Category: Astronomy

Dr. Fred Lawrence Whipple, the oldest living American astronomer and one of the last giants of 20th century astronomy, passed away yesterday at the age of 97 following a prolonged illness. He was Phillips Professor of Astronomy Emeritus at Harvard University and a Senior Physicist at SAO.

“Fred Whipple was one of those rare individuals who affected our lives in many ways. He predicted the coming age of satellites, he revolutionized the study of comets and as Director of the Smithsonian Astrophysical Observatory, he helped form the Harvard-Smithsonian Center for Astrophysics,” says Charles Alcock, current Director of the Harvard-Smithsonian Center for Astrophysics (CfA).

A discoverer of six comets, Whipple may be best known for his comet research. Five decades ago, he first suggested that comets were “icy conglomerates,” what the press called “dirty snowballs.” His dirty snowball theory caught the imagination of the public even as it revolutionized comet science.

Whipple’s change of concept from the generally accepted “flying sandbank” model was “one of the most important contributions to solar system studies in the 20th century,” says Dr. Brian Marsden, director of the Minor Planet Center located at SAO. “I think many people would agree that that was a really shining moment in his scientific career.” A 2003 survey by The Astrophysical Journal showed that Whipple’s 1950 and 1951 scientific papers on the “icy conglomerate” model were the most cited papers in past 50 years.

Whipple’s comet work continued for a lifetime. In 1999, he was named to work on NASA’s Contour mission, becoming the oldest researcher ever to accept such a post.

Never one to limit his work to one area of research, Whipple also contributed to more earthly challenges. During World War II, Whipple co-invented a cutting device that converted lumps of tinfoil into thousands of fragments known as chaff. Allied aircraft would release chaff to confuse enemy radar. Whipple was particularly proud of this invention, for which President Truman awarded him a Certificate of Merit in 1948.

Whipple also strongly influenced the early era of spaceflight. Mindful of the damage to spacecraft from meteors, in 1946 he invented the Meteor Bumper, a thin outer skin of metal. Also known as the Whipple Shield, this mechanism explodes a meteor on contact, preventing the spacecraft from receiving catastrophic damage. Improved versions of it are still in use today.

Whipple and a handful of other scientists had the foresight to envision the era of artificial satellites. Only Whipple had both the imagination and the managerial skill to organize a worldwide network of amateur astronomers to track these then hypothetical objects and to determine their orbits. When Sputnik I was successfully launched on 4 October 1957, Whipple’s group was the only one prepared. Cambridge fast became a nerve center of the earliest part of the space age. Whipple and some of his staff were even featured on the cover of Life magazine for their satellite tracking prowess.

Later, also under his leadership, SAO developed an optical tracking system for satellites using a network of Baker-Nunn cameras. That network achieved spectacular success. “It tracked satellites so well that astronomers were able to determine the exact shape of the Earth from its gravitational effects on satellite orbits,” says Dr. Myron Lecar of SAO.

For his work on the network, Whipple received from President John F. Kennedy in 1963 the Distinguished Federal Civilian Service award. “I think that was my most exciting moment, when I was able to invite my parents and my family to the Rose Garden for the award ceremony,” Whipple said in a 2001 interview.

Born in Red Oak, Iowa, on November 5, 1906, Whipple studied at Occidental College and earned his undergraduate degree in mathematics at the University of California at Los Angeles, prior to moving to Berkeley to obtain his Ph.D. degree in 1931. He then moved to Harvard College Observatory in Cambridge, Massachusetts.

Whipple directed the Smithsonian Astrophysical Observatory (SAO) from 1955 to 1973, before it joined with the Harvard College Observatory to form the Harvard-Smithsonian Center for Astrophysics (CfA).

“Fred Whipple was a truly extraordinary person among extraordinary people. He was gifted with great scientific imagination, superb analytical skills, and excellent management acumen,” says Dr. Irwin Shapiro, who served as CfA director from 1983 to 2004.

In the late 1960s, Whipple selected Mount Hopkins in southern Arizona as the site for a new SAO astronomical facility. Whipple was part of the group that initiated a novel and low-cost approach to building large telescopes first realized in the construction of the Multiple Mirror Telescope, a joint project of SAO and the University of Arizona. Mt. Hopkins Observatory was renamed Fred Lawrence Whipple Observatory in 1981.

Dr. George Field, the first CfA director, says of Whipple, “He will be remembered by a generation of scientists for his leadership and for his keen insight. He was admired by his friends and colleagues for his integrity, and for doggedly pursuing his research into his nineties.”

In 1946 Whipple married Babette F. Samelson, by whom he had two daughters, Sandra and Laura. He also had a son, Earle Raymond, by his first marriage.

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: Harvard-Smithsonian Center for Astrophysics News Release

Observations by an international team of astronomers with the UVES spectrometer on ESO’s Very Large Telescope at the Paranal Observatory (Chile) have thrown new light on the earliest epoch of the Milky Way galaxy.

The first-ever measurement of the Beryllium content in two stars in a globular cluster (NGC 6397) – pushing current astronomical technology towards the limit – has made it possible to study the early phase between the formation of the first generation of stars in the Milky Way and that of this stellar cluster. This time interval was found to amount to 200 – 300 million years.

The age of the stars in NGC 6397, as determined by means of stellar evolution models, is 13,400 ? 800 million years. Adding the two time intervals gives the age of the Milky Way, 13,600 ? 800 million years.

The currently best estimate of the age of Universe, as deduced, e.g., from measurements of the Cosmic Microwave Background, is 13,700 million years. The new observations thus indicate that the first generation of stars in the Milky Way galaxy formed soon after the end of the ~200 million-year long “Dark Ages” that succeeded the Big Bang.

The age of the Milky Way
How old is the Milky Way ? When did the first stars in our galaxy ignite ?

A proper understanding of the formation and evolution of the Milky Way system is crucial for our knowledge of the Universe. Nevertheless, the related observations are among the most difficult ones, even with the most powerful telescopes available, as they involve a detailed study of old, remote and mostly faint celestial objects.

Globular clusters and the ages of stars

Modern astro physics is capable of measuring the ages of certain stars, that is the time elapsed since they were formed by condensation in huge interstellar clouds of gas and dust. Some stars are very “young” in astronomical terms, just a few million years old like those in the nearby Orion Nebula. The Sun and its planetary system was formed about 4,560 million years ago, but many other stars formed much earlier. Some of the oldest stars in the Milky Way are found in large stellar clusters, in particular in “globular clusters” (PR Photo 23a/04), so called because of their spheroidal shape.

Stars belonging to a globular cluster were born together, from the same cloud and at the same time. Since stars of different masses evolve at different rates, it is possible to measure the age of globular clusters with a reasonably good accuracy. The oldest ones are found to be more than 13,000 million years old.

Still, those cluster stars were not the first stars to be formed in the Milky Way. We know this, because they contain small amounts of certain chemical elements which must have been synthesized in an earlier generation of massive stars that exploded as supernovae after a short and energetic life. The processed material was deposited in the clouds from which the next generations of stars were made, cf. ESO PR 03/01.

Despite intensive searches, it has until now not been possible to find less massive stars of this first generation that might still be shining today. Hence, we do not know when these first stars were formed. For the time being, we can only say that the Milky Way must be older than the oldest globular cluster stars.

But how much older?

Beryllium to the rescue
What astrophysicists would like to have is therefore a method to measure the time interval between the formation of the first stars in the Milky Way (of which many quickly became supernovae) and the moment when the stars in a globular cluster of known age were formed. The sum of this time interval and the age of those stars would then be the age of the Milky Way.

New observations with the VLT at ESO’s Paranal Observatory have now produced a break-through in this direction. The magic element is “Beryllium”!

Beryllium is one of the lightest elements [2] – the nucleus of the most common and stable isotope (Beryllium-9) consists of four protons and five neutrons. Only hydrogen, helium and lithium are lighter. But while those three were produced during the Big Bang, and while most of the heavier elements were produced later in the interior of stars, Beryllium-9 can only be produced by “cosmic spallation”. That is, by fragmentation of fast-moving heavier nuclei – originating in the mentioned supernovae explosions and referred to as energetic “galactic cosmic rays” – when they collide with light nuclei (mostly protons and alpha particles, i.e. hydrogen and helium nuclei) in the interstellar medium.

Galactic cosmic rays and the Beryllium clock
The galactic cosmic rays travelled all over the early Milky Way, guided by the cosmic magnetic field. The resulting production of Beryllium was quite uniform within the galaxy. The amount of Beryllium increased with time and this is why it might act as a “cosmic clock”.

The longer the time that passed between the formation of the first stars (or, more correctly, their quick demise in supernovae explosions) and the formation of the globular cluster stars, the higher was the Beryllium content in the interstellar medium from which they were formed. Thus, assuming that this Beryllium is preserved in the stellar atmosphere, the more Beryllium is found in such a star, the longer is the time interval between the formation of the first stars and of this star.

The Beryllium may therefore provide us with unique and crucial information about the duration of the early stages of the Milky Way.

A very difficult observation
So far, so good. The theoretical foundations for this dating method were developed during the past three decades and all what is needed is then to measure the Beryllium content in some globular cluster stars.

But this is not as simple as it sounds! The main problem is that Beryllium is destroyed at temperatures above a few million degrees. When a star evolves towards the luminous giant phase, violent motion (convection) sets in, the gas in the upper stellar atmosphere gets into contact with the hot interior gas in which all Beryllium has been destroyed and the initial Beryllium content in the stellar atmosphere is thus significantly diluted. To use the Beryllium clock, it is therefore necessary to measure the content of this element in less massive, less evolved stars in the globular cluster. And these so-called “turn-off (TO) stars” are intrinsically faint.

In fact, the technical problem to overcome is three-fold: First, all globular clusters are quite far away and as the stars to be measured are intrinsically faint, they appear quite faint in the sky. Even in NGC6397, the second closest globular cluster, the TO stars have a visual magnitude of ~16, or 10000 times fainter than the faintest star visible to the unaided eye. Secondly, there are only two Beryllium signatures (spectral lines) visible in the stellar spectrum and as these old stars do contain comparatively little Beryllium, those lines are very weak, especially when compared to neighbouring spectral lines from other elements. And third, the two Beryllium lines are situated in a little explored spectral region at wavelength 313 nm, i.e., in the ultraviolet part of the spectrum that is strongly affected by absorption in the terrestrial atmosphere near the cut-off at 300 nm, below which observations from the ground are no longer possible.

It is thus no wonder that such observations had never been made before, the technical difficulties were simply unsurmountable.

VLT and UVES do the job
Using the high-performance UVES spectrometer on the 8.2-m Kuyen telescope of ESO’s Very Large Telescope at the Paranal Observatory (Chile) which is particularly sensitive to ultraviolet light, a team of ESO and Italian astronomers [1] succeeded in obtaining the first reliable measurements of the Beryllium content in two TO-stars (denoted “A0228” and “A2111”) in the globular cluster NGC 6397 (PR Photo 23b/04). Located at a distance of about 7,200 light-years in the direction of a rich stellar field in the southern constellation Ara, it is one of the two nearest stellar clusters of this type; the other is Messier 4.

The observations were done during several nights in the course of 2003. Totalling more than 10 hours of exposure on each of the 16th-magnitude stars, they pushed the VLT and UVES towards the technical limit. Reflecting on the technological progress, the leader of the team, ESO-astronomer Luca Pasquini, is elated: “Just a few years ago, any observation like this would have been impossible and just remained an astronomer’s dream!”

The resulting spectra (PR Photo 23c/04) of the faint stars show the weak signatures of Beryllium ions (Be II). Comparing the observed spectrum with a series of synthetic spectra with different Beryllium content (in astrophysics: “abundance”) allowed the astronomers to find the best fit and thus to measure the very small amount of Beryllium in these stars: for each Beryllium atom there are about 2,224,000,000,000 hydrogen atoms.

Beryllium lines are also seen in another star of the same type as these stars, HD 218052, cf. PR Photo 23c/04. However, it is not a member of a cluster and its age is by far not as well known as that of the cluster stars. Its Beryllium content is quite similar to that of the cluster stars, indicating that this field star was born at about the same time as the cluster.

From the Big Bang until now
According to the best current spallation theories, the measured amount of Beryllium must have accumulated in the course of 200 – 300 million years. Italian astronomer Daniele Galli, another member of the team, does the calculation: “So now we know that the age of the Milky Way is this much more than the age of that globular cluster – our galaxy must therefore be 13,600 ? 800 million years old. This is the first time we have obtained an independent determination of this fundamental value!”.

Within the given uncertainties, this number also fits very well with the current estimate of the age of the Universe, 13,700 million years, that is the time elapsed since the Big Bang. It thus appears that the first generation of stars in the Milky Way galaxy was formed at about the time the “Dark Ages” ended, now believed to be some 200 million years after the Big Bang.

It would seem that the system in which we live may indeed be one of the “founding” members of the galaxy population in the Universe.

More Information
The research presented in this press release is discussed in a paper entitled “Be in turn-off stars of NGC 6397: early Galaxy spallation, cosmochronology and cluster formation” by L. Pasquini and co-authors that will be published in the European research journal “Astronomy & Astrophysics” (astro-ph/0407524).

Original Source: ESO News Release

The sharpest image ever taken of a dust disk around another star has revealed structures in the disk which are signs of unseen planets.

Dr. Michael Liu, an astronomer at the University of Hawaii’s Institute for Astronomy, has acquired high resolution images of the nearby star AU Microscopii (AU Mic) using the Keck Telescope, the world’s largest infrared telescope. At a distance of only 33 light years, AU Mic is the nearest star possessing a visible disk of dust. Such disks are believed to be the birthplaces of planets.

“We cannot yet directly image young planets around AU Mic, but they cannot completely hide from us either. They reveal themselves through their gravitational influence, forming patterns in the sea of dust grains orbiting the star,” said Dr. Liu.

The results will be published in the August 12th online Science Express and in the September print edition of Science.

A dust disk ordinarily would appear relatively featureless and symmetric, because any disturbances would be smoothed out as the material orbits the star. However, this is not observed in the case of AU Mic. Instead, Dr. Liu has found its disk is uneven and possesses clumps. These structures arise and are maintained due to the gravitational influence of unseen planetary companions.

The clumps in AU Mic’s disk lie at separations of 25 to 40 Astronomical Units away from the central star (where one Astronomical Unit is the distance from the Earth to the Sun), or about 2 to 4 billion miles. In our own solar system, this corresponds to the regions where Neptune and Pluto reside.

AU Mic is a dim red star, with only half the mass and one-tenth the energy output as the Sun. Previous studies have shown that AU Mic is about 12 million years old, an epoch believed to be an active phase of planet formation. In comparison, our Sun is about 4.6 billion years old, or about 400 times older, and planet formation has long since ended.

“By studying very young stars like AU Mic, we gain insight into the planet formation process as it is occurring. As a result, we learn about the birth of our own solar system and its planets,” said Liu.

The images alone cannot yet tell us what kinds of planets are present, only that the planets are massive enough to gravitationally alter the distribution of the dust. However, many structures in the AU Mic disk are observed to be elliptical (non-circular), indicating that the planetary orbits are elliptical. This is different than in our own solar system, where most planets follow circular orbits.

Images of disks around nearby stars are very rare. Earlier this year, Dr. Liu and his colleagues announced the discovery of the large dusty disk around AU Mic. The light from AU Mic’s disk arises from small dust particles which reflect the light of the central star. The new images are 30 times sharper than the earlier ones, enabling discovery of the clumps in the inner disk of AU Mic.

Dr. Liu used the Keck II Telescope located on Mauna Kea, Hawaii for this research. The two Keck Telescopes are the largest infrared telescopes in the world, each with a primary mirror of 10-meter (33 feet) in diameter. The telescopes are equipped with adaptive optics, a powerful technology which corrects astronomical images for the blurring caused by the Earth’s turbulent atmosphere.

The resulting infrared images are the sharpest ever obtained of a circumstellar disk, with an angular resolution of 1/25 of an arcsecond, about 1/500,000 the diameter of the full moon. If a person’s vision were as sharp as the Keck adaptive optics system, he would be able to read a magazine that was one mile away. In the case of AU Mic, the Keck images can see features as small as 0.4 Astronomical Units, less than half the distance from the Earth to the Sun.

“It is remarkable how quickly Adaptive Optics at Keck has come from being an exotic demonstration technology to producing scientific results of unprecedented quality,” said Dr. Frederic H. Chaffee, the director of the W. M. Keck Observatory. “We are entering a new age of high resolution imaging in astronomy. Dr. Liu’s breathtaking images of possible planets in formation around AU Mic would have been unimaginable from any telescope — space-based or on Earth — a few short years ago. This is an exciting time for us all.”

A preprint of the paper may be found at the astro-ph website

http://arxiv.org/abs/astro-ph/0408164

This work was supported in part by the National Science Foundation.

Original Source: IfA News Release

Globular star clusters are like spherical cathedrals of light – collections of millions of stars lumped into a space only a few dozen light-years across. If the Earth resided within a globular cluster, our night sky would be alight with thousands of stars more brilliant than Sirius.

Our own Milky Way Galaxy currently holds about 200 globular clusters, but once possessed many more. According to the hierarchical theory of galaxy formation, galaxies have grown larger over time by consuming smaller dwarf galaxies and star clusters. And sometimes, it seems that the unfortunate prey is not swallowed whole but instead is munched like a peach, stripped of its outer layers to leave behind only the pit. New research by Paul Martini (Harvard-Smithsonian Center for Astrophysics) and Luis Ho (Observatories of the Carnegie Institution of Washington) shows that some globular clusters may be remnants of dwarf galaxies that were stripped of their outer stars, leaving only the galaxy’s nucleus behind.

Martini and Ho reported their results in the July 20, 2004, issue of The Astrophysical Journal.

Their findings hint at an important yet puzzling connection between the largest globular clusters and the smallest dwarf galaxies. “Star clusters and galaxies are quite different from a structural standpoint – star clusters are much more centrally concentrated, for example – and so the mechanisms that form them must be quite different. Identification of star clusters in the same mass range as galaxies is a very important step toward understanding how both types of objects form,” says Martini.

For their investigation, the team studied 14 globular clusters in the large elliptical galaxy Centaurus A (NGC 5128) using the 6.5-meter-diameter Magellan Clay telescope at Carnegie’s Las Campanas Observatory, Chile. The clusters were selected for their brightness, and since brighter clusters tend to contain more stars and more mass, were expected to be massive. Yet their results surprised even Martini and Ho, showing that the globular clusters of Centaurus A are much more massive than most globulars in the Local Group of galaxies (which includes the Milky Way and the Andromeda Galaxy).

“The essence of our findings is that these 14 globulars are 10 times more massive than the smaller globulars in our neighborhood, and whatever process makes them can produce some really huge objects – they begin to overlap with the smallest galaxies,” says Martini.

Martini also points out the recent discovery of a suspected intermediate-mass black hole in the Andromeda Galaxy globular cluster known as G1, which offers further evidence linking globular clusters to dwarf galaxies. The presence of a moderate-sized black hole is more understandable if it once occupied the center of a dwarf galaxy – a galaxy that lost its outer stars to the pull of Andromeda, leaving it only a shadow of its former self.

Ho, a co-discoverer of the intermediate-mass black hole in G1, adds, “One of the most surprising findings is that the black hole in G1 obeys the same tight correlation between black hole mass and host galaxy mass that has been well established for supermassive black holes in the centers of big galaxies. This puzzling result is more understandable if G1 was once the nucleus of a larger galaxy. A very interesting question is whether some of the massive clusters in Centaurus A also contain central black holes.”

Although most of our Galaxy’s globular clusters are much smaller than those of Centaurus A, the largest Milky Way globulars (such as the omega Centauri star cluster) rival those of the elliptical galaxy. The similarities between massive globulars in both galaxies may point to similar formation mechanisms. Future studies of these most massive globular clusters will explore connections between the formation processes for star clusters and galaxies.

Centaurus A is located approximately 12.5 million light-years away. It is about 65,000 light-years across and is more massive than the Milky Way and Andromeda galaxies put together. Centaurus A possesses a total of about 2000 globular clusters – more than all of the galaxies in the Local Group combined. Recent Spitzer Space Telescope observations of Centaurus A uncovered evidence that it merged with a spiral galaxy about 200 million years ago.

Original Source: Harvard-Smithsonian CfA News Release

A new image from NASA’s Spitzer Space Telescope shows the shimmering embers of a dying star, and in their midst a mysterious doughnut-shaped ring.

“Spitzer’s infrared vision has revealed what could not be seen before – a massive ring of material that was expelled from the dying star,” said Dr. Joseph Hora, a Spitzer scientist at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. “The composition of the ring and how it formed are mysteries we hope to address with further Spitzer studies.”

The new picture is available online at http://photojournal.jpl.nasa.gov/catalog/PIA06755.

The dying star is part of a “planetary nebula” called NGC 246. When a star like our own Sun begins to run out of fuel, its core shrinks and heats up, boiling off the star’s outer layers. Leftover material shoots outward, expanding in shells around the star. This ejected material is then bombarded with ultraviolet light from the central star’s fiery surface, producing huge, glowing clouds – planetary nebulas – that look like giant jellyfish in space.

These cosmic beauties last a relatively brief time, about a few thousand years, in the approximately 10-billion-year lifetime of a star. The name “planetary nebula” came from early astronomers who thought the rounded clouds looked like planets.

NGC 246 is located 1,800 light-years away in the Cetus constellation of our galaxy. Previous observations of this object by visible-light telescopes showed a glistening orb of gas and dust surrounding a central, compact star.

By cutting through the envelope of dust with its infrared eyes, Spitzer provides a more transparent view through and behind the nebula. “What we have seen with Spitzer is totally unexpected,” said Hora. “Although previous observations showed the nebula had a patchy appearance, Spitzer has revealed a ring component of this dying star, possibly consisting of hydrogen molecules.”

In the new false-color picture, the ring appears clumpy and red and off-center from the central star, while fluorescent, or ionized, gases are green. The central star is the left white spot in the middle of the cloud.

Ultimately, these data will help astronomers better understand how planetary nebulas take shape, and how they nourish new generations of stars. A scientific paper on this and other planetary nebulas observed by Spitzer will be published on Sept. 1 in The Astrophysical Journal Supplement, along with 75 other papers reporting Spitzer early mission results.

Launched August 25, 2003, the Spitzer Space Telescope is the fourth of NASA’s Great Observatories, a program that also includes the Hubble Space Telescope, the Chandra X-ray Observatory and the Compton Gamma Ray Observatory. Spitzer is also part of NASA’s Origins Program, which seeks to answer the questions: Where did we come from? Are we alone?

The Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. JPL is a division of Caltech. Spitzer’s infrared array camera, which took the new picture of NGC 246, was built by NASA Goddard Space Flight Center, Greenbelt, Md. The camera’s development was led by Dr. Giovanni Fazio of Harvard-Smithsonian Center for Astrophysics.

Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu.

Original Source: NASA/JPL News Release

Astronomers have shown for the first time that even the smallest galaxies in the Universe have complex structures that indicate a complex history. Using the Subaru Telescope, a team of astronomers from the National Astronomical Observatory of Japan, the Institute of Physics in Lithuania, the University of Durham, Paris Observatory, Kyoto University, Gunma Astronomical Observatory, and the University of Tokyo have discovered an extended halo of stars with a sharp cutoff in the dwarf irregular galaxy Leo A, a member of the Local Group of galaxies that includes the Milky Way. The discovery challenges current scenarios of galaxy formation by showing that instead of being the preservers of pristine building blocks that combined to form larger galaxies, dwarf irregular galaxies have their own history of build-up.

Understanding galaxy formation and evolution on time scales comparable to the age of the Universe is one of astronomy’s greatest challenges. In the scenarios of standard cosmology (Note 1), galaxies are built up via hierarchical merging: small primordial density fluctuations in the smooth distribution of matter in the early Universe grow and combine to form larger structures like the Milky Way. The most numerous type of galaxies in the universe — dwarf irregular galaxies (Note 2) — are supposed to preserve their properties unchanged over billions of years and represent pristine primeval building blocks. This is one reason why astronomers have recently been studying dwarf irregular galaxies with great interest.

The team led by Professors Nobuo Arimoto (National Astronomical Observatory of Japan) and Vladas Vansevicius (Institute of Physics, Lithuania) has studied Leo A — an isolated and extremely gas rich dwarf irregular galaxy with only 0.01% of the mass of the Milky Way and a low fraction chemical elements produced by earlier generations of stars. These characteristics suggest that this galaxy has been evolving without significant interaction with other galaxies. This galaxy has been believed to have quite a simple structure, in contrast to large disk galaxies like the Milky Way. However, this view needs to be changed due to deep imaging of the outer regions of this galaxy with the Subaru Telescope.

Prior to these observations, Leo A was already known to have a large angular size (7′ x 5′; Note 3) and Subaru Telescope equipped with its Prime Focus Camera (Suprime-Cam) was an ideal instrument to study the stars at the galaxy’s outer limits (Fig. 1). A single exposure with Suprime-Cam covers a field of view of 34′ x 27′ (pixel size 0”.2 x 0”.2) with high sensitivity. The team acquired optical images of the dwarf irregular galaxy Leo A with three broad band filters in November 2001. In order to trace the entire extent of the old stars in Leo A, the team employed red giant branch (RGB) stars which are evolved low-mass stars with very high luminosity and are expected to represent well the extended structures of galaxies. They investigated inside an ellipse of semi-major axis a = 12′ which fully covers the galaxy, and detected 1394 RGB stars distributed symmetrically and smoothly within this field.

Fig. 2 shows the radial profile of the surface number density of the red giant stars. The team found significantly larger disk structure (with a semi-major axis of 5.5′) than previously known (3.5′). Moreover, the deep observations permitted the discovery of a new stellar component in dwarf irregular galaxies, which the team calls a ?halo? (5.5′-7.5′), which has a less steep slope in the number density of RGB stars. The halo component ends at 8′ from the center of the galaxy with a sharp cutoff in the RGB star distribution. The existence of such a halo structure in dwarf irregular galaxies had been unconfirmed before these observations.

The size of Leo A revealed by these new observations is twice as large as its previously accepted size, suggesting that even in the nearby universe we see galaxies only as ?tips of icebergs” that are actually a few times more extended.

The newly discovered halo with a sharp stellar cutoff and the disk of the dwarf irregular galaxy Leo A closely resembles the structure as well as stellar and gaseous content found in large full-fledged disk galaxies like the Milky Way. The complicated structure of large massive galaxies has been believed to be a result of the merging of less massive galaxies like dwarf irregular ones. However, this study clearly reveals that the dwarf irregular galaxy Leo A already has disk and halo components, and suggests complex build-up histories for even very low mass galaxies like Leo A, which are supposed to form directly from the primordial density fluctuations in the early universe (Note 1), and challenges contemporary understanding of galaxy evolution. Professors N. Arimoto and V. Vansevicius believe Leo A is a ?Rosetta stone? (Note 4) for understanding the process of galaxy formation and evolution.

The scientific paper on this research has been accepted for publication in the August 20, 2004, Astrophysical Journal Letters (Volume 611, Number 2, L93).

Original Source: Subaru News Release

A gamma-ray burst detected by ESA’s Integral gamma-ray observatory on 3 December 2003 has been thoroughly studied for months by an armada of space and ground-based observatories. Astronomers have now concluded that this event, called GRB 031203, is the closest cosmic gamma-ray burst on record, but also the faintest. This also suggests that an entire population of sub-energetic gamma-ray bursts has so far gone unnoticed…

Cosmic gamma-ray bursts (GRBs) are flashes of gamma rays that can last from less than a second to a few minutes and occur at random positions in the sky. A large fraction of them is thought to result when a black hole is created from a dying star in a distant galaxy. Astronomers believe that a hot disc surrounding the black hole, made of gas and matter falling onto it, somehow emits an energetic beam parallel to the axis of rotation.

According to the simplest picture, all GRBs should emit similar amounts of gamma-ray energy. The fraction of it detected at Earth should then depend on the ‘width’ (opening angle) and orientation of the beam as well as on the distance. The energy received should be larger when the beam is narrow or points towards us and smaller when the beam is broad or points away from us. New data collected with ESA’s high energy observatories, Integral and XMM-Newton, now show that this picture is not so clear-cut and that the amount of energy emitted by GRBs can vary significantly. “The idea that all GRBs spit out the same amount of gamma rays, or that they are ‘standard candles’ as we call them, is simply ruled out by the new data,” said Dr Sergey Sazonov, from the Space Research Institute of the Russian Academy of Sciences, Moscow (Russia) and the Max-Planck Institute for Astrophysics, Garching near Munich (Germany).

Sazonov and an international team of researchers studied the GRB detected by Integral on 3 December 2003 and given the code-name of GRB 031203. Within a record 18 seconds of the burst, the Integral Burst Alert System had pinpointed the approximate position of GRB 031203 in the sky and sent the information to a network of observatories around the world. A few hours later one of them, ESA’s XMM-Newton, determined a much more precise position for GRB 031203 and detected a rapidly fading X-ray source, which was subsequently seen by radio and optical telescopes on the ground.

This wealth of data allowed astronomers to determine that GRB 031203 went off in a galaxy less than 1300 million light years away, making it the closest GRB ever observed. Even so, the way in which GRB 031203 dimmed with time and the distribution of its energy were not different from those of distant GRBs. Then, scientists started to realise that the concept of the ‘standard candle’ may not hold. “Being so close should make GRB 031203 appear very bright, but the amount of gamma-rays measured by Integral is about one thousand times less than what we would normally expect from a GRB,” Sazonov said.

A burst of gamma rays observed in 1998 in a closer galaxy appeared even fainter, about one hundred times less bright than GRB 031203. Astronomers, however, could not conclusively tell whether that was a genuine GRB because the bulk of its energy was emitted mostly as X-rays instead of gamma-rays. The work of Sazonov’s team on GRB 031203 now suggests that intrinsically fainter GRBs can indeed exist.

A team of US astronomers, coordinated by Alicia Soderberg from the California Institute of Technology, Pasadena (USA), studied the ‘afterglow’ of GRB 031203 and gave further support to this conclusion. The afterglow, emitted when a GRB’s blastwave shocks the diffuse medium around it, can last weeks or months and progressively fades away. Using NASA’s Chandra X-ray Observatory, Soderberg and her team saw that the X-ray brightness of the afterglow was about one thousand times fainter than that of typical distant GRBs. The team’s observations with the Very Large Array telescope of the National Radio Astronomy Observatory in Socorro (USA) also revealed a source dimmer than usual.

Sazonov and Soderberg explain that their teams looked carefully for signs that GRB 031203 could be tilted in such a way that most of its energy would escape Integral’s detection. However, as Sazonov said, “the fact that most of the energy that we see is emitted in the gamma-ray domain, rather than in the X-rays, means that we are seeing the beam nearly on axis.” It is, therefore, unlikely that much of its energy output can go unnoticed.

This discovery suggests the existence of a new population of GRBs much closer but also dimmer than the majority of those known so far, which are very energetic but distant. Objects of this type may also be very numerous and thus produce more frequent bursts.

The bulk of this population has so far escaped our attention because it lies at the limit of detection with past and present instruments. Integral, however, may be just sensitive enough to reveal a few more of them in the years to come. These could be just the tip of the iceberg and future gamma-ray observatories, such as the planned NASA’s Swift mission, should be able to extend this search to a much larger volume of the Universe and find many more sub-energetic GRBs.

Original Source: ESA News Release

On the evidence to date, our solar system could be fundamentally different from the majority of planetary systems around stars because it formed in a different way. If that is the case, Earth-like planets will be very rare. After examining the properties of the 100 or so known extrasolar planetary systems and assessing two ways in which planets could form, Dr Martin Beer and Professor Andrew King of the University of Leicester, Dr Mario Livio of the Space Telescope Science Institute and Dr Jim Pringle of the University of Cambridge flag up the distinct possibility that our solar system is special in a paper to be published in the Monthly Notices of the Royal Astronomical Society.

In our solar system, the orbits of all the major planets are quite close to being circular (apart from Pluto’s, which is a special case), and the four giant planets are a considerable distance from the Sun. The extrasolar planets detected so far – all giants similar in nature to Jupiter ? are by comparison much closer to their parent stars, and their orbits are almost all highly elliptical and so very elongated.

“There are two main explanations for these observations,” says Martin Beer. “The most intriguing is that planets can be formed by more than one mechanism and the assumption astronomers have made until now – that all planets formed in basically the same way – is a mistake.”

In the picture of planet formation developed to explain the solar system, giant planets like Jupiter form around rocky cores (like the Earth), which use their gravity to pull in large quantities of gas from their surroundings in the cool outer reaches of a vast disc of material. The rocky cores closer to the parent star cannot acquire gas because it is too hot there and so remain Earth-like.

The most popular alternative theory is that giant planets can form directly through gravitational collapse. In this scenario, rocky cores – potential Earth-like planets – do not form at all. If this theory applies to all the extrasolar planet systems detected so far, then none of them can be expected to contain an Earth-like planet that is habitable by life of the kind we are familiar with.

However, the team are cautious about jumping to a definite conclusion too soon and warn about the second possible explanation for the apparent disparity between the solar system and the known extrasolar systems. Techniques currently in use are not yet capable of detecting a solar-system look-alike around a distant star, so a selection effect might be distorting the statistics – like a fisherman deciding that all fish are larger than 5 inches because that is the size of the holes in his net.

It will be another 5 years or so before astronomers have the observing power to resolve the question of which explanation is correct. Meanwhile, the current data leave open the possibility that the solar system is indeed different from other planetary systems.

Original Source: RAS News Release

Today, August 2nd 2004, particle physicists from the UK and around the world working on the BABAR experiment at the Stanford Linear Accelerator Center (SLAC) in the USA, announced exciting new results demonstrating a dramatic difference in the behaviour of matter and antimatter. Their discovery may help to explain why the Universe we live in is dominated by matter, rather than containing equal parts matter and anti-matter.

SLAC’s PEP-II accelerator collides electrons and their antimatter counterparts, positrons, to produce an abundance of exotic heavy particle and anti-particle pairs known as B and anti-B mesons. These rare forms of matter and antimatter are short-lived, decaying in turn to other lighter subatomic particles, such as kaons and pions, which can be seen in the BABAR experiment.

“If there were no difference between matter and antimatter, both the B meson and the anti-B meson would exhibit exactly the same pattern of decays. However, our new measurement shows an example of a large difference in decay rates instead.” said Marcello Giorgi, of SLAC, Pisa University and INFN, Spokesman of BABAR.

By sifting through the decays of more than 200 million pairs of B and anti-B mesons, experimenters have discovered striking matter-antimatter asymmetry. “We found 910 examples of the B meson decaying to a kaon and a pion, but only 696 examples for the anti-B”, explained Giorgi. “The new measurement is very much a result of the outstanding performance of SLAC’s PEP-II accelerator and the efficiency of the BABAR detector. The accelerator is now operating at 3 times its design performance and BABAR is able to record about 98% of collisions.”

While BABAR and other experiments have observed matter-antimatter asymmetries before, this is the first time that a difference has been found by simple counting of the number of decays of B and anti-B mesons to the same final state. This effect is known as direct CP violation and is found to be 13%; a similar effect occurs for decays of Kaons and antiKaons but only at the level of 4 parts in a million!

“This is a strong, convincing signal of direct CP violation in B decays, a type of matter-antimatter asymmetry which was expected to exist but has not been observed before. With this discovery the full pattern of matter-antimatter asymmetries is coming together into a coherent picture. I am very excited and pleased as one of my postgraduate students, Carlos Chavez who is currently at SLAC, was directly involved.” said Christos Touramanis of the University of Liverpool.

Dan Bowerman, a member of the BABAR team from Imperial College adds “When the universe began with the big bang, matter and antimatter were created in equal amounts. However, all observations indicate that we live in a universe made only of matter. So we have to ask, what happened to the antimatter? The work at BABAR is bringing us closer to answering this question.”

Subtle differences between the behaviour of matter and antimatter must be responsible for the matter-antimatter imbalance that developed in our universe. But our current knowledge of these differences is incomplete and insufficient to account for the observed matter domination. CP violation is one of the three conditions outlined by Russian physicist Andrei Sakharov to account for the observed imbalance of matter and antimatter in the universe.

Professor Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council which funds UK participation in BABAR said: “We still don’t understand fully how the matter dominated Universe we live in has evolved. However this new result, and recent related measurements in BABAR and other experiments around the world, have greatly advanced our understanding in this area. There is still much to discover and learn on this fundamental issue.”

Original Source: PPARC News Release

Astronomers studying data from the National Science Foundation’s Very Long Baseline Array (VLBA) and other telescopes have concluded that a binary pair of stars forming an energetic microquasar was blasted out of the cluster in which it was born by a supernova explosion some 1.7 million years ago. This is the first time that a fast-moving stellar pair has been tracked back to a specific star cluster.

The scientists analyzed numerous observations of a microquasar called LSI +61 303, and concluded that it is moving away from a star cluster named IC 1805 at nearly 17 miles per second.

A microquasar is a pair of stars, one of which is either a dense neutron star or a black hole, in which material sucked from a “normal” star forms a rapidly-rotating disk around the denser object. The disk becomes so hot it emits X-rays, and also spits out “jets” of subatomic particles at nearly the speed of light.

“In this case, both the microquasar and the star cluster are about 7,500 light-years from Earth and the characteristics of the ‘normal’ star in the microquasar match those of the other stars in the cluster, so we feel confident that the microquasar was shot out from a birthplace in this cluster,” said Felix Mirabel, an astrophysicist at the Institute for Astronomy and Space Physics of Argentina and French Atomic Energy Commission. Mirabel worked with Irapuan Rodrigues, of the Federal University of Rio Grande do Sul, Brazil, and Qingzhong Liu of the Purple Mountain Observatory in Nanjing, China. The astronomers reported their results in the August 1 issue of the scientific journal Astronomy & Astrophysics.

Many neutron stars have been found to be moving rapidly through the sky, leading scientists to conclude that the supernova explosions that produced them were asymmetric, giving a “kick” to the star. LSI +61 303’s motion has carried it about 130 light-years from the cluster IC 1805. The cluster is in the constellation Cassiopeia.

LSI +61 303 contains, the astronomers say, either a black hole or a neutron star with twice the mass of the Sun, orbiting a normal star 14 times more massive than the Sun every 26.5 days. The supernova explosion that produced the black hole or neutron star blew away about twice the mass of the Sun.

The black hole or neutron star originally was much more massive than its companion. The scientists still are unsure about how massive it was. Some evidence, they say, indicates that it was formed only four or five million years ago and exploded a million or so years ago. In that case, the star would have been 60 or more times more massive than the Sun, and would have expelled some 90 percent of its initial mass before the supernova explosion.

On the other hand, they say, the star may have formed some 10 million years ago, in which case it would have been 15-20 times more massive than the Sun.

“Studying this system and hopefully others like it that may be found will help us to understand both the evolution of stars before they explode as supernovae and the physics of the supernova explosions themselves,” Mirabel said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release