* Astronomy

Title: A resolved outflow of matter from a Brown Dwarf
Authors: Emma T. Whelan, Thomas P.Ray, Francesca Bacciotti, Antonella Natta, Leonardo Testi, Sofia Randich

The birth of stars involves not only accretion but also, counter-intuitively, the expulsion of matter in the form of highly supersonic outflows.
Although this phenomenon has been seen in young stars, a fundamental question is whether it also occurs amongst newborn brown dwarfs: these are the so-called 'failed stars', with masses between stars and planets, which never manage to reach temperatures high enough for normal hydrogen fusion to occur.
Recently, evidence for accretion in young brown dwarfs has mounted, and their spectra show lines that are suggestive of outflows. Here we report spectro-astrometric data that spatially resolve an outflow from a brown dwarf. The outflow's characteristics appear similar to, but on a smaller scale than, outflows from normal young stars. This result suggests that the outflow mechanism is universal, and perhaps relevant even to the formation of planets.

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Astronomers clinched the case for jet by studying the brown dwarf circled in this image of Rho Ophiuchi (Image: David Malin, AAT)


Brown dwarfs are intermediate in mass between normal stars and planets.
Their mass is so low (between 13 and 75 times the mass of Jupiter) that a brown dwarf's core is too cool to ignite hydrogen, hence they are sometimes called failed stars.
But astronomers are divided over what stunts their growth.

The birth of a normal star is accompanied by both accretion and expulsion of matter. There is growing evidence for small accretion disks around brown dwarfs, so the question arises: do brown dwarfs have outflows too?

Emma Whelan of Ireland's Dublin Institute for Advanced Studies (DIAS) show that they do, and that their outflows are miniature versions of those produced by T Tauri stars (precursors to stars like the Sun).

Some think they condense from gas and dust clouds like other stars but are simply limited by the size of the clouds. Others say they are tossed out of their birth clouds prematurely by gravitational tussles with other growing stars in the cloud.
Recent observations of large, dusty discs around brown dwarfs have bolstered the former theory, as only small - if any - discs are thought to be able to survive a violent ejection in the latter scenario.

Emma Whelan has observed a jet stretching 1.5 billion kilometres from a young brown dwarf in a stellar nursery called Rho Ophiuchi, mimicking a process seen in young stars. The observation suggests brown dwarfs form like stars - and even hints that jets might once have gushed from planets such as Jupiter and Saturn.

Similar jets have been detected around young, massive stars and are thought to form from material in the disc that swirls around them. The stars grow when matter falls on them from the disc, but the stars' magnetic fields funnel about a tenth of that matter back through the jets.

"They form like ordinary stars rather than being ejected. But it might end up that brown dwarfs form by two methods." - Thomas Ray, DIAS, team member.

"This new result confirms beautifully, for the first time, that young brown dwarfs indeed harbour extended jets, further strengthening the analogy with young stars" - Ray Jayawardhana, astronomer at the University of Toronto in Canada, (not part of team).

Previous observations had hinted at brown dwarf jets, but they had been too faint to see directly. Astronomers clinched the case for this jet by studying the brown dwarf's spectrum for three hours using one of the Very Large Telescope's 8-metre dishes in Chile. They measured material rushing away from the brown dwarf at 45 kilometres per second.

Jets are also seen spewing from some galaxies, apparently fed by material falling towards colossal black holes at their centres.



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The colours of the Rho Ophiuchi clouds highlight the many processes that occur there. The blue regions shine primarily by reflected light. Blue light from the star Rho Ophiuchi and nearby stars reflects more efficiently off this portion of the nebula than red light.
The red and yellow regions shine primarily because of emission from the nebula's atomic and molecular gas. Light from nearby blue stars - more energetic than the bright star Antares - knocks electrons away from the gas, which then shines when the electrons recombine with the gas. The dark regions are caused by dust grains - born in young stellar atmospheres - which effectively block light emitted behind them.
The Rho Ophiuchi star clouds, about 500 light years away, are in front of the globular cluster M4 visible above on far lower left, are even more colourful than humans can see - the clouds emits light in every wavelength band from the radio to the gamma-ray.


"If we see the mechanism in brown dwarfs, ordinary stars and massive black holes, there's no reason it couldn't continue downwards to the planetary stage" - Thomas Ray.
The presence of moons around giant planets like Jupiter and Saturn suggest they were surrounded by dusty discs, and those discs could have fuelled jets in their youth.

All jets can reveal crucial information about the objects that spawned them. "The nice thing about jets is we can tell things like densities, temperatures, pressures, elements, even the mass-flow rates" of the material inside them. And knots of mass in the jets can reveal "episodes when the star has been really active and accreting material, like a fossil record," - Thomas Ray.

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