Our ideas of what is conceivable in the universe are being significantly challenged by an object that is orbiting a star 1,400 light-years away.
It is a brown dwarf, a peculiar class of objects that lie between planets and stars, but because of the close proximity of its host star, which is extremely hot, it has a temperature that exceeds 8,000 Kelvin (7,727 degrees Celsius, or 13,940 degrees Fahrenheit). This temperature is hot enough to split the molecules in its atmosphere into their constituent atoms.
That is considerably hotter than the Sun’s surface, where temperatures are a relatively cool 5,778 Kelvin. This brown dwarf is actually the hottest object of its sort that we have ever discovered, breaking records for temperature.
Even while brown dwarfs are frequently hotter than planets, they burn at a lower temperature than the coolest red dwarf stars, meaning that they cannot possibly reach Sun-like temperatures by internal fusion.
The object has been designated WD0032-317B by an international team under the direction of astronomer Na’ama Hallakoun of the Weizmann Institute of Science in Israel.
The discovery, according to the team, can aid in our understanding of what transpires in Jupiter-like gas giants orbiting extremely hot, massive stars, whose characteristics, such as activity and rotation rate, make them difficult to see.
Huge volumes of ultraviolet light are radiated onto planets that are in close orbit to their sun. Thermal dissociation, a process that causes the molecules in their atmospheres to be pulled apart, can result from this.
But our understanding of this harsh environment is limited. It can be challenging to separate stellar activity from signals from a circling exoplanet when they are so near to a very brilliant star.
One exoplanet known to us is hot enough to experience thermal dissociation. That is KELT-9b, an exoplanet that experiences dayside temperatures that exceed 4,600 Kelvin (4,327 degrees Celsius, or 7,820 degrees Fahrenheit), thanks to its orbit around a blue supergiant star.
That is hotter than the majority of stars, which have a maximum surface temperature of roughly 4,000 Kelvin. Red dwarfs are the most prevalent stars in the galaxy.
Brown dwarfs in binary systems with white dwarf stars, however, might be one way to examine these severe regimes. White dwarfs are far smaller than blue supergiants like KELT-9, which makes them dimmer and makes it easier to extract the signal from any superheated companion objects.
A brown dwarf is neither quite a planet nor quite a star. A planet-like object can have enough pressure and heat in its core to start deuterium fusion if it is around 13 times the mass of Jupiter.
That is a ‘heavy’ isotope of hydrogen, and the temperature and pressure needed to fuse it are very different from those needed to fuse the ordinary hydrogen that burns in stars’ cores.
Brown dwarfs have temperatures of about 2,500 Kelvin and can grow to be about the size of 80 Jupiter masses. They shine in the infrared but are cooler and fainter than red dwarfs.
On the other hand, for stars like the Sun, white dwarfs represent the end of their lives. The star’s outer layers are ejected when the hydrogen in its core runs out, leaving the core to collapse into a very dense object around the size of Earth because it is no longer supported by the pressure of fusion.
White dwarfs are incredibly hot, with temperatures comparable to those of blue supergiants, and they glow with residual heat, yet the dying process is very intense.
This gets us to WD0032-317, a low-mass white dwarf star that is extremely hot. Its mass is almost 40% that of the Sun, and it burns at a temperature of 37,000 Kelvin.
Data from the Ultra-Violet-Visual Echelle Spectrograph (UVES) instrument on the Very Large Telescope of the European Southern Observatory in the early 2000s revealed that WD0032-317 was moving, possibly being pulled around by an unidentified orbiting companion. That partner may have been a brown dwarf, according to recent findings in the near infrared.
New observations of the star were obtained by Hallakou and her team using UVES, and they discovered that the companion is a brown dwarf with a mass of between 75 and 88 Jupiters with an incredibly fast orbit of just 2.3 hours.
A sort of smoking star served as the detection’s smoking gun. Astronomers could observe the gas it produces when the star evaporates it while the brown dwarf’s day side is towards us.
The brown dwarf is tidally locked as a result of how close it is to the star. This implies that one side—the day side—is always towards the star and the other side is always at night. The results from the team’s analysis of the extremely high temperatures involved are staggering.
The companion’s heated day-side temperature can range from 7,250 to 9,800 Kelvin, which is as hot as an A-type star, with a night-side temperature of 1,300 to 3,000 Kelvin, or a temperature difference of 6,000 K, which is about four times larger than that of KELT-9b, according to the authors of the paper.
T through M dwarfs are covered under the nightside temperature range. The irradiated companion’s ‘equilibrium’ black-body temperature is approximately 5,100 Kelvin, which is hotter than any known giant planet and about 1,000 Kelvin hotter than KELT-9b. This results in a flux of extreme ultraviolet light that is approximately 5,600 times higher than that of the unirradiated companion.
The fact that WD0032-317B is hotter than any other planet or brown dwarf makes it a prime candidate for research into how extremely hot stars can evaporate their lower-mass neighbors. According to the researchers, WD0032-317B can be studied to better comprehend unusual outlier objects like KELT-9b.
The research has been published in Nature Astronomy.
