https://news.westernu.ca/2021/04/brown-dwarf-discovery/
More massive than most planets, brown dwarfs can spin at upwards of 200,000 mph, but there may be a limit to how fast they can go.
Brown dwarfs are failed stars about the size of Jupiter, with a much larger mass but not quite large enough to become stars. Like the sun and Jupiter, they are composed mainly of hydrogen gas, perhaps with swirling cloud belts.
Illustration by Robert Hurt (IPAC/Caltech)
Using data from NASA’s Spitzer Space Telescope, a team led by Western space scientists Megan Tannock and Stanimir Metchev has identified the three fastest-spinning brown dwarfs ever found. More massive than most planets but not quite heavy enough to ignite like stars, brown dwarfs are cosmic in-betweeners. And though they aren’t as well-known as stars and planets to most people, they are thought to number in the billions in our galaxy.
Megan Tannock
“We seem to have come across a speed limit on the rotation of brown dwarfs,” said Tannock, a Western PhD candidate. “Despite extensive searches, from our own team and others, no brown dwarfs have been found to rotate any faster. In fact, any faster spins may lead to the brown dwarf tearing itself apart.”
The speed measurements were made using data from Spitzer, which NASA retired in January 2020. (The brown dwarfs were discovered by the ground-based Two Micron All Sky Survey, or 2MASS, which ran until 2001). The team then corroborated their unusual findings through observations with the ground-based Gemini North and Magellan telescopes.
https://youtu.be/UnG86tNpe4c
Brown dwarfs, like stars or planets, are already spinning when they form. As they cool down and contract, they spin faster, just like when a spinning ice skater draws her arms into her body. Scientists have measured the spin rates of about 80 brown dwarfs, and they vary from less than two hours (including the three new entries) to tens of hours.
With so much variety among the brown dwarf speeds already measured, it surprised the authors of the new study that the three fastest brown dwarfs ever found have almost the exact same spin rate (about one full rotation per hour) as each other. This cannot be attributed to the brown dwarfs having formed together or being at the same stage in their development, as they are physically different: One is a warm brown dwarf, one is cold, and the other falls between them. Since brown dwarfs cool as they age, the temperature differences suggest these brown dwarfs are different ages.
The authors aren’t chalking this up to coincidence. They think the members of the speedy trio have all reached a spin speed limit; beyond this, a brown dwarf could break apart.
All rotating objects generate centripetal force, which increases the faster the object spins. On a carnival ride, this force can threaten to throw riders from their seats; in stars and planets, it can tear the object apart. Before a spinning object breaks apart it will often start bulging around its midsection as it deforms under the pressure. Scientists call this oblation. Saturn, which rotates once every 10 hours like Jupiter, has a perceptible oblation. Based on the known characteristics of the brown dwarfs, they likely have similar degrees of oblation, according to the paper authors.
https://youtu.be/5ITpH-ViE1k
Reaching the Speed Limit
Considering that brown dwarfs tend to speed up as they age, are these objects regularly exceeding their spin speed limit and being torn apart? In other rotating cosmic objects, like stars, there are there natural braking mechanisms that stop them from destroying themselves. It’s not clear yet if similar mechanisms exist in brown dwarfs.
“It would be pretty spectacular to find a brown dwarf rotating so fast it is tossing its atmosphere out into space,” said Tannock. “But so far, we haven’t found such a thing. I think that must mean that either something is slowing the brown dwarfs down before they hit that extreme or that they can’t get that fast in the first place. The result of our paper supports some sort of limit on the rotation rate, but we’re not sure of the reason yet.”
Brown dwarfs are more massive than planets but not quite as massive as stars. Generally speaking, they have between 13 and 80 times the mass of Jupiter. A brown dwarf becomes a star if its core pressure gets high enough to start nuclear fusion. Image by NASA/JPL-Caltech
The maximum spin rate of any object is determined not only by its total mass but by how that mass is distributed. That’s why, when very rapid spin rates are involved, understanding a brown dwarf’s interior structure becomes increasingly important: The material inside likely shifts and deforms in ways that could change how fast the object can spin. Similar to gas planets such as Jupiter and Saturn, brown dwarfs are composed mostly of hydrogen and helium.
But they are also significantly denser than most giant planets. Scientists think the hydrogen in the core of a brown dwarf is under such tremendous pressures that it starts behaving like a metal rather than an inert gas: It has free-floating conducting electrons, much like a copper conductor. That changes how heat is conducted through the interior, and with very fast spin rates, may also affect how the mass inside an astronomical object is distributed.
Stanimir Metchev
Physicists use observations, laboratory data, and mathematics to create models of what brown dwarf interiors should look like and how they should behave, even under extreme conditions. But current models show that the maximum brown dwarf spin speed should be about 50 per cent to 80 cent faster than the one-hour rotation period described in the new study.
“It is possible that these theories don’t have the full picture yet,” said Metchev. “Some unappreciated factor may be coming into play that doesn’t let the brown dwarf spin faster.” Additional observations and theoretical work may yet reveal whether there’s some braking mechanism that stops brown dwarfs from self-destruction and whether there are brown dwarfs spinning even faster in the darkness.
Recommend this post and follow Sputnik's Orbit
Posted by Chuck in Space
No comments:
Post a Comment