http://www.skyandtelescope.com.au 39And 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
previous observations, which often missed 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-
degenerate scenarios each produce Type Ia supernovae.
Ryan Foley’s studies of sodium in supernova remnants, for
example, indicate that roughly 25% have the telltale outflows
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 asurvey with the Hubble Space Telescope that took a second
look at the locations where Type Ia supernovae had once
exploded. Because SN 2011kx only displayed hydrogen —
and hence proof of a gaseous companion — roughly 60
days after its explosion, she worried that astronomers
might have missed that signature in other supernovae that
weren’t observed for so long. But after observing 70 different
supernovae that exploded one to three years ago, she found
only one that contained those telltale hydrogen emission
lines. Assuming that such lines come from a large star that
triggered the Type Ia supernovae, such scenarios occur less
than 5% of the time.
Moreover, Graham notes that the finding only proves a
large star is nearby, not that it pushed the star to explode.
“It’s like a guilt-by-association situation,” she says, “which
isn’t a strong case.” It appears that, while possible, the single-
degenerate scenario may be rare.Twists and turns
Now that astronomers have determined that both scenarios
are at play, there are still a number of other questions —
mostly about how the explosion occurs. Does it start at the
centre of the white dwarf, for example, or does it start at theGREGG DINDERMAN /
S&T
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