The latest star data from the Gaia space observatory has for the first time allowed astronomers to generate a massive three-dimensional atlas of binary stars widely separated about 3,000 light-years from Earth, 1.3 million of them.
The one-of-a-kind atlas, created by Kareem El-Badry, a Ph.D. in astrophysics. A 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. Before Gaia, the latest compilation of nearby binary stars, gathered with data from the now-defunct Hipparcos satellite, included about 200 probable pairs.
“This is just a massive increase in the sample size,” El-Badry said. “And it’s an increase in the types of evolutionary phases that we find binaries in. In our sample, we only have 17,000 white dwarfs. This is a much larger census.”
White dwarfs are the final stages of most stars; the sun will likely end up as a compact white dwarf in 5 billion years. El-Badry’s atlas includes 1,400 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 refer to them as main sequence stars, because they cluster 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 demograph the population of these stellar twins, asking questions like: 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, because white dwarfs can be aged more precisely than is possible with regular stars. Main sequence stars, like the sun, can look the same for billions, or even tens of billions, of years, while white dwarfs change; For one thing, they cool down at a well-defined rate. And since the 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 any planet around the stars.
“For a white dwarf, in general, it’s easy to know how old it is, not just how old it is since it became a white dwarf, but what its total age is,” he said. “You can also measure their masses, because white dwarfs have a well-known mass-radius relationship.”
As an example, El-Badry and his colleagues recently used the Gaia data to estimate the age of a Jupiter-sized gas giant discovered by the TESS satellite around a pair of K-white dwarfs. That exoplanet, TOI-1259Ab, turned out to be about 4 billion years old, based on the age of the white dwarf.
“In this catalog, there are something like 15 systems like this: star plus planet plus white dwarf,” he said, “and there are a few hundred others that are star plus planet plus another star. Those are also potentially interesting because, in some cases, the other star will do something dynamically to the planet. “
The new catalog of nearby binary stars has been accepted for publication in the magazine. Monthly Notices from the Royal Astronomical Society.
El-Badry also collaborated with Jackie Faherty, a scientist and educator at the American Museum of Natural History in New York City, to create a video of all the millions of binary stars around Earth, representing a good chunk of the whole of Milky. Way Galaxy.
Until Gaia was launched by the European Space Agency in 2013 to accurately measure the distances and movements of millions of nearby stars, the only way to find binaries was to look for stars together in the sky. This can be tricky, because stars viewed very close to Earth could be hundreds or thousands of light-years from each other, simply located on the same site line.
Discarding a random alignment requires a long time of observation to confirm that the two candidates are actually at the same distance and moving together. Due to the movement of the Earth around the sun, nearby stars appear to change position in the sky, and that parallax can be used to calculate how far away they are. The star’s motion across the sky, known as its proper motion, helps determine its speed.
Gaia performs this tedious astrometry continuously for all nearby stars in the sky, 24 hours a day, 7 days a week, from its orbit at the Earth-Sun Lagrange point. However, the space telescope study is most useful for stars that are within about 3,000 light years of Earth, because beyond that, the parallax is often too small to measure.
El-Badry first searched for binary stars in Gaia data after the mission’s second launch of star measurements in 2018, with the help of his colleagues Hans-Walter Rix, director of the Max-Planck Institute for Astronomy in Heidelberg, Germany, and Tyler Heintz. graduate student at Boston University. They developed computational techniques to identify stars that move together through space and at the same distance from Earth. Basically, the technique projects the motion of each star over thousands of years, based on its current current motion, and extracts stars that are moving in the same direction. If they also happen to be the same distance based on parallax, they are probably linked together, he said.
He and his colleagues focus primarily on wide binaries, those separated by a distance of 10 AU (astronomical units) or more, that is, 10 or more times the distance between Earth and the sun (93 million miles). Stars closer than that usually appear as a point of light and require other spectroscopic techniques to distinguish if they are true binaries.
For the first time on the latest Gaia data, El-Badry got up at 3 a.m. on the launch date, December 3 of last year, and joined 100 other astronomers from 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 about 1.8 million binary candidates from Gaia’s catalog of 1.8 billion stars, so El-Badry first had to assess the probability that some of the pairs were the same distance and moving in similar directions only. by chance, not because they were paired. He estimates that nearly 1.3 million pairs had at least a 90% chance of being linked, and 1.1 million had a 99% chance.
“About half of all Sun-like stars are binary, many of them too close to distinguish, but we found that something like 25% of all Sun-like stars have a binary partner at separations of more than 30 AU, about the distance to Pluto., “he said. “The distribution peaks with 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 1,000 AU of each other.
One conclusion, he said, is that the new analysis confirms something hinted at in the 2018 data: Many pairs of binary stars are very similar in mass.
“One thing we already found to be cool, we found it out with Gaia DR2, but now we can study it better with this sample, is that binaries like to be identical twins,” he said. “That’s really weird, because most of these are separated by hundreds or thousands of AUs, so they are so far apart that according to conventional star formation theories their masses should be random. But the data tells a different story: they know something about the masses of their peers. “
The implication, he said, is that they formed much closer together in a process that tended to equalize their masses and then fell apart, perhaps due to interactions with other nearby stars.
The compilation of binary stars also allowed El-Badry to verify the reported uncertainties in Gaia’s measurements of stellar positions, which may help other researchers using the data.
White dwarf atmospheres could contain the pulverized crusts of their dead planets
Kareem El-Badry et al, One Million Gaia eDR3 Binaries: Sample Selection and Validation of Gaia Parallax Uncertainties, Monthly Notices from the Royal Astronomical Society (2021). DOI: 10.1093 / mnras / stab323
Provided by the University of California – Berkeley
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