Sky & Telescope - March 2023 - 10

NASA / ESA / JOSEPH OLMSTED (STSCI)
NEWS NOTES
This artist's illustration
shows a white dwarf star
accreting planetary debris.
GALAXIES
M77: A Galactic Particle
Accelerator
ASTRONOMERS HAVE TRACED dozens
of neutrinos back to the nearby galaxy
NGC 1068 (M77), only the second individual
source known to produce copious
amounts of the uncharged, almost
massless particle.
Traveling at near-light speed, neuSTARS
Ancient
White Dwarf Hosts Planetary Graveyard
THE DEATH THROES of other suns
can quite literally be the end of the
world for their planets, as they are
vaporized or placed on infalling
orbits. Now, scientists report in the
December Monthly Notices of the Royal
Astronomical Society an analysis of one
of the oldest of these planetary graveyards:
a red-hued white dwarf about
91 light-years away that's polluted
with planetary debris.
The European Space Agency's Gaia
spacecraft first identified this white
dwarf, WD J2147−4035, as having
unusual properties. Abbigail Elms
(University of Warwick, UK) analyzed
this and another unusual one,
WD J1922+0233, which is bluer and
a bit farther away at 128 light-years.
Both stellar remnants were already
known to be contaminated and cool.
To take a closer look at both of
the white dwarfs' composition and
temperature, Elms and colleagues analyzed
spectra from the Very Large Telescope
as well as brightness measurements
from the Dark Energy Camera
at the Cerro Tololo Inter-American
Observatory, both in Chile.
" WD J2147−4035 is the reddest and
faintest [polluted] white dwarf discovered
in our galaxy, " says Elms. It's also
10 MARCH 2023 * SKY & TELESCOPE
the coolest polluted white dwarf yet
identified. While white dwarfs start
out blazing hot, they can only cool
over time, and Elms' team estimates
that the reddish white dwarf has been
cooling for more than 10 billion years.
The bluer white dwarf is the secondcoolest
polluted white dwarf known,
and Elms' team estimates it's 9 billion
years old.
There are only a handful of polluted
white dwarfs anywhere near as
cool, says Siyi Xu (Gemini Observatory),
who wasn't involved in the
study. But she adds that uncertainties
involved in calculating the age from a
white dwarf's temperature mean that
it's hard to say with certainty whether
the objects Elms' team described are
for sure the oldest known.
Observing white dwarfs consuming
their ancient planetary systems
provides a rare chance to see what the
small bodies were made of. Elms' team
found that, while the debris falling
onto the bluer star has a composition
similar to that of the Earth's continental
crust, the redder one's rubble has a
makeup that's harder to explain, with
unusually high amounts of potassium
and lithium.
¢ELISE CUTTS
trinos pass through almost everything
in their way, only rarely interacting
with other particles. It takes a collection
of more than 5,000 light-sensitive
detectors buried in one cubic kilometer
of Antarctic ice - the IceCube Neutrino
Observatory - to register the
tiny flashes that mark speeding muons
(heavy electrons), produced in turn by
neutrino interactions.
As reported in the November 4th
Science, the IceCube Collaboration
collected 79 high-energy neutrinos
between 2011 and 2020 from the direction
of M77. The high-energy neutrinos
were likely a byproduct of particle collisions
near the supermassive black hole
that sits in the galaxy's core.
Four years ago, IceCube identified the
first known source of cosmic neutrinos:
TXS 0506+056 (S&T: Nov. 2018, p. 11).
This active galaxy is known as a blazar
because its central black hole is pointing
one of its plasma jets toward Earth.
More recently, scientists have reported a
more general link between the distribution
of distant blazars on the sky and
the arrival directions of high-energy
neutrinos (https://is.gd/blazars).
But the new discovery suggests
that blazars aren't the only neutrino
factories out there. Indeed, we need
more than blazars to explain all the
high-energy neutrinos IceCube detects,
writes Kohta Murase (Penn State) in an
accompanying commentary in Science.
Future observations, in particular
from Europe's Cubic Kilometre Neutrino
Telescope (KM3NeT) that's under
construction on the bottom of the Mediterranean
Sea, will probably clinch the
case; it will have a much better directional
sensitivity than IceCube.
¢GOVERT SCHILLING
https://www.is.gd/blazars
Sky & Telescope - March 2023

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