Learn the trick to finding loose stars

Learn the trick to finding loose stars

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Artist’s depiction of the Be star and its disk (top right) orbiting a dim, hot, abstract star (bottom left). Credit: Painting by William Bounds

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Artist’s depiction of the Be star and its disk (top right) orbiting a dim, hot, abstract star (bottom left). Credit: Painting by William Bounds

Scientists working with powerful telescopes at the Center for High-Resolution Angular Astronomy Array (CHARA) in Georgia have completed a survey of a group of stars suspected to have eaten up most of the gas from companion stars orbiting them. These sensitive measurements have directly revealed the weak glow of detuned stars.

The new research, led by postdoctoral researcher Robert Clement, was published in Astrophysical Journal. This work defines new orbits for abstract dwarf stars orbiting rapidly rotating massive stars, leading to a new understanding of the life path of nearby binary stars.

Working with colleagues at the CHARA array at Mount Wilson, California, Clement pointed high-powered telescopes at a group of relatively nearby B emission line stars, or “be stars” for short. These are rapidly rotating stars that are thought to harbor unusual companions in orbit.

Be stars likely form in intense interactions between close pairs of stars. Astronomers have found that many stars occur in such pairs, a trend that is especially true among stars larger than our Sun. Couples with small separations face a turbulent fate because their size grows as they grow older and can reach a similar dimension of separation.

When this happens, gas from the developing star can cross the gap between the pair so that the companion can enjoy the transmitted gas stream. This cannibalization process will eventually strip the mass donor star of almost all of its gas and will leave behind the small hot core of the former nuclear burnout center.


CHARA matrix measurements (red ellipsoids) of the motion of the naked star (dashed line) orbiting the star Be HR2142 (yellow star) every 81 days. The small black star symbols are the calculated positions of the stripped companion during the time of our observations. The orbit is circular, but it appears elliptical because it is tilted relative to the plane of the sky. The top and right axes show the apparent physical separation in astronomical units (AU, the average distance between the Earth and the Sun) while the bottom and left axes show the angular separation in angular units of milliarcseconds (mas). For comparison, the angular diameter of a full moon in the sky is about 2 million milliarcseconds. Credit: Robert Clement

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CHARA matrix measurements (red ellipsoids) of the motion of the naked star (dashed line) orbiting the star Be HR2142 (yellow star) every 81 days. The small black star symbols are the calculated positions of the stripped companion during the time of our observations. The orbit is circular, but it appears elliptical because it is tilted relative to the plane of the sky. The top and right axes show the apparent physical separation in astronomical units (AU, the average distance between the Earth and the Sun) while the bottom and left axes show the angular separation in angular units of milliarcseconds (mas). For comparison, the angular diameter of a full moon in the sky is about 2 million milliarcseconds. Credit: Robert Clement

Astronomers expected that the mass transfer current would cause the companion star to spin upward and become rotating very quickly. Some of the fastest rotating stars are found to be Be stars. Be stars rotate so fast that some of their gas is ejected from their equatorial regions to form a gas ring that revolves around them.

Until now, this expected stage in the lives of close binary pairs has eluded astronomers because the separations between the stars are too small to see with conventional telescopes and because naked stellar corpses are hidden in the glare of their bright companions. However, Georgia State’s CHARA Array telescopes have provided researchers with a way to find naked stars.

The CHARA array uses six telescopes spread across the summit of Mount Wilson to act like one massive telescope, 330 meters in diameter. This gives astronomers the ability to separate the light of star pairs even with very small angular displacements. Clement also used the MIRC-X and MYSTIC cameras, made at the University of Michigan and the University of Exeter in the United Kingdom, which can record the optical signal of both very bright and very dim objects close to each other.

The researchers wanted to determine whether Be stars were spun by mass transfer and host naked stars orbiting around them. Clement began a two-year monitoring program at CHARA, and his work quickly bore fruit. He discovered the faint light of its naked companions in nine of 37 Be stars. He focused on seven of these targets and was able to follow the orbital motion of the stellar body around the Be star.

“Orbits are important because they allow us to determine the masses of stellar pairs,” Clement said. “Our mass measurements indicate that naked stars have lost almost everything. In the case of HR2142, the naked star has likely decreased 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 from 34 – 331 m in length and up to 10 potential triangles in the lock-in phase. credit: 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 from 34 – 331 m in length and up to 10 potential triangles in the lock-in phase. credit: Astrophysical Journal (2024). doi: 10.3847/1538-4357/ad13ec

Bare stars have not been discovered around every Be star, and researchers believe that in some of these cases, the corpse turned into a small white dwarf star, too faint to be detected even with the CHARA array. In other cases, the interaction may be so intense that the stars merge to form a single, rapidly rotating star.

more information:
Robert Clement et al., CHARA Interferometry Program on Classical Be Multistars: New Discoveries and Orbits of Isolated Subdwarf Companions, Astrophysical Journal (2024). doi: 10.3847/1538-4357/ad13ec

Magazine information:
Astrophysical Journal

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