arXiv subjects

Yuanwei Wu

Publications and source records attributed to Yuanwei Wu.

25 records · Page 2Linked to original sources

arXiv2017

BPGrad: Towards Global Optimality in Deep Learning via Branch and Pruning

Understanding the global optimality in deep learning (DL) has been attracting more and more attention recently. Conventional DL solvers, however, have not been developed intentionally to seek for such global optimality. In this paper we propose a novel approximation algorithm, BPGrad, towards optimizing deep models globally via branch and pruning. Our BPGrad algorithm is based on the assumption of Lipschitz continuity in DL, and as a result it can adaptively determine the step size for current gradient given the history of previous updates, wherein theoretically no smaller steps can achieve the global optimality. We prove that, by repeating such branch-and-pruning procedure, we can locate the global optimality within finite iterations. Empirically an efficient solver based on BPGrad for DL is proposed as well, and it outperforms conventional DL solvers such as Adagrad, Adadelta, RMSProp, and Adam in the tasks of object recognition, detection, and segmentation.

stat.ML↗

arXiv2017

SiO Maser Survey towards off-plane O-rich AGBs around the orbital plane of the Sagittarius Stellar Stream

We conducted an SiO maser survey towards 221 O-rich AGB stars with the aim of identifying maser emission associated with the Sagittarius stellar stream. In this survey, maser emission was detected in 44 targets, of which 35 were new detections. All of these masers are within 5 kpc of the Sun. We also compiled a Galactic SiO maser catalogue including ~2300 SiO masers from the literature. The distribution of these SiO masers give a scale height of 0.40 kpc, while 42 sources deviate from the Galactic plane by more than 1.2 kpc, half of which were found in this survey. Regarding SiO masers in the disc, we found both the rotational speeds and the velocity dispersions vary with the Galactic plane distance. Assuming Galactic rotational speed $Θ$0 = 240 km/s , we derived the velocity lags are 15 km/s and 55 km/s for disc and off-plane SiO masers respectively. Moreover, we identified three groups with significant peculiar motions (with 70% confidence). The most significant group is in the thick disc that might trace stream/peculiar motion of the Perseus arm. The other two groups are mainly made up of off-plane sources. The northern and southern off-plane sources were found to be moving at ~33 km/s and 54 km/s away from the Galactic plane, respectively. Causes of these peculiar motions are still unclear. For the two off-plane groups, we suspect they are thick disc stars whose kinematics affected by the Sgr stellar stream or very old Sgr stream debris.

astro-ph.GA↗

arXiv2017

Vision-based Real-Time Aerial Object Localization and Tracking for UAV Sensing System

The paper focuses on the problem of vision-based obstacle detection and tracking for unmanned aerial vehicle navigation. A real-time object localization and tracking strategy from monocular image sequences is developed by effectively integrating the object detection and tracking into a dynamic Kalman model. At the detection stage, the object of interest is automatically detected and localized from a saliency map computed via the image background connectivity cue at each frame; at the tracking stage, a Kalman filter is employed to provide a coarse prediction of the object state, which is further refined via a local detector incorporating the saliency map and the temporal information between two consecutive frames. Compared to existing methods, the proposed approach does not require any manual initialization for tracking, runs much faster than the state-of-the-art trackers of its kind, and achieves competitive tracking performance on a large number of image sequences. Extensive experiments demonstrate the effectiveness and superior performance of the proposed approach.

cs.CV↗

arXiv2016

On the Relationship of UC H II Regions and Class II Methanol Masers: I. Source Catalogs

We conducted VLA C-configuration observations to measure positions and luminosities of Galactic Class II 6.7 GHz methanol masers and their associated ultra-compact H II regions. The spectral resolution was 3.90625 kHz and the continuum sensitivity reached 45 \uJypb. We mapped 372 methanol masers with peak flux densities of more than 2 Jy selected from the literature, 367 of them were detected. Absolute positions have nominal uncertainties of 0.3 arcsec. In this first paper on the data analysis, we present three catalogs, the first gives information on the strongest feature of 367 methanol maser sources, and the second on all detected maser spots. The third catalog present derived data of the 279 radio continuum sources found in the vicinity of maser sources. Among them, 140 show evidence of physical association with maser sources. Our catalogs list properties including distance, flux density, radial velocity and the distribution of masers on the Galactic plane is then provided as well. We found no significant relationship between luminosities of masers and their associated radio continuum counterparts.

astro-ph.GA↗

arXiv2015

The East-Asian VLBI Network

The East-Asian VLBI Network (EAVN) is the international VLBI facility in East Asia and is conducted in collaboration with China, Japan, and Korea. The EAVN consists of VLBI arrays operated in each East Asian country, containing 21 radio telescopes and three correlators. The EAVN will be mainly operated at 6.7 (C-band), 8 (X-band), 22 (K-band), and 43 GHz (Q-band), although the EAVN has an ability to conduct observations at 1.6 - 129 GHz. We have conducted fringe test observations eight times to date at 8 and 22 GHz and fringes have been successfully detected at both frequencies. We have also conducted science commissioning observations of 6.7 GHz methanol masers in massive star-forming regions. The EAVN will be operational from the second half of 2017, providing complementary results with the FAST on AGNs, massive star-forming regions, and evolved stars with high angular resolution at cm- to mm-wavelengths.

astro-ph.IM↗

arXiv2013

Herschel/HIFI observations of a new interstellar water maser: the 5(32)-4(41) transition at 620.701 GHz

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have performed mapping observations of the 620.701 GHz 5(32)-4(41) transition of ortho-H2O within a roughly 1.5 x 1.5 arcmin region encompassing the Kleinmann-Low nebula in Orion, and pointed observations of that transition toward the Orion South condensation and the W49N region of high-mass star formation. Using the Effelsberg 100 m radio telescope, we obtained ancillary observations of the 22.23508 GHz 6(16)-5(23) water maser transition; in the case of Orion-KL, the 621 GHz and 22 GHz observations were carried out within 10 days of each other. The 621 GHz water line emission shows clear evidence for strong maser amplication in all three sources, exhibiting narrow (roughly 1 km/s FWHM) emission features that are coincident (kinematically and/or spatially) with observed 22 GHz features. Moreover, in the case of W49N - for which observations were available at three epochs spanning a two year period - the spectra exhibited variability. The observed 621 GHz/22 GHz line ratios are consistent with a maser pumping model in which the population inversions arise from the combined effects of collisional excitation and spontaneous radiative decay, and the inferred physical conditions can plausibly arise in gas heated by either dissociative or non-dissociative shocks. The collisional excitation model also predicts that the 22 GHz population inversion will be quenched at higher densities than that of the 621 GHz transition, providing a natural explanation for the observational fact that 22 GHz maser emission appears to be a necessary but insufficient condition for 621 GHz maser emission.

astro-ph.GA↗

arXiv2011

The Bar and Spiral Structure Legacy (BeSSeL) Survey: Mapping the Milky Way with VLBI Astrometry

Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our Galaxy and to determine fundamental parameters such as the rotation velocity ($Θ_0$) and curve and the distance to the Galactic center (R$_0$). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R$_0$ and $Θ_0$ need a substantial revision. In particular the combination of 8.5 kpc and 220 \kms\, can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr\,A* gives best values of R$_0$ = 8.3 $\pm$ 0.23 kpc and $Θ_0$ = 239 or 246 $\pm$ 7 \kms, for Solar motions of V$_ \odot$ = 12.23 and 5.25 \kms, respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) Survey.

astro-ph.GA↗