Sky & Telescope - December 2023 - 11

IN BRIEF
ALMA Reveals Planet
Beginnings
STARS
Strange Star Is a Powerful Magnet
THERE'S AN ODD STAR 3,300 lightyears
away in the southern constellation
of Monoceros, the Unicorn.
The star, known as HD 45166,
appears to be in the Wolf-Rayet class.
These helium-dominated stars blow
away violent winds, leaving a distinctive
signature in the star's spectrum.
However, while most Wolf-Rayet stars
hold more than eight Suns' worth of
mass, this one has much less than that.
Its low mass could be explained if this
peculiar object formed via a collision of
two lower-mass stars. But if that were
the case, it shouldn't be able to make
such a strong wind - and its spectrum
says it is.
Tomer Shenar (University of Amsterdam)
and colleagues used new observations
as well decades of older data from
a plethora of telescopes to examine the
star's magnetic field, publishing the
results in the August 17th Science.
Most stars actually aren't all that
magnetic - an average area on the Sun,
for example, has a field of only about
1 Gauss, about twice the strength of
Earth's surface field. A tenth of massive
stars are more magnetic, though, and
these are likely the progenitors of the
extreme magnetism found in magnetars,
neutron stars with fields 100 trillion
times stronger than the Sun's.
pThis artist impression shows the powerfully
magnetic star HD 45166. The magnetic field
traps particles blowing away from the star,
enshrouding the star in a gaseous shell.
Shenar's team thus used an instrument
on the Canada-France-Hawaii
Telescope to look for a magnetic field's
chemical fingerprints in the spectrum
of HD 45166. And they found it - a
field of a whopping 43,000 Gauss.
" One-thousand-Gauss magnetic fields
are pretty rare amongst massive stars,
so 43,000 Gauss is really off the chart, "
says Paul Crowther (University of Sheffield,
UK), who was not involved in the
Science study.
The star's powerful magnetic field as
well as other data point to its origin in
a stellar merger. The strong field now
traps outflowing material, making its
weak wind appear stronger than it is.
" This paper resolves a long-standing
puzzle about HD 45166, since it has
been recognized as an oddity amongst
Wolf-Rayet stars, " Crowther says.
Eventually, this star will collapse into
a neutron star, and its magnetic field
will strengthen as it does so. The star
may then become a magnetar. However,
Crowther isn't sure of that destiny:
" Lots of complex physics happens during
a supernova explosion, " he says.
¢MONICA YOUNG
In 2014, the young star V960 Monocerotis
began flaring, lighting up unexpected
origins for the planets coming together
around it. Theoretical calculations have
favored the core accretion scenario for
planet formation: Material gloms together
into a core, which then gathers gas around
it. The process takes tens of millions of
years. However, observations of V960
Mon instead favor gravitational instability,
in which disk material collapses suddenly,
forming gas giants in mere thousands of
years. Philipp Weber (University of Santiago,
Chile) and colleagues dug into the
archive of the Atacama Large Millimeter/
submillimeter Array (ALMA), using ALMA's
longer wavelengths to probe denser areas
within the spiral arms in the dust that surrounds
the young star. The ALMA images
show that within the spiral arms, material
is already collapsing into planet-mass
clumps. " No one had ever seen a real
observation of gravitational instability happening
at planetary scales - until now, "
says Weber, who published the results in
the July 20th Astrophysical Journal Letters.
¢MONICA YOUNG
Young Warm Neptune
Losing Its Air
AU Microscopii b is a Neptune-size planet
that orbits its 23 million-year-old red dwarf
star every 8.5 days. That's searingly
close, so it's unsurprising that Hubble
Space Telescope data show the planet
is enveloped in a cloud of escaping gas.
The surprise is that this cloud is only seen
some of the time. Thanks to the planet's
transits, astronomers can detect the spectroscopic
signature of escaping neutral
hydrogen gas; however, the first Hubble
view of AU Mic b showed no escaping
gas. Then, a year and a half later, Hubble
caught a large amount of hydrogen escaping,
with the gas leading the planet in its
orbit. The contrasting observations might
indicate that the star's winds and flares are
shaping and altering the planetary outflow.
Regardless, it is likely that this gradual, albeit
highly variable loss of atmosphere will
ultimately leave only a barren core behind.
Keighley Rockcliffe (Dartmouth College) led
the team that published the results in the
August 1st Astronomical Journal.
¢MONICA YOUNG
sk yand tele scope .o r g * DECEMBER 2023 11
ESO / L. CALÇADA
https://skyandtelescope.org/
Sky & Telescope - December 2023

Table of Contents for the Digital Edition of Sky & Telescope - December 2023
