This is an artist's impression of supernova 1993J, an exploding star in
the galaxy M81 whose light reached us 21 years ago. The supernova
originated in a double-star system where one member was a massive star
that exploded after siphoning most of its hydrogen envelope to its
companion star. After two decades, astronomers have at last identified
the blue helium-burning companion star, seen at the center of the
expanding nebula of debris from the supernova. The Hubble Space
Telescope identified the ultraviolet glow of the surviving companion
embedded in the fading glow of the supernova. Credit: NASA, ESA, and G. Bacon (STScI). Release Images
Supernova 1993J in Spiral Galaxy M81
This Hubble Space Telescope photo composite shows the location of
supernova 1993J inside the majestic spiral galaxy M81. Though
astronomers saw the star explode as a supernova 21 years ago, the glow
of that explosion is still present, as seen in the inset image. The
supernova has faded to the point where astronomers are confident that
they have picked up the ultraviolet glow of a very hot companion star.
This is the first time astronomers have been able to put constraints on
the properties of the companion star in this unusual class of supernova
called Type IIb. Hubble observations in ultraviolet light confirm the
theory that the explosion originated in a double-star system where one
star fueled the mass-loss from the aging primary star.
Photo Credit: NASA, ESA, A. Zezas (CfA), and A. Filippenko (UC Berkeley)
Science Credit: NASA, ESA, and O. Fox (University of California, Berkeley), A. Bostroem (STScI),
S. Van Dyk (Caltech), A. Filippenko (University of California,
Berkeley), C. Fransson (Stockholm University), T. Matheson (NOAO), S.
Cenko (University of California, Berkeley, and NASA/GSFC),
P. Chandra (National Center for Radio Astrophysics/Pune University,
India), V. Dwarkadas (University of Chicago), W. Li and A. Parker
(University of California, Berkeley), and N. Smith (Steward
Observatory)
Scenario for Type IIb SN 1993J
This illustration shows the key steps in the evolution of a Type IIb supernova.
Panel 1: Two very hot stars orbit about each other in a binary system.
Panel 2: The slightly more massive member of the pair evolves into a
bloated red giant and spills the hydrogen in its outer envelope onto
the companion star.
Panel 3: The more massive star explodes as a supernova.
Panel 4: The companion star survives the explosion. Because it has
locked up most of the hydrogen in the system, it is a larger and hotter
star than when it was born. The fireball of the supernova fades. Credit: NASA,
ESA, and A. Feild (STScI)
Astronomers using NASA's Hubble Space Telescope have discovered a
companion star to a rare type of supernova. This observation confirms
the theory that the explosion originated in a double-star system where
one star fueled the mass-loss from the aging primary star.
This detection is the first time astronomers have been able to put
constraints on the properties of the companion star in an unusual class
of supernova called Type IIb. They were able to estimate the surviving
star's luminosity and mass, which provide insight into the conditions
that preceded the explosion.
"A binary system is likely required to lose the majority of the
primary star's hydrogen envelope prior to the explosion. The problem is
that, to date, direct observations of the predicted binary companion
star have been difficult to obtain since it is so faint relative to the
supernova itself," said lead researcher Ori Fox of the University of
California (UC) at Berkeley.
Astronomers estimate that a supernova goes off once every second
somewhere in the universe. Yet they don't fully understand how stars
explode. Finding a "smoking gun" companion star provides important new
clues to the variety of supernovae in the universe. "This is like a
crime scene, and we finally identified the robber," quipped team member
Alex Filippenko, professor of astronomy at UC Berkeley. "The companion
star stole a bunch of hydrogen before the primary star exploded."
The explosion happened in the galaxy M81, which is about 11 million
light-years away from Earth in the direction of the constellation Ursa
Major (the Great Bear). Light from the supernova was first detected in
1993, and the object was designated SN 1993J. It was the nearest known
example of this type of supernova, called a Type IIb, due to the
specific characteristics of the explosion. For the past two decades
astronomers have been searching for the suspected companion, thought to
be lost in the glare of the residual glow from the explosion.
Observations made in 2004 at the W.M. Keck Observatory on Mauna Kea,
Hawaii, showed circumstantial evidence for spectral absorption features
that would come from a suspected companion. But the field of view is
so crowded that astronomers could not be certain if the spectral
absorption lines were from a companion object or from other stars along
the line of sight to SN 1993J. "Until now, nobody was ever able to
directly detect the glow of the star, called continuum emission," Fox
said.
The companion star is so hot that the so-called continuum glow is
largely in ultraviolet (UV) light, which can only be detected above
Earth's absorbing atmosphere. "We were able to get that UV spectrum
with Hubble. This conclusively shows that we have an excess of
continuum emission in the UV, even after the light from other stars has
been subtracted," said team member Azalee Bostroem of the Space
Telescope Science Institute (STScI), in Baltimore, Maryland.
When a massive star reaches the end of its lifetime, it burns though
all of its material and its iron core collapses. The rebounding outer
material is seen as a supernova. But there are many different types of
supernovae in the universe. Some supernovae are thought to have
exploded from a single-star system. Other supernovae are thought to
arise in a binary system consisting of a normal star with a white dwarf
companion, or even two white dwarfs. The peculiar class of supernova
called Type IIb combines the features of a supernova explosion in a
binary system with what is seen when single massive stars explode.
SN 1993J, and all Type IIb supernovae, are unusual because they do
not have a large amount of hydrogen present in the explosion. The key
question has been: how did SN 1993J lose its hydrogen? In the model for
a Type IIb supernova, the primary star loses most of its outer
hydrogen envelope to the companion star prior to exploding, and the
companion continues to burn as a super-hot helium star.
"When I first identified SN 1993J as a Type IIb supernova, I hoped
that we would someday be able to detect its suspected companion star,"
said Filippenko. "The new Hubble data suggest that we have finally done
so, confirming the leading model for Type IIb supernovae."
The team combined ground-based data for the optical light and images
from two Hubble instruments to collect ultraviolet light. They then
constructed a multi-wavelength spectrum that matched what was predicted
for the glow of a companion star.
Fox, Filippenko, and Bostroem say that further research will include
refining the constraints on this star and definitively showing that the
star is present.
The results were published in the July 20 Astrophysical Journal.
For images and more information about Hubble, visit:
http://hubblesite.org/news/2014/38 -
http://www.nasa.gov/hubble
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight
Center in Greenbelt, Md., manages the telescope. STScI conducts Hubble
science operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C.
CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Ori Fox
University of California, Berkeley, California
ofox@astro.berkeley.edu
Alex Filippenko
University of California, Berkeley, California
afilippenko@berkeley.edu
