The night sky may appear velvety dark to us, with stars flashing coldly
against it like cut gems. And it could be somewhat true for some of
them.
A certain kind of dead star eventually hardens and crystallizes as it
cools. A white dwarf that is 104 light-years away and primarily made of
carbon and metallic oxygen has been discovered by astronomers to be doing
just that; its temperature-mass profile indicates that the star's core is
changing into a dense, hard, "cosmic diamond" made of crystallized carbon
and oxygen.
A manuscript on the finding that was approved for publication in the
Monthly Notices of the Royal Astronomical Society is accessible on the
preprint server
arXiv.
The previously known triple HD 190412 has a crystallizing white dwarf
companion,
according to a team of international astronomers
led by Alexander Venner of the University of Southern Queensland in
Australia. "In this work we present the discovery of a new Sirius-like
quadruple system at 32 parsecs distance, composed of a crystallizing white
dwarf companion to the previously known triple HD 190412," the team
writes.
This is the first crystallizing white dwarf whose overall age may be
externally restricted because of its relationship with these main sequence
partners. We take use of this feature by attempting to objectively estimate
a cooling delay caused by core crystallization in the white dwarf.
The universe as a whole must undergo transformation. Every star in the
heavens that shines brilliantly with the light produced by atomic fusion
eventually runs out of fuel for their flames and transforms into something
else.
That object is a white dwarf star for the great majority of stars, which
are those with masses less than or equal to eight times that of the Sun and
include the Sun.
When the star's fuel runs out, its outer layers are ejected into space,
leaving the remaining core to collapse when the star's fusion-driven
pressure is no longer sufficient to support it. This ultradense object will
be about the size of Earth (or the Moon! ), but it will contain as much mass
as 1.4 Suns.
Although the mass of white dwarf stars is very compacted, a phenomenon
known as electron degeneracy pressure prevents it from collapsing much
farther. This prevents the white dwarf from being even denser, as observed
in a neutron star or black hole, because no two electrons can occupy the
same states.
Despite being faint, white dwarf stars are still visible due to lingering
heat. They gradually cool and are predicted to transform into stars known as
black dwarfs when they completely lose their heat and turn into a frigid
mass of crystalline carbon.
Since the universe is just around 13.8 billion years old and this process
is thought to take a quadrillion years (or a million billion years), we
don't expect to see one any time soon.
What we can do is spot the telltale traces of crystallization beginning in
the centers of the white dwarfs we can see in our neighborhood.
The carbon and oxygen atoms inside the white dwarf cease moving about
freely and begin to establish bonds, organizing themselves into a crystal
lattice as the white dwarf crystallizes. This process results in the release
of energy, which is then converted to heat.
White dwarf stars experience a kind of plateau or slowdown in their cooling
as a result, which may be seen in the star's color and brightness and causes
it to look younger than it actually is.
It is necessary to know a star's precise distance in order to evaluate its
brightness precisely; in recent years, this has become much more feasible
thanks to the Gaia mission's high-precision stellar mapping work.
As a result, scientists can now recognize crystallizing white dwarfs with
far more assurance.
Using the Gaia data, Venner and his colleagues were looking for numerous
star systems by finding stars whose relationships to other stars may have
been hazy.
And they discovered that HD 190412, which had previously been believed to
be a
system of three stars, was gravitationally tied up with a recently discovered white dwarf star
(remember, these things are really faint).
The triplet became a quadruplet once the white dwarf, now known as HD
190412 C, was found, but there was more going on. It appears to be
crystallizing based on its characteristics.
White dwarfs have a density of about one million kilograms per cubic meter,
but diamond has a density of roughly 3,500 kilograms per cubic meter, hence
it is uncertain whether or not the crystal in that white dwarf is made of
diamond. There are denser allotropes of carbon, but there is also a lot of
diamond floating around in space.
The team was able to externally restrict the white dwarf's age, which has
never been done previously for a known crystallizing white dwarf, thanks to
the other three stars in the system.
The age of the system is around 7.3 billion years. It seems that the white
dwarf is 4.2 billion years old. According to the researchers, the 3.1
billion year difference indicates that the white dwarf's cooling pace has
slowed by around 1 billion years due to crystallization.
Although the date alone is insufficient to change our theories about white
dwarf crystallization, the finding and the system's closeness to Earth raise
the possibility that there are many more systems like it that we may use as
benchmarks for this intriguing process.
We think that the fact that this system was found at a distance of only 32
parsecs indicates that there are probably many other Sirius-like systems
with crystallizing white dwarfs.
The researchers argue
that future discoveries could therefore enable more robust evaluations of
white dwarf crystallization models.
We draw the conclusion that the HD 190412 system's discovery has provided a
fresh perspective on crystallizing white dwarfs.
