833 research outputs found
Charge dynamics in two-electron quantum dots
We investigate charge dynamics in a two-electron double quantum dot. The
quantum dot is manipulated by using a time-dependent external voltage that
induces charge oscillations between the dots. We study the dependence of the
charge dynamics on the external magnetic field and on the periodicity of the
external potential. We find that for suitable parameter values, it is possible
to induce both one-electron and two-electron oscillations between the dots.Comment: 4 pages, 7 figures, proceedings of the Quantum Dot 2010 conferenc
On the stability of non-isothermal Bonnor-Ebert spheres. II. The effect of gas temperature on the stability
Aims. We investigate the stability of non-isothermal Bonnor-Ebert spheres
with a model that includes a self-consistent calculation of the gas
temperature. This way we can discard the assumption of equality between the
dust and gas temperatures, and study the stability as the gas temperature
changes with chemical evolution of the gas.
Methods. We use a gas-grain chemical model including a time-dependent
treatment of depletion onto grain surfaces, which strongly influences the gas
temperature as the main coolant, CO, depletes from the gas. Dust and gas
temperatures are solved with radiative transfer. For comparison with previous
work, we assume that the cores are deeply embedded in a larger external
structure, corresponding to visual extinction mag.
Results. We find that the critical non-dimensional radius derived
here is similar to our previous work where we assumed ; the values lie below the isothermal critical value
, but the difference is less than 10%. Chemical evolution does
not affect notably the stability condition of low-mass cores (<0.75 ).
For higher masses the decrease of cooling owing to CO depletion causes
substantial temporal changes in the temperature and density profiles of the
cores. In the mass range 1-2 , decreases with chemical
evolution, whereas above 3 , instead increases. We also find
that decreasing increases the gas temperature especially
when the gas is chemically old, causing to increase with respect to
models with higher . The derived values are close
to . The density contrast between the core center and edge varies
between 8 to 16 depending on core mass and the chemical age of the gas,
compared to the constant value 14.1 for the isothermal BES.Comment: 7 pages, 5 figures; accepted for publication in A&A; abstract
(heavily) abridged for arXi
Radio continuum imaging of the R CrA star-forming region with the ATCA
The aim of this study is to investigate the nature of radio sources
associated with young stellar objects (YSOs) belonging to the R CrA cluster. By
combining the centimetre radio data with the wealth of shorter wavelength data
accumulated recently we wish to refine estimates of the evolutionary stages of
the YSOs. Fluxes and spectral indices for the brightest radio sources were
derived from the observations at 3, 6, and 20 cm using the ATCA. Seven of
detected sources can be assigned to YSOs, which have counterparts in the
X-rays, infrared or submm. One of the YSOs, Radio Source 9, is a Class 0
candidate, and another, IRS 7B, is suggested to be in the Class 0/I transition
stage. IRS 7B is associated with extended radio lobes at 6 and 20 cm. The lobes
may have a gyrosynchrotron emission component, which could be understood in
terms of Fermi accleration in shocks. The Class I objects detected here seem to
be a mixed lot. One of these, the wide binary IRS 5, shows a negative spectral
index, rapid variability, and a high degree of circular polarisation with
% on one of the days of observation. These signs of magnetic
activity suggest that at least one of the binary components has advanced beyond
the Class I stage. The radio source without YSO assigment, Radio Source 5, has
been suggested to be a brown dwarf. The radio properties, in particular its
persistent strong emission, do not support this classification. The radio
characteristics of the detected YSOs roughly agree with the scheme where the
dominant emission mechanism changes with age. The heterogeneity of the Class I
group can possibly be explained by a drastic decline in the jet activity during
this stage, which also changes the efficiency of free-free absorption around
the protostar.Comment: Accepted for publication in A&A (8 pages, 4 figures, 4 tables
Covariant Dirac Operators on Quantum Groups
We give a construction of a Dirac operator on a quantum group based on any
simple Lie algebra of classical type. The Dirac operator is an element in the
vector space U_q(\g) \otimes \mathrm{cl}_q(\g) where the second tensor factor
is a -deformation of the classical Clifford algebra. The tensor space
U_q(\g) \otimes \mathrm{cl}_q(\g) is given a structure of the adjoint module
of the quantum group and the Dirac operator is invariant under this action. The
purpose of this approach is to construct equivariant Fredholm modules and
-homology cycles. This work generalizes the operator introduced by Bibikov
and Kulish in \cite{BK}
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