Pre-supernova stars may show signs of instability for months before the big explosion
In the most common type of supernova, the iron core of a massive star suddenly collapses in on itself and the outer layers are thrown out into space in a spectacular explosion. New research led by Weizmann Institute of Science researchers shows that the stars that become these “core-collapse supernovae” might exhibit instability for several months before the big event, spewing material into space and creating a dense gas shell around themselves. The scientists think that many massive stars, including the red super-giants that are the most common progenitors of the core-collapse supernovae, may begin the process this way.
This insight into the conditions leading up to core collapse arose from a unique collaboration called the intermediate Palomar Transient Factory, a fully automated sky survey that utilizes the telescopes of the Palomar Observatory in Southern California. Astrophysicists halfway around the globe, in Israel, are on call for the telescope, which scans the California night sky for the sudden appearance of new astronomical “transients” that were not visible before – which can indicate new supernovae. In October 2013, Dr. Ofer Yaron of Weizmann Institute’s Department of Particle Physics and Astrophysics, got the message that a potential supernova had been sighted. He immediately alerted Dr. Dan Perley, who was observing that night with the Keck telescope in Hawaii, and NASA’s Swift satellite. At the Keck Observatory, researchers soon began to record the spectra of the event. Because the global team had started observing only three hours into the blast, the picture they assembled was the most detailed ever of the core-collapse process. “We had x-rays, ultraviolet, four spectroscopic measurements from between six and 10 hours post-explosion to work with,” says Dr. Yaron.
In a study recently published in Nature Physics, Dr. Yaron, Weizmann Profs. Avishay Gal-Yam and Eran Ofek, and their teams, together with researchers from the California Institute of Technology and other institutes in the United States, Denmark, Sweden, Ireland, Israel, and the UK, analyzed the unique dataset they had collected from the very first days of the supernova.
The time window was crucial: it enabled the team to detect material that had surrounded the star pre-explosion, as it heated up, became ionized, and was eventually overtaken by the expanding cloud of stellar matter. Comparing the observed early spectra and light-curve data with existing models, accompanied by later radio observations, led the researchers to conclude that the explosion was preceded by a period of instability lasting for around a year. This instability caused material to be expelled from the surface layers of the star, forming the circumstellar shell of gas that was observed in the data. Because this was found to be a relatively standard type II supernova, the researchers believe that the instability they revealed may be a regular warm-up act for the immanent explosion.
“We still don’t really understand the process by which a star explodes as a supernova,” says Dr. Yaron. “These findings are raising new questions, for example, about the final trigger that tips the star from merely unstable to explosive.” He continues, “With our globe-spanning collaboration that enables us to alert various telescopes to train their sights on the event, we are getting closer and closer to understanding what happens in that instant, how massive stars end their life and what leads up to the final explosion.”
Prof. Avishay Gal-Yam’s research is supported by the Benoziyo Endowment Fund for the Advancement of Science; the Yeda-Sela Center for Basic Research; the Deloro Institute for Advanced Research in Space and Optics; and Paul and Tina Gardner. Prof. Gal-Yam is the recipient of the Helen and Martin Kimmel Award for Innovative Investigation.
Dr. Eran Ofek’s research is supported by the Helen Kimmel Center for Planetary Science; Paul and Tina Gardner, Austin, TX; Ilan Gluzman, Secaucus, NJ; and the estate of Raymond Lapon.