Cassini Finds Mingling Moons May Share a Dark Past

This collage showcases several of Saturn's moons

Credit: NASA/JPL/Space Science Institute

Despite the incredible diversity of Saturn's icy moons, theirs is a story of great interaction. Some of them are pock-marked, some seemingly dirty, others pristine, one spongy, one two-faced, some still spewing with activity and some seeming to be captured from the far reaches of the solar system. Yet many of them have a common thread -- black "stuff" coating their surfaces.

"We are beginning to unravel the mysteries of these different and strange moons," said Rosaly Lopes, Cassini scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. She coordinated a special section of 14 papers about Saturn's icy moons that appears in the February issue of the journal Icarus. Taken together, the papers bring an idea that Cassini scientist Bonnie Buratti calls "the ecology of the Saturn system" to the forefront. "Ecology is about your entire environment -- not just one body, but how they all interact," said Buratti. "The Saturn system is really interesting, and if you look at the surfaces of the moons, they seem to be altered in ways that aren't intrinsic to them. There seems to be some transport in this system."

Though the details of that transport are not yet clear, mounting evidence suggests that some mechanism has spread the mysterious dark material found on several of the moons from one to another; the material may even have a common cometary origin. Along those lines, several of the new papers focus on similarities between the dark material found on different moons -- on Hyperion and Iapetus, for example, or between Phoebe and Iapetus.

Roger Clark of the U.S. Geological Survey in Denver goes further, saying, "We see the same spectral signature on all the moons that have coatings of dark material." Clark is lead author of one of the new papers, which focuses on Saturn's moon Dione. His team found the dark material there to be extremely fine-grained, making up only a very thin layer on the moon's trailing side. Its distribution and composition, as measured by the Cassini visual and infrared mapping spectrometer, indicate that the dark material is not native to Dione. And scientists see many of the same signatures there that appear on the moons Phoebe, Iapetus, Hyperion and Epimetheus, and also in Saturn's F-ring.

As for where this material comes from and what the dark material is, Clark said, "It's a mystery, which makes it intriguing. We're still trying to find the exact match." The visual and infrared spectrometer detected unique absorption bands in the dark material within the Saturn system, which scientists have not seen anywhere else in the solar system. "The data keep getting better and better," he said. "We're ruling things out and figuring out pieces." So far, the team has identified bound water and, possibly, ammonia in the dark material.

Ongoing geologic activity is another component of Saturn's ecology as some of the moons continue to feed the planet's rings, which in turn affect many of the moons.

Clark's team reports tentative evidence to support the hypothesis presented earlier this year that Dione is still geologically active. In one series of observations, the infrared spectrometer detected a cloud of methane and water ice encircling Dione in its orbit within the outer portions of Saturn's E-ring.

Of course the big story is the icy plumes spewing from the warm, south polar region of Enceladus. These plumes are believed to be feeding the E-ring. A paper led by Frank Postberg of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, says there are traces of organic compounds or silicate materials within the water ice-dominated E-ring, close to Enceladus. This implies that the moon's rocky core and liquid water are dynamically interacting. The finding could bolster a theory that Dennis Matson and Julie Castillo of JPL put forth this year, which said that a warm, organic brew might lie just below Enceladus' surface.

Cassini's next close study of an icy moon is the highly-anticipated flyby of Enceladus scheduled for March 12. During that flyby, Cassini will pass by the active moon at a distance of only 50 kilometers (30 miles) at its point of closest approach, and at a distance of around 200 kilometers (120 miles) when it passes through the plumes.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter was designed, developed and assembled at JPL.

Contacts: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

carolina.martinez@jpl.nasa.gov

Hubble Discovers 67 New Gravitationally Lensed Galaxies in the Distant Universe

Credit: NASA, ESA, C. Faure (Zentrum Für Astronomie, University of Heidelberg),
J.P. Kneib (Laboratoire d'Astrophysique de Marseille)

Astronomers using NASA's Hubble Space Telescope have compiled a large catalog of gravitational lenses in the distant universe. The catalog contains 67 new gravitationally lensed galaxy images found around massive elliptical and lenticular-shaped galaxies. This sample demonstrates the rich diversity of strong gravitational lenses. If this sample is representative, there would be nearly half a million similar gravitational lenses over the whole sky.

The lenses come from a recently completed, large set of observations as part of a huge project to survey a single 1.6-square-degree field of sky (nine times the area of the full Moon) with several space-based and Earth-based observatories. The COSMOS project, led by Nick Scoville at the California Institute of Technology, used observations from several observatories including the Hubble Space Telescope, the Spitzer Space Telescope, the XMM-Newton spacecraft, the Chandra X-ray Observatory, the Very Large Telescope (VLT), the Subaru Telescope, and the Canada-France-Hawaii Telescope.

A team of European astronomers led by Jean-Paul Kneib (Laboratoire d'Astrophysique de Marseille) and Cecile Faure (Zentrum für Astronomie, University of Heidelberg) analyzed the results from Hubble's Advanced Camera for Surveys (ACS). From ACS high-resolution images, complemented by the extensive ground-based follow-up observations, astronomers have identified 67 strong gravitationally lensed galaxies. These lenses were found around very massive galaxies that are usually elliptical or lenticular in shape and have a deficiency of gas and dust.

The strong lensing produced by massive galaxies are much more common than the usual giant "arc" gravitationally lensed galaxies that Hubble has previously observed; but they are generally more difficult to find as they extend over a smaller area and have a wide variety of shapes.

Gravitational lensing occurs when light traveling toward us from a distant galaxy is magnified and distorted as it encounters a massive object between the galaxy and us. These gravitational lenses often allow astronomers to peer much farther back into the early universe than they would normally be able to do.

The massive objects that create the lenses are usually huge clusters of massive galaxies. "We typically see the gravitational lens create a series of bright arcs or spots around a galaxy cluster. What we are observing here is a similar effect but on a much smaller scale — happening only around a single but very massive galaxy," said Kneib.

Of the 67 gravitational lenses identified in the COSMOS survey, the most impressive lenses show the distorted and warped light of one or two background galaxies. At least four of the lenses produce Einstein rings, a complete circular image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the Hubble Space Telescope are all aligned perfectly.

Hubble astronomers went through a unique process to identify these incredible natural lenses. First, possible galaxies were identified from a galaxy catalog, comprising more than 2 million galaxies. "We then had to look through each individual COSMOS image by eye and identify any potential strong gravitational lenses," said Faure. Finally, checks were made to see if the foreground galaxy and the lensed galaxy were really different or just one galaxy with an odd shape. "With this sample of gravitational systems identified by the human eye, we now plan to use the sample of lenses to train robot software to find more of these lenses across the entire Hubble image archive, and we may find even more strong lensing systems in the COSMOS field," added Kneib.

The new results confirm that the universe is filled with gravitational lensing systems. Extrapolating these new findings to the whole sky predicts no less than half a million similar lenses in total.

The study of these gravitational lenses will give astronomers a first-rate opportunity to probe the dark matter distribution around galactic lenses. Once astronomers find even larger numbers of these smaller, stronger lenses, they can be used to create a census of galaxy masses in the universe to test the predictions of cosmological models.

The research team consists of Cecile Faure (Zentrum Für Astronomie Heidelberg), Jean- Paul Kneib (Laboratoire d'Astrophysique de Marseille & the California Institute of Technology), Giovanni Covone (Laboratoire d'Astrophysique de Marseille & INAF, Osservatorio Astronomico di Capodimonte), Lidia Tasca (Laboratoire d'Astrophysique de Marseille), Alexie Leauthaud (Laboratoire d'Astrophysique de Marseille), Peter Capak (California Institute of Technology), Knud Jahnke (Max-Planck-Institut für Astronomie), Vernesa Smolcic (Max-Planck-Institut für Astronomie), Sylvain de la Torre (Laboratoire d'Astrophysique de Marseille), Richard Ellis (California Institute of Technology), Alexis Finoguenov (Max-Planck-Institut für extraterrestrische Physik), Catherine Heymans (Department of Physics & Astronomy, University of British Columbia), Anton Koekemoer (Space Telescope Science Institute), Oliver Le Fevre (Laboratoire d'Astrophysique de Marseille), Richard Massey (California Institute of Technology), Yannick Mellier (Institut d'Astrophysique de Paris), Alexandre Refregier (Service d'Astrophysique, CEA/Saclay), Jason Rhodes (California Institute of Technology), Nick Scoville (California Institute of Technology), Eva Schinnerer (Max- Planck-Institut für Astronomie), James Taylor (California Institute of Technology & Department of Physics and Astronomy, University of Waterloo), Ludovic Van Waerbeke (Department of Physics & Astronomy, University of British Columbia), and Jakob Walcher (Laboratoire d'Astrophysique de Marseille).

Many, Perhaps Most, Nearby Sun-Like Stars May Form Rocky Planets

This artist's concept illustrates the idea that rocky, terrestrial worlds like the inner planets in our solar system may be plentiful, and diverse, in the universe.

Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
Rocky, Terrestrial Worlds

Astronomers have discovered that terrestrial planets might form around many, if not most, of the nearby sun-like stars in our galaxy. These new results suggest that worlds with potential for life might be more common than we thought.

University of Arizona, Tucson, astronomer Michael Meyer and his colleagues used NASA's Spitzer Space Telescope to determine whether planetary systems like ours are common or rare in our Milky Way galaxy. They found that at least 20 percent, and possibly as many as 60 percent, of stars similar to the sun are candidates for forming rocky planets.

Meyer is presenting the findings at the annual meeting of the American Association for the Advancement of Science in Boston. The results appear in the Feb. 1 issue of Astrophysical Journal Letters.

The astronomers used Spitzer to survey six sets of stars, grouped depending on their age, with masses comparable to our sun. The sun is about 4.6 billion years old. "We wanted to study the evolution of the gas and dust around stars similar to the sun and compare the results with what we think the solar system looked like at earlier stages during its evolution," Meyer said.

The Spitzer telescope does not detect planets directly. Instead it detects dust -- the rubble left over from collisions as planets form -- at a range of infrared wavelengths. The hottest dust is detected at the shortest wavelengths, between 3.6 microns and 8 microns. Cool dust is detected at the longest wavelengths, between 70 microns and 160 microns. Warm dust can be traced at 24-micron wavelengths. Because dust closer to the star is hotter than dust farther from the star, the "warm" dust likely traces material orbiting the star at distances comparable to the distance between Earth and Jupiter.

"We found that about 10 to 20 percent of the stars in each of the four youngest age groups shows 24-micron emission due to dust," Meyer said. "But we don't often see warm dust around stars older than 300 million years. The frequency just drops off.

"That's comparable to the time scales thought to span the formation and dynamical evolution of our own solar system," he added. "Theoretical models and meteoritic data suggest that Earth formed over 10 to 50 million years from collisions between smaller bodies."

In a separate study, Thayne Currie and Scott Kenyon of the Smithsonian Astrophysical Observatory, Cambridge, Mass., and their colleagues also found evidence of dust from terrestrial planet formation around stars from 10 to 30 million years old. "These observations suggest that whatever led to the formation of Earth could be occurring around many stars between three million and 300 million years old," Meyer said.

Kenyon and Ben Bromley of the University of Utah, Salt Lake City, have developed planet formation models that provide a plausible scenario. Their models predict warm dust would be detected at 24-micron wavelengths as small rocky bodies collide and merge. "Our work suggests that the warm dust Meyer and colleagues detect is a natural outcome of rocky planet formation. We predict a higher frequency of dust emission for the younger stars, just as Spitzer observes," said Kenyon.

The numbers on how many stars form planets are ambiguous because there's more than one way to interpret the Spitzer data, Meyer said. The warm-dust emission that Spitzer observed around 20 percent of the youngest cohort of stars could persist as the stars age. That is, the warm dust generated by collisions around stars three to 10 million years old could carry over and show up as warm dust emission seen around stars in the 10- to 30- million-year-old range and so on. Interpreting the data this way, about one out of five sun-like stars is potentially planet-forming, Meyer said.

There's another way to interpret the data. "An optimistic scenario would suggest that the biggest, most massive disks would undergo the runaway collision process first and assemble their planets quickly. That's what we could be seeing in the youngest stars. Their disks live hard and die young, shining brightly early on, then fading," Meyer said. "However, smaller, less massive disks will light up later. Planet formation in this case is delayed because there are fewer particles to collide with each other."

If this is correct and the most massive disks form their planets first and the wimpiest disks take 10 to 100 times longer, then up to 62 percent of the surveyed stars have formed, or may be forming, planets. "The correct answer probably lies somewhere between the pessimistic case of less than 20 percent and optimistic case of more than 60 percent," Meyer said.

The next critical test of the assertion that terrestrial planets like Earth could be common around stars like the sun will come next year with the launch of NASA's Kepler mission.

Meyer's 13 co-authors include John Carpenter of the California Institute of Technology in Pasadena. NASA's Jet Propulsion Laboratory in Pasadena manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Caltech manages JPL for NASA.

Viewer's Guide: Total Lunar Eclipse Feb. 20

How a lunar eclipse works.

Why the moon sometimes glows red during a total lunar eclipse.

From North America, look southeast at mid-eclipse to see the Moon nestled between Regulus and Saturn.

On Wednesday night, Feb. 20, for the third time in the past year, the moon will become completely immersed in the Earth's shadow, resulting in a total lunar eclipse.

As is the case with all lunar eclipses, the region of visibility will encompass more than half of our planet. Nearly a billion people in the Western Hemisphere, more than 1.5 billion in Europe and Africa, and perhaps another half-billion in western Asia will be able to watch — weather permitting — as the brilliant mid-winter full moon becomes a shadow of its former self and morphs into a glowing coppery ball.

Almost everyone in the Americas and Western Europe will have a beautiful view of this eclipse if bad weather doesn't spoil the show. The moon will be high in a dark evening sky as viewed from most of the United States and Canada while most people are still awake and about.

Local conditions

The only problematic area will be along the Oregon and northern California coast, where the first partial stage of the eclipse will already be under way when the moon rises and the sun sets on Wednesday evening. But if you have an open view low to the east, even this situation will only add to the drama, for as twilight fades, these far-Westerners will see the shadow-bitten moon coming into stark view low above the landscape. And by late twilight observers will have a fine view of the totally eclipsed lunar disk glowing red and dim low in the eastern sky.

Alaskans will also see the moon rise during the eclipse; in fact, much of western Alaska will see the moon rise while completely immersed in the Earth's shadow.

For Hawaiians, moonrise unfortunately comes just after the end of totality, with the moon gradually ascending the sky and its gradual emergence from the shadow readily visible.

Western Europe and Africa also will get a good view of the eclipse, but at a less convenient time: before dawn on Thursday morning, Feb. 21.

Total triangle

Moreover, this eclipse comes with a rare bonus. The planet Saturn (magnitude +0.2) and the bright bluish star, Regulus (magnitude +1.4) will form a broad triangle with the moon's ruddy disk.

Careful watchers will notice the moon changing its position with respect to the star and planet as it moves eastward through the Earth's shadow.

Saturn's position will also depend somewhat on your location. Seen from North America, the great ringed planet will be 3.5 degrees above and to the left of the moon's center at mid-totality (3:26 Universal time February 21st). At the same moment, Regulus will sit just 2.8 degrees above and to the right of the moon.

Some old-time astronomy buffs may remember from 40 years ago a total lunar eclipse with the moon sitting only about a degree from Spica — a gorgeous celestial tableau! More recently, in 1996, a totally eclipsed moon passed within 2 degrees of Saturn.

But this upcoming double event will be the only one of its kind occurring within the next millennium!

Colors and brightness

There is nothing complicated about how to view this celestial spectacle. Unlike an eclipse of the sun, which necessitates special viewing precautions in order to avoid eye damage, an eclipse of the moon is perfectly safe to watch. All you'll need to watch are your eyes, but binoculars or a telescope will give a much nicer view.

A careful description of the colors seen on the totally eclipsed moon and their changes is valuable. The hues depend on the optical equipment used, usually appearing more vivid with the naked eye than in telescopes. The French astronomer, Andre Danjon, introduced the following five-point scale of lunar luminosity ("L") to classify eclipses:

L = 0: Very dark eclipse, moon almost invisible, especially in mid-totality.

L = 1: Dark eclipse, gray or brownish coloration, details distinguishable only with difficulty.

L = 2: Deep red or rust-colored eclipse, with a very dark central part in the shadow, and outer edge of the umbra relatively bright.

L = 3: Brick red eclipse, usually with a bright or yellow rim to the shadow.

L = 4: Very bright copper-red or orange eclipse, with a bluish very bright shadow rim.

Examine the moon at mid-totality and also near the beginning and end of totality to get an impression of both the inner and outer umbra.

At mid-totality, the darkness of the sky is very impressive. Faint stars, which were completely washed-out by the brilliant moonlight prior to the eclipse, become visible and the surrounding landscape takes on a somber hue. As totality ends, the eastern edge of the moon begins to emerge from the umbra, and the sequence of events repeats in reverse order until the spectacle is over.

Unless airborne volcanic aerosols or other unusual atmospheric effects influence its appearance, the moon's disk should appear moderately bright, especially right around the beginning and end of totality. The lower part of the moon will likely appear brightest and glowing a ruddy or coppery hue, while the upper half of the moon should look more gray or chocolate in color.

Eclipse schedule

The eclipse will begin when the moon enters the faint outer portion, or penumbra of the Earth's shadow. The penumbra, however, is all but invisible to the eye until the moon becomes deeply immersed in it. Sharp-eyed viewers may get their first glimpse of the penumbra as a delicate shading on the left part of the moon's disk about 20 minutes before the start of the partial eclipse (when the round edge of the central shadow or umbra, first touches the moon's left edge). During the partial eclipse, the penumbra should be readily visible as a dusky border to the dark umbral shadow.

The moon will enter Earth's much darker umbral shadow at 1:43 on Feb. 21 by Greenwich or Universal time, which is 8:43 p.m. on Feb. 20 in the Eastern time zone, 7:43 p.m. Central time, 6:43 p.m. Mountain time and 5:43 p.m. Pacific time.

Seventy-eight minutes later the moon is entirely within the shadow, and sails on within it for 51 minutes (about average for a total lunar eclipse), until it begins to find its way out at the lower left (southeastern) edge.

The moon be completely free of the umbra by 9:09 p.m. Pacific time or 12:09 a.m. (Feb. 21) Eastern time.

The vaguer shading of the inner penumbra can continue to be readily detected for perhaps another 20 minutes or so after the end of umbral eclipse. Thus, the whole experience ends toward 12:30 a.m. for the East (with the re-brightened moon now sloping down along the high arc it describes across the sky), or during the mid-evening hours for the West.

For Europe and Africa, the mid-point of this eclipse occurs roughly between midnight and dawn on the morning of Feb. 21, and as such the moon will still be well placed in the western sky. At the moment of mid-totality (3:26 UT), the moon will stand directly overhead from a point in the Atlantic Ocean roughly several hundred miles to the northeast of the coast of Suriname.

There will be a partial eclipse of the moon that will be visible across much of Europe and Asia on the night of Aug. 16-17. About 81 percent of the moon's diameter will become immersed in the umbra, leaving only the upper part of the moon visible.

In 2009, there will be four lunar eclipses, one a slight partial and the three others which will be of the penumbral variety meaning that at best only a vague hint of a light shading or smudginess on the moon's disk might be detected — if anything at all.

But not until Dec. 21, 2010 will there be another total lunar eclipse; that one too will again favor the Americas.

So although we've had a veritable plethora of total eclipses of late, keep in mind that after next Wednesday, you'll have to wait almost three years until your next chance to see another.

By Joe Rao SPACE.com
Skywatching Columnist

Mars Water Was Very Salty

BOSTON - If there was life in surface water on Mars early on, it might have enjoyed a very buoyant, salty ride, if it could thrive in such a hostile environment at all, new research suggests.

Minerals in sedimentary rocks found at Mars' Meridiani Planum by NASA's Opportunity rover suggest they formed in extremely salty water, even saltier than the oceans on Earth, said Andrew Knoll of Harvard University, who is part of a team of scientists taking a closer look at the geological data collected by the Mars Exploration Rovers mission.

Water was definitely present at least for short periods of time at Meridiani Planum, he said, but no one has known how habitable it might have been nor whether it was around long enough for life to take hold and endure.

So Knoll and his colleagues looked at data that reveal the chemistry of the salts in the rocks there as a gauge of the salinity of the ancient water. That allowed the scientists to estimate how salty the brines were at the time the minerals were deposited.

"The punchline is it was really salty," Knoll told a gathering of reporters here at the annual meeting of the American Association for the Advancement of Science, "salty enough that only a handful of terrestrial organisms would have a ghost of chance of surviving there when conditions were at their best."

Constrains the possibilities

There are a couple dozen organisms on Earth that can tolerate the salinity levels determined by Knoll and his team to have been present when the Meridiani Planum rocks were deposited, he said.

The research, to be presented today in detail to scientists as well, is currently under review at a top research journal, Knoll said.

Knoll and his colleagues also have found several different classes of minerals that reveal how little the rocks at Meridiani Planum were altered since they were exposed to the elements, he said. Water at this location was "rare and transient," according to the research, detailed in an online paper in the Journal of Geophysical Research — Planets.

The best era during which to look for evidence of Martian life would be in Mars' earliest history, Knoll said, that is, during the first 500 million to 600 million years, before the Meridiani Planum rocks were deposited.

The new findings bring scientists no closer to determining whether there was ever microbial life on Mars, but they do "constrain our thinking about life on Mars," Knoll said, painting a picture of an environment that is very "forbidding" for life.

Double whammy

Previously, Mars mission data have revealed that liquid water at Meridiani Planum was very acidic.

"So it is doubly bad if it's acidic and salty," Knoll said.

"I'm not sure the effects are necessarily additive, but certainly there are limits to the physiological ways that microorganisms can adapt to tolerate acidity," he said. "There are also limits to ways that they can adapt to tolerate highly saline environments and those are completely different physiological systems. In a sense, it makes it harder to adapt that there are two biochemical systems that would need adapting."

Scientists are more optimistic that life might have had a chance on Mars in its early history compared to later on. In earlier times, Mars' environment was probably wetter, less oxidizing (a condition that is challenging for most life) and less acidic, but "that doesn't mean it was terrific. It just wasn't as bad as it got later," Knoll said.

Meanwhile, Mars scientists and mission engineers gathered at the AAAS meeting today also looked forward to the May 25 arrival of NASA's Phoenix Lander at Mars' north pole and discussed work on the Mars Science Laboratory (MSL) mission, set to launch in September 2009.

MSL will carry a new rover to Mars that is far more complex and sophisticated, and five times heavier, than each of the MER rovers on the red planet now. The MER rovers are learning more about the habitability of the planet in the distant past, said Richard Cook, MER project manager.

Instruments on MSL will focus on detecting organic material at the planet and collecting more precise data on the minerals at Mars.

As for the MER rovers, Spirit is hunkered down for winter, parked on a north-facing slope and serving as a weather station to monitor a nearby dune field. And Opportunity is descending down the wall of Victoria Crater, investigating its finely layered rock stacks, made mostly of sulphate salts, said Steve Squyres of Cornell University, head of the science team for the MER mission.

The rovers nearly died last summer during intense dust storms on Mars that nearly stole all their ability to gather power, but they again outperformed expectations and endured. In fact, it looks good for them to be operational when Phoenix arrives in late spring.

"The rovers have lasted so long that I am never willing to make predictions about how long these things will last," Squyres said. "... It's been four years. You do the math. I think it's pretty likely."

By Robin Lloyd
Staff Writer
www.space.com

Astronomers Find One of the Youngest and Brightest Galaxies in the Early Universe

Credit: NASA; ESA; L. Bradley (Johns Hopkins University);
R. Bouwens (University of California, Santa Cruz); H. Ford (Johns Hopkins University);
and G. Illingworth (University of California, Santa Cruz)

NASA's Hubble and Spitzer space telescopes, with a boost from a natural "zoom lens," have uncovered what may be one of the youngest and brightest galaxies ever seen in the middle of the cosmic "dark ages," just 700 million years after the beginning of our universe.

The detailed images from Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) reveal an infant galaxy, dubbed A1689-zD1, undergoing a firestorm of star birth during the dark ages, a time shortly after the Big Bang but before the first stars reheated the cold, dark universe. Images from NASA's Spitzer Space Telescope's Infrared Array Camera provided strong additional evidence that it was a young star- forming galaxy in the dark ages.

"We certainly were surprised to find such a bright young galaxy 12.8 billion years in the past," said astronomer Garth Illingworth of the University of California, Santa Cruz, and a member of the research team. "This is the most detailed look to date at an object so far back in time."

"The Hubble images yield insight into the galaxy's structure that we cannot get with any other telescope," added astronomer Rychard Bouwens of the University of California, Santa Cruz, one of the co-discoverers of this galaxy.

The new images should offer insights into the formative years of galaxy birth and evolution and yield information on the types of objects that may have contributed to ending the dark ages. The faraway galaxy also is an ideal target for Hubble's successor, the James Webb Space Telescope (JWST), scheduled to launch in 2013.

During its lifetime, the Hubble telescope has peered ever farther back in time, viewing galaxies at successively younger stages of evolution. These snapshots have helped astronomers create a scrapbook of galaxies from infancy to adulthood. The new Hubble and Spitzer images of A1689-zD1 show a time when galaxies were in their infancy.

Current theory holds that the dark ages began about 400,000 years after the Big Bang, as matter in the expanding universe cooled and formed clouds of cold hydrogen. These cold clouds pervaded the universe like a thick fog.

At some point during this era, stars and galaxies started to form. Their collective light reheated the foggy, cold hydrogen, ending the dark ages about a billion years after the Big Bang.

"This galaxy presumably is one of the many galaxies that helped end the dark ages," said astronomer Larry Bradley of Johns Hopkins University in Baltimore, Md., and leader of the study. "Astronomers are fairly certain that high-energy objects such as quasars did not provide enough energy to end the dark ages of the universe. But many young star- forming galaxies may have produced enough energy to end it."

The galaxy is so far away it did not appear in images taken with Hubble's Advanced Camera for Surveys, because its light is stretched to invisible infrared wavelengths by the universe's expansion. It took Hubble's NICMOS, Spitzer, and a trick of nature called gravitational lensing to see the faraway galaxy.

The astronomers used a relatively nearby massive cluster of galaxies known as Abell 1689, roughly 2.2 billion light-years away, to magnify the light from the more distant galaxy directly behind it. This natural telescope is called a gravitational lens.

Though the diffuse light of the faraway object is nearly impossible to see, gravitational lensing has increased its brightness by nearly 10 times, making it bright enough for Hubble and Spitzer to detect. A telltale sign of the lensing is the smearing of the images of galaxies behind Abell 1689 into arcs by the gravitational warping of space by the intervening galaxy cluster.

The images reveal bright, dense clumps of hundreds of millions of massive stars in a compact region about 2,000 light-years across, which is only a fraction of the width of our Milky Way Galaxy. This type of galaxy is not uncommon in the early universe, when the bulk of star formation was taking place, Bradley and Illingworth said.

Spitzer's images show that the galaxy's mass is typical to that of galaxies in the early universe. Its mass is equivalent to several billions of stars like our Sun, or just a tiny fraction of the mass of the Milky Way.

"This observation confirms previous Hubble studies that star birth happens in very tiny regions compared with the size of the final galaxy," Illingworth said.

Even with the increased magnification from the gravitational lens, Hubble's sharp "eye" can only see knots of the brightest, heftiest stars in the galaxy. The telescope cannot pinpoint fainter, lower-mass stars, individual stars, or the material surrounding the star- birthing region. To see those things, astronomers will need the infrared capabilities of NASA's JWST. The planned infrared observatory will have a mirror about seven times the area of Hubble's primary mirror and will collect more light from faint galaxies. JWST also will be able to view even more remote galaxies whose light has been stretched deep into infrared wavelengths that are out of the reach of NICMOS.

"This galaxy will certainly be one of the first objects that will be observed by JWST," said team member Holland Ford of Johns Hopkins University. "This galaxy is so bright that JWST will see its detailed structure. This object is a pathfinder for JWST for deciphering what is happening in young galaxies."

The astronomers noted that the faraway galaxy also would be an ideal target for the Atacama Large Millimeter Array (ALMA), which, when completed in 2012, will be the most powerful radio telescope in the world. "ALMA and JWST working together would be an ideal combination to really understand this galaxy," Illingworth said, noting that "JWST's images and ALMA's measurement of the gas motions will provide revolutionary insights into the very youngest galaxies."

The astronomers will conduct follow-up observations with infrared spectroscopy to confirm the galaxy's distance using the Keck telescope atop Mauna Kea in Hawaii.

The results will be published in the Astrophysical Journal.

Light echoes whisper the distance to a star

Astronomers calibrate the distance scale of the Universe

Taking advantage of the presence of light echoes, a team of astronomers have used an ESO telescope to measure, at the 1% precision level, the distance of a Cepheid - a class of variable stars that constitutes one of the first steps in the cosmic distance ladder.

SO PR Photo 05a/08
The Cepheid Star RS Pup

ESO PR Photo 05b/08
Light Echoes in RS Pup
(NTT/EMMI)

"Our measurements with ESO's New Technology Telescope at La Silla allow us to obtain the most accurate distance to a Cepheid," says Pierre Kervella, lead-author of the paper reporting the result.

Cepheids [1] are pulsating stars that have been used as distance indicators since almost a hundred years. The new accurate measurement is important as, contrary to many others, it is purely geometrical and does not rely on hypotheses about the physics at play in the stars themselves.

The team of astronomers studied RS Pup, a bright Cepheid star located towards the constellation of Puppis ('the Stern') and easily visible with binoculars. RS Pup varies in brightness by almost a factor of five every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15 000 times more luminous.

RS Pup is the only Cepheid to be embedded in a large nebula [2], which is made of very fine dust that reflects some of the light emitted by the star.

Because the luminosity of the star changes in a very distinctive pattern, the presence of the nebula allows the astronomers to see light echoes and use them to measure the distance of the star.

"The light that travelled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time."

This delay is called a 'light echo', by analogy with the more traditional echo, the reflection of sound by, for example, the bottom of a well.

By monitoring the evolution of the brightness of the blobs in the nebula, the astronomers can derive their distance from the star: it is simply the measured delay in time, multiplied by the velocity of light (300 000 km/s). Knowing this distance and the apparent separation on the sky between the star and the blob, one can compute the distance of RS Pup.

From the observations of the echoes on several nebular features, the distance of RS Pup was found to be 6500 light years, plus or minus 90 light years.

"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. "This relation is indeed at the basis of the distance determination of galaxies using Cepheids."

RS Pup is thus distant by about a quarter of the distance between the Sun and the Centre of the Milky Way. RS Pup is located within the Galactic plane, in a very populated region of our Galaxy.

ESO PR Photo 05c/08
Light Echoes
(Artist's Impression)
More Information

"The long-period Galactic Cepheid RS Puppis - I. A geometric distance from its light echoes", P. Kervella et al. is in press in Astronomy and Astrophysics.
The team is composed of Pierre Kervella and Guy Perrin (LESIA, Observatoire de Paris, France), Antoine Mérand (Center for High Angular Resolution Astronomy, Atlanta, Georgia, USA), László Szabados (Konkoly Observatory, Budapest, Hungary), Pascal Fouqué (Observatoire Midi-Pyrénées, Toulouse, France), David Bersier (Liverpool John Moores University, UK), and Emanuela Pompei (ESO).

Notes

[1]: Cepheids are rare and very luminous pulsating stars whose luminosity varies in a very regular way. They are named after the star Delta Cephei in the constellation of Cepheus, the first known variable star of this particular type and bright enough to be easily seen with the unaided eye. Almost a century ago, in 1912, American astronomer Henrietta Leavitt published a relation between the intrinsic brightness and the pulsation period of Cepheids, the longer periods corresponding to the brighter stars. This relation still plays today a central role in the extragalactic distance scale.

[2]: The nebula around RS Pup was discovered in 1961 by Swedish astronomer Bengt Westerlund, who later became ESO Director in Chile (1970-74). Shortly after, in 1972, the American astronomer Robert Havlen, then visiting ESO Chile, published the first study of the nebula in the then rather young European journal Astronomy & Astrophysics.

Science Contact:
Pierre Kervella
Observatoire de Paris-Meudon, France
E-mail: Pierre.Kervella (at) obspm.fr
Phone: +33 1 45 07 79 66

Spitzer Catches Young Stars in Their Baby Blanket of Dust

Credit: NASA/JPL-Caltech/L. Allen (Harvard-Smithsonian Center for Astrophysics)

Newborn stars peek out from beneath their natal blanket of dust in this dynamic image of the Rho Ophiuchi dark cloud from NASA's Spitzer Space Telescope.

Called "Rho Oph" by astronomers, it's one of the closest star-forming regions to our own solar system. Located near the constellations Scorpius and Ophiuchus, the nebula is about 407 light years away from Earth.

Rho Oph is made up of a large main cloud of molecular hydrogen, a key molecule allowing new stars to form out of cold cosmic gas, with two long streamers trailing off in different directions. Recent studies using the latest X-ray and infrared observations reveal more than 300 young stellar objects within the large central cloud. Their median age is only 300,000 years, very young compared to some of the universe's oldest stars, which are more than 12 billion years old.

"Rho Oph is a favorite region for astronomers studying star formation. Because the stars are so young, we can observe them at a very early evolutionary stage, and because the Ophiuchus molecular cloud is relatively close, we can resolve more detail than in more distant clusters, like Orion," said Lori Allen, lead investigator of the new observations, from the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

This false-color image of Rho Oph's main cloud, Lynds 1688, was created with data from Spitzer's infrared array camera, which has the highest spatial resolution of Spitzer's three imaging instruments, and its multiband imaging photometer, best for detecting cooler materials.

The colors in this image reflect the relative temperatures and evolutionary states of the various stars. The youngest stars are surrounded by dusty disks of gas from which they and their potential planetary systems are forming. These young disk systems show up as red in this image. Some of these young stellar objects are surrounded by their own compact nebulae. More evolved stars, which have shed their natal material, are blue.

The extended white nebula in the center right of the image is a region of the cloud glowing in infrared light due to the heating of dust by bright young stars near the cloud's right edge. Fainter, multi-hued diffuse emission fills the image. The color of the nebulosity depends on the temperature, composition and size of the dust grains. Most of the stars forming now are concentrated in a filament of cold, dense gas that shows up as a dark cloud in the lower center and left side of the image against the bright background of the warm dust.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

Spitzer's infrared array camera was built by NASA's Goddard Space Flight Center, Greenbelt, Md. The instrument's principal investigator is Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics. The multiband imaging photometer for Spitzer was built by Ball Aerospace Corporation, Boulder, Colo.; the University of Arizona; and Boeing North American, Canoga Park, Calif. Its principal investigator is George Rieke of the University of Arizona, Tucson.

Rosemary Sullivant 818-354-2274
Jet Propulsion Laboratory, Pasadena, Calif.

White Dwarfs too Close to the Edge

Credit: UCSC

This series of images shows the interaction of a white dwarf star with a black hole. As it passes the black hole, the white dwarf becomes strongly compressed and heated (top left), triggering an explosion. Most of the stellar mass is ejected into space (the "bubble" in the upper right part of the debris in the top right image), while the rest (the cusp-like part of the image) falls toward the black hole. While the ejected matter expands rapidly, the infalling matter builds a violent, thick accretion disk around the black hole.

Unusual supernovae may reveal intermediate-mass black holes in globular clusters.

A strange and violent fate awaits a white dwarf star that wanders too close to a moderately massive black hole. According to a new study, the black hole's gravitational pull on the white dwarf would cause tidal forces sufficient to disrupt the stellar remnant and reignite nuclear burning in it, giving rise to a supernova explosion with an unusual appearance. Observations of such supernovae could confirm the existence of intermediate-mass black holes, currently the subject of much debate among astronomers.

"Our supercomputer simulations show a peculiar supernova that would be a unique signature of an intermediate-mass black hole," says Enrico Ramirez-Ruiz, assistant professor of astronomy and astrophysics at the University of California, Santa Cruz.

Ramirez-Ruiz and his collaborators used detailed computer simulations to follow the entire process of tidal disruption of a white dwarf by a black hole. Their simulations included gas dynamics, gravity, and nuclear physics, requiring weeks of computer time to simulate events that would take place in a fraction of a second.

"Every star that is not too massive ends up as a white dwarf, so they are very common. We were interested in whether tidal disruption can bring this stellar corpse to life again," says Stephen Rosswog, the first author of the paper.

A white dwarf can explode as a type Ia supernova if it accumulates enough mass by siphoning matter away from a companion star. When it reaches a critical mass (about 1.4 times the mass of the Sun), the white dwarf collapses and explodes. Astronomers use these type Ia supernovae as standard candles for cosmic distance measurements because their brightness evolves over time in a predictable manner.

The new paper describes a distinctly different mechanism for igniting a white dwarf, in which tidal disruption by a black hole causes drastic compression of the stellar material. The white dwarf is flattened into a pancake shape aligned in the plane of its orbit around the black hole. As each section of the star is squeezed through a point of maximum compression, the extreme pressure causes a sharp increase in temperatures, which triggers explosive burning.

The explosion ejects more than half of the debris from the disrupted star, while the rest of the stellar material falls into the black hole. The infalling material forms a luminous accretion disk that emits X-rays and should be detectable by the Chandra X-ray Observatory, the researchers say.

"This is a new mechanism for ignition of a white dwarf that results in a very different type of supernova than the standard type Ia, and it is followed by an X-ray source," Ramirez-Ruiz says.

He estimated that this type of event would occur about 100 times less frequently than the standard type Ia supernovae, but should be detectable by future surveys designed to observe large numbers of supernovae. The Large Synoptic Survey Telescope (LSST), planned for completion in 2013, is expected to discover hundreds of thousands of type Ia supernovae per year.

"These exotic creatures will start showing up in the data from the LSST," Ramirez-Ruiz says. "We want to predict the light curves so we can look for them in the survey data."

The mechanism described in the paper requires a black hole that is neither too small nor too big. Such intermediate-mass black holes (500 to 1,000 times the mass of the Sun) may reside in some globular star clusters, but there is much less evidence for their existence than there is for the relatively small stellar black holes (tens of times the mass of the Sun) or for supermassive black holes (a few million times the mass of the Sun), found at the centers of galaxies.

The new paper describes in detail the disruption of a white dwarf with two-tenths the mass of the Sun by a black hole 1,000 times the mass of the Sun. The researchers also found that they can vary the mass of the white dwarf and still get the same outcome--tidal disruption and ignition of the white dwarf.

"We can ignite the whole mass range of white dwarfs if they get close enough to the black hole," Rosswog says.

Site:http://www.astronomy.com

Saturn Has a 'Giant Sponge'

This is a false-color image of jets (blue areas) in the southern hemisphere of Enceladus
taken with the Cassini spacecraft narrow-angle camera on Nov. 27, 2005.
It has been processed to reveal the individual jets that comprise the plume.
Image Credit: NASA/JPL/Space Science Institute

One of Saturn's rings does housecleaning, soaking up material gushing from the fountains on Saturn's tiny ice moon Enceladus, according to new observations from the Cassini spacecraft.

"Saturn's A-ring and Enceladus are separated by 100,000 kilometers (62,000 miles), yet there's a physical connection between the two," says William Farrell of NASA's Goddard Space Flight Center in Greenbelt, Md. "Prior to Cassini, it was believed that the two bodies were separate and distinct entities, but Cassini's unique observations indicate that Enceladus is actually delivering a portion of its mass directly to the outer edge of the A-ring." Farrell is lead author of a paper on this discovery that appeared in Geophysical Research Letters January 23.

This is the latest surprising phenomenon associated with the ice geysers of Enceladus to be discovered or confirmed by Cassini scientists. Earlier, the geysers were found to be responsible for the content of the E-ring. Next, the whole magnetic environment of Saturn was found to be weighed down by the material spewing from Enceladus, which becomes plasma -- a gas of electrically charged particles. Now, Cassini scientists confirm that the plasma, which creates a donut-shaped cloud around Saturn, is being snatched by Saturn's A-ring, which acts like a giant sponge where the plasma is absorbed.

Shot from Enceladus' interior, the gas particles become electrically charged (ionized) by sunlight and collisions with other atoms and electrons. Once electrically charged, the particles feel magnetic force and are swept into the space around Saturn dominated by the planet's powerful magnetic field. There, they are trapped by Saturn's magnetic field lines, bouncing back and forth from pole to pole. The fun ends, however, if their bouncing path carries them inward toward Saturn to the A-ring. There they stick, in essence becoming part of the ring. "Once they get to the outer A-ring, they are stuck," says Farrell.

"This is an example of how Saturn's rings mitigate the overall radiation environment around the planet, sponging up low- and high-energy particles," says Farrell. By contrast, Jupiter has no dense rings to soak up high-energy particles, so that planet's extremely high radiation environment persists.

The Cassini observations confirm a prediction by John Richardson and Slobodan Jurac of the Massachusetts Institute of Technology. In the early 1990's, Hubble Space Telescope observations revealed the presence of a large body of water-related molecules in orbit about 240,000 kilometers (almost 150,000 miles) from Saturn. Richardson and Jurac modeled this water cloud and demonstrated it could migrate inward to the A-ring. "We relied on their predictions to help us interpret our data," said Farrell. "They predicted it, and we were seeing it."

At the time of their prediction, the source of the water cloud was unknown. The source was not identified until 2005 when Cassini discovered the stunning geysers emitted from Enceladus.

Data for the discovery that Saturn's A-ring acts like a sponge were collected in July 2004 when Cassini arrived in orbit around Saturn, making its closest flyby over the A-ring. "We skimmed over the top of that ring fairly close," said Farrell.

Enceladus Afar
Enceladus is seen here as a white disk across the unilluminated side of Saturn's rings
(black and white stripes across the bottom of the image).
This image was taken with the Cassini spacecraft narrow-angle camera on Oct. 27, 2007.
Image Credit: NASA/JPL/Space Science Institute

Hot spots on the inside wall of the plasma donut -- the part colliding with the A-ring -- were emitting radio signals. These signals behaved as a sort of natural radio beacon, indicating the local plasma density at the inner edge of the donut. The signals were detected by Cassini's Radio and Plasma Wave instrument. The team used these signals to monitor the density of the plasma (the higher the frequency, the greater the density) and hence witness the change in gas density with time.

"As we approached the A-ring, the frequency dropped, implying that the plasma density was going down because it was being absorbed by the ring," said Farrell. "What really drove this home was what happened to the signal when we passed over a gap in the rings, called the Cassini division. There, the frequency went higher, implying that the plasma density was going up because plasma was leaking through the gap."

The research was funded by NASA through the Cassini-Huygens project. Cassini-Huygens is an international collaboration among NASA, the European Space Agency, and the Italian Space Agency. The Cassini orbiter was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

For more information:
Written by: Bill Steigerwald, Goddard Space Flight Center.
Media Relations Contact: Carolina Martinez (JPL) 818-354-9382 and Bill Steigerwald (Goddard) 301-286-5017

NGC 4013 and the Tidal Stream

Image Credit & Copyright: R Jay Gabany (Blackbird Observatory) - collaboration;
D.Martínez-Delgado(IAC, MPIA),
M.Pohlen (Cardiff), S.Majewski (U.Virginia), J.Peñarrubia (U.Victoria), C.Palma (Penn State)

Nearly 50 million light-years away in the constellation Ursa Major, NGC 4013 was long considered an isolated island universe.

Seen edge-on, the gorgeous spiral galaxy was known for its flattened disk and central bulge of stars, cut by silhouetted dust lanes. But this deep color image of the region reveals a previously unknown feature associated with NGC 4013, an enormous, faint looping structure extending (above and toward the left) over 80 thousand light-years from the galaxy's center.

A detailed exploration of the remarkable structure reveals it to be a stream of stars originally belonging to another galaxy, likely a smaller galaxy torn apart by gravitational tides as it merged with the larger spiral.

Astronomers argue that the newly discovered tidal stream also explains a warped distribution of neutral hydrogen gas seen in radio images of NGC 4013 and offers parallels to the formation of our own Milky Way galaxy.

Site Astronomy Picture of the Day

Cosmic Finger Taps Our Galaxy's Shoulder

The leading arm of gas streaming from the

Magellanic Clouds is piercing the disk of the Milky Way.

Credit: John Rowe Animations

As if reaching out with a come-hither motion, a giant gas finger emanating from two neighboring galaxies has hooked into the starry disk of the Milky Way and is pulling all three galaxies closer.

This extremity of hydrogen gas is actually the pointy end of the so-called Leading Arm of gas that streams ahead of two irregular galaxies called the Large and Small Magellanic Clouds.

The fate of these nearby galaxies, which are impacted by the Milky Way's gravity, has been somewhat of a mystery. The new finger findings suggest that the Magellanic Clouds will eventually merge with the Milky Way rather than zooming past.

Located about 160,000 light-years from Earth, the Large Magellanic Cloud (LMC) is only one-twentieth the diameter of our galaxy and contains one-tenth as many stars. The Small Magellanic Cloud resides 200,000 light-years from Earth and is about 100 times smaller than the Milky Way.

"We're thrilled because we can determine exactly where this gas is plowing into the Milky Way," said research team leader Naomi McClure-Griffiths of CSIRO's Australia Telescope National Facility.

Called HVC306-2+230, the gas finger is gouging into our galaxy's starry disk about 70,000 light-years away from Earth. In the night sky, the contact point would be nearest the Southern Cross.

Until last year, astronomers thought the Magellanic Clouds had orbited our galaxy many times. This scenario held a gloomy outlook for the clouds, which were said to be doomed to be ripped apart and swallowed by the gravitational goliath.

But then new Hubble Space Telescope measurements revealed the clouds are paying our galaxy a one-time visit rather than being its lunch.

McClure-Griffiths' results, however, are more in line with the previous tale pegging the Milky Way and the Magellanic Clouds as long-time companions. McClure-Griffiths remarks that this isn't the final word and that both theories are still on the table.

By pointing out the spot of contact between the Leading Arm and our galactic disk, the recent study will help astronomers to predict where the clouds themselves will travel in the future.

"We think the Leading Arm is a tidal feature, gas pulled out of the Magellanic Clouds by the Milky Way's gravity," McClure-Griffiths said. "Where this gas goes, we'd expect the clouds to follow, at least approximately."

In the distant future, the three galaxies could become one.

By Jeanna Bryner
Staff Writer
www.space.com

Isolated Galaxy or Corporate Merger? Hubble Spies NGC 1132

Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration

NGC 1132 is dubbed a "fossil group" because it contains enormous concentrations of dark matter, comparable to the dark matter found in an entire group of galaxies. NGC 1132 also has a strong X-ray glow from an abundant amount of hot gas that is normally only found in galaxy groups.

In visible light, however, it appears as a single, isolated, large elliptical galaxy. The origin of fossil-group systems remains a puzzle. They may be the end-products of complete merging of galaxies within once-normal groups. Or, they may be very rare objects that formed in a region or period of time where the growth of moderate-sized galaxies was somehow suppressed, and only one large galaxy formed.

Elliptical galaxies are smooth and featureless. Containing hundreds of millions to trillions of stars, they range from nearly spherical to very elongated shapes. Their overall yellowish color comes from the aging stars. Because ellipticals do not contain much cool gas, they no longer can make new stars.

This image of NGC 1132 was taken with Hubble's Advanced Camera for Surveys. Data obtained in 2005 and 2006 through green and near-infrared filters were used in the composite. In this Hubble image, NGC 1132 is seen among a number of smaller dwarf galaxies of similar color. In the background, there is a stunning tapestry of numerous galaxies that are much larger but much farther away.

NGC 1132 is located approximately 318 million light-years away in the constellation Eridanus, the River.

A Young Erupting Pre-main Sequence Star Takes a (Long) Nap

Figure 1: Two images of V1647 Orionis and McNeil’s Nebula. The image on the left is an optical color composite taken about four years ago with GMOS-North on UT 2004 February 14. The image on the right is also an optical color image taken about one year ago on UT 2007 February 22.

Figure 2: Expanded view of the 2.12-2.35 micron region of the near infrared spectroscopy of V1647

Figure 3: Plot of the 8-13 micron silicate absorption band optical depth extracted from
the mid infrared spectrum.

Figure 4: Optical spectroscopy of V1647 Orionis from GMOS-North obtained on UT 2007 February 22.

A “new” star appeared in the constellation of Orion in late 2003 when the young pre-main sequence star V1647 Orionis went into outburst. The eruption and huge increase in brightness of the object resulted in the appearance of a reflection nebula called “McNeil’s Nebula,” named after the amateur astronomer, Jay McNeil, who discovered the object and alerted the world.

During the outburst the star and nebula remained bright for approximately 18 months before fading rapidly over a six month period. By early 2006 the star and its environment were very similar to their pre-burst stage. The event was monitored and observed with many ground- and space-based facilities and Gemini Observatory played a key role in monitoring the event during its eruptive and quiescent phases. A team led by Colin Aspin (IfA/University of Hawai‘i), Tracy Beck (STScI) and Bo Reipurth (IfA/University of Hawai‘i) spearheaded the monitoring campaign of this unique event.

The eruption of V1647 Orionis is most likely associated with a mass dumping of the inner regions of a heated circumstellar disk onto the young stellar photosphere. The spectacular flaring in brightness of the object is due to a significant increase in accretion luminosity and the clearing or destroying of surrounding dust by an energetic wind that made the star visible. These eruptions are thought to be repetitive and indicative of periods when a significant fraction of the final star’s mass is accreted.

The authors describe three phases for the V1647 Orionis latest eruption:

1. Before November 2004 is the pre-outburst phase
2. From November 2004 to February 2006 is the outburst phase
3. From February 2006 is the quiescent phase

The Gemini observing campaign led by Aspin has revealed some interesting results, particularly for the quiescent period. These include:

• McNeil’s Nebula is faintly visible in these GMOS-N images (Figure 1 right) indicating that the nebular material is still weakly illuminated by the star V1647 Orionis. At the time of acquisition of the GMOS-N imaging and spectroscopic data , V1647 Orionis had an r’ magnitude of 23.3.

• NIRI spectroscopy has revealed for the first time in this type of object the presence of molecular overtone absorption from CO and other key diagnostic atoms like Na and Ca (possibly betraying the photosphere of the star), see Figure 2. The 2um spectroscopy shown in the paper is from IRTF not NIRI. We did publish NIRI spectroscopy but from just after the outburst, not in quiescence.

• The star has a mass of about 0.8 solar mass and its age is about half a million years or less.

• V1647 Orionis in this pre-main sequence phase is about five times more luminous than the Sun.

• Material is falling onto the star at a rate of about one millionth of a solar mass per year.

• Mid infrared observation with MICHELLE/Gemini show evidence of silicate dust evolution over the outburst-to-quiescence period, see Figure 3.
  

In a previous article on V1647 Orionis, Aspin studied a previous outburst of the star which occurred in 1966. It seems that perhaps V1647 Orionis ‘wakes up’ every 37 years but soon (after 1 to 2 years) tires and takes another long nap!

Three Month Composite of Comet Holmes

Credit & Copyright: John Pane

How has Comet Holmes changed? Since brightening unexpectedly by nearly one million fold in late October, the last three months have found the coma of Comet 17P/Holmes both expanding and fading. This spectacular composite image shows how the coma and tail of Comet Holmes have changed.

Due to Earth's changing vantage point, Comet Holmes, out beyond the orbit of Mars, was seen in November nearly head-on, but in recent months is seen more from the side. Additionally, the comet's motion, when combined with Earth's changing perspective, has caused the comet to have shifted relative to the background stars.

The curved path of Comet Holmes shows it to be undergoing apparent retrograde motion as the Earth orbits quickly in front of it. The extent of the coma currently makes Comet Holmes over five times the physical size of our Sun.

Anecdotal evidence holds that the comet is hard to see without long photographic exposures, but on such exposures the comet may still be an impressive sight.

Astronomy Picture of the Day

A Theory of Everything

Astronomers propose an astronomical test of string theory.
Provided by the U of I at Urbana-Champaign

This image shows the how the Big Bang formed our universe.
Astronomy: Roen Kelly

Ancient light absorbed by neutral hydrogen atoms could be used to test certain predictions of string theory, say cosmologists at the University of Illinois. Making the measurements, however, would require a gigantic array of radio telescopes to be built on Earth, in space or on the Moon.

String theory, a theory whose fundamental building blocks are tiny one-dimensional filaments called strings, is the leading contender for a "theory of everything." Such a theory would unify all four fundamental forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity). But finding ways to test string theory has been difficult.

Now, cosmologists at the U of I say absorption features in the 21-centimeter spectrum of neutral hydrogen atoms could be used for such a test.

"High-redshift, 21-centimeter observations provide a rare observational window in which to test string theory, constrain its parameters and show whether or not it makes sense to embed a type of inflation, called brane inflation, into string theory," says Benjamin Wandelt, a professor of physics and of astronomy at the U of I.

"If we embed brane inflation into string theory, a network of cosmic strings is predicted to form," Wandelt says. "We can test this prediction by looking for the impact this cosmic string network would have on the density of neutral hydrogen in the universe."

About 400,000 years after the Big Bang, the universe consisted of a thick shell of neutral hydrogen atoms (each composed of a single proton orbited by a single electron) illuminated by what became known as the cosmic microwave background.

Because neutral hydrogen atoms readily absorb electromagnetic radiation with a wavelength of 21 centimeters, the cosmic microwave background carries a signature of density perturbations in the hydrogen shell, which should be observable today, Wandelt says.

Cosmic strings are filaments of infinite length. Their composition can be loosely compared to the boundaries of ice crystals in frozen water. When water in a bowl begins to freeze, ice crystals will grow at different points in the bowl, with random orientations. When the ice crystals meet, they usually will not be aligned to one another. The boundary between two such misaligned crystals is called a discontinuity or a defect.

Cosmic strings are defects in space. A network of strings is predicted by string theory (and also by other supersymmetric theories known as Grand Unified Theories, which aspire to unify all known forces of nature except gravity) to have been produced in the early universe, but has not been detected so far.

Cosmic strings produce characteristic fluctuations in the gas density through which they move, a signature of which will be imprinted on the 21-centimeter radiation.

The cosmic string network predicted to occur with brane inflation could be tested by looking for the corresponding fluctuations in the 21-centimeter radiation. Like the cosmic microwave background, the cosmological 21-centimeter radiation has been stretched as the universe has expanded. Today, this relic radiation has a wavelength closer to 21 meters, putting it in the long-wavelength radio portion of the electromagnetic spectrum.

To precisely measure perturbations in the spectra would require an array of radio telescopes with a collective area of more than 1,000 square kilometers. Such an array could be built using current technology, Wandelt says, but would be prohibitively expensive.

If such an enormous array were eventually constructed, measurements of perturbations in the density of neutral hydrogen atoms could also reveal the value of string tension, a fundamental parameter in string theory, Wandelt says. "And that would tell us about the energy scale at which quantum gravity begins to become important."

www.astronomy.com

Old galaxies discovered

Astronomers uncover ancestors of Milky Way-type galaxies.
Provided by Rutgers University

Galaxy M74 is representative of spiral galaxies that evolved from
recently discovered galaxies in the early universe.
Credit: odd Boroson/NOAO/AURA/NSF

Astronomers at Rutgers and Penn State universities have discovered galaxies in the distant universe that are ancestors of spiral galaxies like our Milky Way.

These ancient objects, some of the first galaxies ever to form, are being observed as they looked when the universe was a mere 2 billion years old. Today, scientists peg the universe's age at 13.7 billion years, so light from these galaxies traveled almost 12 billion years to reach Earth.

The newly discovered galaxies are quite small, one-tenth the size and one-twentieth the mass of our Milky Way. They also have fewer stars, only one-fortieth as many as are in the Milky Way. From ground-based telescopes, they look like individual stars in size. Recent images made by the Hubble Space Telescope, however, reveal them as regions of active star formation.

"Finding these objects and discovering that they are a step in the evolution of our galaxy is akin to finding a key fossil in the path of human evolution," says Eric Gawiser, assistant professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences.

This image shows an extended, chain-like Lyman alpha emitter.
Credit: NASA/ESA/C. Gronwall/Penn State

The researchers determined that these galaxies were fertile breeding grounds for new stars, which burned hot and bright. These stars ionized the hydrogen atoms around them, stripping them of their electrons and causing them to emit a tell-tale sharp band of ultraviolet light known as Lyman alpha.

The researchers also noted that several of these galaxies, sometimes 10 or more, pulled together over the ensuing few billion years to form a single spiral galaxy.

"The Hubble Space Telescope has delivered striking images of these early galaxies, with 10 times the resolution of ground-based telescopes," says Caryl Gronwall, senior research associate in Penn State's Department of Astronomy & Astrophysics. "They come in a variety of shapes, round, oblong, and even somewhat linear, and we are starting to make precise measurements of their sizes."

The astronomers discovered these galaxies as part of a 5-year-old census of galaxies in the early universe, a project called MUSYC (Multi-Wavelength Survey by Yale and Chile). Gawiser, while a National Science Foundation (NSF) astronomy and astrophysics postdoctoral fellow at Yale, initiated a search for several types of galaxies that could be precursors of Milky Way-type spirals; Gronwall led an investigation into the luminosity, density and distribution of the distinctive Lyman alpha emitters. Their statistical analyses and computer simulations of how galaxies cluster led them to the conclusion they first reported in December 2007: Lyman alpha emitters are the ancestors of spiral galaxies.

This image shows an extended Lyman alpha emitter which shows evidence of merging.
Credit: NASA/ESA/C. Gronwall/Penn State

"We knew by our understanding of cosmological theory that spiral galaxies had to evolve from low-mass galaxies such as these," Gawiser says. "The challenge was to actually find them. We'd seen other early universe galaxies, but they were bigger and destined to evolve into elliptical galaxies, not spirals."

The astronomers undertook four types of observations to find and characterize the objects they were seeking. They performed the first step, actually finding the Lyman alpha-emitting galaxies amid all the visible objects of deep space, using the Blanco 4-meter telescope at the NSF Cerro Tololo Inter-American Observatory in Chile. To measure their distance, they used the Magellan Telescope at Las Campanas Observatory, also in Chile, to measure redshift, an effect that shows how fast an object is receding from view due to a rapidly expanding universe. (The redshift at which they studied these galaxies is 3.1.) To determine how many stars are in the galaxies, they used the NASA Spitzer Space Telescope's Infrared Array Camera. And to determine how big the galaxies are, they used the NASA Hubble Space Telescope's Advanced Camera for Surveys.

"Astronomy has long used a model where big surveys are followed by detailed studies of the interesting objects they find," says Nigel Sharp, program officer in NSF's Division of Astronomical Sciences. "This work nicely couples the large area, wide-field view of our ground-based telescope with the sharp focus of the Hubble, to probe to the faintest light levels. This team has come the closest yet to finding young galaxies that resemble our own Milky Way in its infancy."

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