A magnetic disturbance has been directly observed on a brown dwarf for the first time, showing that these objects behave more like stars than planets. They exhibit strong magnetic fields and possibly, like stars, interact with the discs of gas and dust that surround them in their youth.

“We had no clue if they were like small stars or big planets,” says at the University of Freiburg in Germany. That’s because brown dwarfs are in a grey area: Ìętheir mass is between about 10 and 80 times that of Jupiter, but less than one-tenth that of our sun. They aren’t massive enough to fuse hydrogen the way stars do, though they can briefly fuse the hydrogen isotope deuterium just after they form.

Unlike planets, they form from the contraction of gas clouds rather than the accretion of solid matter. They’re dim, but what light they do emit is due to heat from gas pressure that slowly cools over trillions of years, rather than the energy produced by fusion like in stars.

Berdyugina led a team that used spectral data from Keck telescope observations of a brown dwarf 55 times the mass of Jupiter called LSR J1835+3259, about 18.5 light years away. The light was polarised in a way that revealed a local magnetic field even bigger than the strong ones that accompany sunspots.

Berdyugina says this is so strong because brown dwarfs are relatively dense compared to stars. The field could also have been detected because one of the dwarf’s magnetic poles may have been facing roughly in the direction of Earth when it was observed.

Vanishing magnetism

Over the course of two nights, the magnetic disturbance disappeared, but then one appeared on the other side of the brown dwarf. Berdyugina and her colleagues say that probably happened because the disturbance rotated out of and back into view, but they are not certain.

“This measurement confirms some earlier ideas,” says at the University of Montpellier in France. He notes that radio astronomers have seen what seem to be auroras on brown dwarfs, which would require strong magnetic fields.

“But it also brings new puzzles,” he says. “How can a major magnetic region disappear so rapidly?” He notes that previous brown dwarf observations seemed to point to weaker magnetic fields.

If further observations show that the magnetic disturbance comes back at regular intervals, it means we are seeing it as the brown dwarf rotates and it is part of a global field rather than an intermittent phenomenon.

It could also be a sporadic effect of interaction between the brown dwarf, which scientists estimate is only 22 million years old, and an accretion disc.

Still, this magnetic field helps slot brown dwarfs into one category. “Now we can say they really are like small stars,” Berdyugina says.

Reference:

Read more: Brown dwarfs: From zeroes to astronomical heroes

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Novae – the bright explosions originating from white dwarf stars – have puzzled astronomers because they often shine brighter that should be possible without blowing themselves apart. How they do it comes as a shock.

Ray Li at Michigan State University and his colleagues studied the gamma ray emission and visible light from a nova called ASASSN 16ma. They concluded that the extra power comes from “shocks” – the gas that a nova initially blows out getting slammed from behind by faster blasts of gas.

A nova starts with a white dwarf. White dwarfs are the remains of stars up to 1.4 times the mass of the sun. When the star runs out of fuel it can no longer generate energy and support its own weight. The star collapses and becomes so dense that a single cubic centimeter can weigh 1000 metric tonnes – surface gravity is hundreds of thousands of times that of Earth. If the white dwarf has a companion star, it can pull gas from that star and crush it until that material fuses on the surface and explodes – a runaway reaction that we see as a nova.

Novae can’t be too luminous, though, because at a certain point, theoretically, they should just blow themselves apart. Yet many novae are brighter than theory would suggest.

This is because the nova first ejects gases at a few hundred kilometers per second, says the team. Soon after that, the white dwarf’s solar wind follows – but that’s moving 10 times as fast. The outer envelope of gas is full of charged particles that rebound off that faster gas as it slams into them from behind.

The resulting acceleration of the particles gives off gamma rays, and adds energy to the outer gaseous envelope. On top of that, X-rays emitted by the white dwarf light up the gas. This means that the bulk of the energy comes not from the surface of the white dwarf but from the outer gas envelopes.

“Traditionally, people believe thatÌęthe fusion on the white dwarf surface is the only energy source for theÌęvisible light in a nova,” Li says. “However, inÌęASASSN-16ma, the gamma rays and the optical emission areÌęstronglyÌęcorrelated, suggesting that they have the same origin – the shocks.”

The shocks themselves were already known about, says , who took part in the study, but “nobody thought they were very important”.

The team now wants to observe more novae to see if the hypothesis holds up, says Chomiuk. This will take some time because stellar explosions don’t happen on a regular schedule. Novae, she says, are still poorly understood. “We’re still trying to understand why some of our novae are so luminous and some not,” she says.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-017-0222-1

Read more: Weird ancient burst of light in the sky turns out to be a nova

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Comet-spotting

The “alien megastructure” star that has been puzzling us for the past few years might have a more ordinary explanation: an orbiting Saturn-like planet, complete with wobbling rings.

In 2015, a group led by Tabetha Boyajian, then of Yale University, found that a star called KIC 8462852 had dimmed several times over a few years in a way they couldn’t explain.

The star had been observed by the Kepler space telescope between 2009 and 2013 as it hunted for exoplanets by staring at a patch of sky. When a planet passes in front of a star, an event called a transit, the light intensity dips slightly and then returns to normal.

But KIC 8462852, since dubbed Tabby’s star, didn’t behave that way, with the amount of dimming varying wildly. Speculation abounded, with explanations ranging from exoplanetary comets to a vast orbiting “m±đČ”ČčČőłÙ°ùłÜłŠłÙłÜ°ù±đ” built by an advanced alien civilisation.

Now and his colleagues at the University of Antioquia in Colombia have proposed another possibility: a ringed planet, similar to Saturn, orbiting close to theÌęstar. Such a planet would dim theÌęstar’s light in an irregular way during a transit.

First, the rings would block some of the star’s light, followed by the planet, which would dim it further. Then, after the planet passes, the rings would block some light again.

New angle

But because the rings would be atÌęaÌędifferent angle each time, the smallÌędips at the beginning and end ofÌęthe transits would be larger or smaller. Without seeing many transits,Ìęthere would be no obvious pattern to this.

To test this idea and measure the irregularity, Sucerquia and his colleagues simulated a light curve from a ringed planet about one-tenth the Earth-sun distance from its star. They found another effect: the star would tug on the rings, making them wobble. This would make the silhouette of the rings as seen by anÌęearthbound observer even more irregular from transit to transit.

A ringed planet has been floated to explain Tabby’s star before, but that would have required a world many times the mass of Jupiter, enough to make it a small star. This new-found wobble means the planet could be the mass of Neptune.

at Vanderbilt University in Nashville, Tennessee, is not convinced the explanation works, as even wobbly rings would create a regular dimming pattern in the star’s light. “To my knowledge, there has not yet been a claim of quasiperiodic or periodic dimming in Tabby’s star, as one would expect with something linked to material orbiting that star,” he says.

Sucerquia says there are other explanations for the weird light curve as well – it might even be a moon breaking up. To further solidify his findings, he is comparing new simulations with the data from Tabby’s star and others.

“The point of this work is to show the community that there are mechanisms that can alter the light curves,” Sucerquia says. “These changes can be generated by the dynamics of the moons or the rings, and the changes in these systems can occur in such short scales as to be detected in just a few years.”

Journal reference: arXiv,

Fresh water

Disappointingly dry

Dark matter hitting black holes could be the source of some fast radio bursts – mysterious blasts of radio waves that come from billions of light years away, first detected 10 years ago.

The stuff we see in space – stars, planets and gas – only makes up about 18 percent of the mass of the universe. The rest is dubbed dark matter, which can’t be seen except through its gravitational interactions with everything else.

Nobody knows exactly what dark matter is, but one hypothesis is that it is formed of still-theoretical particles called axions. These particles, if they exist, would be very light, long-lived, and only interact weakly with other matter around them.

at Nishogakusha University in Tokyo says that because the early universe was smaller and offered more chances for axions to attract each other, they would have clumped together to form axion “stars”. Their tendency would be to cluster near the centre of galaxies, making them more likely to pass near the supermassive black holes that sit there and run into the accretion discs of gas that surround them.

“If there are many axion stars in the centres, we expect that some of them collide with the black hole accretion disc,” says Iwazaki. The magnetic field of the disc would cause some axions to decay into individual photons that are then seen on Earth as a fast radio burst (FRB), reaching us at the lower energies of radio wavelengths. There would be enough photons that the signal would still be bright.

This mechanism may also explain why some FRBs repeat at irregular intervals. So far, only one repeating burst has been found: FRB 121102, located in a galaxy some 2.5 billion light years away.

According to Iwazaki, such an effect could result from an axion star passing through a black hole’s accretion disc over and over. It would do so at irregular intervals until the disc’s magnetic field converted enough axions to photons that the axions could no longer form a compact clump.

In 2015, Iwazaki theorised that FRBs were the product of axion stars hitting the magnetic fields of neutron stars, the corpses of stars several times the sun’s mass. That wouldn’t explain the repetition seen from FRB 121102, because neutron stars don’t have accretion discs that would simply pull material off the axions rather than destroying them.

Axion reaction

at the University of Washington in Seattle says Iwazaki’s idea could be supported by mainstream physics, and that suggesting exotic sources for unexplained phenomena can be useful in any case.

One issue with Iwazaki’s model is that he assumes axions interact more strongly with magnetic fields than is suggested by many other theories, says Rosenberg.

That said, axions are an attractive hypothesis for dark matter because they address some open questions in particle physics and don’t require a lot of additions to the standard model, says at the University of Heidelberg in Germany. The standard model is the prevailing theory of particle physics, and has survived every test so far.

But it’s probably better to opt for more conventional explanations first, says at Cornell University in Ithaca, New York, who discovered the repeating FRB. He says the galaxy in which his team found FRB 121102 shares many characteristics with the places where we see certain kinds of supernovae and gamma ray bursts.

If that’s not mere coincidence, he says, the FRBs may also be powered by the types of features found in those locations – such as a young magnetar in a wind nebula or a supernova remnant.

Reference:

Read more: Could fast radio bursts really be powering alien space ships?

Talking to Alpha Centauri

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