537 research outputs found
VLT near- to mid-IR imaging and spectroscopy of the M17 UC1-IRS5 region
We investigate the surroundings of the hypercompact HII region M17 UC1 to
probe the physical properties of the associated young stellar objects and the
environment of massive star formation. Five of the seven point sources in this
region show -band excess emission. Geometric match is found between the H_2
emission and near-IR polarized light in the vicinity of IRS5A, and between the
diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission
is typical for dense PDRs, which are FUV pumped initially and repopulated by
collisional de-excitation. The spectral types of IRS5A and B273A are B3-B7
V/III and G4-G5 III, respectively. The observed infrared luminosity L_IR in the
range 1-20 micron is derived for three objects; we obtain 2.0x10^3 L_\sun for
IRS5A, 13 L_\sun for IRS5C, and 10 L_\sun for B273A. IRS5 might be a young
quadruple system. Its primary star IRS5A is confirmed to be a high-mass
protostellar object (~ 9 M_\sun, ~1x10^5 yrs); it might have terminated
accretion due to the feedback from the stellar activities (radiation pressure,
outflow) and the expanding HII region of M17. UC1 might also have terminated
accretion because of the expanding hypercompact HII region ionized by itself.
The disk clearing process of the low-mass YSOs in this region might be
accelerated by the expanding HII region. The outflows driven by UC1 are running
in south-north with its northeastern side suppressed by the expanding
ionization front of M17; the blue-shifted outflow lobe of IRS5A is seen in two
types of tracers along the same line of sight in the form of H_2 emission
filament and mid-emission. The H_2 line ratios probe the properties of M17 SW
PDR, which is confirmed to have a clumpy structure with two temperature
distributions: warm, dense molecular clumps with n_H>10^5 cm^-3 and T~575 K and
cooler atomic gas with n_H~3.7x10^3-1.5x10^4 cm-3 and T~50-200 K.Comment: accepted for publication in A&A, 19 pages, 15 figures, 5 table
CEN34 -- High-Mass YSO in M17 or Background Post-AGB Star?
We investigate the proposed high-mass young stellar object (YSO) candidate
CEN34, thought to be associated with the star forming region M17. Its optical
to near-infrared (550-2500 nm) spectrum reveals several photospheric absorption
features, such as H{\alpha}, Ca triplet and CO bandheads but lacks any emission
lines. The spectral features in the range 8375-8770{\AA} are used to constrain
an effective temperature of 5250\pm250 (early-/mid-G) and a surface gravity of
2.0\pm0.3 (supergiant). The spectral energy distribution of CEN34 resembles the
SED of a high-mass YSO or an evolved star. Moreover, the observed temperature
and surface gravity are identical for high-mass YSOs and evolved stars. The
radial velocity relative to LSR (V_LSR) of CEN34 as obtained from various
photospheric lines is of the order of -60 km/s and thus distinct from the +25
km/s found for several OB stars in the cluster and for the associated molecular
cloud. The SED modeling yields ~ 10^{-4} M_sun of circumstellar material which
contributes only a tiny fraction to the total visual extinction (11 mag). In
the case of a YSO, a dynamical ejection process is proposed to explain the
V_LSR difference between CEN34 and M17. Additionally, to match the temperature
and luminosity, we speculate that CEN34 had accumulated the bulk of its mass
with accretion rate > 4x10^{-3} M_sun/yr in a very short time span (~ 10^3
yrs), and currently undergoes a phase of gravitational contraction without any
further mass gain. However, all the aforementioned characteristics of CEN34 are
compatible with an evolved star of 5-7 M_sun and an age of 50-100 Myrs, most
likely a background post-AGB star with a distance between 2.0 kpc and 4.5 kpc.
We consider the latter classification as the more likely interpretation.
Further discrimination between the two possible scenarios should come from the
more strict confinement of CEN34's distance.Comment: 8 pages, 8 figures, 2 tables; accepted by A&
Learning Only On Boundaries: a Physics-Informed Neural operator for Solving Parametric Partial Differential Equations in Complex Geometries
Recently deep learning surrogates and neural operators have shown promise in
solving partial differential equations (PDEs). However, they often require a
large amount of training data and are limited to bounded domains. In this work,
we present a novel physics-informed neural operator method to solve
parametrized boundary value problems without labeled data. By reformulating the
PDEs into boundary integral equations (BIEs), we can train the operator network
solely on the boundary of the domain. This approach reduces the number of
required sample points from to , where is the domain's
dimension, leading to a significant acceleration of the training process.
Additionally, our method can handle unbounded problems, which are unattainable
for existing physics-informed neural networks (PINNs) and neural operators. Our
numerical experiments show the effectiveness of parametrized complex geometries
and unbounded problems
Uncertainty Quantification for Maxwell\u27s Equations
This dissertation study three different approaches for stochastic electromagnetic fields based on the time domain Maxwell\u27s equations and Drude\u27s model: stochastic Galerkin method, stochastic collocation method, and Monte Carlo class methods. For each method, we study its regularity, stability, and convergence rates. Numerical experiments have been presented to verify our theoretical results. For stochastic collocation method, we also simulate the backward wave propagation in metamaterial phenomenon. It turns out that the stochastic Galerkin method admits the best accuracy property but hugest computational workload as the resultant PDEs system is usually coupled. The Monte Carlo class methods are easy to implement and do parallel computing but the accuracy is relatively low. The stochastic collocation method inherits the advantages of both of these two methods
Fabrication of a microresonator-fiber assembly maintaining a high-quality factor by CO2 laser welding
We demonstrate fabrication of a microtoroid resonator of a high-quality
(high-Q) factor using femtosecond laser three-dimensional (3D) micromachining.
A fiber taper is reliably assembled to the microtoroid using CO2 laser welding.
Specifically, we achieve a high Q-factor of 2.12*10^6 in the
microresonator-fiber assembly by optimizing the contact position between the
fiber taper and the microtoroid.Comment: 7 pages, 5 figure
On-chip electro-optic tuning of a lithium niobate microresonator with integrated in-plane microelectrodes
We demonstrate electro-optic tuning of an on-chip lithium niobate
microresonator with integrated in-plane microelectrodes. First two metallic
microelectrodes on the substrate were formed via femtosecond laser process.
Then a high-Q lithium niobate microresonator located between the
microelectrodes was fabricated by femtosecond laser direct writing accompanied
by focused ion beam milling. Due to the efficient structure designing, high
electro-optical tuning coefficient of 3.41 pm/V was observed.Comment: 6 pages, 3 figure
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