1,171 research outputs found
Dust-to-gas ratio resurgence in circumstellar disks due to the formation of giant planets: the case of HD 163296
The amount of dust present in circumstellar disks is expected to steadily
decrease with age due to the growth from micron-sized particles to
planetesimals and planets. Mature circumstellar disks, however, can be observed
to contain significant amounts of dust and possess high dust-to-gas ratios.
Using HD 163296 as our case study, we explore how the formation of giant
planets in disks can create the conditions for collisionally rejuvenating the
dust population, halting or reversing the expected trend. We combine N-body
simulations with statistical methods and impact scaling laws to estimate the
dynamical and collisional excitation of the planetesimals due to the formation
of HD 163296's giant planets. We show that this process creates a violent
collisional environment across the disk that can inject collisionally produced
second-generation dust into it, significantly contributing to the observed
dust-to-gas ratio. The spatial distribution of the dust production can explain
the observed local enrichments in HD 163296's inner regions. The results
obtained for HD 163296 can be extended to any disk with embedded forming giant
planets and may indicate a common evolutionary stage in the life of such
circumstellar disks. Furthermore, the dynamical excitation of the planetesimals
could result in the release of transient, non-equilibrium gas species like H2O,
CO2, NH3 and CO in the disk due to ice sublimation during impacts and, due to
the excited planetesimals being supersonic with respect to the gas, could
produce bow shocks in the latter that could heat it and cause a broadening of
its emission lines.Comment: 18 pages, 9 figures, 2 tables. Accepted for publication on The
Astrophysical Journa
Looking for outflow and infall signatures in high mass star forming regions
(Context) Many physical parameters change with time in star forming regions.
Here we attempt to correlate changes in infall and outflow motions in high mass
star forming regions with evolutionary stage using JCMT observations. (Aims)
From a sample of 45 high mass star forming regions in three phases of
evolution, we investigate the presence of established infall and outflow
tracers to determine whether there are any trends attributable to the age of
the source. (Methods) We obtained JCMT observations of HCO+/H13CO+ J=4-3 to
trace large scale infall, and SiO J=8-7 to trace recent outflow activity. We
compare the infall and outflow detections to the evolutionary stage of the host
source (high mass protostellar objects, hypercompact HII regions and
ultracompact HII regions). We also note that the integrated intensity of SiO
varies with the full width at half maximum of the H13CO+. (Results) We find a
surprising lack of SiO detections in the middle stage (Hypercompact HII
regions), which may be due to an observational bias. When SiO is detected, we
find that the integrated intensity of the line increases with evolutionary
stage. We also note that all of the sources with infall signatures onto
Ultracompact HII regions have corresponding outflow signatures as well.Comment: 9 pages, 5 figures, 5 tables. Accepted by A&
Protostellar clusters in intermediate-mass (IM) star forming regions
The transition between the low density groups of T Tauri stars and the high
density clusters around massive stars occurs in the intermediate-mass (IM)
range (M2--8 M). High spatial resolution studies of IM young
stellar objects (YSO) can provide important clues to understand the clustering
in massive star forming regions.
Aims: Our aim is to search for clustering in IM Class 0 protostars. The high
spatial resolution and sensitivity provided by the new A configuration of the
Plateau de Bure Interferometer (PdBI) allow us to study the clustering in these
nearby objects.
Methods: We have imaged three IM Class 0 protostars (Serpens-FIRS 1, IC 1396
N, CB 3) in the continuum at 3.3 and 1.3mm using the PdBI. The sources have
been selected with different luminosity to investigate the dependence of the
clustering process on the luminosity of the source.
Results: Only one millimeter (mm) source is detected towards the low
luminosity source Serpens--FIRS 1. Towards CB 3 and IC1396 N, we detect two
compact sources separated by 0.05 pc. The 1.3mm image of IC 1396 N, which
provides the highest spatial resolution, reveal that one of these cores is
splitted in, at least, three individual sources.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letters (Special Feature IRAM/PdB
The effect of local optically thick regions in the long-wave emission of young circumstellar disks
Multi-wavelength observations of protoplanetary disks in the sub-millimeter
continuum have measured spectral indices values which are significantly lower
than what is found in the diffuse interstellar medium. Under the assumption
that mm-wave emission of disks is mostly optically thin, these data have been
generally interpreted as evidence for the presence of mm/cm-sized pebbles in
the disk outer regions. In this work we investigate the effect of possible
local optically thick regions on the mm-wave emission of protoplanetary disks
without mm/cm-sized grains. A significant local increase of the optical depth
in the disk can be caused by the concentration of solid particles, as predicted
to result from a variety of proposed physical mechanisms. We calculate the
filling factors and implied overdensities these optically thick regions would
need to significantly affect the millimeter fluxes of disks, and we discuss
their plausibility. We find that optically thick regions characterized by
relatively small filling factors can reproduce the mm-data of young disks
without requesting emission from mm/cm-sized pebbles. However, these optically
thick regions require dust overdensities much larger than what predicted by any
of the physical processes proposed in the literature to drive the concentration
of solids. We find that only for the most massive disks it is possible and
plausible to imagine that the presence of optically thick regions in the disk
is responsible for the low measured values of the mm spectral index. For the
majority of the disk population, optically thin emission from a population of
large mm-sized grains remains the most plausible explanation. The results of
this analysis further strengthen the scenario for which the measured low
spectral indices of protoplanetary disks at mm wavelengths are due to the
presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres
A haptic-enabled multimodal interface for the planning of hip arthroplasty
Multimodal environments help fuse a diverse range of sensory modalities, which is particularly important when integrating the complex data involved in surgical preoperative planning. The authors apply a multimodal interface for preoperative planning of hip arthroplasty with a user interface that integrates immersive stereo displays and haptic modalities. This article overviews this multimodal application framework and discusses the benefits of incorporating the haptic modality in this area
On the shape of the mass-function of dense clumps in the Hi-GAL fields. II. Using Bayesian inference to study the clump mass function
Context. Stars form in dense, dusty clumps of molecular clouds, but little is
known about their origin, their evolution and their detailed physical
properties. In particular, the relationship between the mass distribution of
these clumps (also known as the "clump mass function", or CMF) and the stellar
initial mass function (IMF), is still poorly understood. Aims. In order to
better understand how the CMF evolve toward the IMF, and to discern the "true"
shape of the CMF, large samples of bona-fide pre- and proto-stellar clumps are
required. Two such datasets obtained from the Herschel infrared GALactic Plane
Survey (Hi-GAL) have been described in paper I. Robust statistical methods are
needed in order to infer the parameters describing the models used to fit the
CMF, and to compare the competing models themselves. Methods. In this paper we
apply Bayesian inference to the analysis of the CMF of the two regions
discussed in Paper I. First, we determine the Bayesian posterior probability
distribution for each of the fitted parameters. Then, we carry out a
quantitative comparison of the models used to fit the CMF. Results. We have
compared the results from several methods implementing Bayesian inference, and
we have also analyzed the impact of the choice of priors and the influence of
various constraints on the statistical conclusions for the preferred values of
the parameters. We find that both parameter estimation and model comparison
depend on the choice of parameter priors. Conclusions. Our results confirm our
earlier conclusion that the CMFs of the two Hi-GAL regions studied here have
very similar shapes but different mass scales. Furthermore, the lognormal model
appears to better describe the CMF measured in the two Hi-GAL regions studied
here. However, this preliminary conclusion is dependent on the choice of
parameters priors.Comment: Submitted for publication to A&A on November 12, 2013. This paper
contains 11 pages and 7 figure
Control of natural circulation loops by electrohydrodynamic pumping
The paper analyses the effect of electrohydrodynamic (EHD) pumping on the control of natural circulation loops (NCLs). The two major objectives of the investigation are: finding the optimal configuration of an EHD pump and demonstrating that the NCL flow direction can be inverted by exploiting the EHD phenomena. In the initial experimental set-up, we measured the static pressure rise given by an EHD pump made of three consecutive modules of point-ring electrodes for different dielectric fluids and electrode materials. When reversing the polarity of the applied DC voltage, we observed opposite pumping directions, suggesting the presence of two distinct EHD phenomena, inducing motion on opposite directions: ion-drag pumping and conduction pumping. The former was identified as a more efficient process compared to the latter. Based on these preliminary experiments, we built a NCL, operating with the fluid HFE-7100. Two oppositely mounted optimised pumping sections could be alternately activated, to promote clockwise or anticlockwise motion. In the first series of tests, alternately, the pumping sections were triggered prior to the heat input. In any case, the circulation followed the EHD pumping direction. In other tests, the electric field was applied when natural circulation was already present and the flow was reversed by means of opposite EHD pumping, at both polarities. Simply inverting the polarity of the applied voltage, we could alternate ion-drag and conduction pumping; in this way, we easily controlled the direction of motion by means of a single EHD pumping device
New constraints on dust grain size and distribution in CQ Tau
Grain growth in circumstellar disks is expected to be the first step towards
the formation of planetary systems. There is now evidence for grain growth in
several disks around young stars. Radially resolved images of grain growth in
circumstellar disks are believed to be a powerful tool to constrain the dust
evolution models and the initial stage for the formation of planets. In this
paper we attempt to provide these constraints for the disk surrounding the
young star CQ Tau. This system was already suggested from previous studies to
host a population of grains grown to large sizes. We present new high angular
resolution (0.3-0.9 arcsec) observations at wavelengths from 850um to 3.6cm
obtained at the SMA, IRAM-PdBI and NRAO-VLA interferometers. We perform a
combined analysis of the spectral energy distribution and of the
high-resolution images at different wavelengths using a model to describe the
dust thermal emission from the circumstellar disk. We include a prescription
for the gas emission from the inner regions of the system. We detect the
presence of evolved dust by constraining the disk averaged dust opacity
coefficient beta (computed between 1.3 and 7mm) to be 0.6+/-0.1. This confirms
the earlier suggestions that the disk contains dust grains grown to significant
sizes and puts this on firmer grounds by tightly constraining the gas
contamination to the observed fluxes at mm-cm wavelengths. We report some
evidence of radial variations in dust properties, but current resolution and
sensitivity are still too low for definitive results.Comment: 9 pages, A&A in pres
High Angular Resolution Mid-infrared Imaging of Young Stars in Orion BN/KL
We present Keck LWS images of the Orion BN/KL star forming region obtained in
the first multi-wavelength study to have 0.3-0.5" resolution from 4.7 to 22
microns. The young stellar objects designated infrared source-n and radio
source-I are believed to dominate the BN/KL region. We have detected extended
emission from a probable accretion disk around source-n but infer a stellar
luminosity on the order of only 2000 Lsun. Although source-I is believed to be
more luminous, we do not detect an infrared counterpart even at the longest
wavelengths. However, we resolve the closeby infrared source, IRc2, into an arc
of knots ~1000 AU long at all wavelengths. Although the physical relation of
source-I to IRc2 remains ambiguous, we suggest these sources mark a high
density core (10^7-10^8 pc^-3 over 1000 AU) within the larger BN/KL star
forming cluster. The high density may be a consequence of the core being young
and heavily embedded. We suggest the energetics of the BN/KL region may be
dominated by this cluster core rather than one or two individual sources.Comment: 13 pages including 3 color figures. Accepted to The Astrophysical
Journal Letters pending slight reduction in length. High resolution figures
(jpeg) may be found at
http://cfa-www.harvard.edu/~lincoln/keck.bnkl.midir.ppr
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