arXiv subjects

Ye Xu

Publications and source records attributed to Ye Xu.

At least 109 records · Page 6Linked to original sources

arXiv2016

An Outer Arm in the Second Galactic Quadrant: Structure

The lack of arm tracers, especially the remote tracers, is one of the most difficult problems preventing us from studying the structure of the Milky Way. Fortunately, with its high-sensitivity CO survey, the Milky Way Imaging Scroll Painting (MWISP) project offers such an opportunity. Since completing about one-third of its mission, an area of l=[100,150] deg, b=[-3,5] deg has nearly been covered. The Outer arm of the Milky Way first clearly revealed its shape in the second galactic quadrant in the form of molecular gas --- this is the first time that the Outer arm has been reported in such a large-scale mapping of molecular gas. Using the 115 GHz 12CO(1-0) data of MWISP at the LSR velocity ~= [-100,-60] km s^-1 and in the area mentioned above, we have detected 481 molecular clouds in total, and among them 332 (about 69\%) are newly detected and 457 probably belong to the Outer arm. The total mass of the detected Outer arm clouds is ~ 3.1*10^6 M_sun. Assuming that the spiral arm is a logarithmic spiral, the pitch angle is fitted as ~ 13.1 deg. Besides combining both the CO data from MWISP and the 21 cm HI data from the Canadian Galactic Plane Survey (CGPS), the gas distribution, warp, and thickness of the Outer arm are also studied.

astro-ph.GA↗

arXiv2016

Search by Ideal Candidates: Next Generation of Talent Search at LinkedIn

One key challenge in talent search is how to translate complex criteria of a hiring position into a search query. This typically requires deep knowledge on which skills are typically needed for the position, what are their alternatives, which companies are likely to have such candidates, etc. However, listing examples of suitable candidates for a given position is a relatively easy job. Therefore, in order to help searchers overcome this challenge, we design a next generation of talent search paradigm at LinkedIn: Search by Ideal Candidates. This new system only needs the searcher to input one or several examples of suitable candidates for the position. The system will generate a query based on the input candidates and then retrieve and rank results based on the query as well as the input candidates. The query is also shown to the searcher to make the system transparent and to allow the searcher to interact with it. As the searcher modifies the initial query and makes it deviate from the ideal candidates, the search ranking function dynamically adjusts an refreshes the ranking results balancing between the roles of query and ideal candidates. As of writing this paper, the new system is being launched to our customers.

cs.IR↗

arXiv2015

Planck cold clumps in the $λ$ Orionis complex: I. Discovery of an extremely young Class 0 protostellar object and a proto-brown dwarf candidate in a bright rimmed clump PGCC G192.32-11.88

We are performing a series of observations with ground-based telescopes toward Planck Galactic cold clumps (PGCCs) in the $λ$ Orionis complex in order to systematically investigate the effects of stellar feedback. In the particular case of PGCC G192.32-11.88, we discovered an extremely young Class 0 protostellar object (G192N) and a proto-brown dwarf candidate (G192S). G192N and G192S are located in a gravitationally bound bright-rimmed clump. The velocity and temperature gradients seen in line emission of CO isotopologues indicate that PGCC G192.32-11.88 is externally heated and compressed. G192N probably has the lowest bolometric luminosity ($\sim0.8$ L$_{\sun}$) and accretion rate (6.3$\times10^{-7}$ M$_{\sun}$~yr$^{-1}$) when compared with other young Class 0 sources (e.g. PACS Bright Red sources (PBRs)) in the Orion complex. It has slightly larger internal luminosity ($0.21\pm0.01$ L$_{\sun}$) and outflow velocity ($\sim$14 km~s$^{-1}$) than the predictions of first hydrostatic cores (FHSCs). G192N might be among the youngest Class 0 sources, which are slightly more evolved than a FHSC. Considering its low internal luminosity ($0.08\pm0.01$ L$_{\odot}$) and accretion rate (2.8$\times10^{-8}$ M$_{\sun}$~yr$^{-1}$), G192S is an ideal proto-brown dwarf candidate. The star formation efficiency ($\sim$0.3\%-0.4\%) and core formation efficiency ($\sim$1\%) in PGCC G192.32-11.88 are significantly smaller than in other giant molecular clouds or filaments, indicating that the star formation therein is greatly suppressed due to stellar feedback.

astro-ph.GA↗

arXiv2015

Evaluation of the neutron background in cryogenic Germanium target for WIMP direct detection when using a reactor neutrino detector as a neutron veto system

A direct WIMP (Weakly Interacting Massive Particle) detector with a neutron veto system is designed to better reject neutrons. An experimental configuration is studied in the present paper: 984 Ge modules are placed inside a reactor neutrino detector. In order to discriminate between nuclear and electron recoil, both ionization and heat signatures are measured using cryogenic germanium detectors in this detection. The neutrino detector is used as a neutron veto device.The neutron background for the experimental design has been estimated using the Geant4 simulation. The results show that the neutron background can decrease to O(0.01) events per year per tonne of high purity Germanium. We calculate the sensitivity to spin-independent WIMP-nucleon elastic scattering. An exposure of one tonne $\times$ year could reach a cross-section of about 2$\times$$10^{-11}$ pb.

astro-ph.IM↗

arXiv2015

Strain fluctuations and elastic moduli in disordered solids

Recently there has been a surge in interest in using video-microscopy techniques to infer the local mechanical properties of disordered solids. One common approach is to minimize the difference between particle vibrational displacements in a local coarse-graining volume and the displacements that would result from a best-fit affine deformation. Effective moduli are then be inferred under the assumption that the components of this best-fit affine deformation tensor have a Boltzmann distribution. In this paper, we combine theoretical arguments with experimental and simulation data to demonstrate that the above does not reveal information about the true elastic moduli of jammed packings and colloidal glasses.

cond-mat.soft↗

arXiv2015

CO Core Candidates in the Gemini Molecular Cloud

We present observations of a 4 squared degree area toward the Gemini cloud obtained using J = 1-0 transitions of $^{12}$CO, $^{13}$CO and C$^{18}$O. No C$^{18}$O emission was detected. This region is composed of 36 core candidates of $^{13}$CO. These core candidates have a characteristic diameter of 0.25 pc, excitation temperatures of 7.9 K, line width of 0.54 km s$^{-1}$ and a mean mass of 1.4 M$_{\sun}$. They are likely to be starless core candidates, or transient structures, which probably disperse after $\sim$10$^6$ yr.

astro-ph.GA↗

arXiv2014

A Possible Extension of the Scutum-Centaurus Arm into the Outer Second Quadrant

Combining HI data from the Canadian Galactic Plane Survey~(CGPS) and CO data from the Milky Way Imaging Scroll Painting (MWISP) project, we have identified a new segment of a spiral arm between Galactocentric radii of 15 and 19~kpc that apparently lies beyond the Outer Arm in the second Galactic quadrant. Over most of its length, the arm is 400-600 pc thick in $z$. The new arm appears to be the extension of the distant arm recently discovered by Dame & Thaddeus (2011) as well as the Scutum-Centaurus Arm into the outer second quadrant. Our current survey identified a total of 72 molecular clouds with masses on the order of 10$^2$-10$^4$M$_{\sun}$ that probably lie in the new arm. When all of the available data from the CO molecular clouds are fit, the best$-$fitting spiral model gives a pitch angle of 9.3$^{\circ}$ $\pm$0.7$^{\circ}$.

astro-ph.GA↗

arXiv2014

CO observations of a sample towards nearby galaxies

We have simultaneously observed $^{12}$CO, $^{13}$CO, and C$^{18}$O (J=1$-$0) rotational transitions in the centers of a sample of 58 nearby spiral galaxies using the 13.7-m millimeter-wave telescope of the Purple Mountain Observatory. Forty-two galaxies were detected in $^{13}$CO emission, but there was a null detection for C$^{18}$O emission with a sigma upper limit of 2 mK. Central beam ratios, \R, of $^{12}$CO and $^{13}$CO range mostly from 5 to 13, with an average value of 8.1$\pm$4.2, which is slightly lower than previous estimates for normal galaxies. Clear correlations are found between $^{12}$CO and $^{13}$CO luminosities. An average $X$-factor of $1.44\pm0.84\times10^{20}\ \mathrm{cm}^{-2} (\mathrm{K} \mathrm{km} \mathrm{s}^{-1})^{-1}$ is slightly lower than that in the Milky Way.

astro-ph.GA↗

arXiv2014

Diagnosing hyperuniformity in two-dimensional disordered jammed-packings of soft spheres

Hyperuniformity characterizes a state of matter for which density fluctuations diminish towards zero at the largest length scales. However, the task of determining whether or not an experimental system is hyperuniform is experimentally challenging due to finite-resolution, noise and sample-size effects that influence characterization measurements. Here we explore these issues, employing video optical microscopy to study hyperuniformity phenomena in disordered two-dimensional jammed packings of soft spheres. Using a combination of experiment and simulation we characterize the detrimental effects of particle polydispersity, image noise, and finite-size effects on the assignment of hyperuniformity, and we develop a methodology that permits improved diagnosis of hyperuniformity from real-space measurements. The key to this improvement is a simple packing reconstruction algorithm that incorporates particle polydispersity to minimize free volume. In addition, simulations show that hyperuniformity can be ascertained more accurately in direct space than in reciprocal space as a result of finite sample-size. Finally, experimental colloidal packings of soft polymeric spheres are shown to be hyperuniform.

cond-mat.soft↗

arXiv2014

Evaluation of the neutron background in CsI target for WIMP direct detection when using a reactor neutrino detector as a neutron veto system

A direct WIMP (Weakly Interacting Massive Particle) detector with a neutron veto system is designed to better reject neutrons. An experimental configuration is studied in the present paper: a WIMP detectors with CsI(Na) target is placed inside a reactor neutrino detector. The neutrino detector is used as a neutron veto device. The neutron background for the experimental design has been estimated using the Geant4 simulation. The results show that the neutron background can decrease to O(0.01) events per year per tonne of CsI(Na). We calculate the sensitivity to spin-independent WIMP-nucleon elastic scattering. An exposure of one tonne $\times$ year could reach a cross-section of about 3$\times$$10^{-11}$ pb.

astro-ph.IM↗

arXiv2013

Molecular Clouds in the North American and Pelican Nebulae: Structures

We present observations of 4.25 square degree area toward the North American and Pelican Nebulae in the $J = 1-0$ transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O. Three molecules show different emission area with their own distinct structures. These different density tracers reveal several dense clouds with surface density over 500 $M_\odot$ pc$^{-2}$ and a mean H$_2$ column density of 5.8, 3.4, and 11.9$\times10^{21}$ cm$^{-2}$ for $^{12}$CO, $^{13}$CO, and C$^{18}$O, respectively. We obtain a total mass of $5.4\times10^4 M_\odot$ ($^{12}$CO), $2.0\times10^4 M_\odot$ ($^{13}$CO), and $6.1\times10^3 M_\odot$ (C$^{18}$O) in the complex. The distribution of excitation temperature shows two phase of gas: cold gas ($\sim$10 K) spreads across the whole cloud; warm gas ($>$20 K) outlines the edge of cloud heated by the W80 H II region. The kinetic structure of the cloud indicates an expanding shell surrounding the ionized gas produced by the H II region. There are six discernible regions in the cloud including the Gulf of Mexico, Caribbean Islands and Sea, Pelican's Beak, Hat, and Neck. The areas of $^{13}$CO emission range within 2-10 pc$^2$ with mass of (1-5)$\times10^3 M_\odot$ and line width of a few km s$^{-1}$. The different line properties and signs of star forming activity indicate they are in different evolutionary stages. Four filamentary structures with complicated velocity features are detected along the dark lane in LDN 935. Furthermore, a total of 611 molecular clumps within the $^{13}$CO tracing cloud are identified using the ClumpFind algorithm. The properties of the clumps suggest most of the clumps are gravitationally bound and at an early stage of evolution with cold and dense molecular gas.

astro-ph.SR↗

arXiv2012

A search for 95 GHz class I methanol masers in molecular outflows

We have observed a sample of 288 molecular outflow sources including 123 high-mass and 165 low-mass sources to search for class I methanol masers at 95 GHz transition and to investigate relationship between outflow characteristics and class I methanol maser emission with the PMO-13.7m radio telescope. Our survey detected 62 sources with 95 GHz methanol masers above 3$σ$ detection limit, which include 47 high-mass sources and 15 low-mass sources. Therefore the detection rate is 38% for high-mass outflow sources and 9% for low-mass outflow sources, suggesting that class I methanol maser is relatively easily excited in high-mass sources. There are 37 newly detected 95 GHz methanol masers (including 27 high-mass and 10 low-mass sources), 19 of which are newly identified (i.e. first identification) class I methanol masers (including 13 high-mass and 6 low-mass sources). Statistical analysis for the distributions of maser detections with the outflow parameters reveals that the maser detection efficiency increases with outflow properties (e.g. mass, momentum, kinetic energy and mechanical luminosity of outflows etc.). Systematic investigations of relationships between the intrinsic luminosity of methanol maser and the outflow properties (including mass, momentum, kinetic energy, bolometric luminosity and mass loss rate of central stellar sources) indicate a positive correlations. This further supports that class I methanol masers are collisionally pumped and associated with shocks, where outflows interact with the surrounding ambient medium.

astro-ph.SR↗

arXiv2012

A Double-Jet System in the G31.41+0.31 Hot Molecular Core

This work presents a detailed study of the gas kinematics towards the "Hot Molecular Core" (HMC) G31.41+0.31 via multi-epoch VLBI observations of the H2O 22 GHz and CH3OH 6.7 GHz masers, and single-epoch VLBI of the OH 1.6 GHz masers. Water masers present a symmetric spatial distribution with respect to the HMC center, where two nearby (0.2" apart), compact, VLA sources (labeled "A" and "B") are previously detected. The spatial distribution of a first group of water masers, named "J1", is well fit with an elliptical profile, and the maser proper motions mainly diverge from the ellipse center, with average speed of 36 km s-1. These findings strongly suggest that the "J1" water maser group traces the heads of a young (dynamical time of 1.3 10^3 yr), powerful (momentum rate of ~0.2 M_sun yr-1 km s-1), collimated (semi-opening angle ~10 deg) jet emerging from a MYSO located close (within 0.15") to the VLA source "B". Most of the water features not belonging to "J1" present an elongated (about 2" in size), NE--SW oriented (PA = 70 deg), S-shape distribution, which we denote with the label "J2". The elongated distribution of the "J2" group and the direction of motion, approximately parallel to the direction of elongation, of most "J2" water masers suggests the presence of another collimated outflow, emitted from a MYSO near the VLA source "A". The orientation of the "J2" jet agrees well with that (PA = 68 deg) of the well-defined V_LSR gradient across the HMC revealed by previous interferometric, thermal line observations. Furthermore, the "J2" jet is powerful enough to sustain the large momentum rate, 0.3 M_sun yr-1 km s-1, estimated assuming that the V_LSR gradient represents a collimated outflow. These two facts lead us to favour the interpretation of the V_LSR gradient across the G31.41+0.31 HMC in terms of a compact and collimated outflow.

astro-ph.SR↗

arXiv2012

Scaling of Traction Forces with Size of Cohesive Cell Colonies

To understand how the mechanical properties of tissues emerge from interactions of multiple cells, we measure traction stresses of cohesive colonies of 1-27 cells adherent to soft substrates. We find that traction stresses are generally localized at the periphery of the colony and the total traction force scales with the colony radius. For large colony sizes, the scaling appears to approach linear, suggesting the emergence of an apparent surface tension of order 1E-3 N/m. A simple model of the cell colony as a contractile elastic medium coupled to the substrate captures the spatial distribution of traction forces and the scaling of traction forces with the colony size.

cond-mat.soft↗

arXiv2012

A 95 GHz Class I Methanol Maser Survey Toward A Sample of GLIMPSE Point Sources Associated with BGPS Clumps

We report a survey with the Purple Mountain Observatory (PMO) 13.7-m radio telescope for class I methanol masers from the 95 GHz (8_0 - 7_1 A^+) transition. The 214 target sources were selected by combining information from both the Spitzer GLIMPSE and 1.1 mm BGPS survey catalogs. The observed sources satisfy both the GLIMPSE mid-IR criteria of [3.6]-[4.5]>1.3, [3.6]-[5.8]>2.5, [3.6]-[8.0]>2.5 and 8.0 um magnitude less than 10, and also have an associated 1.1 mm BGPS source. Class I methanol maser emission was detected in 63 sources, corresponding to a detection rate of 29% for this survey. For the majority of detections (43), this is the first identification of a class I methanol maser associated with these sources. We show that the intensity of the class I methanol maser emission is not closely related to mid-IR intensity or the colors of the GLIMPSE point sources, however, it is closely correlated with properties (mass and beam-averaged column density) of the BGPS sources. Comparison of measures of star formation activity for the BGPS sources with and without class I methanol masers indicate that the sources with class I methanol masers usually have higher column density and larger flux density than those without them. Our results predict that the criteria log(S_{int})<-38.0+1.72log(N_{H_{2}}^{beam}) and log(N_{H_{2}}^{beam})>22.1, which utilizes both the integrated flux density (S_{int}) and beam-averaged column density (N_{H_{2}}^{beam}) of the BGPS sources, are very efficient for selecting sources likely to have an associated class I methanol maser. Our expectation is that searches using these criteria will detect 90% of the predicted number of class I methanol masers from the full BGPS catalog (~ 1000), and do so with a high detection efficiency (~75%).

astro-ph.GA↗

arXiv2011

Evaluation of the Neutron Background in a Direct WIMP Detector with Neutron Veto System Based on Gd-doped Liquid Scintillator

A direct WIMP (Weakly Interacting Massive Particle) detector with a neutron veto system is designed to better reject neutrons. Two experimental configurations are studied in the present paper: one is for an Xe detector enclosed in a Gd-loaded scintillator and the other one is for an Xe detector placed inside a reactor neutrino detector. The Gd-doped liquid scintillator (or the neutrino detector) is used as a neutron veto device. The neutron backgrounds for the two experimental designs have been estimated using Geant4 simulations. The results show that the neutron backgrounds can decrease to O(0.1) events per year per tonne of liquid Xenon. We calculate the sensitivities to spin-independent WIMP-nucleon elastic scattering. An exposure of one tonne $\times$ year could reach a cross-section of about 6$\times$$10^{-11}$ pb.

astro-ph.IM↗

arXiv2011

The Deformation of an Elastic Substrate by a Three-Phase Contact Line

Young's classic analysis of the equilibrium of a three-phase contact line ignores the out-of-plane component of the liquid-vapor surface tension. While it has long been appreciated that this unresolved force must be balanced by elastic deformation of the solid substrate, a definitive analysis has remained elusive because conventional idealizations of the substrate imply a divergence of stress at the contact line. While a number of theories of have been presented to cut off the divergence, none of them have provided reasonable agreement with experimental data. We measure surface and bulk deformation of a thin elastic film near a three-phase contact line using fluorescence confocal microscopy. The out-of-plane deformation is well fit by a linear elastic theory incorporating an out-of-plane restoring force due to the surface tension of the gel. This theory predicts that the deformation profile near the contact line is scale-free and independent of the substrate elastic modulus.

cond-mat.soft↗

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↗