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Eric Olson
ELKK Meteorites
http://www.star-bits.com
> 
> 
> Contact: James Hathaway
> hathaway_at_asu.edu
> 480-965-6375
> Arizona State University
> May 20, 2004
> 
> Theory proposes new view of sun and Earth's creation
> 
> Like most creation stories, this one is dramatic: we began, not as a mere
> glimmer buried in an obscure cloud, but instead amidst the glare and turmoil
> of restless giants.
> 
> Or so says a new theory, supported by stunning astronomical images and hard
> chemical analysis. For years most astronomers have imagined that the Sun and
> Solar System formed in relative isolation, buried in a quiet, dark corner of
> a less-than-imposing interstellar cloud. The new theory challenges this
> conventional wisdom, arguing instead that the Sun formed in a violent
> nebular environment - a byproduct of the chaos wrought by intense
> ultraviolet radiation and powerful explosions that accompany the short but
> spectacular lives of massive, luminous stars.
> 
> The new theory is described in a "Perspectives" article appearing in the May
> 21 issue of Science. The article was written by a group of Arizona State
> University astronomers and meteorite researchers who cite recently
> discovered isotopic evidence and accumulated astronomical observations to
> argue for a history of development of the Sun, the Earth and our Solar
> System that is significantly different from the traditionally accepted
> scenario.
> 
> If borne out by future work, this vision of our cosmic birth could have
> profound implications for understanding everything from the size and shape
> of our solar system to the physical makeup of the Earth and the development
> of the chemistry of life.
> 
> "There are two different sorts of environment where low-mass stars like the
> Sun form," explained ASU astronomer Jeff Hester, the essay's lead author.
> "In one kind of star-forming environment, you have a fairly quiescent
> process in which an undisturbed molecular cloud slowly collapses, forming a
> star here? a star there. The other type of environment in which Sun-like
> stars form is radically different. These are more massive regions that form
> not only low-mass stars, but luminous high-mass stars, as well."
> 
> More massive regions are very different because once a high-mass star forms,
> it begins pumping out huge amounts of energy that in turn completely changes
> the way Sun-like stars form in the surrounding environment. "People have
> long imagined that the Sun formed in the first, more quiescent type of
> environment," Hester noted, "but we believe that we have compelling evidence
> that this is not the case."
> 
> Critical to the team's argument is the recent discovery in meteorites of
> patterns of isotopes that can only have been caused by the radioactive decay
> of iron-60, an unstable isotope that has a half life of only a million and a
> half years. Iron-60 can only be formed in the heart of a massive star and
> thus the presence of live iron-60 in the young Solar System provides strong
> evidence that when the Sun formed (4.5 billion years ago) a massive star was
> nearby.
> 
> Hester's coauthors on the Science essay include Steve Desch, Kevin Healy,
> and Laurie Leshin. Leshin is a cosmochemist and director of Arizona State
> University's Center for Meteorite Studies. "One of the exciting things about
> the research is that it is truly transdisciplinary, drawing from both
> astrophysics and the study of meteorites - rocks that you can pick up and
> hold in your hand - to arrive at a new understanding of our origins," noted
> Leshin.
> 
> When a massive star is born, its intense ultraviolet radiation forms an "HII
> region" - a region of hot, ionized gas that pushes outward through
> interstellar space. The Eagle Nebula, the Orion Nebula, and the Trifid
> Nebula are all well-known examples of HII regions. A shock wave is driven in
> advance of the expanding HII region, compressing surrounding gas and
> triggering the formation of new low-mass stars. "We see triggered low-mass
> star formation going on in HII regions today," said Healy, who recently
> completed a study of radio observations of this process at work.
> 
> The star does not have much time to get its act together, though. Within
> 100,000 years or so, the star and what is left of its small natal cloud will
> be uncovered by the advancing boundary of the HII region and exposed
> directly to the harsh ultraviolet radiation from the massive star. "We see
> such objects emerging from the boundaries of HII regions,'' Hester said.
> "These are the 'evaporating gaseous globules' or 'EGGs' seen in the famous
> Hubble image of the Eagle Nebula."
> 
> EGGs do not live forever either. Within about ten thousand years an EGG
> evaporates, leaving behind only the low-mass star and its now-unprotected
> protoplanetary disk to face the brunt of the massive star's wrath. Like a
> chip of dry ice on a hot day, the disk itself now begins to evaporate,
> forming a characteristic tear-drop-shaped structure like the "proplyds" seen
> in Hubble images of the Orion Nebula. "Once we understood what we were
> looking at, we realized that we had a number of images of EGGs caught just
> as they were turning into proplyds," said Hester. "The evolutionary tie
> between these two classes of objects is clear."
> 
> Within another ten thousand years or so the proplyd, too, is eroded away.
> All that remains is the star itself, surrounded by the inner part of the
> disk (comparable in size to our Solar System), which is able to withstand
> the continuing onslaught of radiation. It is from this disk and in this
> environment that planets may form.
> 
> The process leaves a Sun-like star and its surrounding disk sitting in the
> interior of a low density cavity with a massive star close at hand. Massive
> stars die young, exploding in violent events called "supernovas." When a
> supernova explodes it peppers surrounding infant planetary systems with
> newly synthesized chemical elements - including short-lived radioactive
> isotopes such as iron-60.
> 
> "This is where the meteorite data come in," said Hester. "When we look at
> HII regions we see that they are filled with young, Sun-like stars, many of
> which are known to be surrounded by protoplanetary disks. Once you ask the
> question, 'what is going to happen when those massive stars go supernova?',
> the answer is pretty obvious. Those young disks are going to get enriched
> with a lot of freshly-made elements."
> 
> "When you then pick up a meteorite and find a mix of materials that can only
> be easily explained by a nearby supernova, you realize that you are looking
> at the answer to a very longstanding question in astronomy and planetary
> science," Desch added.
> 
> "So from this we now know that if you could go back 4.5 billion years and
> watch the Sun and Solar System forming, you would see the kind of
> environment that you see today in the Eagle or Trifid nebulas," said Hester.
> 
> "There are many aspects of our Solar System that seem to make sense in light
> of the new scenario," notes Leshin. "For example, this might be why the
> outer part of the Solar System - the Kuiper Belt - seems to end abruptly.
> Ultraviolet radiation would also have played a role in the organic chemistry
> of the young solar system, and could explain other peculiar effects such as
> anomalies in the abundances of isotopes of oxygen in meteorites."
> 
> One of the most intriguing speculations is that the amount of radioactive
> material injected into the young solar system by a supernova might have
> profoundly influenced the habitability of Earth itself. Heat released by the
> decay of this material may have been responsible for "baking out" the
> planetesimals from which the earth formed, and in the process determining
> how much water is on Earth today.
> 
> "It is kind of exciting to think that life on Earth may owe its existence to
> exactly what sort of massive star triggered the formation of the Sun in the
> first place, and exactly how close we happened to be to that star when it
> went supernova," mused Hester. "One thing that is clear is that the
> traditional boundaries between fields such as astrophysics, meteoritics,
> planetary science, and astrobiology just got less clear-cut. This new
> scenario has a lot of implications, and makes a lot of new predictions that
> we can test."
> 
> If it is accepted, the new theory may also be of use in looking for life in
> the universe beyond. "We want to know how common Earth-like planets are. The
> problem with answering that question is that if you don't know how
> Earth-like planets are formed - if you don't understand their connection
> with astrophysical environments - then all you can do is speculate," Hester
> said.
> 
> "We think that we're starting to see a very specific causal connection
> between astrophysical environments and the things that have to be in place
> to make a planet like ours."
> 
>                                      ###
> 
> Sources: Jeff Hester, 480-965-0741, jhester_at_asu.edu
> Laurie Leshin, 480-965-0796, laurie.leshin_at_asu.edu
> 
> Images: http://clas.asu.edu/newsevents/pressreleases/photos/HII/
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