4,589 research outputs found
RW Aur A from the X-Wind Point of View: General Features
In this paper, the RW Aur A microjet is studied from the point of view of
X-wind models. The archived HST/STIS spectra of optical forbidden lines [O I],
[S II], and [N II] from RW Aur A, taken in Cycle 8 with seven parallel slits
along the jet axis, spaced at 0".07 apart, were analyzed. Images,
position-velocity diagrams, and line ratios among the species were constructed,
and compared with synthetic observations generated by selected solutions of the
X-wind. Prominent features arising in a steady state X-wind could be identified
within the convolved images, full-widths at half maxima and high-velocity peaks
on both of the redshifted and blueshifted jets. The well-known asymmetric
velocity profiles of the opposite jets are built into the selected models. We
discuss model selections within the existing uncertainties of stellar
parameters and inclination angle of the system. In this framework, the
mass-loss rates that were inferred to be decreasing along the jet axis in the
literature are the results of slowly decreasing excitation conditions and
electron density profiles. Despite the apparent asymmetry in terminal
velocities, line intensities and mass-loss rates, the average linear momenta
from the opposite sides of the jet are actually balanced. These previously
hard-to-explain features of the asymmetric RW Aur A jet system now find a
different but self-consistent interpretation within the X-wind framework.Comment: 31 pages, 9 figures, 5 tables; accepted for publication in ApJ (Send
correspondence to: [email protected]
Phase-reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138
We investigate a phase-reference VLBI observation that was conducted at 15.4
GHz by fast switching VLBA antennas between the compact steep-spectrum radio
source 3C 138 and the quasar PKS 0528+134 which are about 4 away on the
sky. By comparing the phase-reference mapping with the conventional hybrid
mapping, we demonstrate the feasibility of high precision astrometric
measurements for sources separated by 4. VLBI phase-reference mapping
preserves the relative phase information, and thus provides an accurate
relative position between 3C 138 and PKS 0528+134 of
and
(J2000.0) in right ascension and declination, respectively. This gives an
improved position of the nucleus (component A) of 3C 138 in J2000.0 to be
RA= and Dec= under the
assumption that the position of calibrator PKS 0528+134 is correct. We further
made a hybrid map by performing several iterations of CLEAN and
self-calibration on the phase-referenced data with the phase-reference map as
an input model for the first phase self-calibration. Compared with the hybrid
map from the limited visibility data directly obtained from fringe fitting 3C
138 data, this map has a similar dynamic range, but a higher angular
resolution. Therefore, phase-reference technique is not only a means of phase
connection, but also a means of increasing phase coherence time allowing
self-calibration technique to be applied to much weaker sources.Comment: 9 pages plus 2 figures, accepted by PASJ (Vol.58 No.6
Calculation of Critical Nucleation Rates by the Persistent Embryo Method: Application to Quasi Hard Sphere Models
We study crystal nucleation of the Weeks-Chandler-Andersen (WCA) model, using
the recently introduced Persistent Embryo Method (PEM). The method provides
detailed characterization of pre-critical, critical and post-critical nuclei,
as well as nucleation rates that compare favorably with those obtained using
other methods (umbrella sampling, forward flux sampling or seeding). We further
map our results to a hard sphere model allowing to compare with other existing
predictions. Implications for experiments are also discussed.Comment: 27 pages, 11 figure
The impacts of infectious disease pandemic on China’s edible vegetable oil futures markets: A long-term perspective
For the extremely important role of China in global edible vegetable
oil market and its decisive measures in the epidemic controlling
and stable economic recovery during the COVID-19
pandemic, the aim of this article is to inspect the quantitative
impacts of infectious disease pandemic on the returns, volatilities
and correlations of China’s edible vegetable oil futures markets by
using a DCC-MVGARCH-X model incorporating Baidu searching
index as the proxy of pandemic severity. Our empirical results
show that infectious disease pandemic does have significantly
positive impacts on the returns and volatilities of China’s soybean,
canola and palm oil futures markets. Second, there are significant
volatility spillover effects among the three vegetable oils, suggesting
strong contagion effect from one oil market to the others.
Third, soybean oil and palm oil show the largest correlation, while
the dependence between canola oil and palm oil is the smallest
one among the three pairwise correlations. Moreover, no matter
to consider epidemic situation in China or in global environment,
infectious disease pandemic has significant effects on these
correlations
Distribution-Free Model-Agnostic Regression Calibration via Nonparametric Methods
In this paper, we consider the uncertainty quantification problem for
regression models. Specifically, we consider an individual calibration
objective for characterizing the quantiles of the prediction model. While such
an objective is well-motivated from downstream tasks such as newsvendor cost,
the existing methods have been largely heuristic and lack of statistical
guarantee in terms of individual calibration. We show via simple examples that
the existing methods focusing on population-level calibration guarantees such
as average calibration or sharpness can lead to harmful and unexpected results.
We propose simple nonparametric calibration methods that are agnostic of the
underlying prediction model and enjoy both computational efficiency and
statistical consistency. Our approach enables a better understanding of the
possibility of individual calibration, and we establish matching upper and
lower bounds for the calibration error of our proposed methods. Technically,
our analysis combines the nonparametric analysis with a covering number
argument for parametric analysis, which advances the existing theoretical
analyses in the literature of nonparametric density estimation and quantile
bandit problems. Importantly, the nonparametric perspective sheds new
theoretical insights into regression calibration in terms of the curse of
dimensionality and reconciles the existing results on the impossibility of
individual calibration. To our knowledge, we make the first effort to reach
both individual calibration and finite-sample guarantee with minimal
assumptions in terms of conformal prediction. Numerical experiments show the
advantage of such a simple approach under various metrics, and also under
covariates shift. We hope our work provides a simple benchmark and a starting
point of theoretical ground for future research on regression calibration.Comment: Accepted at NeurIPS 2023 and update a camera-ready version; Add some
experiments and literature review
A novel voice coil motor-driven compliant micropositioning stage based on flexure mechanism
This paper presents a 2-degrees of freedom flexure-based micropositioning stage with a flexible decoupling mechanism. The stage is composed of an upper planar stage and four vertical support links to improve the out-of-plane stiffness. The moving platform is driven by two voice coil motors, and thus it has the capability of large working stroke. The upper stage is connected with the base through six double parallel four-bar linkages mechanisms, which are orthogonally arranged to implement the motion decoupling in the x and y directions. The vertical support links with serially connected hook joints are utilized to guarantee good planar motion with heavy-loads. The static stiffness and the dynamic resonant frequencies are obtained based on the theoretical analyses. Finite element analysis is used to investigate the characteristics of the developed stage. Experiments are carried out to validate the established models and the performance of the developed stage. It is noted that the developed stage has the capability of translational motion stroke of 1.8 mm and 1.78 mm in working axes. The maximum coupling errors in the x and y directions are 0.65% and 0.82%, respectively, and the motion resolution is less than 200 nm. The experimental results show that the developed stage has good capability for trajectory tracking
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