Andrew Yee[_1_]
January 8th 08, 05:32 AM
Public Affairs Office
Harvard-Smithsonian Center for Astrophysics
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
For Release: Thursday, November 01, 2007
Release No.: 2007-29
White Dwarf "Sibling Rivalry" Explodes into Supernova
Cambridge, MA -- Astronomers at the Harvard-Smithsonian Center for
Astrophysics (CfA) have found that a supernova discovered last year was
caused by two colliding white dwarf stars. The white dwarfs were siblings
orbiting each other. They slowly spiraled inward until they merged, touching
off a titanic explosion. CfA observations show the strongest evidence yet of
what was, until now, a purely theoretical mechanism for creating a
supernova.
"This finding shows that nature may be richer than we suspected, with more
than one way to make a white dwarf explode," said Harvard graduate student
and first author Malcolm Hicken.
The paper describing this discovery appeared in the November 1 issue of The
Astrophysical Journal Letters.
Astronomers characterize an observed supernova based on whether its spectrum
shows evidence of hydrogen (Type II) or not (Type I). In Type II, a massive,
short-lived star undergoes core collapse and explodes. In the conventional
picture for Type Ia, the most common supernovae lacking hydrogen, a white
dwarf star collects gas from a companion star until it undergoes
catastrophic nuclear fusion and blasts itself apart.
The new find, supernova 2006gz, was classified as a Type Ia due to the lack
of hydrogen and other characteristics. However, an analysis combining CfA
data with measurements from The Ohio State University suggested that SN
2006gz was unusual and deserved a closer look.
Most importantly, SN 2006gz showed the strongest spectral signature of
unburned carbon ever seen. Merging white dwarfs are expected to have carbon
outside their densest regions. The powerful explosion from the inside then
should push off the outmost carbon-rich layers.
The spectrum of SN 2006gz also showed evidence for compressed layers of
silicon. Silicon was created during the explosion and then compressed by a
shock wave that rebounded from the surrounding layers of carbon and oxygen.
Computer models for merging white dwarfs predict both the carbon and silicon
spectral signatures.
Additionally, SN 2006gz was brighter than expected, indicating that its
progenitor exceeded the 1.4 solar mass Chandrasekhar limit -- the upper
bound for a single white dwarf. Only one other potential example of a
super-Chandrasekhar supernova has been seen: SN 2003fg. While observations
of that event were suggestive, the data from SN 2006gz are much stronger.
"Our case is different. Although 2006gz is also extra bright, the chemistry
we see, particularly unburned carbon, is well observed and very unusual,"
said Harvard astronomer Robert Kirshner, a member of the discovery team.
In addition to providing the first example of a new way to make supernovae,
SN 2006gz holds implications for the field of cosmology. Type Ia supernovae
typically have a narrow spread in brightness, which makes them useful as
"standard candles" for calculating cosmic distances. It was the study of
Type Ia supernovae that led to the discovery of dark energy, the mysterious
force causing the expansion of the universe to accelerate.
If Type Ia supernovae are more varied than previously expected, then
astronomers must be extra cautious when using them to study the cosmos.
"Supernova 2006gz stands out from normal Type Ia objects and wouldnt be
included in cosmology studies," commented Hicken. "But we have to be careful
not to mistake a double white dwarf explosion for a single white dwarf
blast. SN 2006gz was easy to recognize, but there may be less clear-cut
cases."
The full list of authors of the study is: Malcolm Hicken, Stephane Blondin
and Robert Kirshner (CfA); Peter Garnavich (University of Notre Dame); Jose
Prieto and Darren DePoy (The Ohio State University); and Jerod Parrent
(University of Oklahoma). This research was supported by the National
Science Foundation.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
[NOTE: Images supporting this release are available at
http://cfa-www.harvard.edu/press/2007/pr200729_images.html ]
Harvard-Smithsonian Center for Astrophysics
For more information, contact:
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
For Release: Thursday, November 01, 2007
Release No.: 2007-29
White Dwarf "Sibling Rivalry" Explodes into Supernova
Cambridge, MA -- Astronomers at the Harvard-Smithsonian Center for
Astrophysics (CfA) have found that a supernova discovered last year was
caused by two colliding white dwarf stars. The white dwarfs were siblings
orbiting each other. They slowly spiraled inward until they merged, touching
off a titanic explosion. CfA observations show the strongest evidence yet of
what was, until now, a purely theoretical mechanism for creating a
supernova.
"This finding shows that nature may be richer than we suspected, with more
than one way to make a white dwarf explode," said Harvard graduate student
and first author Malcolm Hicken.
The paper describing this discovery appeared in the November 1 issue of The
Astrophysical Journal Letters.
Astronomers characterize an observed supernova based on whether its spectrum
shows evidence of hydrogen (Type II) or not (Type I). In Type II, a massive,
short-lived star undergoes core collapse and explodes. In the conventional
picture for Type Ia, the most common supernovae lacking hydrogen, a white
dwarf star collects gas from a companion star until it undergoes
catastrophic nuclear fusion and blasts itself apart.
The new find, supernova 2006gz, was classified as a Type Ia due to the lack
of hydrogen and other characteristics. However, an analysis combining CfA
data with measurements from The Ohio State University suggested that SN
2006gz was unusual and deserved a closer look.
Most importantly, SN 2006gz showed the strongest spectral signature of
unburned carbon ever seen. Merging white dwarfs are expected to have carbon
outside their densest regions. The powerful explosion from the inside then
should push off the outmost carbon-rich layers.
The spectrum of SN 2006gz also showed evidence for compressed layers of
silicon. Silicon was created during the explosion and then compressed by a
shock wave that rebounded from the surrounding layers of carbon and oxygen.
Computer models for merging white dwarfs predict both the carbon and silicon
spectral signatures.
Additionally, SN 2006gz was brighter than expected, indicating that its
progenitor exceeded the 1.4 solar mass Chandrasekhar limit -- the upper
bound for a single white dwarf. Only one other potential example of a
super-Chandrasekhar supernova has been seen: SN 2003fg. While observations
of that event were suggestive, the data from SN 2006gz are much stronger.
"Our case is different. Although 2006gz is also extra bright, the chemistry
we see, particularly unburned carbon, is well observed and very unusual,"
said Harvard astronomer Robert Kirshner, a member of the discovery team.
In addition to providing the first example of a new way to make supernovae,
SN 2006gz holds implications for the field of cosmology. Type Ia supernovae
typically have a narrow spread in brightness, which makes them useful as
"standard candles" for calculating cosmic distances. It was the study of
Type Ia supernovae that led to the discovery of dark energy, the mysterious
force causing the expansion of the universe to accelerate.
If Type Ia supernovae are more varied than previously expected, then
astronomers must be extra cautious when using them to study the cosmos.
"Supernova 2006gz stands out from normal Type Ia objects and wouldnt be
included in cosmology studies," commented Hicken. "But we have to be careful
not to mistake a double white dwarf explosion for a single white dwarf
blast. SN 2006gz was easy to recognize, but there may be less clear-cut
cases."
The full list of authors of the study is: Malcolm Hicken, Stephane Blondin
and Robert Kirshner (CfA); Peter Garnavich (University of Notre Dame); Jose
Prieto and Darren DePoy (The Ohio State University); and Jerod Parrent
(University of Oklahoma). This research was supported by the National
Science Foundation.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
[NOTE: Images supporting this release are available at
http://cfa-www.harvard.edu/press/2007/pr200729_images.html ]