Great balls of fire! NASA's Hubble Space Telescope has detected superhot blobs of gas, each twice as massive as the planet Mars, being ejected near a dying star. The plasma balls are zooming so fast through space it would take only 30 minutes for them to travel from Earth to the moon. This stellar "cannon fire" has continued once every 8.5 years for at least the past 400 years, astronomers estimate.
The fireballs present a puzzle to astronomers, because the ejected
material could not have been shot out by the host star, called V Hydrae.
The star is a bloated red giant, residing 1,200 light-years away,
which has probably shed at least half of its mass into space during its
death throes. Red giants are dying stars in the late stages of life that
are exhausting their nuclear fuel that makes them shine. They have
expanded in size and are shedding their outer layers into space.
The current best explanation suggests the plasma balls were launched
by an unseen companion star. According to this theory, the companion
would have to be in an elliptical orbit that carries it close to the red
giant's puffed-up atmosphere every 8.5 years. As the companion enters
the bloated star's outer atmosphere, it gobbles up material. This
material then settles into a disk around the companion, and serves as
the launching pad for blobs of plasma, which travel at roughly a
half-million miles per hour.
This star system could be the archetype to explain a dazzling variety
of glowing shapes uncovered by Hubble that are seen around dying stars,
called planetary nebulae, researchers say. A planetary nebula is an
expanding shell of glowing gas expelled by a star late in its life.
"We knew this object had a high-speed outflow from previous data, but
this is the first time we are seeing this process in action," said
Raghvendra Sahai of NASA's Jet Propulsion Laboratory in Pasadena,
California, lead author of the study. "We suggest that these gaseous
blobs produced during this late phase of a star's life help make the
structures seen in planetary nebulae."
Hubble observations over the past two decades have revealed an
enormous complexity and diversity of structure in planetary nebulae. The
telescope's high resolution captured knots of material in the glowing
gas clouds surrounding the dying stars. Astronomers speculated that
these knots were actually jets ejected by disks of material around
companion stars that were not visible in the Hubble images. Most stars
in our Milky Way galaxy are members of binary systems. But the details
of how these jets were produced remained a mystery.
"We want to identify the process that causes these amazing
transformations from a puffed-up red giant to a beautiful, glowing
planetary nebula," Sahai said. "These dramatic changes occur over
roughly 200 to 1,000 years, which is the blink of an eye in cosmic
time."
Sahai's team used Hubble's Space Telescope Imaging Spectrograph
(STIS) to conduct observations of V Hydrae and its surrounding region
over an 11-year period, first from 2002 to 2004, and then from 2011 to
2013. Spectroscopy decodes light from an object, revealing information
on its velocity, temperature, location, and motion.
The data showed a string of monstrous, superhot blobs, each with a
temperature of more than 17,000 degrees Fahrenheit — almost twice as hot
as the surface of the sun. The researchers compiled a detailed map of
the blobs' location, allowing them to trace the first behemoth clumps
back to 1986.
"The observations show the blobs moving over time," Sahai
said. "The STIS data show blobs that have just been ejected, blobs that
have moved a little farther away, and blobs that are even farther away."
STIS detected the giant structures as far away as 37 billion miles away
from V Hydrae, more than eight times farther away than the Kuiper Belt
of icy debris at the edge of our solar system is from the sun.
The blobs expand and cool as they move farther away, and are then not
detectable in visible light. But observations taken at longer
sub-millimeter wavelengths in 2004, by the Submillimeter Array in
Hawaii, revealed fuzzy, knotty structures that may be blobs launched 400
years ago, the researchers said.
Based on the observations, Sahai and his colleagues Mark Morris of
the University of California, Los Angeles, and Samantha Scibelli of the
State University of New York at Stony Brook developed a model of a
companion star with an accretion disk to explain the ejection process.
"This model provides the most plausible explanation because we know
that the engines that produce jets are accretion disks," Sahai
explained. "Red giants don't have accretion disks, but many most likely
have companion stars, which presumably have lower masses because they
are evolving more slowly. The model we propose can help explain the
presence of bipolar planetary nebulae, the presence of knotty jet-like
structures in many of these objects, and even multipolar planetary
nebulae. We think this model has very wide applicability."
A surprise from the STIS observation was that the disk does not fire
the monster clumps in exactly the same direction every 8.5 years. The
direction flip-flops slightly from side-to-side to back-and-forth due to
a possible wobble in the accretion disk. "This discovery was quite
surprising, but it is very pleasing as well because it helped explain
some other mysterious things that had been observed about this star by
others," Sahai said.
Astronomers have noted that V Hydrae is obscured every 17 years, as
if something is blocking its light. Sahai and his colleagues suggest
that due to the back-and-forth wobble of the jet direction, the blobs
alternate between passing behind and in front of V Hydrae. When a blob
passes in front of V Hydrae, it shields the red giant from view.
"This accretion disk engine is very stable because it has been able
to launch these structures for hundreds of years without falling apart,"
Sahai said. "In many of these systems, the gravitational attraction can
cause the companion to actually spiral into the core of the red giant
star. Eventually, though, the orbit of V Hydrae's companion will
continue to decay because it is losing energy in this frictional
interaction. However, we do not know the ultimate fate of this
companion."
The team hopes to use Hubble to conduct further observations of the V
Hydrae system, including the most recent blob ejected in 2011. The
astronomers also plan to use the Atacama Large Millimeter/submillimeter
Array (ALMA) in Chile to study blobs launched over the past few hundred
years that are now too cool to be detected with Hubble.
The team's results appeared in the August 20, 2016, issue of The Astrophysical Journal.
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, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington, D.C
Felicia Chou
NASA Headquarters, Washington, D.C.
felicia.chou@nasa.gov
202-358-0257
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, California
elizabeth.r.landau@jpl.nasa.gov
818-354-6425
Source: HubbleSite
