1,221 research outputs found
Potential Vorticity Evolution of a Protoplanetary Disk with An Embedded Protoplanet
We present two-dimensional inviscid hydrodynamic simulations of a
protoplanetary disk with an embedded planet, emphasizing the evolution of
potential vorticity (the ratio of vorticity to density) and its dependence on
numerical resolutions. By analyzing the structure of spiral shocks made by the
planet, we show that progressive changes of the potential vorticity caused by
spiral shocks ultimately lead to the excitation of a secondary instability. We
also demonstrate that very high numerical resolution is required to both follow
the potential vorticity changes and identify the location where the secondary
instability is first excited. Low-resolution results are shown to give the
wrong location. We establish the robustness of a secondary instability and its
impact on the torque onto the planet. After the saturation of the instability,
the disk shows large-scale non-axisymmetry, causing the torque on the planet to
oscillate with large amplitude. The impact of the oscillating torque on the
protoplanet's migration remains to be investigated.Comment: 17 pages total with 9 figures (Fig.4,5,9 are in .jpg), accepted to
Ap
A shared neural substrate for action verbs and observed actions in human posterior parietal cortex
High-level sensory and motor cortical areas are activated when processing the meaning of language, but it is unknown whether, and how, words share a neural substrate with corresponding sensorimotor representations. We recorded from single neurons in human posterior parietal cortex (PPC) while participants viewed action verbs and corresponding action videos from multiple views. We find that PPC neurons exhibit a common neural substrate for action verbs and observed actions. Further, videos were encoded with mixtures of invariant and idiosyncratic responses across views. Action verbs elicited selective responses from a fraction of these invariant and idiosyncratic neurons, without preference, thus associating with a statistical sampling of the diverse sensory representations related to the corresponding action concept. Controls indicated that the results are not the product of visual imagery or arbitrary learned associations. Our results suggest that language may activate the consolidated visual experience of the reader
On the relation of optical obscuration and X-ray absorption in Seyfert galaxies
The optical classification of a Seyfert galaxy and whether it is considered
X-ray absorbed are often used interchangeably. But there are many borderline
cases and also numerous examples where the optical and X-ray classifications
appear to be in conflict. In this article we re-visit the relation between
optical obscuration and X-ray absorption in AGNs. We make use of our "dust
color" method (Burtscher et al. 2015) to derive the optical obscuration A_V and
consistently estimated X-ray absorbing columns using 0.3--150 keV spectral
energy distributions. We also take into account the variable nature of the
neutral gas column N_H and derive the Seyfert sub-classes of all our objects in
a consistent way.
We show in a sample of 25 local, hard-X-ray detected Seyfert galaxies (log
L_X / (erg/s) ~ 41.5 - 43.5) that there can actually be a good agreement
between optical and X-ray classification. If Seyfert types 1.8 and 1.9 are
considered unobscured, the threshold between X-ray unabsorbed and absorbed
should be chosen at a column N_H = 10^22.3 / cm^2 to be consistent with the
optical classification.
We find that N_H is related to A_V and that the N_H/A_V ratio is
approximately Galactic or higher in all sources, as indicated previously. But
in several objects we also see that deviations from the Galactic ratio are only
due to a variable X-ray column, showing that (1) deviations from the Galactic
N_H/A_V can simply be explained by dust-free neutral gas within the broad line
region in some sources, that (2) the dust properties in AGNs can be similar to
Galactic dust and that (3) the dust color method is a robust way to estimate
the optical extinction towards the sublimation radius in all but the most
obscured AGNs.Comment: 7 pages, 3 figures, accepted for publication by A&A; updated PDF to
include abstrac
Principle of Maximum Entropy Applied to Rayleigh-B\'enard Convection
A statistical-mechanical investigation is performed on Rayleigh-B\'enard
convection of a dilute classical gas starting from the Boltzmann equation. We
first present a microscopic derivation of basic hydrodynamic equations and an
expression of entropy appropriate for the convection. This includes an
alternative justification for the Oberbeck-Boussinesq approximation. We then
calculate entropy change through the convective transition choosing mechanical
quantities as independent variables. Above the critical Rayleigh number, the
system is found to evolve from the heat-conducting uniform state towards the
convective roll state with monotonic increase of entropy on the average. Thus,
the principle of maximum entropy proposed for nonequilibrium steady states in a
preceding paper is indeed obeyed in this prototype example. The principle also
provides a natural explanation for the enhancement of the Nusselt number in
convection.Comment: 13 pages, 4 figures; typos corrected; Eq. (66a) corrected to remove a
double counting for ; Figs. 1-4 replace
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