159 research outputs found
ALMA CO J=6-5 observations of IRAS16293-2422: Shocks and entrainment
Observations of higher-excited transitions of abundant molecules such as CO
are important for determining where energy in the form of shocks is fed back
into the parental envelope of forming stars. The nearby prototypical and
protobinary low-mass hot core, IRAS16293-2422 (I16293) is ideal for such a
study. The source was targeted with ALMA for science verification purposes in
band 9, which includes CO J=6-5 (E_up/k_B ~ 116 K), at an unprecedented spatial
resolution (~0.2", 25 AU). I16293 itself is composed of two sources, A and B,
with a projected distance of 5". CO J=6-5 emission is detected throughout the
region, particularly in small, arcsecond-sized hotspots, where the outflow
interacts with the envelope. The observations only recover a fraction of the
emission in the line wings when compared to data from single-dish telescopes,
with a higher fraction of emission recovered at higher velocities. The very
high angular resolution of these new data reveal that a bow shock from source A
coincides, in the plane of the sky, with the position of source B. Source B, on
the other hand, does not show current outflow activity. In this region, outflow
entrainment takes place over large spatial scales, >~ 100 AU, and in small
discrete knots. This unique dataset shows that the combination of a
high-temperature tracer (e.g., CO J=6-5) and very high angular resolution
observations is crucial for interpreting the structure of the warm inner
environment of low-mass protostars.Comment: Accepted for publication in A&A Letter
Kinetics of the Multiferroic Switching in MnWO
The time dependence of switching multiferroic domains in MnWO has been
studied by time-resolved polarized neutron diffraction. Inverting an external
electric field inverts the chiral magnetic component within rise times ranging
between a few and some tens of milliseconds in perfect agreement with
macroscopic techniques. There is no evidence for any faster process in the
inversion of the chiral magnetic structure. The time dependence is well
described by a temperature-dependent rise time suggesting a well-defined
process of domain reversion. As expected, the rise times decrease when heating
towards the upper boundary of the ferroelectric phase. However, switching also
becomes faster upon cooling towards the lower boundary, which is associated
with a first-order phase transition
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Neural correlates of the rubber hand illusion in amputees: a report of two cases
One of the current challenges in the field of advanced prosthetics is the development of artificial limbs that provide
the user with detailed sensory feedback. Sensory feedback from our limbs is not only important for proprioceptive
awareness and motor control, but also essential for providing us with a feeling of ownership or simply put, the
sensation that our limbs actually belong to ourselves. The strong link between sensory feedback and ownership
has been repeatedly demonstrated with the so-called rubber hand illusion (RHI), during which individuals are
induced with the illusory sensation that an artificial hand is their own. In healthy participants, this occurs via
integration of visual and tactile signals, which is primarily supported by multisensory regions in premotor and
intraparietal cortices. Here, we describe a functional magnetic resonance imaging (fMRI) study with two upper
limb amputees, showing for the first time that the same brain regions underlie ownership sensations of an artificial
hand in this population. Albeit preliminary, these findings are interesting from both a theoretical as well as a clinical
point of view. From a theoretical perspective, they imply that even years after the amputation, a few seconds of
synchronous visuotactile stimulation are sufficient to activate hand-centered multisensory integration mechanisms.
From a clinical perspective, they show that a very basic sensation of touch from an artificial hand can be obtained
by simple but precisely targeted stimulation of the stump, and suggest that a similar mechanism implemented in
prosthetic hands would greatly facilitate ownership sensations and in turn, acceptance of the prosthesis
The Earliest Phases of Star formation (EPoS): Temperature, density, and kinematic structure of the star-forming core CB 17
Context: The initial conditions for the gravitational collapse of molecular
cloud cores and the subsequent birth of stars are still not well constrained.
The characteristic cold temperatures (about 10 K) in such regions require
observations at sub-millimetre and longer wavelengths. The Herschel Space
Observatory and complementary ground-based observations presented in this paper
have the unprecedented potential to reveal the structure and kinematics of a
prototypical core region at the onset of stellar birth.
Aims: This paper aims to determine the density, temperature, and velocity
structure of the star-forming Bok globule CB 17. This isolated region is known
to host (at least) two sources at different evolutionary stages: a dense core,
SMM1, and a Class I protostar, IRS.
Methods: We modeled the cold dust emission maps from 100 micron to 1.2 mm
with both a modified blackbody technique to determine the optical
depth-weighted line-of-sight temperature and column density and a ray-tracing
technique to determine the core temperature and volume density structure.
Furthermore, we analysed the kinematics of CB17 using the high-density gas
tracer N2H+.
Results: From the ray-tracing analysis, we find a temperature in the centre
of SMM1 of 10.6 K, a flat density profile with radius 9500 au, and a central
volume density of n(H) = 2.3x10^5 cm-3. The velocity structure of the N2H+
observations reveal global rotation with a velocity gradient of 4.3 km/s/pc.
Superposed on this rotation signature we find a more complex velocity field,
which may be indicative of differential motions within the dense core.
Conclusions: SMM is a core in an early evolutionary stage at the verge of
being bound, but the question of whether it is a starless or a protostellar
core remains unanswered.Comment: published in A&
Connecting the Dots: Analyzing Synthetic Observations of Star-Forming Clumps in Molecular Clouds
In this paper, we investigate the extent to which observations of molecular
clouds can correctly identify and measure star-forming clumps. We produced a
synthetic column density map and a synthetic spectral-line data cube from the
simulated collapse of a 5000 M molecular cloud. By correlating the
clumps found in the simulation to those found in the synthetic observations,
clump masses derived from spectral-line data cubes were found to be quite close
to the true physical properties of the clumps. We also find that the `observed'
clump mass function derived from the column density map is shifted by a factor
of ~ 3 higher than the true clump mass function, due to projection of
low-density material along the line of sight. Alves et al. (2007) first
proposed that a shift of a clump mass function to higher masses by a factor of
3 can be attributed to a star formation efficiency of 30 %. Our results
indicate that this finding may instead be due to an overestimate of clump
masses determined from column density observations.Comment: 8 pages, 7 figures, Accepted for publication in the Astrophysical
Journa
Boundary layer structure in turbulent thermal convection and its consequences for the required numerical resolution
Results on the Prandtl-Blasius type kinetic and thermal boundary layer
thicknesses in turbulent Rayleigh-B\'enard convection in a broad range of
Prandtl numbers are presented. By solving the laminar Prandtl-Blasius boundary
layer equations, we calculate the ratio of the thermal and kinetic boundary
layer thicknesses, which depends on the Prandtl number Pr only. It is
approximated as for and as for
, with . Comparison of the Prandtl--Blasius velocity
boundary layer thickness with that evaluated in the direct numerical
simulations by Stevens, Verzicco, and Lohse (J. Fluid Mech. 643, 495 (2010))
gives very good agreement. Based on the Prandtl--Blasius type considerations,
we derive a lower-bound estimate for the minimum number of the computational
mesh nodes, required to conduct accurate numerical simulations of moderately
high (boundary layer dominated) turbulent Rayleigh-B\'enard convection, in the
thermal and kinetic boundary layers close to bottom and top plates. It is shown
that the number of required nodes within each boundary layer depends on Nu and
Pr and grows with the Rayleigh number Ra not slower than \sim\Ra^{0.15}. This
estimate agrees excellently with empirical results, which were based on the
convergence of the Nusselt number in numerical simulations
Field induced spin reorientation and giant spin-lattice coupling in EuFe2As2
We have studied a EuFe2As2 single crystal by neutron diffraction under
magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin
reorientation is observed in the presence of a magnetic field along both the a
and c axes, respectively. Above critical field, the ground state
antiferromagnetic configuration of Eu moments transforms into a
ferromagnetic structure with moments along the applied field direction. The
magnetic phase diagram for Eu magnetic sublattice in EuFe2As2 is presented. A
considerable strain (0.9%) is induced by the magnetic field, caused by
the realignment of the twinning structure. Furthermore, the realignment of the
twinning structure is found to be reversible with the rebound of magnetic
field, which suggested the existence of magnetic shape-memory effect. The Eu
moment ordering exhibits close relationship with the twinning structure. We
argue that the Zeeman energy in combined with magnetic anisotropy energy is
responsible for the observed spin-lattice coupling.Comment: 5 pages, 4 figure
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