Binary stars are all around us, a new map of neighborhood solar shows

The latest star data from Gaia’s Space Observatory for the first time allowed astronomers to generate a massive 3D atlas of widely separated binary stars about 3,000 light years from Earth – 1.3 million between them.

The one-of-a-kind atlas, created by Kareem El-Badry, doctor of astrophysics. student at the University of California, Berkeley, should be a boon to those who study binary stars – which make up at least half of all sun-like stars – and white dwarfs, exoplanets and stellar evolution, in general. Prior to Gaia, the last compilation of nearby binary stars, assembled using data from the now-defunct Hipparcos satellite, included around 200 probable pairs.

“This is just a massive increase in sample size,” El-Badry said. “And that’s an increase in the types of evolutionary phases that we find binaries in. In our sample, we have 17,000 white dwarfs alone. That’s a much larger census.”

White dwarfs are the last stages of most stars; the sun will probably end up becoming a compact white dwarf in 5 billion years. The El-Badry Atlas includes 1400 systems consisting of two white dwarfs and 16,000 binaries consisting of a white dwarf and another type of star

However, the vast majority of the 2.6 million individual stars are still in their prime. Astronomers call them main sequence stars because they cluster together along a line when plotted on a graph showing temperature versus brightness.

With such a large sample size, El-Badry said, it is possible to make a demographics of the population of these stellar twins, asking questions such as: What is the distribution of the mass ratios of the two stars in all? these binary systems? How are their separations or eccentricities distributed?

El-Badry plans to focus on white dwarf binaries in the future, as white dwarfs may be age more precisely than is possible with regular stars. Main sequence stars like the sun can look alike for billions, if not tens of billions of years, while white dwarfs change – on the one hand, they cool down at a well-defined rate. And because binary pairs are born at the same time, the age of the white dwarf tells astronomers the age of its main sequence twin, or of all the planets around the stars.

“For a white dwarf, in general, it’s easy to tell how old she is, not just how old she was since she became a white dwarf, but what her total age is,” he says. “You can also measure their masses, because white dwarfs have a well understood mass-to-radius relationship.”

As an example, El-Badry and his colleagues recently used data from Gaia to estimate the age of a Jupiter-sized gas giant discovered by the TESS satellite around a pair of white K-dwarfs. . This exoplanet, TOI-1259Ab, was found to be about 4 billion years old, based on the age of the white dwarf.

“In this catalog there is something like 15 systems like this: star plus planet plus white dwarf,” he said, “and there are a few hundred that are star plus planet plus another star. . case the other star will dynamically do something to the planet. “

The new catalog of nearby binary stars has been accepted for publication in the journal Monthly notices from the Royal Astronomical Society.

El-Badry also collaborated with Jackie Faherty, scientist and educator at the American Museum of Natural History in New York City, to create a video overview of all the millions of binary stars around Earth, which is a good chunk of the ‘set of Milky. Galaxy Way.

Binary stars

Until Gaia was launched by the European Space Agency in 2013 to accurately measure the distances and motions of millions of nearby stars, the only way to find binaries was to look for nearby stars in the sky. This can be tricky, as stars that appear to be very close to Earth could be hundreds to thousands of light years apart, just lying along the same site line.

Eliminating a random alignment requires a lot of observation time to confirm that the two candidates are really at the same distance and are moving together. Due to the movement of the Earth around the sun, nearby stars appear to change position in the sky, and this parallax can be used to calculate how far away they are. The movement of the star in the sky, known as proper movement, helps determine its speed.

Gaia performs this tedious astrometry continuously for all nearby stars in the sky, 24/7, from its orbit at the Earth-Sun point of Lagrange. Studying the space telescope is, however, very useful for stars located around 3000 light years from Earth, because beyond that parallax is usually too small to measure.

El-Badry first searched Gaia’s data for binary stars after the mission’s second release of star measurements in 2018, with help from colleagues Hans-Walter Rix, director of the Max- Institute. Planck for astronomy in Heidelberg, Germany, and Tyler Heintz, a graduate student at Boston University. They developed computational techniques to identify stars moving together in space and at the same distance from Earth. The technique essentially projects the motion of each star over thousands of years, based on its current motion, and extracts stars that are moving in the same direction. If they also turn out to be the same distance based on parallax, they are likely related to each other, he said.

He and his colleagues mainly focus on wide binaries – those separated by a distance of 10 AU (astronomical units) or more – that is, 10 times or more the distance between Earth and the sun (93 million miles). Stars closer than this usually appear as a point of light and require other spectroscopic techniques to distinguish whether they are true binaries.

To get a first feel for Gaia’s latest data, El-Badry got up at 3 a.m. on the release date, December 3 of last year, and joined hundreds of other astronomers around the world. on Zoom. He quickly ran pre-programmed queries on the data to extract the catalog information he needed to create the 3D map.

Initial queries returned some 1.8 million binary candidates from Gaia’s 1.8 billion star catalog, so El-Badry first had to assess the likelihood that some of the pairs were the same distance and moving. in similar directions by chance, and not because they are paired. He estimates that nearly 1.3 million pairs had at least a 90% chance of being tied, and 1.1 million had a 99% chance.

“About half of all sun-like stars are binary, many of which are too close to be distinguished, but we find that 25% of all sun-like stars have a binary companion at separations greater than 30 AU, roughly the distance from Pluto, “he said. “The distribution peaks at a separation of 30 or 50 AU.”

Some pairs are separated by as much as one parsec – 260,000 AU, or 3.26 light years – although most are within 1000 AU of each other.

One thing to remember, he said, is that the new analysis confirms something that was hinted at in the 2018 data: Many pairs of binary stars have very similar mass.

“One thing we’ve already found that is cool – we found out with Gaia DR2, but now we can study it better with this sample – is that binaries like to be identical twins,” he said. . “It’s really strange, because most of them are separated by hundreds or thousands of AUs, so they’re so far apart that, according to conventional star formation theories, their masses should be random. on the masses of their companions. “

The implication, he said, is that they formed much closer to each other in a process that tended to equalize their masses and then migrate separately, possibly due to interactions with others. nearby stars.

Compiling binary stars also allowed El-Badry to verify reported uncertainties in Gaia’s measurements of stellar positions, which may help other researchers using the data.