36,022 research outputs found
Effect of dust grain porosity on the appearance of protoplanetary disks
We theoretically analyze protoplanetary disks consisting of porous dust
grains. In the analysis of observations of protoplanetary disks the dust phase
is often assumed to consist of spherical grains, allowing one to apply the Mie
scattering formalism. However, in reality, the shape of dust grains is expected
to deviate strongly from that of a sphere. We investigate the influence of
porous dust grains on the temperature distribution and observable appearance of
protoplanetary disks for dust grain porosities of up to 60 %. We performed
radiative transfer modeling to simulate the temperature distribution, spectral
energy distribution, and spatially resolved intensity and polarization maps.
The optical properties of porous grains were calculated using the method of
discrete dipole approximation. We find that the flux in the optical wavelength
range is for porous grains higher than for compact, spherical grains. The
profile of the silicate peak at 9.7 um strongly depends on the degree of grain
porosity. The temperature distribution shows significant changes in the
direction perpendicular to the midplane. Moreover, simulated polarization maps
reveal an increase of the polarization degree by a factor of about four when
porous grains are considered, regardless of the disk inclination. The
polarization direction is reversed in selected disk regions, depending on the
wavelength, grain porosity, and disk inclination. We discuss several possible
explanations of this effect and find that multiple scattering explains the
effect best. Porosity influences the observable appearance of protoplanetary
disks. In particular, the polarization reversal shows a dependence on grain
porosity. The physical conditions within the disk are altered by porosity,
which might have an effect on the processes of grain growth and disk evolution.Comment: 12 pages, 18 figure
Detecting planets in protoplanetary disks: A prospective study
We investigate the possibility to find evidence for planets in circumstellar
disks by infrared and submillimeter interferometry. We present simulations of a
circumstellar disk around a solar-type star with an embedded planet of 1
Jupiter mass. The three-dimensional (3D) density structure of the disk results
from hydrodynamical simulations. On the basis of 3D radiative transfer
simulations, images of this system were calculated. The intensity maps provide
the basis for the simulation of the interferometers VLTI (equipped with the
mid-infrared instrument MIDI) and ALMA. While MIDI/VLTI will not provide the
possibility to distinguish between disks with or without a gap on the basis of
visibility measurements, ALMA will provide the necessary basis for a direct gap
detection.Comment: 5 page
Magnetic fields in molecular clouds: Limitations of the analysis of Zeeman observations
Context. Observations of Zeeman split spectral lines represent an important
approach to derive the structure and strength of magnetic fields in molecular
clouds. In contrast to the uncertainty of the spectral line observation itself,
the uncertainty of the analysis method to derive the magnetic field strength
from these observations is not been well characterized so far.
Aims. We investigate the impact of several physical quantities on the
uncertainty of the analysis method, which is used to derive the line-of-sight
(LOS) magnetic field strength from Zeeman split spectral lines.
Methods. We simulate the Zeeman splitting of the 1665 MHz OH line with the 3D
radiative transfer (RT) extension ZRAD. This extension is based on the line RT
code Mol3D (Ober et al. 2015) and has been developed for the POLArized
RadIation Simulator POLARIS (Reissl et al. 2016).
Results. Observations of the OH Zeeman effect in typical molecular clouds are
not significantly affected by the uncertainty of the analysis method. We
derived an approximation to quantify the range of parameters in which the
analysis method works sufficiently accurate and provide factors to convert our
results to other spectral lines and species as well. We applied these
conversion factors to CN and found that observations of the CN Zeeman effect in
typical molecular clouds are neither significantly affected by the uncertainty
of the analysis method. In addition, we found that the density has almost no
impact on the uncertainty of the analysis method, unless it reaches values
higher than those typically found in molecular clouds. Furthermore, the
uncertainty of the analysis method increases, if both the gas velocity and the
magnetic field show significant variations along the line-of-sight. However,
this increase should be small in Zeeman observations of most molecular clouds
considering typical velocities of ~1 km/s.Comment: 9 pages, 6 figure
Observability of characteristic binary-induced structures in circumbinary disks
Context: A substantial fraction of protoplanetary disks forms around stellar
binaries. The binary system generates a time-dependent non-axisymmetric
gravitational potential, inducing strong tidal forces on the circumbinary disk.
This leads to a change in basic physical properties of the circumbinary disk,
which should in turn result in unique structures that are potentially
observable with the current generation of instruments.
Aims: The goal of this study is to identify these characteristic structures,
to constrain the physical conditions that cause them, and to evaluate the
feasibility to observe them in circumbinary disks.
Methods: To achieve this, at first two-dimensional hydrodynamic simulations
are performed. The resulting density distributions are post-processed with a 3D
radiative transfer code to generate re-emission and scattered light maps. Based
on these, we study the influence of various parameters, such as the mass of the
stellar components, the mass of the disk and the binary separation on
observable features in circumbinary disks.
Results: We find that the Atacama Large (sub-)Millimetre Array (ALMA) as well
as the European Extremely Large Telescope (E-ELT) are capable of tracing
asymmetries in the inner region of circumbinary disks which are affected most
by the binary-disk interaction. Observations at submillimetre/millimetre
wavelengths will allow the detection of the density waves at the inner rim of
the disk and the inner cavity. With the E-ELT one can partially resolve the
innermost parts of the disk in the infrared wavelength range, including the
disk's rim, accretion arms and potentially the expected circumstellar disks
around each of the binary components
Interferometric Detection of Planets/Gaps in Protoplanetary Disks
We investigate the possibility to find evidence for planets in circumstellar
disks by infrared and submillimeter interferometry. Hydrodynamical simulations
of a circumstellar disk around a solar-type star with an embedded planet of 1
Jupiter mass are presented. On the basis of 3D radiative transfer simulations,
images of this system are calculated. These intensity maps provide the basis
for the simulation of the interferometers VLTI (equipped with the mid-infrared
instrument MIDI) and ALMA. While ALMA will provide the necessary basis for a
direct gap and therefore indirect planet detection, MIDI/VLTI will provide the
possibility to distinguish between disks with or without accretion on the
central star on the basis of visibility measurements.Comment: 4 pages, TeX (or Latex, etc); to appear in proceedings of "Scientific
Frontiers in Research on Extrasolar Planets
Supercurrent through grain boundaries in the presence of strong correlations
Strong correlations are known to severely reduce the mobility of charge
carriers near half-filling and thus have an important influence on the current
carrying properties of grain boundaries in the high- cuprates. In this
work we present an extension of the Gutzwiller projection approach to treat
electronic correlations below as well as above half-filling consistently. We
apply this method to investigate the critical current through grain boundaries
with a wide range of misalignment angles for electron- and hole-doped systems.
For the latter excellent agreement with experimental data is found. We further
provide a detailed comparison to an analogous weak-coupling evaluation.Comment: 4 pages, 3 figure
Collapse and revival oscillations as a probe for the tunneling amplitude in an ultra-cold Bose gas
We present a theoretical study of the quantum corrections to the revival time
due to finite tunneling in the collapse and revival of matter wave interference
after a quantum quench. We study hard-core bosons in a superlattice potential
and the Bose-Hubbard model by means of exact numerical approaches and
mean-field theory. We consider systems without and with a trapping potential
present. We show that the quantum corrections to the revival time can be used
to accurately determine the value of the hopping parameter in experiments with
ultracold bosons in optical lattices.Comment: 10 pages, 12 figures, typos in section 3A correcte
Tracing planet-induced structures in circumstellar disks using molecular lines
Circumstellar disks are considered to be the birthplace of planets. Specific
structures like spiral arms, gaps, and cavities are characteristic indicators
of planet-disk interaction. Investigating these structures can provide insights
into the growth of protoplanets and the physical properties of the disk. We
investigate the feasibility of using molecular lines to trace planet-induced
structures in circumstellar disks. Based on 3D hydrodynamic simulations of
planet-disk interactions, we perform self-consistent temperature calculations
and produce N-LTE molecular line velocity-channel maps and spectra of these
disks using our new N-LTE line radiative transfer code Mol3D. Subsequently, we
simulate ALMA observations using the CASA simulator. We consider two nearly
face-on inclinations, 5 disk masses, 7 disk radii, and 2 different typical
pre-main-sequence host stars (T Tauri, Herbig Ae). We calculate up to 141
individual velocity-channel maps for five molecules/isotopoloques in a total of
32 rotational transitions to investigate the frequency dependence of the
structures indicated above. We find that the majority of protoplanetary disks
in our parameter space could be detected in the molecular lines considered.
However, unlike the continuum case, gap detection is not straightforward in
lines. For example, gaps are not seen in symmetric rings but are masked by the
pattern caused by the global (Keplerian) velocity field. We identify specific
regions in the velocity-channel maps that are characteristic of planet-induced
structures. Simulations of high angular resolution molecular line observations
demonstrate the potential of ALMA to provide complementary information about
the planet-disk interaction as compared to continuum observations. In
particular, the detection of planet-induced gaps is possible under certain
conditions.(abridged)Comment: 19 pages, 19 figures, accepted for publication in A&
Spectral Dependence of Polarized Radiation due to Spatial Correlations
We study the polarization of light emitted by spatially correlated sources.
We show that in general polarization acquires nontrivial spectral dependence
due to spatial correlations. The spectral dependence is found to be absent only
for a special class of sources where the correlation length scales as the
wavelength of light. We further study the cross correlations between two
spatially distinct points that are generated due to propagation. It is found
that such cross correlation leads to sufficiently strong spectral dependence of
polarization which can be measured experimentally.Comment: 5 pages, 4 figure
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