324 research outputs found
Abundances in Halo Stars, with Special Focus on the Abundances of Lithium & Beryllium
No full textBonifacio, Piercarlo. (2012). Abundances in Halo Stars, with Special Focus on the Abundances of Lithium & Beryllium. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/128778
Prominent activity of the blazar OJ 287 in 2005: XMM-Newton and multiwavelength observations
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GRBs and the Ghost of the Fireball
No full textGamma Ray Burst has been widely believed in last decade to be super-explosions: the Fireball. We are argue on the contrary that GRBs (as well as Soft Gamma Repeaters SGR) are precessing Gamma Jets. We remind the list of contradiction that Fireball and its smaller galactic version, the Magnetar, have to face. In particular the existence of weak isolated X-ray precursor signal before the main Gamma Ray Burst and (rare SGR) events disagree with any explosive, one shoot, scenarios either isotropic or wide-beamed. We interpret them as earlier marginal blazing of outlying X conical Jet tails of precessing, spinning gamma Jet
The first galactic stars and chemical enrichment in the halo
No full textInvited Review at IAU Symposium 265 , "Chemical Abundances in the Universe: Connecting First Stars to Planets", K. Cunha, M. Spite & B. Barbuy, eds. Cambridge University Press p. 81The cosmic microwave background and the cosmic expansion can be interpreted as evidence that the Universe underwent an extremely hot and dense phase about 14 Gyr ago. The nucleosynthesis computations tell us that the Universe emerged from this state with a very simple chemical composition: H, 2H, 3He, 4He, and traces of 7Li. All other nuclei where synthesised at later times. Our stellar evolution models tell us that, if a low-mass star with this composition had been created (a ``zero-metal'' star) at that time, it would still be shining on the Main Sequence today. Over the last 40 years there have been many efforts to detect such primordial stars but none has so-far been found. The lowest metallicity stars known have a metal content, Z, which is of the order of 10e-4Z_Sun. These are also the lowest metallicity objects known in the Universe. This seems to support the theories of star formation which predict that only high mass stars could form with a primordial composition and require a minimum metallicity to allow the formation of low-mass stars. Yet, since absence of evidence is not evidence of absence, we cannot exclude the existence of such low-mass zero-metal stars, at present. If we have not found the first Galactic stars, as a by product of our searches we have found their direct descendants, stars of extremely low metallicity (Z<=10e-3Z_Sun). The chemical composition of such stars contains indirect information on the nature of the stars responsible for the nucleosynthesis of the metals. Such a fossil record allows us a glimpse of the Galaxy at a look-back time equivalent to redshift z=10, or larger. The last ten years have been full of exciting discoveries in this field, which I will try to review in this contribution
Low Metallicity Stars in our Galaxy
No full textThe advent of 8 m class telescopes has allowed the detailed spectroscopic
study of sizeable numbers of extremely metal-poor Galactic stars which
are the witnesses of the formation of the early Galaxy. Their chemical
composition displays some distinctive trends which should provide
a strong constraint on the physical nature of the first generation(s) of
stars and on their nucleosynthetic output. I will review recent results
in the field following the periodic table, from lithium to uranium and
shortly comment on the intriguing classes of Carbon Enhanced Metal Poor (CEMP)
stars, for many of which there is no analogue among solar metallicity stars.
In spite of these exciting results, the number of known stars of metallicity
below [Fe/H] = -3.3 remains quite small and it would be desirable to discover
more, both to clearly understand the metal-weak tail of Halo metallicity
distribution and to clarify the abundance trends at the lowest metallicities.
Most of these extremely rare objects have been discovered
by the wide field objective prism surveys, HK survey and Hamburg-ESO survey.
In the near future the Sloan Digital Sky Survey and its continuation SEGUE
are expected to boost significantly the numbers of known extremely metal poor
stars. We are living exciting times but an even more exciting future
lies ahead
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