16,318 research outputs found
Anomalous globular clusters: insights from neutron capture elements abundances
Thanks to the large amount of spectroscopic and photometric data assembled in
the last couple of decades, the assumption that all globular clusters (GCs)
contain a simple mono-metallic stellar population has been modified. Besides
the common variations in the elements created/destroyed in the H-burning
processes, spreads and/or multi-modalities in heavier elements have been
detected in a few objects. Among the most remarkable chemical inhomogeneity in
these anomalous objects is the internal variation in the neutron-capture
(n-capture) elements, that can provide some information about the material from
which stars were born. I report a summary of the chemical pattern observed in
GCs where variations in n-capture have been detected, and the connection
between these chemical features and the distribution of stars along the
color-magnitude diagrams in the context of the lively debate on multiple
stellar populations.Comment: 5 pages, 4 fugures, proceedings of the meeting "Reading the book of
globular clusters with the lens of stellar evolution", Rome, Nov. 26-28,
2012, to be published in Memorie della Societa' Astronomica Italian
Charge and Magnetic Flux Correlations in Chern-Simons Theory with Fermions
Charge and magnetic flux bearing operators are introduced in Chern-Simons
theory both in its pure form and when it is coupled to fermions. The magnetic
flux creation operator turns out to be the Wilson line. The euclidean
correlation functions of these operators are shown to be local and are
evaluated exactly in the pure case and through an expansion in the inverse
fermion mass whenever these are present. Physical states only occur in the
presence of fermions and consist of composite charge-magnetic flux carrying
states which are in general anyonic. The large distance behavior of the
correlation functions indicates the condensation of charge and magnetic flux.Comment: Latex, 17 page
Chemical composition of stellar populations in Omega Centauri
We derive abundances of Fe, Na, O, and s-elements from GIRAFFE@VLT spectra
for more than 200 red giant stars in the Milky Way satellite Omega Centauri.
Our preliminary results are that: (i) we confirm that Omega Centauri exibiths
large star-to-star metallicity variations ( 1.4 dex); (ii) the
metallicity distribution reveals the presence of at least five stellar
populations with different [Fe/H]; (iii) a clear Na-O anticorrelation is
clearly observed for the metal-poor and metal-intermediate populations while
apparently the anticorrelation disappears for the most metal-rich populations.
Interestingly the Na level grows with iron.Comment: 2 pages, 2 figures. To appear in the proceedings of IAU Symp. 268
"Light elements in the Universe" (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22
M22 is an anomalous globular cluster that hosts two groups of stars with
different metallicity and s-element abundance. The star-to-star light-element
variations in both groups, with the presence of individual Na-O and C-N
anticorrelations, demonstrates that this Milky-Way satellite has experienced a
complex star-formation history. We have analysed FLAMES/UVES spectra for seven
stars covering a small color interval, on the reddest horizontal-branch (HB)
portion of this cluster and investigated possible relations between the
chemical composition of a star and its location along the HB. Our chemical
abundance analysis takes into account effects introduced by deviations from the
local-thermodynamic equilibrium (NLTE effects), that are significant for the
measured spectral lines in the atmospheric parameters range spanned by our
stars. We find that all the analysed stars are barium-poor and sodium-poor,
thus supporting the idea that the position of a star along the HB is strictly
related to the chemical composition, and that the HB-morphology is influenced
by the presence of different stellar populations.Comment: 21 pages, 3 figures, accepted for publication in Ap
Constraints on the slope of the dark halo mass function by microlensing observables
We investigate the dark halo lens mass function (MF) for a wide class of
spheroidal non singular isothermal models comparing observed and observable
microlensing quantities for MACHO observations towards LMC and taking into
account the detection efficiency. We evaluate the microlensing observable
quantities, i.e. observable optical depth, number of events and mean duration,
for models with homogenous power - law MF changing the upper and lower mass
limits and the flattening of the dark halo. By applying the simple technique of
the inverse problem method we are then able to get some interesting constraints
on the slope of the MF and on the dark halo mass fraction f made out
by MACHOs consistently with previous results.Comment: 10 LaTex pages, 2 postscript figures, accepted on 21/5/2001 for
pubblication on A&A; title changed, completely revised version : a new
definition of observable optical depth is used and all the MACHO results from
5.7 years of observations are used to constrain the slope of the dark halo
mass functio
Modern mechanisms make manless Martian mission mobile: Spin-off spells stairclimbing self-sufficiency for earthbound handicapped
Concepts were developed for three wheel chairs from progressively improving designs of a proposed unmanned roving vehicle for the surface exploration of Mars; as a spin-off, a concept for a stair-climbing wheel chair was generated. The mechanisms employed in these are described. The Mars mission is envisioned using the booster rockets and aeroshell of the Viking missions
Recherches sur les sérums des cobayes immunisés et sur les sérums des cobayes hyperimmunisés contre le charbon
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