657 research outputs found
Interpreting Brightness Asymmetries in Transition Disks: Vortex at Dead Zone or Planet-carved Gap Edges?
Recent sub-millimeter observations show non-axisymmetric brightness
distributions with a horseshoe-like morphology for more than a dozen transition
disks. The most accepted explanation for the observed asymmetries is the
accumulation of dust in large-scale vortices. Protoplanetary disks vortices can
form by the excitation of Rossby-wave instability in the vicinity of a steep
pressure gradient, which can develop at the edges of a giant planet carved gap
or at the edges of an accretionally inactive zone. We studied the formation and
evolution of vortices formed in these two distinct scenarios by means of
two-dimensional locally isothermal hydrodynamic simulations. We found that the
vortex formed at the edge of a planetary gap is short-lived, unless the disk is
nearly inviscid. In contrast, the vortex formed at the outer edge of a dead
zone is long-lived. The vortex morphology can be significantly different in the
two scenarios: the vortex radial and azimuthal extensions are ~1.5 and ~3.5
times larger for the dead zone edge compared to gap models. In some particular
cases, the vortex aspect ratios can be similar in the two scenarios, however,
the vortex azimuthal extensions can be used to distinguish the vortex formation
mechanisms. We calculate predictions for vortex observability in the
sub-millimeter continuum with ALMA. We found that the azimuthal and radial
extent of brightness asymmetry correlates with vortex formation process, within
the limitations of alpha-viscosity prescription.Comment: 13 pages, 10 figures, accepted for publication in Ap
Knot Floer homology detects fibred knots
Ozsv\'ath and Szab\'o conjectured that knot Floer homology detects fibred
knots in . We will prove this conjecture for null-homologous knots in
arbitrary closed 3--manifolds. Namely, if is a knot in a closed 3--manifold
, is irreducible, and is monic, then is fibred.
The proof relies on previous works due to Gabai, Ozsv\'ath--Szab\'o, Ghiggini
and the author. A corollary is that if a knot in admits a lens space
surgery, then the knot is fibred.Comment: version 4: incorporates referee's suggestions, to appear in
Inventiones Mathematica
The partially asymmetric zero range process with quenched disorder
We consider the one-dimensional partially asymmetric zero range process where
the hopping rates as well as the easy direction of hopping are random
variables. For this type of disorder there is a condensation phenomena in the
thermodynamic limit: the particles typically occupy one single site and the
fraction of particles outside the condensate is vanishing. We use extreme value
statistics and an asymptotically exact strong disorder renormalization group
method to explore the properties of the steady state. In a finite system of
sites the current vanishes as , where the dynamical exponent,
, is exactly calculated. For the transport is realized by active particles, which move with a constant velocity, whereas for
the transport is due to the anomalous diffusion of a single Brownian
particle. Inactive particles are localized at a second special site and their
number in rare realizations is macroscopic. The average density profile of
inactive particles has a width of, , in terms of the
asymmetry parameter, . In addition to this, we have investigated the
approach to the steady state of the system through a coarsening process and
found that the size of the condensate grows as for large
times. For the unbiased model is formally infinite and the coarsening is
logarithmically slow.Comment: 12 pages, 9 figure
Weakly coupled, antiparallel, totally asymmetric simple exclusion processes
We study a system composed of two parallel totally asymmetric simple
exclusion processes with open boundaries, where the particles move in the two
lanes in opposite directions and are allowed to jump to the other lane with
rates inversely proportional to the length of the system. Stationary density
profiles are determined and the phase diagram of the model is constructed in
the hydrodynamic limit, by solving the differential equations describing the
steady state of the system, analytically for vanishing total current and
numerically for nonzero total current. The system possesses phases with a
localized shock in the density profile in one of the lanes, similarly to
exclusion processes endowed with nonconserving kinetics in the bulk. Besides,
the system undergoes a discontinuous phase transition, where coherently moving
delocalized shocks emerge in both lanes and the fluctuation of the global
density is described by an unbiased random walk. This phenomenon is analogous
to the phase coexistence observed at the coexistence line of the totally
asymmetric simple exclusion process, however, as a consequence of the
interaction between lanes, the density profiles are deformed and in the case of
asymmetric lane change, the motion of the shocks is confined to a limited
domain.Comment: 14 pages, 15 figures, to appear in Phys. Rev.
ALMA observations of the molecular gas in the debris disk of the 30 Myr old star HD 21997
The 30 Myr old A3-type star HD 21997 is one of the two known debris dust
disks having a measurable amount of cold molecular gas. With the goal of
understanding the physical state, origin, and evolution of the gas in young
debris disks, we obtained CO line observations with the Atacama Large
Millimeter/submillimeter Array (ALMA). Here we report on the detection of 12CO
and 13CO in the J=2-1 and J=3-2 transitions and C18O in the J=2-1 line. The gas
exhibits a Keplerian velocity curve, one of the few direct measurements of
Keplerian rotation in young debris disks. The measured CO brightness
distribution could be reproduced by a simple star+disk system, whose parameters
are r_in < 26 AU, r_out = 138 +/- 20 AU, M_*=1.8 +0.5 -0.2 M_Sun, and i = 32.6
+/- 3.1 degrees. The total CO mass, as calculated from the optically thin C18O
line, is about (4-8) x 10^-2 M_Earth, while the CO line ratios suggest a
radiation temperature on the order of 6-9 K. Comparing our results with those
obtained for the dust component of the HD 21997 disk from the ALMA continuum
observations by Mo\'or et al., we conclude that comparable amounts of CO gas
and dust are present in the disk. Interestingly, the gas and dust in the HD
21997 system are not co-located, indicating a dust-free inner gas disk within
55 AU of the star. We explore two possible scenarios for the origin of the gas.
A secondary origin, which involves gas production from colliding or active
planetesimals, would require unreasonably high gas production rates and would
not explain why the gas and dust are not co-located. We propose that HD 21997
is a hybrid system where secondary debris dust and primordial gas coexist. HD
21997, whose age exceeds both the model predictions for disk clearing and the
ages of the oldest T Tauri-like or transitional gas disks in the literature,
may be a key object linking the primordial and the debris phases of disk
evolution.Comment: 8 pages, 4 figures, accepted for publication in Ap
Thermodynamics of layered Heisenberg magnets with arbitrary spin
We present a spin-rotation-invariant Green-function theory of long- and
short-range order in the ferro- and antiferromagnetic Heisenberg model with
arbitrary spin quantum number S on a stacked square lattice. The thermodynamic
quantities (Curie temperature T_C, N\'eel temperature T_N, specific heat C_V,
intralayer and interlayer correlation lengths) are calculated, where the
effects of the interlayer coupling and the S dependence are explored. In
addition, exact diagonalizations on finite two-dimensional (2D) lattices with
S>=1 are performed, and a very good agreement between the results of both
approaches is found. For the quasi-2D and isotropic 3D magnets, our theory
agrees well with available quantum Monte Carlo and high-temperature
series-expansion data. Comparing the quasi-2D S=1/2 magnets, we obtain the
inequalities T_N>T_C and, for small enough interlayer couplings, T_N<T_C. The
results for C_V and the intralayer correlation length are compared to
experiments on the quasi-2D antiferromagnets Zn_2VO(PO_4)_2 with S=1/2 and
La_2NiO_4 with S=1, respectively.Comment: 10 pages, 8 figures, 3 tables, submitted to Physical Review
Receptors of peptides as therapeutic targets in epilepsy research
Neuropeptides are signaling molecules participating in the modulation of synaptic transmission. Neuropeptides are stored in dense core synaptic vesicles, the release of which requires profound excitation. Only in the extracellular space, neuropeptides act on G-protein coupled receptors to exert a relatively slow action both pre- and postsynaptically. Consequently, neuropeptide modulators are ideal candidates to influence epileptic tissue overexcited during seizures. Indeed, a number of neuropeptides have receptors implicated in epilepsy and many of them are considered to participate in endogenous neuroprotective actions. Neuropeptide receptors, present in the hippocampus, the most frequent focus of seizures in temporal lobe epilepsy, received the largest attention as potential anti-epileptic targets. Receptors of hippocampal neuropeptides, somatostatin, neuropeptide Y, galanin, dynorphin, enkephalin, substance P, cholecystokinin, vasoactive intestinal polypeptide, and receptors of some neuropeptides, which are also hormones such as ghrelin, angiotensins, corticotropin- releasing hormone, adrenocorticotropin, thyrotropin-releasing hormone, oxytocin and vasopressin involved in epilepsy are discussed in the review article. Activation and inhibition of receptors by oral application of peptides as drugs is typically not efficient because of low bioavailability: rapid degradation and insufficient penetration of peptides through the blood-brain barrier. Recent progress in the development of non-peptide agonists and antagonists of neuropeptide receptors as well as gene therapeutic approaches leading to the local production of agonists and antagonists within the central nervous system will also be discussed
Precise laser spectroscopy of the antiprotonic helium atom and CPT test on antiproton mass and charge
We have measured twelve transition frequencies of the antiprotonic helium
atom (pbar-He+) with precisions of 0.1--0.2 ppm using a laser spectroscopic
method. The agreement between the experiment and theories was so good that we
can put a limit on the proton-antiproton mass (or charge) difference. The new
limit is expected to be much smaller than the already published value, 60 ppb.Comment: proceeding of the conference, "PANIC02
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