First Step In Brown Dwarf Formation Seen, French Astronomers Say Brown Dwarf Could Be Closer To Earth Than Proxima Centauri

An international team led by the laboratory AIM (1) (CEA-CNRS-Université Paris Diderot) just saw the very first step in the formation of a brown dwarf, one of those very low-mass stars, that fall between star and planet. Astronomers know how to  detect brown dwarfs, despite their low radiation, however their formation and birth remains a mystery. 

Image of the cloud Oph B-11, obtained by the interferometer IRAM to the wavelength of 3.2 mm. The contours are the intensity and size of the source. The blue ellipse, top left, shows the resolution of the instrument and the cross marks the coincidence with a source already detected in sub-millimeter waves.


© Ph.André / CEA-SAP

It is thanks to the great IRAM interferometer (CNRS) (2), on the Plateau de Bure, that the astrophysicists were able to locate a condensing gas and dust, indicating the formation of a brown dwarf. This discovery and its interpretation are presented in the journal Science on July 6, 2012.

Stars of all masses can form in the Universe. Some are stars, other planets. The border is a function of their mass. The "in-between" is a mass range between 13 and 80 times the mass of Jupiter, the largest planet in the solar system, weighing less than 8% of the Sun.

 Neither really star or a planet, the brown dwarf is warm enough for the fusion of deuterium, but that does not trigger the fusion reaction of hydrogen, a reaction which is the energy source of stars. 

The big dark cloud Rho Ophiuchi, observed here in the mid-infrared spectrum, was  where the brown dwarf is located, in the region (red circle) of Oph pre-B-11, at an average distance of 450 light years from Earth.

© NASA / JPL

The object, 'C,' has both the field of intermediate mass and a heat signature distinct from that of the planets, characterizing it as a  brown dwarfs. Brown dwarfs radiate so little heat and llight that it was not until 1995 that the first, named Teide-1, was detected in the heart of the cluster of Pleiades by its infrared emission. Since then, hundreds have been discovered by large infrared surveys, but the exact mechanism of their formation is not well established. 

To observe this early phase of star formation, where these objects are still "cold" without a source of internal energy, the researchers used the IRAM interferometer great (Millimeter Radio Astronomy Institute) at Plateau de Bure (Hautes-Alpes, France), operating in the millimeter wave range. In addition to this range of wavelength particularly suitable, the IRAM interferometer provides, through its six mobile antennas 15 meters in diameter, a resolution unattainable with single antenna telescopes, such as the Herschel Space Observatory for instance. 

The brown dwarf formation observed by the researchers, called Oph B-11, is a condensation of gas and dust located within a large region of star formation, the Rho Ophiuchi dark cloud, about 450 light years from Earth. The very good resolution of the IRAM interferometer was used to estimate its size is about 140 astronomical units (AU (3)) of the approximate size of the solar system: reported in the Rho Ophiuchi cloud, C is a very compact size. As Oph B-11 is not detected in the infrared, including by the Herschel satellite, astronomers infer its temperature does not exceed 10K (-263 ° C). These features, together with the millimeter emission detected with the IRAM interferometer, indicate that the mass of the cloud does not exceed 2-3% the mass of the Sun, exactly the area (or size)of brown dwarfs. 

" We chose a particular area of Ophiuchus, the L1688 region, because there reigns an additional pressure that could favor the formation of a brown dwarf; In addition, we had already identified a source, but through another telescope, whose resolution does not allow measure the size of the object , "says Philippe André, CEA-IRFU. " This is the first time we discovered a fragment of cloud and dense compact enough to form a brown dwarf by collapsing gas and particules, just as more massive stars form." 

This discovery demonstrates that at least some brown dwarfs formed just like the other stars. However, this hypothesis was discarded because far more gravity of a fragment of a cloud of very low mass seemed insufficient to cause its collapse. The researchers believe that other forces than that of  gravity could contribute to the formation of stars, including the turbulent motion of matter in dark clouds. 

This turbulence could also be responsible for filament material in these clouds recently discovered by Herschel. Brown dwarfs, which are very faint stars, are just beginning to be studied.Their number is estimated in the galaxy between 50 to 100 billion or 20 to 40% of the stars of the Galaxy, and it is possible that one of these yet undetected brown dwarfs is closer to Earth than the nearest star currently known, Proxima Centauri.

Notes:
(1) Laboratory Astrophysics, instrumentation and modeling (CEA / CNRS / Université Paris Diderot).
(2) Institute of Radioastronomy millimeter (CNRS).
(3) AU Astronomical Unit, which means the average distance from Earth to the Sun about 150 million kilometers.

Contacts and sources: 

CNRS

References: "Interferometric Identification of a Pre-Brown Dwarf", Philippe Andre (Service-Laboratory Astrophysics AIM, CEA-IRFU), Derek Ward-Thompson (University of Central Lancashire, UK), Jane Greaves (Physics and Astronomy, University of St Andrews, St Andrews, UK) - published in the July 6 (Vol. 337) of the journal Science.