Star Twins

PHYSICS

Star Twins

Amélie Gadsby

New analysis by a theoretical physicist from UC Berkeley and a radio astronomer from the Smithsonian Astrophysical Observatory at Harvard suggests that all stars are born as twins – although not the identical kind. This means that our Sun most likely was too. The search for our Sun’s pair, a star dubbed Nemesis, has been ongoing for many years. It was supposed to have caused an asteroid to move into the Earth’s orbit and collide with us, killing the dinosaurs, however it has never been found. This was asserted thanks to a radio survey of a giant molecular cloud in the constellation Perseus, which is filled with recently formed stars. It is 600 light years from Earth and roughly 50 light years wide. The only statistical model that could reproduce the Perseus constellation was one in which all stars initially form as wide binaries, which break apart or shrink over millions of years. These wide binaries were at least 500 astronomical units apart, therefore a wide binary companion to the Sun would have been 17x further than Neptune. We can therefore assume that it must have mixed with other stars in theMilky Way, which is why it has never been seen since.

According to first author Sarah Savoy, "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." This could help us to discover more about the formations of galaxies, not just stars. In 1993-1995, Pavel Kroupa carried out stellar-dynamic computations on the interaction of stars that were recently freed from their gas clouds after formation, which led him to conclude that all stars are born as binaries. However, there has been little evidence since. Stars are usually born in dense egg shapes called dense cores, which are part of huge clouds of hydrogen gas. These clouds look like black holes through a telescope, because the dust blocks light from stars both inside the cloud and behind it. However, it is possibly to probe them using radio telescopes as the clouds absorb and re-emit optical and infrared radiation at far higher wavelengths. Last year, astronomers used the Very Large Array in New Mexico to carry out a survey called VANDAM of all stars less than 4 million years old (young stars) in the Perseus cloud. Co-author Steven Stahler contacted Savoy who was part of the VANDAM team, andgathered a census on Class 0 (stars less than 500,000 years old) and Class 1 (stars between 500,000-1 million years old), combined with observations on the dense cores. They discovered that all of the wide (over 500 AU) binary systems were formed from Class 0 stars, whereas the Class 1 binary stars tended to be only 200 AU apart. The Class 0 stars were also aligned along the long axis of the dense core, but not the Class 1 stars.

Radio image of a triple star system forming in the Perseus cloud

Barnard 68 cloud blocks out light, in a similar way to the Perseus cloud

PHYSICS

They continued modelling different scenarios to explain this distribution and concluded that the only way to explain this is that all stars of a similar mass to the Sun start off as wide Class 0 binaries. 6 in 10 will break apart over time, the rest will form tight binaries. Single, low-mass stars are therefore formed from the break-up of 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," Stahler said. "These centres of higher density at some point collapse in on themselves because of their self-gravity to form Class 0 stars." Therefore, this theory implies that the dense cores convert twice as much mass into stars than we had previously believed. In the next few years, studies from the VLA and ALMA telescope in Chile andthe SCUBA-2 survey in Hawaii will provide more insight into dense cores and star formation. Sources:

https://phys.org/news/2017-06-evidence-stars-born-pairs.html

https://cerncourier.com/a/evidence-suggests-all-stars-born-in-pairs/