Sky & Telescope - February 2019 - 14

NEWS NOTES
SUPERNOVAE

Low-key Blast Marks Possible Birth of Neutron Star Duo
Kishalay De (Caltech) and colleagues
estimate that SN iPTF14gqr released
only one-ﬁfth of the mass of the Sun
- most supernovae expel several Suns'
worth of gas. What's more, the team
deduced that the star was about 1.5
times as massive as the Sun shortly
before the explosion. But for a star to go
boom, its original mass needs to be at
least eight times as massive as the Sun.
Something must have stripped the star
of most of its mass before it died. The
most likely candidate, the team argues
in the October 12th Science, is a companion neutron star.
Here's how the team thinks the
story played out: Two massive stars
once orbited each other. One of them
exploded, leaving its core behind as a
neutron star. This neutron star then
stripped its buddy of most of its gas. By

MAGELLANIC CLOUDS

Evidence Mounts for Magellanic Collision
THE PAST DECADE HAS seen astrono-

mers' understanding of the Magellanic
Clouds - two dwarf galaxies near the
Milky Way - completely overthrown,
resulting in new revelations about the

violent and ongoing formation of our
own galaxy. Now, a team led by Sally
Oey (University of Michigan) has
made a discovery that bears out predictions that these two galaxies once collided: A big chunk of the Small Magellanic Cloud (SMC) is moving toward
the Large Magellanic Cloud (LMC) and
the Magellanic Bridge of gas and stars
that joins them.
Astronomical wisdom once held that
the LMC and SMC had been orbiting
the Milky Way for billions of years,
but a landmark study in 2007 showed
that the dwarf galaxies are likely falling
toward the Milky Way for the ﬁrst time
(S&T: Oct. 2012, p. 28). That realization introduced a new puzzle: If the
Magellanic Bridge wasn't stripped from
the LMC and SMC by the powerful
gravity of the Milky Way, as previously
suspected, how did it get there?
t This picture of the Small Magellanic Cloud is
composed of two images from the Digitized Sky
Survey 2.

FE B RUA RY 2 019 * SK Y & TELESCOPE

p The moments before (left), during (middle),
and after (right) Supernova iPTF14gqr appeared
in the outskirts of a spiral galaxy.

the time the second star exploded - and
became known as SN iPTF14gqr - there
wasn't much gas left. The explosion
would also have produced a neutron star,
leaving two neutron stars in a tight orbit.
"This is the ﬁrst example of a supernova that produced a compact binary
system that is tight enough to eventually
merge and produce gravitational waves,"
De says. Unfortunately, at nearly 1 billion light-years away, the high-energy
light from any putative neutron stars is
much too faint to detect.
■ CHRISTOPHER CROCKETT

In 2012, researchers offered an
answer. The Magellanic Clouds must
have had a close encounter with each
other in the recent past - perhaps even
a direct collision - to pull the bridge's
stars and gas away so strongly.
In the November 1st Astrophysical
Journal Letters, Oey and her collaborators present support for this hypothesis
based on stellar position and velocity data from the Gaia satellite. They
discovered that many of the stars in the
eastward "wing" of the SMC are moving in concert, a result that can only be
explained by the LMC's gravitational
force acting on the galaxy globally.
With further study, astronomers
might be able to use the observed bulk
motion to understand the direct collision in even more detail: how long ago
it occurred, at what speed, and at what
angle. The better astronomers understand the past trajectories of these
galaxies, the more precisely they can
predict their future fates, which will
likely see them accumulated into the
Milky Way.
■ EMILY SANDFORD

SUPER NOVA: SDSS / CA LTECH / K ECK; M AG ELL A NIC
CLOUD: DAVIDE DE M A RTIN (ESA / HUBBLE ) / DSS2

LAST YEAR, A TSUNAMI of gravitational waves washed over Earth, heralding the collision of two neutron stars in
a far-off galaxy. Such collisions appear
to be the birthplace of many of the
heaviest elements such as gold, platinum, and uranium (S&T: Feb. 2018, p.
32). Now an unusual, rapidly brightening supernova might help researchers
understand how such neutron star duos
arise in the ﬁrst place.
The supernova, designated iPTF14gqr, went off in the outskirts of a
galaxy some 900 million light-years
away. Detected in October 2014, this
supernova seemed odd right away. Most
supernovae take a few weeks to hit
their peak brightness. This one did so
in less than 7 days, suggesting it had
only a relative paucity of debris to clear
out of the way.

Sky & Telescope - February 2019

Table of Contents for the Digital Edition of Sky & Telescope - February 2019