Universe Today

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