14,831 research outputs found
Elastic properties of carbon nanotubes and their heterojunctions
Comprehensive studies on the modelling and numerical simulation of the mechanical behaviour under tension, bending and torsion of single-walled carbon nanotubes and their heterojunctions are performed. It is proposed to deduce the mechanical properties of the carbon nanotubes heterojunctions from the knowledge of the mechanical properties of the single-walled carbon nanotubes, which are their constituent key unit
Detection of new eruptions in the Magellanic Clouds LBVs R 40 and R 110
We performed a spectroscopic and photometric analysis to study new eruptions
in two luminous blue variables (LBVs) in the Magellanic Clouds. We detected a
strong new eruption in the LBV R40 that reached in 2016, which is
around mag brighter than the minimum registered in 1985. During this new
eruption, the star changed from an A-type to a late F-type spectrum. Based on
photometric and spectroscopic empirical calibrations and synthetic spectral
modeling, we determine that R\,40 reached ~K
during this new eruption. This object is thereby probably one of the coolest
identified LBVs. We could also identify an enrichment of nitrogen and r- and
s-process elements. We detected a weak eruption in the LBV R 110 with a maximum
of mag in 2011, that is, around mag brighter than in the
quiescent phase. On the other hand, this new eruption is about mag
fainter than the first eruption detected in 1990, but the temperature did not
decrease below 8500 K. Spitzer spectra show indications of cool dust in the
circumstellar environment of both stars, but no hot or warm dust was present,
except by the probable presence of PAHs in R\,110. We also discuss a possible
post-red supergiant nature for both stars
Retired galaxies: not to be forgotten in the quest of the star formation -- AGN connection
We propose a fresh look at the Main Galaxy Sample of the Sloan Digital Sky
Survey by packing the galaxies in stellar mass and redshift bins. We show how
important it is to consider the emission-line equivalent widths, in addition to
the commonly used emission-line ratios, to properly identify retired galaxies
(i.e. galaxies that have stopped forming stars and are ionized by their old
stellar populations) and not mistake them for galaxies with low-level nuclear
activity. We find that the proportion of star-forming galaxies decreases with
decreasing redshift in each mass bin, while that of retired galaxies increases.
Galaxies with have formed all their stars at
redshift larger than 0.4. The population of AGN hosts is never dominant for
galaxy masses larger than . We warn about the effects of
stacking galaxy spectra to discuss galaxy properties. We estimate the lifetimes
of active galactic nuclei (AGN) relying entirely on demographic arguments ---
i.e. without any assumption on the AGN radiative properties. We find
upper-limit lifetimes of about 1--5 Gyr for detectable AGN in galaxies with
masses between --. The lifetimes of the AGN-dominated
phases are a few yr. Finally, we compare the star-formation histories of
star-forming, AGN and retired galaxies as obtained by the spectral synthesis
code STARLIGHT. Once the AGN is turned on it inhibits star formation for the
next 0.1 Gyr in galaxies with masses around , 1
Gyr in galaxies with masses around .Comment: accepted for MNRAS figure resolution has been degraded with respect
to what will be published in MNRA
Structural and optical properties of europium doped zirconia single crystals fibers grown by laser floating zone
Yttria stabilized zirconia single crystal fibers doped with europium ions were developed envisaging optical applications. The laser floating zone technique was used in order to grow millimetric high quality single crystal fibers. The as-grown fibers are completely transparent and inclusion free, exhibiting a cubic structure. Under ultraviolet (UV) excitation, a broad emission band appears at 551 nm. The europium doped fibers are translucent with a tetragonal structure and exhibit an intense red emission at room temperature under UV excitation. The fingerprint transition lines between the 5D0 and 7FJ(0–4) multiplets of the Eu3+ ions are observed with the main emission line at ∼ 606 nm due to 5D0→7F2 transition. Photoluminescence excitation and wavelength dependent the photoluminescence spectra confirm the existence of different Eu3+ optical centers.
© 2011 American Institute of PhysicsFCT-PTDC/CTM/66195/2006FCT-SFRH/BD/45774/200
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