Astronomers Catch a Small Star Slowly Feeding on Its Brown Dwarf Companion

Astronomers have spotted a small star steadily pulling material off a nearby brown dwarf, the first time such a gentle, ongoing transfer has been seen between two low-mass objects. The system, called ZTF J0440+2325, lies roughly 300 light-years away in the Milky Way. The team, led by researchers at the Massachusetts Institute of Technology, reported the finding in the journal Nature Astronomy, and MIT described it in an announcement this month. ScienceDaily carried the institute’s account on Saturday.

Brown dwarfs are objects too massive to be planets but too small to sustain the nuclear fusion that powers ordinary stars. In this pair, the star has about 85 times the mass of Jupiter and the brown dwarf about 25 times. They orbit each other every 87 minutes or so, so tightly that the entire orbit would fit inside the sun.

“This is the first time we’ve caught a low-mass star actively accreting from another low-mass object,” said Aaron Householder, an MIT graduate student who led the work with Kevin Burdge, an assistant professor of physics.

The discovery began with an odd signal. The Zwicky Transient Facility, a sky survey, recorded a light curve that rose and fell in a repeating, triangle-shaped pattern. The researchers first suspected a so-called black widow binary, but the small wobble they measured in the star’s motion ruled that out.

Instead, they concluded that material peeling off the brown dwarf falls straight onto the star’s surface, creating a hot spot. As the brown dwarf circles, that bright spot rotates in and out of view, producing the triangular pattern. Computer simulations in which particles leaving the brown dwarf fell onto the star supported the explanation.

The flow is tiny by cosmic standards, about one hundred-thousandth of Earth’s mass each year. Because of that slow pace, the researchers say the star could keep feeding for hundreds of thousands of years and possibly for billions. “Instead of swallowing the thing up, the star can gradually eat it,” Burdge said in MIT’s announcement.

That long-term estimate comes from modeling rather than direct observation, and ScienceDaily’s account notes that the star “could” continue feeding that long rather than that it will. Still, the finding fills a gap between two familiar outcomes: companions that orbit their stars at a safe distance, and those that are swallowed quickly. The team says that middle path may help explain how stars interact with brown dwarfs and planets that wander too close, and it plans to search for more systems like this one.

Collaborators came from the California Institute of Technology, the University of Hawaii, Spain’s Instituto de Astrofísica de Canarias and the Harvard-Smithsonian Center for Astrophysics, among others, with partial funding from the National Science Foundation.