“Standard” Candles
18 JUNE 2019 • SKY & TELESCOPE
a typical Type Ia. But then, roughly 60 days after it exploded,
its spectrum started to shapeshift, revealing bright hydrogen
emission lines. The ballooning cloud of debris had slammed
into shells of circumstellar gas that had almost certainly been
expelled from a companion red giant star.
It was quite the surprise: Astronomers had generally
favored one scenario or the other. Instead, they had copycat
killers on their hands. “I think this is a case where Occam’s
razor has failed us,” says Andrew Howell (University of Cali-
fornia, Santa Barbara).
Still, scientists only had 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 fi nd
more. He predicted that when the white dwarf explodes, the
fi reball of expanding ejecta will slam into the surviving com-panion 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 defi nite 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 chang-
ing luminosity revealed a temporary jump in brightness that
matched the prediction for what would happen as the super-
nova blast struck a companion star. It was further evidence
that Type Ia supernovae can form with the help of a large star.
And it was not the only one. Last year, astronomers also
spotted evidence for this elusive blue bump within a Type Ia
supernova’s light curve that had been closely monitored by
NASA’s planet-hunting Kepler space telescope. Unlike previ-
ous observations, which often miss the supernova’s earliest
moments, Kepler imaged the star before it exploded and every
30 minutes thereafter — providing the best evidence yet that
a star can explode with the help of a large companion.
With a growing number of observations supporting each
scenario, many astronomers now suspect that doomed white
dwarfs tango with a variety of companions, large and small.
That has caused researchers to move on to the next game:
determining how often single-degenerate and double-degen-
erate scenarios each produce Type Ia supernovae. Ryan Foley’s
studies of sodium in supernova remnants, for example, indi-
cate that roughly 25% have the telltale outfl ows of gas they’d
expect from the winds blowing off red giants or other non-
degenerate stars. As such, he argues that roughly a quarter of
Type Ia supernovae are triggered by larger companions.
Melissa Graham (University of Washington) thinks the
number is much lower, however. She recently conducted aNormal
supernova001234567123
Days after explosionU
ltravio
let^ brightness(billions^ofSuns)4567Supernova
2017cbvpTHE BLUE BUMP Type Ia supernovae brighten and fade in a pre-
dictable pattern. Usually, their ultraviolet brightness follows the purple
curve. But SN 2017cbv was abnormally bright its fi rst two days, a boost
astronomers think comes from the explosion’s debris slamming into a
companion star.uSCENARIO #2½: DOUBLE DETONATION
In a twist on the double-degenerate scenario, one
white dwarf may steal from the thin layer of helium
enveloping a second, less massive (but wider)
dwarf (A). Eventually enough gas piles up on the
more massive dwarf to trigger an explosion (B).
This surface explosion sends a shock wave deep
into the dwarf that kicks off a detonation in the
carbon core, producing a supernova (C).GREGG^
DINDERMAN/^ S&T
(^2
)A