Astronomers using NASA's Hubble Space Telescope have taken attendance in a class of brown dwarfs and found indications that these odd and elusive objects also tend to be loners. The Hubble census -- the most complete to date -- provides new and compelling evidence that stars and planets form in different ways.
NASA Headquarters, Washington, DC
Space Telescope Science Institute, Baltimore, MD
National Optical Astronomy Observatory, Tucson, AZ
August 23, 2000
The Hubble census -- the most complete to date -- provides new and compelling evidence that stars and planets form in different ways.
"Because the brown dwarfs bridge the gap between stars and planets, their properties reveal new and unique insights into how stars and planets form," said Joan Najita of the National Optical Astronomy Observatory (NOAO) in Tucson, AZ. Her study with fellow NOAO researcher Glenn Tiede and John Carr of the Naval Research Laboratory, Washington, DC, will appear in the October issue of the Astrophysical Journal.
Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light that they emit is so faint it's hard to tell how many of them are scattered throughout the galaxy, and how they're formed.
The Hubble census finds that, like stars, there are more low-mass brown dwarfs than high-mass ones, and this trend continues down to low, nearly "planetary" masses. "In this respect, the isolated, or free-floating, brown dwarfs found by Hubble appear to represent the low-mass counterparts of the more massive stars," added Najita. "This suggests that stars and free-floating brown dwarfs form in the same way."
However, the Hubble finding also offers the strongest evidence so far that free-floating brown dwarfs are far different than the recently discovered planets that orbit nearby stars. Najita's team found brown dwarfs more often alone than in orbit around other stars. "This suggests that the extra-solar planets and, by extension, the planets in our own solar system, formed very differently from how the Sun and other stars formed," Najita noted.
Only a few years ago, it was commonly believed that brown dwarfs are rare, perhaps because the process that makes stars "stops working" at lower masses. "Nature does not discriminate between stars that can shine by fusion and lower-mass objects that are unable to do so," said Najita. "In fact, the universe easily makes brown dwarfs of all masses, from the most massive to the least."
The study also found that brown dwarfs are unlikely to contribute significantly to the mysterious, unseen "dark matter" that dominates the mass of our galaxy and the universe. Although Hubble found that brown dwarfs are abundant, it turns out that they are not common enough to explain the dark matter. Najita and her colleagues conclude that brown dwarfs probably contribute less than 0.1 percent of the mass of our Milky Way's halo.
The inventory was carried out using Hubble's infrared vision to measure the brightness and temperature of stars in the cluster IC 348, located in the constellation Perseus. Because the cluster is young, the brown dwarfs in the cluster are intrinsically brighter, which made it easy to detect about 30 brown dwarfs. A critical step in the observation was picking out the brown dwarfs from the clutter of background stars. To tackle this problem, Najita and colleagues developed a new technique using Hubble's NICMOS camera. The procedure measures the strength of an infrared water-absorption band in the atmospheres of the stars. The strength of the band is a sensitive measure of each star's temperature.
"The ability to measure the temperature of each star solved several problems simultaneously," Najita said. "In addition to helping us distinguish the cluster brown dwarfs from background stars, we were also able to measure the masses of the brown dwarfs without having to assume their age. This greatly improved our mass estimates."
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy (AURA), Inc., for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NOAO is operated for the National Science Foundation by AURA, Inc.
July 21, 2000
Youngest Brown Dwarf Yet in a Multiple Stellar System
July 11, 2000
Surprised scientists made provocative observations of an X-ray flare from a celestial object called a brown dwarf -- the first ever seen from such an object -- giving them strong hints of the tangled magnetic fields that may exist inside, according to an article to be published in the July 20 issue of the Astrophysical Journal Letters.
"We were most surprised by the fact that it was a flare,""said Lars Bildsten, co-author and professor of physics at the Institute for Theoretical Physics at the University of California, Santa Barbara.
"At best we expected a few photons every hour," said Bildsten. "Instead, we saw nothing for nine hours and then a bright flare that lasted nearly two hours. If the observation had been shorter, we would have nothing to report."
"We were shocked," said Robert Rutledge, of the California Institute of Technology in Pasadena, and lead author of the paper. "We didn't expect to see flaring from such a lightweight object. This is really the 'mouse that roared.'"
This first X-ray flare ever seen from a brown dwarf, or failed star, was detected by NASA's Chandra X-ray Observatory, the telescope that was launched nearly a year ago. The bright X-ray flare has implications for understanding the explosive activity and origin of magnetic fields of extremely low mass stars, according to the team of four who made the discovery.
"It was as if we were searching for a dim bulb and instead found a bright flash of light," said Bildsten.
"Less massive than stars but more massive than planets, brown dwarfs were long assumed to be rare," explained principal investigator Gibor Basri in the April issue of Scientific American. "New sky surveys, however, show that the objects may be as common as stars."
Chandra detected no X-rays at all from the brown dwarf known as "LP 944-20" for the first nine hours of a twelve hour observation, then the source flared dramatically before it faded away over the next two hours. The energy emitted in the brown dwarf flare was comparable to a small solar flare and is believed to come from a twisted magnetic field.
"This is the strongest evidence yet that brown dwarfs and possibly young giant planets have magnetic fields, and that a large amount of energy can be released in a flare," said Eduardo Martin, of Caltech, also a member of the team.
Professor Gibor Basri of the University of California, Berkeley, the principal investigator for this observation, speculated that "the flare could have its origin in the turbulent magnetized hot material beneath the surface of the brown dwarf. A sub-surface flare could heat the atmosphere, allowing currents to flow and give rise to the X-ray flare -- like a stroke of lightning."
Basri, an expert in brown dwarfs wrote an article describing them in the April issue of Scientific American. In that article he explains:
"A brown dwarf is a failed star. A star shines because of the thermonuclear reactions in its core, which release enormous amounts of energy by fusing hydrogen into helium. For the fusion reactions to occur, though, the temperature in the star's core must reach at least three million kelvins. And because core temperature rises with gravitational pressure, the star must have a minimum mass: about 75 times the mass of the planet Jupiter, or about 7 percent of the mass of our sun. A brown dwarf just misses that mark -- it is heavier than a gas-giant planet but not quite massive enough to be a star."
The brown dwarf, named LP 944-20, is about 500 million years old and has a mass that is about 60 times that of Jupiter, or 6 percent of the sun's mass. Its diameter is one-tenth that of the sun and has a rotation period of less than five hours. Located in the constellation Fornax in the southern skies, LP 944-20 is one of the best studied brown dwarfs because it is only 16 light years from Earth.
The researchers explained that the absence of X-rays from LP 944-20 during the non-flaring period is in itself a significant result. It sets the lowest limit on steady X-ray power produced by a brown dwarf, and shows that million degree Celsius upper atmospheres, or coronas, cease to exist as the surface temperature of a brown dwarf cools below about 2500 degrees Celsius.
"This is an important confirmation of the trend that hot gas in the atmospheres of lower mass stars is produced only in flares," said Bildsten.
Since brown dwarfs have too little mass to sustain significant nuclear reactions in their cores, their primary source of energy is the release of gravitational energy as they slowly contract -- at a rate of a few inches per year. They are very dim -- one hundredth of 1 percent as luminous as the sun -- and of great interest to astronomers because they are poorly understood and probably a very common class of objects that are intermediate between normal stars and giant planets.
The 12-hour observation of brown dwarf LP 944-20 was made on December 15, 1999, using the Advanced CCD Imaging Spectrometer (ACIS). The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge University, and Pennsylvania State University, University Park. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
NASA Headquarters, Washington, DC
Marshall Space Flight Center, Huntsville, AL
Chandra X-ray Observatory Center, Center for Astrophysics, Cambridge, MA
July 11, 2000
"We were shocked," said Dr. Robert Rutledge of the California Institute of Technology (Caltech) in Pasadena, CA, the lead author on the discovery paper to appear in the July 20 issue of Astrophysical Journal Letters. "We didn't expect to see flaring from such a lightweight object. This is really the mouse that roared."
The study of the bright X-ray flare will increase understanding of the explosive activity and origin of magnetic fields of extremely low-mass stars.
Chandra detected no X-rays at all from the object called LP 944-20 for the first nine hours of a twelve-hour observation, and then the source flared dramatically before it faded away over the next two hours.
The energy emitted in the brown dwarf flare was comparable to a small solar flare, and was a billion times greater than observed X-ray flares from Jupiter. The flaring energy is believed to come from a twisted magnetic field. "This is the strongest evidence yet that brown dwarfs and possibly young giant planets have magnetic fields, and that a large amount of energy can be released in a flare," said Dr. Eduardo Martin, also of Caltech and a member of the team.
Professor Gibor Basri of the University of California, Berkeley, the principal investigator for this observation, speculated that the flare "could have its origin in the turbulent, magnetized hot material beneath the surface of the brown dwarf. A sub-surface flare could heat the atmosphere, allowing currents to flow and give rise to the X-ray flare -- like a stroke of lightning."
LP 944-20 is about 500 million years old and has a mass about 60 times that of Jupiter, or 6 percent of that of the Sun. Its diameter is about one-tenth that of the Sun and it has a rotation period of less than five hours. Located in the constellation Fornax in the southern skies, LP 944-20 is one of the best-studied brown dwarfs because it is only 16 light years from Earth.
The absence of X-rays from LP 944-20 during the non-flaring period is in itself a significant result. It sets the lowest limit on steady X-ray power produced by a brown dwarf, and shows that the million-degree Celsius upper atmospheres, or coronas, cease to exist as the surface temperature of a brown dwarf cools below about 2500 degrees Celsius.
"This is an important confirmation of the trend that hot gas in the atmospheres of lower-mass stars is produced only in flares," said Professor Lars Bildsten of the University of California, Santa Barbara, also a member of the team.
Brown dwarfs have too little mass to sustain significant nuclear reactions in their cores. Their primary source of energy is the release of gravitational energy as they slowly contract. They are very dim -- less than a tenth of a percent as luminous as the Sun -- and of great interest to astronomers because they are poorly understood and probably a very common class of objects intermediate between normal stars and giant planets.
The 12-hour observation of LP 944-20 was made on December 15, 1999, using the Advanced CCD Imaging Spectrometer (ACIS).
The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and Pennsylvania State University, University Park. NASA's Marshall Space Flight Center in Huntsville, AL, manages the Chandra program. TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA.
Images associated with this release, including high-resolution digital versions of the X-ray image (JPG, 300 dpi TIFF), are available on the Internet at:
Royal Astronomical Society Press Notice
9 May 2000
Brown dwarf 'missing link' discovered
Astronomers have identified three brown dwarfs of a type never before
observed, so filling in what has until now been an elusive 'missing link'
in the range of properties of known brown dwarfs. The discovery resulted
from a collaboration between astronomers using the United Kingdom
Infrared Telescope (UKIRT) in Hawaii and scientists associated with the
Sloan Digital Sky Survey (SDSS).
Brown dwarfs are intriguing objects, intermediate between stars and planets. Often picturesquely described as 'failed stars', they are more massive than Jupiter, the largest planet in the solar system, but they fall short of the minimum mass a true star needs -- 8% of the Sun's mass. Stars can shine constantly for billions of years because they generate nuclear energy from the fusion of hydrogen into helium. But brown dwarfs cannot sustain nuclear power production. After a modest initial flush, they cool off and become progressively fainter.
Young brown dwarfs are now known to exist in the hundreds in the Sun's neighbourhood. They have surface temperatures that range down from about 3,500 K (3,200 degrees C) to 1,500 K (1,200 degrees C). Over most of this range their appearances are similar to cool stars of the same temperature. However, as the surface of a brown dwarf cools below 1,500 K, a dramatic chemical change takes place: large amounts of methane form, considerably altering the appearance of the brown dwarf.
The first methane-dominated brown dwarf to be discovered was found orbiting a nearby star by astronomers at Caltech in 1995. More have been found by astronomers at Caltech and Johns Hopkins University since early 1999, largely through two ongoing surveys of the night sky -- the Sloan Digital Sky Survey operating a single dedicated telescope in New Mexico, and the 2 Micron All-sky Survey (2MASS), which operates one telescope in Arizona and one in Chile. The methane brown dwarfs have turned out to be almost identical to each other. Their spectra are very similar to those of the giant Jupiter-like planets, even though they are considerably warmer.
The three newly discovered brown dwarfs bridge the gap between the young, warmer group and the cooler methane group. They are not identical, but form a sequence linking the warmer more star-like and the cooler more planet-like types.
Teams of astronomers have been searching intensively for such transition objects over the last year. In February 2000, following the discovery of several new brown dwarf candidates by the Sloan Survey, infrared measurements by Dr Sandy Leggett at UKIRT indicated that three of them might be this sought-after type. Infrared spectra were taken at UKIRT by the observing team of Leggett, Dr Thomas Geballe of the Gemini Observatory in Hawaii, Professor Gillian Knapp of Princeton University and Alexander McDaniel, a Princeton University undergraduate student, working with Xiaohui Fan (Princeton graduate student) and Dr David Golimowski and Dr Todd Henry at the Johns Hopkins University.
The spectra clearly revealed that the properties of these three brown dwarfs fall between the warmer and cooler groups previously known. Both methane and carbon monoxide show up weakly. Methane is absent in the warmer group and strong in the cooler group, while carbon monoxide is the other way around -- strong in the warmer group and not seen in the cooler group.
A paper reporting these findings will be published in the Astrophysical Journal. Reports are also being presented at a meeting in Jackson Hole, Wyoming (28 May - 1 June) and at the 196th meeting of the American Astronomical Society in Rochester, New York, 4 - 8 June. Detailed analysis of the spectra is under way to deduce more about the nature of these objects, which may resemble Jupiter and Saturn shortly after they formed about 5 billion years ago.
A system of classification has been devised for brown dwarfs, which builds on the established method of classifying stars (types O, B, A, F, G, K, M in order of decreasing temperature) and is similarly based on spectral features. New classes introduced following the discovery of brown dwarfs are L and T, only loosely defined at present.
Cool dwarf stars and the younger, warmer brown dwarfs have similar appearances and share portions of the M and L classifications. The M-type objects, with surface temperatures ranging down to 2,100 K (1,800 degrees C) have water and strong oxide features in their spectra. They may be stars or brown dwarfs, depending on their mass.
The next cooler group, with temperatures of roughly 1,500 to 2,100 K (1,200 to 1,800 degrees C) are the L dwarfs (L0 to L8), have spectra characterized by hydride features and even deeper water bands. The coolest, least massive stars fall into the warmer half of this temperature range. Their temperatures cannot be lower than 1,800 K (1,500 degrees C). Objects with temperatures between 1,500 and 1,800 K (1,200 and 1,500 degrees C) must be L-type brown dwarfs.
The methane (T-type) brown dwarfs found to date are the coolest objects so far detected. Their surface temperatures range down from about 1000 K to 800 K (700 to 500 degrees C). Their spectra show strong absorption by methane and water.
The 'missing link' objects found in the study reported here are believed to have surface temperatures in the range 1,000 to 1,500 K (700 to 1,200 degrees C).
2. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Particle Physics and Astronomy Research Council. The Sloan Digital Sky Survey is a joint project of the University of Chicago, Fermilab, Institute for Advanced Study, Japan Participation Group, Johns Hopkins University, Max-Planck Institute for Astronomy, Princeton University, the United States Naval Observatory and the University of Washington.
Epping NSW 1710, Australia
May 7, 1999
Dr Tinney of the Anglo-Australian Observatory and Mr Andrew Tolley, a student at the University of Oxford, recently observed a brown dwarf and noted changes in its surface chemistry as it rotated. They made their observations with Australia's largest optical telescope, the Anglo-Australian Telescope (AAT), located at Siding Spring outside Coonabarabran NSW.
Brown dwarfs are failed stars with masses in between that of Jupiter-like planets and normal stars. For decades, scientists have suspected that brown dwarfs exist but the first confirmed detection came only a few years ago. Today, after 30 years of searching, less than 50 brown dwarfs have been discovered. Of these, only a handful are bright enough and close enough to be studied for weather.
Tinney's team is leading Southern Hemisphere attempts to learn more about brown dwarfs. They are very faint, and extremely difficult to detect, and for Tinney and Tolley to have noted surface changes is remarkable.
Brown dwarfs never made it as stars, being too small to light up their nuclear furnaces. Instead, they just smoulder away in space at temperatures below 2000 degrees -- less than a third that of a typical star like the sun. 'Proper' stars are so hot that their surfaces are a completely smooth mix of vapourised material. But the outer layers of brown dwarfs are cool enough for chemicals to 'rain' out as smoke-like particles.
Tinney said, "Brown dwarfs are too small and far away to see the clouds. We detected them indirectly through the effects they have on the brown dwarf's atmosphere. We looked for a changing pattern of chemistry in the atmosphere of one brown dwarf, called LP944-20, as it rotated."
With a special instrument developed by the AAT, called the Taurus Tuneable Filter, Tinney and Tolley were able to 'tune into' a very narrow wavelength band and accurately measure tiny fluctuations. The narrow band chosen matched that of a tracer molecule called titanium oxide. The strength of this tracer allowed astronomers to track the formation of cloud particles.
Now that the technique has been honed, the astronomers are looking to other brown dwarfs. "We plan to study at least two more brown dwarfs in the next few months," Tinney concluded.
Background material on brown dwarfs
Background material on titanium oxide
Information and photos are also available on the ScienceNOW! website.
The left panel shows the weather patterns we might expect on a brown dwarf if it looked like Jupiter. The right panel shows observed variations seen as the brown dwarf LP944-20 rotates. The arrows highlight strong episodes of cloud passage where very different signals are seen in the two colours observed.
New Mexico State University
Las Cruces, New Mexico
December 15, 1998
Brown dwarfs, sometimes known as failed stars, have a reputation for being the dim bulbs of the heavens. That's one reason the first real specimen, a brown dwarf named Gliese 229B, was discovered only three years ago.
But Gliese 229B presented a puzzle to New Mexico State University astronomer Mark Marley and his colleagues as they studied the strange new object, because it seemed even darker than expected.
"Brown dwarfs are supposed to be dim, but it was turning out to be much, much darker than we would have thought in the optical part of the spectrum," Marley said.
In a classic example of the high-tech detective work today's astronomers use to analyze distant objects, Marley and two colleagues have determined that the brown dwarf suffers from a malady similar to one Los Angeles is famous for -- a hazy atmosphere. "The compounds are different, but it's like the red haze you see when you fly into Los Angeles," he said.
While L.A.'s smog is caused by sunlight reacting with auto emissions and other particles in the air over the city, the brown dwarf's red haze is thought to be caused by a different sort of chemical reaction. It appears that gases in the brown dwarf's atmosphere, primarily methane, react with light from a nearby star that Gl229B orbits, causing them to form more complicated molecules that clump together to form extremely small drops, Marley said -- drops about one-hundredth the size of those that form clouds in the Earth's atmosphere. The drops tend to block the visible light from the brown dwarf but are transparent in other parts of the spectrum, he said.
Results of the analysis of Gl229B's atmosphere were published in the Dec. 11 issue of Science, the weekly journal of the American Association for the Advancement of Science. The article was written by Caitlin Griffith of Northern Arizona University's Department of Physics and Astronomy, Roger Yelle of Boston University's Center for Space Physics, and Marley, a planetary scientist in NMSU's Department of Astronomy.
Solving this particular mystery about this particular brown dwarf, Marley said, adds to scientists' understanding of the universe around us. Since Gliese 229B was discovered, by a team of Cal Tech and Johns Hopkins scientists, "there are now dozens of brown dwarfs that have been discovered, and it's important to understand what their spectra can tell us about them," Marley said. "If these guys turn out to be a common part of the universe, we have to get a basic understanding of what's going on in their atmospheres, how hot they are, what they're made of."
Brown dwarfs are too small and cool to be stars and too massive to be planets. Scientists believe they form the same way stars do, but never accumulate enough mass to sustain nuclear fusion at their cores. They seem to share some characteristics with giant planets like Jupiter.
Drawing on his research on Jupiter and other planets, Marley has developed computer models that help astronomers examine newly discovered objects such as brown dwarfs and planets orbiting stars beyond our solar system. His collaborators on the latest brown dwarf project have models that complement his. Using data obtained by the Keck Telescope in Hawaii, they found that Gliese 229B fit their models in most respects, but not in the optical part of the spectrum.
The optical part of the spectrum includes light waves that are visible to the human eye, plus a section of the spectrum between visible light and the infrared region that is not visible to the human eye. The brown dwarf's darkness in this part of the spectrum could not be caused by clouds, the scientists concluded. Its atmosphere is too warm to contain ice clouds like those on Jupiter and too cool to contain silicate clouds like those on low mass stars. Also, in parts of the spectrum where the brown dwarf is brighter, such as the near-infrared part of the spectrum, "the data look like there are no clouds at all -- a perfectly clear atmosphere," Marley said.
"So there was this puzzle," he said. "It seemed to be cutting off light in one region (of the spectrum) but in other areas it looked just fine."
While the astronomers are confident they have solved the puzzle of Gliese 229B's atmosphere, by analysis of its spectrum and an understanding of how different particles scatter light, the results don't necessarily apply to other brown dwarfs, Marley said.
"Most of the other brown dwarfs that have been discovered are isolated," he said. Gliese 229B is orbiting a nearby star, and ultraviolet light from the star is a factor in the chemical reaction in the brown dwarf's atmosphere.
"And this one is still the coldest one so far," he added -- another factor in the atmospheric makeup.
Max Planck Institute for Extraterrestrial Physics
Garching bei Munchen/Germany
Observations carried out with the ROSAT X-ray satellite, and with several telescopes of the European Southern Observatory in La Silla, Chile, have revealed the first brown dwarf known to emit X-rays. The brown dwarf, called Cha H-alpha 1, is a very young member of the Chamaeleon dark cloud number I (Cha I), a star forming region located 550 light-years away from us.
Brown dwarfs are objects intermediate between stars and planets. The temperatures and pressures at their centers are insufficient to sustain the nuclear reactions that provide stars with their longlasting source of energy. Since central temperature and pressure are essentially a function of the mass of the object, the borderline between stars and brown dwarfs can be drawn at a mass that theoretical models accurateley place at 7.5% of the mass of our Sun, or about eight times the mass of Jupiter.
Lacking the source of energy that provides stars with a force preventing collapse under their own weight, brown dwarfs continue to shrink, cool, and fade out almost indefinitely after they are formed. It is precisely their very low luminosity what makes brown dwarfs so difficult to detect: it was not until late 1995 that the first bona-fide brown dwarf was identified. Since then, the number of known brown dwarfs has kept increasing, reaching about one dozen nowadays. When they are very young, brown dwarfs shrink rather rapidly, releasing large amounts of gravitational potential energy that makes them relatively bright and hot. Thus, apromising way of detecting brown dwarfs is to look at nearby regions where star fomation has taken place recently or is still going on, such as Chamaeleon I.
Meanwhile, an investigation also aimed at the detection of the faintest members of Chamaeleon I had been carried out by Dr. Fernando Comeron, of the European Southern Observatory in Garching bei München. Comeron tried to identify these objects by means of two features often found in young stars: the excess luminosity displayed at infrared wavelengths, due to the circumstellar material left over from the process of star formation, and the emission in H-alpha , a spectral line resulting when free protons and electrons become bound to form hydrogen atoms. For this purpose, Comerón had surveyed the entral region of Chamaeleon I using the infrared camera at the 2.2 m telescope in La Silla, and had obtained objective prism spectra of the same area in the wavelength region around the H-alpha line using the 1.5 m Danish telescope, also in La Silla. These surveys hat revealed two new faint objects with infrared excess luminosity, and six new H-alpha emission objects. Their luminosity placed them near, and possibly below, the theoretical borderline separating stars from brown dwarfs in a region of the age of Chamaeleon I.
The comparison between the ROSAT observations and the surveys carried out from La Silla yielded the surprising result that the faintest object detected with H-alpha emission was coincident, with the strongest X-ray source in that area of the sky without a previously identified visible counter part.
This was an exciting discovery, but it was made uncertain by the fact that Cha H-alpha 1 was too faint to allow a good quality spectrum to be obtained with the 1.5 m Danish telescope. Such a spectrum was necessary to determine the temperature of the object, which would in turn allow the comparison with theoretical models and therefore the estimate of its mass and its age. Fortunately, in May 1998 it was possible to obtain new spectra, in the visible and infrared spectral ranges, using the ESO 3.5 m New Technology Telescope in La Silla. These spectra allow the temperature of Cha H-alpha 1 to be determined with an accuracy of about 150 degrees Kelvin. The derived temperature and luminosity of Cha H-alpha 1, when compared to the predictions of theoretical models, tell us that Cha H-alpha 1 is a brown dwarf with a mass of only 4 to 5 % of the mass of the Sun, and an age of one million years.
One of these properties is the X-ray emission, which is known to be very common among young stars with masses of a few tenths of that of the Sun. Such stars are fully convective, meaning that the gas in their interior forms ascending and descending currents extending from deep inside the star to the surface. Such large scale motions of ionized gas, coupled with the general rotation of the star, can generate a strong magnetic field by dynamo effect. The magnetic field transports energy from the interior of the star to the chromosphere, a layer of hot gas lying above the atmosphere of the star. It is precisely the energy conveyed by the magnetic field, when deposited in the chromosphere, what produces the very high temperatures which give rise to X-ray emission. A similar mechanism of energy transport also operates in our Sun, although its details are not well understood. The detection of X-ray emission, and its correlation with various properties of the star such as its luminosity or its rotational period, can be of a great help to understand the details of the process of magnetic field generation and the internal structure of the star.
According to this general picture, one would expect that brown dwarfs, which are fully convective as well, should also possess magnetic fields and the hot chromospheres responsible for the X-ray emission. So far, this had been only theoretical conjecture without an observational confirmation. It had been suggested, on the other hand, that the fact that brown dwarfs cannot sustain nuclear reactions in their cores may have a negative impact on the process that generates the magnetic field. The role of rotation in generating the magnetic field has been also controversial: theoreticians disagree on whether fast rotation should enhance the magnetic field, or rather inhibit it, due to the difficulty of sustaining a well ordered convective pattern inside a rapidly rotating object. Answering to these questions requires observations of actual brown dwarfs and of their X-ray emission produced by magnetic activity. The results of Neuhauser and Comeron, obtained with ROSAT and several ESO telescopes on La Silla, revealing brown dwarfs as a new class of X-ray emitting sources, are thus an important step towards a detailed understanding of these faint but important objects.
Ralph Neuhauser, Fernando Comeron: ROSAT X-ray Detection of a Young Brown Dwarf in the Chamaeleon I Dark Cloud, Science, Vol. 282, 2 October 1998, 83 - 85
University of California-Los Angeles
June 5, 1998
"It's remarkable that it took so many years to find the nearest cluster of young stars to the Earth," Zuckerman said. "You would think the nearest ones would be much easier to find than distant clusters, but in this case it is not so."
Stars of this type -- called T Tauri stars -- are formed from a huge cloud of cold molecular gas, yet the astronomers have found no evidence of the molecular cloud that produced this star cluster. It is the strange absence of molecular gas that has delayed the discovery of this nearby cluster.
"One of the defining characteristics of newly formed stars is that they are located in or near cold clouds of molecular gas and dust particles," Webb said. "Because the newly discovered stars are in our own back yard, only 150 light years away, a giant cloud should be easily seen -- but there is no sign of it. Stars this young shouldn't have traveled far from their mother cloud. How it could dissipate so quickly is a real mystery."
Could the stars have been produced in some other way than by a molecular cloud? Zuckerman said there is "no known exception" to a cluster of stars being formed by a molecular cloud, and that an alternative source of star creation is the "last thing" astronomers would suspect.
Another surprise is the brown dwarf -- a celestial object larger than a planet but not quite a star. Seeing one near a bright star is like "trying to pick out a firefly buzzing around a searchlight," Zuckerman said. Astronomers have been searching for brown dwarfs for years, but have photographed very few (this one is only the third) in orbit around a star.
The object is moderately far from the star -- about 100 times the distance between the Earth and the Sun. Few things in astronomy are certain, and there is a remote possibility that the cool object is merely a foreground or background object, but the astronomers doubt that. If it is verified to be a companion to CoD-33 7795, then it will likely be the lowest mass object orbiting a star, other than the Sun, ever directly imaged through a telescope.
Before this research -- which was supported by the National Science Foundation and NASA -- astronomers knew of the existence of several of these young stars nearby but could not be certain that they were related. The new discoveries have approximately doubled the number of known young stars and established that they form a bonafide cluster. "We would have been happy to find one or two additional members of the cluster," Zuckerman said. "To double the number of known members of the cluster is far beyond what we anticipated."
In addition, they have determined that HR 4796, a star that received substantial attention in April, is also a member of this cluster. HR 4796 has a disk of dusty material around it that astronomers suspect contains coalescing planets. Other stars in the cluster, because of their youth, are also strong candidates to have disks containing planets in formation, Zuckerman and Webb believe.
With no cold molecular cloud to point the way, Webb and his colleagues instead relied on the fact that young stars usually emit abundant X-rays to target potential members of the cluster -- called the TW Hydrae Association, after the first member to be discovered, the variable star TW Hydrae. The astronomers studied an immense region of space, mostly in the southern constellation of Hydra. In addition to using X-ray emission, the research team used other techniques, including an analysis of the motion of the stars across the sky, to narrow down a list of some 350 X-ray emitters to about 50 candidates that warranted further study. Among these 50 were found the new cluster members.
The nearest young stars that astronomers typically study are about 450-500 light years from Earth. The 19 known stars and the brown dwarf in the TW Hya Association, by contrast, are about three times closer. Many stars in the sky are even closer to us than this, but all are much older.
The team's findings are based mainly on data obtained with the Low Resolution Imaging Spectrometer on the 10-meter (400-inch) Keck II telescope at the Mauna Kea Observatory in Hawaii. In addition, the researchers used data from the Infrared Telescope Facility, also at Mauna Kea, Germany's Roentgen X-ray Satellite and the Southern Proper Motion database archives maintained by Yale University.
Future studies will be aimed at finding the star's birth site. The results could have important and unanticipated implications for understanding the processes by which stars form.
"These are the closest known T Tauri stars and will make excellent laboratories for studies of the formation of stars and planetary systems," Webb said.
Around TW Hydrae itself, dust and gas exist with a rich chemistry, Zuckerman and colleagues reported last year in the journal Science. This chemistry is believed to be similar to that in our own solar system 10 million or so years after the formation of the sun.
The research team -- which also includes Imants Platis, a postdoctoral scholar at Yale; and UCLA graduate students Jennifer Patience, Michael Schwartz, and Russel White -- will continue to study the cluster and its mysterious origin. "We've found quite a few of the stars in the cluster, but there are more -- maybe a lot more," Webb commented.
Royal Astronomical Society Press Notices
Tuesday 8th April 1997
The team of astronomers at Leicester are Drs Martin Cossburn, Simon Hodgkin, Richard Jameson and David Pinfield. David Pinfield will describe the new discovery at the UK's National Astronomy Meeting at the University of Southampton on Tuesday 8th April, and a paper on it will be published later in the Monthly Notices of the Royal Astronomical Society. The team is involved in an international project to search for brown dwarfs. Astronomers would like to know just how common these objects are. It is possible that they may make some significant contribution towards the unseen dark matter in galaxies, but this is as yet highly uncertain.
PIZ1 was discovered with the 2.5-metre Isaac Newton Newton Telescope on the island of La Palma during an observing run in December 1995 and January 1996. (Images were taken through filters known as I and Z, hence the origin of the name.) Further observations were made with the United Kingdom Infrared Telescope (UKIRT) in Hawaii in October 1996 and spectra were taken with the 4-metre William Herschel Telescope on La Palma in November 1996.
Brown dwarfs are sometimes referred to as 'failed stars' -- in the sense that they are not massive enough for the nuclear fusion of hydrogen to take place in their cores. So once a brown dwarf has formed, it has no ongoing source of energy and spends the rest of its lifetime cooling down, getting fainter all the time. So the best time to find a brown dwarf is when it is young and at its brightest. The Pleiades cluster has proved a particularly good hunting ground, because the stars in it are relatively young on the astronomical scale -- only 100 million years, and it is near enough for faint brown dwarfs to be detectable. Three other brown dwarfs have been discovered in the Pleiades already.
Even the brightest brown dwarfs are difficult to detect because they give out so little radiation. The search has gone on over the last 15 years, with limited success in terms of definite identifications. Recent detections have been as a direct result of improvements in the sensitivity of detectors and the use of larger telescopes.
Early indications from the international search suggest that many more new objects of similar mass to PIZ 1 are being found.
ESO Press Release 07/97
28 April 1997
Text and photos with all links are available on the ESO website.
Until now, very few Brown Dwarfs have been securely identified as such. Two are members of double-star systems, and a few more are located deep within the Pleiades star cluster. Now, however, Maria Teresa Ruiz of the Astronomy Department at Universidad de Chile (Santiago de Chile), using telescopes at the ESO La Silla observatory, has just discovered one that is all alone and apparently quite near to us. Contrary to the others which are influenced by other objects in their immediate surroundings, this new Brown Dwarf is unaffected and will thus be a perfect object for further investigations that may finally allow us to better understand these very interesting celestial bodies.
It has been suggested that Brown Dwarfs may constitute a substantial part of the unseen dark matter in our Galaxy. This discovery may therefore also have important implications for this highly relevant research area.
For this project, the Chilean astronomer obtained large-field photographic exposures with the 1-m ESO Schmidt telescope at La Silla, each covering a sky area of 5.5 deg x 5.5 deg. When comparing plates of the same sky field obtained at time intervals of several years , she was able to detect, among the hundreds of thousands of stellar images on the plates, a few faint ones whose positions had changed a little in the meantime. The search technique is based on the fact that such a shift is a good indicator of the object being relatively nearby. It must therefore also be intrinsically faint, i.e. a potential White Dwarf candidate.
On every pair of plates, approximately twenty faint moving objects were detected with proper motions  of more than 0.25 arcsec per year. Indeed, follow-up spectroscopic observations showed that about 20 percent of these or about four per plate were White Dwarfs. Until now, a total of forty new White Dwarfs have been discovered during this very successful project, i.e. over ten times more than originally expected.
To her great surprise, the spectrum was of a type never seen before and certainly not that of a White Dwarf or any other easily identifiable type of star (cf. ESO Press Photo 12/97). In particular, there were no signs of spectral bands of titanium oxide (TiO) or vanadium oxide (VO) which are common in very cool stars, nor of the spectral lines seen in White Dwarfs. On the other hand, an absorption line of the short-lived element lithium was identified, as well as a hydrogen line in emission.
However, when the colour of this mysterious object was measured in different wavebands, it was found to be very red and quite similar to that of one of the two known Brown Dwarfs in double star systems. The presence of the lithium line in the spectrum is also an indication that it might be of that type.
The astronomer now decided to give the new object the name KELU-1; this word means `red' in the language of the Mapuche people, the ancient population in the central part of Chile. Its visual magnitude is 22.3, i.e. more than 3 million times fainter than what can be seen with the unaided eye.
In early April, additional infrared observations with the UKIRT (UK Infrared Telescope) on Mauna Kea (Hawaii) by Sandra K. Leggett (Joint Astrophysical Centre, Hilo, Hawaii, USA) confirmed the Brown Dwarf nature of KELU-1, in particular through the unambiguous detection of methane (CH4) bands in its spectrum.
This is also the reason that some astronomers consider Brown Dwarfs in the Milky Way and other galaxies as an important component of the `dark matter' whose presence is infered from other indirect measurements but has never been directly observed.
It is assumed that the mass limit that separates nuclear-burning stars and slowly contracting Brown Dwarfs is at about 90 times the mass of the giant planet Jupiter, or 8 percent of that of the Sun.
During recent years, several Brown Dwarf candidates have been de-masked as low-mass stars and only recently a few Brown Dwarfs were identified in the Pleiades star cluster. Those Brown Dwarfs are quite young and therefore comparatively hotter and brighter.
Contrarily, KELU-1 is most probably somewhat older and its unique location so close to us greatly facilitates future investigations. Moreover, it is not at all `disturbed' by the presence of other objects in its immediate surroundings, as this is the case for all other known objects of this type.
It will now be important to obtain accurate measurements of KELU-1's parallax, that is, the small annual change of its position in the sky that is caused by the Earth's motion around the Sun and thus the viewing angle of an Earth-based observer. This should be possible within the next year.
Moreover, high resolution spectral investigations with large telescope facilities, soon to include the ESO Very Large Telescope at the Paranal observatory in northern Chile, will now for the first time enable us to investigate the processes that take place in the relatively cold upper layers of Brown Dwarfs. For instance, the observed presence of lithium shows that its atmosphere must be different from that of low-mass stars.
Although the mass density of Brown Dwarfs derived from this estimate is insufficient to constitute all the `dark matter' in the Milky Way Galaxy, it is consistent with the most recent estimates of the local mass density, both observed and as infered from dynamical considerations of the motions of stars in the solar neighborhood.
 This is done by means of a so-called blink-comparator, an optical device in which the two plates are placed. A tilting mirror allows to view the same sky field alternately on the two plates. Any celestial object that has changed its position will appear to `jump' back and forth and can thus be identified.
 A proper motion in the sky of 0.25 arcsec/year corresponds to a transversal speed of about 12 km/sec if the object is located at a distance of 10 parsec, or 32.6 light-years. The largest known proper motion of an object outside the solar system is that of Barnard's Star at about 10 arcsec/year.
 For instance, as the mineral perovskite.
ESO Press Photo 11/97
Observations described in ESO Press Release 07/97 (28 April 1997) have shown that this object is located at a distance of only about 10 parsec from the Sun. Moreover, it is single and is therefore not disturbed by any other objects in its neighbourhood.
This image was obtained on March 15, 1997, through a near-infrared `gunn-i' filter with the EFOSC1 multimode instrument at the ESO 3.6-m telescope at La Silla. The exposure lasted 40 seconds and was made during good sky conditions. The field measures approx. 5 x 5 arcmin; North is up and East is left.
This is the caption to ESO PR Photo 11/97 [JPEG, 144k] which accompanies ESO Press Release 07/97 (28 April 1997). It is also available in a high-resolution version [JPEG; 2505 x 3000 pix; 1.3Mb] for reproduction purposes. It may be reproduced, if credit is given to the European Southern Observatory.
Copyright Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
ESO Press Photo 12/97
A comparison with the spectrum in the visual to near-infrared region (4000 - 10000 A, i.e. 400 - 1000 nm) with that of a normal, cool dwarf star of spectral type dM6e shows significant differences. In particular, it does not display the bands of titanium oxide which are seen in the dwarf star.
The insert shows the visual part of the spectrum (4000 - 7000 A) enlarged. The bands of CaOH and neutral sodium (Na) are seen, and also an absorption line of neutral lithium. There is a weak H-alpha emission line from neutral hydrogen. This is the first blue spectrum ever obtained of a Brown Dwarf object.
This tracing is based on two 30-min spectral exposures in the visual region and two 20-min exposures in the near-infrared region. The other spectrum is of another high-proper-motion object identified during the Calan-ESO proper-motion survey carried out with the ESO 1-m Schmidt telescope and herewith classified as of the dwarf M-type.
This is the caption to ESO PR Photo 12/97 [GIF, 35k] which accompanies ESO Press Release 07/97 (28 April 1997). It may be reproduced, if credit is given to the European Southern Observatory.
Copyright Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany