45 research outputs found
Schrödinger-Poisson-Vlasov-Poisson correspondence
The Schr\"odinger-Poisson equations describe the behavior of a superfluid
Bose-Einstein condensate under self-gravity with a 3D wave function. As
, being the boson mass, the equations have been postulated to
approximate the collisionless Vlasov-Poisson equations also known as the
collisionless Boltzmann-Poisson equations. The latter describe collisionless
matter with a 6D classical distribution function. We investigate the nature of
this correspondence with a suite of numerical test problems in 1D, 2D, and 3D
along with analytic treatments when possible. We demonstrate that, while the
density field of the superfluid always shows order unity oscillations as
due to interference and the uncertainty principle, the potential
field converges to the classical answer as . Thus, any dynamics
coupled to the superfluid potential is expected to recover the classical
collisionless limit as . The quantum superfluid is able to
capture rich phenomena such as multiple phase-sheets, shell-crossings, and warm
distributions. Additionally, the quantum pressure tensor acts as a regularizer
of caustics and singularities in classical solutions. This suggests the
exciting prospect of using the Schr\"odinger-Poisson equations as a low-memory
method for approximating the high-dimensional evolution of the Vlasov-Poisson
equations. As a particular example we consider dark matter composed of
ultra-light axions, which in the classical limit () is expected
to manifest itself as collisionless cold dark matter
Remnant radio-loud AGN in the Herschel-ATLAS field
Only a small fraction of observed active galactic nuclei (AGN) display large-scale radio emission associated with jets, yet these radio-loud AGN have become increasingly important in models of galaxy evolution. In determining the dynamics and energetics of the radio sources over cosmic time, a key question concerns what happens when their jets switch off. The resulting âremnant' radio-loud AGN have been surprisingly evasive in past radio surveys, and therefore statistical information on the population of radio-loud AGN in their dying phase is limited. In this paper, with the recent developments of Low-Frequency Array (LOFAR) and the Very Large Array, we are able to provide a systematically selected sample of remnant radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection method, we constrain the upper limit on the fraction of remnants in our radio-loud AGN sample to 9âperâcent, implying that the extended lobe emission fades rapidly once the core/jets turn off. We also find that our remnant sample has a wide range of spectral indices (â1.5 â©œ α1400150 â©œ â0.5), confirming that the lobes of some remnants may possess flat spectra at low frequencies just as active sources do. We suggest that, even with the unprecedented sensitivity of LOFAR, our sample may still only contain the youngest of the remnant population
Unveiling the Role of the Magnetic Field at the Smallest Scales of Star Formation
We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of polarized dust emission from the protostellar source Ser-emb 8 at a linear resolution of 140 au. Assuming models of dust-grain alignment hold, the observed polarization pattern gives a projected view of the magnetic field structure in this source. Contrary to expectations based on models of strongly magnetized star formation, the magnetic field in Ser-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA data with previous observational studies, we can connect magnetic field structure from protostellar core (Ì80,000 au) to disk (Ì100 au) scales. We compare our observations with four magnetohydrodynamic gravo-turbulence simulations made with the AREPO code that have initial conditions ranging from super-AlfvĂ©nic (weakly magnetized) to sub-AlfvĂ©nic (strongly magnetized). These simulations achieve the spatial dynamic range necessary to resolve the collapse of protostars from the parsec scale of star-forming clouds down to the Ì100 au scale probed by ALMA. Only in the very strongly magnetized simulation do we see both the preservation of the field direction from cloud to disk scales and an hourglass-shaped field at <1000 au scales. We conduct an analysis of the relative orientation of the magnetic field and the density structure in both the Ser-emb 8 ALMA observations and the synthetic observations of the four AREPO simulations. We conclude that the Ser-emb 8 data are most similar to the weakly magnetized simulations, which exhibit random alignment, in contrast to the strongly magnetized simulation, where the magnetic field plays a role in shaping the density structure in the source. In the weak-field case, it is turbulenceânot the magnetic fieldâthat shapes the material that forms the protostar, highlighting the dominant role that turbulence can play across many orders of magnitude in spatial scale.Astronom
Regulation of Black Hole Winds and Jets Across the Mass Scale
We present a study of the mechanical power generated by both winds and jets
across the black hole mass scale. We begin with the study of ionized X-ray
winds and present a uniform analysis using Chandra grating spectra. The high
quality grating spectra facilitate the characterization of the outflow
velocity, ionization and column density of the absorbing gas. We find that the
kinetic power of the winds scales with increasing bolometric luminosity as
log(L_wind) \propto (1.58 \pm 0.07) log(L_Bol). This means that SMBH may be
more efficient than stellar-mass black holes in launching winds. In addition,
the simplicity of the scaling may suggest common driving mechanisms across the
mass scale. For comparison, we next examine jet production, estimating jet
power based on the energy required to inflate local bubbles. The jet relation
is log(L_Jet)\propto (1.18\pm0.24) log(L_Bol). The energetics of the bubble
associated with Cygnus X-1 are particularly difficult to determine, and the
bubble could be a background SNR. If we exclude Cygnus X-1, then the jets
follow a consistent relation to the winds within errors but with a higher
normalization, log(L_Jet) \propto (1.34 \pm 0.50) log(L_Bol). The formal
consistency in the wind and jet scaling relations suggests that a common
launching mechanism may drive both flows; magnetic processes are viable
possibilities. We also examine winds with especially high velocities, v >
0.01c. These ultra-fast outflows tend to resemble the jets more than the winds,
indicating we may be observing a regime in which winds become jets. This study
allows for the total power from black hole accretion, both mechanical and
radiative, to be characterized in a simple manner and suggests a possible
connection between winds and jets. Finally, we find at low Eddington fractions,
the jet power is dominant, and at high Eddington fractions the wind power is
dominant.Comment: 24 pages, 10 figures, Accepted to ApJ on 16 Nov 201
Direct observation shows superposition and large scale flexibility within cytoplasmic dynein motors moving along microtubules
Cytoplasmic dynein is a dimeric AAA+ motor protein that performs critical roles in eukaryotic cells by moving along microtubules using ATP. Here using cryo-electron microscopy we directly observe the structure of Dictyostelium discoideum dynein dimers on microtubules at near-physiological ATP concentrations. They display remarkable flexibility at a hinge close to the microtubule binding domain (the stalkhead) producing a wide range of head positions. About half the molecules have the two heads separated from one another, with both leading and trailing motors attached to the microtubule. The other half have the two heads and stalks closely superposed in a front-to-back arrangement of the AAA+ rings, suggesting specific contact between the heads. All stalks point towards the microtubule minus end. Mean stalk angles depend on the separation between their stalkheads, which allows estimation of inter-head tension. These findings provide a structural framework for understanding dyneinâs directionality and unusual stepping behaviour
A moving mesh unstaggered constrained transport scheme for magnetohydrodynamics
We present a constrained transport (CT) algorithm for solving the 3D ideal magnetohydrodynamic (MHD) equations on a moving mesh, which maintains the divergence-free condition on the magnetic field to machine-precision. Our CT scheme uses an unstructured representation of the magnetic vector potential, making the numerical method simple and computationally efficient. The scheme is implemented in the moving mesh code arepo. We demonstrate the performance of the approach with simulations of driven MHD turbulence, a magnetized disc galaxy, and a cosmological volume with primordial magnetic field. We compare the outcomes of these experiments to those obtained with a previously implemented Powell divergence-cleaning scheme. While CT and the Powell technique yield similar results in idealized test problems, some differences are seen in situations more representative of astrophysical flows. In the turbulence simulations, the Powell cleaning scheme artificially grows the mean magnetic field, while CT maintains this conserved quantity of ideal MHD. In the disc simulation, CT gives slower magnetic field growth rate and saturates to equipartition between the turbulent kinetic energy and magnetic energy, whereas Powell cleaning produces a dynamically dominant magnetic field. Such difference has been observed in adaptive-mesh refinement codes with CT and smoothed-particle hydrodynamics codes with divergence-cleaning. In the cosmological simulation, both approaches give similar magnetic amplification, but Powell exhibits more cell-level noise. CT methods in general are more accurate than divergence-cleaning techniques, and, when coupled to a moving mesh can exploit the advantages of automatic spatial/temporal adaptivity and reduced advection errors, allowing for improved astrophysical MHD simulations