Category: Astronomy

Galaxies are not randomly distributed. Image credit: IAC Click to enlarge
Although the galaxies we see in the night sky seem randomly strewn across the heavens, they’re actually organized into large scale structures that look like cosmic filaments. These filaments and walls surround huge bubble-like voids that lack any large structures at all. European astronomers measured the orientation of thousands of galaxies, and found that many are oriented in the direction of these linear filaments.

Astronomers from the University of Nottingham, UK, and the Instituto de Astrofisica de Canarias (Spain), have found the first observational evidence that galaxies are not randomly oriented.

Instead, they are aligned following a characteristic pattern dictated by the large-scale structure of the invisible dark matter that surrounds them.

This discovery confirms one of the fundamental aspects of galaxy formation theory and implies a direct link between the global properties of the Universe and the individual properties of galaxies.

Galaxy formation theories predicted such an effect, but its empirical verification has remained elusive until now. The results of this work were published the 1 April issue of Astrophysical Journal Letters.

Nowadays, matter is not distributed uniformly throughout space but is instead arranged in an intricate “cosmic web” of filaments and walls surrounding bubble-like voids. Regions with high galaxy concentrations are known as galaxy clusters whereas low density regions are termed voids.

This inhomogeneous distribution of matter is called the “Large-scale distribution of the Universe.” When the Universe is considered as whole, this distribution has a similar appearance to a spider’s web or the neural network of the brain. But it was not always like this.

After the Big Bang, when the Universe was much younger, matter was distributed homogeneously. As the Universe was evolving, gravitational pulls began to compress the matter in certain regions of space, forming the large-scale structure that we currently observe.

According to these models and theories a direct consequence of this process is that galaxies should be preferentially oriented perpendicularly to the direction of the linear filaments.

Several observational studies have looked for a preferential spatial orientation (or alignment) of galaxy rotation axes with respect to their surrounding large-scale structures. However, none of them have been successful, due to the difficulties associated with trying to characterise the filaments.

The research conducted by the astrophysical group formed by Ignacio Trujillo (University of Nottingham, UK), Conrado Carretero and Santiago G. Patiri, (both from the Instituto de Astrofisica de Canarias, Spain) has been able to measure this effect, confirming theoretical predictions.

To achieve this goal, they used a new technique based on the analysis of the huge voids that are found in the large-scale structure of the Universe. These voids have been detected by searching for large regions of space depleted of bright galaxies.

In addition, they took advantage of information provided by the two largest sky surveys yet undertaken: the Sloan Digital Sky Survey and the Two Degree Field Survey. These surveys contain positional information for more than half a million galaxies located within a distance of one billion light-years of the Earth.

Other parameters provided by the surveys, such as the position angle and the ellipticity of the objects, were used to estimate the orientation of the disk galaxies.

“We found that there is an excess of disk galaxies that are highly inclined relative to the plane defined by the large-scale structure surrounding them,” explained Dr. Trujillo. “Their rotation axes are mainly oriented in the direction of the filaments.

“Our work provides important confirmation of the tidal torque theory which explains how galaxies have acquired their current spin,” said Trujillo.

“The spin of the galaxies is believed to be intrinsically linked to their morphological shapes. So, this work is a step forward on our understanding of how galaxies have reached their current shapes.”

Dr. Ignacio Trujillo has a research assistant position, funded by PPARC, in the School of Physics and Astronomy at the University of Nottingham.

An abstract of the paper is available on the web at:
http://xxx.lanl.gov/abs/astro-ph/0511680

Original Source: RAS News Release

Deep Impact. Image credit: NASA. Click to enlarge
When Deep Impact collided with Tempel 1, it released an amazing amount of water vapour from the comet – as much as 250,000 tonnes were blasted into space. These measurements were made by NASA’s Swift satellite, which normally locates and observes gamma ray bursts. Swift, like almost every other telescope on Earth and in space was pointed at Comet Tempel 1 when Deep Impact smashed into it last July. Swift monitored the X-ray emissions before and after the collision, and used that to measure the amount of water vapour ejected.

Over the weekend of 9-10 July 2005 a team of UK and US scientists, led by Dr. Dick Willingale of the University of Leicester, used NASA’s Swift satellite to observe the collision of NASA’s Deep Impact spacecraft with comet Tempel 1. Reporting today (Tuesday) at the UK 2006 National Astronomy Meeting in Leicester, Dr. Willingale revealed that the Swift observations show that the comet grew brighter and brighter in X-ray light after the impact, with the X-ray outburst lasting a total of 12 days.

“The Swift observations reveal that far more water was liberated and over a longer period than previously claimed,” said Dick Willingale.

Swift spends most of its time studying objects in the distant Universe, but its agility allows it to observe many objects per orbit. Dr. Willingale used Swift to monitor the X-ray emission from comet Tempel 1 before and after the collision with the Deep Impact probe.

The X-rays provide a direct measurement of how much material was kicked up after the impact. This is because the X-rays were created by the newly liberated water as it was lifted into the comet’s thin atmosphere and illuminated by the high-energy solar wind from the Sun.

“The more material liberated, the more X-rays are produced,” explained Dr. Paul O’Brien, also from the University of Leicester.

The X-ray power output depends on both the water production rate from the comet and the flux of subatomic particles streaming out of the Sun as the solar wind. Using data from the ACE satellite, which constantly monitors the solar wind, the Swift team managed to calculate the solar wind flux at the comet during the X-ray outburst. This enabled them to disentangle the two components responsible for the X-ray emission.

Tempel 1 is usually a rather dim, weak comet with a water production rate of 16,000 tonnes per day. However, after the Deep Impact probe hit the comet this rate increased to 40,000 tonnes per day over the period 5-10 days after impact. Over the duration of the outburst, the total mass of water released by the impact was 250,000 tonnes.

One objective of the Deep Impact mission was to determine what causes cometary outbursts. A simple theory suggests that such outbursts are caused by the impact of meteorites on the comet nucleus. If this is the case, Deep Impact should have initiated an outburst.

Although the impact was observed across the electromagnetic spectrum, most of what was seen was directly attributable to the impact explosion. After 5 days, optical observations showed that the comet was indistinguishable from its state prior to the collision. This was in stark contrast to the X-ray observations.

The analysis of the X-ray behaviour by the Swift team indicates that the collision produced an extended X-ray outburst largely because the amount of water produced by the comet had increased.

“A collision such as Deep Impact can cause an outburst, but apparently something rather different from the norm can also happen,” said Dr. Willingale. “Most of the water seen in X-rays came out slowly, possibly in the form of ice-covered dust grains.”

Original Source: RAS News Release

Spiral galaxy NGC 1300. Image credit: Hubble. Click to enlarge
Researchers have harnessed the power one of the world’s fastest supercomputers – the Earth Simulator – to model the growth of galaxies in the early Universe. The team simulated the process right from the beginning, shortly after the Big Bang, when clumps of gas came together to form stars which then merged into larger and larger collections, and finally became galaxies. They found that galaxies like the Milky Way probably have the same composition now as they did only a billion years after the Big Bang.

Two astronomers have performed one of the world’s largest astrophysics simulations to date in order to model the growth of galaxies. Using the “Earth Simulator” supercomputer in Japan, which is also used for climate modelling and simulating seismic activity, Masao Mori of the University of California at Los Angeles and Masayuki Umemura at the University of Tsukuba have calculated how galaxies evolved from just 300 million years after the Big bang to the present day. The results show that galaxies may have evolved much faster than currently believed (Nature 440 644).

According to the “hierarchical” model, galaxies are formed via a bottom-up process that starts with the formation of small clumps of gas and stars that then merge into bigger systems. Mori and Umemura simulated this process using a powerful 3D hydrodynamic code combined with a “spectral synthesis” code for an astrophysical plasma in order to take into account the dynamical and chemical evolution of a primordial galaxy. The Earth-Simulator simulation was performed with an ultra-high resolution based on 1024 “grid points”, making it one of the biggest calculations ever performed in astrophysics.

Mori and Masayuki set up the initial conditions in their simulation based on a cold dark matter universe, the parameters of which are determined by measurements of the cosmic microwave background. These observations, first made in 2003, show that we are living in a flat universe comprising just 4% ordinary matter, 22% dark matter and 74% dark energy – in agreement with the standard model of cosmology. The researchers then directly compared their numerical results with observations of primitive galaxies called Lyman-alpha emitters and “Lyman break” galaxies, which astronomers find in the most distant and therefore oldest parts of the universe.

The results show that the primordial bubbles of gas that formed in the early universe just 300 millions years after the Big Bang do indeed look like Lyman-alpha emitters. After about 1 billion years, the simulations show that these galaxies mutate into Lyman break galaxies. Finally, after 10 billion years of evolution, the structures resemble present-day elliptical galaxies.

The simulation also predicts the mixture of chemical elements in the galaxy at each stage of its evolution, and suggests that our Milky Way has roughly the same composition today as it did when it was just 1 billion years old. Until now, galaxies were thought to have evolved gradually and become enriched in heavier elements beyond hydrogen and helium over a period of 10 billion years by repeated star formation and supernova explosions.

“Our finding shows that galaxy formation proceeded much faster and that a large amount of heavy elements were produced in galaxies in just 1 billion years,” says Mori.

Original Source: Institute of Physics

NASA’s first look at a lonely neutron star. Image credit: NASA/HST Click to enlarge
The most powerful explosions in the Universe are the mysterious gamma ray bursts, which astronomers now think are collisions between neutron stars. A new simulation has calculated that in the moments after a collision, the explosion generates a magnetic field 1000 million million times more powerful than the Earth’s magnetic field – the strongest magnetic fields in the Universe. The simulation took weeks on a supercomputer to calculate just a few milliseconds of a collision between neutron stars.

Scientists from The University of Exeter and the International University, Bremen have discovered what is thought to be the strongest magnetic field in the Universe. In a paper in the journal Science, Dr Daniel Price and Professor Stephan Rosswog show that violent collisions between neutron stars in the outer reaches of space create this field, which is 1000 million million times larger than our earth’s own magnetic field. It’s thought that these collisions could be behind some of the brightest explosions in the Universe since the Big Bang, so-called short Gamma-ray bursts.

Dr Daniel Price, of the School of Physics at The University of Exeter, said: “We have managed to simulate, for the first time, what happens to the magnetic field when neutron stars collide, and it seems possible that the magnetic field produced could be sufficient to spark the creation of Gamma-ray bursts. Gamma-ray bursts are the most powerful explosions we can detect but until recently little to nothing has been known about how they are generated. It’s thought that strong magnetic fields are essential in producing them, but until now no one has shown how fields of the required intensity could be created.”

He continues: “What really surprised us was just how fast these tremendous fields are generated – within one or two milliseconds after the stars hit each other.”

Prof Stephan Rosswog, of the International University, Bremen, Germany, adds: “Even more incredible is that the magnetic field strengths reached in the simulations are just lower limits on the strengths that may be actually be produced in nature. It has taken us months of nearly day and night programming to get this project running – just to calculate a few milliseconds of a single collision takes several weeks on a supercomputer.”

The remnants of supernovae, neutron stars are formed when massive stars run out of nuclear fuel and explode, shedding their outer layers and leaving behind a small but extremely dense core. When two neutron stars are left orbiting each other, they will spiral slowly together, resulting in these massive collisions.

Original Source: University of Exeter

Astronomers from Boston University have carefully mapped the giant gas clouds in our region of the Milky Way, offering clues to the environment that helped create our Solar System. The team used a large radio telescope that captures high frequency radio waves. When viewed at this wavelength, the clouds are far more transparent, and their inner structure is revealed. All of the clouds they’ve studied so far are lumpy, and will eventually be the birthplaces of stars.

A team of astronomers from Boston University’s Institute for Astrophysical Research has produced the clearest map to-date of the giant gas clouds in the Milky Way that serve as the birthplaces of stars. Using a powerful telescope, the astronomers tracked emissions of a rare form of carbon monoxide called 13CO to chart a portion of our home galaxy and its star-forming molecular clouds.

The researchers hope the new illustration will aid in the identification of additional clouds and study of their internal structure to better understand the origin of stars like the sun, which began its life in such a cloud about 5 billion years ago. The data and images are published in the March issue of the Astrophysical Journal Supplement.

The eight-year project, called the Boston University-Five College Radio Astronomy Observatory (FCRAO) Galactic Ring Survey (GRS), was led by a team of astronomers based at BU, the University of Cologne in Germany, and the University of Massachusetts.

To produce the detailed image, the astronomers mapped the location of 13CO in the Milky Way using a large radio telescope operated by the FCRAO of the University of Massachusetts that captures and images radio emissions at a frequency near 100,000 MHz – about 1,000 times higher than FM stations. When viewed in the emission from 13CO, the clouds are far more transparent than the more traditionally studied 12CO which allowed the team to peer more deeply into their interior.

“The value of such high range imaging is that it enables us to identify the underlying patterns of gas distribution and speeds that point toward the key physical processes occurring within the molecular gas phase of the interstellar medium,” said Dr. Mark Heyer, a researcher from UMass involved in the project.

Using a new receiver developed at UMass, the astronomers could depict the structure of the clouds faster and with much finer detail than any previous attempts. As an added benefit, the distribution of the clouds also delineates the spiral structure of the Milky Way.

“Ironically, because we live inside the Milky Way, we know more about the shapes of far more distant galaxies better than our own,” said James Jackson, astronomy professor at BU and lead investigator of the study. “The GRS map helps us better understand the configuration of our home galaxy and its components.”

“Upon seeing the GRS image, I knew right away it was something terrific. It was like the first time I put on glasses as a kid, and wondered how I ever got along without knowing about every shape, contour and detail of the world around me,” said Dr. Ronak Shah, a researcher from BU who worked on the project. “The GRS has that affect on a lot of us. We thought we understood the Milky Way and then the GRS revealed so much more detail to explore.”

According to Dr. Robert Simon, now at the University of Cologne, but who started the project with Jackson in 1998 at BU, the information from the GRS will constitute an important new database for the study of molecular clouds and Milky Way structure for generations of astronomers.

The scientists are now closely analyzing the image and one of the initial findings is the probable identification of dark, cold molecular clouds in the earliest stages of star development.

“Data from the Galactic Ring Survey have shown that these clouds are the counterparts to active, bright star-forming clouds, but because they have not yet been heated by the embedded stars, they are much colder and quieter,” said Jackson. “Follow-up studies of these clouds will provide additional important clues about the origin of stars since we’ll be able to examine them at an earlier point in their life.”

Another interesting result is that all of the molecular clouds studied so far have similar lumpy structures, regardless of their size, mass, and star-forming activity. These lumps will eventually become stars and, according to the researchers, this similarity suggests that all clouds form stars of various masses in roughly the same proportion.

The Milky Way is a vast disk of 100 billion stars, gas, and dust and because it is flat, the map is long and narrow. Since most of the Galaxy lies in the southern skies, unreachable from Northern Hemisphere telescopes, and because many of the molecular gas clouds are concentrated toward its inner regions, only a portion was imaged.

The Institute for Astrophysical Research (IAR) was founded in 1998 in order to promote and facilitate research and education in astrophysics at Boston University. The IAR supports research by BU Astronomy faculty members, graduate and undergraduate students, and postdoctoral and senior research associates. In addition, the IAR manages and coordinates the use of astrophysical research facilities and promotes the design, development, and operation of instruments and telescopes for astronomical research.

Founded in 1839, Boston University is an internationally recognized institution of higher education and research. With more than 30,000 students, it is the fourth largest independent university in the United States. BU contains 17 colleges and schools along with a number of multi-disciplinary centers and institutes which are central to the school’s research and teaching mission.

Original Source: Boston University

HD98618 would look almost identical to our Sun. Image credit: SOHO Click to enlarge.
When astronomers start searching for evidence of live orbiting other stars, they’ll start with familiar terrain: other stars like our Sun. Astronomers from the Australian National University have identified a nearby candidate that’s a virtual twin of our Sun in age, size, temperature and chemistry; although, it’s 2% more massive. The star, HD98618, is located 126 light-years away in the constellation Ursa Major (the Big Dipper), and is bright enough to see with binoculars.

ANU astronomers have discovered a nearby solar twin which may shed light on the search for planets that are similar to Earth and that may even support life.

HD98618 is only the second star found so far that is almost identical to the Sun in age, size, temperature and chemistry, according to the researchers Dr Jorge Meléndez, Ms Katie Dodds-Eden and Mr José Robles, from the Research School of Astronomy and Astrophysics.

“This solar twin doesn’t only have the same mass as the Sun, it was also formed with the same ‘chemical recipe’. So this star was equipped in the same way as the Sun to form Earth-like planets,” Mr Robles said.

“Hopefully, as new planet finding techniques are developed and refined, astronomers will find whether HD98618 hosts terrestrial planets, which may even contain life.”

HD98618 lies a mere 126 light-years away in the northern constellation of Ursa Major (the ‘Big Dipper’). It is bright enough to see in binoculars, but only in the Northern Hemisphere.

The researchers believe that HD98618 is about four billion years-old, about 10 per cent younger than our own Sun. Its chemical properties are almost identical to the Sun and to the other closest Sun twin, a star known as 18 Scorpii, which was discovered a decade ago.

“It means that hypothetical terrestrial planets around this solar twin may have had enough time to develop some kind of complex life, assuming the time-scale for complex life formation is similar to Earth’s,” Dr Meléndez said.

The team says that focused observations of the two stars by planet-hunter teams could reveal or rule out within a few years giant planets, such as our own Jupiter, around HD98618. “18 Scorpii and HD98618 offer hope to find solar systems similar to our own in the Universe,” Dr Meléndez said.

The discovery also has implications for research in other areas. Solar twins are ideal for the absolute calibration of astronomical measuring instruments. They can provide data useful in modelling the solar phenomena that may affect climate change, and will help settle the argument about the uniqueness or otherwise of our Sun and Solar System.

“We had a number of candidates with similar properties to the Sun, but while we held out hope for each star that it would turn out to be really special, it was not at all certain to happen. HD 98618 was one of the last of our candidates to be analysed, so it was quite a surprise when we discovered how it stood out from the other candidates, together with 18 Scorpii. It was very exciting – I had to blink twice to be sure I wasn’t imagining it,” Ms Dodds-Eden said.

The researchers made the discovery using the largest telescope in the world, the 10m Keck I telescope on the summit of Hawaii’s dormant Mauna Kea volcano.

Their paper detailing the discovery will be published in Astrophysical Journal Letters. Related images are available from the ANU Media Office.

Original Source: ANU News Release

An illustration showing a quasar at the center of the galaxy. Image credit: NASA Click to enlarge
Sometimes the supermassive black holes at the hearts of galaxies are quiet, and nearly invisible. Other times they’re actively gobbling up material, blazing as quasars in the X-ray spectrum. NASA’s Chandra X-Ray Observatory has observed one of these transition times, when the heated material around the supermassive black hole is beginning to ignite. It’s likely the galaxy recently collided or merged with another galaxy, and the turbulence caused material to fall into the black hole.

An artist’s illustration depicts a quasar in the center of a galaxy that has turned on and is expelling gas at high speeds in a galactic superwind. Clouds of hot, X-ray producing gas detected by Chandra around the quasars 4C37.43 and 3C249.1, provide strong evidence for such superwinds.

The X-ray features seen at five, six, ten and eleven o’clock in the 4C37.43 image are located tens of thousands of light years from the central supermassive black hole that powers the quasar. They are likely due to shock waves in the superwind.

Mergers of galaxies are a possible cause for the ignition, or turn-on, of quasars. Computer simulations show that a galactic merger drives gas toward the central region where it triggers a burst of star formation and provides fuel for the growth of a central black hole.

The inflow of gas into the black hole releases tremendous energy, and a quasar is born. The power output of the quasar dwarfs that of the surrounding galaxy and pushes gas out of the galaxy in a galactic superwind.

Over a period of about 100 million years, the superwind will drive most of the gas away from the central regions of the galaxy, quenching both star formation and further supermassive black hole growth. The quasar phase will end and the galaxy will settle down to a relatively quiet life.

Original Source: Chandra X-ray Observatory

A hot gas cloud whirling around a miniature ‘cannibal’ star. Image credit: ESA Click to enlarge
ESA’s XMM-Newton space telescope has observed the tiny cores of dead stars wrapped up in a nice warm blanket of superheated gas. These “low-mass X-ray binary” are pulling a steady stream of material from a larger companion star, and then whipping it up into a disk. This observation answers the question of why these dead stars sometimes blink off in the X-ray spectrum. That’s the time when we’re seeing this disk edge-on, and it’s obscuring our view of the star.

ESA’s XMM-Newton has seen vast clouds of superheated gas, whirling around miniature stars and escaping from being devoured by the stars’ enormous gravitational fields – giving a new insight into the eating habits of the galaxy’s ‘cannibal’ stars.

The clouds of gas range in size from a few hundred thousand kilometres to a few million kilometres, ten to one hundred times larger than the Earth. They are composed of iron vapour and other chemicals at temperatures of many millions of degrees.

“This gas is extremely hot, much hotter than the outer atmosphere of the Sun,” said Maria Diaz Trigo of ESA’s European Science and Technology Research Centre (ESTEC), who led the research.

ESA’s XMM-Newton x-ray observatory made the discovery when it observed six so-called ‘low-mass X-ray binary’ stars (LMXBs). The LMXBs are pairs of stars in which one is the tiny core of a dead star.

Measuring just 15?20 kilometres across and comparable in size to an asteroid, each dead star is a tightly packed mass of neutrons containing more than 1.4 times the mass of the Sun.

Its extreme density generates a powerful gravitational field that rips gas from its ‘living’ companion star. The gas spirals around the neutron star, forming a disc, before being sucked down and crushed onto its surface, a process known as ‘accretion’.

The newly discovered clouds sit where the river of matter from the companion star strikes the disc. The extreme temperatures have ripped almost all of the electrons from the iron atoms, leaving them carrying extreme electrical charges. This process is known as ‘ionisation’.

The discovery solves a puzzle that has dogged astronomers for several decades. Certain LMXBs appear to blink on and off at X-ray wavelengths. These are ‘edge-on’ systems, in which the orbit of each gaseous disc lines up with Earth.

In previous attempts to simulate the blinking, clouds of low-temperature gas were postulated to be orbiting the neutron star, periodically blocking the X-rays. However, these models never reproduced the observed behaviour well enough.

XMM-Newton solves this by revealing the ionised iron. “It means that these clouds are much hotter than we anticipated,” said Diaz. With high-temperature clouds, the computer models now simulate much better the dipping behaviour.

Some 100 known LMXBs populate our galaxy, the Milky Way. Each one is a stellar furnace, pumping X-rays into space. They represent a small-scale model of the accretion thought to be taking place in the very heart of some galaxies. One in every ten galaxies shows some kind of intense activity at its centre.

This activity is thought to be coming from a gigantic black hole, pulling stars to pieces and devouring their remains. Being much closer to Earth, the LMXBs are easier to study than the active galaxies.

“Accretion processes are still not well understood. The more we understand about the LMXBs, the more useful they will be as analogues to help us understand the active galactic nuclei,” says Diaz.

Original Source: ESA Portal

An image of the cool brown dwarf orbiting a star near the Sun. Image credit: UA Steward Observatory. Click to enlarge
Astronomers have discovered a brown dwarf in our galactic neighbourhood, only 12.7 light years away – this makes it the second closest brown dwarf ever discovered. The failed star is circling another star that was only recently discovered in the southern constellation Pavo. The primary star is small, with only 1/10th the mass of our Sun, and the brown dwarf orbits at 4.5 times the distance of the Earth to the Sun.

Astronomers have discovered a unique “brown dwarf” right in our solar neighborhood.

If your city were the galaxy, it would be like finding someone you didn’t know about living upstairs in your house, one of the discoverers said.

The rare object is only 12.7 light years from Earth, circling a primary star that itself was discovered only recently in the southern hemisphere constellation Pavo (the Peacock).

Only one other brown dwarf system has been found closer to Earth, and it’s only marginally closer.

The primary star is only one-tenth the mass of our sun. This is the first time astronomers have found a cool brown dwarf companion to such a low-mass star. Until now, none has been found orbiting stars less than half the mass of our sun.

The brown dwarf is 4.5 AU from the star, or four and one-half times farther from its star than Earth is from our sun. Astronomers estimate that the brown dwarf is between nine and 65 times as massive as Jupiter.

Brown dwarfs are neither planets nor stars. They are dozens of times more massive than our solar system’s largest planet, Jupiter, but too small to be self-powered by hydrogen fusion like stars.

Only about 30 similarly cool brown dwarfs have been found anywhere in the sky, and only about 10 have been discovered orbiting stars.

“Besides being extremely close to Earth and in orbit around a very low-mass star, this object is a ‘T dwarf ‘ – a very cool brown dwarf with a temperature of about 750 degrees Celsius (1,382 degrees Fahrenheit),” said Beth Biller, a graduate student at The University of Arizona.

“It is also likely the brightest known object of its temperature because it is so close,” Biller said. “And it’s a rare example of a brown dwarf companion within 10 astronomical units of its primary star.”

Biller, along with Markus Kasper of the European Southern Observatory (ESO) and Laird Close of UA’s Steward Observatory, led the team who discovered the brown dwarf, designated SCR 1845-6357B.

“What’s really exciting about this is that we found the brown dwarf around one of the 25 stellar systems nearest to the sun,” Close said. “Most of these nearby stars have been known for decades, and only just recently a handful of new objects have been found in our local neighborhood.”

Close said, “If you think of the galaxy as being the size of Tucson, it’s kind of like finding someone living in the upstairs of your house that you didn’t know about before.”

Close helped develop the special adaptive optics camera, the NACO Simultaneous Differential Imager(SDI), that the team used to image the brown dwarf. The camera is used on ESO’s Very Large Telescope (VLT) in Chile. Another SDI camera is used at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz.

“This is also a valuable object to the scientific community because its distance is well known,” said ESO’s Markus Kasper. This will allow astronomers to measure the brown dwarf’s luminosity accurately and, eventually, to calculate its orbital motion, Kasper said. “These properties are vital for understanding the nature of brown dwarfs.”

The discovery of this brown dwarf suggests there may be more cool brown dwarfs in binary systems than single brown dwarfs floating free in the solar neighborhood, Close said. A “binary system” is where a brown dwarf revolves around a star or another brown dwarf.

Astronomers now have found five cool brown dwarfs in binary systems but only two single, isolated cool brown dwarfs within 20 light years of the sun, Close noted. They can expect to find more T dwarf companions in some newly found stellar systems within 33 light years of our solar system, he added.

Evidence that T dwarfs in binary systems outnumber single, isolated T dwarfs in the solar neighborhood has ramifications for theories that predict single brown dwarfs will form more often than binary ones, Close said.

The NACO Simultaneous Differential Imager(SDI) uses adaptive optics to remove the blurring effects of Earth’s atmosphere to produce extremely sharp images. The camera enhances the ability of the VLT to detect faint companions that would otherwise be lost in the glare of their primary stars.

Close and Rainer Lenzen of the Max Planck Institute for Astronomy in Heidelberg, Germany, developed the SDI camera to search for methane-rich extrasolar planets. The SDI camera splits light from a single object into four identical images, then passes the beams through three slightly different methane-sensitive filters. When the filtered light beams hit the detector array, astronomers subtract the images so the bright star disappears and its far dimmer, methane-rich companion pops into view.

The team will publish the discovery in the Astrophysical Journal Letters in the article, “Discovery of a Very Nearby Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357.” In addition to Biller, Kasper and Close, team members include Wolfgang Brandner of the Max Planck Institute in Heidelberg, Germany, and Stephan Kellner of the W.M. Keck Observatory in Waimea, Hawaii.

Original Source: UA News Release

Inward migration of a group of protoplanets, where they’re represented by white circles. Image credit: QMUL Click to enlarge
Astronomers think they’ve got a handle on many aspects of planetary formation. But two British researchers have discovered a problem with the formation of gas giant planets. Under their model, the cores of these massive planets should be drawn inward by their parent star in only 100,000 years – not nearly enough time to form into a stable orbit. It could be that the first generations of planets never get past the “clump” stage before they’re destroyed. It’s only the later generations that actually survive long enough to become planets.

Two British astronomers, Paul Cresswell and Richard Nelson present new numerical simulations in the framework of the challenging studies of planetary system formation. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star. Their results will soon be published in Astronomy & Astrophysics.

In an article to be published in Astronomy & Astrophysics, two British astronomers present new numerical simulations of how planetary systems form. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star.

The current picture of how planetary systems form is as follows: i) dust grains coagulate to form planetesimals of up to 1 km in diameter; ii) the runaway growth of planetesimals leads to the formation of ~100 ? 1000 km-sized planetary embryos; iii) these embryos grow in an “oligarchic” manner, where a few large bodies dominate the formation process, and accrete the surrounding and much smaller planetesimals. These “oligarchs” form terrestrial planets near the central star and planetary cores of ten terrestrial masses in the giant planet region beyond 3 astronomical units (AU).

However, these theories fail to describe the formation of gas giant planets in a satisfactory way. Gravitational interaction between the gaseous protoplanetary disc and the massive planetary cores causes them to move rapidly inward over about 100,000 years in what we call the “migration” of the planet in the disc. The prediction of this rapid inward migration of giant protoplanets is a major problem, since this timescale is much shorter than the time needed for gas to accrete onto the forming giant planet. Theories predict that the giant protoplanets will merge into the central star before planets have time to form. This makes it very difficult to understand how they can form at all.

For the first time, Paul Cresswell and Richard Nelson examined what happens to a cluster of forming planets embedded in a gaseous protoplanetary disc. Previous numerical models have included only one or two planets in a disc. But our own solar system, and over 10% of the known extrasolar planetary systems, are multiple-planet systems. The number of such systems is expected to increase as observational techniques of extrasolar systems improve. Cresswell and Nelson’s work is the first time numerical simulations have included such a large number of protoplanets, thus taking into account the gravitational interaction between the protoplanets and the disc, and among the protoplanets themselves.

The primary motivation for their work is to examine the orbits of protoplanets and whether some planets could survive in the disc for extended periods of time. Their simulations show that, in very few cases (about 2%), a lone protoplanet is ejected far from the central star, thus lengthening its lifetime. But in most cases (98%), many of the protoplanets are trapped into a series of orbital resonances and migrate inward in lockstep, sometimes even merging with the central star.

Cresswell and Nelson thus claim that gravitational interactions within a swarm of protoplanets embedded in a disc cannot stop the inward migration of the protoplanets. The “problem” of migration remains and requires more investigation, although the astronomers propose several possible solutions. One may be that several generations of planets form and that only the ones that form as the disc dissipates survive the formation process. This may make it harder to form gas giants, as the disc is depleted of the material from which gas giant planets form. (Gas giant formation may still be possible though, if enough gas lies outside the planets’ orbits, since new material may sweep inward to be accreted by the forming planet). Another solution might be related to the physical properties of the protoplanetary disc. In their simulations, the astronomers assumed that the protoplanetary disc is smooth and non-turbulent, but of course this might not be the case. Large parts of the disc could be more turbulent (as a consequence of instabilities caused by magnetic fields), which may prevent inward migration over long time periods.

This work joins other studies of planetary system formation that are currently being done by a European network of scientists. Our view of how planets form has drastically changed in the last few years as the number of newly discovered planetary systems has increased. Understanding the formation of giant planets is currently one of the major challenges for astronomers.

Original Source: Astronomy & Astrophysics