Sky and Telescope - April 2016 - 10

News Notes

STELLAR I Runaways' Telltale Shocks

NASA / JPL-CALTECH / UCLA

Zeta Ophiuchi (blue star in the center of the image) and its accompanying bow shock, as seen
at infrared wavelengths by the WISE space telescope.

Astronomers have now found dozens
of stars zipping through space at 30 kilometers per second (67,000 mph) or faster
relative to their surroundings. These
speedsters, called runaway stars, tend
to be in isolated regions of the galaxy,
though their paths often lead directly
away from stellar clusters or even our
galaxy's central black hole.
Astronomers had previously discovered about 20 of these stars, through pure
luck. But now they have a new technique:
searching for the bow shocks the stars
create in interstellar gas and dust as they
whiz through this material.
The star Zeta (ζ) Ophiuchi inspired
the new approach. It's a hot, massive O
star with a relative motion fast enough
- roughly 24 km/s - to be "supersonic."
As this massive star plows through
space, astronomers discovered, its strong
outﬂowing wind causes interstellar gas
and dust to stack up in front of it, like the
shock front of air that piles up ahead of
a high-performance jet. This arc-shaped
gas compresses, heats up, and shines
with infrared light, emission that reveals
both the mass and velocity of the star.
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April 2016 sky & telescope

With Zeta Ophiuchi's example in
hand, Grace Olivier (Case Western
Reserve University) and colleagues
decided to look for bow shocks in order
to search for runaway stars. They turned
to archival infrared data from the Spitzer
and WISE space observatories and found
more than 200 images of fuzzy red arcs.
They then used the Wyoming Infrared
Observatory's 2.3-meter telescope to look
for the culprits behind 80 of them.
The team was surprised to ﬁnd that
"more than 95% of these bow shocks
have a hot, massive, runaway candidate
at their center," team member William
Chick (University of Wyoming) said during a press conference January 5th at the
American Astronomical Society meeting
in Kissimmee, Florida. "It may be that
our Milky Way is swarming with these
hot runaway stars."
The team plans to extend its search
to include the entire galactic plane. With
more runaways spotted, the researchers
will be able to trace the stars' motions
backward in order to ﬁnd the source of
their accelerating kicks.
■ SHANNON HALL

MISSIONS I China's Dark
Matter Probe in Orbit
On December 17th, China launched its
ﬁrst astrophysics payload. The Dark Matter Particle Explorer (DAMPE) will detect
energetic particles known as cosmic
rays, as well as gamma-ray photons from
supernovae, pulsars, and other astrophysical sources - including, perhaps,
elusive dark matter.
DAMPE is a collaboration among
universities in Switzerland and Italy and
the Chinese Academy of Sciences. (NASA
is restricted from trading technology with
China due to the 1976 International Trade
in Arms Regulations, a key reason why
China isn't a partner on the International
Space Station.)
DAMPE joins the Alpha Magnetic Spectrometer and the Calorimetric Electron
Telescope, both aboard the International
Space Station, in taking the dark matter
hunt to space. It's the ﬁrst of a ﬁve-mission
series, with the next two payloads - a
quantum-communications satellite and an
X-ray telescope - set to launch this year.
■ DAVID DICKINSON

BLACK HOLES I
Magnetic Fields Near
the Event Horizon
Astronomers have detected magnetic
ﬁelds dancing in the skirts of the Milky
Way's central black hole. The work is part
of the Event Horizon Telescope (S&T:
(
Feb.
2012, p. 20), a planet-spanning network of
radio antennas aiming to "see" a supermassive black hole - or, rather, its telltale
silhouette amid the glow of hot gas surrounding it like a superheated tutu.
Michael Johnson (Harvard-Smithsonian
Center for Astrophysics) and other members of the EHT collaboration used telescopes in Hawai'i, California, and Arizona
to peer into the inner sanctum of our
galaxy's black hole, Sagittarius A*. With
only three sites and a few nights' worth of
observations, they couldn't reconstruct a
complete image - instead of seeing the

Sky and Telescope - April 2016

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