It has long been known in the astronomical community that many stars are binaries, orbiting each other because of their mutual gravitational pull. A forthcoming paper by UC Berkeley theoretical astrophysicist Steven Stahler and Harvard radio astronomer Sarah Sadavoy cites evidence that all stars began life in such a partnership, including our sun.
In their paper, the scientists first combined observational data from two surveys in which Sadavoy had participated. Combining the data allowed them to see the spatial relationship of very young stars to the gas clouds that formed them. They then performed a statistical analysis to understand better how the clouds actually created these stars. All this was done by email; the two scientists have yet to meet.
The first survey, called VANDAM, imaged young stars using their emission at radio wavelengths. This emission was captured by the VLA — the “very large array” of radio dishes, located in the New Mexico desert. Using such an extended array, rather than a single telescope, allows astronomers to see the separation of binaries, two orbiting stars that appear close together in the sky.
The VANDAM survey focused on the Perseus molecular cloud. This region was selected because of its relatively high volume of young stars, Sadavoy said.
The second survey, the Gould Belt Survey, looked at the same molecular cloud using the infrared James Clerk Maxwell Telescope, located near the summit of the Mauna Kea in Hawaii. A camera on this telescope detected shorter-wavelength radio emission from heated dust grains. In this way, researchers could image the relatively small cloud structures (called “dense cores”) that form both individual stars and binaries. These dense cores appear “egg-shaped” in the sky.
According to Sadavoy, the pairing of these two surveys, which utilized two different types of telescopes, is a unique approach that wouldn’t have been possible even a couple of years ago.
“Now we had really good data,” Stahler said. “We combined the VLA positions of young stars with images of their parent dense cores — kind of a binary result, you might say.”
Some of the stars are single, and some are in binaries with varying separations. Interestingly, according to Stahler, the stars in binaries with the widest separation are also the youngest, called “Class 0.”
Sadavoy and Stahler found that these widest and youngest binaries tend to align with the central axis of their egg-shaped parent dense cores. In contrast, somewhat older binaries (consisting of “Class 1” stars) do not align in this way. These two results indicate that dense cores first create stars along their central axes.
As the stars in binaries age, they can drift together, and they can also change their orientation, a process that occurs within about a million years.
Sadavoy and Stahler then tested a number of theoretical models to account for the observed populations of young stars, both Class 0 and 1, and both single and binary. The only scenario that worked was one in which the dense cores only produce wide, Class 0 binaries.
According to the model, these binaries do one of two things as they age. Either the orbiting stars approach one another, creating a tighter binary, or the original pair breaks apart, stars going their separate ways.
Thus, the scientists concluded, any young, single star is not primordial, but the product of a dissolving binary.
“It is more probable for a binary to break up than to shrink, according to our model,” Stahler said. Our own sun, a typical star, thus also began as a binary star whose companion star has now drifted far away.
Why some binary stars remain together while others break apart remains to be determined.
About half of all stars are members of binaries. This improved understanding of binary origins can help other researchers more accurately model distant galaxies, according to astrophysics postdoctoral research fellow Ryan Trainor, who studies galaxy formation.
“Distant galaxies are younger galaxies, which means they have a lot of very young stars within them — which are also more likely to be binaries,” Trainor said.
Stahler and Sadavoy next want to look at young stars and dense cores within other nearby regions to make sure that their model continues to explain the observations. The pair also want to look at the Perseus cloud in more depth, comparing binaries with single stars, hoping to understand how binary breakup occurs.