March 1, 2024

Learning the Trick for Finding Cannibalized Stars

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Artist’s rendering of a Be star and its disk (top right) orbited by a faint, hot, stripped star (bottom left). Credit: Painting by William Pounds

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Artist’s rendering of a Be star and its disk (top right) orbited by a faint, hot, stripped star (bottom left). Credit: Painting by William Pounds

Scientists working with the powerful telescopes at Georgia’s Center for High Angular Resolution Astronomy (CHARA) have completed a survey of a group of stars suspected of having devoured most of the gas from orbiting companion stars. These sensitive measurements directly detected the faint glow of the cannibalized stars.

The new research, led by postdoctoral researcher Robert Klement, is published in The Astrophysical Journal. The work identifies new orbits of stripped subdwarf stars that circle massive, rapidly rotating stars, leading to new understanding of the life trajectories of nearby binary stars.

Working with colleagues at the CHARA Array in Mount Wilson, California, Klement pointed high-powered telescopes at a collection of relatively nearby B emission line stars, or “Be stars” for short. These are rapidly rotating stars believed to harbor unusual orbiting companions.

Be stars are likely formed in intense interactions between close pairs of stars. Astronomers have found that many stars occur in these pairs, a trend that is especially true among stars more massive than our Sun. Pairs with small separations face a tumultuous fate because they increase in size as they age and can reach a size similar to that of the separation. .

When this happens, gas from the growing star can cross the gap between the pair, so the companion can bask in the flow of transferred gas. This cannibalization process will eventually strip the mass-donating star of almost all of its gas and leave behind the tiny hot core of its former nuclear combustion center.


CHARA measurements (red ellipses) of the motion of the naked star (dashed line) that orbits the star Be HR2142 (yellow star) every 81 days. The small black star symbols are the calculated positions of the stripped companion during the period of our observations. The orbit is circular, but appears elliptical because it is tilted with respect to the plane of the sky. The top and right axes show the apparent physical separation in astronomical units (AU, the average Earth-Sun distance), while the bottom and left axes give the angular separation in the angular units of milliarcseconds (mas). For comparison, the full Moon in the sky has an angular diameter of about 2 million milliarcseconds. Credit: Robert Klement

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CHARA measurements (red ellipses) of the motion of the naked star (dashed line) that orbits the star Be HR2142 (yellow star) every 81 days. The small black star symbols are the calculated positions of the stripped companion during the period of our observations. The orbit is circular, but appears elliptical because it is tilted with respect to the plane of the sky. The top and right axes show the apparent physical separation in astronomical units (AU, the average Earth-Sun distance), while the bottom and left axes give the angular separation in the angular units of milliarcseconds (mas). For comparison, the full Moon in the sky has an angular diameter of about 2 million milliarcseconds. Credit: Robert Klement

Astronomers predicted that the mass transfer flow causes the companion star to spin and become a very fast rotator. Some of the fastest rotating stars are found as Be stars. Stars rotate so quickly that some of their gas is expelled from their equatorial zones to form an orbiting ring of gas.

Until now, this predicted phase in the lives of close binary pairs has eluded astronomers because the stars’ separations are too small to observe with conventional telescopes and because the naked stellar corpses are hidden in the glow of their bright companions. However, Georgia’s CHARA Array telescopes offered researchers the means to find the stripped stars.

The CHARA Array uses six telescopes spread across the summit of Mount Wilson to function as a huge single telescope measuring 330 meters in diameter. This gives astronomers the ability to separate light from pairs of stars even at very small angular displacements. Klement also used the MIRC-X and MYSTIC cameras – built at the University of Michigan and the University of Exeter in the United Kingdom – which can record the light signal from very bright and very faint objects close to each other.

The researchers wanted to determine whether the Be stars were generated by mass transfer and whether they hosted stripped stars. Klement began a two-year observation program at CHARA and his work quickly bore fruit. He discovered the faint light of naked companions in nine of the 37 Be stars. He focused on seven such targets and was able to track the orbital motion of the stellar corpse around the Be star.

“Orbits are important because they allow us to determine the masses of stellar pairs,” Klement said. “Our mass measurements indicate that the stripped stars have lost almost everything. In the case of the star HR2142, the stripped star likely went from 10 times the mass of the Sun to about one solar mass.”


The six CHARA telescopes are arranged in a Y-shaped configuration, providing 15 baselines ranging between 34 and 331 meters in length and up to 10 possible closing phase triangles. Credit: The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad13ec

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The six CHARA telescopes are arranged in a Y-shaped configuration, providing 15 baselines ranging between 34 and 331 meters in length and up to 10 possible closing phase triangles. Credit: The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad13ec

No stripped stars have been detected around all Be stars, and researchers believe that in some of these cases the corpse has transformed into a small white dwarf star, too faint to be detected even with the CHARA Array. In other cases, the interaction may have been so intense that the stars merged to become one rapidly rotating star.

Klement is now extending the search for orbiting stripped stars to Be stars in the southern sky, using the European Southern Observatory’s Very Large Telescope Interferometer in Chile.

He is also working with Luqian Wang at the Yunnan Observatories in China on research using NASA’s Hubble Space Telescope to detect the faint light of the rogue companions. Because these corpses are hot, they are relatively brighter in the ultraviolet wavelengths that can only be observed with the Hubble Space Telescope.

“This search for Be stars – and the discovery of nine faint companion stars – truly demonstrates the power of CHARA,” said Alison Peck, program director in the National Science Foundation’s Division of Astronomical Sciences, which supports the CHARA Array. “Using the array’s exceptional angular resolution and high dynamic range allows us to answer questions about star formation and evolution that have never been possible to answer before.”

Douglas Gies, director of the CHARA Array, said the search has finally uncovered a key hidden stage in the lives of nearby stellar pairs.

“The CHARA Array survey of Be stars directly revealed that these stars were created through a total mass transfer transformation,” said Gies. “We are now seeing, for the first time, the result of the stellar feast that gave rise to the stripped stars.”

More information:
Robert Klement et al, The CHARA Array Interferometric Program on the Multiplicity of Classic Be Stars: New Detections and Orbits of Stripped Subdwarf Companions, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad13ec

Diary information:
Astrophysical Journal

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