A team headed up at Western University in Canada has found three brown dwarfs that are spinning faster than ever seen before – completing a full rotation once per hour. The brown dwarfs were identified by NASA’s Spitzer Space Telescope and later studied with ground-based telescopes, such as the Gemini North telescope on the summit of Mauna Kea in Hawaii. The discovery, led by graduate student Megan Tannock, could help determine the fastest speed at which a brown dwarf can rotate before tearing itself apart.

So why are brown dwarfs so interesting? These substellar objects are more massive than planets but not quite as massive as stars. Like stars, they form from the collapse of a giant molecular cloud. But unlike stars, they do not have enough mass for nuclear fusion of hydrogen into helium to occur. The three newly-discovered brown dwarfs have roughly same diameter as Jupiter, but are between 40 and 70 times more massive.

Bright spots

Like stars and planets, brown dwarfs are already spinning at the beginning of their lifetime. As they age, brown dwarfs cool and contract, and their spin rates increase in order to conserve angular momentum.

The astronomers used the Spitzer Space Telescope’s IRAC instrument to observe each brown dwarf photometrically and measure the speed of its rotation. They did this by examining the brightness of features, such as spots on the surface, which changes according to whether the spot is facing towards or away from us. By measuring these repeated patterns of brightness variation, the team determined that the brown dwarfs were spinning with a period of one hour.

The spin rates of approximately 80 brown dwarfs have been measured to date, with varying speeds of up to tens of hours. The previously fastest known brown dwarfs completed full rotations every 1.4 hours. With their record-breaking rotation rate of once per hour, these three new brown dwarfs are spinning at an astonishing rate of about 100 km/s.

Tannock explains that, in order to confirm the results were “not a half-period caused by a repeated spot pattern”, the astronomers also measured spectral changes caused by the Doppler effect. They compared near-infrared spectral observations from the Gemini North and Magellan Baade ground-based telescopes with the spectra predicted from computer models.

Proposed speed limits

Considering the wide range of brown dwarf spin rates, it came as a surprise to the researchers that these three brown dwarfs shared similar rotation times. Assessing the age of each brown dwarf, using temperatures and surface gravities determined from their spectra, suggested that the three did not share the same age. The question then arises of why do they have similar spin rates?

The answer may lie in the centripetal force that’s generated by all rotating objects. Centripetal force increases as an object’s spin (and for a brown dwarf, its age) increases, eventually resulting in the object being ripped apart. Before this occurs, however, the midsection of the object will begin to bulge, a feature called oblation. The researchers measured this feature to determine how close the brown dwarfs were to the breakup point. Finding that the three brown dwarfs have similar degrees of oblation, they suggest that all three could be approaching a spin speed limit.

Tannock believes that “there is likely a braking mechanism, and brown dwarfs can’t actually spin so fast that they fly apart”. In other rotating cosmic objects such as low-mass stars, their magnetic fields cause large amounts of braking. “Brown dwarfs have strong magnetic fields in their interiors, and they are also fully convective on the inside,” says Tannock, noting that brown dwarfs could therefore exhibit a similar braking effect to low-mass stars.

Some scientists have suggested that a breakup would occur at a period of 20 minutes, with current models indicating that brown dwarfs could potentially spin 50–80% faster than these three. However, periods between 20 minutes and one hour are yet to be observed.

As such, Tannock suggests that “the minimum rotation period is around one hour”, and that further measurements could help determine whether there is a limit to the rotation speed. The future James Webb Space Telescope could possibly measure the low-amplitude variations, while theoretical work could help further understand the physics of brown dwarfs’ internal structures. “We are looking forward to seeing what people come up with,” say the study authors.