Using Spitzer's infrared eyes, they found so-called infrared echoes, which occur when a flash of light from the supernova blasts through clouds, heating them up and causing them to glow in infrared. As the light rolls outward, the infrared echoes continue to flare up and travel away from the star.
Thursday, May 29, 2008
Using Spitzer's infrared eyes, they found so-called infrared echoes, which occur when a flash of light from the supernova blasts through clouds, heating them up and causing them to glow in infrared. As the light rolls outward, the infrared echoes continue to flare up and travel away from the star.
Wednesday, May 28, 2008
NASA's Spitzer Space Telescope has found a bizarre ring of material around the magnetic remains of a star that blasted to smithereens.
The stellar corpse, called SGR 1900+14, belongs to a class of objects known as magnetars. These are the cores of massive stars that blew up in supernova explosions, but unlike other dead stars, they slowly pulsate with X-rays and have tremendously strong magnetic fields.
"The universe is a big place and weird things can happen," said Stefanie Wachter of NASA's Spitzer Science Center at the California Institute of Technology, Pasadena, who found the ring serendipitously. "I was flipping through archived Spitzer data of the object, and that's when I noticed it was surrounded by a ring we'd never seen before." Wachter is lead author of a paper about the findings in this week's Nature.
Wachter and her colleagues think that the ring, which is unlike anything ever seen before, formed in 1998 when the magnetar erupted in a giant flare. They believe the crusty surface of the magnetar cracked, sending out a flare, or blast of energy, that excavated a nearby cloud of dust, leaving an outer, dusty ring. This ring is oblong, with dimensions of about seven by three light-years. It appears to be flat, or two-dimensional, but the scientists said they can't rule out the possibility of a three-dimensional shell.
"It's as if the magnetar became a huge flaming torch and obliterated the dust around it, creating a massive cavity," said Chryssa Kouveliotou, senior astrophysicist at NASA's Marshall Space Flight Center, Huntsville, Ala., and a co-author of the paper. "Then the stars nearby lit up a ring of fire around the dead star, marking it for eternity."
The discovery could help scientists figure out if a star's mass influences whether it becomes a magnetar when it dies. Though scientists know that stars above a certain mass will "go supernova," they do not know if mass plays a role in determining whether the star becomes a magnetar or a run-of-the-mill dead star. According to the science team, the ring demonstrates that SGR 1900+14 belongs to a nearby cluster of young, massive stars. By studying the masses of these nearby stars, the scientists might learn the approximate mass of the original star that exploded and became SGR 1900+14.
"The ring has to be lit up by something, otherwise Spitzer wouldn't have seen it," said Enrico Ramirez-Ruiz of the University of California, Santa Cruz. "The nearby massive stars are most likely what's heating the dust and lighting it up, and this means that the magnetar, which lies at the exact center of the ring, is associated with the massive star-forming region."
Rings and spheres are common in the universe. Young, hot stars blow bubbles in space, carving out dust into spherical shapes. When stars die in supernova explosions, their remains are blasted into space, forming short-lived beautiful orbs called supernova remnants. Rings can also form around exploded stars whose expanding shells of debris ram into pre-existing dust rings, causing the dust to glow, as is the case with the supernova remnant called 1987A.
But the ring around the magnetar SGR 1900+14 fits into none of these categories. For one thing, supernova remnants and the ring around 1987A cry out with X-rays and radio waves. The ring around SGR 1900+14 only glows at specific infrared wavelengths that Spitzer can see.
At first, the astronomers thought the ring must be what's called an infrared echo. These occur when an object sends out a blast wave that travels outward, heating up dust and causing it to glow with infrared light. But when they went back to observe SGR 1900+14 later, the ring didn't move outward as it should have if it were an infrared echo.
A closer analysis of the pictures later revealed that the ring is most likely a carved-out cavity in a dust cloud -- a phenomenon that must be somewhat rare in the universe since it had not been seen before. The scientists plan to look for more of these rings.
"This magnetar is still alive in many ways," said Ramirez-Ruiz. "It is interacting with its environment, making a big impact on the young star-forming region where it was born."
Other paper authors include V. Dwarkadas of the University of Chicago, Ill.; J. Granot of the University of Hertfordshire, England; S.K. Patel of the Optical Sciences Corporation, Huntsville, Ala.; and D. Figer of the Rochester Institute of Technology, N.Y. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center. Caltech manages JPL for NASA. Spitzer's infrared array camera, which made the observations, was built by NASA's Goddard Space Flight Center, Greenbelt, Md. Its principal investigator is Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics.
Jet Propulsion Laboratory, Pasadena, Calif.
Saturday, May 24, 2008
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/HST
The New Horizons researchers combined observations from their Pluto-bound spacecraft, which flew past Jupiter in February 2007; data from the Hubble Space Telescope orbiting Earth, and the European Southern Observatory’s Very Large Telescope, perched on an Atacama Desert mountain in Chile. This is the first time that high resolution, close–up imaging of the Little Red Spot has been combined with powerful Earth–orbital and ground-based imagery made at ultraviolet through mid–infrared wavelengths.
Jupiter’s "LRS" is an anticyclone, a storm whose winds circulate in the opposite direction to that of a cyclone – counterclockwise, in this case. It is nearly the size of Earth and as red as the similar, but larger and more well known, Great Red Spot (or GRS). The dramatic evolution of the LRS began with the merger of three smaller white storms that had been observed since the 1930s. Two of these storms coalesced in 1998, and the combined pair merged with a third major Jovian storm in 2000. In late 2005 – for reasons still unknown — the combined storm turned red.
The new observations confirm that wind speeds in the LRS have increased substantially over the wind speeds in the precursor storms, which had been observed by NASA’s Voyager and Galileo missions in past decades. Researchers measured the latest wind speeds and directions using two image mosaics from New Horizons' telescopic Long Range Reconnaissance Imager (LORRI), taken 30 minutes apart in order to track the motion of cloud features. New Horizons obtained the images from a distance of approximately 2.4 million kilometers (1.5 million miles) from Jupiter at a resolution of 14.4 kilometers (8.9 miles) per pixel. The LRS' maximum winds speeds of about 384 miles per hour (between 155 – 190 meters per second) far exceed the156 mile-per-hour threshold that would make it a Category 5 storm on Earth.
"This storm is still developing, and some of the changes remain mysterious,” says Dr. Andrew Cheng of the Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., who led the study team. “This unique set of observations is giving us hints about the storm's structure and makeup; from this, we expect to learn much more about how these large atmospheric disturbances form on worlds across the solar system."
Jupiter's venerable Great Red Spot has decreased steadily in size over the past several decades. In addition, a rare "global upheaval" in Jupiter's atmosphere began before New Horizons visited last year. This upheaval involved the disappearance of activity in the South Equatorial Belt (which left the GRS as an isolated storm), the appearance of a south tropical disturbance north of the Little Red Spot, and other spectacular cloud changes.
"This was a rare opportunity to combine observations from a powerful suite of instruments, as Jupiter will not be visited again by a spacecraft until 2016 at the earliest," says Cheng, whose team publishes its work in the June 2008 Astronomical Journal.
Scientists combined LORRI image maps of cloud motions with visible-color images from Hubble, and mid-infrared images from the Very Large Telescope. The latter technique allows scientists to "see" thermal structure and dynamics beneath the visible cloud layers, because thermal infrared wavelengths (indicating heat) can pass through the higher clouds. "The new observations confirm that the thermal structures, wind speeds, and cloud features of the LRS are very similar to those of the GRS," says Dr. Hal Weaver, a member of the study team from APL and the New Horizons project scientist. "Both the LRS and the GRS extend into the stratosphere, to far higher altitudes than for the smaller storms on Jupiter."
The observations offer clues to the mystery of why the GRS, and now also the LRS, may be so red. The wind speeds and overall strength of the LRS increased substantially in the seven years between the Galileo and the New Horizons observations, during which the storm became red. "This supports the idea that a common dynamical mechanism explains the reddening of the two largest anticyclonic systems on Jupiter, one possibility of which is that storm winds dredge up material from below," says Dr. Amy Simon-Miller of NASA’s Goddard Space Flight Center, Greenbelt, Md.
In their report, the scientists also wonder about the future evolution of Jupiter’s two giant storms. The LRS already rivals the steadily shrinking GRS in size and wind speed. The new thermal and wind field observations hint at an interaction between the south tropical disturbance, the Little Red Spot, and a warm cyclonic region south of the LRS, forming a complex that could dwarf the Great Red Spot.
"The Great Red Spot may not always be the largest and strongest storm on Jupiter,” says Dr. Glenn Orton of NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Continued monitoring of Jupiter's constantly evolving atmosphere will surely yield more surprises."
New Horizons is the first mission in NASA's New Frontiers Program of medium-class spacecraft exploration projects. Dr. Alan Stern leads the mission and science team as principal investigator; APL manages the mission for NASA’s Science Mission Directorate. The mission team also includes Southwest Research Institute, Ball Aerospace Corporation, the Boeing Company, NASA Goddard Space Flight Center, NASA Jet Propulsion Laboratory, Stanford University, KinetX Inc. (navigation team), Lockheed Martin Corporation, University of Colorado, the U.S. Department of Energy, and a number of other firms, NASA centers, and university partners.
M. Buckley, Johns Hopkins University Applied Physics Laboratory
(240) 228-7536 or (443) 778-7536
Friday, May 23, 2008
At the beginning of this month, the IEEE/ION Position, Location and Navigation Symposium (PLANS) 2008 conference in Monterey, California featured two interesting concepts for the use of these highly accurate X-ray sources. The first proposal called "Noise Analysis for X-ray Navigation Systems" headed by John Hanson of CrossTrac Engineering, introduces a scaled-up version of terrestrial GPS, using pulsars rather than man-made satellites. The system is called X-ray navigation, or "XNAV" for short. Primarily focusing on space missions beyond Jupiter, XNAV would use the Solar System as the base co-ordinate and then measure the phase of the incoming X-ray emission from the mapped pulsars. As the X-ray pulses are so accurate, onboard systems could measure and compare the signal from multiple pulsar sources and automatically deduce the position of the spacecraft to a high degree of certainty. I suppose it would be an advanced 3D version of the traditional sextant as used by ships to measure the elevation of stars above the Earth's horizon.
The second concept entitled "Online Time Delay Estimation of Pulsar Signals for Relative Navigation using Adaptive Filters", is headed by Amir Emadzadeh at the UCLA Electrical Engineering Department. Emadzadeh suggests that the location of two spacecraft can be worked out if both ships are looking at the same, known pulsar. The periodic emission measured by both ships will have a differential time delay proportional to the distance between the ships. In addition, the UCLA group suggest a method to derive their relative inertial position by observing a distribution of X-ray sources throughout the cosmos.
These are very interesting concepts, but until we begin routinely venturing beyond the orbit of Jupiter I doubt we'll see these ideas come to fruition any time soon…
Thursday, May 22, 2008
Credit: M. Wong and I. de Pater (University of California, Berkeley)
This third red spot, which is a fraction of the size of the two other features, lies to the west of the Great Red Spot in the same latitude band of clouds.
The new red spot was previously a white oval-shaped storm. The change to a red color indicates its swirling storm clouds are rising to heights like the clouds of the Great Red Spot. One possible explanation is that the red storm is so powerful it dredges material from deep beneath Jupiter's cloud tops and lifts it to higher altitudes where solar ultraviolet radiation — via some unknown chemical reaction — produces the familiar brick color.
Detailed analysis of the visible-light images taken by Hubble's Wide Field Planetary Camera 2 on May 9 and 10, and near-infrared adaptive optics images taken by the W.M. Keck telescope on May 11, is revealing the relative altitudes of the cloud tops of the three red ovals. Because all three oval storms are bright in near-infrared light, they must be towering above the methane in Jupiter's atmosphere, which absorbs the Sun's infrared light and so looks dark in infrared images.
Turbulence and storms first observed on Jupiter more than two years ago are still raging, as revealed in the latest pictures. The Hubble and Keck images also reveal the change from a rather bland, quiescent band surrounding the Great Red Spot just over a year ago to one of incredible turbulence on both sides of the spot.
Red Spot Jr. appeared in spring of 2006. The Great Red Spot has persisted for as long as 200 to 350 years, based on early telescopic observations. If the new red spot and the Great Red Spot continue on their courses, they will encounter each other in August, and the small oval will either be absorbed or repelled from the Great Red Spot. Red Spot Jr. which lies between the two other spots, and is at a lower latitude, will pass the Great Red Spot in June.
The Hubble and Keck images may support the idea that Jupiter is in the midst of global climate change, as first proposed in 2004 by Phil Marcus, a professor of mechanical engineering at the University of California, Berkeley. The planet's temperatures may be changing by 15 to 20 degrees Fahrenheit. The giant planet is getting warmer near the equator and cooler near the South Pole. He predicted that large changes would start in the southern hemisphere around 2006, causing the jet streams to become unstable and spawn new vortices.
For additional information, contact:
Space Telescope Science Institute, Baltimore, Md.
University of California, Berkeley, Calif.
Mike Wong/Imke de Pater
University of California, Berkeley, Calif.
Wednesday, May 21, 2008
In January of 2008 Soderberg was expecting to study a month-supernova that was already underway. But as she and her assistant studied the X-ray emissions conveyed from space by NASA's Swift satellite, they saw an extremely bright light that seemed to jump out of the sky. They didn't know it at the time, but they had just become the first astronomers to have caught a star in the act of exploding.
Soderberg regards the discovery as a case of extreme serendipity. The satellite was pointing in the right place at the right time, she said, because she had asked Neil Gehrels, Swift's lead scientist at NASA's Goddard Space Flight Center to turn it that way to look at another supernova. And while she was away lecturing, she had asked her colleague, Edo Berger, to keep an eye on the data for her.
"It's a really lucky chain of events — a surprise," said Soderberg, who is leading the group studying the explosion. "It was all over in a matter of minutes."
Other observatories also turned their telescopes toward this stellar explosion, making detailed observations of the event, including the Hubble Space Telescope, the Chandra X-ray Observatory, Palomar's 60- and 200-nch telescopes, the Gemini Observatory and Kitt 1 Telescope in Hawaii, and the Very Large Array and Apache Point Observatories in New Mexico. This will allow a very detailed study of this event.
A typical supernova occurs when the core of a massive star runs out of nuclear fuel and collapses under its own gravity to form an ultradense object known as a neutron star. The newborn neutron star compresses and then rebounds, triggering a shock wave that plows through the star's gaseous outer layers and blows the star to smithereens. Until now, astronomers have only been able to observe supernovae brightening days or weeks after the event, when the expanding shell of debris is energized by the decay of radioactive elements forged in the explosion.
Tuesday, May 20, 2008
Although the universe contains billions of galaxies, only a small amount of its matter is locked up in these behemoths. Most of the universe's matter that was created during and just after the Big Bang must be found elsewhere.
Now, in an extensive search of the local universe, astronomers say they have definitively found about half of the missing normal matter, called baryons, in the spaces between the galaxies. This important component of the universe is known as the "intergalactic medium," or IGM, and it extends essentially throughout all of space, from just outside our Milky Way galaxy to the most distant regions of space observed by astronomers.
The questions "where have the local baryons gone, and what are their properties?" are being answered with greater certainty than ever before.
"We think we are seeing the strands of a web-like structure that forms the backbone of the universe," Mike Shull of the University of Colorado explained. "What we are confirming in detail is that intergalactic space, which intuitively might seem to be empty, is in fact the reservoir for most of the normal, baryonic matter in the universe."
Hubble observations made nearly a decade ago by Todd Tripp and colleagues first reported finding the hottest portion of this missing matter in the local universe. That study utilized spectroscopic observations of one quasar to look for absorbing intergalactic gas along the path to the quasar.
In the May 20 issue of The Astrophysical Journal, Charles Danforth and Shull report on observations taken along sight-lines to 28 quasars. Their analysis represents the most detailed observations to date of how the IGM looks within about four billion light-years of Earth.
Baryons are protons, neutrons, and other subatomic particles that make up ordinary matter such as hydrogen, helium, and heavier elements. Baryonic matter forms stars, planets, moons, and even the interstellar gas and dust from which new stars are born.
Astronomers caution that the missing baryonic matter is not to be confused with "dark matter," a mysterious and exotic form of matter that is only detected via its gravitational pull.
Danforth and Shull, of the Department of Astrophysical and Planetary Sciences at the University of Colorado in Boulder, looked for the missing baryonic matter by using the light from distant quasars (the bright cores of galaxies with active black holes) to probe spider-web-like structure that permeates the seemingly invisible space between galaxies, like shining a flashlight through fog.
Using the Space Telescope Imaging Spectrograph (STIS) aboard NASA's Hubble Space Telescope and NASA's Far Ultraviolet Spectroscopic Explorer (FUSE), the astronomers found hot gas, mostly oxygen and hydrogen, which provide a three-dimensional probe of intergalactic space. STIS and FUSE found the spectral "fingerprints" of intervening oxygen and hydrogen superimposed on the quasars' light.
The bright quasar light was measured to penetrate more than 650 filaments of hydrogen in the cosmic web. Eighty-three filaments were found laced with highly ionized oxygen in which five electrons have been stripped away.
The presence of highly ionized oxygen (and other elements) between the galaxies is believed to trace large quantities of invisible, hot, ionized hydrogen in the universe. These vast reservoirs of hydrogen have largely escaped detection because they are too hot to be seen in visible light, yet too cool to be seen in X-rays.
The oxygen "tracer" was probably created when exploding stars in galaxies spewed the oxygen back into intergalactic space where it mixed with the pre-existing hydrogen via a shockwave which heated the oxygen to very high temperatures.
The team also found that about 20 percent of the baryons reside in the voids between the web-like filaments. Within these voids could be faint dwarf galaxies or wisps of matter that could turn into stars and galaxies in billions of years.
Probing this vast cosmic web will be a key goal for the Cosmic Origins Spectrograph (COS), a new science instrument that astronauts plan to install on Hubble during Servicing Mission 4 later this year.
"COS will allow us to make more robust and more detailed core samples of the cosmic web," Shull said. "We predict that COS will find considerably more of the missing baryonic matter."
"Our goal is to confirm the existence of the cosmic web by mapping its structure, measuring the amount of heavy metals found in it, and measuring its temperature. Studying the cosmic web gives us information on how galaxies built up over time."
The COS team hopes to observe 100 additional quasars and build up a survey of more than 10,000 hydrogen filaments in the cosmic web, many laced with heavy elements from early stars.
Space Telescope Science Institute, Baltimore, Md. 410-338-4493/4514
University of Colorado, Boulder, Colo. 303-492-7827
Monday, May 19, 2008
The star, known as EV Lacertae, isn’t much to write home about. It’s a run-of-the-mill red dwarf, by far the most common type of star in the universe. It shines with only one percent of the Sun’s light, and contains only a third of the Sun’s mass. At a distance of only 16 light-years, EV Lacertae is one of our closest stellar neighbors. But with its feeble light output, its faint magnitude-10 glow is far below naked-eye visibility.
"Here’s a small, cool star that shot off a monster flare. This star has a record of producing flares, but this one takes the cake," says Rachel Osten, a Hubble Fellow at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Md. "Flares like this would deplete the atmospheres of life-bearing planets, sterilizing their surfaces."
The flare was first seen by the Russian-built Konus instrument on NASA’s Wind satellite in the early morning hours of April 25. Swift’s X-ray Telescope caught the flare less than two minutes later, and quickly slewed to point toward EV Lacertae. When Swift tried to observe the star with its Ultraviolet/Optical Telescope, the flare was so bright that the instrument shut itself down for safety reasons. The star remained bright in X-rays for 8 hours before settling back to normal.
EV Lacertae can be likened to an unruly child that throws frequent temper tantrums. The star is relatively young, with an estimated age of a few hundred million years. The star rotates once every four days, which is much faster than the sun, which rotates once every four weeks. EV Lacertae’s fast rotation generates strong localized magnetic fields, making it more than 100 times as magnetically powerful as the Sun’s field. The energy stored in its magnetic field powers these giant flares.
EV Lacertae’s constellation, Lacerta, is visible in the spring for only a few hours each night in the Northern Hemisphere. But if the star had been more easily visible, the flare probably would have been bright enough that the star could have been seen with the naked eye for one to two hours.
The flare’s incredible brightness enabled Swift to make detailed measurements. "This gives us a golden opportunity to study a stellar flare on a second-by-second basis to see how it evolved," says Stephen Drake of NASA Goddard.
Since EV Lacertae is 15 times younger than our Sun, it gives us a window into our solar system’s early history. Younger stars rotate faster and generate more powerful flares, so in its first billion years the sun must have let loose millions of energetic flares that would have profoundly affected Earth and the other planets.
Flares release energy across the electromagnetic spectrum, but the extremely high gas temperatures produced by flares can only be studied with high-energy telescopes like those on Swift. Swift's wide field and rapid repointing capabilities, designed to study gamma-ray bursts, make it ideal for studying stellar flares. Most other X-ray observatories have studied this star and others like it, but they have to be extremely lucky to catch and study powerful flares due to their much smaller fields of view.
Says Eric Feigelson of Penn State University in University Park, Pa., "I find it remarkable that a satellite designed to detect the explosive birth of black holes in distant galaxies can also detect explosions on stars in the immediate neighborhood of our Sun."
Goddard Space Flight Center
Credit: Nichole King (STScI) et al., Mayall Telescope, KPNO, NOAO, NSF
On Earth, neon is known for being flashy. Any Las Vegas tourist knows that signs sporting this noble gas are hard to miss, but in space this is not always true. Neon is the fifth most abundant element in the cosmos, but until recently, astronomers couldn't seem to get a precise measurement of it in the Universe.
Now, new research shows that NASA's Spitzer Space Telescope has a "sweet spot" for detecting neon in star-forming regions.
"By the good fortune of excellent instrument planning, Spitzer's infrared spectrometer instrument can measure all the dominant forms of neon in star-forming regions, at the exact same time and area," says Dr. Robert Rubin, of the NASA Ames Research Center in Moffett Field, Calif.
Rubin and his collaborators used Spitzer's spectrometer to measure neon and sulfur abundances in 25 star-forming regions across the nearby spiral galaxy M33. For the first time, they found that the ratio of neon to sulfur in all of these areas is relatively constant at about 16. They note that this observational result is not surprising because it is consistent with current models for how these chemical elements are created in the cosmos.
"Both neon and sulfur are produced in nuclear reactions deep inside the core of the most massive stars following the stages where carbon and oxygen are synthesized by nuclear processes in these stars. Theory predicts that neon and sulfur yields depend very little on how enriched in the 'heavy elements' -- all chemical elements except hydrogen and helium -- that the star begins its life with. Thus, it is expected that the neon to sulfur ratio remains relatively constant throughout a galaxy," says Rubin.
Team members believe that this research may eventually provide valuable insights into the amount of neon in our Sun, which is currently a controversial topic among astronomers.
Seeing Neon Lights
Because cosmic neon can take on a variety of "forms" in a given region of space, astronomers must account for all of its possibilities to get an accurate measurement of it. The neon atom contains 10 electrons, and an energetic environment can rip one or multiple electrons off. This altered atom is called ionized neon.
According to Rubin, neon atoms in star-forming regions exist primarily in two states -- singly and doubly ionized neon, meaning that it is missing one and two electrons, respectively. While prior infrared missions could only detect one form of neon at a time in any given area, Spitzer can see all the dominant forms at the same exact time. This makes Spitzer's measurements more precise. The telescope's unique sensitivity also allows it to measure this ratio for very distant objects.
"In addition to neon, Spitzer's infrared spectrometer can do nearly as well to measure the sulfur abundance for star-forming regions at the same time and location... Spitzer is a lean, mean neon and sulfur abundance machine," says Rubin. Sulfur is the eighth most abundant element in the cosmos.
By taking accurate measurements of both neon and sulfur in a star-forming region, astronomers can then create an abundance ratio, which helps them keep everything in perspective.
"The rationale behind ratios is to take away the variability due to other effects such as distance. If we looked at nearby star-forming regions everything would look brighter, not because there is more neon and sulfur, but because it is closer. By putting neon in a ratio with sulfur, we normalize this measurement and put everything on the same level," says Rubin.
Team members chose to compare neon to sulfur because Spitzer was able to measure all the dominant forms of these elements at the same time, in the same area.
Rubin and his collaborators will continue their research with Spitzer in the next few months by observing nearly two dozen star-forming regions in the nearby galaxy NGC 6822. A paper on the M33 research findings will be in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society.
Other authors on the paper include Janet Simpson, Sean Colgan, Ian McNabb, Edwin Erickson, Michael Haas, and Robert Citron, of the NASA Ames Research Center. Reginald Dufour and Gregory Brunner, of Rice University. Adalbert Pauldrach, of the University of Munich.
A copy of the paper is posted at: http://arxiv.org/abs/0804.0828.
Friday, May 16, 2008
What appears to be the hole of an elongated smoke ring in this National Radio Astronomy Observatory image really is an enormous, nearly empty, bubble blown into the dusty, gas disk of our Milky Way Galaxy.
Such interstellar bubbles are sculpted by the force of the wind and radiation from typically a few dozen hot, massive stars along with the explosive impact of dying stars which are called supernovae.
The force sweeps up the disk's gas that is in its path, creating a gas shell surrounding a bubble.
The neighbourhood of our own solar system resides in such a cavity. However the shell in this image, catalogued (using its coordinates) as galactic shell GS 62.1+0.2-18, is located at a distance of 30,000 light years from Earth, and measures 1,100 by 520 light years.
Despite its distance, this "smoke ring" appears so large on the sky that the apparent width of the full moon would fit eight times inside it. The bright yellowish-orange dots scattered across this image are clusters of young, massive stars surrounded by hot gas and are called nebulae.
Astronomers from the International/VLA Galactic Plane Survey have determined that none of these clusters harbor the stars that blew the giant shell since none of the clusters are at the same distance as the shell. Indeed they all are located closer to the Earth than the shell is.
Probably, the stars that blew GS 62.1+0.2-18's hole perished as supernova explosions. This image shows only a small part of a survey which uses both the Very Large Array and the Green Bank Telescopes to trace, in detail, the cool gas in our Galaxy.
This gas has been coloured purple, blue and green in this image. In order to show the locations of star clusters, the image of gas was overlaid with 2 additional images.
The one of radio emission associated with regions of hot gas was coloured orange, while heated dust, imaged in infrared by the Midcourse Space Experiment satellite, was coloured red.
The Principal Investigator for the survey is A. R. Taylor. This study is published by VGPS investigators J. M. Stil, A. R. Taylor, J. M. Dickey, D. W. Kavars, P. G. Martin, T. A. Rothwell, A. I. Boothroyd, Felix J. Lockman, and N. M. McClure-Griffiths in the Astronomical Journal, Volume 132, number 3, page 1158.
Investigator(s): J. M. Stil, A. R. Taylor, J. M. Dickey, D. W. Kavars, P. G. Martin, T. A. Rothwell, A. I. Boothroyd, Felix J. Lockman, and N. M. McClure-Griffiths
About this image:
This figure shows a Very Large Array 90cm image of 40 square degrees of the inner Galactic plane. This image covers about 200 times the area of the full moon and has a resolution of 42 seconds of arc (about the angular size of Jupiter viewed from Earth).
The most prominent sources are the ionized HII regions that form around massive stars and supernova remnants which are leftover from the death throes of massive stars.
From a variety of diagnostics including the number of massive stars in our Galaxy, there should be many more supernova remnants than are currently known.
The purpose of this project was to overcome the observational selection effects such as poor sensitivity and resolution that have long been assumed to be responsible for the dearth of observed supernova remnants.
From this very high fidelity image we have been able to discover 35 previously unknown remnants, a 300% increase for this region of the plane and an overall 15% increase in the total known in the Galaxy.
It has only recently become possible to create such high fidelity images at low radio frequencies (where supernova remnants are brightest) due to significant software improvements and increased computing power.
Investigator(s): Crystal Brogan (NRAO), Yosi Gelfand (CfA), Bryan Gaensler (CfA), Namir Kassim (NRL), Joe Lazio (NRL)
The image is a composite of a radio image constructed from observations taken in several configurations of the Very Large Array at a wavelength of 20 cm for the MAGPIS survey with mid-infrared data taken as part of the GLIMPSE survey conducted by the Spitzer Space Telescope.
The radio data are coded red, the long-wavelength infrared data (at 8 micrometers) green, and the shorter wavelength infrared data blue-white; yellow regions in the image show places where both radio and infrared emission is prominent.
Normal stars are brightest at the shortest wavelengths, showing up as the myriad of blue-white points. Birthsites of the youngest massive stars show as yellow clumps -- radiation from the newborn stars heats surrounding dust producing infrared emission, while the ultraviolet light from these stars separates electrons from hydrogen atoms giving rise to radio emission.
More mature stars have managed to destroy the dust nearby leaving red cores surrounded by yellow, then green, shells as the temperature drops far from the stars.
The prominent red arcs mark the sites where massive stars have died in titanic explosions and blasted their gas light years into space at thousands of miles per second; their radio emission is produced as electrons, accelerated to nearly the speed of light by the outward moving blast waves, spiral in the Galactic magnetic field.
The diffuse green glow reveals the tiny dust particles that suffuse interstellar space along the band of the Milky Way; dark filaments superposed on this emission show regions where the gas and dust are so thick that no light can get through -- regions in which future generations of stars will form.
The striking image, a composite of ultraviolet data from the Galaxy Evolution Explorer and radio data from the National Science Foundation's Very Large Array in New Mexico, shows the Southern Pinwheel galaxy, also known simply as M83.
In the new view, the main spiral, or stellar, disk of M83 looks like a pink and blue pinwheel, while its outer arms appear to flap away from the galaxy like giant red streamers. It is within these so-called extended galaxy arms that, to the surprise of astronomers, new stars are forming.
"It is absolutely stunning that we find such an enormous number of young stars up to 140,000 light-years away from the center of M83," said Frank Bigiel of the Max Planck Institute for Astronomy in Germany, lead investigator of the new Galaxy Evolution Explorer observations. For comparison, the diameter of M83 is only 40,000 light-years across.
The new image is online at http://www.nasa.gov/mission_pages/galex/20080416.html .
Some of the "outback" stars in M83's extended arms were first spotted by the Galaxy Evolution Explorer in 2005. Remote stars were also discovered around other galaxies by the ultraviolet telescope over subsequent years. This came as a surprise to astronomers because the outlying regions of a galaxy are assumed to be relatively barren and lack high concentrations of the ingredients needed for stars to form.
The newest Galaxy Evolution Explorer observations of M83 (colored blue and green) were taken over a longer period of time and reveal many more young clusters of stars at the farthest reaches of the galaxy. To better understand how stars could form in such unexpected territory, Bigiel and his colleagues turned to radio observations from the Very Large Array (red). Light emitted in the radio portion of the electromagnetic spectrum can be used to locate gaseous hydrogen atoms, or raw ingredients of stars. When the astronomers combined the radio and Galaxy Evolution Explorer data, they were delighted to see they matched up.
"The degree to which the ultraviolet emission and therefore the distribution of young stars follows the distribution of the atomic hydrogen gas out to the largest distances is absolutely remarkable," said Fabian Walter, also of the Max Planck Institute for Astronomy, who led the radio observations of hydrogen in the galaxy.
The astronomers speculate that the young stars seen far out in M83 could have formed under conditions resembling those of the early universe, a time when space was not yet enriched with dust and heavier elements.
"Even with today's most powerful telescopes, it is extremely difficult to study the first generation of star formation. These new observations provide a unique opportunity to study how early generation stars might have formed," said co-investigator Mark Seibert of the Observatories of the Carnegie Institution of Washington in Pasadena.
M83 is located 15 million light-years away in the southern constellation Hydra.
Other investigators include: Barry Madore of The Observatories of the Carnegie Institution of Washington; Armando Gil de Paz of the Complutense University of Madrid, Spain; David Thilker of Johns Hopkins University, Baltimore; Elias Brinks of the University of Hertfordshire, England; and Erwin de Blok of the University of Cape Town, South Africa.
The California Institute of Technology in Pasadena leads the Galaxy Evolution Explorer mission and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory, also in Pasadena, manages the mission and built the science instrument. Caltech manages JPL for NASA. The mission was developed under NASA's Explorers Program managed by NASA's Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission.
The Very Large Array is part of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Additional information about the Galaxy Evolution Explorer is online at http://www.nasa.gov/galex and http://www.galex.caltech.edu
Provided by National Radio Astronomy Observatory
"Our ideas about how the fastest-spinning pulsars are produced do not predict either the kind of orbit or the type of companion star this one has," said David Champion of the Australia Telescope National Facility. "We have to come up with some new scenarios to explain this weird pair."
Astronomers first detected the pulsar, called J1903+0327, as part of a long-term survey using the National Science Foundation's Arecibo radio telescope in Puerto Rico. They made the discovery in 2006 doing data analysis at McGill University, where Champion worked at the time. They followed up the discovery with detailed studies using the Arecibo telescope, the NSF's Robert C. Byrd Green Bank Telescope (GBT) in West Virginia, the Westerbork radio telescope in the Netherlands, and the Gemini North optical telescope in Hawaii.
The pulsar, a city-sized superdense stellar corpse left over after a massive star exploded as a supernova, is spinning on its axis 465 times every second. Nearly 21,000 light-years from Earth, it is in a highly-elongated orbit that takes it around its companion star once every 95 days. An infrared image made with the Gemini North telescope in Hawaii shows a Sun-like star at the pulsar's position. If this is an orbital companion to the pulsar, it is unlike any companions of other rapidly rotating pulsars. The pulsar, a neutron star, also is unusually massive for its type.
"This combination of properties is unprecedented. Not only does it require us to figure out how this system was produced, but the large mass may help us understand how matter behaves at extremely high densities," said Scott Ransom of the National Radio Astronomy Observatory.
Pulsars are neutron stars whose strong magnetic fields channel lighthouse-like beams of light and radio waves that whirl around as the star spins. Typical pulsars spin a few times a second, but some, like PSR J1903+0327, are much faster, rotating hundreds of times a second. They are called millisecond pulsars.
Astronomers think most millisecond pulsars are sped up by material falling onto them from a companion star. This requires the pulsar to be in a tight orbit around its companion that becomes more and more circular with time. The orbits of some millisecond pulsars are the most perfect circles in the Universe, so the elongated orbit of the new pulsar is a mystery.
"What we have found is a millisecond pulsar that is in the wrong kind of orbit around what appears to be the wrong kind of star," Champion said. "Now we have to figure out how this strange system was produced."
The scientists are considering three possibilities. The first, that the pulsar simply was born spinning quickly, seems unlikely to them. Another possibility, they say, is that the pulsar was formed in a tight group of stars known as a globular cluster, where it had a companion that spun it up. Later, a close encounter with another star in the cluster stripped it
of its companion and flung it out of the cluster. For several reasons, including the fact that they don't see a nearby cluster from which it could have come, they don't like that explanation either.
A third scenario says the pulsar may be part of a triple, not double, star system. The pulsar's 95-day orbit would be around a neutron star or white dwarf, not the Sun-like star seen in the infrared image. The Sun-like star would then be in a more-distant orbit around the pulsar and its companion.
"We've found about 50 pulsars in binary systems. We may now have found our first pulsar in a stellar triple system," Ransom said.
The international research team is busy trying to get their answers. They will study the star in the infrared image further to confirm the indications that it is similar to our Sun and that it actually is a companion to the pulsar. Additional radio observations will study the pulsar's orbit and seek to measure its motion in space.
To see an image of the pulsar, click here.
Wednesday, May 14, 2008
Radio (NSF/NRAO/VLA/Cambridge/D.Green et al.);
This makes the original explosion the most recent supernova in the Galaxy, as measured in Earth's time-frame (referring to when events are observable at Earth). Equivalently, this is the youngest known supernova remnant in the Galaxy (140 years old), easily beating the previous record of about 330 years for Cassiopeia A. The rapid expansion and young age for G1.9+0.3 was recently confirmed by a new VLA image obtained in early 2008.
This artist's impression shows a view looking down on the Milky Way galaxy. The position of the Sun is shown, as are the approximate positions and names (shown in orange) of historical supernovas. These are stellar explosions that are thought to have occurred in the last 2,000 years and may have been seen by early astronomers. The estimated position of the recently discovered G1.9+0.3 is shown in black. Although the distance to this remnant is uncertain, the angle is accurately known. Note that G1.9+0.3 is the only object that is found in the bulge of the galaxy. (Credit: NASA/CXC/M.Weiss)
Optical Image of Milky Way Galaxy
The original supernova explosion was not seen in optical light about 140 years ago because it occurred close to the center of the Galaxy, and is embedded in a dense field of gas and dust. This made the supernova about a trillion times fainter, in optical light, than if it had been unobscured. However, X-rays and radio waves from the resulting supernova remnant easily penetrate this dust and gas.
On the right is an infrared image from the Two Micron All Sky survey (2MASS), where the colors represent different infrared wavelengths. The center of the galaxy is the bright red spot in the upper right and the location of G1.9+0.3 is shown by the box in the lower left, less than two degrees away (corresponding to about a thousand light years at the distance of the galactic center). More stars are visible in this 2MASS image than in an optical image, where obscuration by dust and gas is more prominent. Also, note the difference in orientation: in the close-up view of G1.9+0.3, north is up and east is to the left, while in the 2MASS image north is to the left and east is down.
Supernova remnants are caused when the debris thrown outwards by the explosion crashes into surrounding material, generating a shell of hot gas and high-energy particles that glows brightly in X-rays, radio waves and other wavelengths for thousands of years. In the case of G1.9+0.3 the material is expanding outwards at almost 35 million miles per hour, or about 5% the speed of light, an unprecedented expansion speed for a supernova remnant. Another superlative for G1.9+0.3 is that it has generated the most energetic electrons ever seen in a supernova remnant.
The incredible images from NASA's "Great Observatories" and many other NASA space- and ground-based telescopes are now available to the public in an educational and innovative manner through the release of the free WorldWide Telescope software from Microsoft.
Views of the cosmos from such observatories as NASA's Hubble Space Telescope, Spitzer Space Telescope, and Chandra X-ray Observatory can all be accessed through the same intuitive interface of exploring the night sky. Several all-sky surveys are also available through the WorldWide Telescope, including the Two Micron All-Sky Survey and the Infrared Astronomical Satellite survey. The rich multimedia software enables browsing through the visible, infrared, x-ray and other views of the universe, allowing for direct comparison of multi-wavelength observations that reveal surprising contrasts.
Other innovative features include guided tours created by scientists and educators. These tours guide users through various aspects of astronomy with narration, music, text and graphics. Members of the public, including children, will also be able to make their own tours to share with others.
The Two Micron All-Sky Survey is a collaborative effort between the University of Massachusetts, Amherst, and the Infrared Processing and Analysis Center in Pasadena, Calif., operated by NASA's Jet Propulsion Laboratory and the California Institute of Technology, both in Pasadena.
The Infrared Astronomical Satellite is a joint project between NASA, the Netherlands and the United Kingdom. Its data are archived at the Infrared Processing and Analysis Center.
JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech, which manages JPL for NASA.
The WorldWide Telescope is available as of May 13, 2008, at www.worldwidetelescope.org.
Tuesday, May 13, 2008
The most energetic particles in the universe have regained some of their former mystery. Last year, it seemed that the origin of these particles had finally been tracked down to a set of giant black holes in nearby galaxies, but a new study casts doubt on that conclusion.
Ultra-high-energy cosmic rays, or UHECRs, are individual sub-atomic particles with energies up to about 1020 electron volts, far beyond anything achieved in particle accelerators.
When they hit the Earth's atmosphere, they produce a shower of other particles, and the Pierre Auger Observatory in Argentina has spotted more of these events than any other detector.
In November 2007, an Auger team looked at the arrival directions of the 27 highest-energy cosmic rays, and found that they fit a suggestive pattern. Most came from within 3° of the directions of nearby active galaxies, which hold supermassive black holes at their cores and emit many kinds of radiation. So it seemed that the galaxies were emitting UHECRs too.
Now researchers led by Igor Moskalenko of Stanford University in California, US, have looked more closely at these particular active galaxies, as well as others along the same line of sight. They find that they are an unremarkable bunch. "The sample consists mainly of low-power active galaxies," says Moskalenko.
Such weak active galaxies are common – so common in fact that astronomers expect to find several within 3° of any random direction on the sky. "The correlation found by the Auger group is likely to be a chance coincidence," Moskalenko told New Scientist.
Furthermore, Moskalenko believes that the small, weak active galaxies simply lack the firepower to generate the highest energy cosmic rays. For one thing, they show no signs of high-energy gamma-ray emission, which he believes should go along with cosmic-ray acceleration.
Instead, he suggests that UHECRs are more likely to come from the more energetic breeds of active galaxy, such as quasars and radio galaxies, especially those that squirt out high-speed jets of material, which are already known to emit gamma rays.
Some of the cosmic rays seen by Auger do coincide with examples of this tooled-up type of active galaxy. At least four of them could have been fired at us by Centaurus A, a radio galaxy only 12 million light years away.
Moskalenko and colleagues say the other UHECRs could also be from nearby active galaxies with jets, even if their arrival directions don't coincide with such sources.
That's because these cosmic rays are probably protons or heavier charged particles, whose paths are bent by magnetic fields. Moskalenko points out that the strength and alignment of intergalactic magnetic fields is not well known, so they might be able to bend the paths of cosmic rays beyond 3°. Then, radio galaxies such as Centaurus A could in effect be shooting around the corner at us.
But Auger team member Jim Hinton of Leeds University, UK, disagrees with Moskalenko's interpretation of the data. He says the fact that the galaxies are weak suggests that most should not be able to produce even a single ultra-high energy cosmic ray that could make its way to a detector on Earth.
That explains why Auger has detected only 27 UHECRs, when there are far more active galaxies that theoretically produce them. "Which ones are seen is largely a matter of chance," Hinton told New Scientist.
"This also implies that UHECR-accelerating [active galaxies] are common in nature – ie, they are not just the powerful radio galaxies," Hinton continued. "This implication is something that has surprised (and led to criticism by) many authors – but nature may just be like that!"
Even if the UHECR puzzle is not yet solved, it might be soon – the Auger Observatory, which released its first results in 2005, is almost completed and is amassing cosmic ray data faster than before.
Monday, May 12, 2008
Merging system's interaction sets standard for galaxy evolution.
Provided by European Space Agency
The Antennae Galaxies are among the closest known merging systems. Also known as NGC 4038 and NGC 4039, the two began interacting a few hundred million years ago, creating one of the most impressive sights in the night sky. They are used by scientists as a standard against which to validate theories of galactic evolution.
An international group of scientists led by Ivo Saviane from the European Southern Observatory used Hubble's Advanced Camera for Surveys and Wide Field Planetary Camera 2 to observe individual stars spawned by the colossal cosmic collision in the Antennae Galaxies. By measuring the colors and brightnesses of red giant stars in the system, the scientists found that the Antennae are much closer than previously thought: 45 million light-years instead of the previous best estimate of 65 million light-years.
The team targeted a region in the relatively quiescent outer regions in the southern tidal tail, away from the active central regions. This tail consists of material thrown from the main galaxies as they collided. The scientists needed to observe regions with older red giant stars to derive an accurate distance. Red giants are known to reach a standard brightness, which can then be used to infer their distance.
The previous distance to the Antennae Galaxy was about 65 million light-years, although values as high as 100 million light years have been used. Our Sun is only 8 light-minutes away from us, so the Antennae Galaxies may seem rather distant. But if we consider that we already know of galaxies more than 10 billion light-years away, the two galaxies are really our neighbors.
The new, smaller distance makes the Antennae Galaxies less extreme in terms of the physics needed to explain the observed phenomena. For instance, its infrared radiation is now that expected of a standard early merging event rather than that of an ultraluminous infrared galaxy. The size of the star clusters formed as a consequence of the Antennae merger now agree with those of clusters created in other mergers instead of being 1.5 times as large.
The Antennae Galaxies are named for the two long tails of stars, gas and dust that resemble the antennae of an insect. These "antennae" are a physical result of the collision between the two galaxies. Studying their properties gives us a preview of what may happen when our Milky Way Galaxy collides with the neighboring Andromeda galaxy in several billion years.