2019-07-01_Australian_Sky_&_Telescope

(singke) #1

40 AUSTRALIAN SKY & TELESCOPE July 2019


A


interface between the core and the envelope? Could there be
two explosions, one that starts on the surface that sparks an
explosion below?
The possibilities have caused many scientists to argue that
perhaps a number of scenarios — each a variant on the single-
and double-degenerate scenarios but with a unique explosion
mechanism — produce Type Ia supernovae. “I would be
surprised if there were just two paths, actually,” says Saurabh
Jha (Rutgers).
Take a new twist on the double-degenerate scenario.
Typically, models suggest that the two white dwarfs spiral
toward each other and merge. But Ken Shen (University of
California, Berkeley) has been honing a slightly different idea
for roughly a decade, one that relies on an often-overlooked
element within white dwarfs: helium. White dwarfs are
mostly composed of carbon and oxygen, but a thin layer of
helium often overlies the bulk. Shen thinks that one white
dwarf may steal enough of the other’s helium to ignite its
outer layer. This detonation squeezes the white dwarf, sending
a shock wave deep below that ignites a second explosion —
the Type Ia detonation — in the star’s core.
With the primary white dwarf annihilated, the companion
— which until now had been orbiting the dwarf at rapid
speeds — has nothing to hold it in place and shoots off into
space. Shen even published a study in 2018 that shows three
such white dwarfs speeding through the galaxy. Not only are
they quick-moving, but they possess bizarre characteristics
that can be perfectly explained if they underwent such a
catastrophic event.
Stuart Sim says this model is his personal favourite.
Although other theories also suggest that a white dwarf
can siphon matter from a second, less massive dwarf, the
scenarios still require that the primary dwarf reaches critical
mass in order to detonate its core. But because the helium
model relies on an explosion on the surface to squeeze the
core, the explosion can occur well before the star hits that
mass. Given that there should be more low-mass white

dwarfs than high-mass white dwarfs in the universe, which
might create duos that never reach critical mass, that’s an
attractive feature.
These scenarios might sound wild, and they are: None
of these ideas is exactly natural. “If you were to talk to a
17th-century physicist about all the things that happen in
the universe, I doubt any of these things would have come
to mind,” Sim says. “They have been thought up by people
who are scratching their heads thinking about how we can
arrange conditions for this rather spectacular but strange
thing to occur.”

Blazing forward
Although scientists do not fully understand the various
channels that lead to Type Ia supernovae, the explosions’ role
as cosmic milestones is on solid ground. “No one needs to
give back their Nobel Prize or anything like that,” Foley says.
That’s because empirically-speaking, astronomers know
the brightness of every Type Ia supernova’s flame fairly well.
A better understanding of what ignites that flame will only
improve the precision of those measurements — especially if
those systems change over time.
Currently, astronomers think that double-degenerate
systems might take longer to explode than single-degenerate
systems, because it can take billions of years after the white
dwarfs form for them to spiral in close enough to each other
for something explosive to happen. That could explain why
so many Type Ia supernovae in today’s universe seem to
come from double-degenerate systems, not single-degenerate
ones. It also suggests that a larger fraction might have come
from single-degenerate systems in the early universe. If one
pairing explodes at a slightly different luminosity than the
other, then it could affect the precision of our cosmological
measurements.
And that precision is crucial in illuminating dark
energy, the poorly understood phenomenon that propels
the expansion of the universe. A precise measurement of

XSCENARIO #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)

‘STANDARD’ CANDLES
Free download pdf