arXiv subjects

Wei Zhu

Publications and source records attributed to Wei Zhu.

At least 289 records · Page 16Linked to original sources

arXiv2019

Spitzer Microlensing of MOA-2016-BLG-231L : A Counter-Rotating Brown Dwarf Binary in the Galactic Disk

We analyze the binary microlensing event MOA-2016-BLG-231, which was observed from the ground and from Spitzer. The lens is composed of very low-mass brown dwarfs (BDs) with $M_1 = 21^{+12}_{-5} \ M_J$ and $M_2 = 9^{+5}_{-2}\ M_J$, and it is located in the Galactic disk $D_{\rm L} = 2.85^{+0.88}_{-0.50}\ {\rm kpc}$. This is the fifth binary brown dwarf discovered by microlensing, and the BD binary is moving counter to the orbital motion of disk stars. Constraints on the lens physical properties come from late time, non-caustic-crossing features of the Spitzer light curve. Thus, MOA-2016-BLG-231 shows how Spitzer plays a crucial role in resolving the nature of BDs in binary BD events with short timescale ($\lesssim 10$ days).

astro-ph.SR↗

arXiv2019

Influence of Stellar Metallicity on Occurrence Rates of Planets and Planetary Systems

We study the influence of stellar metallicity on the fraction of stars with planets (i.e., the occurrence rate of planetary systems) and the average number of planets per star (i.e., the occurrence rate of planets). The former directly reveals the planet formation efficiency, whereas the latter reveals the final product of formation and evolution. We show that these two occurrence rates have different dependences on stellar metallicity. Specifically, the fraction of stars with planets rises gradually with metallicity, from $\sim$25% to $\sim$36% for 0.4 dex of [Fe/H] for all Kepler-like planets (period $P<400$ days and radius $R_{\rm p}\gtrsim R_\oplus$). The average number of planets per star reaches a plateau (or possibly starts declining) at [Fe/H]$\gtrsim0.1$. This is plausibly caused by the emergence of distant giant planets at high metallicities, given that the close-in small planets and the distant giants preferentially co-exist in the same system.

astro-ph.EP↗

arXiv2019

$Spitzer$ Microlensing Parallax for OGLE-2017-BLG-0896 Reveals a Counter-Rotating Low-Mass Brown Dwarf

The kinematics of isolated brown dwarfs in the Galaxy, beyond the solar neighborhood, is virtually unknown. Microlensing has the potential to probe this hidden population, as it can measure both the mass and five of the six phase-space coordinates (all except the radial velocity) even of a dark isolated lens. However, the measurements of both the microlens parallax and finite-source effects are needed in order to recover the full information. Here, we combine $Spitzer$ satellite parallax measurement with the ground-based light curve, which exhibits strong finite-source effects, of event OGLE-2017-BLG-0896. We find two degenerate solutions for the lens (due to the known satellite-parallax degeneracy), which are consistent with each other except for their proper motion. The lens is an isolated brown dwarf with a mass of either $18\pm1M_J$ or $20\pm1M_J$. This is the lowest isolated-object mass measurement to date, only $\sim$45\% more massive than the theoretical deuterium-fusion boundary at solar metallicity, which is the common definition of a free-floating planet. The brown dwarf is located at either $3.9\pm0.1$ kpc or $4.1\pm0.1$ kpc toward the Galactic bulge, but with proper motion in the opposite direction of disk stars, with one solution suggesting it is moving within the Galactic plane. While it is possibly a halo brown dwarf, it might also represent a different, unknown population.

astro-ph.SR↗

arXiv2019

Phase self-aligned continuous-variable measurement-device-independent quantum key distribution

Continuous-variable measurement-independent-device quantum key distribution (CV-MDI-QKD) can offer high secure key rate at metropolitan distance and remove all side channel loopholes of detection as well. However, there is no complete experimental demonstration of CV-MDI-QKD due to the remote distance phase-locking techniques challenge. Here, we present a new optical scheme to overcome this difficulty and also removes the requirement of two identical independent lasers. We anticipate that our new scheme can be used to demonstrate the in-field CV-MDI-QKD experiment and build the CV-MDI-QKD network with untrusted source.

quant-ph↗

arXiv2019

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two-dimensional materials and to combine their best in van der Waals heterostructures, are in great demand as being unique platform to discover new physical phenomena and to design novel functionalities in interface-based devices. Herein, PbI2 crystals as thin as few-layers are first synthesized, particularly through a facile low-temperature solution approach with the crystals of large size, regular shape, different thicknesses and high-yields. As a prototypical demonstration of flexible band engineering of PbI2-based interfacial semiconductors, these PbI2 crystals are subsequently assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS2 is strongly enhanced in MoS2/PbI2 stacks, while a dramatic photoluminescence quenching of WS2 and WSe2 is revealed in WS2/PbI2 and WSe2/PbI2 stacks. This is attributed to the effective heterojunction formation between PbI2 and these monolayers, but type I band alignment in MoS2/PbI2 stacks where fast-transferred charge carriers accumulate in MoS2 with high emission efficiency, and type II in WS2/PbI2 and WSe2/PbI2 stacks with separated electrons and holes suitable for light harvesting. Our results demonstrate that MoS2, WS2, WSe2 monolayers with very similar electronic structures themselves, show completely distinct light-matter interactions when interfacing with PbI2, providing unprecedent capabilities to engineer the device performance of two-dimensional heterostructures.

cond-mat.mes-hall↗

arXiv2018

KMT-2017-BLG-0165Lb: A Super-Neptune mass planet Orbiting a Sun-like Host Star

We report the discovery of a low mass-ratio planet $(q = 1.3\times10^{-4})$, i.e., 2.5 times higher than the Neptune/Sun ratio. The planetary system was discovered from the analysis of the KMT-2017-BLG-0165 microlensing event, which has an obvious short-term deviation from the underlying light curve produced by the host of the planet. Although the fit improvement with the microlens parallax effect is relatively low, one component of the parallax vector is strongly constrained from the light curve, making it possible to narrow down the uncertainties of the lens physical properties. A Bayesian analysis yields that the planet has a super-Neptune mass $(M_{2}=34_{-12}^{+15}~M_{\oplus})$ orbiting a Sun-like star $(M_{1}=0.76_{-0.27}^{+0.34}~M_{\odot})$ located at $4.5~{\rm kpc}$. The blended light is consistent with these host properties. The projected planet-host separation is $a_{\bot}={3.45_{-0.95}^{+0.98}}~{\rm AU}$, implying that the planet is located outside the snowline of the host, i.e., $a_{sl}\sim2.1~{\rm AU}$. KMT-2017-BLG-0165Lb is the sixteenth microlensing planet with mass ratio $q<3\times10^{-4}$. Using the fifteen of these planets with unambiguous mass-ratio measurements, we apply a likelihood analysis to investigate the form of the mass-ratio function in this regime. If we adopt a broken power law for the form of this function, then the break is at $q_{\rm br}\simeq0.55\times10^{-4}$, which is much lower than previously estimated. Moreover, the change of the power law slope, $ζ>3.3$ is quite severe. Alternatively, the distribution is also suggestive of a "pile-up" of planets at Neptune-like mass ratios, below which there is a dramatic drop in frequency.

astro-ph.EP↗

arXiv2018

A convergence framework for inexact nonconvex and nonsmooth algorithms and its applications to several iterations

In this paper, we consider the convergence of an abstract inexact nonconvex and nonsmooth algorithm. We promise a pseudo sufficient descent condition and a pseudo relative error condition, which are both related to an auxiliary sequence, for the algorithm; and a continuity condition is assumed to hold. In fact, a lot of classical inexact nonconvex and nonsmooth algorithms allow these three conditions. Under a special kind of summable assumption on the auxiliary sequence, we prove the sequence generated by the general algorithm converges to a critical point of the objective function if being assumed Kurdyka- Lojasiewicz property. The core of the proofs lies in building a new Lyapunov function, whose successive difference provides a bound for the successive difference of the points generated by the algorithm. And then, we apply our findings to several classical nonconvex iterative algorithms and derive the corresponding convergence results

math.OC↗

arXiv2018

Room-temperature Composite-pulses for Robust Diamond Magnetometry

The sensitivity of a practical quantum magnetometer is challenged by both inhomogeneous coupling between sensors and environment and errors in quantum control. Based on the physical criteria of modern quantum sensing, we present a robust and effective composite-pulse for high fidelity operation against inhomogeneous broadening of sensor and control errors via optimized modulation of control field. Such a technique is verified on nitrogen-vacancy (NV) center in diamond to keep almost perfect operation up to a detuning as larger as $110\%$. The sensitivity of the magnetometer with NV center ensemble is experimentally improved by a factor of $4$, comparing to dynamical decoupling with normal rectangular pulse. Our work marks an important step towards high trustworthy ultra-sensitive quantum sensing with imperfect quantum control in realistic systems. The used principle however, is universal and not restricted to NV center ensemble magnetometer.

quant-ph↗

arXiv2018

Kmt-2016-blg-1397b: Kmtnet-only discovery of a microlens giant planet

We report the discovery of a giant planet in the KMT-2016-BLG-1397 microlensing event, which was found by The Korea Microlensing Telescope Network (KMTNet) alone. The time scale of this event is t_E = 40.0 +- 0.5 days and the mass ratio between the lens star and its companion is q = 0.016 +- 0.002. The planetary perturbation in the light curve is a smooth bump, resulting in the classical binary-lens/binary-source (2L1S/1L2S) degeneracy. We measure the V - I color of the (putative) two sources in the 1L2S model, and then effectively rule out the binary source solution. The finite-source effect is marginally detected. Combined with the limits on the blend flux and the probability distribution of the source size normalized by the Einstein radius rho, a Bayesian analysis yields the lens mass M_L = 0.45+0.33-0.28 M_sun, at distance of D_L = 6.60+1.10-1.30 kpc. Thus the companion is a super-Jupiter of a mass m_p = 7.0+5.2-4.3 M_J , at a projected separation r = 5.1+1.5-1.7 AU, indicating that the planet is well beyond the snow line of the host star.

astro-ph.EP↗

arXiv2018

Looking for the gluon condensation signature in proton using the Earth limb gamma-ray spectra

A new type of gamma ray spectrum is predicted in a general hadronic framework if considering the gluon condensation effects in proton. The result presents the power-law with a sharp break in the gamma ray spectra at the TeV-band. We suggest to probe this GC-signature in the Earth limb gamma-ray spectra using the DArk Matter Particle Explorer and the CALorimetric Electron Telescope on orbit.

hep-ph↗

arXiv2018

Stop memorizing: A data-dependent regularization framework for intrinsic pattern learning

Deep neural networks (DNNs) typically have enough capacity to fit random data by brute force even when conventional data-dependent regularizations focusing on the geometry of the features are imposed. We find out that the reason for this is the inconsistency between the enforced geometry and the standard softmax cross entropy loss. To resolve this, we propose a new framework for data-dependent DNN regularization, the Geometrically-Regularized-Self-Validating neural Networks (GRSVNet). During training, the geometry enforced on one batch of features is simultaneously validated on a separate batch using a validation loss consistent with the geometry. We study a particular case of GRSVNet, the Orthogonal-Low-rank Embedding (OLE)-GRSVNet, which is capable of producing highly discriminative features residing in orthogonal low-rank subspaces. Numerical experiments show that OLE-GRSVNet outperforms DNNs with conventional regularization when trained on real data. More importantly, unlike conventional DNNs, OLE-GRSVNet refuses to memorize random data or random labels, suggesting it only learns intrinsic patterns by reducing the memorizing capacity of the baseline DNN.

cs.CV↗

arXiv2018

The Gluon condensation in high energy cosmic rays

The gluon condensation (GC)-effects in high energy cosmic rays are investigated. After a brief review of the GC, several examples including gamma-, electron-, and positron-spectra in a broad GeV$\sim $TeV region can be explained by the GC-effects. We find that the GC may break the power-law of the cosmic ray spectra if the energy of accelerated protons exceeds the GC-threshold. The GC is a new phenomenon that is not yet known, it provides a new window to understand the high energy cosmic ray spectra.

hep-ph↗

arXiv2018

Kepler-730b is probably a hot Jupiter with a small companion

Here we draw attention to a candidate system with one hot Jupiter and one small, nearby companion, revealed by the recent Kepler data release (DR25). The hot Jupiter, Kepler-730b, has radius $R_{\rm p}=11.36^{+1.14}_{-0.98}~R_\oplus$ and orbital period $P=6.492$ d, and the newly discovered companion, KOI-929.02, has $R_{\rm p}=1.45^{+0.15}_{-0.20}~R_\oplus$ and $P=2.852$ d. No transit timing variation (TTV) was detected because of the marginal detection of the small companion transit, but this small companion passed all the validation tests. This system is probably another planetary system with hot Jupiter and small, nearby companion, after the outstanding WASP-47 system, and so far the only such system (out of 46) in the prime Kepler mission. The nature of this system, if confirmed, would suggest that hot Jupiters with small, nearby companions are probably more common than we used to believe. There remains a possibility that the small companion actually transits a star that is different from the hot Jupiter host, and follow-up observations with 10-m telescopes can potentially resolve this issue.

astro-ph.EP↗

arXiv2018

Dark Passengers in Stellar Surveys

We develop stellar population models to predict the number of binaries with a single luminous member is Gaia and Hipparcos. Our models yield dozens of detections of black hole - luminous companion binaries (BHLC) and hundreds to thousands of neutron star - luminous companion binaries (NSLC) with Gaia. Interestingly, our models also yield a single detection of BHLC binary with Hipparcos, and a few NSLC binaries. We also show how the statistical distribution of detected binaries with a single luminous companion can be used to constrain the formation process of neutron stars and black holes.

astro-ph.SR↗

arXiv2018

The Super Earth-Cold Jupiter Relations

We report relations between inner ($<1$ au) super Earths (planets with mass/radius between Earth and Neptune) and outer ($>1$ au) giant planets (mass $>0.3~M_{\rm J}$, or cold Jupiters) around Sun-like stars, based on data from both ground-based radial velocity (RV) observations and the Kepler mission. We find that cold Jupiters appear three times more often around hosts of super Earths than they do around field stars. Given the prevalence of the super Earth systems, their cold Jupiters can account for nearly all cold Jupiters. In other words, cold Jupiters are almost certainly ($\sim90\%$) companied by super Earths. A few corollaries follow: (1) around metal-rich ([Fe/H]$>0.1$) stars, the fraction of super Earths with cold Jupiters can rise to $60\%$ or higher; (2) the inner architecture can be strongly impacted by the outer giant and we report some observational evidence for this; (3) planetary systems like our own, with cold Jupiters but no super Earths, should be rare ($\sim1\%$). The strong correlation between super Earths and cold Jupiters establish that super Earths and cold Jupiters do not compete for solid material, rather, they share similar origins, with the cold Jupiter formation requiring a somewhat more stringent condition. Lastly, we propose a few immediate observational tests of our results, using ground-based RV observations and ongoing/planned space missions.

astro-ph.EP↗

arXiv2018

Deep Neural Nets with Interpolating Function as Output Activation

We replace the output layer of deep neural nets, typically the softmax function, by a novel interpolating function. And we propose end-to-end training and testing algorithms for this new architecture. Compared to classical neural nets with softmax function as output activation, the surrogate with interpolating function as output activation combines advantages of both deep and manifold learning. The new framework demonstrates the following major advantages: First, it is better applicable to the case with insufficient training data. Second, it significantly improves the generalization accuracy on a wide variety of networks. The algorithm is implemented in PyTorch, and code will be made publicly available.

cs.LG↗

arXiv2018

Electronic nature of coverage-dependent nanosurface effect by cooperative orbital redistribution

Nanomaterial surface states can effectively modify or even dominate their physical and chemical properties due to large surface-to-volume ratios. Such surface effects are highly dependent on particle size and ligand coverage, yet the underlying electronic-level mechanism still remains unknown. Using TiO2 nanosheet as a model system, we reveal the electronic nature of coverage-dependent nanosurface effects through varying ligand coverage and probing the modified surface bonding and electronic band structures with near-edge X-ray absorption fine structure. We discover experimentally that surface ligands can competitively polarize the 3d orbitals of surface Ti atoms into chemisorption states, which is cooperative with increased ligand coverages. Such coverage-dependent cooperative orbital redistribution accounts for various nanosurface effects on regulating the electronic structure, surface reactivity, optical property, and chemisorption of nanomaterials.

cond-mat.mtrl-sci↗

arXiv2018

About 30% of Sun-like Stars Have Kepler-like Planetary Systems: A Study of their Intrinsic Architecture

We constrain the intrinsic architecture of Kepler planetary systems by modeling the observed multiplicities of the transiting planets (tranets) and their transit timing variations (TTVs). We robustly determine that the fraction of Sun-like stars with Kepler-like planets, $η_{\rm Kepler}$, is $30\pm3\%$. Here Kepler-like planets are planets that have radii $R_{\rm p} \gtrsim R_\oplus$ and orbital periods $P<400$~days. Our result thus significantly revises previous claims that more than 50\% of Sun-like stars have such planets. Combining with the average number of Kepler planets per star ($\sim0.9$), we obtain that on average each planetary system has $3.0\pm0.3$ planets within 400 days. We also find that the dispersion in orbital inclinations of planets within a given planetary system, $σ_{i,k}$, is a steep function of its number of planets, $k$. This can be parameterized as $σ_{i,k}\propto k^α$ and we find that $-4<α<-2$ at 2-$σ$ level. Such a distribution well describes the observed multiplicities of both tranets and TTVs with no excess of single tranets. Therefore we do not find evidence supporting the so-called "Kepler dichotomy." Together with a previous study on orbital eccentricities, we now have a consistent picture: the fewer planets in a system, the hotter it is dynamically. We discuss briefly possible scenarios that lead to such a trend. Despite our Solar system not belonging to the Kepler club, it is interesting to notice that the Solar system also has three planets within 400 days and that the inclination dispersion is similar to Kepler systems of the same multiplicity.

astro-ph.EP↗