Sky & Telescope - April 2022 - 12

NEWS NOTES
STARS
Stellar Remnant at the
Heart of " The Cow "
A TITANIC EXPLOSION in a remote
galaxy that occurred in June 2018 was
most likely the birth cry of a neutron
star or a black hole, according to a
detailed analysis of high-energy X-rays
captured from the fading blast.
Discovered by the Asteroid Terrestrial-impact
Last Alert System (ATLAS)
and cataloged as AT2018cow, the explosion
occurred in a spiral arm of the
inconspicuous galaxy CGCG 137-068,
which is 200 million light-years away in
Hercules. But this was no regular supernova:
" The Cow, " as it was nicknamed,
brightened much faster and was much
bluer and more luminous than any
exploding star observed before.
Astronomers have found similar
" fast blue optical transients " (FBOTs) in
archival data, but they saw the Cow in
real time. It was by far the most luminous
FBOT observed.
On December 13th in Nature Astronomy,
a team led by Dheeraj Pasham
(MIT) argued that a compact object lies
at the core of the blast. Pasham and
colleagues analyzed two months' worth
of high-energy X-ray data collected by
NASA's Neutron Star Interior Composition
Explorer (NICER) X-ray telescope
aboard the International Space Station.
The astronomers found a rapid flickering
(224 cycles per second), from which
they estimated that the size of the X-ray
emitting region is no larger than 1,300
kilometers (810 miles) across.
This size rules out an extended
gaseous envelope, as well as a black
hole more massive than 850 Suns. The
slightly irregular flickering likely arose
from a rapidly rotating accretion disk,
though it's unclear whether the material
orbits a neutron star or black hole.
" The authors have done a thorough
job exploring different options, " says
EXOPLANETS
Giant Planet Imaged Around Massive Stars
Chryssa Kouveliotou (George Washington
University), who was not involved
in the study. " I think it is still too early
to call what the spinning object is. We
probably need to observe more such
systems to make a final decision. "
¢GOVERT SCHILLING
The labeled planet at lower right, designated b Centauri
(AB)b, orbits its stars 100 times farther out than
Jupiter circles the Sun, at 560 astronomical units. The
dot at upper right is a background star.
b
ASTRONOMERS HAVE IMAGED a
giant planet around a massive pair of
stars collectively known as b Centauri.
The discovery in the December
9th Nature challenges our notions of
how planets form.
Markus Janson (Stockholm University)
and colleagues saw the planet
in observations taken in 2019 and
2021 as part of the B-star Exoplanet
Abundance Study (BEAST). The real
kicker, though, was an observation
taken two decades earlier as part of a
different project. Including the latter
observation enabled the researchers to
confirm that the planet is both moving
with the b Centauri system and
orbiting its central stars. The stars
together are up to 10 times the mass
of the Sun, making this system the
most massive known to host planets.
The planet, dubbed b Centauri
pAn artist's impression of the mysterious burst
dubbed AT2018cow
12 APRIL 2022 * SKY & TELESCOPE
(AB)b to denote that it circles both
stars, has one of the widest known
orbits, comparable to that of Sedna
in the outer solar system. But the
planet's mass, deduced from the
glow leftover from its formation, is
between 9.3 and 12.5 Jupiters - over
a million times more massive than
the far-out Kuiper Belt object.
The mass-orbit juxtaposition puts
planet formation scenarios to the
test. Astronomers think giant planets
form via core accretion, in which
smaller grains stick together, merging
their way up to planet sizes before
attracting a gaseous outer layer.
But if a planet forms too far
out, there won't be much gas for
it to attract. This is especially true
around massive stars, whose intense
radiation quickly dissipates gas. Then
again, since disks disperse from the
inside out, perhaps a wide-orbiting
planet has more time to grow.
It's also possible that material collapsed
suddenly in a process known
as gravitational instability.
Previous studies of less massive
stars have shown that bigger stars
tend to host bigger planets in wider
orbits. However, exoplanet expert
Joshua Winn (Princeton) cautions
that we need more exoplanets orbiting
truly massive stars to draw firm
conclusions.
¢MONICA YOUNG
STAR AND PLANET: ESO / JANSON ET AL. / NATURE 2021; EXPLODING STAR: NATIONAL
ASTRONOMICAL OBSERVATORY OF JAPAN
Sky & Telescope - April 2022

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