Brown dwarf aurora may help characterize distant exoplanets
The discovery of apowerful aurora surrounding a distant failed star may in future aidastronomers in their hunt for habitable planets. The aurora is thefirst to be discovered around a brown dwarf, known as LSRJ 1835+3259(LSRJ). It's a type of star that shares many characteristics with knownexoplanets, and the technique used to observe the phenomenon couldone day be a factor in determining whether a planet could sustainlife.
There are untoldbillions of stars in our Milky Way that exist in a baffling range ofshapes and sizes, from enormous red supergiants to tiny yetincredibly dense neutron stars. However, the category of star thatmost of us are familiar with is the yellow dwarf – theclassification of our own Sun.
A brown dwarf isessentially a star that failed to sustain enough mass to kick startthe hydrogen nuclear fusion reaction at its core required to make itburn. Due to this evolutionary failure, a brown dwarf exhibitscharacteristics common with both stars and large planets. Thanks tothese similarities, the discovery of an aurora present on a browndwarf could have implications in the ongoing search for habitableexoplanets.
A paper covering therecent observations of LSRJ asserts that the aurora, the first toever be discovered around a brown dwarf, should also be presentaround other very faint analogues of the quasi-star, and that we mayin the future be able to use similar radio emissions to characterizedistant worlds.
"For the coolestbrown dwarfs we've discovered, their atmosphere is pretty similar towhat we would expect for many exoplanets, and you can actually lookat a brown dwarf and study its atmosphere without having a starnearby that's a factor of a million times brighter obscuring yourobservations," states assistant professor of astronomy atCaltech, Gregg Hallinan.
The aurora on LSRJ,which Hallinan and his team believe to be hundreds of thousands oftimes more powerful than any phenomena of its kind in our solarsystem, was detected using a combination of optical and radiotelescopes.
The seeds for thediscovery were planted back in the early 2000s, when radio waves werefirst seen emitting from a brown dwarf. This baffled astronomers, asthe bodies lack the standard method of radio wave emission exhibitedby stars such as our own Sun that creates the waves through therelease of charged particles and solar flares.
In 2006, Hallinanrevealed that the enigmatic brown dwarf radio emissions seemed topulse in a similar manner to those from planets in our solar systemknown to host aurora. The new study employed the Very Large Array(VLA) located at the National Radio Astronomy Observatory, NewMexico, to characterize the radio emissions from LSRJ. Once again,Hallinan observed radio pulsing as the brown dwarf rotated.
Simultaneously, LSRJwas subjected to surveillance from the Palomar Observatory's Haletelescope, which recorded variations in brightness across the h-alphaemission line. The teamthen employed optical telescopes located at the Keck Observatory,Hawaii, to make timed observations of the brown dwarf's brightness.The results of the study led to one clear conclusion – that thepulsing radio emissions are the signature of an incredibly powerfulaurora.
Whilst the discovery ofthe aurora may answer the riddle of the radio emissions, it does notexplain how an aurora is able to form on a brown dwarf. Ordinarily,for an aurora to form, stellar winds from a nearby star carry chargedparticles into a planet's magnetosphere, allowing them to excite thegas atoms within to produce colorful emissions such as the auroraborealis.
However, with LSRJthere is no nearby star to produce the stellar wind, and so the teamare uncertain as to how the aurora is being produced. One possibleexplanation holds that a planet orbiting the brown dwarf could havewhipped up enough of a current to drive charged particles into themagnetosphere, but further observations will be needed to solve theriddle.
In the future, Hallinanand his team hope to use low-frequency radio observations from thenewly-constructed Owens Valley Long Wavelength Array, located inCalifornia. This would allow them to measure the strength of an exoplanet's magnetic fieldin much the same way that optical and radio wave observations wereused to characterize LSRJ.
"That could be particularly interestingbecause whether or not a planet has a magnetic field may be animportant factor in habitability," explains Hallinan. "I'mtrying to build a picture of magnetic field strength and topology andthe role that magnetic fields play as we go from stars to browndwarfs and eventually right down into the planetary regime."
A paper outlining the discovery has been publishedin the online journal Nature.