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Yong Zhang

Publications and source records attributed to Yong Zhang.

At least 631 records · Page 35Linked to original sources
arXiv2017

The LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in the Southern Galactic Cap I. The Spectroscopic Redshift Catalog

We present a spectroscopic redshift catalog from the LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in the Southern Galactic Cap (SGC), which is designed to observe all sources (Galactic and extra-galactic) by using repeating observations with a limiting magnitude of $r=18.1~mag$ in two $20~deg^2$ fields. The project is mainly focusing on the completeness of LAMOST ExtraGAlactic Surveys (LEGAS) in the SGC, the deficiencies of source selection methods and the basic performance parameters of LAMOST telescope. In both fields, more than 95% of galaxies have been observed. A post-processing has been applied to LAMOST 1D spectrum to remove the majority of remaining sky background residuals. More than 10,000 spectra have been visually inspected to measure the redshift by using combinations of different emission/absorption features with uncertainty of $σ_{z}/(1+z)<0.001$. In total, there are 1528 redshifts (623 absorption and 905 emission line galaxies) in Field A and 1570 redshifts (569 absorption and 1001 emission line galaxies) in Field B have been measured. The results show that it is possible to derive redshift from low SNR galaxies with our post-processing and visual inspection. Our analysis also indicates that up to 1/4 of the input targets for a typical extra-galactic spectroscopic survey might be unreliable. The multi-wavelength data analysis shows that the majority of mid-infrared-detected absorption (91.3%) and emission line galaxies (93.3%) can be well separated by an empirical criterion of $W2-W3=2.4$. Meanwhile, a fainter sequence paralleled to the main population of galaxies has been witnessed both in $M_r$/$W2-W3$ and $M_*$/$W2-W3$ diagrams, which could be the population of luminous dwarf galaxies but contaminated by the edge-on/highly inclined galaxies ($\sim30\%$).

astro-ph.GA↗
arXiv2017

On the growth rate of periodic orbits for vector fields

We establish the relationship between the growth rate of periodic orbits and the topological entropy for $C^1$ generic vector fields: this extends a classical result of Katok for $C^{1+α}(α>0)$ surface diffeomorphisms to $C^1$ generic vector fields of any dimension. The main difficulty comes from the existence of singularities and the shear of the flow.

math.DS↗
arXiv2017

Mapping the Milky Way with LAMOST II: the stellar halo

The radial number density and flattening of the Milky Way's stellar halo is measured with $\mathrm{5351}$ metal-poor ([Fe/H]$<-1$) K giants from LAMOST DR3, using a nonparametric method which is model independent and largely avoids the influence of halo substucture. The number density profile is well described by a single power law with index $5.03^{+0.64}_{-0.64}$, and flattening that varies with radius. The stellar halo traced by LAMOST K giants is more flattened at smaller radii, and becomes nearly spherical at larger radii. The flattening, $q$, is about 0.64, 0.8, 0.96 at $r=15$, 20 and 30 kpc (where $r=\sqrt{R^2+\left[Z/q\left(r\right)\right]^2}$), respectively. Moreover, the leading arm of the Sagittarius dwarf galaxy tidal stream in the north, and the trailing arm in the south, are significant in the residual map of density distribution. In addition, an unknown overdensity is identified in the residual map at (R,Z)=(30,15) kpc.

astro-ph.GA↗
arXiv2017

On the Origin of the 3.3 Micron Unidentified Infrared Emission Feature

The 3.3 $μ$m unidentified infrared emission feature is commonly attributed to C-H stretching band of aromatic molecules. Astronomical observations have shown that this feature is composed of two separate bands at 3.28 and 3.30 $μ$m and the origin of these two bands is unclear. In this paper, we perform vibrational analyses based on quantum mechanical calculations of 153 organic molecules, including both pure aromatic molecules and molecules with mixed aromatic/olefinic/aliphatic hydridizations. We find that many of the C-H stretching vibrational modes in polycyclic aromatic hydrocarbon (PAH) molecules are coupled. Even considering the un-coupled modes only, the correlation between the band intensity ratios and the structure of the PAH molecule is not observed and the 3.28 and 3.30 $μ$m features cannot be directly interpreted in the PAH model. Based on these results, the possible aromatic, olefinic and aliphatic origins of the 3.3 $μ$m feature are discussed. We suggest that the 3.28 $μ$m feature is assigned to aromatic C-H stretch whereas the 3.30 $μ$m feature is olefinic. From the ratio of these two features, the relative olefinic to aromatic content of the carrier can be determined.

astro-ph.GA↗
arXiv2017

Heron triangle and rhombus pairs with a common area and a common perimeter

By Fermat's method, we show that there are infinitely many Heron triangle and $θ$-integral rhombus pairs with a common area and a common perimeter. Moreover, we prove that there does not exist any integral isosceles triangle and $θ$-integral rhombus pairs with a common area and a common perimeter.

math.NT↗
arXiv2017

Microscopic structure differences in CZTSe quaternary alloys prepared by different techniques revealed by spatially-resolved laser-induced-modification Raman spectroscopy

While producing comparable efficiencies and showing similar properties when probed by conventional techniques, such as Raman, photoluminescence and X-ray diffraction, two thin film solar cell materials with complex structures, such as quaternary compound CZTSe, may in fact differ significantly in their microscopic structures. In this work, laser induced modification Raman spectroscopy, coupled with high spatial resolution and high temperature capability, is demonstrated as an effective tool to obtain important structure information beyond that the conventional characterization techniques can offer, and thus to reveal the microscopic scale variations between nominally similar alloys. Specifically, CZTSe films prepared by sputtering and co-evaporation methods that exhibited similar Raman and XRD features were found to behave very differently under high laser power and high temperature Raman probe, because the differences in their microscopic structures lead to different structure modifications in response to the external stimuli, such as light illumination and temperature. They were also shown to undergo different degree of plastic changes and have different thermal conductivities as revealed by spatially-resolved Raman spectroscopy.

cond-mat.mtrl-sci↗
arXiv2017

Squeezed back-to-back correlations of $ϕϕ$ in Au+Au and d+Au collisions at the Relativistic Heavy Ion Collider energies

We investigate the squeezed back-to-back correlations (BBC) of $ϕϕ$, caused by the mass modification of the particles in the source medium, in the heavy-ion collisions of Au+Au and d+Au at the Relativistic Heavy Ion Collider (RHIC) energies. The BBC functions are calculated using the modified masses extracted from experimental data and the source space-time distributions provided by the viscous hydrodynamic code VISH2+1. Our investigations indicate that the BBC of $ϕϕ$ may perhaps be observed in the collisions of d+Au and the peripheral collisions of Au+Au at the RHIC. We suggest to measure the BBC experimentally for understanding the mass modifications of the $ϕ$ meson in the collisions.

nucl-th↗
arXiv2017

Arithmetic properties of polynomials

In this paper, first, we prove that the Diophantine system \[f(z)=f(x)+f(y)=f(u)-f(v)=f(p)f(q)\] has infinitely many integer solutions for $f(X)=X(X+a)$ with nonzero integers $a\equiv 0,1,4\pmod{5}$. Second, we show that the above Diophantine system has an integer parametric solution for $f(X)=X(X+a)$ with nonzero integers $a$, if there are integers $m,n,k$ such that \[\begin{cases} \begin{split} (n^2-m^2) (4mnk(k+a+1) + a(m^2+2mn-n^2)) &\equiv0\pmod{(m^2+n^2)^2},\\ (m^2+2mn-n^2) ((m^2-2mn-n^2)k(k+a+1) - 2amn) &\equiv0 \pmod{(m^2+n^2)^2}, \end{split} \end{cases}\] where $k\equiv0\pmod{4}$ when $a$ is even, and $k\equiv2\pmod{4}$ when $a$ is odd. Third, we get that the Diophantine system \[f(z)=f(x)+f(y)=f(u)-f(v)=f(p)f(q)=\frac{f(r)}{f(s)}\] has a five-parameter rational solution for $f(X)=X(X+a)$ with nonzero rational number $a$ and infinitely many nontrivial rational parametric solutions for $f(X)=X(X+a)(X+b)$ with nonzero integers $a,b$ and $a\neq b$. At last, we raise some related questions.

math.NT↗
arXiv2017

Mapping the Milky Way with LAMOST I: Method and overview

We present a statistical method to derive the stellar density profiles of the Milky Way from spectroscopic survey data, taking into account selection effects. We assume that the selection function of the spectroscopic survey is based on photometric colors and magnitudes and possibly altered during observations and data reductions. Then the underlying selection function for a line-of-sight can be well recovered by comparing the distribution of the spectroscopic stars in a color-magnitude plane with that of the photometric dataset. Subsequently, the stellar density profile along a line-of-sight can be derived from the spectroscopically measured stellar density profile multiplied by the selection function. The method is validated using Galaxia mock data with two different selection functions. We demonstrate that the derived stellar density profiles well reconstruct the true ones not only for the full targets, but also for the sub-populations selected from the full dataset. Finally, the method is applied to map the density profiles for the Galactic disk and halo, respectively, using the LAMOST RGB stars. The Galactic disk extends to about R=19 kpc, where the disk still contributes about 10% to the total stellar surface density. Beyond this radius, the disk smoothly transitions to the halo without any truncation, bending, or broken. Moreover, no over-density corresponding to the Monoceros ring is found in the Galactic anti-center direction. The disk shows moderate north-south asymmetry at radii larger than 12 kpc. On the other hand, the R-Z tomographic map directly shows that the stellar halo is substantially oblate within a Galactocentric radius of 20 kpc and gradually becomes nearly spherical beyond 30 kpc.

astro-ph.GA↗
arXiv2017

Search for Hydrogenated C$_{60}$ (fulleranes) in Circumstellar Envelopes

The recent detection of fullerene (C$_{60}$) in space and the positive assignment of five diffuse interstellar bands to C$_{60}^+$ reinforce the notion that fullerene-related compounds can be efficiently formed in circumstellar envelopes and be present in significant quantities in the interstellar medium. Experimental studies have shown that C$_{60}$ can be readily hydrogenated, raising the possibility that hydrogenated fullerenes (or fulleranes, C$_{60}$H$_m$, $m=1-60$) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C$_{60}$H$_m$. Our results show that the four mid-infrared bands from the C$_{60}$ skeletal vibrations remain prominent in slightly hydrogenated C$_{60}$, but their strengths diminish in different degrees with increasing hydrogenation. It is therefore possible that the observed infrared bands assigned to C$_{60}$ could be due to a mixture of fullerenes and fulleranes. This provides a potential explanation for the observed scatter of the C$_{60}$ band ratios. Our calculations suggest that a feature around 15$μ$m due to the breathing mode of heavily hydrogenated C$_{60}$ may be detectable astronomically. A preliminary search for this feature in 35 C$_{60}$ sources is reported.

astro-ph.SR↗
arXiv2017

Candidate members of the Pal 5, GD-1, Cetus Polar, and Orphan tidal stellar halo streams from Sdss DR9, Lamost DR3 and APOGEE catalogues

We present candidate members of the Pal 5, GD-1, Cetus Polar, and Orphan tidal stellar streams found in LAMOST DR3, SDSS DR9 and APOGEE catalogs. In LAMOST DR3, we find 20, 4, 24 high confidence candidates of tidal streams GD-1, Cetus Polar and Orphan respectively. We also list from the SDSS DR9 spectroscopic catalog 59, 118, 10 high confidence candidates of tidal streams Cetus Polar, Orphan and Pal 5, respectively. Furthermore, we find 7 high confidence candidates of the Pal 5 tidal stream in APOGEE data. Compared with SDSS, the new candidates from LAMOST DR3 are brighter, so that together, more of the color-magnitude diagram, including the giant branch can be explored. Analysis of SDSS data shows that there are 3 metallicity peaks of the Orphan stream and also shows some spatial separation. LAMOST data confirms multiple metallicities in this stream. The metallicity, given by the higher resolution APOGEE instrument, of the Pal 5 tidal stream is [Fe/H] $\sim -1.2$, higher than that given earlier by SDSS spectra. Many previously unidentified stream members are tabulated here for the first time, along with existing members, allowing future researchers to further constrain the orbits of these objects as they move within the Galaxy's dark matter potential.

astro-ph.GA↗
arXiv2017

Non-weakly amenable Beurling algebras

Weak amenability of a weighted group algebra, or a Beurling algebra, is a long-standing open problem. The commutative case has been extensively investigated and fully characterized. We study the non-commutative case. Given a weight function $ω$ on a locally compact group $G$, we characterize derivations from $L^1(G,ω)$ into its dual in terms of certain functions. Then we show that for a locally compact IN group $G$, if there is a non-zero continuous group homomorphism $φ$: $G\to \mathbb{C}$ such that $φ(x)/ω(x)ω(x^{-1})$ is bounded on $G$, then $L^1(G,ω)$ is not weakly amenable. Some useful criteria that rule out weak amenability of $L^1(G,ω)$ are established. Using them we show that for many polynomial type weights the weighted Heisenberg group algebra is not weakly amenable, neither is the weighted $\boldsymbol{ax+b}$ group algebra. We further study weighted quotient group algebra $L^1(G/H,\hatω)$, where $\hatω$ is the canonical weight on $G/H$ induced by $ω$. We reveal that the kernel of the canonical homomorphism from $L^1(G,ω)$ to $L^1(G/H,\hatω)$ is complemented. This allows us to obtain some sufficient conditions under which $L^1(G/H,\hatω)$ inherits weak amenability of $L^1(G,ω)$. We study further weak amenability of Beurling algebras of subgroups. In general, weak amenability of a Beurling algebra does not pass to the Beurling algebra of a subgroup. However, in some circumstances this inheritance can happen. We also give an example to show that weak amenability of both $L^1(H,ω|_H)$ and $L^1(G/H,\hatω)$ does not ensure weak amenability of $L^1(G,ω)$.

math.FA↗
arXiv2017

Cavity mediated effective interaction between phonon and magnon

Optomagnonics and optomechanics have various applications ranging from tunable light sources to optical manipulation for quantum information science. Here, we propose a hybrid system with the interaction between phonon and magnon which could be tuned by the electromagnetic field based on the radiation pressure and magneto-optical effect. The self energy of the magnon and phonon induced by the electromagnetic field and the influence of the thermal noise are studied. Moreover, the topological features of the hybrid system is illustrated with resort to the dynamical encircling with the exceptional points, and the chiral characteristics under these encirclements are found.

quant-ph↗
arXiv2017

Central Beurling algebras: Weak amenability of the central Beurling algebras on [FC]$^-$ groups

We study weak amenability of central Beurling algebras $ZL^1(G,ω)$. The investigation is a natural extension of the known work on the commutative Beurling algebra $L^1(G,ω)$. For [FC]$^-$ groups we establish a necessary condition and for [FD]$^-$ groups we give sufficient conditions for the weak amenability of $Z\L1o$. For a compactly generated [FC]$^-$ group with the polynomial weight $ω_α(x) = (1 + |x|)^α$, we prove that $ZL^1(G,ω_α)$ is weakly amenable if and only if $α< 1/2$.

math.FA↗
arXiv2017

New Formulas for Amplitudes from Higher-Dimensional Operators

In this paper we study tree-level amplitudes from higher-dimensional operators, including $F^3$ operator of gauge theory, and $R^2$, $R^3$ operators of gravity, in the Cachazo-He-Yuan formulation. As a generalization of the reduced Pfaffian in Yang-Mills theory, we find a new, gauge-invariant object that leads to gluon amplitudes with a single insertion of $F^3$, and gravity amplitudes by Kawai-Lewellen-Tye relations. When reduced to four dimensions for given helicities, the new object vanishes for any solution of scattering equations on which the reduced Pfaffian is non-vanishing. This intriguing behavior in four dimensions explains the vanishing of graviton helicity amplitudes produced by the Gauss-Bonnet $R^2$ term, and provides a scattering-equation origin of the decomposition into self-dual and anti-self-dual parts for $F^3$ and $R^3$ amplitudes.

hep-th↗
arXiv2017

Affiliation networks with an increasing degree sequence

Affiliation network is one kind of two-mode social network with two different sets of nodes (namely, a set of actors and a set of social events) and edges representing the affiliation of the actors with the social events. Although a number of statistical models are proposed to analyze affiliation networks, the asymptotic behaviors of the estimator are still unknown or have not been properly explored. In this paper, we study an affiliation model with the degree sequence as the exclusively natural sufficient statistic in the exponential family distributions. We establish the uniform consistency and asymptotic normality of the maximum likelihood estimator when the numbers of actors and events both go to infinity. Simulation studies and a real data example demonstrate our theoretical results.

stat.ME↗