first seem as casting a doubt to the whole concept of dark energy, scientists are quick to point out that the foundations for the latter’s existence are secure. “The accelerating Universe will not now go away. [Scientists] will not have to give back their Nobel prizes,” says Dr Brad Tucker, an astronomer at the Australian National University in Canberra, Australia, and member of Olling’s team. “The new results will actually help us to better understand the physics of supernovae, and figure out what is this dark energy that is dominating the Universe.”
Of equal importance is the fact that current space telescopes, like Kepler, whose primary role has been a very different one, can play an important role on helping to uncover long-held mysteries about the Universe, in many novel ways. “Kepler’s unprecedented pre-event supernova observations and Swift’s agility in responding to supernova events have both produced important discoveries at the same time but at very different wavelengths,” says Dr. Paul Hertz, Director of NASA’s Astrophysics Division, at the agency’s headquarters, in Washington, D.C. “Not only do we get insight into what triggers a Type Ia supernova, but these data allow us to better calibrate Type Ia supernovae as standard candles, and that has implications for our ability to eventually understand the mysteries of dark energy.”
As is always the case in cosmology and astrophysics, seemingly small and inconsequential findings tend to lead to important and ground-breaking results.