Brown dwarf hotter than the sun discovered in binary system
A binary system 1400 light years away consisting of a brown dwarf and white dwarf has been discovered. The brown dwarf, which orbits its low-mass white dwarf companion, is the hottest of its kind with day-side temperatures reaching a scolding 8000K. Typical temperatures for brown dwarfs are a mere 2500K, and even the sun is only lukewarm in comparison with a temperature of 5772K.
Such extreme heat is not powered by internal nuclear reactions, but rather due to the close orbit the brown dwarf has to its companion, WD 0032-317. Since the system is tidally locked, the same side of the brown dwarf is pelted with radiation which results in impressively high temperatures. As a consequence, the night side is much cooler at around 2000K.
The system was first observed in the early 2000s when astronomers were using ESO’s Very Large Telescope in Chile to survey white dwarfs. After being discovered by the Ultra-Violet-Visual Echelle Spectrograph (UVES), the white dwarf gained the attention of scientists due to its hot temperatures of 37,000K. A white dwarf is a star in the final stage of its life. Once a star runs out of fuel for nuclear fusion, it expands as a red giant. Shortly after, the outer layers are blown away to reveal a dense, inert core. The system was originally labelled as a binary system consisting of two white dwarfs, however, later analysis revealed signs of a brown dwarf companion.
Brown dwarfs are objects in between that of a star and planet, also known as ‘failed stars’. Although they are much more massive than planets, they cannot generate enough pressure and heat to ignite fusion beyond deuterium burning. Along with being the hottest brown dwarf found, the failed star in the WD 0032-317 binary system may also be among the largest yet, with a mass 75-88x that of Jupiter.
Follow-up observations of the system were taken, revealing emission emanating from the day side of the brown dwarf. This was originally missed since observations were taken of the night side. “We could see an emission line coming from the irradiated side of the companion,” said Na’ama Hallakoun, study lead author and observational astrophysicist at the Weizmann Institute of Science in Israel. “I was puzzled. My first thought was to think that I had done something wrong during the redemption process of the data.”
This system can be used as an important tool to help astronomers better understand exoplanets that take close-in orbits to hot, massive host stars. Such an example are ultra-hot Jupiter systems, where intense UV stellar radiation can evaporate the atmospheres of the exoplanet, even vaporising the planetary material. However, this process is hard to observe and study, and as a result, only one system of this kind has been observed: KELT-9b, an exoplanet so hot that it leaves a trail of material behind in its orbit. The difficulties of observing these systems arise due to a combination of the glare and rapid rotation of host stars, which make it hard to discern the wobble of the star when measuring the radial velocity signal. Therefore, using white dwarf - brown dwarf binary systems is the next best thing. “Jupiter analogues give you an indirect way of studying the atmospheres of giant planets because brown dwarfs should have atmospheres very similar to those of gas giant planets,” said Hallakoun.
This discovery also provides insight into stellar evolution. Using current models, the brown dwarf is a few billion years old, however, the white dwarf is younger at 1 billion years. Since the white dwarf is 0.4 solar masses, it cannot have existed on its own, since a star of this mass would need longer than the age of the universe to enter its white dwarf phase. The research team has suggested that the brown dwarf played a role in the evolution of its companion, sharing a common envelope until around 1 million years ago. This common envelope developed when the primary star expanded into a red giant, engulfing the brown dwarf.
“The brown dwarf may have helped the primary star shed some of its mass and become a white dwarf earlier than expected for a single star” Hallakoun says. “It’s a really young, fast common-envelope system. And we also hope that this system and other such systems will help us better constrain the theoretical models we have to explain binary motions.”
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Cover image: NASA/JPL-Caltech
Journal source: N. Hallakoun et al. An irradiated-Jupiter analogue hotter than the Sun. Nat Astron, published online August 14, 2023; doi: 10.1038/s41550-023-02048-z