423 research outputs found
Dynamical description of the buildup process in resonant tunneling: Evidence of exponential and non-exponential contributions
The buildup process of the probability density inside the quantum well of a
double-barrier resonant structure is studied by considering the analytic
solution of the time dependent Schr\"{o}dinger equation with the initial
condition of a cutoff plane wave. For one level systems at resonance condition
we show that the buildup of the probability density obeys a simple charging up
law, where is the
stationary wave function and the transient time constant is exactly
two lifetimes. We illustrate that the above formula holds both for symmetrical
and asymmetrical potential profiles with typical parameters, and even for
incidence at different resonance energies. Theoretical evidence of a crossover
to non-exponential buildup is also discussed.Comment: 4 pages, 2 figure
El test de transformación de los linfocitos (TTL): Algunas aplicaciones del mismo en el laboratorio de análisis biológicos
Se estudió el test de transformación de linfocitos (TTL) analizándose algunos de los resultados obtenidos en sujetos no alérgicos y enfermos sensibilizados a diferentes sustancias: alergias alimenticias, alergias inhalantes, sustancias nitrogenadas específicas, sensibilizaciones medicamentosas y frente a células tumorales irradiadas empleando poblaciones linfoides autólogas. Se realizan finalmente unas consideraciones generales a las técnicas.The test transformation lymphocite (TTL) was studied, and the results obtained were analyzed in subjects with different substances (foods specific mitogens, inhalants alergens, drugs) and against irradiated tumor cells using autologous linfoide population.
Finally general considerations to the technique were made
Nebular emission lines in IRAS 17347-3139
We report the detection of nebular emission lines in the optical and
mid-infrared spectra of IRAS 17347-3139, a heavily obscured OH/IR star which
may be rapidly evolving from the AGB to the PN stage. The presence of emission
lines is interpreted as a clear indication that the ionization of its
circumstellar envelope has already started. This source belongs to the rare
class of objects known as `OHPNe' displaying both OH maser and radio continuum
emission. However, unlike the rest of stars in this class, prominent C-rich
dust features are detected in its mid-infrared spectrum, which makes the
analysis of this star particularly interesting.Comment: 2 pages, 2 figures, Proc. IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge
Univ. Press
Spitzer/IRS spectroscopy of high mass precursors to planetary nebulae
We present Spitzer/IRS observations of a small sample of heavily obscured
IRAS sources displaying both the infrared and OH maser emission characteristic
of OH/IR stars on the asymptotic giant branch (AGB), but also radio continuum
emission typical of ionized planetary nebulae (PNe), the so-called OHPNe. Our
observations show that their mid-infrared spectra are dominated by the
simultaneous presence of strong and broad amorphous silicate absorption
features together with crystalline silicate features, originated in their
O-rich circumstellar shells. Out of the five sources observed, three of them
are clearly non-variable at infrared wavelengths, confirming their post-AGB
status, while the remaining two still show strong photometric fluctuations, and
may still have not yet departed from the AGB. One of the non-variable sources
in the sample, IRAS 17393-2727, displays a strong [Ne II] nebular emission at
12.8 microns, indicating that the ionization of its central region has already
started. This suggests a rapid evolution from the AGB to the PN stage. We
propose that these heavily obscured OHPNe represent the population of high mass
precursors to PNe in our Galaxy.Comment: To appear in The Astrophysical Journal Letters (scheduled in the 2007
September 1 issue
New groups of planetary nebulae with peculiar dust chemistry towards the Galactic bulge
We investigate Galactic bulge planetary nebulae without emission-line central
stars for which peculiar infrared spectra have been obtained with the Spitzer
Space Telescope, including the simultaneous signs of oxygen and carbon based
dust. Three separate sub-groups can be defined characterized by the different
chemical composition of the dust and the presence of crystalline and amorphous
silicates.
We find that the classification based on the dust properties is reflected in
the more general properties of these planetary nebulae. However, some observed
properties are difficult to relate to the common view of planetary nebulae. In
particular, it is challenging to interpret the peculiar gas chemical
composition of many analyzed objects in the standard picture of the evolution
of planetary nebulae progenitors.
We confirm that the dual-dust chemistry phenomenon is not limited to
planetary nebulae with emission-line central stars.Comment: 17 pages, 13 figure
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