7,438 research outputs found
Compact stellar systems in the Fornax cluster: a UV perspective
In recent years, increasing evidence for chemical complexity and multiple
stellar populations in massive globular clusters (GCs) has emerged, including
extreme horizontal branches (EHBs) and UV excess. Our goal is to improve our
understanding of UV excess in the regime of both massive GCs and ultra-compact
dwarf galaxies (UCDs). To this end, we use deep archival GALEX data of the
central Fornax cluster to measure NUV and FUV magnitudes of UCDs and massive
GCs. We obtain NUV photometry for a sample of 35 compact objects with
-13.5<M_V<-10 mag. Of those, 21 objects also have FUV photometry. Roughly half
of the sources fall into the UCD luminosity regime (M_V <=-11 mag). We find
that seven out of 17 massive Fornax GCs exhibit a NUV excess with respect to
expectations from stellar population models, even for models with enhanced
Helium abundance. This suggests that not only He-enrichment has contributed to
forming the EHB population of these GCs. The GCs extend to stronger UV excess
than GCs in M31 and massive GCs in M87, at the 97% confidence level. Most of
the UCDs with FUV photometry also show evidence for UV excess, but their UV
colours can be matched by isochrones with enhanced Helium abundances and old
ages 12-14 Gyrs. We find that Fornax compact objects with X-ray emission
detected from Chandra images are almost disjunct in colour from compact objects
with GALEX UV detection, with only one X-ray source among the 35 compact
objects. However, since this source is one of the three most UV bright GCs, we
cannot exclude that the physical processes causing X-ray emission also
contribute to some of the observed UV excess.Comment: Research Note, 7 pages, 3 figures, accepted for publication in A&
Towards an -theorem for granular gases
The -theorem, originally derived at the level of Boltzmann non-linear
kinetic equation for a dilute gas undergoing elastic collisions, strongly
constrains the velocity distribution of the gas to evolve irreversibly towards
equilibrium. As such, the theorem could not be generalized to account for
dissipative systems: the conservative nature of collisions is an essential
ingredient in the standard derivation. For a dissipative gas of grains, we
construct here a simple functional related to the original ,
that can be qualified as a Lyapunov functional. It is positive, and results
backed by three independent simulation approaches (a deterministic spectral
method, the stochastic Direct Simulation Monte Carlo technique, and Molecular
Dynamics) indicate that it is also non-increasing. Both driven and unforced
cases are investigated
Oscillations in a maturation model of blood cell production.
We present a mathematical model of blood cell production which describes both the development of cells through the cell cycle, and the maturation of these cells as they differentiate to form the various mature blood cell types. The model differs from earlier similar ones by considering primitive stem cells as a separate population from the differentiating cells, and this formulation removes an apparent inconsistency in these earlier models. Three different controls are included in the model: proliferative control of stem cells, proliferative control of differentiating cells, and peripheral control of stem cell committal rate. It is shown that an increase in sensitivity of these controls can cause oscillations to occur through their interaction with time delays associated with proliferation and differentiation, respectively. We show that the characters of these oscillations are quite distinct and suggest that the model may explain an apparent superposition of fast and slow oscillations which can occur in cyclical neutropenia. © 2006 Society for Industrial and Applied Mathematics
Analytical approach to soliton ratchets in asymmetric potentials
We use soliton perturbation theory and collective coordinate ansatz to
investigate the mechanism of soliton ratchets in a driven and damped asymmetric
double sine-Gordon equation. We show that, at the second order of the
perturbation scheme, the soliton internal vibrations can couple {\it
effectively}, in presence of damping, to the motion of the center of mass,
giving rise to transport. An analytical expression for the mean velocity of the
soliton is derived. The results of our analysis confirm the internal mode
mechanism of soliton ratchets proposed in [Phys. Rev. E {\bf 65} 025602(R)
(2002)].Comment: 9 figures. Submitted to Phys. Rev.
Effects of random localizing events on matter waves: formalism and examples
A formalism is introduced to describe a number of physical processes that may
break down the coherence of a matter wave over a characteristic length scale l.
In a second-quantized description, an appropriate master equation for a set of
bosonic "modes" (such as atoms in a lattice, in a tight-binding approximation)
is derived. Two kinds of "localizing processes" are discussed in some detail
and shown to lead to master equations of this general form: spontaneous
emission (more precisely, light scattering), and modulation by external random
potentials. Some of the dynamical consequences of these processes are
considered: in particular, it is shown that they generically lead to a damping
of the motion of the matter-wave currents, and may also cause a "flattening" of
the density distribution of a trapped condensate at rest.Comment: v3; a few corrections, especially in Sections IV and
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