First Evidence of Nova Cycle.
Hibernating-Star Explosion Provides First Evidence of Nova Cycle. The awakening of a classical nova from hibernation
The explosion of a “hibernating” star is revealing insight into how these star eruptions evolve, according to a new study.
A team of astronomers has found evidence of additional mini outbursts
leading up to the so-called classical nova — or final explosion of a
white dwarf star, the planet-size remains of a star that has burned up
its nuclear fuel. Their data provide the first direct evidence for the
nova hibernation hypothesis, a theory that suggests a cyclical evolution
of such stars.
A classical nova occurs when a
white dwarf star gains matter from its secondary star (a red dwarf) over
a period of time, causing a thermonuclear reaction on the surface that
ventually erupts in a single visible outburst. This creates a 10,000
fold brightness.
The nova — called V1213 Cen, or Nova Centauri 2009 — erupted in 2009,
but astronomers at the University of Warsaw had been monitoring its
source star since 2003. By collecting data for several years before and
after the star’s eruption, the researchers were able to study the
evolution of this type of nova.
Classical novas,such as Nova Centauri 2009, occur in binary star
systems, or systems in which two stars orbit around a common center of
mass. A white dwarf and another small star orbit a common center of mass
while the white dwarf pulls matter away from its companion. Hydrogen
from the second star accumulates on the white dwarf’s surface, where
fusion leads to a big explosion. Unlike the much more powerful supernova
explosions, which signify the deaths of stars, nova eruptions don’t
necessary destroy their stellar parents.
“These are the brightest and most frequent stellar eruptions in the
galaxy, and they’re often visible to the naked eye,” said Przemek Mróz,
the lead author of the study, which is detailed online in the Aug. 17
edition of the journal Nature.
The smaller explosions seen in the years leading up to the big eruption
of Nova Centauri 2009 are called dwarf novas. Rather than detonating on
the white dwarf’s surface, these explosions occur in the accretion disk —
the rotating cloud of star stuff that the white dwarf pulls away from
its companion, Mróz said. But the timing of nova eruptions remains
unpredictable.
Upper panels: Snapshots of a nova
lifecycle. Lower panel: The Milky Way over the Warsaw Telescope dome,
Las Campanas Observatory.
Mróz and colleagues studied the dwarf nova outbursts by creating a light
curve, or a graph that shows variations in the amount of light coming
from a variable star over a period of time. A light curve provides clues
to the binary stars’ fluctuating mass-transfer rate, or the rate at
which one star takes matter away from the other. “Dwarf nova outbursts
appear when the mass-transfer rate between the stars is unstable, so
each little bump in the light curve is produced by a portion of matter
falling onto the surface,” he explained.
The results of their study provided the first direct evidence for
changes in the mass-transfer rate before, during and after the classical
nova eruption, the researchers said. Understanding changes in the
mass-transfer rate can help astronomers predict when a nova might erupt,
they added.
The observations also provide strong evidence for what scientists call
the nova hibernation hypothesis, which suggests that the mass-transfer
rate increases after the explosion, and then decreases significantly
over a million years. Then, according to the theory, the star enters the
hibernation phase.
These events create a cycle, ultimately leading to yet another nova explosion, the theory goes.
“This discovery wouldn’t have been possible without the long-term
observations by the Optical Gravitational Lensing Experiment(OGLE),” an
observational project based at the University of Warsaw, which has been
surveying the universe since 1992, Mróz said.
First Evidence of Nova Cycle.
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