* Astronomy

Astronomers have discovered that the large disks of gas and dust around young stars will fragment if two young stars pass close to each other and form smaller brown dwarfs stars with disks of their own. The news was announced this week at the Canadian Astronomical Society in Calgary, Alta, by James Wadsley, assistant professor of Physics & Astronomy at McMaster University, and his student Sijing Shen.

"This is an exciting discovery because it may be the dominant way brown dwarfs are made. The challenge to theorists was to explain not only the origin of brown dwarfs but also the details telescopes are seeing: brown dwarfs with disks and the systems of many dwarfs orbiting a single regular star. We've done that" - James Wadsley.

Brown dwarf stars are as common in number as large stars but are no more than 8 percent of the mass of the Sun. Their low mass prevents nuclear fusion in their core so they don't shine like regular stars. Regular stars form from cold dense cores in giant molecular gas clouds. The natural mass of a core is expected to be large, closer to that of a regular star than a brown dwarf so something extra was required to understand the origin of brown dwarfs.

Using SHARCNET (Shared Hierarchical Academic Research Computing Network) parallel computing facilities at McMaster, Shen and Wadsley simulated several encounters between young stars with disks at unprecedented resolution, seeing gas pile-ups, drawn-out tidal arms and huge masses of gas driven closer to the stars. Amid this chaos several small objects were seen to form, from Jupiter-sized objects up to brown dwarfs. Reports from lower resolution simulations by other groups had shown no indication of disks. However, in every case, the new objects had disks with sizes ranging up to 18 astronomical units (the size of Saturn's orbit). As these rapidly spinning disks evolve they should produce jets of gas and even result in the formation of planets orbiting the brown dwarfs. Both these things have been observed in nature.

"We had no idea the simulated results would be so beautiful and complex, and then we found out that observations were revealing brown dwarfs with disks that matched what we were seeing " - Sijing Shen, who is studying for her PhD in Physics & Astronomy at McMaster.

The simulated objects would either leave the stars on their own or in groups, or remain as multiple brown dwarf companions to a star. Telescopes have detected up to three brown dwarfs orbiting a regular star. Thus the brown dwarfs and planets in the simulations are remarkably similar to what is observed. However, it remains to be determined exactly how often such encounters occur in nature and what fraction of those encounters reliably produce brown dwarfs. For this, Shen and Wadsley are planning a much larger set of encounter simulations using SHARCNET's new supercomputers.

Source: McMaster University

The eclipsing binary brown dwarfs 1,400 light-years away in the Orion nebula, have masses of 55 times Jupiter's mass, and 35 times Jupiter's mass (with a 10 percent margin of error).



The astronomers also measured the light spectrum variations and determined the dwarfs' surface temperatures. The heavier of the two has a temperature of 2,650 degrees Kelvin and the smaller, 2,790 degrees K.


Position (2000): R.A. = 05 h 35 m 21 s.84 Dec. = -05° 46 ' 08 ".5

Astronomers have made the first direct measurements of the mass of a pair of brown dwarfs in the Orion Nebula. They used precise timings of the orbital period during which one eclipsed the other, dimming their collective brightness to weigh the failed stars.

Brown dwarfs have a masses between 13 to 75 times the mass of Jupiter.
Scientists believe brown dwarfs form through the collapse of clouds of interstellar gas and dust, like stars. But they lack the mass needed to initiate the nuclear reactions that convert hydrogen to helium in stars.


The I-band light curve of 2MASS J05352184–0546085. In total, 1590 individual flux measurements were obtained on 280 separate nights over 10 years, with an average cadence of 5-6 measurements per night. The typical uncertainty on the measurements is 2%. A time-series analysis reveals a period of P = 9.779621 ± 0.000042 days. The data here are phased relative to the time of periastron passage, as determined from the orbit solution

"If we want to understand how stars like the Sun come to be, we need to also understand when and under what circumstances they fail to be born" - Keivan Stassun at Vanderbilt University in Nashville, Tennessee, US.

Keivan Stassun and Robert Mathieu of University of Wisconsin-Madison and Jeff Valenti of the Space Telescope Science Institute in Baltimore, Maryland, both in the US using the 0.9-metre telescope at Kitt Peak National Observatory and a trio of telescopes at the Cerro Tololo Inter-American Observatory in the Chilean Andes, found that the eclipsing binary brown dwarfs to be about 5% and 3.5% the mass of the Sun, respectively. It is the first time scientists have directly measured a brown dwarf's mass.

The researchers made more than 1500 measurements of the brown dwarfs located in the stellar nursery of the Orion Nebula, over 280 nights between 1994 and 2005.

"The reason that is ultimately important is that these two brown dwarfs now serve as a kind of Rosetta Stone" - Keivan Stassun.

Since the researchers measured the diameter, brightness, surface temperature and mass of each dwarf in the pair, they hope to use the results to evaluate suspected brown dwarfs for which a mass cannot be directly determined.
The observations agree with brown dwarf theory. The newly found brown dwarfs have nearly the same diameter as the Sun and are thought to be only a few million years old.
They are expected to collapse down over a couple hundred million years to become more planet-sized.

But the authors admit that theory and observation did not line up perfectly. Although theory predicts more massive brown dwarfs will be hotter. Surprisingly, the more massive brown dwarf in this system is found to be cooler than its lower-mass companion. This may indicate a younger age for the more massive brown dwarf, which would suggest that these brown dwarfs were not born together

The team suggests several possible explanations for the unexpected result. It could be that the more massive brown dwarf is a bit older, and therefore farther along in the shrinking and cooling process. In that case, the two brown dwarfs would not have been born together.

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A group of astronomers led by Neal Evans of The University of Texas at Austin has used NASA’s Spitzer Space Telescope to show that brown dwarfs form like stars — by pulling in matter from a collapsing gas cloud and forming disks of potentially planet-forming material around themselves — and that such disks are common around young brown dwarfs.
The formation process that creates brown dwarfs has long been a mystery.

"Brown dwarfs are found in the ‘no-man’s land’ between stars and planets. They have masses too small to be a star, but too large to be a planet" - Neal Evans.

Two lines of evidence point to a star-like formation process for brown dwarfs: the presence of disks found around brown dwarfs, and the discovery of extremely dim objects forming inside clouds of gas and dust — objects too dim to be protostars.
Both lines of evidence come from Spitzer observations by Evans’ team, a group of about 60 astronomers from various institutions, called the “Cores to Disks” or “c2d” Spitzer Legacy Project.

"None of these objects could have been found without the unprecedented sensitivity of the instruments on the Spitzer Space Telescope" - Neal Evans.

First, the new c2d Spitzer observations, combined with supporting observations from a number of ground-based telescopes, show that a substantial number of brown dwarfs are surrounded by disks of dusty material, similar to those found around forming stars.
C2d team members Katelyn Allers, Jacqueline Kessler-Silacci, Daniel Jaffe, and Lucas Cieza of The University of Texas at Austin found about a dozen disk-surrounded brown dwarfs in the southern-hemisphere constellations Chamaeleon, Lupus, and Ophiuchus. Some of the brown dwarfs have a mass of five to 10 Jupiters, and are only a few million years old — young, astronomically speaking.

The disk discoveries were made by comparing observations of these objects at different wavelengths. The brown dwarfs were first studied in near-infrared light using the four-meter Blanco Telescope at the National Science Foundation’s Cerro Tololo Interamerican Observatory in Chile.
Astronomers used the near-infrared information to predict how much mid-infrared light they should give off. The Spitzer observations showed that the objects gave off much more mid-infrared light than expected. This can be explained by the presence of a disk around the brown dwarf. Disks are made up of dust, which absorbs light radiated from the brown dwarf and re-emits it at lower energies — that is, in the particular mid-infrared wavelengths detectable by Spitzer.

The group also found these objects are less massive than the smallest stars.

"You can’t weigh these brown dwarfs directly. We used theoretical models to figure out that they may have masses as low as five to 10 Jupiter masses. The disks around the brown dwarfs are analogous to the disks around very young Sun-like stars; disks that we believe provide the raw materials for planets" - Katelyn Allers.

Daniel Apai of the University of Arizona and his collaborators announced in October 2005 that they had found evidence that disks around more massive brown dwarfs might form planets.
Allers’ discoveries broaden the original finding of a disk around the more massive brown dwarf OTS 44 by Kevin Luhman of Penn State, announced in February 2005, and his more recent discovery of a less massive disk-surrounded brown dwarf. The c2d announcement shows these are part of a wide-spread phenomenon — not oddballs, but the norm.

The presence of these disks around brown dwarfs challenges one idea for their formation, namely ejection caused by gravitational interactions inside a region of star formation densely packed with stars. These results conflict with that theory in three ways: First, computer models show that it would be difficult for ejected brown dwarfs to keep their disks. Second, one of Allers’ brown dwarfs is in a wide binary system, which is difficult for the ejection model to produce. Finally, neither Lupus nor Chamaeleon are forming stars in the dense clusters the ejection model requires.

The discovery of a substantial number of disks around even very low mass brown dwarfs increases the likelihood that the alternative formation scenario applies: that brown dwarfs form more or less like stars do, by accreting matter from a collapsing cloud of gas and dust — or, in the jargon of star-formation researchers, a “core.”

"These results suggest an origin for brown dwarfs similar to that of stars: a collapsing ‘core’ of gas and dust. If this is right, we should see evidence for very low mass objects in cores" - Neal Evans

Mass is hard to measure in the very early stages of brown-dwarf formation. But astronomers know that forming objects give off light in amounts related to their mass and the rate at which they are accreting new material onto themselves. So a low mass, accreting object would be very faint.

Evidence for such tiny, dim objects exists. The first discovery of a very dim object (called L1014-IRS) forming inside what was previously thought to be a “starless core” in early Spitzer images was made by c2d team member Chadwick Young of Nicholls State University, and collaborators and announced in November 2004. Now, c2d team members have found about a dozen very faint objects that may be brown dwarfs in this earlier disk phase, embedded in cores of gas and dust. Once again, this shows that L1014-IRS, like OTS 44, is not an oddball, but the norm.

The new examples were found by c2d team members Tyler Bourke, Tracy Huard, and Philip Myers of the Harvard-Smithsonian Centre for Astrophysics; Michael Dunham of UT-Austin; and Jens Kauffmann of the Max-Planck-Institut für Radioastronomie. These findings suggest a new class of objects is emerging. Dubbed “Very Low Luminosity Objects,” or “VeLLOs,” they have less than one-tenth the Sun’s luminosity.

These are unlikely to be stars in a very early stage of formation.

"Accreting protostars are much more luminous than they will be when they become stars. So finding such a low luminosity in these objects is surprising. It implies that the product of the current mass and the rate at which mass is being added is unusually low"- Neal Evans

These studies show that the VeLLOs embedded in what were thought of as “starless cores” may be earlier stages of the disk-surrounded brown dwarfs found by Katelyn Allers and her c2d collaborators. In fact, further studies by Bourke and Huard show strong evidence for a disk around L1014-IRS, as announced in October 2005.
“Cores to Disks” is one of six Spitzer Legacy Science Projects selected in November 2000 to complete major surveys with Spitzer. The c2d team was awarded 400 hours of Spitzer observations, and produces data freely available to all astronomers.

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