New evidence that all stars are born in pairs

New evidence that all stars are born in pairs

Radio image of a very young binary star system, less than
about 1 million years old, that formed within a dense core
(oval outline) in the Perseus molecular cloud. All stars likely
form as binaries within dense cores. Credit: SCUBA-2 survey
image by Sarah Sadavoy, CfA

Did our sun have a twin when it was born 4.5 billion years
ago?

Almost certainly yes—though not an identical twin. And so did
every other sunlike star in the universe, according to a new
analysis by a theoretical physicist from UC Berkeley and a
radio astronomer from the Smithsonian Astrophysical Observatory
at Harvard University.

Many have companions, including our nearest
neighbor, Alpha Centauri, a triplet system. Astronomers have
long sought an explanation. Are binary and triplet star systems
born that way? Did one star capture another? Do binary stars
sometimes split up and become single stars?

Astronomers have even searched for a companion to our sun, a
star dubbed Nemesis because it was supposed to have kicked an
asteroid into Earth’s orbit that collided with our planet and
exterminated the dinosaurs. It has never been found.

The new assertion is based on a radio survey of a giant
filled with recently formed
stars in the constellation Perseus, and a mathematical model
that can explain the Perseus observations only if all sunlike
stars are born with a companion.

“We are saying, yes, there probably was a Nemesis, a long time
ago,” said co-author Steven Stahler, a UC Berkeley research
astronomer.

“We ran a series of statistical models to see if we could
account for the relative populations of young single stars and
binaries of all separations in the Perseus molecular cloud, and
the only model that could reproduce the data was one in which
all stars form initially as wide binaries. These systems then
either shrink or break apart within a million years.”

A radio image of a triple star system forming within a
dusty disk in the Perseus molecular cloud obtained by the Atacama
Large Millimeter/submillimeter Array (ALMA) in Chile. Credit:
Bill Saxton, ALMA (ESO/NAOJ/NRAO), NRAO/AUI/NSF

In this study, “wide” means that the two stars are separated by
more than 500 astronomical units, or AU, where one astronomical
unit is the average distance between the sun and Earth (93
million miles). A wide binary companion to our sun would have
been 17 times farther from the sun than its most distant planet
today, Neptune.

Based on this model, the sun’s sibling most likely escaped and
mixed with all the other stars in our region of the Milky Way
galaxy, never to be seen again.

“The idea that many stars form with a companion has been
suggested before, but the question is: how many?” said first
author Sarah Sadavoy, a NASA Hubble fellow at the Smithsonian
Astrophysical Observatory. “Based on our simple model, we say
that nearly all stars form with a companion. The Perseus cloud
is generally considered a typical low-mass star-forming region,
but our model needs to be checked in other clouds.”

The idea that all stars are born in a litter has implications
beyond , including the very
origins of galaxies, Stahler said.

Stahler and Sadavoy posted their findings in April on the arXiv
server. Their paper has been accepted for publication in the
Monthly Notices of the Royal Astronomical Society.

Stars birthed in ‘dense cores’

Astronomers have speculated about the origins of binary and
multiple star systems for hundreds of years, and in recent
years have created computer simulations of collapsing masses of
gas to understand how they condense under gravity into stars.
They have also simulated the interaction of many young stars
recently freed from their gas clouds. Several years ago, one
such computer simulation by Pavel Kroupa of the University of
Bonn led him to conclude that all stars are born as binaries.

This infrared image from the Hubble Space Telescope
contains a bright, fan-shaped object (lower right quadrant)
thought to be a binary star that emits light pulses as the two
stars interact. The primitive binary system is located in the IC
348 region of the Perseus molecular cloud and was included in the
study by the Berkeley/Harvard team. Credit: NASA, ESA and J.
Muzerolle, STScI

Yet direct evidence from observations has been scarce. As
astronomers look at younger and younger stars, they find a
greater proportion of binaries, but why is still a mystery.

“The key here is that no one looked before in a systematic way
at the relation of real young stars to the clouds that spawn
them,” Stahler said. “Our work is a step forward in
understanding both how binaries form and also the role that
binaries play in early stellar evolution. We now believe that
most stars, which are quite similar to our own sun, form as
binaries. I think we have the strongest evidence to date for
such an assertion.”

According to Stahler, astronomers have known for several
decades that stars are born inside egg-shaped cocoons called
dense cores, which are sprinkled throughout immense clouds of
cold, molecular hydrogen that are the nurseries for young
stars. Through an optical telescope, these clouds look like
holes in the starry sky, because the dust accompanying the gas
blocks light from both the stars forming inside and the stars
behind. The clouds can, however, be probed by radio telescopes,
since the cold dust grains in them emit at these radio
wavelengths, and radio waves are not blocked by the dust.

The Perseus molecular cloud is one such stellar nursery, about
600 light-years from Earth and about 50 light-years long. Last
year, a team of astronomers completed a survey that used the
Very Large Array, a collection of radio dishes in New Mexico,
to look at star formation inside the cloud. Called VANDAM, it
was the first complete survey of all young stars in a molecular
cloud, that is, stars less than about 4 million years old,
including both single and mulitple stars down to separations of
about 15 astronomical units. This captured all multiple stars
with a separation of more than about the radius of Uranus’
orbit—19 AU—in our solar system.

Stahler heard about the survey after approaching Sadavoy, a
member of the VANDAM team, and asking for her help in observing
young stars inside dense cores. The VANDAM survey produced a
census of all Class 0 stars – those less than about 500,000
years old – and Class I stars – those between about 500,000 and
1 million years old. Both types of stars are so young that they
are not yet burning hydrogen to produce energy.

Sadavoy took the results from VANDAM and combined them with
additional observations that reveal the egg-shaped cocoons
around the young stars. These additional observations come from
the Gould Belt Survey with SCUBA-2 on the James Clerk Maxwell
Telescope in Hawaii. By combining these two data sets, Sadavoy
was able to produce a robust census of the binary and
single-star populations in Perseus, turning up 55 young stars
in 24 multiple-star systems, all but five of them binary, and
45 single-star systems.

Using these data, Sadavoy and Stahler discovered that all of
the widely separated binary systems—those with stars separated
by more than 500 AU—were very young systems, containing two
Class 0 stars. These systems also tended to be aligned with the
long axis of the egg-shaped dense core. The slightly older
Class I binary stars were closer together, many separated by
about 200 AU, and showed no tendency to align along the egg’s
axis.

A dark molecular cloud, Barnard 68, is filled with gas and
dust that block the light from stars forming inside as well as
stars and galaxies located behind it. These and other stellar
nurseries, like the Perseus molecular cloud, can only be probed
by radio waves. Credit: FORS Team, 8.2-meter VLT Antu, ESO

“This has not been seen before or tested, and is super
interesting,” Sadavoy said. “We don’t yet know quite what it
means, but it isn’t random and must say something about the way
wide binaries form.”

Egg-shaped cores collapse into two centers

Stahler and Sadavoy mathematically modeled various scenarios to
explain this distribution of stars, assuming typical formation,
breakup and orbital shrinking times. They concluded that the
only way to explain the observations is to assume that all
stars of masses around that of the sun start off as wide Class
0 binaries in egg-shaped dense cores, after which some 60
percent split up over time. The rest shrink to form tight
binaries.

“As the egg contracts, the densest part of the egg will be
toward the middle, and that forms two concentrations of density
along the middle axis,” he said. “These centers of higher
density at some point collapse in on themselves because of
their self-gravity to form Class 0 stars.”

“Within our picture, single low-mass, sunlike stars are not
primordial,” Stahler added. “They are the result of the breakup
of . ”

Their theory implies that each dense core, which typically
comprises a few solar masses, converts twice as much material
into stars as was previously thought.

Stahler said that he has been asking radio astronomers to
compare dense cores with their embedded young stars for more
than 20 years, in order to test theories of binary star
formation. The new data and model are a start, he says, but
more work needs to be done to understand the physics behind the
rule.

Such studies may come along soon, because the capabilities of a
now-upgraded VLA and the ALMA telescope in Chile, plus the
SCUBA-2 survey in Hawaii, “are finally giving us the data and
statistics we need. This is going to change our understanding
of dense cores and the embedded stars within them,” Sadavoy
said.

Explore further:

No close partner for young, massive stars in Omega Nebula

More information: Embedded Binaries and Their Dense
Cores. arxiv.org/abs/1705.00049

Provided by:
University of California – Berkeley