38 AUSTRALIAN SKY& TELESCOPE July 2019
XSCENARIO #1: SINGLE DEGENERATE
A white dwarf paired with a much larger
star — such as an aged red giant — can
siphon gas from the companion star and
skirt itself in a disk (A). As the disk gas
falls onto the white dwarf, the temperature
and density build until carbon fusion
ignites and the white dwarf explodes with
a standard luminosity (B).XSCENARIO #2: DOUBLE
DEGENERATE Over billions of years, two
white dwarfs will inspiral toward each
other by emitting gravitational waves
(A). Eventually they will collide and be
destroyed (B). Because the white dwarfs
don’t have to reach a critical density
and temperature in order to explode,
the resulting flash could have a range
of luminosities depending on the white
dwarfs’ masses.killers on their hands. “I think this is a case where Occam’s
razor has failed us,” says Andrew Howell (University of
California, Santa Barbara).
Still, scientists had only one clear example of a Type Ia
supernova sparked by a large star. In 2010, Daniel Kasen
(University of California, Berkeley) suggested a way to find
more. He predicted that when the white dwarf explodes,
the fireball of expanding ejecta will slam into the surviving
companion star — a run-in that heats up the ejecta and
causes it to brighten. That event should produce an abnormal
blue bump in the supernova’s early light curve.
Astronomers searched for this signal in existing data sets
but found no definite examples — until 2017. On March
10th, David Sand (University of Arizona) discovered a Type Ia
supernova on the outskirts of the spiral galaxy NGC 5643. With
help from the Las Cumbres Observatory, a network of (then) 18
telescopes around the world that monitors objects continuously,
he and his colleagues observed the supernova every 6 hours for
5 days. The resulting chart of its changing luminosity revealed
a temporary jump in brightness that matched the prediction for
what would happen as the supernova blast struck a companion
star. It was further evidence that Type Ia supernovae can form
with the help of a large star.EXPLOSION GRAPHIC: TATIANAZAETS / ISTOCK / GETTY IMAGES PLUST TEL 001‘STANDARD’ CANDLESA
whitedwarfsthathita specificmas
led the Chandrasekhar mass. But in
eaches that limit: If it did, gravity wo
not explode. Instead, as the dwarf a
ekhar mass, it contracts, causing th
density to spike. At a certain point
to the Chandrasekhar mass — car
es and blows the white dwarf apart.difference and continue to use the objects in cosmology. As
many scientists say, they’re not standard candles, but they’re
standardisable candles.
Still, scientists did not know why some Type Ia supernovae
reached different brightnesses, causing many to reconsider
an alternate possibility: White dwarfs were instead dancing a
deadly waltz with their own kind.
Because the dwarfs collide in this scenario, the final
system won’t necessarily hit the same critical conditions
every time. White dwarfs in a binary can vary in weight from
0.3 to 1.4 times the mass of the Sun, so the final system could
theoretically vary from 0.6 to 2.8 Suns (although having two
massive white dwarfs paired up is highly unlikely). Convert
that to energy and you have a wide range of luminosities,
thus explaining the oddities discovered in 1991.
By 2011, the pendulum had swung toward this so-called
double-degenerate scenario, where ‘degenerate star’ refers here
to a white dwarf (as opposed to the single-degenerate scenario,
where there is only one dwarf and a large star). Not only
can low-mass companions explain the range of brightnesses,
but they also explain why astronomers had failed to spot
bereaved companion stars within the debris of ancient
supernova remnants.
Indeed, when Nugent discovered the nearby supernova,
he was able to scour previous images to search for signs of a
companion. But Hubble images of the Pinwheel Galaxy taken
before the blast revealed no trace of a star at SN 2011fe’s
location, leavin prit could be no
larger than a sene pair can tango
er wasfarfro closed.Co siderae
,
its spe hydrogen
emissi slammed
into sh tainly been
expelled from a companion red giant star.
It was quite the surprise: Astronomers had generally
favoured one scenario or the other. Instead, they had copycat