Category: Astronomy

Image credit: Chandra

The latest photos released from the Chandra X-Ray Observatory show x-rays produced from a brown dwarf which is orbiting a binary star system at a distance of 2.75 times that of the Pluto Charon orbit around the Sun. The star system is 180 light-years away from Earth in the constellation of Hydra. The brown dwarf, TWA 5B, is a failed star between 15 and 40 times the size of Jupiter.

A Chandra observation revealed X-rays produced by TWA 5B, a brown dwarf star orbiting a young binary star system known as TWA 5A. The star system is 180 light years from the Earth and a member of a group of about a dozen young stars in the Hydra constellation. The brown dwarf orbits the binary star system at a distance about 2.75 times that of Pluto’s orbit around the Sun.

The sizes of the sources in the image are due to an instrumental effect that causes the spreading of pointlike sources. For a comparison of the actual size of TWA 5B to the Sun and the planet Jupiter, see the illustration below.

Brown dwarfs are often referred to as “failed stars” because they are under the mass limit (about 80 Jupiter masses, or 8 percent of the mass of the Sun) needed to spark the nuclear fusion of hydrogen to helium which supplies the energy for stars such as the Sun. Lacking any central energy source, brown dwarfs are intrinsically faint and draw their energy from a very gradual shrinkage or collapse.

Young brown dwarfs, like young stars, have turbulent interiors. When combined with rapid rotation, this turbulent motion can lead to a tangled magnetic field that can heat their upper atmospheres, or coronas, to a few million degrees Celsius. The X-rays from both TWA 5A and TWA 5B are from their hot coronas.

TWA 5B is estimated to be only between 15 and 40 times the mass of Jupiter, making it one of the least massive brown dwarfs known. Its mass is rather near the boundary (about 12 Jupiter masses) between planets and brown dwarfs, so these results could have implications for the possible X-ray detection of very massive planets around stars.

Original Source: Chandra News Release

Image credit: Subaru Telescope

The Subaru telescope, based in Japan, has detected the most distant galaxy ever recorded at 12.8 billion light-years away. The Subaru Deep Field project team uncovered 70 candidate distant objects, by using a special filter which only allows light of a very specific wavelength to pass through – one that corresponds to objects which are approximately 13 billion light-years away.

Subaru telescope has found a galaxy 12.8 billion light years away (a redshift of 6.58; see note 1), the most distant galaxy ever observed. This discovery is the first result from the Subaru Deep Field Project, a research project of the Subaru Telescope of the National Astronomical Observatory of Japan which operates the Subaru telescope. The Subaru Deep Field (SDF) project team found approximately 70 distant galaxy candidates by attaching a special filter designed to detect galaxies around 13 billion light years away on a camera with a wide field of view. Follow-up observations with a spectrograph confirmed that two out of nine of the candidates are in fact distant galaxies. One of these is the most distant galaxy ever observed. This discovery raises the expectation that the project will be able to find a large number of distant galaxies that will help unravel the early history of the universe in a statistically meaningful manner.

The SDF project is an observatory project of the National Astronomical Observatory of Japan designed to showcase the abilities of Subaru telescope and to resolve fundamental astronomical questions that are difficult to address through Subaru’s regular time allocation system. Most research programs on Subaru telescope are selected through a competitive time allocation process called Open Use, which is open to all astronomers but allows a maximum of only three observing nights every six months. By pooling together observing nights reserved for the observatory and astronomers that contributed to the establishment of Subaru Telescope, an observatory project can address questions that require greater telescope resources than the typical research proposal. The SDF project’s main goal is to detect a large number of the most distant galaxies detectable and to understand their properties and their impact on the evolution of the universe. The speed of light is the fundamental limit to how fast information can travel (see note 2). When we detect light from a galaxy 13 billion light years away, that means we are seeing the galaxy as it was 13 billion years ago. Looking for ever more distant galaxies means looking at galaxies at earlier and earlier times in the universe.

The SDF observations took advantage of the fact that light from distant galaxies have a characteristic wavelength and shape. Astronomers think that the earliest galaxies rapidly formed stars from hydrogen, the dominant form of matter in the universe. The light from these stars would have excited any hydrogen remaining around them to higher energy states and even ionize it. When excited hydrogen returns to lower energy states, it emits light at several distinct wavelengths. However, most of this light would escape the young galaxy as an emission line at 122 nanometers because “bluer” light with shorter wavelengths and higher energy can re-excite other hydrogen atoms. Since the universe is expanding, the farther away a galaxy is from us, the faster it is moving away from us. Because of this movement, light from distant galaxies are doppler shifted to longer, or redder wavelengths, and this emission line is “redshifted” to a longer wavelength that is characteristic of the galaxy’s distance and the galaxy itself appears redder. As the light travels the long distance from its origin to Earth, light at the higher energy side, or blue side of the emission line, can be absorbed by the neutral hydrogen in intergalactic space. This absorption gives the emission line a distinctive asymmetrical look. A overall red appearance and a strong emission line at a particular wavelength with a particular asymmetrical shape is the signature of a distant new born galaxy.

To detect the most distant galaxies ever observed, the SDF team developed a special filter that only passes light with the narrow wavelength range of 908 to 938 nanometers. These wavelengths correspond to the 122 nanometer emission line after travelling a distance of 13 billion light years. The team installed the special filter, and two other filters at shorter and longer wavelengths bracketing the special filter, on Subaru telescope’s Suprime-Cam, Subaru Prime Focus Camera, and carried out an extensive observing program from April through May 2002. Suprime-Cam has the capability of imaging an area of the sky as large as the full moon in one exposure, a unique capability among instruments on 8-m class and larger telescopes, and is extremely well suited for surveys of very faint objects over large areas of the sky. By observing an area of the sky the size of the moon for up to 5.8 hours in each filter, the team was able to detect over 50,000 objects, including many extremely faint galaxies. By selecting galaxies that were bright only in the special filter and preferentially red, the team found 70 candidates for galaxies at a redshift of 6.6 (or a distance of 13 billion light years; see figure 1).

In June 2002, the team used FOCAS, the Faint Object Camera and Spectrograph on Subaru telescope, to observe 9 of the 70 candidates, and confirmed the generally red appearance and an emission line with a distinctive asymmetry in 2 objects (see figure 2), and determined that their redshifts are 6.58 and 6.54. The light from these galaxies was emitted 12.8 billion years ago when the universe was only 900 million years old. The previously observed most distant galaxy, with a redshift of 6.56, was discovered by looking at a large cluster of galaxies that can amplify light from more distant galaxies with a gravitational lensing effect. (See our press release from May 2002, http://www.naoj.org/Latestnews/200205/UH/index.html.)The SDF observations is the first time multiple galaxies at such a great distance have been observed, and without the help of gravitational lensing. The galaxy with a redshift of 6.58 is the most distant galaxy ever observed.

The SDF team expects to find many more distant galaxies through continued observations. Before the first stars and galaxies formed, the universe was in a stage that Astronomers call “the dark ages of the universe”. Determining when the dark ages ended is one of the most important astronomical questions of our time. Core members of the team, Keiichi Kodaira from the Graduate University of Advanced Studies in Japan, Nobunari Kashikawa from the National Astronomical Observatory of Japan, and Yoshiaki Taniguchi from Tohoku University hope that by detecting a statistically significant number of distant galaxies, they can begin to characterize the galaxies that heralded the end of the universe’s dark ages.

Original Source: Subaru News Release

Image credit: ESO

New observations from the Japanese 8-m Subaru telescope and the ESO Very Large Telescope (VLT) have revealed new details in the Virgo galaxy cluster – located 50 million light years away. One insight is that massive young stars seem to be able to form in isolation, far away from the brighter parts of galaxies.

At a distance of some 50 million light-years, the Virgo Cluster is the nearest galaxy cluster. It is located in the zodiacal constellation of the same name (The Virgin) and is a large and dense assembly of hundreds of galaxies.

The “intracluster” space between the Virgo galaxies is permeated by hot X-ray emitting gas and, as has become clear recently, by a sparse “intracluster population of stars”.

So far, stars have been observed to form in the luminous parts of galaxies. The most massive young stars are often visible indirectly by the strong emission from surrounding cocoons of hot gas, which is heated by the intense radiation from the embedded stars. These “HII regions” (pronounced “Eitch-Two” and so named because of their content of ionized hydrogen) may be very bright and they often trace the beautiful spiral arms seen in disk galaxies like our own Milky Way.

New observations by the Japanese 8-m Subaru telescope and the ESO Very Large Telescope (VLT) have now shown that massive stars can also form in isolation, far from the luminous parts of galaxies. During a most productive co-operation between astronomers working at these two world-class telescopes, a compact HII region has been discovered at the very boundary between the outer halo of a Virgo cluster galaxy and Virgo intracluster space.

This cloud is illuminated and heated by a few hot and massive young stars. The estimated total mass of the stars in the cloud is only a few hundred times that of the Sun.

Such an object is rare at the present epoch. However, there may have been more in the past, at which time they were perhaps responsible for the formation of a fraction of the intracluster stellar population in clusters of galaxies. Massive stars in such isolated HII regions will explode as supernovae at the end of their short lives, and enrich the intracluster medium with heavy elements.

Observations of two other Virgo cluster galaxies, Messier 86 and Messier 84, indicate the presence of other isolated HII regions, thus suggesting that isolated star formation may occur more generally in galaxies. If so, this process may provide a natural explanation to the current riddle why some young stars are found high up in the halo of our own Milky Way galaxy, far from the star-forming clouds in the main plane.

The Virgo Cluster
The galaxies in the Universe are rarely isolated – they prefer company. Many are found within dense structures, referred to as galaxy clusters, cf. e.g., ESO PR Photo 16a/99.

The galaxy cluster nearest to us is seen in the direction of the zodiacal constellation Virgo (The Virgin), at a distance of approximately 50 million light-years. PR Photo 04a/03 (from the Wide Field Imager camera at the ESO La Silla Observatory) shows a small sky region near the centre of this cluster with some of the brighter cluster galaxies. PR Photo 04b/03 displays an image of a larger field (partially overlapping Photo 04a/03) in the light of ionized hydrogen – it was obtained by the Japanese 8.2-m Subaru telescope on Mauna Kea (Hawaii, USA). The field includes some of the large galaxies in this cluster, e.g., Messier 86, Messier 84 and NGC 4388. In order to show the faintest possible hydrogen emitting objects embedded in the outskirts of bright galaxies, their smooth envelopes have been “subtracted” during the image processing. This is why they look quite different in the two photos.

Clusters of galaxies are believed to have formed because of the strong gravitational pull from dark and luminous matter. The Virgo cluster is considered to be a relatively young cluster, because studies of the distribution of its member galaxies and X-ray investigations of hot cluster gas have revealed small “subclusters of galaxies” around the major galaxies Messier 87, Messier 86 and Messier 49. These subclusters are yet to merge to form a dense and smooth galaxy cluster.

The Virgo cluster is apparently cigar-shaped, with its longest dimension of about 10 million light-years near the line-of-sight direction – we see it “from the end”.

Stars in intracluster space
Galaxy clusters are dominated by dark matter. The largest fraction of the luminous (i.e. “visible”) cluster mass is made up of the hot gas that permeates all of the cluster. Recent observations of “intracluster” stars have confirmed that, in addition to the individual galaxies, the Virgo cluster also contains a so-called “diffuse stellar component”, which is located in the space between the cluster galaxies.

The first hint of this dates back to 1951 when Swiss astronomer Fritz Zwicky (1898-1974), working at the 5-m telescope at Mount Palomar in California (USA), claimed the discovery of diffuse light coming from the space between the galaxies in another large cluster of galaxies, the Coma cluster. The brightness of this intracluster light is 100 times fainter than the average night-sky brightness on the ground (mostly caused by the glow of atoms in the upper terrestrial atmosphere) and its measurement is difficult even with present technology. We now know that this intracluster glow comes from individual stars in that region.

Planetary nebulae
More recently, astronomers have undertaken a new and different approach to detect the elusive intracluster stars. They now search for Sun-like stars in their final dying phase during which they eject their outer layers into surrounding space. At the same time they unveil their small and hot stellar core which appears as a “white dwarf star”.

Such objects are known as “planetary nebulae” because some of those nearby, e.g. the “Dumbbell Nebula” (cf. ESO PR Photo 38a/98) resemble the disks of the outer solar system planets when viewed in small telescopes.

The ejected envelope is illuminated and heated by the very hot star at its centre. This nebula emits strongly in characteristic emission lines of oxygen (green; at wavelengths 495.9 and 500.7 nm) and hydrogen (red; the H-alpha line at 656.2 nm). Planetary nebulae may be distinguished from other emission nebulae by the fact that their main green oxygen line at 500.7 nm is normally about 3 to 5 times brighter than the red H-alpha line.
Search for intracluster planetary nebulae

An international team of astronomers [2] is now carrying out a very challenging research programme, aimed at finding intracluster planetary nebulae. For this, they observe the regions between cluster galaxies with specially designed, narrow-band optical filters tuned to the wavelength of the green oxygen lines.

The main goal is to study the overall properties of the diffuse stellar component in the nearby Virgo cluster. How much diffuse light comes from the intracluster space, how is it distributed within the cluster, and what is its origin?

Because the stars in this region are apparently predominantly old, the most likely explanation of their presence in this region is that they formed inside individual galaxies, which were subsequently stripped of many of their stars during close encounters with other galaxies during the initial stages of cluster formation. These “lost” stars were then dispersed into intracluster space where we now find them.

The Subaru observations
Japanese and European astronomers used the Suprime-Cam wide-field mosaic camera at the 8-m Subaru telescope (Mauna Kea, Hawaii, USA) to search for intracluster planetary nebulae in one of the densest regions of the Virgo cluster, cf. PR Photo 04b/03. They needed a telescope of this large size in order to select such objects and securely discriminate them from the thousands of foreground stars in the Milky Way and background galaxies.

In particular, by observing in two narrow-band filters sensitive to oxygen and hydrogen, respectively, the planetary nebulae visible in this field could be “separated” from distant (high-redshift) background galaxies, which do not have strong emission in both the green and red band. It is very time-consuming to observe the weak H-alpha emission and this can only be done with a big telescope.

Some 40 intracluster planetary nebulae candidates were found in this field which had the expected oxygen/H-alpha line intensity ratios of 3 – 5, such as those depicted PR Photo 04d/03. Unexpectedly, however, the data also showed a small number of star-like emission objects with oxygen/H-alpha line ratios of about 1. This is more typical of a cloud of ionized gas around young, massive stars – like the so-called HII regions in our own galaxy, the Milky Way.

However, it would be very unusual to find such star formation regions in the intracluster region, so follow-up spectroscopic observations were clearly needed for confirmation.

The VLT measurements
The only way to make sure that these unusual objects are actually powered by young stars is by a detailed spectroscopical study, analyzing the emitted light over a wide range of wavelengths. One of the objects was observed in this way in April 2002 with the FORS2 multi-mode instrument at the 8.2-m VLT YEPUN telescope at the ESO Paranal Observatory (Chile).

This was a most challenging observation, even for this very powerful facility, requiring several hours of exposure time. The brightness of the faint object (the flux of the oxygen [OIII 500.7]-line) was comparable to that of a 60-Watt light bulb at a distance of about 6.6 million km, i.e., about 17 times farther than the Moon.

The recorded (long-slit) spectrum (PR Photo 04e/03) is indeed that of an HII region, with characteristic emission lines from hydrogen, oxygen and sulphur, and with underlying blue “continuum” emission from hot, young stars. This is the first concrete evidence that some of the ionized hydrogen gas in the intracluster medium near NGC 4388 is heated by massive stars, rather than radiation from the nucleus of the galaxy.

Comparing the spectrum with simple starburst models showed that this HII region is “powered” by one or two hot and massive (O-type) stars. The best-fitting starburst model implies an estimated total mass of young stars of some 400 solar masses with an age of about 3 million years. The object is obviously very compact – it is indeed unresolved in all the images. The inferred radius of the HII region is about 11 light-years.

Young stars form far from galaxies
This compact star-forming region is located about 3.4 arcmin north and 0.9 arcmin west of the galaxy NGC 4388, corresponding to a distance of some 82,000 light-years (projected) from the main star-forming regions in this galaxy. The small cloud is moving away from us with an observed velocity of 2670 km/sec. This is considerably faster than the mean velocity of the Virgo cluster (about 1200 km/sec) but similar to that of NGC 4388 (2520 km/sec), indicating that it is probably falling through the Virgo cluster core together with NGC 4388, but it cannot have moved far during the comparatively short lifetime of its massive stars.

It is not known whether it once was or still is bound to NGC 4388, or whether it only belonged to the surroundings that fell into the Virgo cluster with this galaxy. In any case, the existence of this HII region is a clear demonstration that stars can form in the “diffuse” outskirts of galaxies, if not in intracluster space.

Because of internal dynamical processes, the stars in this object cannot remain forever in a dense cluster. Within a few hundred million years they will disperse and mix with the diffuse stellar population nearby. This isolated star formation is therefore likely to contribute to the intracluster stellar population, either directly, or after having moved away from the halo of NGC 4388.

This mode of isolated star formation does not contribute much to the total intracluster light emission – at the current rate it can explain only a small fraction of the diffuse light now observed in this region. However, it may have been more significant in the past, when protogalaxies and proto-galaxy groups, rich in neutral gas and with gas clouds at large distances from their centers, fell into the forming Virgo cluster for the first time.

Prospects
The existence of isolated compact HII regions like this one is important as a very different site of star formation than those normally seen in galaxies. The massive stars born in such isolated clouds will explode as supernovae and enrich the Virgo intracluster medium with metals.

Other possible – but not yet spectroscopically verified – compact HII regions in the halos of both Messier 86 and Messier 84 have been detected during this work. This finding thus also calls into question the current use of emission-line planetary nebulae luminosities as a distance indicator; to obtain the best possible accuracy, it will henceforth be necessary to weed out possible HII regions in the samples.

If compact HII regions exist generally in galaxies, they may possibly be the birthplaces of some of the young stars now observed in the halo of our Milky Way galaxy, high above the main plane. Observational programmes with both the Subaru and VLT telescopes are now planned to discover more of these interesting objects and to explore their properties.

Original Source: ESO News Release

Image credit: ESO

A team of European astronomers have located the closest brown dwarf star ever discovered. Epsilon Indi, located only 12 light-years from the Earth, was thought to be a single star, but the ESO team spotted its companion. Epsilon Indi B is 45 times the mass of Jupiter and takes 400 years to orbit the main star.

A team of European astronomers [2] has discovered a Brown Dwarf object (a ‘failed’ star) less than 12 light-years from the Sun. It is the nearest yet known.

Now designated Epsilon Indi B, it is a companion to a well-known bright star in the southern sky, Epsilon Indi (now “Epsilon Indi A”), previously thought to be single. The binary system is one of the twenty nearest stellar systems to the Sun.

The brown dwarf was discovered from the comparatively rapid motion across the sky which it shares with its brighter companion : the pair move a full lunar diameter in less than 400 years. It was first identified using digitised archival photographic plates from the SuperCOSMOS Sky Surveys (SSS) and confirmed using data from the Two Micron All Sky Survey (2MASS). Follow-up observations with the near-infrared sensitive SOFI instrument on the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory confirmed its nature and has allowed measurements of its physical properties.

Epsilon Indi B has a mass just 45 times that of Jupiter, the largest planet in the Solar System, and a surface temperature of only 1000 ?C. It belongs to the so-called ‘T dwarf’ category of objects which straddle the domain between stars and giant planets.

Epsilon Indi B is the nearest and brightest T dwarf known. Future studies of the new object promise to provide astronomers with important new clues as to the formation and evolution of these exotic celestial bodies, at the same time yielding interesting insights into the border zone between planets and stars.

Tiny moving needles in giant haystacks
Imagine you are a professional ornithologist, recently returned home from an expedition to the jungles of South America, where you spent long weeks using your high-powered telephoto lenses searching for rare species of birds. Relaxing, you take a couple of wide-angle snapshots of the blooming flowers in your back garden, undistracted by the common blackbird flying across your viewfinder. Only later, when carefully comparing those snaps, you notice something tiny and unusually coloured, flittering close behind the blackbird: you’ve discovered an exotic, rare bird, right there at home.

In much the same way, a team of astronomers [2] has just found one of the closest neighbours to the Sun, an exotic ‘failed star’ known as a ‘brown dwarf’, moving rapidly across the sky in the southern constellation Indus (The Indian). Interestingly, at a time when telescopes are growing larger and are equipped with ever more sophisticated electronic detectors, there is still much to be learned by combining old photographic plates with this modern technology.

Photographic plates taken by wide-field (“Schmidt”) telescopes over the past decades have been given a new lease on life through being digitised by automated measuring machines, allowing computers to trawl effectively through huge and invaluable data archives that are by far not yet fully exploited [3]. For the Southern Sky, the Institute for Astronomy in Edinburgh (Scotland, UK) has recently released scans made by the SuperCOSMOS machine of plates spanning several decades in three optical passbands. These data are perfectly suited to the search for objects with large proper motions and extreme colours, such as brown dwarfs in the Solar vicinity.
Everything is moving – a question of perspective

In astronomy, the `proper motion’ of a star signifies its apparent motion on the celestial sphere; it is usually expressed in arcseconds per year [4]. The corresponding, real velocity of a star (in kilometres per second) can only be estimated if the distance is known.

A star with a large proper motion may indicate a real large velocity or simply that the star is close to us. By analogy, an airplane just after takeoff has a much lower true speed than when it’s cruising at high altitude, but to an observer watching near an airport, the departing airplane seems to be moving much more quickly across the sky.

Proxima Centauri, our nearest stellar neighbour, is just 4.2 light-years away (cf. ESO PR 22/02) and has a proper motion of 3.8 arcsec/year (corresponding to 23 km/sec relative to the Sun, in the direction perpendicular to the line-of-sight). The highest known proper motion star is Barnard’s Star at 6 light-years distance and moving 10 arcsec/year (87 km/sec relative to the Sun). All known stars within 30 light-years are high-proper-motion objects and move at least 0.2 arcsec/year.

Trawling for fast moving objects
For some time, astronomers at the Astrophysical Institute in Potsdam have been making a systematic computerised search for high-proper-motion objects which appear on red photographic sky plates, but not on the equivalent blue plates. Their goal is to identify hitherto unknown cool objects in the Solar neighbourhood.

They had previously found a handful of new objects within 30 light-years in this way, but nothing as red or moving remotely as fast as the one they have now snared in the constellation of Indus in the southern sky. This object was only seen on the very longest-wavelength plates in the SuperCOSMOS Sky Survey database. It was moving so quickly that on plates taken just two years apart in the 1990s, it had moved almost 10 arcseconds on the sky, giving a proper motion of 4.7 arcsec/year. It was also very faint at optical wavelengths, the reason why it had never been spotted before. However, when confirmed in data from the digital Two Micron All Sky Survey (2MASS), it was seen to be much brighter in the infrared, with the typical colour signature of a cool brown dwarf.

At this point, the object was thought to be an isolated traveller. However, a search through available online catalogues quickly revealed that just 7 arcminutes away was a well-known star, Epsilon Indi. The two share exactly the same very large proper motion, and thus it was immediately clear the two must be related, forming a wide binary system separated by more than 1500 times the distance between the Sun and the Earth.

Epsilon Indi is one of the 20 nearest stars to the Sun at just 11.8 light years [5]. It is a dwarf star (of spectral type K5) and with a surface temperature of about 4000 ?C, somewhat cooler than the Sun. As such, it often appears in science fiction as the home of a habitable planetary system [6]. That all remains firmly in the realm of speculation, but nevertheless, we now know that it most certainly has a very interesting companion.

This is a remarkable discovery: Epsilon Indi B is the nearest star-like source to the Sun found in 15 years, the highest proper motion source found in over 70 years, and with a total luminosity just 0.002% that of the Sun, one of the intrinsically faintest sources ever seen outside the Solar System!

After Proxima and Alpha Centauri, the Epsilon Indi system is also just the second known wide binary system within 15 light years. However, unlike Proxima Centauri, Epsilon Indi B is no ordinary star.

Brown dwarfs: cooling, cooling, cooling …
Within days of its discovery in the database, the astronomers managed to secure an infrared spectrum of Epsilon Indi B using the SOFI instrument on the ESO 3.5-m New Technology Telescope (NTT) at the La Silla Observatory (Chile). The spectrum showed the broad absorption features due to methane and water steam in its upper atmosphere, indicating a temperature of ‘only’ 1000 ?C. Ordinary stars are never this cool – Epsilon Indi B was confirmed as a brown dwarf.

Brown dwarfs are thought to form in much the same way as stars, by the gravitational collapse of clumps of cold gas and dust in dense molecular clouds. However, for reasons not yet entirely clear, some clumps end up with masses less than about 7.5% of that of our Sun, or 75 times the mass of planet Jupiter. Below that boundary, there is not enough pressure in the core to initiate nuclear hydrogen fusion, the long-lasting and stable source of power for ordinary stars like the Sun. Except for a brief early phase where some deuterium is burned, these low-mass objects simply continue to cool and fade slowly away while releasing the heat left-over from their birth.

Theoretical discussions of such objects began some 40 years ago. They were first named ‘black dwarfs’ and later ‘brown dwarfs’, in recognition of their predicted very cool temperatures. However, they were also predicted to be very faint and very red, and it was only in 1995 that such objects began to be detected.

The first were seen as faint companions to nearby stars, and then later, some were found floating freely in the Solar neighbourhood. Most brown dwarfs belong to the recently classified spectral types L and T, below the long-known cool dwarfs of type M. These are very red to human eyes, but L and T dwarfs are cooler still, so much so that they are almost invisible at optical wavelengths, with most of their emission coming out in the infrared. [7].
How massive is Epsilon Indi B?

The age of most brown dwarfs detected to date is unknown and thus it is hard to estimate their masses. However, it may be assumed that the age of Epsilon Indi B is the same as that of Epsilon Indi A, whose age is estimated to be 1.3 billion years based on its rotational speed. Combining this information with the measured temperature, brightness, and distance, it is then possible to determine the mass of Epsilon Indi B using theoretical models of brown dwarfs.

Two independent sets of models yield the same result: Epsilon Indi B must have a mass somewhere between 4-6% of that of the Sun, or 40-60 Jupiter masses. The most likely value is around 45 Jupiter masses, i.e. well below the hydrogen fusion limit, and definitively confirming this new discovery as a bona-fide brown dwarf.

The importance of Epsilon Indi B
PR Photo 03c/03 shows the current census of the stars in the solar neighbourhood. All these stars have been known for many years, including GJ1061, which, however, only had its distance firmly established in 1997. The discovery of Epsilon Indi B, however, is an extreme case, never before catalogued, and the first brown dwarf to be found within the 12.5 light year horizon.

If current predictions are correct, there should be twice as many brown dwarfs as main sequence stars. Consequently, Epsilon Indi B may be the first of perhaps 100 brown dwarfs within this distance, still waiting to be discovered!

Epsilon Indi B is an important catch well beyond the cataloguing the Solar neighbourhood. As the nearest and brightest known brown dwarf and with a very accurately measured distance, it can be subjected to a wide variety of detailed observational studies. It may thus serve as a template for more distant members of its class.

With the help of Epsilon Indi B, astronomers should now be able to see further into the mysteries surrounding the formation and evolution of the exotic objects known as brown dwarfs, halfway between stars and giant planets, the physics of their inner cores, and the weather and chemistry of their atmospheres.

An historical note – the southern constellation Indus
The constellation Indus lies deep in the southern sky, nestled between three birds, Grus (The Crane), Tucana (The Toucan) and Pavo (The Peacock), cf. PR Photo 03d/03.

First catalogued in 1595-1597 by the Dutch navigators Pieter Dirkszoon Keyser and Frederick de Houtman, this constellation was added to the southern sky by Johann Bayer in his book ‘Uranometria’ (1603) to honour the Native Americans that European explorers had encountered on their travels.

In particular, it has been suggested that it is specifically the native peoples of Tierra del Fuego and Patagonia that are represented in Indus, just over two thousand kilometres south of La Silla where the first spectroscopic observations of Epsilon Indi B were made some 400 years later.

In the later drawing by Bode shown here, Epsilon Indi, the fifth brightest star in Indus, is associated with one of the arrows in the Indian’s hand.

Original Source: ESO News Release

Image credit: Hubble

Just in case you were worried, it appears that a supernova would have to be really really close to the Earth before it could cause massive damage to our environment. Scientists at NASA and Kansas University used data from a recent exploding star to determine that a supernova would have to be only 26 light-years away in order to strip away the ozone layer – an encounter that only happens once every 670 million years.

We have one less thing to worry about. While the cosmic debris from a nearby massive star explosion, called a supernova, could destroy the Earth’s protective ozone layer and cause mass extinction, such an explosion would have to be much closer than previously thought, new calculations show.

Scientists at NASA and Kansas University have determined that the supernova would need to be within 26 light years from Earth to significantly damage the ozone layer and allow cancer-causing ultraviolet radiation to saturate the Earth’s surface.

An encounter with a supernova that close only happens at a rate of about once in 670 million years, according to Dr. Neil Gehrels of NASA’s Goddard Space Flight Center in Greenbelt, Md., who presents these findings today at the American Astronomical Society meeting in Seattle.

“Perhaps a nearby supernova has bombarded Earth once during the history of multicellular life with its punishing gamma rays and cosmic rays,” said Gehrels. “The possibility for mass extinction is indeed real, yet the risk seems much lower than we have thought.”

The new calculations are based largely on advances in atmospheric modeling, analysis of gamma rays produced by a supernova in 1987 called SN1987a, and a better understanding of galactic supernova locations and rates. A supernova is an explosion of a star at least twice as massive as our Sun.

Previous estimates from the 1970s stated that supernovae as far as 55 light years from Earth could wipe out up to 90 percent of the atmosphere for hundreds of years. The damage would be from gamma rays and cosmic rays, both prodigiously emitted by supernovae. Gamma rays are the most energetic form of light. Cosmic rays are atomic particles, the fastest-moving matter in the Universe, produced when the expanding shell of gas from the exploded star runs into surrounding dust and gas in the region. Gamma rays, moving at light speed, would hit the Earth’s atmosphere first, followed closely by the cosmic rays moving at close to light speed.

Gamma-ray light particles (called photons) and the cosmic-ray particles can wreak havoc in the upper atmosphere, according to Dr. Charles Jackman of NASA Goddard, who provided the atmospheric analysis needed for the new calculation.

The particles collide with nitrogen gas (N2) and break the molecule into highly-reactive nitrogen atoms (N). The nitrogen atoms then react fairly quickly with oxygen gas (O2) to form nitric oxide (NO) and, subsequently, other nitrogen oxides (NOx). The nitrogen oxide molecules can then destroy ozone (O3) through a catalytic process. This means that a single NOx molecule can destroy an ozone molecule and remain intact to destroy hundreds of more ozone molecules.

The new calculations — based on the NASA Goddard two-dimensional photochemical transport model — show that a supernova within 26 light years from Earth could wipe out 47 percent of the ozone layer, allowing approximately twice the amount of cancer-causing ultraviolet radiation to reach the Earth’s surface. Excessive UV radiation is harmful to both plants and animals, thus a doubling of UV levels would be a significant problem to life on Earth.

The gamma-ray irradiation would last 300 to 500 days. The ozone layer would then repair itself, but only to endure cosmic-ray bombardment shortly after, lasting at least 10 years. (Cosmic rays are electrically charged particles whose paths are influenced by magnetic fields, and the extent of such fields in the interstellar medium is not well understood.)

The calculations simultaneously point to the resilience of the ozone layer as well as its fragility in a violent Universe, said Dr. Claude Laird of the University of Kansas, who developed the gamma-ray and cosmic ray input code and performed the atmospheric model simulations. Although the ozone layer should recover relatively rapidly once the particle influx tapers off — within about one to two years, the Goddard models show — even this short period of time is sufficient to cause significant and lasting damage to the biosphere.

“The atmosphere usually protects us from gamma rays, cosmic rays, and ultraviolet radiation, but there’s only so much hammering it can take before Earth’s biological defenses break down,” he said.

Dr. John Cannizzo of NASA Goddard and University of Maryland, Baltimore Country, initiated and coordinated the new calculations. “I’ve long been fascinated by the possibility of extinction from something as remote as a star explosion,” he said. “With this updated calculation, we essentially worked backwards to determine what level of ozone damage would be needed to double the level of ultraviolet radiation reaching the Earth’s surface and then determined how close a supernova would need to be to cause that kind of damage.”

These results will appear in the Astrophysical Journal 2003, March 10, vol. 585. Co-authors include Barbara Mattson of NASA Goddard (via L3 Com Analytics Corporation) and Wan Chen of Sprint IP Design in Reston, Virginia.

Original Source: NASA News Release

Astronomers from the University of Mexico have found a distant star system where a small, young star has been flung out of its binary star system by the gravitational interaction with its neighbours. The star, called T Tauri Component Sb, has 20% the mass of the Sun, and was part of a group of stars 450 light years from the Earth. The team has been tracking the path of the rogue star since 1983, and watched it slingshot past one star and head out into space.

Image credit: Hubble

The new Advanced Camera for Surveys (ACS) on the Hubble Space Telescope has revealed a clear disk of dust around a young, 5 million year old star. Astronomers believe these disks are the birthplaces of planets. The star, called HD 141569A is part of a triple star system located 320 light-years away in the constellation of Libra.

NASA Hubble Space Telescope’s new Advanced Camera for Surveys (ACS) has given astronomers their clearest view yet of the dust disk around a young, 5-million-year-old star. Such disks are expected to be the birthplace of planets. The star, called HD 141569A, lies 320 light-years away in the constellation Libra and appears to be a member of a triple-star system.

The star HD 141569A was first identified as a candidate for a circumstellar disk in 1986, from observations done with the NASA/Netherlands/United Kingdom Infrared Astronomy Satellite (IRAS). An excess of infrared radiation associated with the star provides telltale evidence for the presence of a dust disk. Hubble’s Near Infrared Camera and Multi-Object Spectrometer photographed the disk in 1999 and revealed two concentric rings divided by a dark lane. This was interpreted as evidence of dynamical sculpting by one or more planets.

The ACS reveals that the disk’s structure is much more complex than previously thought. The disk is actually a tightly wound spiral structure. The outer regions of the disk reveal two diffuse spiral arms, one of which appears to be associated with the nearby double star system (HD 141569BC) seen at the upper left. The apparent connection between the disk and the double star suggest that an interaction with the double star may be responsible for the structures seen in the disk.

However, previous mid-IR images of the disk show that it is relatively clear of dust within approximately 2.8 billion miles of the star. This inner region may have been swept clear by one or more unseen planets.

These observations of the disk were obtained with the ACS’s High Resolution Camera (HRC) coronagraph. The photo on the left is a processed visible light image. In the photo on the right, the disk has been geometrically altered to simulate a face-on view, and false-color has been applied to enhance the disk structure. The black center marks regions where light from the star has been masked out. These images are the first results of a survey of disks around young main-sequence stars being conducted by the ACS science team.

Original Source: Hubble News Release

Image credit: NASA

A team of astronomers from the California Institute of Technology in Pasadena presented images today of a rare Hyper Extremely Red Object (Hero). This dim object, located near a galaxy 10 billion light years away is traveling away from us at almost the speed of light. In fact, it’s so far, and moving so fast, it has gone way past being red-shifted – it’s only visible in infrared light.

Heroes are usually confined to comic books and movies, but as the saying goes, we all need one. So astronomers have turned to the deep, dark cosmos to find their heroic figure — the “Hyper Extremely Red Object,” or “Hero.”

At the American Astronomical Society winter meeting in Seattle today, an astronomer from the California Institute of Technology in Pasadena reports the discovery of a Hero near the radio galaxy 53W002, more than 10 billion light years away. This marks the first time a Hero has been found near a radio galaxy, suggesting that radio galaxies — which are optically dim but have strong radio emissions — may provide a guidepost for scouting out other Hero objects.

“Hero objects are intriguing. Like comic book heroes, they travel really fast — almost at the speed of light. They are virtually invisible to our eyes and they are very mysterious. Most importantly, this type of Hero may hold a key for understanding how the first galaxies formed and evolved in the universe,” said Dr. Myungshin Im, a staff research scientist at the Space Infrared Telescope Facility Science Center, located at Caltech.

So far, the astronomical version of a hero has taken on the unassuming guise of a small, glowing, red patch in deep space. More advanced infrared telescopes like NASA’s Space Infrared Telescope Facility, managed by the Jet Propulsion Laboratory, Pasadena, Calif., and launching in spring 2003, may, among other things, lift this red veil and reveal these remote objects for what they really are — quite possibly the universe’s earliest stars and galaxies.

Due to expansion of the universe after the Big Bang, a distant object in the universe races away from us so fast that any visible light from it “redshifts” — in other words, a light source becomes redder when it recedes from observers on Earth, and, conversely, bluer when it approaches. So when a visible light source moves away from us at nearly the speed of light, it often appears in infrared wavelengths. Big Bang theory also implies that the farther away an object is, the faster it moves away from us.

53W002_HERO1, the designation for the newly found Hero, is so far away and moves so fast it appears as a faint infrared source. In fact, it took two powerful telescopes equipped with infrared cameras to spot it in the deep sky. Im discovered 53W002_HERO1 from images taken by the near-infrared camera and multi-object spectrometer on NASA’s Hubble Space Telescope and the cooled infrared spectrograph and camera attached to the Subaru 8-meter (26-feet) telescope atop Mauna Kea in Hawaii. Dr. Toru Yamada and collaborators at the National Astronomical Observatory of Japan provided Im with the Subaru data.

The more distant a cosmic object is, the further in the past we see it. But for Im and colleagues to glean information about the early universe from 53W002_HERO1, they first need to determine its intrinsic color — that is, how would this astronomical hero appear to a human observer nearby?

It could be red, indicating either dust-obscured galaxies cocooning intense star formation, or older galaxies filled with an overabundance of elderly, reddish stars, both of which would lie about 10 billion light-years away. If the former condition exists, astronomers will appreciate the degree to which dust hid star formation during that epoch. However, if the latter holds then scientists can trace back to a time when a significant population of stars were born.

Another possibility is that a Hero might really be blue — a very young galaxy populated with fresh, super-hot blue stars at a distant 13 to 14 billion light years. In this instance, we may be witnessing the formation of the universe’s very first galaxies.

To determine whether 53W002_HERO1 is intrinsically red or blue, Im and his colleagues will peer at these mysterious objects in the redder part of infrared, a feat that requires a view from above Earth’s infrared-absorbing atmosphere. This will be accomplished with the Space Infrared Telescope Facility.

Original Source: NASA/JPL News Release

Image credit: Rensselaer Polytechnic Institute

Astronomers announced today that they have discovered a giant ring of stars circling the Milky Way. They believe this ring could contain as many as 500 million stars, and was formed when our galaxy collided with a smaller, dwarf galaxy several billion years ago. Other galaxies have been seen with a halo of stars, including Andromeda.

A previously unseen band of stars beyond the edge of the Milky Way galaxy has been discovered by a team of scientists from Rensselaer Polytechnic Institute, Fermi National Accelerator Laboratory, and the Sloan Digital Sky Survey (SDSS). The discovery could help to explain how the galaxy was assembled 10 billion years ago.

This ring around the Milky Way galaxy discovered by the Sloan Digital Sky Survey may be what’s left of a collision between our galaxy and a smaller, dwarf galaxy that occurred billions of years ago. It’s an indication that at least part of our galaxy was formed by many smaller or dwarf galaxies mixing together, explained investigators Heidi Jo Newberg of Rensselaer Polytechnic Institute and Brian Yanny of the Fermi National Accelerator Laboratory’s Experimental Astrophysics Group. For illustration purposes, the sun is approximately 30,000 light years from the center of the galaxy. Traveling from Earth at the speed of light, it would take 40,000 light years to reach the newly-discovered ring of stars.

Hidden from view behind stars and gas on the same visual plane as the Milky Way, this ring of stars is approximately 120,000 light years in diameter, says Heidi Newberg, associate professor of physics and astronomy at Rensselaer and a co-lead investigator on the project. Traveling from Earth at the speed of light, it would take 40,000 light years to reach the ring.

“These stars may be what’s left of a collision between our galaxy and a smaller, dwarf galaxy that occurred billions of years ago,” says Newberg. “It’s an indication that at least part of our galaxy was formed by many smaller or dwarf galaxies mixing together.”

The ring of stars is probably the largest of a series of similar structures being found around the galaxy. Investigators believe that as smaller galaxies are pulled apart, the remnants dissolve into streams of stars around larger galaxies. Gravity, primarily from unseen dark matter, holds the ring in a nearly circular orbit around the Milky Way.

“What’s new is the position of the star belt on the outskirts of the Milky Way, an ideal position to study the distribution and amount of dark and light mass within the band,” said Brian Yanny, a scientist at Fermilab’s Experimental Astrophysics Group and a co-lead investigator on the project.

Newberg and Yanny presented their findings today at the American Astronomical Society meeting in Seattle, Washington.

Evidence of this new unexpected band of stars hidden by the Milky Way comes from multi-color photo imagery of hundreds of square degrees of sky and hundreds of spectroscopic exposures from the Sloan Digital Sky Survey, the largest international collaborative astronomical survey ever undertaken.

For four years Newberg, Yanny, and a collaboration of SDSS scientists have been examining the distribution of stars in the Milky Way. At the outer edge of the galaxy in the direction of the constellation Monoceros (the Unicorn) they found tens of thousands of unexpected stars that altered then-standard galactic models.

Three-dimensional mapping from the SDSS revealed the excess stars were actually parts of a separate structure outside the Milky Way.

“The large area covered by the Sloan Survey and the accuracy of the multi-colored observations has allowed us to revisit some classic questions, questions from 50 to 100 years ago,” Yanny said. “What does our Milky Way look like as a whole? How did it form? Did it form in one ‘whoosh,’ or was it built up slowly via mergers of collapsing dwarf galaxies? And how does the mysterious dark (invisible) matter affect the distribution of stars?”

Original Source: SDSS News Release

Image credit: Hubble

A new paper published by Dartmouth university researcher Brian Chaboyer reports that our universe might be dominated by “dark energy”; a mysterious force that seems to be causing objects in the universe to accelerate away from each other. The researchers came to this conclusion by calculating the age of distant globular clusters, and matching it to the expansion age of the universe. The numbers only match if the universe has been accelerating up until now.

A Dartmouth researcher is building a case for a “dark energy”-dominated universe. Dark energy, the mysterious energy with unusual anti-gravitational properties, has been the subject of great debate among cosmologists.

Brian Chaboyer, Assistant Professor of Physics and Astronomy at Dartmouth, with his collaborator Lawrence Krauss, Professor of Physics and Astronomy at Case Western Reserve University, have reported their finding in the January 3, 2003, issue of Science. Combining their calculations of the ages of the oldest stars with measurements of the expansion rate and geometry of the universe lead them to conclude that dark energy dominates the energy density of the universe.

?This finding provides strong support for a universe which is dominated by a kind of energy we?ve never directly observed,? says Chaboyer. ?Observations of distant supernova have suggested for a few years that dark energy dominates the universe, and our finding provides independent evidence that the universe is dominated by this type of energy we do not understand.?

The researchers came to this conclusion as they were refining their calculations for the age of globular clusters, which are groups of about 100,000 or more stars found in the outskirts of the Milky Way, our galaxy. Because this age (about 12 billion years old) is inconsistent with the expansion age for a flat universe (only about 9 billion years old), Krauss and Chaboyer came to the conclusion that the universe is expanding more quickly now than it did in the past.

The only explanation, according to Chaboyer and Krauss, for an accelerating universe is that the energy content of a vacuum is non-zero with a negative pressure, in other words, dark energy. This negative pressure of the vacuum grows in importance as the universe expands and causes the expansion to accelerate.

Original Source: Dartmouth College News Release