This white dwarf spins in just 25 seconds!

This white dwarf spins in just 25 seconds

25 seconds. It does not take more for this amazing white dwarf to completely turn on itself. A record! All thanks to a companion bigger than her and to a strange mechanism called“Magnetic propeller”.

A white dwarf, it’s a star which burned all its fuel. After losing its outer layers, it continues to shrink and cool for millions of years. And J0240 + 1952 is one of them. About the size of our Earth, but 200,000 times more massive. But what makes her really special is her amazing rotation speed. As our planet turns on itself in 24 hours, this white dwarf star does it in just … 25 seconds!

The researchers from the University of Warwick (United Kingdom) explain this extreme speed by the presence, near J0240 + 1952, of another star. The white dwarf, indeed, aspires to matter of his companion. A plasma falling at high speed on its equator and provides him with the formidable energy which feeds its rotation.

From “Magnetic propellers” the sky?

It is moreover this plasma which allowed astronomers to measure the rotational speed of J0240 + 1952. At one point in its history, the star developed a strong magnetic field. As a result, much of the aspirated plasma is actually propelled into space at some 3,000 km / s. The rest flows to the magnetic poles of the white dwarf. Forming bright spots and causing identifiable pulsations in the light that reaches us. As long as we use the most sensitive of our instruments.

In some 70 years of observations, this is only the second time that researchers have noted this so-called“Magnetic propeller”. But the first time they can actually confirm the high speed of rotation of the white dwarf. “Based on our first observation, we had built a model. He predicted that the white dwarf had to spin very quickly. This second observation confirms it “, enthuses Tom Marsh, physicist, in a communicated. What now to consider this mechanism of“Magnetic propeller” as a generic property of binary systems with white dwarf meeting certain conditions.

The three fastest spinning brown dwarfs ever measured

Using the space telescope Spitzer, scientists have identified the three brown dwarfs which spin the fastest on themselves. Having almost exactly the same spin speed (about one spin per hour), they could be a sign of a spin speed limit beyond which a brown dwarf could disintegrate.

Item Adrien coffinet published on 04/17/2021

Brown dwarfs are objects of intermediate mass Between the planets and the stars. Three of these objects, discovered in 2001 by the 2MASS statement, turn out to be the brown dwarfs at the fastest rotation known to date.

These three objects spin around in just over an hour. For comparison, the fastest brown dwarfs known so far turn in about 1.4 hours and Jupiter in 9.9 hours. In doing so, the equator of the largest member of the trio rotates at more than 360,000 km / h, against 1,670 km / h for the terrestrial equator and 45,300 km / h for that of Jupiter.

A probable speed limit of rotation

Using data of the space telescope Spitzer and Gemini North and Magellan ground telescopes, Megan E. Tannock and colleagues measured the rotation period about 80 brown dwarfs, and it varies from less than two hours to several tens of hours. With such diversity, the authors of this new study were surprised that the three fastest brown dwarfs had almost exactly the same spin speed. This similarity cannot be attributed to the fact that brown dwarfs would have formed together or would be at the same stage of their development because they are physically different: one is hot, another is cold and the last is in between. Since the brown dwarfs cool down as they get older, the differences in temperature suggest that these brown dwarfs have different ages.

Comparison of the flattening and rotation of 2MASS J0348-6022, the fastest rotating brown dwarf known, with those of Jupiter and Saturn. © Nasa / JPL-Caltech

For the authors, this is no coincidence: these objects would have reached a speed limit beyond which a brown dwarf could disintegrate. All rotating objects generate a centripetal force, which increases with the speed of rotation, which can “tear” the object. Before a rotating object breaks, it will usually warp, with an equatorial diameter greater than its polar diameter. Just like saturn, which turns in about ten hours like Jupiter, these brown dwarfs are therefore probably a little flattened.

An insurmountable limit?

Since brown dwarfs tend to speed up with age, do these objects regularly exceed their spin speed limit and get “torn”? In other rotating celestial objects, such as the stars, there are natural braking mechanisms that prevent them from destroying each other. It is not clear if similar mechanisms exist in brown dwarfs.

It would be quite spectacular to find a brown dwarf spinning so fast that she ejects her atmosphere in the space, said Megan Tannock, currently doing a PhD at The University of Western Ontario. But so far, we haven’t found any. I think that must mean that something is slowing brown dwarfs down before they reach that extreme or they can’t go that fast in the first place. The result of our article supports some sort of limit on the turnover rate, but we are still not sure why. “.

To determine the rotation period of 2MASS J0348-6022, the scientists used the change in its brightness over time as measured by Spitzer. They also compared the spectrum of the brown dwarf to that expected according to its speed of rotation to confirm their result. This rapid rotation leads to the slight flattening of the brown dwarf, similarly to Jupiter and Saturn. © Nasa, JPL-Caltech

The maximum rotational speed of an object is determined not only by its total mass, but also by how that mass is distributed. It is therefore important to understand the internal structure of brown dwarfs which, like Jupiter and Saturn are mainly composed ofhydrogen and D’helium. To the pressures in the hearts of brown dwarfs, hydrogen would behave like a metal rather than a gas inert. It changes the way heat is conducted inside and, with very fast rotational speeds, it could affect the mass distribution inside as well.

According to current models, the maximum rotational speed of brown dwarfs should be greater than the one hour rotation period described in the new study by about 50 to 80%. “ It is possible that these theories do not yet have a complete overview. Said Stanimir Metchev, co-author of the article and holder of the Canada Research Chair in extrasolar planets at the Institute for Earth and Space Exploration at Western University. ” Some factors not taken into account could come into play, which would not allow the brown dwarf to turn faster. Further observations and theoretical work could reveal whether there is a braking mechanism that prevents brown dwarfs from self-destructing and whether there are brown dwarfs that spin even faster.

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