2,665 research outputs found
Effect of Bohm potential on a charged gas
Bohm's interpretation of Quantum Mechanics leads to the derivation of a
Quantum Kinetic Equation (QKE): in the present work, propagation of waves in
charged quantum gases is investigated starting from this QKE. Dispersion
relations are derived for fully and weakly degenerate fermions and bosons
(these latter above critical temperature), and the differences underlined. Use
of a kinetic equation permits investigation of "Landau-type" damping: it is
found that the presence of damping in fermion gases is dependent upon the
degree of degeneracy, whereas it is always present in boson gases. In fully
degenerate fermions a phenomenon appears that is akin to the "zero sound"
propagation.Comment: 11 pages, no figures, pdf forma
Time and time-frequency analysis of near-infrared signals for the assessment of ozone autohemotherapy long-term effects in multiple sclerosis
Ozone autohemotherapy is an emerging therapeutic technique that is gaining increasing importance in treating neurological disorders. A validated and standard methodology to assess the effect of such therapy on brain metabolism and circulation is however still lacking. We used a near-infrared spectroscopy system (NIRS) to monitor the cerebral oxygenation of 9 subjects: 4 remitting-relapsing multiple sclerosis (MS) sufferers and 5 controls. Subjects were tested before, during, and after ozone autohemotherapy. We monitored the concentration changes in the level of oxygenated and deoxygenated haemoglobin, and in the level of the Cytochrome-c-oxidase (CYT-c). From the time and time-frequency analysis of the NIRS signals we extracted 128 variables, which were used to characterize the metabolic brain pattern during the therapy. We showed that by using only 7 NIRS variables out of 128 it is possible to characterize the metabolic brain pattern of the two groups of subjects. The MS subjects showed a marked increase of the CYT-c activity and concentration about 40 minutes after the end of the autohemotherapy, possibly revealing a reduction of the chronic oxidative stress level typical of MS sufferers. From a technical point of view, this preliminary study showed that NIRS could be useful to show the effects of ozone autohemotherapy at cerebral level, in a long term monitoring. The clinical result of this study is the quantitative measurement of the CYT-c level changes in MS induced by ozone autohemotherap
Physical Properties of Galactic Planck Cold Cores revealed by the Hi-GAL survey
Previous studies of the initial conditions of massive star formation have
mainly targeted Infrared-Dark Clouds (IRDCs) toward the inner Galaxy. This is
due to the fact that IRDCs were first detected in absorption against the bright
mid-IR background, requiring a favourable location to be observed. By
selection, IRDCs represent only a fraction of the Galactic clouds capable of
forming massive stars and star clusters. Due to their low dust temperatures,
IRDCs are bright in the far-IR and millimeter and thus, observations at these
wavelengths have the potential to provide a complete sample of star-forming
massive clouds across the Galaxy. Our aim is to identify the clouds at the
initial conditions of massive star formation across the Galaxy and compare
their physical properties as a function of their Galactic location. We have
examined the physical properties of a homogeneous galactic cold core sample
obtained with the Planck satellite across the Galactic Plane. With the use of
Herschel Hi-GAL observations, we have characterized the internal structure of
them. By using background-subtracted Herschel images, we have derived the H2
column density and dust temperature maps for 48 Planck clumps. Their basic
physical parameters have been calculated and analyzed as a function of location
within the Galaxy. These properties have also been compared with the empirical
relation for massive star formation derived by Kauffmann & Pillai (2010). Most
of the Planck clumps contain signs of star formation. About 25% of them are
massive enough to form high mass stars. Planck clumps toward the Galactic
center region show higher peak column densities and higher average dust
temperatures than those of the clumps in the outer Galaxy. Although we only
have seven clumps without associated YSOs, the Hi-GAL data show no apparent
differences in the properties of Planck cold clumps with and without star
formation.Comment: 22 pages, 11 figures, accepted for publication in A&
Growth and equilibrium size of water droplets in air
A model is presented to describe the growth in time of the average water drop in supersaturated air, and predict their radius at equilibrium. Many previous works consider the growth of an isolated drop, whereas in the present work
the effect of the presence of a large number of drops, with the ensuing depletion in water content in the surrounding air, is considered: it is shown that the effect of depletion is crucial to obtain the equilibrium radius. Preliminary results, obtained under some simplifying assumptions, are presented: expressions accounting for this
depletion effect are given for the time evolution of the liquid-water temperature and of the number of water molecules in the drop and drop radius near equilibrium, and
for their asymptotic equilibrium values
Phonon-induced electron relaxation in weakly-confined single and coupled quantum dots
We investigate charge relaxation rates due to acoustic phonons in
weakly-confined quantum dot systems, including both deformation potential and
piezoelectric field interactions. Single-electron excited states lifetimes are
calculated for single and coupled quantum dot structures, both in homonuclear
and heteronuclear devices. Piezoelectric field scattering is shown to be the
dominant relaxation mechanism in many experimentally relevant situations. On
the other hand, we show that appropriate structure design allows to minimize
separately deformation potential and piezolectric field interactions, and may
bring electron lifetimes in the range of microseconds.Comment: 20 pages (preprint format), 7 figures, submitted to Physical Review
Two massive star-forming regions at early evolutionary stages
We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301
and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent
massive star-forming regions in early stages of evolution (due to previously
undetected radio emission at the 1-sigma level of 2 mJy per beam). Also
reported are 1.2-millimeter continuum and a series of molecular-line
observations made with the SEST telescope. For IRAS 15596-5301, the
observations reveal the presence of three distinct compact radio-continuum
sources associated with a dense molecular core. We suggest that this core
contains a cluster of B stars which are exciting compact HII regions that are
in pressure equilibrium with the dense molecular surroundings. No radio
continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2
mJy). However, a dense molecular core has been detected. The high luminosity
and lack of radio emission from this massive core suggests that it hosts an
embedded young massive protostar that is still undergoing an intense accretion
phase. This scenario is supported by the observed characteristics of the line
profiles and the presence of a bipolar outflow detected from observations of
the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-
forming region in a very early evolutionary stage, being the precursor of an
ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical
Journa
High Resolution Observations of the Massive Protostar in IRAS18566+0408
We report 3 mm continuum, CH3CN(5-4) and 13CS(2-1) line observations with
CARMA, in conjunction with 6 and 1.3 cm continuum VLA data, and 12 and 25
micron broadband data from the Subaru Telescope toward the massive proto-star
IRAS18566+0408. The VLA data resolve the ionized jet into 4 components aligned
in the E-W direction. Radio components A, C, and D have flat cm SEDs indicative
of optically thin emission from ionized gas, and component B has a spectral
index alpha = 1.0, and a decreasing size with frequency proportional to
frequency to the -0.5 power. Emission from the CARMA 3 mm continuum, and from
the 13CS(2-1), and CH3CN(5-4) spectral lines is compact (i.e. < 6700 AU), and
peaks near the position of VLA cm source, component B. Analysis of these lines
indicates hot, and dense molecular gas, typical for HMCs. Our Subaru telescope
observations detect a single compact source, coincident with radio component B,
demonstrating that most of the energy in IRAS18566+0408 originates from a
region of size < 2400 AU. We also present UKIRT near-infrared archival data for
IRAS18566+0408 which show extended K-band emission along the jet direction. We
detect an E-W velocity shift of about 10 km/sec over the HMC in the CH3CN lines
possibly tracing the interface of the ionized jet with the surrounding core
gas. Our data demonstrate the presence of an ionized jet at the base of the
molecular outflow, and support the hypothesis that massive protostars with
O-type luminosity form with a mechanism similar to lower mass stars
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