A Dublin institute is to the forefront in the study of the 'failed star' known as the Black Dwarf. Researchers at the Dublin Institute for Advanced Studies have made important new discoveries about the small, indistinct "failed stars" known to astronomers as a Brown Dwarf, writes Dick Ahlstorm
Their findings have revealed a great deal about these objects, how they form and whether they are more like a small star or a big planet.
"People really don't understand how these Brown Dwarfs form," says PhD student in the institute's school of cosmic physics Emma Whelan. They are small, visually weak and very difficult to study as a result, she says.
That is what makes the findings by Whelan and her supervisor at the Institute, Prof Tom Ray, so important. Their discovery, using telescope data recorded by collaborators at the Osservatorio Astrofisico di Arcetri in Italy, was published last month in the journal, Nature.
"Most of what we know about how stars form is from low mass stars," says Whelan. Star birth occurs inside huge clouds of dust and debris such as the Giant Molecular Clouds of our Milky Way Galaxy. The dust begins to collapse in on itself due to gravitational pull.
If enough mass accumulates there may be sufficient heat and gravity to trigger the hydrogen fusion that typifies all true stars. More dust circulating around the star may also go on to collapse into circling planets.
Brown Dwarfs form when too little material has accumulated to trigger hydrogen fusion, Whelan says. These failed stars are intermediate between a low mass star and a oversized planet, with a typical Brown Dwarf having a mass of between 20 and 70 times that of Jupiter.
"The most important question relating to these objects must be how exactly do they form, more like stars or more like planets," says Whelan. "This puzzle has repercussions for the theory of star formation as a whole."
One clue to this puzzle is seen repeatedly in "baby" stars, protostars that have only just formed. They all start by accumulating mass from the surrounding molecular clouds, but counter- intuitively they all also eject powerful outflows of fast-moving material back out towards the cloud.
The disc of matter that will go on to form a protostar is rotating and some of the infalling matter is launched back out along the magnetic field lines surrounding the protostar as a result of centrifugal force. These jets or outflows produce huge, fast moving streamers, measured in parsecs (3 x 1013km) and speeds measured at tens of kilometres a second.
These jets have an important purpose. They take angular momentum away from the rotating protostar, slowing it and thereby allowing it to continue pulling more matter into the forming star. The jets are also thought to blow back the surrounding dust cloud, allowing the star to emerge into the open.
This outflow mechanism was known to exist in low mass and very large stars and also in those gravitational monsters, Black Holes. "People wondered if Brown Dwarfs had outflows as well but they are small and are visually weak," says Whelan.
Whelan and Ray answered this fundamental question with their study however. They used recorded spectrographic data captured by a large telescope in Chile by the Italian team. The Dublin researchers used a technique known as "spectro-astrometry", a method that allows the measurement of distances from spectroscopic data. "It is the only way to see the outflow from a Brown Dwarf," says Whelan.
The data was for Brown Dwarf rho-Oph 102, located in the rho-Ophiuchi molecular cloud. They managed to identify an outflow from the body moving at about 40km per second and reaching out a full 15 times the distance between the earth and the sun or about 2,244 million kilometres.
"It is quite a big deal," says Whelan. The first proof that Brown Dwarfs can produce outflows may provide answers to a number of questions.
The finding adds weight to theorists who hold that Brown Dwarfs form in much the same way as low mass stars, she says.
More significantly, it also shows that something as small as a Brown Dwarf can produce outflows using the same mechanism as the giant stars and Black Holes, despite the 10 orders of magnitude size difference between these bodies. "It is the same mechanism irrespective of mass," says Whelan.
"This important mechanism is thus very robust and it does not now seem impossible that outflows may also accompany the formation of planets. The idea that perhaps Jupiter or Saturn once were the drivers of such impressive flows is indeed tantalising."