NASA's Kepler spacecraft was designed to find exoplanets by looking for stars that dim as a planet crosses the star's face. Fortuitously, the same design makes it ideal for spotting other astronomical transients – objects that brighten or dim over time. A new search of Kepler archival data has uncovered an unusual super-outburst from a previously unknown dwarf nova .
The system brightened by a factor of 1,600 over less than a day before slowly fading away. The
star system in question consists of a white dwarf star with a brown
dwarf companion about one-tenth as massive as the white dwarf. A white
dwarf is the leftover core of an aging Sun-like star and contains about a
Sun's worth of material in a globe the size of Earth. A brown dwarf is
an object with a mass between 10 and 80 Jupiters that is too small to
undergo nuclear fusion.
The brown dwarf circles the white dwarf
star every 83 minutes at a distance of only 250,000 miles (400,000 km) –
about the distance from Earth to the Moon. They are so close that the
white dwarf's strong gravity strips material from the brown dwarf,
sucking its essence away like a vampire. The stripped material forms a
disk as it spirals toward the white dwarf (known as an accretion disk).
It
was sheer chance that Kepler was looking in the right direction when
this system underwent a super-outburst, brightening by more than 1,000
times. In fact, Kepler was the only instrument that could have witnessed
it, since the system was too close to the Sun from Earth's point of
view at the time. Kepler's rapid cadence of observations, taking data
every 30 minutes, was crucial for catching every detail of the outburst.
The
event remained hidden in Kepler's archive until identified by a team
led by Ryan Ridden-Harper of the Space Telescope Science Institute
(STScI), Baltimore, Maryland, and the Australian National University,
Canberra, Australia. "In a sense, we discovered this system
accidentally. We weren't specifically looking for a super-outburst. We
were looking for any sort of transient," said Ridden-Harper.
Kepler
captured the entire event, observing a slow rise in brightness followed
by a rapid intensification. While the sudden brightening is predicted
by theories, the cause of the slow start remains a mystery. Standard
theories of accretion disk physics don't predict this phenomenon, which
has subsequently been observed in two other dwarf nova super-outbursts.
"These
dwarf nova systems have been studied for decades, so spotting something
new is pretty tricky," said Ridden-Harper. "We see accretion disks all
over – from newly forming stars to supermassive black holes – so it's
important to understand them."
Theories suggest that a
super-outburst is triggered when the accretion disk reaches a tipping
point. As it accumulates material, it grows in size until the outer edge
experiences gravitational resonance with the orbiting brown dwarf. This
might trigger a thermal instability, causing the disk to get
superheated. Indeed, observations show that the disk's temperature rises
from about 5,000–10,000° F (2,700–5,300° C) in its normal state to a
high of 17,000–21,000° F (9,700–11,700° C) at the peak of the
super-outburst.
This type of dwarf nova system is relatively rare,
with only about 100 known. An individual system may go for years or
decades between outbursts, making it a challenge to catch one in the
act.
"The detection of this object raises hopes for detecting even
more rare events hidden in Kepler data," said co-author Armin Rest of
STScI.
The team plans to continue mining Kepler data, as well as
data from another exoplanet hunter, the Transiting Exoplanet Survey
Satellite (TESS) mission, in search of other transients.
"The
continuous observations by Kepler/K2, and now TESS, of these dynamic
stellar systems allows us to study the earliest hours of the outburst, a
time domain that is nearly impossible to reach from ground-based
observatories," said Peter Garnavich of the University of Notre Dame in
Indiana.
This work was published in the Oct. 21, 2019 issue of the Monthly Notices of the Royal Astronomical Society.
The
Space Telescope Science Institute is expanding the frontiers of space
astronomy by hosting the science operations center of the Hubble Space
Telescope, the science and operations center for the James Webb Space
Telescope, and the science operations center for the future Wide Field
Infrared Survey Telescope (WFIRST). STScI also houses the Mikulski
Archive for Space Telescopes (MAST) which is a NASA-funded project to
support and provide to the astronomical community a variety of
astronomical data archives, and is the data repository for the Hubble,
Webb, Kepler, K2, TESS missions and more.
Contact:
Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland
410-338-4366
cpulliam@stsci.edu
