467 research outputs found
Phonon resonances in atomic currents through Bose-Fermi mixtures in optical lattices
We present an analysis of Bose-Fermi mixtures in optical lattices for the
case where the lattice potential of the fermions is tilted and the bosons (in
the superfluid phase) are described by Bogoliubov phonons. It is shown that the
Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark
states; these transitions are accompanied by energy dissipation into the
superfluid and result in a net atomic current along the lattice. We derive a
general expression for the drift velocity of the fermions and find that the
dependence of the atomic current on the lattice tilt exhibits negative
differential conductance and phonon resonances. Numerical simulations of the
full dynamics of the system based on the time-evolving block decimation
algorithm reveal that the phonon resonances should be observable under the
conditions of a realistic measuring procedure.Comment: 8 pages, 5 figure
Gas vs dust sizes of protoplanetary disks: effects of dust evolution
The extent of the gas in protoplanetary disks is observed to be universally
larger than the extent of the dust. This is often attributed to radial drift
and grain growth of the mm grains, but line optical depth produces a similar
observational signature. We investigate in what parts of the disk structure
parameter space dust evolution and line optical depth are the dominant drivers
of the observed gas and dust size difference. Using the thermochemical model
DALI with dust evolution included we ran a grid of models aimed at reproducing
the observed gas and dust size dichotomy. The relation between R_dust and dust
evolution is non-monotonic and depends on the disk structure. R_gas is directly
related to the radius where the CO column density drops below 10^15 cm^-2 and
CO becomes photodissociated. R_gas is not affected by dust evolution but scales
with the total CO content of the disk. R_gas/R_dust > 4 is a clear sign for
dust evolution and radial drift in disks, but these cases are rare in current
observations. For disks with a smaller R_gas/R_dust, identifying dust evolution
from R_gas/R_dust requires modelling the disk structure including the total CO
content. To minimize the uncertainties due to observational factors requires
FWHM_beam 10 on the 12CO
emission moment zero map. For the dust outer radius to enclose most of the disk
mass, it should be defined using a high fraction (90-95%) of the total flux.
For the gas, any radius enclosing > 60% of the 12CO flux will contain most of
the disk mass. To distinguish radial drift and grain growth from line optical
depth effects based on size ratios requires disks to be observed at high enough
angular resolution and the disk structure should to be modelled to account for
the total CO content of the disk.Comment: 18 pages, 27 figures, accepted in A&
Gas Density Perturbations Induced by One or More Forming Planets in the AS 209 Protoplanetary Disk as Seen with ALMA
The formation of planets occurs within protoplanetary disks surrounding young
stars, resulting in perturbation of the gas and dust surface densities. Here,
we report the first evidence of spatially resolved gas surface density
() perturbation towards the AS~209 protoplanetary disk from the
optically thin CO () emission. The observations were carried out
at 1.3~mm with ALMA at a spatial resolution of about 0.3\arcsec
0.2\arcsec (corresponding to 38 25 au). The CO
emission shows a compact (60~au), centrally peaked emission and an outer
ring peaking at 140~au, consistent with that observed in the continuum emission
and, its azimuthally averaged radial intensity profile presents a deficit that
is spatially coincident with the previously reported dust map. This deficit can
only be reproduced with our physico-thermochemical disk model by lowering
by nearly an order of magnitude in the dust gaps. Another
salient result is that contrary to CO, the DCO () emission
peaks between the two dust gaps. We infer that the best scenario to explain our
observations (CO deficit and DCO enhancement) is a gas
perturbation due to forming-planet(s), that is commensurate with previous
continuum observations of the source along with hydrodynamical simulations. Our
findings confirm that the previously observed dust gaps are very likely due to
perturbation of the gas surface density that is induced by a planet of at least
0.2~M in formation. Finally, our observations also show the
potential of using CO isotopologues to probe the presence of saturn mass
planet(s)
A deep Herschel/PACS observation of CO(40-39) in NGC 1068: a search for the molecular torus
Emission from high-J CO lines in galaxies has long been proposed as a tracer
of X-ray dominated regions (XDRs) produced by AGN. Of particular interest is
the question of whether the obscuring torus, which is required by AGN
unification models, can be observed via high-J CO cooling lines. Here we report
on the analysis of a deep Herschel-PACS observation of an extremely high J CO
transition (40-39) in the Seyfert 2 galaxy NGC 1068. The line was not detected,
with a derived 3 upper limit of . We apply an XDR model in order to
investigate whether the upper limit constrains the properties of a molecular
torus in NGC 1068. The XDR model predicts the CO Spectral Line Energy
Distributions for various gas densities and illuminating X-ray fluxes. In our
model, the CO(40-39) upper limit is matched by gas with densities , located at from the AGN,
with column densities of at least . At such high
column densities, however, dust absorbs most of the CO(40-39) line emission at
m. Therefore, even if NGC 1068 has a molecular torus
which radiates in the CO(40-39) line, the dust can attenuate the line emission
to below the PACS detection limit. The upper limit is thus consistent with the
existence of a molecular torus in NGC 1068. In general, we expect that the
CO(40-39) is observable in only a few AGN nuclei (if at all), because of the
required high gas column density, and absorption by dust.Comment: 22 pages, accepted for publication in Ap
Multidimensional chemical modelling, II. Irradiated outflow walls
Observations of the high-mass star forming region AFGL 2591 reveal a large
abundance of CO+, a molecule known to be enhanced by far UV (FUV) and X-ray
irradiation. In chemical models assuming a spherically symmetric envelope, the
volume of gas irradiated by protostellar FUV radiation is very small due to the
high extinction by dust. The abundance of CO+ is thus underpredicted by orders
of magnitude. In a more realistic model, FUV photons can escape through an
outflow region and irradiate gas at the border to the envelope. Thus, we
introduce the first 2D axi-symmetric chemical model of the envelope of a
high-mass star forming region to explain the CO+ observations as a prototypical
FUV tracer. The model assumes an axi-symmetric power-law density structure with
a cavity due to the outflow. The local FUV flux is calculated by a Monte Carlo
radiative transfer code taking scattering on dust into account. A grid of
precalculated chemical abundances, introduced in the first part of this series
of papers, is used to quickly interpolate chemical abundances. This approach
allows to calculate the temperature structure of the FUV heated outflow walls
self-consistently with the chemistry.
Synthetic maps of the line flux are calculated using a raytracer code.
Single-dish and interferometric observations are simulated and the model
results are compared to published and new JCMT and SMA observations. The
two-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dish
observations and also explains the non-detection by the SMA. We conclude that
the observed CO+ line flux and its narrow width can be interpreted by emission
from the warm and dense outflow walls irradiated by protostellar FUV radiation.Comment: Accepted by ApJ. 17 pages, 11 figures. A version with higher
resolution images is available from
http://www.astro.phys.ethz.ch/staff/simonbr/papgridII.pdf . First paper of
this series of papers available at arXiv:0906.058
HO distribution in the disc of HD 100546 and HD 163296: the role of dust dynamics and planet--disc interaction
[Abridged] Far-infrared observations with Herschel revealed a surprisingly
low abundance of cold-water reservoirs in protoplanetary discs. On the other
hand, a handful of discs show emission of hot water transitions excited at
temperatures above a few hundred Kelvin. In particular, the protoplanetary
discs around the Herbig Ae stars HD 100546 and HD 163296 show opposite trends
in terms of cold versus hot water emission: in the first case, the ground-state
transitions are detected and the high-J lines are undetected, while the trend
is opposite in HD 163296. We performed a spectral analysis using the
thermo-chemical model DALI. We find that HD 163296 is characterised by a
water-rich (abundance ) hot inner disc (within the snowline)
and a water-poor () outer disc: the relative abundance may be due
to the thermal desorption of icy grains that have migrated inward. Remarkably,
the size of the HO emitting region corresponds to a narrow dust gap visible
in the millimeter continuum at au with ALMA. The low-J lines detected
in HD 100546 instead imply an abundance of a few in the cold outer
disc ( au). The emitting region of the cold HO transitions is
spatially coincident with that of the HO ice previously seen in the
near-infrared. Notably, millimetre observations with ALMA reveal the presence
of a large dust gap between nearly 40 and 150 au, likely opened by a massive
embedded protoplanet. In both discs, we find that the warm molecular layer in
the outer region (beyond the snow line) is highly depleted of water molecules,
implying an oxygen-poor chemical composition of the gas. We speculate that
gas-phase oxygen in the outer disc is readily depleted and its distribution in
the disc is tightly coupled to the dynamics of the dust grains.Comment: Accepted for publication on A&
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