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TOPIC: The first stars


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Posts: 131433
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NGC 6397
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Title: The Space Motion of the Globular Cluster NGC 6397
Authors: Jasonjot S. Kalirai, Jay Anderson, Harvey B. Richer, Ivan R. King, James P. Brewer, Giovanni Carraro, Saul D. Davis, Gregory G. Fahlman, Brad M. S. Hansen, Jarrod R. Hurley, Sebastien Lepine, David B. Reitzel, R. Michael Rich, Michael M. Shara, Peter B. Stetson

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 ± 0.04 mas/yr and mu_delta = -17.34 ±  0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.

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RE: The first stars
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Title: First stars as a possible origin for the helium-rich population in omega Centauri
Authors: Ena Choi, Sukyoung K. Yi

The most massive Galactic globular cluster omega Cen appears to have two, or perhaps more, distinct main sequences. Its bluest main sequence is at the centre of debate because it has been suggested to have an extremely high helium abundance of Y ~ 0.4. The same helium abundance is claimed to explain the presence of extreme horizontal branch stars of omega Cen as well. This demands a relative helium to metal enrichment of deltaY/deltaZ ~ 70; that is, more than one order of magnitude larger than the generally accepted value. Candidate solutions, namely, AGB stars, massive stars, and supernovae, have been suggested; but in this study, we show that none of them is a viable channel, in terms of reproducing the high value of deltaY/deltaZ for the constrained age difference between the red and blue populations. Essentially no populations with an ordinary initial mass function, including those candidates, can produce such a high deltaY/deltaZ because they all produce metals as well as helium. As an alternative, we investigate the possibility of the stochastic ``first star'' contamination to the gas from which the younger generation of omega Cen formed. This requires the assumption that Population III star formation episode overlaps with that of Population II. While the required condition appears extreme, very massive objects in the first star generation provide a solution that is at least as plausible as any other suggestions made before.

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Title: The first generation of stars in LCDM cosmology
Authors: L. Gao, T. Abel, C. S. Frenk, A. Jenkins, V. Springel, N. Yoshida
Version 2

We have performed a large set of high-resolution cosmological simulations using smoothed particle hydrodynamics to study the formation of the first luminous objects in the LCDM cosmology. We follow the collapse of primordial gas clouds in eight early structures and document the scatter in the properties of the first star-forming clouds. Our first objects span formation redshifts from z~10 to z~50 and cover an order of magnitude in halo mass. We find that the physical properties of the central star-forming clouds are very similar in all of the simulated objects despite significant differences in formation redshift and environment. The physical properties of the clouds have little correlation with spin, mass, or assembly history of the host halo. The collapse of protostellar objects at higher redshifts progresses much more rapidly due to the higher densities, which accelerates the formation of molecular hydrogen, enhances initial cooling and shortens the dynamical timescales. The mass of the star-forming clouds cover a broad range, from a few hundred to a few thousand solar masses, and exhibit various morphologies: some of have disk-like structures nearly rotational supported; others form flattened spheroids; still others form bars. All of them develop a single proto-stellar 'seed' which does not fragment into multiple objects up to the moment that the central gas becomes optically thick to H2 cooling lines. At this time, the instantaneous mass accretion rate onto the centre varies significantly from object to object, with disk-like structures have the smallest mass accretion rates. The formation epoch and properties of the star-forming clouds are sensitive to the values of cosmological parameters.

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Title: The First Stars in the Universe and Cosmic Reionisation
Author: Rennan Barkana (Tel Aviv University)

The earliest generation of stars, far from being a mere novelty, transformed the universe from darkness to light. The first atoms to form after the Big Bang filled the universe with atomic hydrogen and a few light elements. As gravity pulled gas clouds together, the first stars ignited and their radiation turned the surrounding atoms into ions. By looking at gas between us and distant galaxies, we know that this ionisation eventually pervaded all space, so that few hydrogen atoms remain today between galaxies. Knowing exactly when and how it did so is a primary goal of cosmologists, because this would tell us when the early stars formed and in what kinds of galaxies. Although this ionisation is beginning to be understood by using theoretical models and computer simulations, a new generation of telescopes is being built that will map atomic hydrogen throughout the universe.

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Posts: 131433
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NGC6397
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NASA's Hubble Space Telescope has uncovered what astronomers are reporting as the dimmest stars ever seen in any globular star cluster. Globular clusters are spherical concentrations of hundreds of thousands of stars.

These clusters formed early in the 13.7-billion-year-old universe. The cluster NGC 6397 is one of the closest globular star clusters to Earth. Seeing the whole range of stars in this area will yield insights into the age, origin, and evolution of the cluster.

Stars that don't have enough mass never shine, dying billions of years before their bigger counterparts. But astronomers have never been able to measure the exact mass limit, because the lightest stars that do shine can be simply too faint to detect.
Now, new images show for the first time how big a star must be to avoid impending doom.
Reporting in the journal Science, astronomers have viewed high quality pictures of some of the faintest stars in our galaxy for the first time.
The images come from the dimmest members of the NGC 6397 cluster in the constellation Ara - ancient stars that orbit the Milky Way's centre in a close-knit group.

NGC 6397
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Credit NASA
Position (2000): R.A. 17h 40m 41s.36 Dec. -53° 40' 25".3

The new pictures finally answer one of astronomy's burning questions. By calculating the mass of the faintest ancient stars, researchers can work out the minimum mass needed for a star to survive. Potential stars that fall just short of the limit die before they're even born.
Almost everything about a star's fate is determined by the mass of the gas cloud from which it is formed.
Gravity pulls the gas into a giant ball, or protostar. As the ball gets bigger and more solid, its centre becomes extremely hot.
For large protostars, the heat becomes so intense that the star begins to burn hydrogen by fusion, and so starts to shine. These stars can sustain themselves for billions of years, because their heat is self-replenishing. Some could live longer than the estimated lifespan of our Universe - in effect, forever.
Small protostars never make it this far. Their cores are just not hot enough for hydrogen fusion, so they never light up. They quickly stop shrinking and fade into brown dwarfs, or giant planets.
A small difference in mass can therefore mean the difference between effective immortality and an untimely death.
But how big is big enough? The problem has been that those stars that do start to shine - but have only just enough mass - burn very faintly and are nearly impossible to see.



The long-awaited new images finally lay this question to rest, say the authors. The dimmest stars were measured as being 8.3% of the Sun's mass. All protostars that are smaller than this are headed for life as a brown dwarf.
The pictures also provide a spectacular new record of the end of a star's life.
Large stars, which burn out more quickly, can become white dwarfs- glowing cinders which slowly fade with age. Astronomers had predicted that these should turn blue as they move towards death.
The new findings provide the first images of this signature colour change, confirming expectations.
These ancient white dwarfs, 8500 light-years away, which have never been seen before, are amongst the Universe's oldest stars. Now that astronomers can work out how long they have lived, they can refine estimates of the age of the Universe.

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ISOSS J18364-0221
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Scientists have secured their first look at the birth of monstrous stars that shine 100 000 times more brightly than the Sun, thanks to ESA’s Infrared Space Observatory (ISO).

The discovery allows astronomers to begin investigating why only some regions of space promote the growth of these massive stars.
Space is littered with giant clouds of gas. Occasionally, regions within these clouds collapse to form stars.

"One of the major questions in the field of study is why do some clouds produce high- and low-mass stars, whilst others form only low-mass stars?" - Oliver Krause, Max-Planck-Institut für Astronomie, Heidelberg and Steward Observatory, Arizona.

The conditions necessary to form high-mass stars are difficult to deduce because such stellar monsters form far away and are shrouded behind curtains of dust. Only long wavelengths of infrared radiation can escape from these obscuring cocoons and reveal the low temperature dust cores that mark the sites of star formation. This radiation is exactly what ISO’s ISOPHOT far-infrared camera has collected.

Stephan Birkmann, Oliver Krause and Dietrich Lemke, all of the Max-Planck-Institut für Astronomie, Heidelberg, used ISOPHOT’s data to zero-in on two intensely cold and dense cores, each containing enough matter to form at least one massive star.


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The ISOSS J18364--0221 region contains two dense cores, each containing enough matter to become at least one massive star. The background image was taken by the Calar Altar 3.5 metre telescope during October 2003 and June 2004. The white contours show cold dust and were taken by the James Clerk Maxwell Telescope during May 2003. The diffuse and elongated emission displayed in green is due to shocked molecular hydrogen (was also observed at Calar Alto) and this traces the outflow of the obscured central source.
Credits: Birkmann/Krause/Lemke (Max-Planck-Insitut für Astronomie)


"This opens up a new era for the observations of the early details of high-mass star formation" - Oliver Kraus.

The data was collected in the ISOPHOT Serendipity Survey (ISOSS), a clever study pioneered by Lemke. He realised that when ISO was turning from one celestial object to another, valuable observing time was being lost. He organised for ISOPHOT’s far-infrared camera to continuously record during such slews and beam this data to Earth.
During the ISO mission, which lasted for two and a half years during 1995–98, the spacecraft made around 10 000 slews, providing a web of data across the sky for the previously unexplored window of infrared emission at 170 micrometres. This wavelength is 310 times longer than optical radiation and reveals cold dust down to just 10K (–263° Celsius).
A catalogue was produced of the cold sites in the survey.
Birkmann and his colleagues investigated this catalogue and found fifty potential places of high-mass stellar birth. A campaign of follow-up observations using ground-based telescopes revealed that object ISOSS J18364-0221 was in fact two cold dense cores that looked suspiciously like those associated with the birth of low-mass stars, but containing much more mass.

The first core is at 16.5 Kelvin (–256.5° Celsius). It contains seventy-five times the mass of the Sun and shows signs of gravitational collapse. The second one is around 12K (–261° Celsius) and contains 280 solar masses. The team are currently studying the other potential sites.

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Posts: 131433
Date:
HE1327-2326
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The star, HE1327-2326, in the constellation Hydra sets a new record for being the most heavy element-deficient star ever found. Its chemical composition, as measured with the Subaru Telescope High Dispersion Spectrograph, provides evidence of nucleosynthesis by the first generations of stars in the universe, and places new constraints on their masses and metal enrichment history in the very early universe. The new star HE1327-2326 has an unexpectedly low abundance of the metal lithium and an unexpectedly high amount of the metal strontium for such a primitive star.
To explain the strontium, it has been suggested that HE1327-2326 is a binary system. If the primitive star's binary companion had the opportunity to evolve, it might start to synthesise heavier metals including strontium.
The first generation of stars are believed to have formed several hundred million years after the Big Bang, which occurred 13.7 billion years ago. These stars were part of the transition from a universe that consisted only of hydrogen and helium gas to one that contains a variety of elements and objects including stars and galaxies.

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RE: The Oldest Stars
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Calculations Reveal Chemical Inheritance of Oldest Stars
Kenichi Nomoto, Nobuyuki Iwamoto and other researchers from the University of Tokyo and the Japan Atomic Energy Research Institute have found new evidence that two stars that were thought to be among the earliest generations of stars were in fact formed from the explosion of older stars.


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Their new computer simulations of the life and death of first generation stars and the chemical elements they produce match earlier observational data from Subaru telescope.


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He1327-2326, 4000 light-years away in the constellation Hydra, is one of the oldest stars that developed briefly after the emergence of the universe.
It contains only one three hundred-thousandth of the heavy elements in comparison to the sun.
Its 13.6 billion years age was recognized in April 2005.
It replaces He0107-5240 (discovered in 2002) as the oldest and metal poor star.



The 0,8 solar mass star He0107-5240, 36,000 light-years in the constellation Phoenix, now becomes the Second oldest star (13.6 billion years).

The new star HE1327-2326 has an unexpectedly low abundance of the metal lithium and an unexpectedly high amount of the metal strontium for such a primitive star.

"The lithium problem is immediately more troublesome. Many of the primitive stars studied in the past have lithium abundances that are very similar to one another. It's remarkable but apparently what we see in these stars is the tiny amount of lithium produced in the Big Bang. Yet in this star, it's not at that perfect value, so we're a bit confused as to why that might be. When we get such extreme objects as this it really forces the people who model how these elemental abundances come about to take a close look at what they might be missing …" - Timothy Beers, of Michigan State University in East Lansing, US.

One possibility is HE1327-2326 is a binary system.
If the primitive star's binary companion had the opportunity to evolve, it might start to synthesise heavier metals including strontium.
The evolving binary might then cast off its outer envelope, allowing some of the material to be accepted by its primitive companion, explaining the high strontium content.

Read More (Japanese Only)


-- Edited by Blobrana at 20:05, 2005-06-08

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Posts: 131433
Date:
The first stars
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Two stars that were thought to be among the first produced in the universe were in fact formed from the explosion of an older star, according to astrophysicists in Japan. Ken'ichi Nomoto and colleagues from the University of Tokyo came to this conclusion by comparing the chemical composition of the stars with a computer model. The work could help shed more light on the nature of the first stars.

One of the biggest challenges in astronomy is identifying the first stars in the universe -- those that were born from a primordial gas of hydrogen and helium. These "first-generation" stars should contain very few heavier elements, which are known collectively in astrophysics as "metals". There was therefore much interest when two teams of astronomers (the second of which included Nomoto) found two stars, one in 2002 and the other this year, in which the ratio of iron to hydrogen is about a hundred thousand times less than in our Sun.
Now, however, Nomoto and colleagues argue that these "hyper-metal-poor" stars are in fact "second-generation" stars. They have very unusual chemical abundances, including a ratio of carbon to iron that is ten thousand times as much the Sun. The Japanese team believes that the stars were formed from gases that were chemically contaminated by a first-generation star that had died and formed a black hole in a supernova explosion.

According to their model, most of the iron synthesized in the first supernovae "fell back" onto the black holes that were formed, which meant that only a tiny fraction of iron was ejected into interstellar space. The Tokyo group tested its predictions by comparing the observed chemical abundances of the two stars with those obtained in computer calculations.

The result means that the nature of the first stars could now be predicted more quantitatively.
Our study shows that stars 20 to 130 times heavier than the Sun that underwent supernovae explosions and formed black holes played an important role in the earliest chemical enrichment of the universe," - Ken'ichi Nomoto.



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