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Thursday, 13 June 2013

Where Is Dark Matter Most Dense? Subaru Telescope Gets Some Hints by ELIZABETH HOWELL on JUNE 13, 2013

Put another checkmark beside the “cold dark matter” theory. New observations by Japan’s Subaru Telescope are helping astronomers get a grip on the density of dark matter, this mysterious substance that pervades the universe.
We can’t see dark matter, which makes up an estimated 85 percent of the universe, but scientists can certainly measure its gravitational effects on galaxies, stars and other celestial residents. Particle physicists also are on the hunt for a “dark matter” particle — with some interesting results released a few weeks ago.
The latest experiment with Subaru measured 50 clusters of galaxies and found that the density of dark matter is largest in the center of these clusters, and smallest on the outskirts. These measurements are a close match to what is predicted by cold dark matter theory, scientists said.
Cold dark matter assumes that this material can’t be observed in any part of the electromagnetic spectrum, the band of light waves that ranges from high-energy X-rays to low-energy infrared heat. Also, the theory dictates that dark matter is made up of slow-moving particles that, because they collide with each other infrequently, are cold. So, the only way dark matter interacts with other particles is by gravity, scientists have said.
To check this out, Subaru peered at “gravitational lensing” in the sky — areas where the light of background objects are bent around dense, massive objects in front. Galaxy clusters are a prime example of these super-dense areas.
Several dark matter maps: one based on a sample of 50 individual galaxy clusters (left), another looking at an average galaxy cluster (center), and another based on dark matter theory (right). Red is the highest concentration of dark matter, followed by yellow, green and blue. At right, in the middle, is a map based on cold dark matter theory that comes close to the average galaxy cluster observed with the Suburu Telescope.
“The Subaru Telescope is a fantastic instrument for gravitational lensing measurements. It allows us to measure very precisely how the dark matter in galaxy clusters distorts light from distant galaxies and gauge tiny changes in the appearance of a huge number of faint galaxies,” stated Nobuhiro Okabe, an astronomer at Academia Sinica in Taiwan who led the study.
Next, the team members could compare where the matter was most dense with that predicted by cold dark matter theory. To do that, they measured 50 of the most massive, known clusters of galaxies. Then, they measured the “concentration parameter”, or the cluster’s average density.

“They found that the density of dark matter increases from the edges to the center of the cluster, and that the concentration parameter of galaxy clusters in the near universe aligns with CDM theory,” stated the National Astronomical Observatory of Japan.

The next step, researchers stated, is to measure dark matter density in the center of the galaxy clusters. This could reveal more about how this substance behaves. Check out more about this study in Astrophysical Journal Letters.


Alien Life Unlikely Around White and Brown Dwarfs, Study Finds Charles Q. Choi, Astrobiology Magazine Contributor Date: 05 June 2013 Time: 07:00 AM ET

The dead and failed stars known as white dwarfs and brown dwarfs can give off heat that can warm up worlds, but their cooling natures and harsh light make them unlikely to host life, researchers say.
Stars generally burn hydrogen to give off light and heat up nearby worlds. However, there are other bodies in space that can shine light as well, such as the failed stars known as brown dwarfs and the dead stars known as white dwarfs.
White dwarfs are remnants of normal stars that have burned all the hydrogen in their cores. Still, they can remain hot enough to warm nearby planets for billions of years. Planets around white dwarfs might include the rocky cores of worlds that were in orbit before the star that became the white dwarf perished; new planets might also emerge from envelopes of gas and dust around white dwarfs.
Brown dwarfs are gaseous bodies that are larger than the heaviest planets but smaller than the lightest stars. This means they are too low in mass for their cores to squeeze hydrogen with enough pressure to support nuclear fusion like regular stars.
Still, the gravitational energy from their contractions does get converted to heat, meaning they can warm their surroundings. NASA's WISE spacecraft and other telescopes have recently discovered hundreds of brown dwarfs, raising the possibility of detecting exoplanets circling them; scientists have already observed protoplanetary disks around a few of them.
White dwarfs and brown dwarfs are bright enough to support habitable zones — regions around them warm enough for planets to sustain liquid water on their surfaces. As such, worlds orbiting them might be able support alien life as we know it, as there is life virtually everywhere there is water on Earth.
"These planets could be like the Earth, but they are relatively unstudied," said study lead author Rory Barnes, a planetary scientist and astrobiologist at the University of Washington at Seattle.
An added benefit of looking for exoplanets around these dwarfs is that they might be easier to detect than ones around regular stars. These dwarfs are relatively small and faint, meaning any worlds that pass in front of them would dim them more noticeably than planets crossing in front of normal stars.
Shifting habitable zones
However, unlike regular stars, white dwarfs and brown dwarfs cool as they age, meaning their habitable zones will move inward over time. Barnes and his colleague René Heller at the Leibniz Institute for Astrophysics Potsdam in Germany were curious as to whether this complicated the habitability of planets there.
The most obvious peril of a shifting habitable zone is that it could result in a planet getting so cold all the liquid water on its surface freezes solid. There are other dangers, however — as white dwarfs and brown dwarfs cool, the light they give off would change as well, possibly meaning they would end up sterilizing worlds with dangerous, high-energy radiation.
To be specific, extreme ultraviolet rays would break a planet's water apart into hydrogen and oxygen. The hydrogen can escape into space, and without hydrogen to bond with oxygen, the world has no water and is not habitable.
This artist’s impression shows the disc of gas and cosmic dust around a brown dwarf.
CREDIT: ALMA (ESO/NAOJ/NRAO)/M.
Kornmesser (ESO)View full size image
Such exoplanets would resemble Venus, with dry atmospheres dominated by carbon dioxide. Young white dwarf stars would bathe nearby planets in extreme ultraviolet radiation; the situation is less clear with brown dwarfs, Barnes and Heller said.
In addition, because white dwarfs and brown dwarfs are so dim, their habitable zones already start off very near them — about one-hundredth the distance between the sun and Earth, which is about one-thirtieth the distance between the sun and Mercury.
At such close distances, the gravitational pull of the dwarfs will significantly flex and heat planets, just as the moon's gravitational tug on Earth results in tides. Too much heating can cause planets to lose all their water, becoming what Barnes and his colleagues dub "tidal Venuses."
Frustratingly, water-rich planets that lose their hydrogen may develop oxygen-rich atmospheres, which astronomers might mistake as a sign of life, the investigators said. Oxygen usually does not stay long in atmospheres — as such, detecting it on an alien world might suggest to scientists that organisms such as plants exist there to generate the gas. [9 Exoplanets That Could Host Alien Life]
Inhospitable white dwarfs
White dwarfs should tidally heat planets more than brown dwarfs, since white dwarves are so massive, the researchers noted. White dwarfs are only about the size of the Earth, but they are remarkably dense, with masses nearly two-thirds that of the sun.
All in all, the scientists found it unlikely that planets orbiting white dwarfs would ever be truly habitable. When they are young, white dwarfs would blast planets in their habitable zones with ultraviolet rays that would strip the worlds of water; when they grow older, their habitable zones would shift closer to them, and the amount of tidal heating might also end up desiccating any planets residing in those zones.
To look at what planets around white dwarfs might be like, the scientists analyzed two exoplanet candidates orbiting the star KOI 55, which will soon die and become a white dwarf.
The star, which is about half the mass of the sun, lies about 3,850 light years away, and the putative worlds KOI 55.01 and KOI 55.02, discovered by NASA's Kepler mission, are about two-fifths and two-thirds the mass of the Earth, respectively.
If these candidates do exist, these roughly Earth-sized exoplanets will fall within the habitable zone of KOI 55 when it becomes a white dwarf. However, these worlds are currently roasting under the star's heat and probably losing their water, if they had any, and are therefore unlikely to become habitable later, the researchers said. Gravitational interactions between the exoplanets may also cause sterilizing levels of tidal heating. [A World of Kepler Planets (Gallery)]
Planets orbiting brown dwarfs also run the risk of never achieving habitable conditions, but they may have a slightly better chance than worlds around white dwarfs, the researchers found. Catastrophic tidal heating remains a problem — because these stars are dim, planets must orbit relatively close in to receive enough light to be habitable, but the chances are that tidal forces might simply tear apart planets that are so close to their star.
Not impossible
Although the chances for life around white dwarfs and brown dwarfs might look slim, they are not zero, the scientists cautioned. "I'm not arguing that all planets around brown dwarfs and white dwarfs are uninhabitable," Barnes said, "just that they have more hurdles to clear."
For instance, a planet might drift into the habitable zone of a white dwarf from a more distant orbit long after the formation of that dead star. It would still have to contend with tidal heating, but it would have avoided radiation that likely would have sterilized its surface.
"The biggest question in my mind is regarding the loss of water," Barnes added. "We don't have a good handle on that process, and also don't know how much water these planets could have to begin with. Therefore estimating the time it takes to lose all water, and hence be uninhabitable, is difficult to quantify at present. It could be that some of these planets retain enough water that, as the habitable zone reaches them, they could still support life."
"I believe that the topic of habitability of planets around brown dwarfs should be investigated more," said astrophysicist Emeline Bolmont at the Astrophysics Laboratory of Bordeaux in France, who did not take part in this study. "However, we would need observation missions to observe planets around brown dwarfs. Such a mission requires a long observation time, and brown dwarfs are very faint objects, so it will not be easy."
Still, Bolmont said he thought a proposal aiming to observe planets around brown dwarfs might be accepted in a few years. "If a planet were observed in transit, the [soon-to-be launched] James Webb Space Telescope would be able to probe its atmosphere and teach us a lot about its composition," Bolmont said.

More research is needed to understand how planets orbiting white dwarfs and brown dwarfs form, and "particularly the amount of water they form with," Barnes said. "We also need to understand how the high-energy radiation of brown dwarfs evolves with time. This is the energy that can remove water, but we don't have any idea how strong it can be, and how long it lasts."

A Rare Opportunity to Watch a Blue Straggler Forming by SHANNON HALL on JUNE 11, 2013

A unique and enigmatic variety of stars known as blue stragglers appear to defy the normal stellar aging process. Discovered in globular clusters, they appear much younger than the rest of the stellar population. Since their discovery in 1953, astronomers have been asking the question: how do these stars regain their youth?

For years, two theories have persisted. The first theory suggests that two stars collide, forming a single more massive star. The second theory proposes that blue stragglers emerge from binary pairs. As the more massive star evolves and expands, it blows material onto the smaller star. In both theories, the star grows steadily more massive and bluer – it regains its youth.
But now, a surprising finding may lend credence to the second theory. Astronomers at the Nicolaus Copernicus Astronomical Center in Poland recently observed a blue straggler caught in the midst of forming!
The binary system that was studied, known as M55-V60, is located within the globular cluster M55. Dr. Michal Rozyczka, one of the research scientists on the project, told Universe Today, “The system is a showcase example of a blue straggler formed via the theoretically predicted peaceful mass exchange between its components.”
The team used both photometric (the overall light from the system) and spectroscopic (the light spread out into a range of wavelengths) observations. The photometric data revealed the light curve – the change in brightness due to one star passing in front of the other – of the system. This provided evidence that the astronomers were looking at a binary system.
From the spectroscopic data, shifts in wavelength reveal the velocity (along the line of sight) of a source. The research team noted that the system’s center of mass was moving with respect to the binary system. This will occur in a semi-detached binary system, where mass transfers from one star to the other. As it does this, the center of mass will follow the mass-transfer.

From both photometric and spectroscopic observations (which covered more than 10 years!) the team was able to verify that this object is not only a binary, but a semi-detached binary, residing at the edge of M55.
The system is semi-detached with the less massive (secondary) component filling its Roche lobe,” explained Dr. Rozyczka. “The secondary has a tear like shape, with the tip of the tear directed toward the more massive primary. A stream of gas flows out of the tip along a curved path and hits the primary.”
How do we know that it is in fact a blue straggler? The simple answer is that the secondary star, with is gaining mass, appears bluer than normal. This blue straggler is clearly in the process of forming. It is the second observation of such a formation, with the first being V228 in the globular cluster: 47 Tuc.

This research verifies that semi-detached binaries are a viable formation mechanism for blue stragglers. The binary was discovered by happenstance, in a project aimed at determining accurate ages and distances of nearby clusters. It’s certainly a surprising result from the survey.