A lonely pair of gas giants that could never become a star
Star formation processes sometimes give rise to astronomical objects called brown dwarfs. They are smaller and colder than stars, and in the most extreme cases can have masses and temperatures down to those of exoplanets. Like stars, brown dwarfs often wander through space alone, but they can also appear in binary systems, where two brown dwarfs orbit each other and travel together in the galaxy.
Researchers led by Clémence Fontanive of the Center for Space and Habitability (CSH) at the University of Bern have now discovered a curious starless binary system of brown dwarfs. The system, CFHTWIR-Oph 98 (or Oph 98 for short), consists of the two very low-mass objects Oph 98 A and Oph 98 B. It is located 450 light-years from Earth. Researchers were surprised by the fact that Oph 98 A and B orbit each other at a strikingly large distance, about 5 times the distance between Pluto and the Sun (200 AU). The study has been published in The Astrophysical Journal Letters.
The pair is a rare example of two objects that resemble extrasolar giant planets in many respects, orbiting each other without a parent star. The more massive component, Oph 98 A, is a young brown dwarf with 15 times the mass of Jupiter, which is almost exactly on the boundary between brown dwarfs and planets. Its companion, Oph 98 B, is only 8 times heavier than Jupiter.
The components of binary star systems are connected by an invisible bond called gravitational binding energy, and this bond becomes stronger the more massive the objects are or the closer they are to each other. With extremely low masses and a very large separation, Oph 98 has the weakest binding energy of any binary star system known to date.
Clémence Fontanive and her colleagues discovered Oph 98 A’s companion using images from the Hubble Space Telescope. Fontanive explains, «Low-mass brown dwarfs are very cold and emit very little light, only through infrared thermal radiation. This thermal radiation is extremely faint and red, and brown dwarfs are therefore visible only in infrared light.» In addition, the stellar cluster containing the double, Ophiuchus, is embedded in a dense, dusty cloud that scatters visible light. «Infrared observations are the only way to see through this dust,» says the principal investigator. «Also, to detect a system like Oph 98, you need a camera with a very high resolution, because the angle separating Oph 98 A and B is thousands of times smaller than the size of the moon in the sky,» she adds. The Hubble Space Telescope is among the few telescopes capable of observing objects as faint as these brown dwarfs, and capable of resolving such narrow angles.
Because brown dwarfs are cold enough, water vapor forms in their atmospheres, producing distinctive features in the infrared that are commonly used to identify brown dwarfs. However, these water signatures are not easily seen from Earth’s surface. Hubble, located above the atmosphere in the vacuum of space, can be used to study the existence of water vapor in astronomical objects. Fontanive explains, «Both objects looked very red and showed clear signs of water molecules. This immediately confirmed that the faint source we saw next to Oph 98 A was very likely a cold brown dwarf as well, and not a random star that happened to be aligned with the brown dwarf in the sky.»
The binary star system Oph 98 formed just 3 million years ago in the nearby stellar nursery Ophiuchus, making it a newborn on astronomical timescales. The system is thus much younger than the typical time required for planets to form. Brown dwarfs like Oph 98 A are formed by the same mechanisms as stars. Although Oph 98 B is the right size for a planet, the host Oph 98 A is too small to have a large enough reservoir of material to form such a large planet. «This tells us that Oph 98 B, like its host, must have formed by the same mechanisms that create stars, and shows that the processes that create binary stars work in scaled-down versions down to these planetary masses,» Fontanive comments.