A First! --Astronomers Observe a Supernova Collapse in Real Time
"We still don't really understand the process by which a star explodes as a supernova,"
says Ofer Yaron, with the Weizmann Institute's Particle Physics and Astrophysics team, "These
findings are raising new questions, for example, about the final trigger that tips the star
from merely unstable to explosive.
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."
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 so-called core-collapse supernovae might already exhibit instability for several
months before the big event, spewing material into space and creating a dense gas shell
around themselves.
They think that many massive stars, including the red super-giants that are the most common
progenitors of these supernovae, may begin the process this way.
This insight into the conditions leading up to core collapse arose from a unique collaboration
called the Palomar Transient Factory, a fully automated sky survey using 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, Yaron got the message that a potential supernova had been sighted, and
he immediately alerted Dr. Dan Perley who was observing that night with the Keck telescope
in Hawaii, and NASA's Swift Satellite.
At Keck, the researchers soon began to record the spectra of the event.
Because they had started observing only three hours into the blast, the picture the team
managed to assemble was the most detailed ever of the core collapse process.
"We had x-rays, ultraviolet, four spectroscopic measurements from between six and ten hours
post-explosion to work with," says Yaron.
In a study recently published in Nature Physics, Yaron, Weizmann Institute researchers 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 and 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 to the immanent
explosion.
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