Brown dwarfs aplenty in star-forming region

Tricolor composite image of W3 Main where massive stars are being born. Red colored objects to the left of center are extremely young massive stars, surrounded by less massive stars of one million years old. Nebulas with a variety of colors and appearances are ionized gas reflecting light from these stars. Filamentary dark clouds are also conspicuous. The line at bottom left shows a scale of 0.2 parsecs, which is approximately 40 thousand astronomical units. National Astronomical Observatory of Japan


January 29, 2009
Subaru Telescope facility, Hilo, Hawai

To explore dim, distant, low mass stars, a team of Japanese and Indian astronomers used the high sensitivity and spatial resolution of the Cooled Infrared Spectrograph and Camera for OHS (CISCO) at the Subaru Telescope in Hilo, Hawaii, to obtain unprecedented detailed data toward the W3 Main star forming region. W3 Main, located approximately 6,000 light-years away in the constellation Cassiopeia, is a very active and massive star-forming region. To date, this near-infrared image (at right) is the deepest and finest image from a ground-based telescope among the images of massive star forming regions. The deep and high-resolution image shows distinctive reddish and bluish nebulosity features, dark filaments between the diffuse nebulosities, and a significant population of faint stars in W3 Main.

The study has shown for the first time that there is a significant number of brown dwarfs in the W3 Main star forming region. This result is significantly different from that obtained in the cases of Trapezium and IC 348, where a decrease of relative population of brown dwarfs was found. The research findings indicate that a relative number of brown dwarfs may differ among regions in the galaxy. For the future, the team will proceed with the observations toward much more massive star forming regions in remote areas to study whether the results are widespread.

Dwarf stars emit powerful pulse

This image shows what it might look like standing on the surface of a planet orbiting a brown dwarf star. An alien moon can also be seen in the sky. The brown dwarf gives off such feeble visible light it is difficult to see any of the landscape except for the reflection in the water.Brown dwarfs are a type of "failed" star.(Artist View)

Friday, 20 April 2007

The brown dwarfs are behaving like an altogether different and exotic cosmic object called a pulsar.Pulsars are rotating neutron stars that emit a flashing radio signal.
When the rotating beams sweep Earth, astronomers detect the radio pulse, which has been likened to the rotating beacon of a lighthouse.Pulsars are created when a massive star explodes in a supernova and its core collapses into a rapidly spinning neutron star.Brown dwarfs, on the other hand, are stellar also-rans which lack the necessary mass to kick-start nuclear fusion reactions in their cores.Greg Hallinan from the National University of Ireland in Galway and his colleagues used the Very Large Array radio telescope in New Mexico to observe a very cool, rapidly rotating brown dwarf called TVLM 513-46546.

40-year-old problem:

A class of "failed" star called a brown dwarf emits beams of radiation that are thousands of times brighter than any released by the Sun.A bright flash from the brown dwarf was observed roughly every two hours.All the planets with a magnetic field, including Earth, have bright radio emission from their magnetic polar regions.Brown dwarfs are thought to generate their emission in a similar way to pulsars. But here, the emission is many times brighter than that from planets. The radio waves are produced above the object's magnetic poles.
This radio emission requires these brown dwarfs to possess magnetic fields as powerful as those detected at the most magnetically active stars.The periodic pulses detected from brown dwarfs are very similar to those observed from pulsars. But the whole system is on a much slower and smaller scale, so it is easier for astronomers to decipher what is going on.

Link made?:

How pulsars produce their radiation has been a problem in astrophysics for 40 years.
This is because we have little understanding of how hot, electrified gas, or plasma, behaves in the extreme conditions present at a pulsar.Brown dwarfs are now the second class of stellar object known to produce persistent levels of extremely bright, "coherent" radiation.Moreover, this radio signal manifests itself as periodic pulses. However, in the case of brown dwarfs, both the source conditions and the emission mechanism are reasonably well understood.For some time, scientists have wondered if there were similarities between this type of emission and the periodic radio beams from pulsars. Observations of TVLM 513-46546 could provide the first direct evidence for such a link.Dr Hallinan said: "Our research shows that these objects can be fascinating and dynamic systems, and may be the key to unlocking this long-standing mystery of how pulsars produce radio emissions.
"It looks like brown dwarfs are the missing step between the radio emissions we see generated at Jupiter and those we observe from pulsars".

The National University of Ireland astrophysicist presented details of his work at the Royal Astronomical Society's National Astronomy Meeting in Preston.

Brown Dwarfs, Poorly Understood, Poorly Named

A cluster of nearly one thousand newly formed stars is captured in this infrared photograph as it emerges from the gaseous womb from which is was recently born. This extremely young cluster contains the largest known population of objects known as Brown Dwarfs. These are among the faintest sources present in the image.

07 June 2001

What's in a name? Sometimes, not much.

At a gathering in Germany this April of astronomers who study how planets and stars form, a poll was taken to determine whether brown dwarfs needed a new name.

The poll, however informal, represents the scientific community's acknowledgement that brown dwarfs exist in a gray area of definitions. They are cool, dim, but massive objects that so far do a lousy job of bridging our gap in understanding between planets and stars.

"Planetars" was suggested, as was "substellar objects" and host of other names.

Moderate debate ensued. No consensus was reached.

But with today's announcement that a significant number of brown dwarfs have protoplanetary disks around them, and hence must have formed just like stars do, the name "brown dwarf" now seems even less equipped to describe the objects.

Perhaps what brown dwarfs need is a real name, a single word, something memorable. Think planets, stars, comets, asteroids. Catchy names, all.

Charles J. Lada of the Smithsonian Astrophysical Observatory argues that "substellar objects" would now be the most apt moniker.

Prodded to consider that perhaps "substellar objects" wasn't exactly a catch term, Lada scratched his chin, tried to come up with something better, then decided that there was little chance the textbooks would be rewritten anyway.

Then he pointed out that this is all largely a semantic argument. Whatever we call brown dwarfs, they are still just stars that didn't make the grade.

And Nature, for its part, doesn't give a hoot.

"Nature doesn't see a difference between a brown dwarf and a star when it creates them," said August A. Muench of the University of Florida. The disk finding around brown dwarfs is the centerpiece of his doctoral dissertation.

But it sure makes enigmas out of the littler ones.

Astronomers Find the Two Dimmest Stellar Bulbs

ssc2008-22a: Not-So-Bright Bulbs
Credit: NASA/JPL-Caltech

Thursday, December 11, 2008

It's a tie! The new record-holder for dimmest known star-like object in the universe goes to twin "failed" stars, or brown dwarfs, each of which shines feebly with only one millionth the light of our sun.

Previously, astronomers thought the pair of dim bulbs was just one typical, faint brown dwarf with no record-smashing titles. But when NASA's Spitzer Space Telescope observed the brown dwarf with its heat-seeking infrared vision, it was able to accurately measure the object's extreme faintness and low temperature for the first time. What's more, the Spitzer data revealed the brown dwarf is, in fact, twins.

"Both of these objects are the first to break the barrier of one millionth the total light-emitting power of the sun," said Adam Burgasser of the Massachusetts Institute of Technology, Cambridge. Burgasser is lead author of a new paper about the discovery appearing in the Astrophysical Journal Letters.

Brown dwarfs are the misfits of the cosmos. They are compact balls of gas floating freely in space, but they are too cool and lightweight to be stars, and too warm and massive to be planets. The name "brown dwarf" comes from the fact that these small, star-like bodies change color over time as they cool, and thus have no definitive color. In reality, most brown dwarfs would appear reddish if they could be seen with the naked eye. Their feeble light output also means they are hard to find. The first brown dwarf wasn't discovered until 1995. While hundreds are known today, astronomers say there are many more in space still waiting to be discovered.

The newfound dim duo of brown dwarfs, while notable for their exceptional faintness, will probably not be remembered for their name. They are called 2MASS J09393548-2448279 after the Two Micron All-Sky Survey, or "2MASS," the mission partially funded by NASA that first detected the object in 1999.

Astronomers recently used Spitzer's ultrasensitive infrared vision to learn more about the object, which was still thought to be a solo brown dwarf. These data revealed a warm atmospheric temperature of 565 to 635 Kelvin (560 to 680 degrees Fahrenheit). While this is hundreds of degrees hotter than Jupiter, it's still downright cold as far as stars go. In fact, 2MASS J09393548-2448279, or 2M 0939 for short, is one of the coldest star-like bodies measured so far.

To calculate the object's brightness, the researchers had to first determine its distance from Earth. After three years of precise measurements with the Anglo-Australian Observatory in Australia, they concluded that 2M 0939 is the fifth-closest known brown dwarf to us, 17 light-years away toward the constellation Antlia. This distance, together with Spitzer's measurements, told the astronomers the object was both cool and extremely dim.

But something was puzzling. The brightness of the object was twice what would be expected for a brown dwarf with its particular temperature. The solution? The object must have twice the surface area. In other words, it's twins, with each body shining only half as bright, and each with a mass of 30 to 40 times that of Jupiter. Both bodies are one million times fainter than the sun in total light, and at least one billion times fainter in visible light alone.

"These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of," said Burgasser. "And like low-energy fluorescent light bulbs, they emit most of their light in a narrow range of wavelengths, in this case in the infrared."

According to the authors, there are even dimmer brown dwarfs scattered throughout the universe, most too faint to see with current sky surveys. NASA's upcoming Wide-Field Infrared Survey Explorer mission will scan the entire sky at infrared wavelengths, and is expected to uncover hundreds of these inconspicuous characters.

"The holy grail in the study of brown dwarfs is to find out how low you can go in terms of temperature, mass and brightness," said Davy Kirkpatrick, a co-author of the paper at NASA's Infrared Processing and Analysis Center at the California Institute of Technology, Pasadena. "This will tell us more about how brown dwarfs form and evolve."

Other authors of this paper are Chris Tinney of the University of New South Wales, Australia; Michael C. Cushing of the University of Hawaii, Manoa; Didier Saumon of the Los Alamos National Laboratory, NM; Mark S. Marley, NASA Ames Research Center, Moffett Field, Calif.; and Clara S. Bennett of the Massachusetts Institute of Technology.

Brown Dwarfs Do Form Like Stars

Wednesday, December 03, 2008



This artist's conception shows the brown dwarf ISO-Oph 102. Observations by the Submillimeter Array suggest that it is forming like a star, by accumulating material from the surrounding accretion disk (orange) shown here. The brown dwarf sheds angular momentum by ejecting material in two oppositely directed jets (red). Blue bow shocks indicate where those jets are interacting with the interstellar medium. Credit: ASIAA



his artist's conception zooms in on the brown dwarf and its accretion disk. The discovery of a bipolar molecular outflow at ISO-Oph 102 offers the first strong evidence in favor of brown dwarf formation through gravitational collapse. Credit: David A. Aguilar (CfA)

Cambridge, MA - Astronomers have uncovered strong evidence that brown dwarfs form like stars. Using the Smithsonian's Submillimeter Array (SMA), they detected molecules of carbon monoxide shooting outward from the object known as ISO-Oph 102. Such molecular outflows typically are seen coming from young stars or protostars. However, this object has an estimated mass of 60 Jupiters, meaning it is too small to be a star. Astronomers have classified it as a brown dwarf.
Brown dwarfs are on the dividing line between planets and stars, and generally have masses between 15 and 75 Jupiters. (The theoretical minimum mass for a star to sustain nuclear fusion is 75 times Jupiter.) As a result, brown dwarfs are sometimes called failed stars. However, it is not clear whether they form like stars, from the gravitational collapse of gas clouds, or if they form like planets, agglomerating rocky material until they grow massive enough to draw in nearby gas.

A star forms when a cloud of interstellar gas draws itself together through gravity, growing denser and hotter until fusion ignites. If the initial gas cloud is rotating, that rotation will speed up as it collapses inward, much like an ice skater drawing her arms in. In order to gather mass, the young protostar must somehow shed that angular momentum. It does so by spewing material in opposite directions as a bipolar outflow.

A brown dwarf is less massive than a star, so there is less gravity available to pull it together. As a result, astronomers debated whether a brown dwarf could form the same way as a star. Previous observations provided hints that they could. The serendipitous discovery of a bipolar molecular outflow at ISO-Oph 102 offers the first strong evidence in favor of brown dwarf formation through gravitational collapse.

"We thought that any such outflow would be too weak to detect with current facilities and would have to wait until a next-generation instrument like ALMA [the Atacama Large Millimeter Array]," said Ngoc Phan-Bao of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), lead author on the paper announcing the find. "This was a big surprise. Finding the molecular outflow with the SMA shows the extraordinary capabilities of the array."

As might be expected, the outflow contains much less mass than the outflow from a typical star: about 1000 times less, in fact. The outflow rate is also smaller by a factor of 100. In all respects, the molecular outflow of ISO-Oph 102 is a scaled-down version of the outflow process seen in young stars.

"These findings suggest that brown dwarfs and stars aren't different because they formed in different ways," said Paul Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of ASIAA. "They share the same formation mechanism. Whether an object ends up as a brown dwarf or star apparently depends only on the amount of available material."

The paper on ISO-Oph 102 will be published in the December 20 issue of the Astrophysical Journal Letters. Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Closest Brown Dwarf Companion Ever Spotted Around a Star Provokes New Perspective

2002 May 21

Astronomers using adaptive optics technology on the Gemini North Telescope have observed a brown dwarf orbiting a low-mass star at a distance comparable to just three times the distance between the Earth and Sun. This is the closest separation distance ever found for this type of binary system using direct imaging.The record-breaking find is just one of a dozen lightweight binary systems observed in the study. Together, they provide a new perspective on the formation of stellar systems and how smaller bodies in the Universe (including large planets) might form.

"By using Gemini's advanced imaging capabilities, we were able to clearly resolve this binary pair where the distance between the brown dwarf and its parent star is only about twice the distance of Mars from the Sun," said team member Melanie Freed, a graduate student at the University of Arizona in Tucson. With an estimated mass of 38-70 times the mass of Jupiter, the newly identified brown dwarf is located just three times the Sun-Earth distance (or 3.0 Astronomical Units) from its parent star. The star, known as LHS 2397a, is only 46 light-years from Earth. The motion of this object in the sky indicates that it is an old, very low-mass star.

The previous imaging record for the closest distance between a brown dwarf and its parent (a much brighter, Sun-like star) was almost five times greater at 14 AU. One Astronomical Unit (AU) equals the average distance between the Earth and the Sun or about 150 million kilometers (93 million miles).

Often portrayed as "failed stars," brown dwarfs are bigger than giant planets like Jupiter, but their individual masses are less than 8% of the Sun's mass (75 Jupiter masses), so they are not massive enough to shine like a star. Brown dwarfs are best viewed in the infrared because surface heat is released as they slowly contract. The detection of brown dwarf companions within 3 AU of another star is an important step toward imaging massive planets around other stars.

This University of Arizona team led by Dr. Laird Close used the Gemini North Telescope to detect eleven other low mass companions, suggesting that these low-mass binary pairs may be quite common. The discovery of so many low-mass pairs was a surprise, given the argument that most very low-mass stars and brown dwarfs were thought to be solo objects wandering though space alone after being ejected out of their stellar nurseries during the star formation process.

"We have completed the first adaptive optics-based survey of stars with about 1/10th of the Sun's mass, and we found nature does not discriminate against low-mass stars when it comes to making tight binary pairs," said Close, an assistant professor of astronomy at the University of Arizona. Dr. Close is the lead author on a paper presented today at the Brown Dwarfs International Astronomical Union Symposium in Kona, Hawaii, and he is the principal investigator of the low-mass star survey.

The team looked at 64 low-mass stars (originally identified by John Gizis of the University of Delaware) that appeared to be solo stars in the lower resolution images from the 2MASS all-sky infrared survey. Once the team used adaptive optics on Gemini to make images that were ten times sharper, twelve of these stars were revealed to have close companions. Surprisingly, Close's team found that the separation distances between the low mass stars and their companions were significantly less than expected.

"We find companions to low-mass stars are typically only 4 AU from their primary stars, this is surprisingly close together," said team member Nick Siegler, a University of Arizona graduate student. "More massive binaries have typical separations closer to 30 AU, and many binaries are much wider than this." The new Gemini observations, Close said, "imply strongly that low-mass stars do not have companions that are far from their primaries." Similar results had been found previously by a team led by Dr. Eduardo L. Martin of the University of Hawaii Institute for Astronomy in a survey of 34 very low-mass stars and brown dwarfs in the Pleiades cluster carried out with the Hubble Space Telescope. These two surveys together clearly demonstrate that there is an intriguing dearth of brown dwarfs at separations larger than 20 AU from very low-mass stars and other brown dwarfs.

The team projects that one out of every five low-mass stars has a companion with a separation in the range (3-200 AU). Within this separation range, astronomers have observed a similar frequency of more massive stellar companions around larger Sun-like stars.

Taken as a whole, these new results suggest that (contrary to theory) low-mass binaries may form in a process similar to that of more massive binaries. Indeed, this finding adds to growing evidence from other groups that the percentage of binary systems is similar for bodies spanning the range from one solar mass to as little as 0.05 solar masses (or 52 times Jupiter's mass). For example, a group led by Neill Reid of the Space Telescope Science Institute and the University of Pennsylvania has come to a similar conclusion with a smaller sample of 20 even lower-mass stars and brown dwarfs observed with the Hubble Space Telescope.

The fact that low-mass stars have any low-mass brown dwarf companions inside 5 AU is also surprising because the exact opposite is true around Sun-like stars. Very few Sun-like stars have brown dwarf companions inside this distance, according to radial velocity studies. "This lack of brown dwarf companions within 5 AU of Sun-like stars has been called the 'brown dwarf desert'," Close noted. "However, we see there is likely no brown dwarf desert around low-mass stars."

These results form important constraints for theorists working to understand how the mass of a star affects the mass and separation distance of the companions that form with it. "Any accurate model of star and planet formation must reproduce these observations," Close said.

These observations were possible only because of the combination of the University of Hawaii's uniquely sensitive Hokupa'a adaptive optics imaging system and the technical performance of the Gemini telescopes. The Hokupa'a system sensitivity is due to the curvature wavefront sensing concept developed by Dr. Francois Roddier. Adaptive optics is an increasingly crucial technology that eliminates most of the "blurring" caused by the turbulence in the Earth's atmosphere (i.e., the twinkling of the stars). It does this by rapidly adjusting the shape of a special, smaller flexible mirror to match local turbulence, based on real-time feedback to the mirror's support system from observations of the low-mass star. Hokupa'a can count individual photons (particles of light) and so can sharpen accurately even very faint (i.e., low-mass) stars.

The near-infrared adaptive optics images made by the 8-meter Gemini telescope in this survey were twice as sharp as those that can be made at the same wavelengths by the Earth-orbiting, 2.4-meter Hubble Space Telescope. The only ground-based survey of its kind, this work required five nights over one year with the Hokupa'a system at Gemini North.

It is important to note that the distances used here are as measured on the sky. The real orbital separations may be slightly larger once the full orbit of these binaries is known in the future.

The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The telescopes are located at Mauna Kea, Hawaii (Gemini North) and Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.

Brown Dwarf Found Around Nearby, Sun-Like Star

The Gemini North Telescope using the University of Hawaii's Hokupa'a adaptive optics system found a very faint companion orbiting around 15 Sge (left). The same Gemini data have been processed to show the brown dwarf companion more clearly (right). The brown dwarf lies only 0.8 arc seconds from the primary. (The fainter ripples are artifacts of the image processing.) The brown dwarf is located at about the 7:00 position on these images.

Gemini-North adaptive optics image of 15 Sge and its newly found companion (15 Sge B). The data was obtained in the near-infrared, at a wavelength of 2.2 microns. The image has been computer processed to subtract the light from the much brighter primary star in the vicinity of the companion.

This image must be credited to "Gemini Observatory/University of Hawaii Institute for Astronomy/Michael Liu/NSF".

2002 January 07

Astronomers using adaptive optics on the Gemini North and Keck telescopes have taken an image of a brown dwarf orbiting a nearby star similar to the Sun. The faint companion is separated from its parent star by less than the distance between the Sun and the planet Uranus and is the smallest separation brown dwarf companion seen with direct imaging.The research team estimates the mass of the brown dwarf at 55 to 78 times the mass of planet Jupiter. The discovery raises puzzling questions about how the brown dwarf formed, and it adds to the surprising diversity of extrasolar planetary systems being found with cutting-edge observational techniques.

"This discovery implies that brown dwarf companions to average, Sun-like stars exist at a separation comparable to the distance between the Sun and the outer planets in our Solar System," said Michael Liu, the Beatrice Parrent fellow at the University of Hawaii's Institute for Astronomy. Liu is lead author of a paper presented today at a press conference in Washington, DC, at the 199th meeting of the American Astronomical Society.

Found with adaptive optics technology at the Gemini North and Keck Telescopes on Mauna Kea, Hawaii, the brown dwarf is located in the constellation Sagitta (The Arrow) around a star commonly known as 15 Sge. The star, a G-star formally identified as HR 7672, is one to three billion years old, making it slightly younger than the Sun. It is located approximately 58 light-years from Earth.

"This companion is probably too massive to have formed the way we believe that planets do, namely from a circumstellar disk of gas and dust when the star was young," Liu added. "This finding suggests that a diversity of processes act to populate the outer regions of other solar systems. The parent star is very similar to our Sun, yet it has a brown dwarf companion whose mass is dozens of times the combined mass of all the planets in our solar system."

Co-authors on the paper presented today are Debra Fischer, James Graham, James Lloyd and Geoff Marcy of the University of California at Berkeley, and Paul Butler of the Carnegie Institution's Department of Terrestrial Magnetism in Washington, DC.

The brown dwarf orbits the star at an estimated distance of 14 Astronomical Units (AU), 14 times the distance between Earth and the Sun (1 AU = 150 million kilometers or 93 million miles). This makes it the closest substellar object yet seen by direct imaging around a main sequence (stable, hydrogen-burning) star. For comparison, Saturn orbits the Sun at 10 AU, with Uranus the next planet outward at 19 AU.



photo: Keck adaptive optics image of 15 Sge and its companion, also obtained in the near-infrared. The arrow points to the companion, seen as a close point source. (The streaks of light around the primary star are image artifacts produced by the telescope.) Orientation and size are the same as the above Gemini image.

This image must be credited to "W. M. Keck Observatory/University of Hawaii Institute for Astronomy/Michael Liu".

Astronomers believe that brown dwarfs are intermediate objects between planets and stars. Often described as 'failed stars', they are more massive than Jupiter, the largest planet in our Solar System. However, they fall short of the minimum mass need to sustain nuclear fusion, estimated at 8 percent of the Sun's mass. After a modest initial outburst of higher temperatures at birth, brown dwarfs cool off and steadily grow fainter.

While many planets have been found around other stars by radial velocity studies (which search for the very weak wobbling of stars due to an unseen planet), the same studies find almost no brown dwarfs, a phenomenon known as the "brown dwarf desert." However, such work only probes the inner four AU around other stars. Very little is known about region outside of four AU, the domain of giant planets in our own solar system.

Hints of an interesting object around 15 Sge first arose in data gathered ten years ago from Lick Observatory on Mount Hamilton, CA. Marcy, Butler, and Fischer obtained high-precision radial velocity measurements of this star as part of their effort to find planets. While they did not find any planets, they did notice clues of a more massive, distant companion.

In the summer of 2001, Liu took high-resolution pictures of 15 Sge using the University of Hawaii's QUIRC camera and Hoku'pa'a adaptive optics (AO) system on the 8.1-meter Gemini North telescope. Second and third epoch imaging were obtained using the Keck AO system in August and December 2001.

Ground-based astronomical images normally are blurred due to the turbulence of the Earth's atmosphere. AO is an exciting technology that compensates for this effect in real time, correcting the blurring and making some images sharper than even those produced by the Hubble Space Telescope.

Liu noticed a very faint object next to 15 Sge, akin to distinguishing a firefly next to a bright searchlight. However, the object could have been a distant star in the background, merely appearing to be close to 15 Sge when projected on the sky. Over the course of six months, Liu and his collaborators monitored the star with Keck AO and NIRSPEC on the 10-meter Keck II telescope and found that the faint object moved on the sky along with the primary star, proving the two objects were physically associated. A spectrum of the companion indicated a very cool temperature, characteristic of brown dwarfs.

"Only by using adaptive optics to produce very sharp images could we have found this companion," Liu explains. "It is too faint and too close to its parent star to be seen otherwise."

Liu and his collaborators are continuing to search for such objects. "Now that we know brown dwarfs exist in the region of giant planet formation, we would like to understand how often these oddball pairings occur in the Universe, and what that can tell us about the alternate and divergent ways in which solar systems form around Sun-like stars," he says.

Brown dwarf

photo: This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. The brown dwarf, called Gliese 229B, is about 20 to 50 times the mass of Jupiter.

Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars; this upper limit is between 75 and 80 Jupiter masses (MJ). Currently there is some debate as to what criterion to use to define the separation between a brown dwarf from a giant planet at very low brown dwarf masses (~13 MJ ), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 MJ do fuse deuterium and those above ~65 MJ also fuse lithium. The only planets known to orbit brown dwarfs are 2M1207b and MOA-2007-BLG-192Lb.

Brown dwarfs, a term coined by Jill Tarter in 1975, were originally called black dwarfs, a classification for dark substellar objects floating freely in space which were too low in mass to sustain stable hydrogen fusion (the term black dwarf currently refers to a white dwarf that has cooled down so that it no longer emits heat or visible light). Alternative names have been proposed, including Planetar and Substar.

Early theories concerning the nature of the lowest mass stars and the hydrogen burning limit suggested that objects with a mass less than 0.07 solar masses for Population I objects or objects with a mass less than 0.09 solar masses for Population II objects would never go through normal stellar evolution and would become a completely degenerate star (Kumar 1963). The role of deuterium-burning down to 0.012 solar masses and the impact of dust formation in the cool outer atmospheres of brown dwarfs was understood by the late eighties. They would however be hard to find in the sky, as they would emit almost no light. Their strongest emissions would be in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since those earlier times, numerous searches involving various methods have been conducted to find these objects. Some of those methods included multi-color imaging surveys around field stars, imaging surveys for faint companions to main sequence dwarfs and white dwarfs, surveys of young star clusters and radial velocity monitoring for close companions.

For many years, efforts to discover brown dwarfs were frustrating and searches to find them seemed fruitless. In 1988, however, University of California, Los Angeles professors Eric Becklin and Ben Zuckerman identified a faint companion to GD 165 in an infrared search of white dwarfs. The spectrum of GD 165B was very red and enigmatic, showing none of the features expected of a low-mass red dwarf star. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey (2MASS) when Davy Kirkpatrick, out of the California Institute of Technology, and others discovered many objects with similar colors and spectral features.

Today, GD 165B is recognized as the prototype of a class of objects now called "L dwarfs". While the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD 165B would be classified as a brown dwarf or simply a very low mass star, since observationally, it is very difficult to distinguish between the two.

Interestingly, soon after the discovery of GD 165B other brown dwarf candidates were reported. Most failed to live up to their candidacy however, and with further checks for substellar nature, such as the lithium test, many turned out to be stellar objects and not true brown dwarfs. When young (up to a gigayear old), brown dwarfs can have temperatures and luminosities similar to some stars, so other distinguishing characteristics are necessary, such as the presence of lithium. Stars will burn lithium in a little over 100 Myr, at most, while most brown dwarfs will never acquire high enough core temperatures to do so. Thus, the detection of lithium in the atmosphere of a candidate object ensures its status as a brown dwarf.

In 1995 the study of brown dwarfs changed dramatically with the discovery of three incontrovertible substellar objects, some of which were identified by the presence of the 6708 Li line. The most notable of these objects was Gliese 229B which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Saturn's moon, Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype.

Since 1995, when the first brown dwarf was confirmed, hundreds have been identified. Brown dwarfs close to Earth include Epsilon Indi Ba and Bb, a pair of dwarfs gravitationally bound to a sunlike star, around 12 light-years from the Sun.

The standard mechanism for star birth is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts it heats up. The release of gravitational potential energy is the source of this heat. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increases dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the protostar. For most stars, gas and radiation pressure generated by the thermonuclear fusion reactions within the core of the star will support it against any further gravitational contraction. Hydrostatic equilibrium is reached and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.

If, however, the mass of the protostar is less than about 0.08 solar mass, normal hydrogen thermonuclear fusion reactions will not ignite in the core. Gravitational contraction does not heat the small protostar very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum electron degeneracy pressure.Further gravitational contraction is prevented and the result is a "failed star", or brown dwarf that simply cools off by radiating away its internal thermal energy.

Distinguishing high mass brown dwarfs from low mass stars:

Lithium:

Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which achieve the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of Lithium-7 and a proton producing two Helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pioneered by Rafael Rebolo and colleagues.

* However, lithium is also seen in very young stars, which have not yet had a chance to burn it off. Heavier stars like our sun can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size.

* Contrariwise, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 MJ can burn off their lithium by the time they are half a billion years old, thus this test is not perfect.

Methane:

Unlike stars, older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane. Dwarfs confirmed in this fashion include Gliese 229B.

Luminosity:

Main sequence stars cool, but eventually reach a minimum luminosity which they can sustain through steady fusion. This varies from star to star, but is generally at least 0.01% the luminosity of our Sun. Brown dwarfs cool and darken steadily over their lifetimes: sufficiently old brown dwarfs will be too faint to be detectable.

Distinguishing low mass brown dwarfs from high mass planets:

A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter. At the high end of their mass range (60-90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron degeneracy pressure, as it is in white dwarfs; at the low end of the range (1-10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10-15% over the range of possible masses. This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after some time on the order of 10 million years. However, there are other ways to distinguish dwarfs from planets:

Density is a clear giveaway. Brown dwarfs are all about the same radius; so anything that size with over 10 Jupiter masses is unlikely to be a planet.

X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet like temperatures (under 1000 K).

Some astronomers believe that there is in fact no actual black-and-white line separating light brown dwarfs from heavy planets, and that rather there is a continuum. For example, Jupiter and Saturn are both made out of primarily hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune) emit more heat than they receive from the Sun. And all four giant planets have their own "planetary systems" -- their moons. In addition, it has been found that both planets and brown dwarfs can have eccentric orbits.

Currently, the International Astronomical Union considers objects with masses above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas those objects under that mass (and orbiting stars or stellar remnants) are considered planets.

Brown-dwarf desert:

A brown-dwarf desert is an orbital distance around a star at which brown dwarfs cannot exist as binary stars. This is usually up to 5 AU around solar mass stars.

This desert occurs because if a brown dwarf was to form it would have to do so at the same time as its companion star. If the brown dwarf forms within 5 AU of its companion star it would begin migration towards the star and eventually become consumed by the larger star.

It has recently been observed that very-low-mass binaries could destroy the theory of brown-dwarf deserts. This is because low-mass binaries are seen to orbit within 5AU; however, due to the low mass of the larger companion, this matter is still a topic of debate.