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A team of scientists recently discovered
a star that appears to have consumed
its own planets like some sort of infanticidal titan
from Greek mythology.
It's the story of HD240430 and HD240429,
or, as the authors of Oh et al.
2017 have aptly named them, Kronos and Krios.
It's also what we'll be discussing on today's "Space
Time" Journal Club.
[FUTURISTIC MUSIC]
Our story begins where every story
begins, in the heart of a vast cloud of gas and dust
drifting through interstellar space.
It's here that stars are born.
Now, we've talked about this process before.
So in short, essentially all stars
form in these stellar nurseries.
And they form in the many thousands
before being ejected from their birth clusters
to wander the galaxy.
Our sun wanders alone.
And we have no idea where its sibling stars might be.
However, if the gravitational connection
between a pair of stars is strong enough,
they might be ejected from the cluster as a binary pair.
And this is pretty common, actually.
Around half of all stars are binaries.
Binary stars typically have the same chemical composition
as each other, having formed from the same cloud.
They might be a close binary, whizzing around each other
with periods as short as days or even hours.
Or they might be a wide binary, separated
by up to a few light-years and just barely gravitationally
bound.
These wide binaries are actually extremely useful tools
for understanding star formation and the environment
of the galaxy they live in.
They're so loosely bound that even
a small gravitational nudge can break apart
the pair, for example, by interacting
with other massive objects.
So by counting the number of wide binaries
of different separations, we can fine-tune
our theoretical models of the galactic mass distribution,
including the numbers of near-invisible stellar objects
like black holes and neutron stars,
as well as the distribution of gas and dark matter.
They also allow us to test to what
degree stars that formed in the same cloud
share a chemical signature.
If we understand this chemical tagging, perhaps one day
we'll be able to locate the sun's lost siblings.
Wide binaries are useful, and they should also
be very abundant.
But they're hard to identify.
They're so, well, widely separated
that it's hard to tell if a given pair of stars
is actually gravitationally bound.
Semyeong Oh of Princeton University and collaborators
have taken up the hunt for these systems.
Their primary tool is Gaia, an orbiting telescope launched
by the European Space Agency in 2013 on a five-year mission
to measure the precise positions and distances of about
a billion objects in the Milky Way.
They identified plenty of stars that
are close enough together that they
could be gravitationally bound.
And they then made spectroscopic observations
to measure Doppler shift velocities.
That told them which pairs were actually traveling together
rather than passing each other by.
That same spectroscopy also measured chemical composition.
That's how the team came upon HD240430 and HD240429, Kronos
and Krios, two stars nearly two light-years apart
from each other, about 326 light-years
from our solar system.
They're both G-type stars like our sun.
And both are around four billion years old.
The stars' velocities tell us that they're moving in lockstep
together around the galaxy.
That means they're almost certainly a wide binary
pair with an orbital period of around 10,000 years.
So that means they should be made of the same stuff, right?
Well, the researchers checked their spectra,
and this is where things got weird.
Stellar spectra are thick, with sharp emission and absorption
features that result from electron transitions
in atoms in the star's atmosphere.
And the strength of these lines can tell us
the relative abundance of non-hydrogen and helium
elements within the star.
In astro-speak, they tell us the star's metallicity,
although for an astronomer, anything heavier than helium
is called a metal.
So it turns out that Kronos has a higher
metallicity than Krios.
And elements with high condensation temperatures,
like silicon and most actual metals,
were in extreme over-abundance in Kronos.
Low condensation temperature elements, so-called volatiles,
like carbon, nitrogen, or oxygen,
were only slightly more abundant.
Now, we'll get back to this, but it's important.
One volatile, lithium, does show up
in bizarrely high abundance in Kronos.
And that's also important, and we'll also get back to that.
These stars definitely formed from the same molecular cloud.
It's incredibly unlikely that they would end up
tracking each other's motion so closely unless they
were ejected from their birth cluster as a binary pair.
But if they formed from the same cloud,
they should be made of the same stuff, right?
So why are their chemical differences much higher than
ever observed for a wide binary pair?
Oh and collaborators have a story.
Just like its mythological namesake,
perhaps Kronos ate its children, perhaps the extra elements
of the vaporized remains of its own planetary system.
This is a dramatic, but actually plausible, story.
The extra elements found in Kronos
are pretty much exactly what you'd
expect from a star nomming on a bunch of terrestrial planets.
Terrestrial, or rocky, planets are
made of elements with high condensation temperatures.
This is the first stuff to come together
in the early days of an inner solar system
as things are cooling down.
Volatiles are less abundant in those inner rocky planets.
Because they condense at core temperatures,
they remain vaporized in the inner solar system,
but come together further out, giving us gas giants
and comets and stuff.
The researchers tested the hypothesis by throwing a bunch
of Earth-like planets into a sun-like star--
mathematically, I mean.
The Princeton research ethics policies
are strict about actual planetary destruction.
Oh et al. calculated that 15 Earth masses of raw Earth
material would produce the observed abundances
very nicely.
The other compelling bit of evidence
is Kronos's over-abundance of lithium.
Lithium gets depleted in the early years
of stars like the sun.
It gets dragged down to the core by convection,
where it's destroyed in a fusion reaction.
Kronos has an unreasonably high lithium abundance
for a star of its age, while Krios has the expected amount.
Even if Kronos and Krios didn't form together,
that lithium content suggests that Kronos accreted
a bunch of new material sometime well after its formation
and after the formation of any planetary system.
15 Earth masses would about do the trick there as well.
This is all good info to have.
We now know that binary stars can have very different
metallicities to each other.
And we'll need to take this into account for future searches.
So what caused Kronos to devour its worlds?
Well, computer simulations of planet formation
do show that planets can fall into their home stars.
Now, this can happen if a gas giant ends up
in the inner part of the system, either by migration
due to interactions between planets
or if the gas giant is perturbed by a passing
star into an elliptical orbit.
We talk a bit about the effects of Jupiter
on our own solar system in this episode.
Future Gaia data will be used to search for rampaging
outer planets around Kronos.
And it may also reveal other planet-eating stars,
which will shed light on the whole planet formation process.
We'll be sure to keep you updated on future episodes
of "Space Time."
We recently discussed the gradual brightening
of the sun and the inevitable extinction of all life on Earth
that will result. Or is it inevitable?
You guys came to the rescue in the comments
and suggested a number of strategies
for saving the world.
Several of you mentioned the idea of star lifting,
of actually reducing the mass of the sun
to reduce its luminosity and, as a side benefit,
to potentially greatly increase its lifespan.
There are a few ways to do this, but most ideas
involve increasing the rate of the solar wind
while also magnetically driving the outflow towards the poles
so Earth doesn't get blasted.
If you want a very thorough overview of this idea,
check out Isaac Arthur's video on it,
and check out the rest of his awesome channel
while you're at it.
Link in the description.
Andrew Milo recommends we block out
some of the sun's light with orbiting solar farms.
This is probably better than my idea of reflective aerosols
or nano-particles, because A, it doesn't
change the chemistry of the atmosphere, and B,
we get unlimited free energy.
It's also more permanent.
OK, let's do it.
Mark Jeninga has a simpler solution.
We just move the Earth.
Mark points to a paper--
Korycansky, Laughlin, and Adams, 2000-- that
proposes a method to do just this.
It should be possible to deflect an asteroid
into an elliptical orbit that takes it near both the Earth
and Jupiter.
It tugs on the Earth gravitationally,
increasing our angular momentum.
The asteroid loses angular momentum in the process,
but it regains it as it flies by Jupiter.
Essentially, we steal angular momentum
from Jupiter via the asteroid, which increases our orbit.
With a big enough asteroid, one encounter
per several thousand years should
be enough to maintain the current solar flux
by increasing the size of our orbit.
The paper is linked in the description.
TheRealMirCat points out that if Earth
is our only home by the time all of this happens,
we deserve to burn.
Harsh but fair.
So let's get cracking.