61 research outputs found
Volatile snowlines in embedded disks around low-mass protostars
(Abridged*) Models of the young solar nebula assume a hot initial disk with
most volatiles are in the gas phase. The question remains whether an actively
accreting disk is warm enough to have gas-phase water up to 50 AU radius. No
detailed studies have yet been performed on the extent of snowlines in an
embedded accreting disk (Stage 0). Quantify the location of gas-phase volatiles
in embedded actively accreting disk system. Two-dimensional physical and
radiative transfer models have been used to calculate the temperature structure
of embedded protostellar systems. Gas and ice abundances of HO, CO, and
CO are calculated using the density-dependent thermal desorption formulation.
The midplane water snowline increases from 3 to 55 AU for accretion rates
through the disk onto the star between -. CO can remain in the solid phase within the disk for down to AU. Most of the CO
is in the gas phase within an actively accreting disk independent of disk
properties and accretion rate. The predicted optically thin water isotopolog
emission is consistent with the detected HO emission toward the Stage
0 embedded young stellar objects, originating from both the disk and the warm
inner envelope (hot core). An accreting embedded disk can only account for
water emission arising from AU, however, and the extent rapidly
decreases for low accretion rates. Thus, the radial extent of the emission can
be measured with ALMA observations and compared to this limit. Volatiles
sublimate out to 50 AU in young disks and can reset the chemical content
inherited from the envelope in periods of high accretion rates. A hot young
solar nebula out to 30 AU can only have occurred during the deeply embedded
Stage 0, not during the T-Tauri phase of our early solar system.Comment: 15 pages, 10 figures, accepted for publication in A&
Testing protostellar disk formation models with ALMA observations
Abridged: Recent simulations have explored different ways to form accretion
disks around low-mass stars. We aim to present observables to differentiate a
rotationally supported disk from an infalling rotating envelope toward deeply
embedded young stellar objects and infer their masses and sizes. Two 3D
magnetohydrodynamics (MHD) formation simulations and 2D semi-analytical model
are studied. The dust temperature structure is determined through continuum
radiative transfer RADMC3D modelling. A simple temperature dependent CO
abundance structure is adopted and synthetic spectrally resolved submm
rotational molecular lines up to are simulated. All models
predict similar compact components in continuum if observed at the spatial
resolutions of 0.5-1 (70-140 AU) typical of the observations to date. A
spatial resolution of 14 AU and high dynamic range () are
required to differentiate between RSD and pseudo-disk in the continuum. The
peak-position velocity diagrams indicate that the pseudo-disk shows a flatter
velocity profile with radius than an RSD. On larger-scales, the CO isotopolog
single-dish line profiles are similar and are narrower than the observed line
widths of low- lines, indicating significant turbulence in the large-scale
envelopes. However a forming RSD can provide the observed line widths of
high- lines. Thus, either RSDs are common or a higher level of turbulence
( ) is required in the inner envelope compared
with the outer part. Multiple spatially and spectrally resolved molecular line
observations are needed. The continuum data give a better estimate on disk
masses whereas the disk sizes can be estimated from the spatially resolved
molecular lines observations. The general observable trends are similar between
the 2D semi-analytical models and 3D MHD RSD simulations.Comment: 16 pages, 14 figures, accepted for publication, A&
An Inner Disk in the Large Gap of the Transition Disk SR 24S
We report new Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 3
observations at 2.75 mm of the TD around SR 24S with an angular resolution of
0.11'' 0.09'' and a peak signal-to-noise ratio of . We
detect an inner disk and a mostly symmetric ring-like structure that peaks at
0.32'', that is 37 au at a distance of 114.4 pc. The full
width at half maximum of this ring is 28 au. We analyze the observed
structures by fitting the dust continuum visibilities using different models
for the intensity profile, and compare with previous ALMA observations of the
same disk at 0.45 mm and 1.30 mm. We qualitatively compare the results of these
fits with theoretical predictions of different scenarios for the formation of a
cavity or large gap. The comparison of the dust continuum structure between
different ALMA bands indicates that photoevaporation and dead zone can be
excluded as leading mechanisms for the cavity formation in SR 24S disk, leaving
the planet scenario (single or multiple planets) as the most plausible
mechanism. We compared the 2.75 mm emission with published (sub-)centimeter
data and find that the inner disk is likely tracing dust thermal emission. This
implies that any companion in the system should allow dust to move inwards
throughout the gap and replenish the inner disk. In the case of one single
planet, this puts strong constraints on the mass of the potential planet inside
the cavity and the disk viscosity of about 5 and
, respectively.Comment: Accepted to Ap
Size, Topology, and Shape Optimization of Truss Structures using Symbiotic Organisms Search
Truss structures are common in the building industry. One way to contain construction costs is to implement structural optimization. Optimization has to consider cross-sectional size, area, topology, and node coordinates as design variables. However, each truss structure has numerous complex constraints and variables that make optimizing this structure complex and difficult. The metaheuristic method is efficient and effective in solving large and complex problems. This paper tested three metaheuristic algorithms: particle swarm optimization (PSO), differential evolution (DE), and symbiotic organisms search (SOS). Each algorithm was used to optimize a 10-bar planar truss structure and a 15-bar planar truss structure. SOS was found to have the best optimization results, convergence behavior, and consistency
Comparative Study of Particle Swarm Optimization Algorithms in Solving Size, Topology, and Shape Optimization
This paper focuses on optimizing truss structures while propose best PSO variants. Truss optimization is one way to make the design efficient. There are three types of optimization, size optimization, shape optimization, and topology optimization. By combining size, shape and topology optimization, we can obtain the most efficient structure. Metaheuristics have the ability to solve this problem. Particle swarm optimization (PSO) is metaheuristic algorithm which is frequently used to solve many optimization problems. PSO mimics the behavior of flocking birds looking for food. But PSO has three parameters that can interfere with its performance, so this algorithm is not adaptive to diverse problems. Many PSO variants have been introduced to solve this problem, including linearly decreasing inertia weight particles swarm optimization (LDWPSO) and bare bones particles swarm optimization (BBPSO). The metaheuristic method is used to find the solution, while DSM s used to analyze the structure. A 10-bar truss structure and a 39-bar truss structure are considered as case studies. The result indicates that BBPSO beat other two algorithms in terms of best result, consistency, and convergence behaviour in both cases. LDWPSO took second place for the three categories, leaving PSO as the worst algorithm that tested
Revised SED of the triple protostellar system VLA 1623-2417
VLA 16232417 is a triple protostellar system deeply embedded in Ophiuchus
A. Sources A and B have a separation of 1.1", making their study difficult
beyond the submillimeter regime. Lack of circumstellar gas emission suggested
that VLA 16232417 B has a very cold envelope and is much younger than source
A, generally considered the prototypical Class 0 source. We explore the
consequences of new ALMA Band 9 data on the spectral energy distribution (SED)
of VLA 16232417 and their inferred nature. Using dust continuum observations
spanning from centimeter to near-infrared wavelengths, the SED of each
component in VLA 16232417 is constructed and analysed. The ALMA Band 9 data
presented here show that the SED of VLA 16232417 B does not peak at 850
m as previously expected, but instead presents the same shape as VLA
16232417 A at wavelengths shorter than 450 m. The results presented
here indicate that the previous assumption that the flux in and
Spitzer observations is solely dominated by VLA 16232417 A is not valid, and
instead, VLA 16232417 B most likely contributes a significant fraction of
the flux at 450 m. These results, however, do not explain the
lack of circumstellar gas emission and puzzling nature of VLA 16232417 B.Comment: 6 pages, 2 figures, accepted to A&A letters to the edito
A recent accretion burst in the low-mass protostar IRAS 15398-3359: ALMA imaging of its related chemistry
Low-mass protostars have been suggested to show highly variable accretion
rates through-out their evolution. Such changes in accretion, and related
heating of their ambient envelopes, may trigger significant chemical variations
on different spatial scales and from source-to-source. We present images of
emission from C17O, H13CO+, CH3OH, C34S and C2H toward the low-mass protostar
IRAS 15398-3359 on 0.5" (75 AU diameter) scales with the Atacama Large
Millimeter/submillimeter Array (ALMA) at 340 GHz. The resolved images show that
the emission from H13CO+ is only present in a ring-like structure with a radius
of about 1-1.5" (150-200 AU) whereas the CO and other high dipole moment
molecules are centrally condensed toward the location of the central protostar.
We propose that HCO+ is destroyed by water vapor present on small scales. The
origin of this water vapor is likely an accretion burst during the last
100-1000 years increasing the luminosity of IRAS 15398-3359 by a factor of 100
above its current luminosity. Such a burst in luminosity can also explain the
centrally condensed CH3OH and extended warm carbon-chain chemistry observed in
this source and furthermore be reflected in the relative faintness of its
compact continuum emission compared to other protostars.Comment: Accepted for publication in ApJ Letters; 14 pages, 5 figure
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