Most massive binary star identified

  
NASA, ESA, D. Lennon en E. Sabbi (ESA/STScI)

By NOVA, Leiden

Published: April 19, 2013

Astronomers have observed a binary star that potentially weighed 300 to 400 solar masses at birth. The present day total mass of the two stars is between 200 and 300 times that of the Sun, depending on its evolutionary stage, which possibly makes it the most massive binary star known to date.

The massive binary star R144 is located in an outer area of the star-forming region 30 Doradus in the Large Magellanic Cloud. A number of particularly bright stars can be found in the center of that region with a characteristic pattern of spectral lines. The masses of these so-called Wolf-Rayet stars are up to 250 times the mass of the Sun. R144 is the visually brightest light source of this type in the star-forming region 30 Doradus and radiates strongly in X-rays. This was an indication that R144 is a binary star. Scientists have now confirmed this presumption thanks to the discovery of periodic (orbital) changes in the spectrum.

Astronomers obtained spectra of R144 with the X-shooter spectrograph on the European Southern Observatory’s Very Large Telescope. X-shooter is one of the most sensitive spectrographs on Earth and can observe light from the near-ultraviolet to the near-infrared in one shot. “The identification of this candidate would have been a great challenge without X-shooter. This spectrograph makes observations a lot easier and much more efficient, especially because less observation time is required to cover a large spectral range,” said Hugues Sana from the University of Amsterdam.

The spectrum forms the fingerprint of a star. From the changing shape and position of the spectral lines, it becomes clear that R144 is a binary star. The spectral lines also suggest that the binary system is formed by two hydrogen-rich Wolf-Rayet stars with similar masses, and a current total mass of 200 to 300 solar masses. NGC 3603-A1 was formerly known as the most massive binary star, with a total mass that is equal to 212 times the mass of the Sun.

“It is a mystery how extremely massive stars form,” said Frank Tramper from the University of Amsterdam. “According to the most widely accepted theories, stars of hundreds of solar masses can only form in massive star clusters. The fact that R144 lies far out from the central star cluster in 30 Doradus is possibly an indication that these systems can form in isolation.”

“There is an alternative scenario for the formation of R144,” said Alex de Koter, also from the University of Amsterdam, “namely that R144 was formed in the central star cluster, but that it was ejected by dynamical interactions with other massive stars.” The team is already working on follow-up observations to determine whether R144 is indeed a “runaway” star to definitively establish its mass and its other physical properties in order to decide whether R144 really is the most massive double star discovered so far.

Dying supergiant stars implicated in hours-long gamma-ray bursts

GRB 111209A exploded on December 9, 2011. The blast produced high-energy emission for an astonishing seven hours, earning a record as the longest-duration GRB ever observed. This false-color image shows the event as captured by the X-ray Telescope aboard NASA's Swift satellite. //NASA/Swift/B. Gendre (ASDC/INAF-OAR/ARTEMIS)

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: April 17, 2013

Three unusually long-lasting stellar explosions discovered by NASA’s Swift satellite represent a previously unrecognized class of gamma-ray bursts (GRBs). Two international teams of astronomers studying these events conclude that they likely arose from the catastrophic death of supergiant stars hundreds of times larger than the Sun.

GRBs are the most luminous and mysterious explosions in the universe. The blasts emit surges of gamma rays — the most powerful form of light — as well as X-rays, and they produce afterglows that are visible at optical and radio energies. Swift, Fermi, and other spacecraft detect an average of about one GRB each day.

“We have seen thousands of gamma-ray bursts over the past four decades, but only now are we seeing a clear picture of just how extreme these extraordinary events can be,” said Bruce Gendre, a researcher now associated with the French National Center for Scientific Research who led this study while at the Italian Space Agency’s Science Data Center in Frascati, Italy.

Prior to Swift’s launch in 2004, satellite instruments were much less sensitive to GRBs that unfolded over comparatively long timescales.

Traditionally, astronomers have recognized two GRB types, short and long, based on the duration of the gamma-ray signal. Short bursts last two seconds or less and are thought to represent a merger of compact objects in a binary system, with the most likely suspects being neutron stars and black holes. Long GRBs may last anywhere from several seconds to several minutes, with typical durations falling between 20 and 50 seconds. These events are thought to be associated with the collapse of a star many times the Sun’s mass and the resulting birth of a new black hole.

Both scenarios give rise to powerful jets that propel matter at nearly the speed of light in opposite directions. As they interact with matter in and around the star, the jets produce a spike of high-energy light.

Gendre and his colleagues made a detailed study of GRB 111209A, which erupted December 9, 2011, using gamma-ray data from the Konus instrument on NASA’s Wind spacecraft, X-ray observations from Swift and the European Space Agency’s XMM-Newton satellite, and optical data from the TAROT robotic observatory in La Silla, Chile. The burst continued to produce high-energy emission for an astonishing seven hours, making it by far the longest-duration GRB ever recorded.

Another event, GRB 101225A, exploded on Christmas Day in 2010 and produced high-energy emission for at least two hours. Subsequently nicknamed the “Christmas burst,” the event’s distance was unknown, which led two teams to arrive at radically different physical interpretations. One group concluded the blast was caused by an asteroid or comet falling onto a neutron star within our galaxy. Another team determined that the burst was the outcome of a merger event in an exotic binary system located some 3.5 billion light-years away.

“We now know that the Christmas burst occurred much farther off, more than halfway across the observable universe, and was consequently far more powerful than these researchers imagined,” said Andrew Levan from the University of Warwick in Coventry, England.

Using the Gemini North Telescope in Hawaii, Levan and his team obtained a spectrum of the faint galaxy that hosted the Christmas burst. This enabled the scientists to identify emission lines of oxygen and hydrogen and determine how much these lines were displaced to lower energies compared to their appearance in a laboratory. This difference, known to astronomers as a redshift, places the burst some 7 billion light-years away.

As a part of this study, Levan’s team also examined 111209A and the more recent burst 121027A, which exploded October 27, 2012. All show similar X-ray, ultraviolet, and optical emission and all arose from the central regions of compact galaxies that were actively forming stars. The astronomers conclude that all three GRBs constitute a hitherto unrecognized group of “ultralong” bursts.

To account for the normal class of long GRBs, astronomers envision a star similar to the Sun’s size but with many times its mass. The mass must be high enough for the star to undergo an energy crisis, with its core ultimately running out of fuel and collapsing under its own weight to form a black hole. Some of the matter falling onto the nascent black hole becomes redirected into powerful jets that drill through the star, creating the gamma-ray spike, but because this burst is short-lived, the star must be comparatively small.

“Wolf-Rayet stars fit these requirements,” said Levan. “They are born with more than 25 times the Sun’s mass, but they burn so hot that they drive away their deep, outermost layer of hydrogen as an outflow we call a stellar wind.” Stripping away the star’s atmosphere leaves an object massive enough to form a black hole but small enough for the particle jets to drill all the way through in times typical of long GRBs.

Because ultralong GRBs persist for periods up to 100 times greater than long GRBs, they require a stellar source of correspondingly greater physical size. Both groups suggest that the likely candidate is a supergiant, a star with about 20 times the Sun’s mass that still retains its deep hydrogen atmosphere, making it hundreds of times the Sun’s diameter.

Gendre’s team goes further, suggesting that GRB 111209A marked the death of a blue supergiant containing relatively modest amounts of elements heavier than helium, which astronomers call metals.

“The metal content of a massive star controls the strength of its stellar wind, which determines how much of the hydrogen atmosphere it retains as it grows older,” Gendre said. The star’s deep hydrogen envelope would take hours to complete its fall into the black hole, which would provide a long-lived fuel source to power an ultralong GRB jet.

Metal content also plays a strong role in the development of long GRBs, according to a detailed study presented by John Graham and Andrew Fruchter, both from the Space Telescope Science Institute in Baltimore, Maryland.

Stars make heavy elements throughout their energy-producing lives and during supernova explosions, and each generation of stars enriches interstellar gas with a greater proportion of them. While astronomers have noted that long GRBs occur much more frequently in metal-poor galaxies, a few of them have suggested that this pattern is not intrinsic to the stars and their environments.

To examine this possibility, Graham and Fruchter developed a novel method that allowed them to compare galaxies by their underlying rates of star formation. They then examined galaxies that served as hosts for long GRBs and various types of supernovae as well as a control sample of 20,000 typical galaxies in the Sloan Digital Sky Survey.

The astronomers found that 75 percent of long GRBs occurred among the 10 percent of star formation with the lowest metal content. While the study found a few long GRBs in environments with high-metal content, like our galaxy, these occur at only about 4 percent the rate seen in low-metal environments per unit of underlying star formation.

“Most stars form in metal-rich environments, and this has a side effect of decreasing the prevalence of long GRBs as the universe grows older,” Graham said. “And while a nearby long GRB would be catastrophic to life on Earth, our study shows that galaxies like our own are much less likely to produce them.”

The astronomers suspect this pattern reflects a difference in how well a massive star manages to retain its rotation speed. Rising metal content means stronger stellar winds. As these winds push material off the star’s surface, the star’s rotation gradually decreases in much the same way as a spinning ice skater slows when she extends her arms. Stars with more rapid rotation may be more likely to produce a long GRB.

Graham and Fruchter hypothesize that the few long GRBs found in high-metal environments received an assist from the presence of a nearby companion star. By feeding mass — and with it, rotational energy — onto the star that explodes, a companion serves as the physical equivalent of someone pushing a slowly spinning ice skater back up to a higher rotational speed.

GRB 101225A, better known as the "Christmas burst," was an unusually long-lasting gamma-ray burst. Because its distance was not measured, astronomers came up with two radically different interpretations. In the first, a solitary neutron star in our  galaxy shredded and accreted an approaching comet-like body. In the second, a neutron star is engulfed by, spirals into, and merges with an evolved giant star in a distant galaxy. Now, thanks to a measurement of the Christmas burst’s host galaxy, astronomers have determined that it represented the collapse and explosion of a supergiant star hundreds of times larger than the Sun

Kepler discovers smallest "habitable zone" planets

Fig : The diagram compares the planets of the inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a star classified as a K2 dwarf, measuring just two-thirds the size of the Sun and only one-fifth as bright. At 7 billion years old, the star is somewhat older than the Sun. The green areas mark each star's habitable zone. (NASA Ames/JPL-Caltech)

By NASA Headquarters, Washington, D.C., NASA's Ames Research Center in Moffett Field, California 

Published: April 19, 2013

NASA’s Kepler mission has discovered two new planetary systems that include three super-Earth-sized planets in the “habitable zone,” the range of distance from a star where the surface temperature of an orbiting planet might be suitable for liquid water.

The Kepler-62 system has five planets: 62b, 62c, 62d, 62e, and 62f. The Kepler-69 system has two planets: 69b and 69c. Kepler-62e, 62f, and 69c are the super-Earth-sized planets.

Two of the newly discovered planets orbit a star smaller and cooler than the Sun. Kepler-62f is only 40 percent larger than Earth, making it the exoplanet closest to the size of our planet known in the habitable zone of another star. Kepler-62f is likely to have a rocky composition. Kepler-62e orbits on the inner edge of the habitable zone and is roughly 60 percent larger than Earth.

The third planet, Kepler-69c, is 70 percent larger than Earth and orbits in the habitable zone of a star similar to our Sun. Astronomers are uncertain about the composition of Kepler-69c, but its orbit of 242 days around a Sun-like star resembles that of our neighboring planet Venus.

Scientists do not know whether life could exist on the newfound planets, but their discovery signals that astronomers are another step closer to finding a world similar to Earth around a star like our Sun.


“The Kepler spacecraft has certainly turned out to be a rock star of science,” said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington, D.C. “The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity.”

The Kepler space telescope, which simultaneously and continuously measures the brightness of more than 150,000 stars, is NASA’s first mission capable of detecting Earth-sized planets around stars like our Sun.

Orbiting its star every 122 days, Kepler-62e was the first of these habitable zone planets identified. Kepler-62f, with an orbital period of 267 days, was later found by Eric Agol, associate professor of astronomy at the University of Washington.

The scientists have measure the size of Kepler-62f is now measured, but they have yet to determine its mass and composition. Based on previous studies of rocky exoplanets similar in size, however, astronomers are able to estimate its mass by association.

“The detection and confirmation of planets is an enormously collaborative effort of talent and resources, and requires expertise from across the scientific community to produce these tremendous results,” said William Borucki, Kepler science principal investigator at NASA’s Ames Research Center at Moffett Field, California. “Kepler has brought a resurgence of astronomical discoveries, and we are making excellent progress toward determining if planets like ours are the exception or the rule.”

The two habitable zone worlds orbiting Kepler-62 have three companions in orbits closer to their star, two larger than the size of Earth and one about the size of Mars. Kepler-62b, Kepler-62c, and Kepler-62d orbit every five, 12, and 18 days, respectively, making them very hot and inhospitable for life as we know it.

The five planets of the Kepler-62 system orbit a star classified as a K2 dwarf, measuring just two-thirds the size of the Sun and only one-fifth as bright. At 7 billion years old, the star is somewhat older than the Sun. It is about 1,200 light-years from Earth in the constellation Lyra.

A companion to Kepler-69c, known as Kepler-69b, is more than twice the size of Earth and whizzes around its star every 13 days. The Kepler-69 planets’ host star belongs to the same class as our Sun, called G-type. It is 93 percent the size of the Sun and 80 percent as luminous; it's located approximately 2,700 light-years from Earth in the constellation Cygnus.

“We only know of one star that hosts a planet with life — the Sun. Finding a planet in the habitable zone around a star like our Sun is a significant milestone toward finding truly Earth-like planets,” said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, California, and lead author of the Kepler-69 system discovery.

When a planet candidate transits, or passes in front of, the star from the spacecraft’s vantage point, a percentage of light from the star is blocked. The resulting dip in the brightness of the starlight reveals the transiting planet’s size relative to its star. Using the transit method, Kepler has detected 2,740 candidates. Using various analysis techniques, ground telescopes and other space assets, astronomers have confirmed 122 as planets.

Early in the mission, the Kepler telescope primarily found large gas giants in very close orbits of their stars. Known as “hot Jupiters,” these worlds are easier to detect due to their size and very short orbital periods. Earth would take three years to accomplish the three transits required to be accepted as a planet candidate. As Kepler continues to observe, transit signals of habitable zone planets the size of Earth orbiting stars like the Sun will begin to emerge.

 

Ice cloud over Titan's south pole

Fig :  The recently formed south polar vortex stands out in the color-swaddled atmosphere of Saturn's largest moon, Titan, in this natural color view from NASA's Cassini spacecraft. NASA/JPL-Caltech/Space Science Institute

By Jet Propulsion Laboratory, Pasadena, California, NASA's Goddard Space Flight Center, Greenbelt, Maryland

 Published: April 12, 2013

An ice cloud taking shape over Titan’s south pole is the latest sign that the change of seasons is setting off a cascade of radical changes in the atmosphere of Saturn’s largest moon. Made from an unknown ice, this type of cloud has long hung over Titan’s north pole, where it is now fading, according to observations made by the composite infrared spectrometer (CIRS) on NASA’s Cassini spacecraft.

“We associate this particular kind of ice cloud with winter weather on Titan, and this is the first time we have detected it anywhere but the north pole,” said Donald E. Jennings of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

The southern ice cloud, which shows up in the far infrared part of the light spectrum, is evidence that an important pattern of global air circulation on Titan has reversed direction. When Cassini first observed the circulation pattern, warm air from the southern hemisphere was rising high in the atmosphere and was transported to the cold north pole. There, the air cooled and sank down to lower layers of the atmosphere and formed ice clouds. A similar pattern, called a Hadley cell, carries warm, moist air from Earth’s tropics to the cooler middle latitudes.

Based on modeling, scientists had long predicted a reversal of this circulation once Titan’s north pole began to warm and its south pole began to cool. The official transition from winter to spring at Titan’s north pole occurred in August 2009. But because each of the moon’s seasons lasts about 7.5 Earth years, researchers still did not know exactly when this reversal would happen or how long it would take.

The first signs of the reversal came in data acquired in early 2012, which came shortly after the start of southern fall on Titan, when Cassini images and visual and infrared mapping spectrometer data revealed the presence of a high-altitude “haze hood” and a swirling polar vortex at the south pole. Both features have long been associated with the cold north pole. Later, Cassini scientists reported that infrared observations of Titan’s winds and temperatures made by CIRS had provided definitive evidence of air sinking, rather than upwelling, at the south pole. By looking back through the data, the team narrowed down the change in circulation to within six months of the 2009 equinox.

Despite the new activity at the south pole, the southern ice cloud had not appeared yet. CIRS didn’t detect it until about July 2012, a few months after the haze and vortex were spotted in the south.

“This lag makes sense because first the new circulation pattern has to bring loads and loads of gases to the south pole. Then, the air has to sink. The ices have to condense. And the pole has to be under enough shadow to protect the vapors that condense to form those ices,” said Carrie Anderson from Goddard.

At first blush, the southern ice cloud seems to be building rapidly. The northern ice cloud, on the other hand, was present when Cassini first arrived and has been slowly fading the entire time the spacecraft has been observing it.

So far, the identity of the ice in these clouds has eluded scientists, though they have ruled out simple chemicals, such as methane, ethane and hydrogen cyanide, which are typically associated with Titan. One possibility is that “species X,” as some team members call the ice, could be a mixture of organic compounds.

“What’s happening at Titan’s poles has some analogy to Earth and to our ozone holes,” said F. Michael Flasar of Goddard. “And on Earth, the ices in the high polar clouds aren’t just window dressing: They play a role in releasing the chlorine that destroys ozone. How this affects Titan chemistry is still unknown. So it’s important to learn as much as we can about this phenomenon, wherever we find it.”