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Post Info TOPIC: Globular Clusters
Blobrana


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RE: Globular ClusterS
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Title: Massive stars and globular cluster formation
Authors: Kenji Bekki, Masashi Chiba

We first present chemodynamical simulations to investigate how stellar winds of massive stars influence early dynamical and chemical evolution of forming globular clusters (GCs). In our numerical models, GCs form in turbulent,high-density giant molecular clouds (GMCs), which are embedded in a massive dark matter halo at high redshifts. We show how high-density, compact stellar systems are formed from GMCs influenced both by physical processes associated with star formation and by tidal fields of their host halos. We also show that chemical pollution of GC-forming GMCs by stellar winds from massive stars can result in star-to-star abundance inhomogeneities among light elements (e.g., C, N, and O) of stars in GCs. The present model with a canonical initial mass function (IMF) also shows a C-N anticorrelation that stars with smaller [C/Fe] have larger [N/Fe] in a GC. Although these results imply that ''self-pollution'' of GC-forming GMCs by stellar winds from massive stars can cause abundance inhomogeneities of GCs, the present models with different parameters and canonical IMFs can not show N-rich stars with [N/Fe] ~ 0.8 observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the context of massive star formation preceding low-mass one within GC-forming GMCs (''bimodal star formation scenario''). We also show that although almost all stars (~97%) show normal He abundances (Y) of ~0.24 some stars later formed in GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich stars with Y >0.26 is however found to be small (~10^-3) for most models.

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Blobrana


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New Hubble observations of the massive globular cluster NGC 2808 provide evidence that it has three generations of stars instead of one as current theories predict.



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Blobrana


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3D Animation showing the globular cluster NGC 2808 in the constellation of Carina.




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Blobrana


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Posts: 131433
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NGC2808
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This NASA/ESA Hubble Space Telescope image of a dense swarm of stars shows the central region of the globular cluster NGC 2808. Of the 150 known globular clusters in the Milky Way, NGC 2808 is one of the largest.
Astronomers used Hubbles Advanced Camera for Surveys to measure the brightness and colour of the cluster stars. Its resolution allowed them to sort out the different stellar populations.
The Hubble measurements showed three distinct populations, with each successive generation appearing slightly bluer. This colour difference suggests that successive generations contain different amounts of helium.
All the stars in NGC 2808 were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster. This is contrary to conventional ideas that propose that all the stars in a globular cluster originate from the same time and from the same material.
The images were taken in May 2005 and in August and November 2006.

ngc2808
Expand (601kb, 1024 x 768)
Credits: NASA, ESA, G. Piotto (University of Padua) and A. Sarajedini (University of Florida)

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Blobrana


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Posts: 131433
Date:
NGC 2808
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NASA's Hubble Space Telescope has discovered what may be called a multiple 'baby boom' in a globular cluster.
For long, astronomers thought that globular star clusters had a single "baby boom" of stars early in their lives and then settled into a quiet existence.
But now, new observation of the massive globular cluster NGC 2808 has provided fresh evidence that nearly three generation of stars formed very early in the cluster's life.

"We had never imagined that anything like this could happen. This is a complete shock" - Giampaolo Piotto of the University of Padova in Italy and leader of the team that made the discovery.

"The standard picture of a globular cluster is that all of its stars formed at the same time, in the same place, and from the same material, and they have co-evolved for billions of years. This is the cornerstone on which much of the study of stellar populations has been built. So we were very surprised to find several distinct populations of stars in NGC 2808. All of the stars were born within 200 million years very early in the life of the 12.5-billion-year-old massive cluster" - Luigi Bedin, team member and a research scientist with the ESA, the ESO and the Space Telescope Science Institute in Baltimore, US.

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Blobrana


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Astronomers are puzzling over a strange, ancient star cluster that hosts three generations of stars instead of the normal one. Some researchers say it might be the remains of a small galaxy that was dismembered by the Milky Way; if so, it could help clarify the murky picture of our galaxy's beginnings billions of years ago.
Globular star clusters are dense groupings of hundreds of thousands or millions of stars that dot our Milky Way galaxy. They contain the galaxy's most ancient stars, which date back to perhaps just a few hundred million years after the big bang 13.7 billion years ago.
Each cluster was thought to have formed in a single burst of star formation, since that single baby boom should have blown away the gas needed to form future generations of stars.
Now, a Hubble Space Telescope survey of one cluster that contains three distinct generations of stars is challenging that long-held notion.

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Blobrana


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Posts: 131433
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Globular Clusters
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Globular clusters, often regarded as the most boring denizens of our galaxy, have just become a lot more interesting. Rogue, hot stars have been discovered inside them. The tens of thousands of stars that make up each globular cluster were all thought to have formed at the same time, with exactly the same composition, from a single spherical cloud of gas. Then, two years ago, a hint of a rogue population of hot, helium-rich stars was detected in the largest known globular cluster, Omega Centauri. This cluster, however, is known to be exceptional.

"I just thought, OK, the very weird globular cluster is being weird again" - Alison Sills of McMaster University in Hamilton, Ontario, who studies the clusters.

Now, Giampaolo Piotto's team at the University of Padua in Italy has found similar helium-rich stars in another, more ordinary cluster, NGC 2808, suggesting that such stars could exist in all globulars. The team thinks that about a 100 million years after each cluster formed, some of the larger stars sent out a gentle wind laced with helium and heavier elements, which settled at the centre of the cluster and coalesced into a second generation of chemically enriched stars. Similar wind-formed stars could also explain mysterious ultraviolet emissions from elliptical galaxies as some of them would be hot enough to radiate UV light.
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Title: The Chemical Evolution of Helium in Globular Clusters: Implications for the Self-Pollution Scenario
Authors: A. I. Karakas, Y. Fenner, Alison Sills, S. W. Campbell and J. C. Lattanzio

We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y ~ 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.

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Title: Helium enhancements in globular Cluster stars from Asymptotic Giant Branch star pollution
Authors: A. Karakas1, Y. Fenner, Alison Sills, S. W. Campbell and J. C. Lattanzio

Using a chemical evolution model we investigate the intriguing suggestion that there are populations of stars in some globular clusters (e.g. NGC 2808, ! Centauri) with enhanced levels of helium (Y ~ 0.28 to 0.40) compared to the majority of the population that presumably have a primordial helium abundance. We assume that a previous generation of massive low-metallicity Asymptotic Giant Branch (AGB) stars has polluted the cluster gas via a slow stellar wind. We use two independent sets of AGB yields computed from detailed models to follow the evolution of helium, carbon, nitrogen and oxygen in the cluster gas using a Salpeter initial mass function (IMF) and a number of top-heavy IMFs. In no case were we able to fit the observational constraints, Y > 0.30 and C+N+O constant. Depending on the shape of the IMF and the yields, we either obtained Y & 0.30 and large increases in C+N+O or Y < 0.30 and C+N+O constant. These results suggest that either AGB stars alone are not responsible for the large helium enrichment or that any dredge-up from this generation of stars was less than predicted by standard models.

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