1,845,718 research outputs found
Blob formation and acceleration in the solar wind: role of converging flows and viscosity
The effect of viscosity and of converging flows on the formation of blobs in
the slow solar wind is analysed by means of resistive MHD simulations. The
regions above coronal streamers where blobs are formed (Sheeley et al., 1997)
are simulated using a model previously proposed by Einaudi et al. (1999). The
result of our investigation is twofold. First, we demonstrate a new mechanism
for enhanced momentum transfer between a forming blob and the fast solar wind
surrounding it. The effect is caused by the longer range of the electric field
caused by the tearing instability forming the blob. The electric field reaches
into the fast solar wind and interacts with it, causing a viscous drag that is
global in nature rather than local across fluid layers as it is the case in
normal uncharged fluids (like water). Second, the presence of a magnetic cusp
at the tip of a coronal helmet streamer causes a converging of the flows on the
two sides of the streamer and a direct push of the forming island by the fast
solar wind, resulting in a more efficient momentum exchange
Giant Magneto-Oscillations of Electric-Field-Induced Spin Polarization in 2DEG
We consider a disordered two-dimensional electron gas with spin-orbit
coupling placed in a perpendicular magnetic field and calculate the magnitude
and direction of the electric-field-induced spin polarization. We find that in
strong magnetic fields the polarization becomes an oscillatory function of the
magnetic field and that the amplitude of these oscillations is parametrically
larger than the polarization at zero magnetic field. We show that the enhanced
amplitude of the polarization is a consequence of strong electron-hole
asymmetry in a quantizing magnetic field.Comment: 6 pages, 3 figure
Constraints on neutrino decay lifetime using long-baseline charged and neutral current data
We investigate the status of a scenario involving oscillations and decay for
charged and neutral current data from the MINOS and T2K experiments. We first
present an analysis of charged current neutrino and anti-neutrino data from
MINOS in the framework of oscillation with decay and obtain a best fit for
non-zero decay parameter . The MINOS charged and neutral current data
analysis results in the best fit for ~eV, and zero decay parameter, which
corresponds to the limit for standard oscillations. Our combined MINOS and T2K
analysis reports a constraint at the 90\% confidence level for the neutrino
decay lifetime ~s/eV. This is the best limit
based only on accelerator produced neutrinos
Antiproton scattering off and nuclei at low and intermediate energies
Antiproton scattering off ^3\He and ^4\He targets is considered at beam
energies below 300 MeV within the Glauber-Sitenko approach, utilizing the amplitudes of the J\"ulich model as input. A good agreement with available
data on differential \bar p ^4\He cross sections and on \bar p ^3\He and
\pbar ^4\He reaction cross sections is obtained. Predictions for polarized
total He cross sections are presented, calculated within the
single-scattering approximation and including Coulomb-nuclear interference
effects. The kinetics of the polarization buildup is discussed.Comment: 11 pages, 6 figure
A coordinate-dependent superspace deformation from string theory
Starting from a type II superstring model defined on in
a linear graviphoton background, we derive a coordinate dependent -deformed
, superspace. The chiral fermionic coordinates
satisfy a Clifford algebra, while the other coordinate algebra remains
unchanged. We find a linear relation between the graviphoton field strength and
the deformation parameter. The null coordinate dependence of the graviphoton
background allows to extend the results to all orders in .Comment: 14 pages, reference added, accepted for publication in JHE
Modelling Intermediate Age and Old Stellar Populations in the Infrared
We have investigated the spectro-photometric properties of the Asymptotic
Giant Branch (AGB) stars and their contribution to the integrated infrared
emission in simple stellar populations (SSP). Adopting analytical relations
describing the evolution of these stars in the HR diagram and empirical
relations for the mass-loss rate and the wind terminal velocity, we were able
to model the effects of the dusty envelope around these stars, with a minimal
number of parameters. We computed isochrones at different age and initial metal
content. We compare our models with existing infrared colors of M giants and
Mira stars and with IRAS PSC data. Contrary to previous models, in the new
isochrones the mass-loss rate, which establishes the duration of the AGB phase,
also determines the spectral properties of the stars. The contribution of these
stars to the integrated light of the population is thus obtained in a
consistent way. We find that the emission in the mid infrared is about one
order of magnitude larger when dust is taken into account in an intermediate
age population, irrespective of the particular mixture adopted. The dependence
of the integrated colors on the metallicity and age is discussed, with
particular emphasis on the problem of age-metallicity degeneracy. We show that,
contrary to the case of optical or near infrared colors, the adoption of a
suitable pass-band in the mid infrared allows a fair separation of the two
effects. We suggest intermediate redshift elliptical galaxies as possible
targets of this method of solving the age-metallicity dilemma. The new SSP
models constitute a first step in a more extended study aimed at modelling the
spectral properties of the galaxies from the ultraviolet to the far infrared.Comment: 16 pages, 10 figures, to appear in A&
Pure Luminosity Evolution models for faint field galaxy samples
We have examined a set of pure luminosity evolution (PLE) models in order to
explore up to what extent the rapidly increasing observational constraints from
faint galaxy samples can be understood in this simple framework. We find that a
PLE model, in which galaxies evolve mildly in time even in the rest frame UV,
can reproduce most of the observed properties of faint galaxies assuming an
open () universe. In particular, such a model is able to fit
reasonably well the number counts in the , and bands, as
well as the colour and redshift distributions derived from most of the existing
samples. The most significant discrepancy between the predictions of this model
and the data is the distribution of faint -selected galaxies.
Significantly worse fits are obtained with PLE models for the theoretically
attractive value of , although a simple number luminosity evolution
model with a significant amount of merger events fits the data also in this
cosmology.Comment: 15 pages, plain tex (insert encapsulated postscript figures), plus an
extra figure Fig3c.ps and the tex-macro mn.tex uuencoded, gzipp'ed tar file
-- accepted by MNRA
Grain growth in the envelopes and disks of Class I protostars
We present new 3 mm ATCA data of two Class I Young Stellar Objects in the
Ophiucus star forming region: Elias29 and WL12. For our analysis we compare
them with archival 1.1 mm SMA data. In the (u,v) plane the two sources present
a similar behavior: a nearly constant non-zero emission at long baselines,
which suggests the presence of an unresolved component and an increase of the
fluxes at short baselines, related to the presence of an extended envelope. Our
data analysis leads to unusually low values of the spectral index , which may indicate that mm-sized dust grains have already formed
both in the envelopes and in the disk-like structures at such early stages. To
explore the possible scenarios for the interpretation of the sources we perform
a radiative transfer modeling using a Monte Carlo code, in order to take into
account possible deviations from the Rayleigh-Jeans and optically thin regimes.
Comparison between the model outputs and the observations indicates that dust
grains may form aggregates up to millimeter size already in the inner regions
of the envelopes of Class I YSOs. Moreover, we conclude that the embedded
disk-like structures in our two Class Is are probably very compact, in
particular in the case of WL12, with outer radii down to tens of AU.Comment: 12 pages, 8 figures, Accepted for publication in A&
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