Category: Astronomy

An artist’s impression of the dusty disk orbiting IRS 46. Image credit: NASA/JPL-Caltech Click to enlarge
Astronomers at W. M. Keck Observatory have found ??bf? for the first time ??bf? some of the basic compounds necessary to build organic molecules and one of the bases found in DNA within the inner regions of a planet-forming disk. The object, known as “IRS 46,” is located in the Milky Way galaxy, about 375 light years from Earth, in the constellation Ophiuchus. The results will be published in an upcoming issue of the Astrophysical Journal Letters.

“We see prebiotic organic molecules in comets and the gas giant planets in our own solar system and wonder, where did these chemicals come from?” said Dr. Marc Kassis, support astronomer at the W. M. Keck Observatory. “The Spitzer Space Telescope is letting us study these young stellar objects in new and revealing ways, giving us exciting clues about where life may form in the universe.”

The two organic compounds found — acetylene and hydrogen cyanide — are commonly found in our own solar system, such as the atmospheres of the giant gas planets, the icy surfaces of comets, and the atmosphere of Saturn??bf?s largest moon, Titan. Another carbon-containing species detected, carbon dioxide, is widespread in the atmospheres of Venus, the Earth, and Mars.

“If you add hydrogen cyanide, acetylene and water together in a test tube, and give them an appropriate surface on which to be concentrated and react, you’ll get a slew of organic compounds including amino acids and a DNA purine base called adenine,” said Keck Astronomer Dr. Geoffrey Blake, of the California Institute of Technology in Pasadena and co-author of the paper. “Now, we can detect these same molecules in the planet zone of a star hundreds of light-years away.”

The presence of gas-rich disks around young stars is well known, but little is understood about the chemical structure inside. The discovery of acetylene and hydrogen cyanide in one of these disks will help astronomers better understand these disks, where future solar systems may someday form and possibly result in life.

“Spitzer found something very unique — a young protostar with a dusty disk that, when viewed from Earth, appears tilted on the sky, similar to how some galaxies appear,” Kassis explained. “This viewing angle let the team use Keck-NIRSPEC data to study the inner regions of the disk. The results told the team exactly how the disk was moving and suggest there may be a stellar wind coming from the inner region. Keck also helped measure the high temperatures and the particle concentration in the disk.”

The dust and gas surrounding a young star blocks visible light, but lets longer wavelengths, such as infrared light, pass through. Astronomers can find out what this gas and dust is made of by separating the light into its component wavelengths, or colors.

Since 2003, the NASA Spitzer Space Telescope has allowed astronomers to use this technique to study molecular compounds in protoplanetary disks of young stellar objects. The Spitzer “c2d legacy program” has looked at more than 100 sources in five nearby star-forming regions and only one ??bf? IRS 46 ??bf? showed clear evidence of containing the organic compounds in the warm regions close to the star where terrestrial planets are most likely to form.

“This infant system might look a lot like ours did billions of years ago, before life arose on Earth,” said Fred Lahuis of Leiden Observatory in the Netherlands and the SRON Netherlands Institute for Space Research. Lahuis is the lead author of the paper describing the results.

While the precise events leading up to self-replicating nucleic acids remains unclear, the molecules of acetylene (C2H2) and hydrogen cyanide (HCN) have been shown to produce the base compounds necessary to build RNA and DNA. The team found that the abundance of hydrogen cyanide (HCN) was nearly 10,000 times higher than that found in cold interstellar gas from which stars and planets are born.

Models of early solar-system chemistry have historically centered on data from our own primitive solar system, but now discoveries of protoplanetary disks have opened the field to solar systems other than our own. Theoretical models have suggested that large quantities of complex organic molecules would be present in the inner-most regions of these disks, but until now, no observational tests have been possible.

To help determine where, exactly, the organic-rich gas resides in IRS 46, the team also used submillimeter data from the James Clerk Maxwell Telescope on Mauna Kea. The faint signals observed again suggest that the material originates from the inner disk, perhaps no more 10 astronomical units from the parent star, similar in distance to where Saturn orbits the Sun in our own solar system. However, much additional work remains to be done to know this for certain.

“The gases are very warm, close to or somewhat above the boiling point of water on Earth,” said Dr. Adwin Boogert, also of Caltech. “These high temperatures helped to pinpoint the location of the gases in the disk.”

The Keck-NIRSPEC results point to the presence of a stellar wind emerging from the inner region of the disk orbiting IRS 46. The wind may eventually blow away the dusty debris in the disk, perhaps revealing the presence of rocky, Earth-like planets in several million years.

The Jet Propulsion Laboratory manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. JPL is a division of Caltech.

The W. M. Keck Observatory is managed by the California Association for Research in Astronomy, a non-profit 501 (c) (3) corporation. The Keck I and Keck II 10-meter telescopes probe the faintest objects in the optical and infrared Universe.

Original Source: W. M. Keck Observatory

Composite Image of Geminga. Image credit: XMM-Newton Click to enlarge
A team led by Dr. Patrizia Caraveo of the Italian National Institute for Astrophysics (INAF) in Milan discovered this cometary trail with data from NASA’s Chandra X-ray Observatory Archive. The discovery follows the team’s discovery in 2003 using ESA’s XMM-Newton of Geminga’s twin X-ray tails stretching for billions of chilometers.

Together, these observations provide unique insight into the contents and density of the interstellar “ocean” Geminga is plowing through, as well as the physics of Geminga itself. Not only is Geminga close, only about 500 light years from Earth, it is cutting across our line of sight, offering a spectacular view of a pulsar in motion.

“Geminga is the only isolated pulsar we know of showing both a small comet-like trail and a larger tail structure,” said Dr. Andrea De Luca of INAF’s Istituto di Astrofisica Spaziale e Fisica Cosmica, lead author on an article about this discovery in Astronomy and Astrophysics. “This jettison from Geminga’s journey through interstellar space provides unprecedented information about the physics of pulsars.”

A pulsar is a type of rapidly spinning neutron star that emits steady pulses of radiation with each rotation, funnelled along strong magnetic field lines, much like a lighthouse beam sweeping across space. A neutron star is the core remains of an exploded star once at least eight times as massive as the sun.

These dense stars, only about 20 kilometers across, still contain roughly the mass of the sun. Neutron stars contain the densest material known. Like many neutron stars, Geminga got a “kick” from the explosion that created it and has been flying through space like a cannonball ever since.

De Luca said that Geminga’s complex phenomenology of tails and a trail must be from high-energy electrons escaping the pulsar magnetosphere following paths clearly driven by the pulsar??bf?s motion in the interstellar medium.

Most pulsars emit radio waves. Yet Geminga is “radio quiet” and was discovered 30 years ago as a unique “gamma-ray only” source (only later was Geminga seen in the X-ray and optical light wavebands). Geminga generates gamma rays by accelerating electrons and positrons, a type of antimatter, to high speeds as it spins like a dynamo four times per second.

“Astronomers have known that only a fraction of these accelerated particles produce gamma rays, and they have wondered what happens to the remaining ones,” said Caraveo, a co-author on the Astronomy & Astrophysics article. “Thanks to the combined capabilities of Chandra and XMM-Newton, we now know that such particles can escape. Once they reach the shock front, created by the supersonic motion of the star, the particles lose their energy radiating X-rays.”

Meanwhile, an equal number of particles (with a different electric charge) should move in the opposite direction, aiming back at the star. Indeed, when they hit the star’s crust they create tiny hotspots, which have been detected through their varying X-ray emission.

The next generation of high-energy gamma-ray instruments – namely, the planned Italian Space Agency’s AGILE mission and NASA’s GLAST mission – will explore the connection between the X-ray and gamma ray behaviour of pulsars to provide clues to the nature of unknown gamma-ray sources, according to Prof. Giovanni Bignami, a co-author and director of the Centre d’Etude Spatiale des Rayonnements (CESR) in Toulouse, France. Of the 271 higher-energy gamma-ray objects detected by a NASA telescope called EGRET, 170 remained unidentified in other wavebands. These unidentified objects could be “gamma-ray pulsars” like Geminga, whose optical and X-ray light might be visible only because of its nearness to Earth.

Only about a dozen other radio-quiet isolated neutron stars are known, and Geminga is the only one with tails and trails and copious gamma-ray emission. Bignami named Geminga for “Gemini gamma-ray source” in 1973. In his local Milan dialect, the name is a pun on “ghe minga,” which means “it is not there.” Indeed, Geminga was unidentified in other wavelengths until 1993, twenty years after its discovery.

The discovery team also includes Drs. Fabio Mattana and Alberto Pellizzoni of the INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica.

Original Source: INAF News Release

NGC 2467 and Surroundings. Image credit: ESO Click to enlarge
Just like Charles Dickens’ Christmas Carol takes us on a journey into past, present and future in the time of only one Christmas Eve, two of ESO’s telescopes captured various stages in the life of a star in a single image.

ESO’s first image shows the area surrounding the stellar cluster NGC 2467, located in the southern constellation of Puppis (“The Stern”). With an age of a few million years at most, it is a very active stellar nursery, where new stars are born continuously from large clouds of dust and gas.

The image, looking like a colourful cosmic ghost or a gigantic celestial Mandrill, contains the open clusters Haffner 18 (centre) and Haffner 19 (middle right: it is located inside the smaller pink region – the lower eye of the Mandrill), as well as vast areas of ionised gas.

The bright star at the centre of the largest pink region on the bottom of the image is HD 64315, a massive young star that is helping shaping the structure of the whole nebular region.

The first image was taken with the Wide-Field Imager camera at the 2.2m MPG/ESO telescope located at La Silla, in Chile.

Another image of the central part of this area is shown in ESO’s second image. It was obtained with the FORS2 instrument at ESO’s Very Large Telescope on Cerro Paranal, also in Chile.

NGC 2467 and Surroundings. Image credit: ESO Click to enlarge
However, the second image zooms in on the open stellar cluster Haffner 18, perfectly illustrating three different stages of this process of star formation: In the centre of the picture, Haffner 18, a group of mature stars that have already dispersed their birth nebulae, represents the completed product or immediate past of the star formation process. Located at the bottom left of this cluster, a very young star, just come into existence and, still surrounded by its birth cocoon of gas, provides insight into the very present of star birth. Finally, the dust clouds towards the right corner of the image are active stellar nurseries that will produce more new stars in the future.

Haffner 18 contains about 50 stars, among which several short lived, massive ones. The massive star still surrounded by a small, dense shell of hydrogen, has the rather cryptic name of FM3060a. The shell is about 2.5 light-years wide and expands at a speed of 20 km/s. It must have been created some 40,000 years ago. The cluster is between 25,000 and 30,000 light-years away from us.

Original Source: ESO News Release

NGC 2264, the Cone Nebula and Christmas Tree Cluster. Image credit: NASA/JPL-Caltech. Click to enlarge
Astronomers using NASA’s Spitzer Space Telescope have given the world a spectacular new picture of a star-forming region called the “Christmas Tree Cluster,” complete with first-ever views of a group of newborn stars still linked to their siblings.

Spitzer’s cameras are very sensitive to the infrared (heat), allowing astronomers to see through the obscuring gas and dust of the star-forming cloud that swaddles infant stars.

The Christmas Tree Cluster, also known as NGC 2264, is a well-studied region in the Monoceros (the Unicorn) constellation. The Christmas Tree Cluster was so named because it looks like a tree in visible light. The nebula is roughly 2,500 light-years away. That is, the nebula emitted the light in the new Spitzer image 2,500 years ago.

For astronomers studying the development of very young stars — stars less than a few million years old — “This region has it all,” said University of Arizona astronomer Erick T. Young.

“We see the dramatic-looking emission of cold gas — clouds that look like thunderheads. We see when the massive molecular cloud breaks up and begins to condense into clumps of stars,” Young said. “And, for the first time, because of Spitzer’s sensitivity, we can see individual stars roughly the size of our sun tightly packed within those clumps.” The cluster of stars is so tightly packed that they must be less than 100,000 years old, he added.

Astronomers are calling this compact collection of bright protostars within the Christmas Tree Cluster the “Snowflake Cluster” because of how they are spaced. The newborn stars are patterned like a single feathery crystal of snow, or geometrically spaced like spokes in a wheel.

The Spitzer observations show that just as theory predicts, the density and temperature of the initial star-forming cloud dictates the spacing between the protostars.

Young is deputy principal investigator for Spitzer’s Multiband Imaging Photometer (MIPS), a UA-built camera that took the longest wavelengths of infrared light used in Christmas Tree Cluster mosaic. Astronomers combined light from MIPS and Spitzer’s Infrared Array Camera (IRAC), developed by the Smithsonian Astrophysical Observatory, in constructing in the picture.

The infant stars appear as pink and red specks in the snowflake cluster that adorns the larger Christmas Tree Cluster in the IRAC and MIPS image. The larger, yellowish spheres are massive stars within the NGC 2264 region. The organic molecules mixed in with dust that surrounds the cluster are illuminated as wisps of green. The blue dots smeared across the image are older Milky Way stars at various distances along the telescope’s line of sight.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA’s Science Mission Directorate. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech.

Original Source: UA News Release

An accurate illustration of young galaxies twe<lve billion light years awayClick to enlarge/a>
Astronomers have found clear indications that clumps of dark matter are the nursing grounds for new born galaxies about twelve billion light years away. A single nest of dark matter can nurture several young galaxies. These results from researchers at the Space Telescope Science Institute, the National Astronomical Observatory of Japan, and the University of Tokyo confirm predictions of the currently dominant theory of cosmology known as the Cold Dark Matter model.

Recent studies suggest that dark matter out weighs ordinary matter by a factor of seven. Although dark matter cannot be seen directly through a telescope, it reveals itself to astronomers by its strong gravitational pull on nearby stars and gas, and even galaxies.

Galaxies are often clustered together and how they cluster is determined mostly by gravity.

By studying how galaxies cluster, it is possible to determine how dark matter is distributed and how it affects the birth and growth of galaxies. In the past, it was extremely difficult to study the clustering of young galaxies. Young galaxies appear faint due to their great distances, and finding enough of them to study how they cluster was an observational challenge.

Masami Ouchi from the Space Telescope Science Institute and colleagues used the Subaru telescope and its Suprime-Cam camera to study a piece of the sky in the constellation Cetus (the Whale) called the Subaru/XMM-Newton Deep Survey Field (SXDS). This piece of sky covers an area five times the size of the full moon. By taking deep and sensitive images of the field in three colors of visible light, the SXDS team was able to find about seventeen thousand (17,000) young galaxies twelve billion light years away. This number is ten times larger than previous studies of such young galaxies.
Based on these data, the team found that:

1) There are many pairs of galaxies with separations less than eight hundred thousand (800,000) light years.
2) Even at large distances, galaxies are strongly clustered.

Both of these results are expected if the galaxies are nestled within clumps of dark matter. The SXDS team compared the observational results in detail to theoretical predictions based on a Cold Dark Matter model by team member Takashi Hamana and found that the average clump of dark matter nests weighs as much as six hundred billion (600,000,000,000) Suns, and that a single clump of dark matter harbors multiple young galaxies.

Independently, Nobunari Kashikawa from the National Astronomical Observatory of Japan and colleagues also used Subaru’s Suprime-Cam camera to study an area of sky in the constellation Coma Berenices (Berenice’s Hair) called the Subaru Deep Field (SDF). This field is only the size of one full moon but the data available are twice as sensitive as the SXDS field data. The SDF team found about five thousand (5,000) young galaxies at a distance of twelve billion light years, and eight hundred (800) even younger galaxies at a distance of twelve billion five hundred million light years. The SDF team was also able to double check the identities of the young galaxies by taking spectral data of the galaxies with the Subaru and Keck telescopes. The SDF team independently obtained the results 1)+2) described above, and concluded that some single clumps of dark matter harbours multiple young galaxies. In the SDF images, it is possible to see several new born galaxies huddled together in a small area. By comparing the SDF data in detail to high precision computer simulations of the growth of clumps in Cold Dark Matter by team member Masahiro Nagashima of Kyoto University, the SDF team concludes that heavier clumps of dark matter have more bright galaxies, and that this preference produces the correlations found in real observation.

The two teams together have found the first concrete evidence that young galaxies in the early universe are nestled within clumps of dark matter, and that a single clump of dark matter nurses several young galaxies. Both teams took advantage of the Subaru telescope’s unique ability to take deep sensitive images over a large area of sky.

Original Source: NAOJ News Release

An artist’s concept of a hypothetical supernova in our galaxy. Image credit: David A. Aguilar (CfA). Click to enlarge
A team of astronomers has found faint visible echoes of three ancient supernovae by detecting their centuries-old light as it is reflected by clouds of interstellar gas hundreds of light-years removed from the original explosions.

Located in a nearby galaxy in the southern skies of Earth, the three exploding stars flashed into short-lived brilliance at least two centuries ago, and probably longer. The oldest one is likely to have occurred more than six hundred years ago.

The light echoes were discovered by comparing images of the Large Magellanic Cloud (LMC) taken years apart. By precisely subtracting the common elements in each image of the galaxy and looking by eye to see what variable objects remain, the team looked for evidence of invisible dark matter that might distort the light of stars in a transitory way, as part of a sky survey called SuperMACHO.

This careful image analysis also revealed a small number of concentric, circular-shaped arcs that are best explained as light moving outward over time, and being scattered as it encounters dense pockets of cool interstellar dust. Team members then fit perpendicular vectors to the curves of each arc system, which were found to point backwards toward the sites of three supernovae remnants, which were previously known and thought to be relatively young.

“Without the geometry of the light echo, we had no precise way of knowing just how old these supernovae were,” said astronomer Armin Rest of the National Optical Astronomy Observatory (NOAO), lead author of a paper on the discovery in the December 22, 2005, issue of Nature. “Some relatively simple mathematics can help us answer one of the most vexing questions that astronomers can ask-exactly how old is this object that we are looking at?”

Just as a sound echo can occur when sound waves bounce off a distant surface and reflect back toward the listener, a light echo can be seen when light waves traveling through space are reflected back toward the viewer-in this case, the Mosaic digital camera on the National Science Foundation’s Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile.

This technique can be extended to famous supernovae in history. “Imagine seeing light from the same explosion first seen by Johannes Kepler some 400 years ago, or the one recorded by Chinese observers in 1006,” said Christopher Stubbs of the Harvard-Smithsonian Center for Astrophysics (CfA), co-author of the paper and principal investigator for the SuperMACHO program. “These light echoes give us that possibility.”

In principle, astronomers can split the light echo into a spectrum to investigate what type of supernova occurred. “We have the potential with these echoes to determine the star’s cause of death, just like the archaeologists who took a CT scan of King Tut’s mummy to find out how he died,” said co-author Arti Garg of CfA.

Astronomers can also use supernova light echoes to measure the structure and nature of the interstellar medium. Dust and gas between the stars are invisible unless illuminated by some light source, just as fog at night is not noticeable until lit by a car’s headlights. A supernova blast can provide that illumination, lighting up surrounding clouds of matter with its strobe-like flash.

“We see the reflection as an arc because we are inside an imaginary ellipse, with the Earth at one focus of the ellipse and the ancient supernovae at the other,” explained Nicholas Suntzeff of NOAO. “As we look out toward the supernovae, we see the reflection of the light echo only when it intersects the outer surface of the ellipse. The shape of the reflection from our vantage point appears to be a portion of a circle.”

An unusual aspect of the arcs is that they generally appear to move much faster than the speed of light. This does not violate the cosmic speed limit, which states that any object cannot move faster than the speed of light. “What our telescopes see is the reflection moving, and not any physical object,” Suntzeff added. “It is also very exciting that our observations confirm the visionary prediction of Fritz Zwicky in 1940 that light from ancient supernovae could be seen in echoes of the explosion.”

Two additional high-resolution color graphics to illustrate this result are available at http://www.noao.edu/outreach/press/pr05/pr0512.html.

Other co-authors of the Nature paper are Knut Olsen and Chris Smith (CTIO); Jose Luis Prieto (Ohio State University); Douglas Welch (McMaster University, Ontario); Andrew Becker and Gajus Miknaitis (University of Washington); Marcel Bergmann (Gemini Observatory); Alejandro Clocchiatti and Dante Minniti (Pontifica Universidad Catolica de Chile); and, Kem Cook, Mark Huber and Sergei Nikolaev (Lawrence Livermore).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: CfA News Release

The center of our Milky Way galaxy captured by Keck Laser Guide Star. Image credit: W.M. Keck Observatory/UCLA Click to enlarge
UCLA astronomers and colleagues have taken the first clear picture of the center of our Milky Way galaxy, including the area surrounding the supermassive black hole, using a new laser virtual star at the W.M. Keck observatory in Hawaii.

“Everything is much clearer now,” said Andrea Ghez, UCLA professor of physics and astronomy, who headed the research team. “We used a laser to improve the telescope’s vision ? a spectacular breakthrough that will help us understand the black hole’s environment and physics. It’s like getting Lasik surgery for the eyes, and will revolutionize what we can do in astronomy.”

Astronomers are used to working with images that are blurred by the Earth’s atmosphere. However, a laser virtual star, launched from the Keck telescope, can be used to correct the atmosphere’s distortions and clear up the picture. This new technology, called Laser Guide Star adaptive optics, will lead to important advances for the study of planets in our solar system and outside of our solar system, as well as galaxies, black holes, and how the universe formed and evolved, Ghez said.

“We have worked for years on techniques for ‘beating the distortions in the atmosphere’ and producing high-resolution images,” she said. “We are pleased to report the first Laser Guide Star adaptive optics observations of the center of our galaxy.”

Ghez and her colleagues took “snapshots” of the center of the galaxy, targeting the supermassive black hole 26,000 light years away, at different wavelengths. This approach allowed them to study the infrared light emanating from very hot material just outside the black hole’s “event horizon,” about to be pulled through.

“We are learning the conditions of the infalling material and whether this plays a role in the growth of the supermassive black hole,” Ghez said. “The infrared light varies dramatically from week to week, day to day and even within a single hour.”

The research, federally funded by the National Science Foundation, will be published Dec. 20 in the Astrophysical Journal Letters.

The research was conducted using the 10-meter Keck II Telescope, which is the world’s first 10-meter telescope with a laser on it. Laser Guide Star allows astronomers to “generate an artificial bright star” exactly where they want it, which reveals the atmosphere’s distortions.

Since 1995, Ghez has been using the W.M. Keck Observatory to study the galactic center and the movement of 200 nearby stars.

Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light. Black holes cannot be seen directly, but their influence on nearby stars is visible, and provides a signature, Ghez said. The supermassive black hole, with a mass more than 3 million times that of our sun, is in the constellation of Sagittarius. The galactic center is located due south in the summer sky.

The black hole came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object, Ghez said.

Co-authors on the research include UCLA graduate students Seth Hornstein and Jessica Lu; the adaptive optics team at W. M. Keck Observatory: David Le Mignant, Marcos Van Dam and Peter Wizinowich; Antonin Bouchez (formerly with the W. M. Keck Observatory) and Keith Matthews at Caltech; Mark Morris, a UCLA professor of physics and astronomy; and Eric Becklin, a UCLA professor of physics and astronomy.

Ghez provides more information, and images of the galactic center, at http://www.astro.ucla.edu/research/galcenter/.

Original Source: UCLA News Release

Alpha Centauri and the Southern Cross. Image credit: ESO Click to enlarge
Astronomers have used ESO’s Very Large Telescope in Chile and the Anglo-Australian Telescope in eastern Australia as a ‘stellar stethoscope’ to listen to the internal rumblings of a nearby star. The data collected with the VLT have a precision better than 1.5 cm/s, or less than 0.06 km per hour!

By observing the star with two telescopes at the same time, the astronomers have made the most precise and detailed measurements to date of pulsations in a star similar to our Sun. They measured the rate at which the star’s surface is pulsing in and out, giving clues to the density, temperature, chemical composition and rotation of its inner layers – information that could not be obtained in any other way.

The astronomers from Denmark, Australia, and the USA used Kueyen, one of the four 8.2-m Unit Telescopes of ESO’s Very Large Telescope (VLT) at Cerro Paranal in Chile, and the 3.9-m Anglo-Australian Telescope (AAT) in New South Wales (Australia), to study the star Alpha Centauri B, one of our closest neighbours in space, about 4.3 light-years away.

Alpha Centauri is the brighter of the two ‘Pointers’ to the Southern Cross. Alpha Centauri itself is a triple system and Alpha Centauri B is an orange star, a little cooler and a little less massive than the Sun.

Churning gas in the star’s outer layers creates low-frequency sound waves that bounce around the inside of the star, causing it to ring like a bell. This makes the star’s surface pulsate in and out by very tiny amounts – only a dozen metres or so every four minutes. Astronomers can detect these changes by measuring the small, associated wavelength shifts.

The researchers sampled the light from Alpha Centauri B for seven nights in a row, making more than 5 000 observations in all. At the VLT, 3379 spectra were obtained with typical exposure times of 4 seconds and a median cadence of one exposure every 32 seconds! At the AAT 1642 spectra were collected, with typical exposures of 10 s, taken every 90 s.

“From this unique dataset, we were able to determine as many as 37 different patterns (or modes) of oscillation”, says Hans Kjeldsen, from University of Aarhus (Denmark) and lead author of the paper describing the results.

The astronomers also measured the mode lifetimes (how long the oscillations last), the frequencies of the modes, and their amplitudes (how far the surface of the star moves in and out). Such measurements are a huge technical challenge. Indeed, the star’ surface moves slowly, at the tortoise-like speed of 9 cm a second, or about 300 metre an hour. The astronomers borrowed their high-precision measurement technique from the planet-hunters, who also look for slight Doppler shifts in starlight.

“So much of what we think we know about the universe rests on the ages and properties of stars,” said Tim Bedding, from the University of Sydney and co-author of the study. “But there is still a great deal we don’t know about them.”

By using two telescopes at different sites the astronomers were able to observe the Alpha Centauri B as continuously as possible.

“That’s a huge advantage, because gaps in the data introduce ambiguity,” said Bedding. “The success of the observations also depended on the very stable spectrographs attached to the two telescopes – UVES at the VLT and UCLES at the AAT – which analysed the star’s light.”

Original Source: ESO News Release

The locations of our solar system and of W3OH in our galaxy. Image credit: Max Planck Society Click to enlarge
The Perseus spiral arm, the nearest spiral arm in the Milky Way outside the Sun’s orbit, lies only half as far from Earth as some previous results had suggested. An international team of astronomers including scientists from the Max-Planck-Institut f??bf?r Radioastronomie (MPIfR) has recently achieved the most accurate distance measurement ever to the Perseus arm. This was done by use of a vast array of radio telescopes in the USA called the Very Long Baseline Array, observing very bright spots within clouds of gas that contain methyl alcohol in the placental material surrounding a newly formed star called W3OH.

Dr. Xu Ye, an astronomer at Shanghai Observatory now working at the Max-Planck-Institut f??bf?r Radioastronomie and one of the members of the international team who made the measurements, stated that “we measured distance by the simplest and most direct method in astronomy – essentially the technique used by surveyors called triangulation.” Specifically, the team used the changing vantage point of the Earth as it orbits the Sun to form one leg of a triangle. Measuring the change in apparent position of a source, they could calculate the source’s distance by simple trigonometry (resulting in 6357??bf?130 light years).

This result resolves the longstanding problem of the distance to this spiral arm. In thje past, different methods of measuring distance have disagreed by more than a factor of 2. Prof. Karl Menten, another member of the team, states that “this confirms distances based on the apparent luminosity of young stars but disagrees with distances based on a model of the rotation of the Milky Way. The reason for the discrepancy is that young stars in the Perseus spiral arm have unexpectedly large motions.”

The astronomers found that the young star is not moving in a circular orbit around the Milky Way, but deviates by 10% from circular. It is rotating more slowly and “falling” toward the center of the Milky Way. Team member Zheng Xing-Wu of Nanjing University points out that “the simplest explanation is that the cloud of gas out of which the star formed was gravitationally attracted by excess mass of material in the Perseus spiral arm.”

“Studies such as ours are the first steps to accurately map the Milky Way,” says Dr. Mark Reid, a member of the team from the Harvard-Smithsonian Center for Astrophysics. “We have established that the radio telescope we used, the Very Long Baseline Array, can measure distances with unprecedented accuracy–nearly a factor of 100 times better than previously accomplished.” To get a feeling for this measurement one may visualize a person standing on the moon, holding a torch in his stretched-out hand. Let her turn around herself like an ice scater, but only making a single turn in the course of one year. The VLBA measurement is equivalent to measuring the torch’s motion with an accuracy comparable to the torch’s size.

The technique used is Very Long Baseline Interferometry (VLBI), where observations made with many telescopes are combined to achieve the resolution of an extraordinarily large telescope nearly the size of the Earth. The VLBA telescopes stretch from Hawaii over the continental United States to the Virgin Island of St. Croix, producing the resolution of an 8000 km diameter telescope. While the VLBA has extremely high resolution, it requires extremely bright and very compact radio sources such as masers for such measurements (a maser is the microwave equivalent of a laser.) Along with water, methanol is the most widespread maser molecule found in star-forming regions. The methanol spectral line used for the present experiment was discovered in the course of Prof. Menten’s dissertation in the 1980s. In 1988, while working with Dr. Reid, they conducted the first VLBI observations of methanol masers; the target then was also W3OH. “Already then we dreamt of observations such as this one” says Menten.

In fact similar VLBA observations have also been made on water masers in W3OH. This effort, led by the MPIfR’s Kazuya Hachisuka, yielded a distance similar to the methanol masers. “A splendid confirmation!” says Hachisuka. His team also includes Reid and Menten and a number of Japanese scientists.

The methanol observations are only the start of a very large-scale project that Reid and Menten have initiated. It will determine distances and motions of methanol masers all over the Milky Way. It has been granted a large block of VLBA observing time. In addition to the motions on the sky these observations also yield the star’s velocity toward or away from the observer by measuring the Doppler shift of the methanol lines. The resulting three dimensional motions will deliver unique constraints not only on the rotation of the Milky Way but also on the distribution of the unseen Dark Matter that is postulated to surround it.

While the method – simple trigonometry – sounds basic, the transformation into practical results requires a comprehensive understanding of VLBA and all aspects of the observations, including thorough modeling of the Earths’ atmosphere which affects the incoming radio waves. Dr. Reid has dedicated many years of his life to reach the point were programs such as this one can be performed.

Over the years this truly international effort was supported by a Research Prize granted to Dr. Reid by the Alexander von Humboldt Foundation. The cooperation with Shanghai Observatory is supported by a joint program of the Max Planck Society, the Chinese Academy of Sciences, and the Smithsonian Institution’s Visitor Program.

Original Source: Max Planck Society