Sky and Telescope - January 2017 - 18

NEWS NOTES
METEORITES
t An excavation crew pulls "Gancedo,"
estimated to weigh 30 tons, from a known
meteorite strewnfield in Argentina.

Huge "Gancedo"
Found in Argentina
HOLY SPUTTERING SPACE ROCKS!

A team in Argentina has excavated a
monster: a 30-ton chunk of iron-nickel
meteorite. For comparison, the legal
maximum weight for an 18-wheeler in
the United States is 40 tons.
Named "Gancedo" after a nearby
town, the rock was found in the heart of
Campo del Cielo ("Field of Heaven" in

Spanish), a meteorite strewnfield northwest of Buenos Aires whose known
fragments have a total mass of more
than 100 tons. A team from the local
Astronomy Association of Chaco dug
the huge space rock out of the ground
on September 10th, and images of the
find soon flooded the internet.
Gancedo fell to Earth between 4,000
and 6,000 years ago. Locals have known
of the meteorites for centuries, even
making iron tools from bits found in
the strewnfield, which extends over
an ellipse 3 km wide by 19 km long.
Meteorites from here have a coarse,
polycrystalline texture characteristic
of iron-nickel meteorites. They are also
unusually pure, consisting of 93% iron.
Most of the remaining 7% is nickel.

It'll take time to weigh Gancedo
more precisely, but the rock is most
likely the second-largest meteorite
recovered from Campo del Cielo, after
the 37-ton "El Chaco" uncovered in
1980. The title of biggest space rock still
safely belongs to the Hoba meteorite
in Namibia, estimated to weigh more
than 60 tons. It's still half buried at its
discovery site.
■ DAVID DICKINSON

Astronomy Association of Chaco

Tabby's Star Gets Weirder
KIC 8462852 , or Tabby's Star as it's

informally known, made headlines
in 2015 when Tabetha Boyajian (now
Louisiana State University) announced
the discovery of strange dips in the
star's brightness that defied explanation
(S&T: Feb. 2016, p. 14). The mysterious
behavior even prompted speculations
of an alien megastructure blocking the
star's light (S&T: Mar. 2016, p. 16).
The controversy is now building:
independent investigations suggest that
the star has been fading for decades -
why, no one knows.
Volunteers with the Planet Hunters
18

J A N U A R Y 2 0 1 7 * SK Y & TELESCOPE

citizen science project first spotted KIC
8462852's odd behavior while looking for dips in starlight created by a
transiting exoplanet. Typical planetary
transits reduce starlight periodically, by
up to a few percent for a few hours. But
this star was seen to dim irregularly
for days at a time by varying amounts,
even up to 20%.
Yet KIC 8462852 seems to be a normal, 12th-magnitude F-class star. No
planet, dusty disk, or pulsations could
explain the signal. The scenario that
Boyajian's team settled on for publication in 2015 was a comet swarm in the
midst of dissolution. But both she and
others readily admitted the scenario
was contrived.
Soon after, Bradley Schaefer (also
Louisiana State University) sifted
through the glass-plate archive at
Harvard College Observatory and found
that the star has faded over the past 100
years, at a rate of 0.16 magnitude per
century. Heated debate ensued.
Now, Benjamin Montet (Caltech)
and Joshua Simon (Observatories of the
Carnegie Institution of Washington)

have used Kepler observations to confirm
KIC 8462852's fade - but it's dropping
faster than anyone thought.
For the first three years of Kepler
data, the star faded 0.3% per year,
equivalent to 0.37 magnitude per
century and more than double what
Schaefer found. Then, for almost seven
months, the star suddenly began dimming at an accelerated 2.5% per year.
The sudden decline came to a halt
before Kepler's initial mission ended in
2013, but it's unclear if the light curve
flattened out or just returned to its previous, less extraordinary fade.
Appearing in an upcoming Astrophysical Journal Letters, these results
support Schaefer's assertion that the
star is in long-term decline, though
Montet and Simon can't directly confirm the century-long trend because
Kepler's observations are limited to four
years. The study also puts the "comet
swarm" scenario to rest.
"Honestly, I'm all out of good ideas
- all of us are," Boyajian says. "We just
need more data at this point!"
■ MONICA YOUNG

ME TEORITES: MINISTERIO DE GOBIER NO, JUSTICIA Y REL ACIÓN CON L A CO M UNIDA D (2); STELL A R: ESO / L. CA LÇA DA

STELLAR



Sky and Telescope - January 2017

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