Following up on this morning’s post re cosmic rays and the early Earth comes news that the Chandra X-ray Observatory has mapped cosmic ray acceleration in Cassiopeia A, a 325-year-old supernova remnant. The map, showing that electrons are being accelerated close to a theoretically maximum rate, provides evidence that supernova remnants are major contributors of energetic charged particles like cosmic rays.
“Scientists have theorized since the 1960s that cosmic rays must be created in the tangle of magnetic fields at the shock, but here we can see this happening directly,” said Michael Stage of the University of Massachusetts, Amherst. “Explaining where cosmic rays come from helps us to understand other mysterious phenomena in the high-energy universe.”
Image: This extraordinarily deep Chandra image shows Cassiopeia A (Cas A, for short), the youngest supernova remnant in the Milky Way. New analysis shows that this supernova remnant acts like a relativistic pinball machine by accelerating electrons to enormous energies. The blue, wispy arcs in the image show where the acceleration is taking place in an expanding shock wave generated by the explosion. The red and green regions show material from the destroyed star that has been heated to millions of degrees by the explosion. Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.
The remarkable thing about the image above is that the glow generated by cosmic rays is brighter than the superheated gas caught in the supernova shock waves. The data demonstrate the effects of cosmic ray acceleration and have much to teach us about how supernova remnants change over time. And I like the simile used by team member Glenn Allen (MIT), who likened the motion of charged particles as they are accelerated to the action of a pinball machine (see the caption). Are we looking at a primal force in spurring life’s evolution on nearby worlds?
Gamma-ray production in young open clusters: Berk 87, Cyg OB2 and Westerlund 2
Authors: W. Bednarek
(Submitted on 26 Apr 2007)
Abstract: Young open clusters are likely sites of cosmic ray acceleration as indicated by recent detections of the TeV gamma-ray sources in the directions of two open clusters (Cyg OB2 and Westerlund 2) and their directional proximity to some unidentified EGRET sources. In fact, up to now a few different scenarios for acceleration of particles inside open clusters have been considered, i.e. shocks in massive star winds, pulsars and their nebulae, supernova shocks, massive compact binaries. Here we consider in detail the radiation processes due to both electrons and hadrons accelerated inside the open cluster. As a specific scenario, we apply the acceleration process at the shocks arising in the winds of WR type stars. Particles diffuse through the medium of the open cluster during the activity time of the acceleration scenario defined by the age of the WR star. They interact with the matter and radiation, at first inside the open cluster and, later in the dense surrounding clouds. We calculate the broad band spectrum in different processes for three example open clusters (Berk 87, Cyg OB2, Westerlund 2) for which the best observational constraints on the spectra are at present available. It is assumed that the high energy phenomena, observed from the X-ray up to the GeV-TeV gamma-ray energies, are related to each other. We conclude that the most likely description of the radiation processes in these objects is achieved in the hybrid (leptonic-hadronic) model in which leptons are responsible for the observed X-ray and GeV gamma-ray emission and hadrons are responsible for the TeV gamma-ray emission, which is produced directly inside and in dense clouds surrounding the open cluster.
Comments:
14 pages, 7 figures, submitted to MNRAS
Subjects:
Astrophysics (astro-ph)
Cite as:
arXiv:0704.3517v1 [astro-ph]
Submission history
From: Wlodek Bednarek [view email]
[v1] Thu, 26 Apr 2007 11:34:54 GMT (565kb)
http://arxiv.org/abs/0704.3517
10,000 Earths’ Worth of Fresh Dust Found Near Star Explosion
Astronomers have at last found definitive evidence that the universe’s first dust – the celestial stuff that seeded future generations of stars and planets – was forged in the explosions of massive stars.
The findings, made with NASA’s Spitzer Space Telescope, are the most significant clue yet in the longstanding mystery of where the dust in our very young universe came from. Scientists had suspected that exploding stars, or supernovae, were the primary source, but nobody had been able to demonstrate that they can create copious amounts of dust – until now. Spitzer’s sensitive infrared detectors have found 10,000 Earth masses worth of dust in the blown-out remains of the well-known supernova remnant Cassiopeia A.
“Now we can say unambiguously that dust – and lots of it – was formed in the ejecta of the Cassiopeia A explosion. This finding was possible because Cassiopeia A is in our own galaxy, where it is close enough to study in detail,” said Jeonghee Rho of NASA’s Spitzer Science Center at the California Institute of Technology in Pasadena. Rho is the lead author of a new report about the discovery appearing in the Jan. 20 issue of the Astrophysical Journal.
Space dust is everywhere in the cosmos, in our own neck of the universe and all the way back billions of light-years away in our infant universe. Developing stars need dust to cool down enough to collapse and ignite, while planets and living creatures consist of the powdery substance. In our nearby universe, dust is pumped out by dying stars like our sun. But back when the universe was young, sun-like stars hadn’t been around long enough to die and leave dust.
That’s where supernovae come in. These violent explosions occur when the most massive stars in the universe die. Because massive stars don’t live very long, theorists reasoned that the very first exploding massive stars could be the suppliers of the unaccounted-for dust. These first stars, called Population III, are the only stars that formed without any dust.
Other objects in addition to supernovae might also contribute to the universe’s first dust. Spitzer recently found evidence that highly energetic black holes, called quasars, could, together with supernovae, manufacture some dust in their winds (http://www.spitzer.caltech.edu/Media/releases/ssc2007-16/index.shtml) .
Rho and her colleagues analyzed the Cassopeia A supernova remnant, located about 11,000 light-years away. Though this remnant is not from the early universe, its proximity to us makes it easier to address the question of whether supernovae have the ability to synthesize significant amounts of dust. The astronomers analyzed the infrared light coming from Cassiopeia A using Spitzer’s infrared spectrograph, which spreads light apart to reveal the signatures of different elements and molecules. “Because Spitzer is extremely sensitive to dust, we were able to make high-resolution maps of dust in the entire structure,” said Rho.
The map reveals the quantity, location and composition of the supernova remnant’s dust, which includes proto-silicates, silicon dioxide, iron oxide, pyroxene, carbon, aluminium oxide and other compounds. One of the first things the astronomers noticed was that the dust matches up perfectly with the gas, or ejecta, known to have been expelled in the explosion. This is the smoking gun indicating the dust was freshly made in the ejecta from the stellar blast. “Dust forms a few to several hundred days after these energetic explosions, when the temperature of gas in the ejecta cools down,” said Takashi Kozasa, a co-author at the Hokkaido University in Japan.
The team was surprised to find freshly-made dust deeper inside the remnant as well. This cooler dust, mixed in with gas referred to as the unshocked ejecta, had never been seen before.
All the dust around the remnant, both warm and cold, adds up to about three percent of the mass of the sun, or 10,000 Earths. This is just enough to explain where a large fraction, but not all, of the universe’s early dust came from. “Perhaps at least some of the unexplained portion is much colder dust, which could be observed with upcoming telescopes, such as Herschel,” said Haley Gomez, a co-author at University of Wales, Cardiff. The Herschel Space Observatory, scheduled to launch in 2008, is a European Space Agency mission with significant NASA participation.
Rho also said that more studies of other supernovae from near to far are needed to put this issue to rest. She notes that the rate at which dust is destroyed – a factor in determining how much dust is needed to explain the dusty early universe – is still poorly understood.
The principal investigator of the research program, and a co-author of the paper, is Lawrence Rudnick of the University of Minnesota, Twin Cities. Other co-authors include W.T. Reach of the Spitzer Science Center; J. D. Smith of the Steward Observatory, Tucson, Ariz.; T. Delaney of the Massachusetts Institute of Technology, Cambridge; J.A. Ennis of the University of Minnesota; and A. Tappe of the Spitzer Science Center and the Harvard Smithsonian Center for Astrophysics, Cambridge, Mass.
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. Caltech manages JPL for NASA. Spitzer’s infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell. For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .