47,928 research outputs found
A photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun: I. Atmospheric parameters and color similarity to the Sun
Solar twins and analogs are fundamental in the characterization of the Sun's
place in the context of stellar measurements, as they are in understanding how
typical the solar properties are in its neighborhood. They are also important
for representing sunlight observable in the night sky for diverse photometric
and spectroscopic tasks, besides being natural candidates for harboring
planetary systems similar to ours and possibly even life-bearing environments.
We report a photometric and spectroscopic survey of solar twin stars within 50
pc of the Sun. Hipparcos absolute magnitudes and (B-V)_Tycho colors were used
to define a 2 sigma box around the solar values, where 133 stars were
considered. Additional stars resembling the solar UBV colors in a broad sense,
plus stars present in the lists of Hardorp, were also selected. All objects
were ranked by a color-similarity index with respect to the Sun, defined by
uvby and BV photometry. Moderately high-resolution, high-S/N spectra were used
for a subsample of equatorial-southern stars to derive Teff, log g, and [Fe/H]
with average internal errors better than 50 K, 0.20 dex, and 0.08 dex,
respectively. Ages and masses were estimated from theoretical HR diagrams. The
color-similarity index proved very successful. We identify and rank new
excellent solar analogs, which are fit to represent the Sun in the night sky.
Some of them are faint enough to be of interest for moderately large
telescopes. We also identify two stars with near-UV spectra indistinguishable
from the Sun's. We present five new "probable" solar twin stars, besides five
new "possible" twins. Masses and ages for the best solar twin candidates lie
very close to the solar values, but chromospheric activity levels range
somewhat. We propose that the solar twins be emphasized in the ongoing searches
for extra-solar planets and SETI searches.Comment: 25 pages, 15 figures, 14 table
A new data reduction scheme to obtain the mode II fracture properties of Pinus Pinaster wood
In this work a numerical study of the End Notched Flexure (ENF) specimen was performed
in order to obtain the mode II critical strain energy released rate (GIIc) of a Pinus pinaster wood in the RL crack propagation system. The analysis included interface finite elements and a progressive damage
model based on indirect use of Fracture Mechanics.
The difficulties in monitoring the crack length during an experimental ENF test and the inconvenience of performing separate tests in order to obtain the elastic properties are well known. To avoid these
problems, a new data reduction scheme based on the equivalent crack concept was proposed and validated. This new data reduction scheme, the Compliance-Based Beam Method (CBBM), does not require crack measurements during ENF tests and additional tests to obtain elastic properties.FCT - POCTI/EME/45573/200
Finite element analysis of the ECT test on mode III interlaminar fracture of carbon-epoxy composite laminates
In this work a parametric study of the Edge Crack Torsion (ECT) specimen was performed
in order to maximize the mode III component (GIII) of the strain energy release rate for carbon-epoxy laminates.
A three-dimensional finite element analysis of the ECT test was conducted considering a
[90/0/(+45/-45)2/(-45/+45)2/0/90]S lay-up. The main objective was to define an adequate geometry to obtain an almost pure mode III at crack front. The geometrical parameters studied were specimen dimensions, distance between pins and size of the initial crack.
The numerical results demonstrated that the ratio between the specimen length and the initial crack length had a significant effect on the strain energy release rate distributions. In almost all of the tested
configurations, a mode II component occurred near the edges but it did not interfere significantly with the dominant mode III state.FCT - POCTI/EME/45573/200
Copper and Barium Abundances in the Ursa Major Moving Group
We present Cu and Ba abundances for 7 G-K dwarf stars, members of the
solar-metallicity, 0.3 Gyr old Ursa Major Moving Group. All analyzed member
stars show [Ba/Fe] excesses of +0.3-plus, associated with [Cu/Fe] deficiencies
of up to -0.23 dex. The present results suggest that there is an
anti-correlation between the abundances of Cu and the heavy elements produced
by the main component of the neutron capture s-process. Other possible
anomalies are Na and C deficiencies with respect to normal solar-metallicity
stars. The new data do not confirm the recent claim that the group member
HR6094 is a Ba dwarf star.Comment: 8 pages, 6 figures, accepted to MNRA
Constraining Elko Dark Matter at the LHC with Monophoton Events
A mass dimension one fermion, also known as Elko, constitutes a dark matter
candidate which might interact with photons at the tree level in a specific
fashion. In this work, we investigate the constraints imposed by unitarity and
LHC data on this type of interactions using the search for new physics in
monophoton events. We found that Elkos which can explain the dark matter relic
abundance mainly through electromagnetic interactions are excluded at the
95\%CL by the 8 TeV LHC data for masses up to 1 TeV.Comment: 6 pages, 4 figure
A Physical Model for Co-evolution of QSOs and of their Spheroidal Hosts
At variance with most semi-analytic models, in the Anti-hierarchical Baryon
Collapse scenario (Granato et al. 2001, 2004) the main driver of the galaxy
formation and evolution is not the merging sequence but are baryon processes.
This approach emphasizes, still in the framework of the hierarchical clustering
paradigm for dark matter halos, feedback processes from supernova explosions
and from active nuclei, that tie together star formation in spheroidal galaxies
and the growth of black holes at their centers. We review some recent results
showing the remarkably successful predictive power of this scenario, which
allows us to account for the evolution with cosmic time of a broad variety of
properties of galaxies and active nuclei, which proved to be very challenging
for competing models.Comment: Invited talk at the Specola Vaticana Workshop on "AGN and Galaxy
Evolution", Castel Gandolfo, 3-6 October 2005, 10 pages, 2 figure
From First Galaxies to QSOs: feeding the baby monsters
We present a physical model for the coevolution of massive spheroidal
galaxies and active nuclei at their centers. Supernova heating is increasingly
effective in slowing down the star formation and in driving gas outflows in
smaller and smaller dark matter halos. Thus the more massive protogalaxies
virializing at early times are the sites of faster star formation. The
correspondingly higher radiation drag causes a faster angular momentum loss by
the gas and induces a larger accretion rate onto the central black hole. In
turn, the kinetic energy of the outflows powered by the active nuclei can
unbind the residual gas in a time shorter for larger halos. The model accounts
for a broad variety of dynamical, photometric and metallicity properties of
early-type galaxies, for the M_BH -- \sigma relation and for the local
supermassive black-hole mass function.Comment: 6 pages, contributed paper to Proceedings of the Conference on
"Growing Black Holes" held in Garching, Germany, on June 21-25, 2004, edited
by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of "ESO
Astrophysics Symposia
Elaboração de biscoitos com resĂduo da extração de suco de caju.
bitstream/item/27966/1/Com.Tec.214.pdfDisponĂvel tambĂ©m on-line
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