Sky & Telescope - December 2018 - 13

STARS

Auroras Discovered Around Rogue Brown Dwarf
mechanism works
at Jupiter but with
particles coming from
around a set of brown
the planet's moons
dwarfs - including a
rather than the Sun.
"rogue" that wanders
Since SIMP 0136+0933
the galaxy alone -
has no host star, its
indicating surprisingly
aurorae can't come
strong magnetic ﬁelds
from interactions with
in these failed stars.
a stellar wind. PerMelodie Kao (Arihaps it hosts a moon
zona State University)
of its own, but before
and colleagues used
p Artist's conception of the rogue
scientists can come to
the Karl G. Jansky
SIMP 0136+0933
that conclusion, more
Very Large Array to
observations are needed to conﬁrm the
detect high-frequency radio emisorigin of its radio waves.
sions from ﬁve nearby brown dwarfs,
The sheer intensity of the dwarfs'
with masses between 12 and 30 times
radio emissions poses its own puzzle,
the mass of Jupiter. Among these is
presenting "huge challenges to our
SIMP 0136+0933, a possible planet
understanding of the dynamo mechawith 13 times Jupiter's mass, ﬂoating
nism that produces the magnetic ﬁelds
on its own in interstellar space about
in brown dwarfs and exoplanets," says
20 light-years away. The emissions
indicate that the ﬁve objects' magnetic coauthor Gregg Hallinan (Caltech).
When Kao and her team closely
ﬁelds are hundreds to thousands of
examined SIMP 0136+0933 and the
times stronger than those around the
other brown dwarfs, they found they
Sun, powering brilliant, radio-emitcouldn't easily relate their magnetic
ting aurorae similar to the northern
ﬁeld strengths to the objects' masses,
lights on Earth. The results appear
temperatures, or ages. This implies
in the August Astrophysical Journal
that the generation of strong magnetic
Supplement Series.
ﬁelds in these objects is probably much
Aurorae happen when charged
different than the mechanisms that
particles from the Sun stream in
create ﬁelds in the Sun or Earth.
along Earth's magnetic ﬁeld and crash
into our upper atmosphere. A similar
■ JOHN BOCHANSKI

CA LTECH / CHUCK CA RTER / NR AO / AUI / NSF

ASTRONOMERS HAVE
DISCOVERED auroras

But not all black holes are active. It
could be that huge numbers of massstarved black holes, silent in X-rays,
lurk undetected in other dwarfs; thus
the result can't rule out the stellarseed scenario just yet.
Igor Chilingarian (Smithsonian
Astrophysical Observatory) and colleagues added to the debate when they
used X-ray observations to conﬁrm 10
black holes (out of a larger sample of
305 candidates) with between 36,000
and 370,000 Suns' worth of mass in
nearby dwarf galaxies. Their results
appear in the August 10th Astrophysi-

cal Journal. Chilingarian contends
that the mere existence of black
holes on the lower end of this mass
spectrum favors the stellar-seed scenario. Mezcua counters that directcollapse models are ﬂexible enough
to allow for masses that small.
Ultimately, only knowing the
true number of black holes in dwarf
galaxies will settle the debate. But
in the meantime, the trove of new
middleweight black holes will enable
astronomers to investigate trends in
their growth and evolution.
■ MONICA YOUNG

IN BRIEF
Upgrade for Arecibo
Observatory
The iconic Arecibo Observatory has received
a recent boost in funding - $5.8 million from
the National Science Foundation - to help
design and build a new receiver, which will be
installed in 2022. Called a phased-array feed,
the receiver consists of 166 antennas that
will increase the telescope's field of view and
boost its survey speed, making it five times
faster than it is now. A major goal for the new
instrument is to find and monitor millisecond
pulsars, whose minute variations help measure the low-frequency hum of gravitational
waves suffusing the universe. Find more on
how the observatory fared post-hurricane at
https://is.gd/AreciboUpgrade.
■ ELIZABETH HOWELL

AAS Opens Membership
to Amateurs
The American Astronomical Society (AAS) began as an organization of both professionals
and amateur astronomers. Now, the society is
returning to that foundation by opening membership to amateurs via its Amateur Affiliate
Program. Dues are $52 for 2019; inaugural
benefits will include reduced registration fees
for AAS meetings and access to AAS journals.
Additional opportunities are expected once a
critical mass is established. To learn more and
join, go to https://is.gd/AASamateur.
■ MONICA YOUNG

Impact Shaped Ice Giant
Scientists have long known that a long-ago
impact likely caused Uranus's strange tilt.
Jacob Kegerreis (Durham University, UK) and
colleagues explore this concept further by
simulating more than 50 impact scenarios. In
the July 1st Astrophysical Journal, the team
concludes that 4 billion years ago, a rockand-ice protoplanet with at least twice Earth's
mass collided with Uranus and knocked it
over. But the planet held on to most of its
gaseous atmosphere; what little gas it lost
might have helped form the thin rings that
encircle the planet today. The ancient collision
helps explain why the ice giant's atmosphere
is so incredibly cold (57 K, or −357°F), even
for being so far out from the Sun: The simulations suggest that the impactor's debris
formed a thin shell of ice within Uranus that
traps heat emanating from the planet's core.
The impact could also explain the planet's
tilted and off-center magnetic field. Read
more at https://is.gd/icegiantimpact.
■ MONICA YOUNG
sk yandtelescope.com * DECE M BE R 2 018

https://www.is.gd/AreciboUpgrade https://www.is.gd/AASamateur https://www.is.gd/icegiantimpact http://www.skyandtelescope.com

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018