Sky & Telescope - August 2020 - 10

NEWS NOTES

The Origins of
Interstellar Objects

p An artist's impression of the interstellar
object Comet 2I/Borisov

STUDIES OF THE FIRST TWO interstellar visitors to the solar system -
1I/'Oumuamua and 2I/Borisov - are
shedding light on the objects' origins.
An abundance of primordial carbon
monoxide from Comet Borisov suggests
that it formed in the frosty outer fringes
of its parent star's planetary disk. Sepa-

rately, a new theoretical model shows
that the fragmentation of a larger body
could have formed 'Oumuamua.
Two studies published April 20th
in Nature Astronomy measured Comet
Borisov's composition soon after
perihelion last December and found an
unexpected overabundance of carbon
monoxide (CO) compared to water
molecules. Dennis Bodewits (Auburn
University) and colleagues observed
Comet Borisov using Hubble and the
Neil Gehrels Swift Observatory, while
Martin Cordiner (NASA Goddard Space
Flight Center) and colleagues employed
the Atacama Large Millimeter/submillimeter Array (ALMA). Both studies
showed the ratio of CO to water to be
higher than in any comet observed in
the inner solar system. The high CO
levels indicate that Comet Borisov likely
formed beyond the snow line, where
CO turns to ice, in the outer part of a
planetary disk.
The origins of 1I/'Oumuamua are
more difﬁcult to explain (S&T: Oct.
2018, p. 20). This object, now out
of view, was elongated, porous, and
tumbled erratically. Not only that, it

MILKY WAY

Is Our Galaxy Throwing Out Star-Making Material?
A SUBSTANTIAL FRACTION of the
smattering of stars dwelling in the
Milky Way's halo may have an unexpected origin: the galaxy itself. Sijie Yu
(University of California, Irvine) and
colleagues published remarkably realistic simulations supporting this scenario
in the Monthly Notices of the Royal
Astronomical Society.
Most of the Milky Way's stars circle
within the disk and central bulge that
make up the most visible portion of our
galaxy. All around this disk, halo stars
also orbit the center within a roughly
sphere-shape volume, traveling on
highly elongated orbits toward or away
from the galactic center.
Astronomers have long thought that
most of these stars come from dwarf
galaxies falling into the Milky Way

AUGUST 2 02 0 * SK Y & TELESCOPE

(S&T: Apr. 2017, p. 22). Stars may also
form within the halo as infalling gas
clouds compress. What Yu and colleagues propose, though, is the opposite:
They suggest stars are forming in gas
clouds thrown out of the Milky Way.
Yu studied six simulated Milky Way-
like galaxies, part of the so-called "Latte
Suite" from the Feedback in Realistic
Environments 2 simulation. FIRE-2
reproduces the growth and evolution
of galaxies in unprecedented detail,
accounting for both cosmology and
local feedback effects.
Tracking the six simulated galaxies,
the researchers traced individual stars
back in time, ﬁnding that between 5%
and 40% of the halo stars originated in
gas ﬂung out of the galaxy's center. In
the simulations, large outﬂows arising

didn't slow down as much as expected
as it moved away from the Sun, yet it
released only wisps of gas.
Now, Yun Zhang (Chinese Academy
of Sciences) thinks she has a model that
can explain 'Oumuamua's odd properties. Zhang and Douglas Lin (Tsinghua
University, China) describe their idea
April 13th in Nature Astronomy.
During planet formation, a rubblepile comet or asteroid might pass very
near its young host star, close enough
for strong gravitational effects to
stretch the rubble pile until it disrupts.
The star's heat vaporizes volatile ices
on the surface of the elongated fragments, accounting for 'Oumuamua's
lack of comet-like activity. However,
volatiles stored deeper down, such
as water, wouldn't vaporize until the
object reached the inner solar system,
accounting for 'Oumuamua's non-gravitational acceleration.
Not only could this scenario explain
'Oumuamua's odd properties, the
researchers say the escape of such fragments could be a common occurrence
in our galaxy.
■ JEFF HECHT

from rounds of supernova explosions
collapse into stars on their way out to a
galaxy's periphery.
While astronomers have seen clear
evidence of galactic winds in numerous
other galaxies, such as M82, similar
signatures in the Milky Way remain
ambiguous (S&T: Apr. 2014, p. 26). Yu
sees subtler evidence of star-forming
outﬂows in data from the European
Space Agency's Gaia satellite combined
with maps of stellar chemical abundances. But there's no smoking gun yet.
Amina Helmi (University of Groningen, The Netherlands), who was not
involved in this study, says, "I think we
need more data to be able to identify
such stars, particularly chemical abundances of distant halo stars." Nevertheless, with predictive simulations such as
these in hand, astronomers will know
what to look for.
■ MONICA YOUNG

NR AO / AUI / NSF / S. DAG NELLO

SOLAR SYSTEM

Sky & Telescope - August 2020

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