An exploding star, known as a supernova, brightens and dims so predictably that astronomers use them to calibrate distances in space. These violent events are pillars of modern astronomy and physics used to study many cosmological phenomena such as the expansion history of the universe, yet the origin of these natural fireworks remains a mystery. However, an international team of researchers reported in a study published Dec. 15 in the scientific journal Nature that they are a step closer to solving one of the universe’s greatest unsolved mysteries.
Supernovae are classified as either a Type I or Type II supernova, depending upon whether or not hydrogen is observed in their spectra. Hydrogen is the most abundant element in a star, and heavier elements are produced in the centers of stars. The absence of hydrogen indicates that the outer shells of a star have been expelled, a process thought to happen late in a star's lifetime. A Type Ia supernovae also lacks the next heaviest element to hydrogen – helium – and silicon is observed instead. This indicates that Type Ia supernovae are associated with the cores of highly-evolved stars.
Prior to a Type Ia supernova, there are two stars, known as a binary system. One star is a white dwarf; the other is a companion star that is close enough to transfer its own material onto the white dwarf. Once enough mass falls onto the white dwarf, reaching a total of about 1.4x the mass of our sun, it is no longer a stable configuration. The white dwarf collapses in on itself and the star explodes. General consensus holds that Type Ia supernovae result from thermonuclear explosions of a white dwarf in a binary system; however, astrophysicists do not know what the secondary star – the star that dumps mass onto the primary to make it explode – is.
“We use these supernovae for cosmology studies regularly, but we don’t really know what makes them,” says Nathaniel Butler, an assistant professor in the School of Earth and Space Exploration at Arizona State University. “However, we think that we now have a better understanding of the companion star for this one very interesting supernova, and we have obtained unprecedented constraints on the characteristics of that companion star.”
On August 24, 2011an exploding star, SN 2011fe/PTF11kly (hereafter SN 2011fe), was discovered in the Pinwheel Galaxy. This galaxy was intensively monitored over the past decade and was also regularly observed by the Hubble Space Telescope (HST) and Chandra on several occasions. Astronomers immediately saw the potential of the imaging data obtained of this nearby supernova by an automated survey, the Palomar Transient Factory (PTF). Together, these archival data offer a unique opportunity to constrain the nature of the star system of SN 2011fe.
“We think SN 2011fe was the earliest Ia ever detected, identified by PTF within hours of the explosion. This supernova was great because it happened only about 20 million light years away, which is about 40 times closer than typical Ia SNe. So the potential to observe was phenomenal; it was visible through binoculars,” says Butler.
Prior to this event, astrophysicists did not know what type of star that companion was likely to be. However, thanks to the wealth of pre-explosion data, the most precise measurements to date on the system prior to explosion were possible. The data reveal that the secondary star is not a giant star. It is either a white dwarf or a regular (main sequence) star.
“This knowledge is then very useful for modeling the process that leads to type Ia SNe (formation of this type of binary, its evolution), and this knowledge enables calculations of rates, detailed explosion calculations, etc,” says Butler. “We now know, for at least one case and the best to date, that highly-luminous, red-giant stars aren't needed to trigger a type Ia SN.”