1,318 research outputs found
Classical and Quantum-like approaches to Charged-Particle Fluids in a Quadrupole
A classical description of the dynamics of a dissipative charged-particle
fluid in a quadrupole-like device is developed. It is shown that the set of the
classical fluid equations contains the same information as a complex function
satisfying a Schrodinger-like equation in which Planck's constant is replaced
by the time-varying emittance, which is related to the time-varying temperature
of the fluid. The squared modulus and the gradient of the phase of this complex
function are proportional to the fluid density and to the current velocity,
respectively. Within this framework, the dynamics of an electron bunch in a
storage ring in the presence of radiation damping and quantum-excitation is
recovered. Furthermore, both standard and generalized (including dissipation)
coherent states that may be associated with the classical particle fluids are
fully described in terms of the above formalism.Comment: LaTex, to appear in Physica Script
Nonlocal effects in high energy charged particle beams
Within the framework of the thermal wave model, an investigation is made of
the longitudinal dynamics of high energy charged particle beams. The model
includes the self-consistent interaction between the beam and its surroundings
in terms of a nonlinear coupling impedance, and when resistive as well as
reactive parts are included, the evolution equation becomes a generalised
nonlinear Schroedinger equation including a nonlocal nonlinear term. The
consequences of the resistive part on the propagation of particle bunches are
examined using analytical as well as numerical methods.Comment: 6 pages, 6 figures, uses RevTeX
On the Absence of Continuous Symmetries for Noncommutative 3-Spheres
A large class of noncommutative spherical manifolds was obtained recently
from cohomology considerations. A one-parameter family of twisted 3-spheres was
discovered by Connes and Landi, and later generalized to a three-parameter
family by Connes and Dubois-Violette. The spheres of Connes and Landi were
shown to be homogeneous spaces for certain compact quantum groups. Here we
investigate whether or not this property can be extended to the noncommutative
three-spheres of Connes and Dubois-Violette. Upon restricting to quantum groups
which are continuous deformations of Spin(4) and SO(4) with standard
co-actions, our results suggest that this is not the case.Comment: 15 pages, no figure
Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
Planets form in protoplanetary disks and inherit their chemical compositions.
It is thus crucial to map the distribution and investigate the formation of
simple organics, such as formaldehyde and methanol, in protoplanetary disks. We
analyze ALMA observations of the nearby disk-jet system around the T Tauri star
DG Tau in the o-HCO and CHOH E,
A transitions at an unprecedented resolution of ,
i.e., au at a distance of 121 pc. The HCO emission originates from
a rotating ring extending from au with a peak at au, i.e., at
the edge of the 1.3mm dust continuum. CHOH emission is not detected down to
an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para
ratio of 1.8-2.8 the ring- and disk-height-averaged HCO column density is
cm, while that of CHOH is
cm. In the inner au no o-HCO emission
is detected with an upper limit on its beam-averaged column density of
cm. The HCO ring in the disk of DG Tau is
located beyond the CO iceline (R au). This suggests that the
HCO abundance is enhanced in the outer disk due to formation on grain
surfaces by the hydrogenation of CO ice. The emission peak at the edge of the
mm dust continuum may be due to enhanced desorption of HCO in the gas phase
caused by increased UV penetration and/or temperature inversion. The
CHOH/HCO abundance ratio is , in agreement with disk chemistry
models. The inner edge of the HCO ring coincides with the radius where the
polarization of the dust continuum changes orientation, hinting at a tight link
between the HCO chemistry and the dust properties in the outer disk and at
the possible presence of substructures in the dust distribution.Comment: 8 pages, 6 figures, accepted for publication on A&A Letter
Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants
Extensive surveys of star-forming regions with Spitzer have revealed
populations of disk-bearing young stellar objects. These have provided crucial
constraints, such as the timescale of dispersal of protoplanetary disks,
obtained by carefully combining infrared data with spectroscopic or X-ray data.
While observations in various regions agree with the general trend of
decreasing disk fraction with age, the Lupus V and VI regions appeared to have
been at odds, having an extremely low disk fraction. Here we show, using the
recent Gaia data release 2 (DR2), that these extremely low disk fractions are
actually due to a very high contamination by background giants. Out of the 83
candidate young stellar objects (YSOs) in these clouds observed by Gaia, only
five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and
have similar proper motions to other members in this star-forming complex. Of
these five targets, four have optically thick (Class II) disks. On the one
hand, this result resolves the conundrum of the puzzling low disk fraction in
these clouds, while, on the other hand, it further clarifies the need to
confirm the Spitzer selected diskless population with other tracers, especially
in regions at low galactic latitude like Lupus V and VI. The use of Gaia
astrometry is now an independent and reliable way to further assess the
membership of candidate YSOs in these, and potentially other, star-forming
regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter
Stability and collapse of localized solutions of the controlled three-dimensional Gross-Pitaevskii equation
On the basis of recent investigations, a newly developed analytical procedure
is used for constructing a wide class of localized solutions of the controlled
three-dimensional (3D) Gross-Pitaevskii equation (GPE) that governs the
dynamics of Bose-Einstein condensates (BECs). The controlled 3D GPE is
decomposed into a two-dimensional (2D) linear Schr\"{o}dinger equation and a
one-dimensional (1D) nonlinear Schr\"{o}dinger equation, constrained by a
variational condition for the controlling potential. Then, the above class of
localized solutions are constructed as the product of the solutions of the
transverse and longitudinal equations. On the basis of these exact 3D
analytical solutions, a stability analysis is carried out, focusing our
attention on the physical conditions for having collapsing or non-collapsing
solutions.Comment: 21 pages, 14 figure
X-Shooter study of accretion in Chamaeleon I: II. A steeper increase of accretion with stellar mass for very low mass stars?
The dependence of the mass accretion rate on the stellar properties is a key
constraint for star formation and disk evolution studies. Here we present a
study of a sample of stars in the Chamaeleon I star forming region carried out
using the VLT/X-Shooter spectrograph. The sample is nearly complete down to
M~0.1Msun for the young stars still harboring a disk in this region. We derive
the stellar and accretion parameters using a self-consistent method to fit the
broad-band flux-calibrated medium resolution spectrum. The correlation between
the accretion luminosity to the stellar luminosity, and of the mass accretion
rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3,
respectively. These slopes and the accretion rates are consistent with previous
results in various star forming regions and with different theoretical
frameworks. However, we find that a broken power-law fit, with a steeper slope
for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller
than ~ 0.3 Msun, is slightly preferred according to different statistical
tests, but the single power-law model is not excluded. The steeper relation for
lower mass stars can be interpreted as a faster evolution in the past for
accretion in disks around these objects, or as different accretion regimes in
different stellar mass ranges. Finally, we find two regions on the mass
accretion versus stellar mass plane empty of objects. One at high mass
accretion rates and low stellar masses, which is related to the steeper
dependence of the two parameters we derived. The second one is just above the
observational limits imposed by chromospheric emission. This empty region is
located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be
effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with
the one expected for photoevaporation to rapidly dissipate the inner disk.Comment: Accepted for publication on Astronomy & Astrophysics. Abstract
shortened for arxiv constraints. Revised version after language editin
V1647 Orionis: One Year into Quiescence
We present new optical, near-IR, and mid-IR observations of the young
eruptive variable star V1647 Orionis that went into outburst in late 2004 for
approximately two years. Our observations were taken one year after the star
had faded to its pre-outburst optical brightness and show that V1647Ori is
still actively accreting circumstellar material. We compare and contrast these
data with existing observations of the source from both pre-outburst and
outburst phases. From near-IR spectroscopy we identify photospheric absorption
features for the first time that allow us to constrain the classification of
the young star itself. Our best fit spectral type is M0+-2 sub-classes with a
visual extinction of 19+-2 magnitudes and a K-band veiling of rK~1.5+-0.2. We
estimate that V1647Ori has a quiescent bolometric luminosity of ~9.5Lsun and a
mass accretion rate of ~1.10^-6Msun yr^-1. Our derived mass and age, from
comparison with evolutionary models, are 0.8+-0.2 Msun and ~0.5Myrs,
respectively. The presence towards the star of shock excited optical [S II] and
[Fe II] emission as well as near-IR H2 and [Fe II] emission perhaps suggests
that a new Herbig-Haro flow is becoming visible close to the star.Comment: 22 pages, 19 Figures, accepted AJ 13 October 200
Microscopic Derivation of Non-Markovian Thermalization of a Brownian Particle
In this paper, the first microscopic approach to the Brownian motion is
developed in the case where the mass density of the suspending bath is of the
same order of magnitude as that of the Brownian (B) particle. Starting from an
extended Boltzmann equation, which describes correctly the interaction with the
fluid, we derive systematicaly via the multiple time-scale analysis a reduced
equation controlling the thermalization of the B particle, i.e. the relaxation
towards the Maxwell distribution in velocity space. In contradistinction to the
Fokker-Planck equation, the derived new evolution equation is non-local both in
time and in velocity space, owing to correlated recollision events between the
fluid and particle B. In the long-time limit, it describes a non-markovian
generalized Ornstein-Uhlenbeck process. However, in spite of this complex
dynamical behaviour, the Stokes-Einstein law relating the friction and
diffusion coefficients is shown to remain valid. A microscopic expression for
the friction coefficient is derived, which acquires the form of the Stokes law
in the limit where the mean-free in the gas is small compared to the radius of
particle B.Comment: 28 pages, no figure, submitted to Journal of Statistical Physic
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