Devourer of planets? Researchers dub star ‘Kronos’

Sun-like star Kronos shows signs of having ingested 15
Earth masses worth of rocky planets, prompting Princeton
astronomers to nickname it for the Titan who ate his young.
This artist’s rendering of the diverse rocky planets in our
galaxy hints at what Kronos’s planets might have looked like
before the star enveloped them. Credit: NASA/JPL-Caltech/R.
Hurt (SSC-Caltech)

In mythology, the Titan Kronos devoured his children,
including Poseidon (better known as the planet Neptune),
Hades (Pluto) and three daughters.

So when a group of Princeton astronomers discovered twin
, one of which showed signs of having
ingested a dozen or more rocky planets, they named them after
Kronos and his lesser-known brother Krios. Their official
designations are HD 240430 and HD 240429, and they are both
about 350 light years from Earth.

The keys to the discovery were first confirming that the widely
separated pair are in fact a binary pair, and secondly
observing Kronos’ strikingly unusual chemical abundance
pattern, explained Semyeong Oh, a graduate student in
astrophysical sciences who is lead author on a new paper
describing Kronos and Krios. Oh works with David Spergel, the
Charles A. Young Professor of Astronomy on the Class of 1897
Foundation and director of the Flatiron Institute’s Center for
Computational Astrophysics.

Other co-moving star pairs have had different chemistries, Oh
explained, but none as dramatic as Kronos and Krios.

Most stars that are as metal-rich as Kronos “have all the other
elements enhanced at a similar level,” she said, “whereas
Kronos has volatile elements suppressed, which makes it really
weird in the general context of stellar abundance patterns.”

In other words, Kronos had an unusually high level of
rock-forming minerals, including magnesium, aluminum, silicon,
iron, chromium and yttrium, without an equally high level of
volatile compounds—those that are most often found in gas form,
like oxygen, carbon, nitrogen and potassium.

Kronos is already outside the galactic norm, said Oh, and in
addition, “because it has a stellar companion to compare it to,
it makes the case a little stronger.”

Kronos and Krios are far enough apart that some astronomers
have questioned whether the two were in fact a binary pair.
Both are about 4 billion years old, and like our own, slightly
older sun, both are yellow G-type stars. They orbit each other
infrequently, on the order of every 10,000 years or so. An
earlier researcher, Jean-Louis Halbwachs of the Observatoire
Astronomique of Strasbourg, had identified them as
co-moving—moving together—in his 1986 survey, but Oh
independently identified them as co-moving based on
two-dimensional astrometric information from the European Space
Agency’s Gaia mission.

During a group research discussion at the Flatiron Institute, a
colleague suggested pooling their data sets. John Brewer, a
postdoctoral researcher from Yale University visiting at
Columbia University, had been using data from the Keck
Observatory on Mauna Kea, Hawaii, to calculate the
spectrographic chemistries and radial velocities of stars.

“John suggested that maybe we should cross-match my co-moving
catalogue with his chemical-abundance catalogue, because it’s
interesting to ask whether they have the same compositions,” Oh
said.

Binary stars should have matching radial velocities, but that
information hadn’t been available in the Gaia dataset, so
seeing their matching velocities in Brewer’s data supported the
theory that Kronos and Krios, though two light years apart,
were a binary set.

Then the researchers noticed the extreme chemical differences
between them.

Stars HD 240430 and HD 240429, better known as Kronos and
Krios, as they appear in the Space Telescope Science Institute’s
Digitized Sky Survey. Though these binary stars formed together,
their chemical abundances are very different, leading researchers
to conclude that Kronos had absorbed 15 Earth masses worth of
rocky planets. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

“I’m very easily excitable, so as soon as they had the same
and different chemistry,
my mind already started racing,” said Adrian Price-Whelan, a
Lyman Spitzer, Jr. Postdoctoral Fellow in Astrophysical
Sciences and a co-author on the paper.

Oh took more convincing, both scientists recalled. “Semyeong is
careful and was skeptical,” said Price-Whelan, so her first
step was to double-check all the data. Once simple error had
been ruled out, they began entertaining various theories. Maybe
Kronos and Krios had accreted their planetary disks at
different times during stellar formation. That one can’t be
tested, said Price-Whelan, but it seems unlikely.

Maybe they only started moving together more recently, after
trading partners with another pair of binary stars, a process
known as binary exchange. Oh ruled that out with “a simple
calculation,” she said. “She’s very modest,” Price-Whelan
noted.

Oh’s skepticism was finally overcome when she plotted the
chemical abundance pattern as a function of condensation
temperature—the temperatures at which volatiles condense into
solids. Condensation temperatures play a key role in planetary
formation because tend to form where it’s warm—closer
to a star—while gas giants form more easily in the colder
regions far from their star.

She immediately observed that all of the minerals that solidify
below 1200 Kelvin were the ones Kronos was low in, while all
the minerals that solidify at warmer temperatures were
abundant.

“Other processes that change the abundance of elements
generically throughout the galaxy don’t give you a trend like
that,” said Price-Whelan. “They would selectively enhance
certain elements, and it would appear random if you plotted it
versus condensation temperatures. The fact that there’s a trend
there hinted towards something related to planet formation
rather than galactic chemical evolution.”

That was her “Aha!” moment, Oh said. “All of the elements that
would make up a rocky planet are exactly the elements that are
enhanced on Kronos, and the volatile elements are not enhanced,
so that provides a strong argument for a planet engulfment
scenario, instead of something else.”

Oh and her colleagues calculated that gaining this many
rock-forming minerals without many volatiles would require
engulfing roughly 15 Earth-mass planets.

Eating a gas giant wouldn’t give the same result, Price-Whelan
explained. Jupiter, for example, has an inner rocky core that
could easily have 15 Earth masses of rocky material, but “if
you were to take Jupiter and throw it into a star, Jupiter also
has this huge gaseous envelope, so you’d also enhance carbon,
nitrogen—the volatiles that Semyeong mentioned,” he said. “To
flip it around, you have to throw in a bunch of smaller
planets.”

While no known star has 15 Earth-sized planets in orbit around
it, the Kepler space telescope has detected many multi-planet
systems, said Jessie Christiansen, an astronomer at the NASA
Exoplanet Science Institute at the California Institute of
Technology, who was not involved in the research. “I see no
problem with there being more than 15 Earth masses of
accretable material around a solar-type star.” She pointed to
Kepler-11, which has more than 22 Earth masses of material in
six planets with close orbits, or HD 219134, which has at least
15 Earth masses of material in its inner four planets.

“At the moment, we are still at the stage of piecing together
different observations to determine how and when exoplanets
form,” said Christiansen. “It’s difficult to directly observe
planet formation around young stars—they are typically shrouded
in dust, and the stars themselves are very active, which makes
it hard to disentangle any signals from the . So we have to infer what we can from the
limited information we have. If borne out, this new window onto
the masses and compositions of the material in the early stages
of planetary systems may provide crucial constraints for
theories.”

The research also has implication for stellar formation models,
noted Price-Whelan.

“One of the common assumptions—well-motivated, but it is an
assumption—that’s pervasive through galactic astronomy right
now is that stars are born with [chemical] abundances, and they
then keep those abundances,” he said. “This is an indication
that, at least in some cases, that is catastrophically false.”

Explore further:
What
kinds of stars form rocky planets?

Provided by: Princeton
University

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