'Symbiotic stars' caught snacking on each other outside the Milky Way

For the first time, stars snacking on their stellar neighbors outside the Milky Way have had their orbits fully mapped. Using the Sloan Digital Sky Survey, astronomers have identified two pairs of stars beyond the galaxy that are consuming their companions. The new discovery can help astronomers understand if distant galactic environments function similarly to or differently from the Milky Way. It can also provide insight into one of the fundamental methods of measuring distance in the night sky. More than half the stars in the Milky Way come in pairs. While it seems likely that binary stars should make up a significant fraction of other galaxies, scientists have been unable to confirm that because at such large distances ordinary stars are too faint to see.But so-called symbiotic stars, where one companion consumes the other, can be extremely bright, making them easier to observe. "Measuring the orbits of these symbiotic star systems is an important step towards learning whether other galaxies create binary stars like those in the Milky Way," Jasmin Washington, a co-author of the new study and a graduate student at the University of Arizona, said in a statement. She was an undergraduate at the University of Virginia during the project. Washington and her fellow author, Hannah Lewis, a graduate student at the University of Virginia, presented the results on Tuesday (Jan 12) at the 237th meeting of the American Astronomical Society, held virtually last week.

An artistic impression of the Draco C1 symbiotic binary star system showing material flowing off the red giant star onto its white dwarf companion.

(Image: © John Blondin/North Carolina State University)

"We have developed for the first time ever the complete understanding of the architecture of an extragalactic [symbiotic] system," Washington said at the briefing.

Serendipitous snackers

Although a pair of stars may be born together, they can age differently due to their masses. The more massive of the two will quickly burn through its material to reach the end of its lifetime first. If that star is large enough, it will leave behind a compact white dwarf. Although small and dim, white dwarfs can pack the mass of the sun into an object the size of the Earth. If close enough, the gravity of the dense objects can pull material from their companion, creating a signal that astronomers can identify from extremely far away. While astronomers know that stellar pairs are common in the Milky Way, they remain uncertain how large a fraction they make up in other galaxies.

"The properties of binary systems likely depend on the environment that they formed in," Lewis said at the briefing. "Those environmental properties can vary vastly between galaxies."

For the last decade, Sloan Telescope’s Apache Point Observatory Galactic Evolution Explorer (APOGEE) survey has studied the sky, gathering data about hundreds of thousands of stars in the Milky Way and its nearest galactic neighbors. These include the Draco dwarf spheroidal galaxy and the Small Magellanic Cloud (SMC), roughly 260,000 and 200,000 light-years respectively.

"These two galaxies alone show how conditions can vary wildly between systems," Lewis said. Draco is an ancient galaxy, a hundred thousand times smaller than the Milky Way, and is dominated by dark matter rather than stars. The SMC is younger and larger, only 200 times smaller than our galaxy and composed of old and young stars. Both galaxies are home to a symbiotic stellar pair visible to APOGEE, the Draco C1 and LIN 358 pair, respectively.

This graph shows the motions measured by the APOGEE data for the Draco C1 symbiotic binary star system, which has been monitored repeatedly over the last five years. Black dots represent the data, while the blue curve shows the computer model for the orbit of the red giant as it circles the white dwarf, moving toward and away from the observer. (Image credit: Washington et al.)

Slurping material from the neighboring stars only allows astronomers to identify the pair. The Doppler shift — the same phenomena responsible for causing train whistles to reach a higher pitch as they move closer and lower as they move farther away — also causes changes in the frequency of light coming from a star, depending on whether it is moving closer to or farther from the observer. That back-and-forth motion can help astronomers to calculate the full orbit of the binary system and the masses of both stars.

By combing through several years of APOGEE data, Washington realized that the stars in Draco C1 take roughly three Earth years to orbit one another, while LIN 358's components take just over two. The results reveal the first full orbital measurements of any symbiotic star system outside the Milky Way.

"Very few symbiotic stars have ever been monitored long enough for astronomers to watch the full willing dance," Lewis said in a statement. "And no one has ever done this in detail for symbiotic stars in other galaxies."

The new measurements will help astronomers better understand star formation in other galaxies.

"Dwarf galaxies have very different internal environments and evolutionary histories from the Milky Way," Borja Anguiano, also at the University of Virginia, said in the statement. A co-author on the paper, Anguiano originally discovered that APOGEE had serendipitously observed Draco C1 and LIN 358 several times.

"Soon we will have enough orbits mapped for binaries in other galaxies that we may begin to answer the question of whether different types of galaxies are more efficient at making binary stars."

The results from the observations of Draco C1 were published earlier this year in the Astrophysical Journal Letters.

Standard candles

In some symbiotic stars, the white dwarf can slurp enough material from its companion that it explodes in a Type Ia supernova. These extremely bright blasts can be seen across the universe, and they all start out with the same brightness for a nearby observer. Astronomers can use the apparent brightness of the supernova to calculate its distance, making Type Ia supernovas a "standard candle" for measuring the universe.

While Draco C1 and LIN 358 are unlikely to explode as supernovae anytime soon, understanding how they work can provide insights into how these standard candles evolve.

"Because we rely on Type Ia supernovae as distance measurements, it's important that we understand exactly how they work, and what systems we should be looking for as possible supernovae progenitors," Anguiano said. "Being able to study the orbits of symbiotic stars in other galaxies will allow us to confirm whether the process of forming Type Ia supernovae is universal."

Mapping the orbital characteristics of Draco C1 and LIN 358 is a "first incredible step" towards using a decade's worth of APOGEE data to understand binary stars outside the Milky Way, Washington said at the briefing.

"Studying extragalactic symbiotic stars in great detail and being able to precisely derive their orbits and stellar parameters could provide important insights into these cosmic markers," she said.