arXiv subjects

Chao Liu

Publications and source records attributed to Chao Liu.

At least 433 records · Page 24Linked to original sources

arXiv2014

The velocity distribution in the solar neighbourhood from the LAMOST pilot survey

We use about 15,000 F/G nearby dwarf stars selected from the LAMOST pilot survey to map the U-V velocity distribution in the solar neighbourhood. An extreme deconvolution algorithm is applied to reconstruct an empirical multi-Gaussian model. In addition to the well known substructures, e.g., Sirius, Coma Berenices, Hyades-Pleiades over-densities, several new substructures are unveiled. A ripple-like structure from (U, V) = (-120, -5) to (103, -32)km/s is clearly seen in the U-V distribution. This structure seems associated with resonance induced by the Galactic bar, since it is extended in U while having a small dispersion in V at the same time. A ridge structure between (U, V) = (-60, 40) and (-15, 15) km/s is also found. Although similar substructures have been seen in the Hipparcos data, their origin is still unclear. Another compact over-density is seen at (U, V) = (-102, -24). With this large data sample, we find that the substructure located at V~70 km/s and the Arcturus group are essentially parallel in V, which may indicate that they originate from an unrelaxed disk component perturbed by the rotating bar.

astro-ph.GA↗

arXiv2014

Fixing the Reference Frame for PPMXL Proper Motions Using Extragalactic Sources

We quantify and correct systematic errors in PPMXL proper motions using extragalactic sources from the first two LAMOST data releases and the Veron-Cetty & Veron Catalog of Quasars. Although the majority of the sources are from the Veron catalog, LAMOST makes important contributions in regions that are not well-sampled by previous catalogs, particularly at low Galactic latitudes and in the south Galactic cap. We show that quasars in PPMXL have measureable and significant proper motions, which reflect the systematic zero-point offsets present in the catalog. We confirm the global proper motion shifts seen by Wu, Ma, & Zhou (2011), and additionally find smaller-scale fluctuations of the QSO-derived corrections to an absolute frame. We average the proper motions of 158,106 extragalactic objects in bins of 3x3 degrees and present a table of proper motion corrections.

astro-ph.GA↗

arXiv2014

Complex-based analysis of dysregulated cellular processes in cancer

Background: Differential expression analysis of (individual) genes is often used to study their roles in diseases. However, diseases such as cancer are a result of the combined effect of multiple genes. Gene products such as proteins seldom act in isolation, but instead constitute stable multi-protein complexes performing dedicated functions. Therefore, complexes aggregate the effect of individual genes (proteins) and can be used to gain a better understanding of cancer mechanisms. Here, we observe that complexes show considerable changes in their expression, in turn directed by the concerted action of transcription factors (TFs), across cancer conditions. We seek to gain novel insights into cancer mechanisms through a systematic analysis of complexes and their transcriptional regulation. Results: We integrated large-scale protein-interaction (PPI) and gene-expression datasets to identify complexes that exhibit significant changes in their expression across different conditions in cancer. We devised a log-linear model to relate these changes to the differential regulation of complexes by TFs. The application of our model on two case studies involving pancreatic and familial breast tumour conditions revealed: (i) complexes in core cellular processes, especially those responsible for maintaining genome stability and cell proliferation (e.g. DNA damage repair and cell cycle) show considerable changes in expression; (ii) these changes include decrease and countering increase for different sets of complexes indicative of compensatory mechanisms coming into play in tumours; and (iii) TFs work in cooperative and counteractive ways to regulate these mechanisms. Such aberrant complexes and their regulating TFs play vital roles in the initiation and progression of cancer.

q-bio.MN↗

arXiv2014

A Bayesian Method for the Extinction

We propose a Bayesian method to measure the total Galactic extinction parameters, $R_V$ and $A_V$. Validation tests based on the simulated data indicate that the method can achieve the accuracy of around 0.01\,mag. We apply this method to the SDSS BHB stars in the northern Galactic cap and find that the derived extinctions are highly consistent with those from \cite{SFD98}. It suggests that the Bayesian method is promising for the extinction estimation, even the reddening values are close to the observational errors.

astro-ph.IM↗

arXiv2014

The binarity of Milky Way F,G,K stars as a function of effective temperature and metallicity

We estimate the fraction of F,G,K stars with close binary companions by analysing multi-epoch stellar spectra from SDSS and LAMOST for radial velocity (RV) variations. We employ a Bayesian method to infer the maximum likelihood of the fraction of binary stars with orbital periods of 1000 days or shorter, assuming a simple model distribution for a binary population with circular orbits. The overall inferred fraction of stars with such a close binary companion is 43.0% \pm 2.0% for a sample of F, G, K stars from SDSS SEGUE, and 30% \pm 8.0% in a similar sample from LAMOST. The apparent close binary fraction decreases with the stellar effective temperature. We divide the SEGUE and LEGUE data into three subsamples with different metallicity ([Fe/H] < -1.1; -1.1 < [Fe/H] < -0.6; -0.6 < [Fe/H]), for which the inferred close binary fractions are 56% \pm 5.0%, 56.0% \pm 3%, and 30% \pm 5.7%. The metal-rich stars from our sample are therefore substantially less likely to possess a close binary companion than otherwise similar stars drawn from metal-poor populations. The different ages and formation environments of the Milky Way's thin disk, thick disk and halo may contribute to explaining these observations. Alternatively metallicity may have a significant effect on the formation and/or evolution of binary stars.

astro-ph.GA↗

arXiv2014

The Nearest High-Velocity Stars Revealed by LAMOST Data Release 1

We report the discovery of 28 candidate high-velocity stars (HVSs) at heliocentric distances of less than 3 kpc, based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 1. Our sample of HVS candidates covers a much broader color range than the equivalent ranges discussed in previous studies and comprises the first and largest sample of HVSs in the solar neighborhood. The sufficiently accurate observed and derived parameters for all candidates allow us to ascertain their nature as genuine HVSs, while a subset of 12 objects represents the most promising candidates. Our results also highlight the great potential of discovering statistically large numbers of HVSs of different spectral types in LAMOST survey data. This will ultimately enable us to achieve a better understanding of the nature of Galactic HVSs and their ejection mechanisms, and to constrain the structure of the Galaxy.

astro-ph.GA↗

arXiv2014

Comment on "Fault-Tolerate Quantum Private Comparison Based on GHZ States and ECC"

A two-party quantum private comparison scheme using GHZ states and error-correcting code (ECC) was introduced in Li et al.'s paper [Int. J. Theor. Phys. 52: 2818-2815, 2013], which holds the capability of fault-tolerate and could be performed in a none-ideal scenario. However, this study points out there exists a fatal loophole under a special attack, namely the twice-Hadamard-CNOT attack. A malicious party may intercept the other's particles, firstly executes the Hadamard operations on these intercepted particles and his (her) own ones respectively, and then sequentially performs twice CNOT operations on them and the auxiliary particles prepared in advance. As a result, the secret input will be revealed without being detected through measuring the auxiliary particles. For resisting this special attack, an improvement is proposed by applying a permutation operator before TP sends the particle sequences to all the participants.

quant-ph↗

arXiv2014

Improved Deterministic N-To-One Joint Remote Preparation of an Arbitrary Qubit via EPR Pairs

Recently, Bich et al. (Int. J. Theor. Phys. 51: 2272, 2012) proposed two deterministic joint remote state preparation (JRSP) protocols of an arbitrary single-qubit state: one is for two preparers to remotely prepare for a receiver by using two Einstein-Podolsky-Rosen (ERP) pairs; the other is its generalized form in the case of arbitrary N>2 preparers via N ERP pairs. In this paper, Through reviewing and analyzing Bich et al.'s second protocols with N>2 preparers, we find that the success probability P_{suc}=1/4 < 1. In order to solve the problem, we firstly constructed two sets of projective measurement bases: the real-coefficient basis and the complex-coefficient one, and further proposed an improved deterministic N-to-one JRSP protocol for an arbitrary single-qubit state with unit success probability (i.e, P_{suc}=1). Morever, our protocol is also flexible and convenient, and it can be used in a practical network.

quant-ph↗

arXiv2014

Cryptanalysis and improvement of the quantum private comparison protocol based on Bell entangled states

Recently, Liu et al. [Commun. Theor. Phys. 57, 583, 2012] proposed a quantum private comparison protocol based on entanglement swapping of Bell states, which aims to securely compare the equality of two participants' information with the help of a semi-honest third party (TP). However, this study points out there is a fatal loophole in this protocol, i.e., TP can obtain all of the two participants secret inputs without being detected through making a specific Bell-basis measurement. To fix the problem, a simple solution, which uses one-time eavesdropper checking with decoy photons instead of twice eavesdropper checking with Bell states, is demonstrated. Compared with the original protocol, it also reduces the Bell states consumption and simplifies the steps in the protocol.

quant-ph↗

arXiv2014

The K giant stars from the LAMOST survey data I: identification, metallicity, and distance

We present a support vector machine classifier to identify the K giant stars from the LAMOST survey directly using their spectral line features. The completeness of the identification is about 75% for tests based on LAMOST stellar parameters. The contamination in the identified K giant sample is lower than 2.5%. Applying the classification method to about 2 million LAMOST spectra observed during the pilot survey and the first year survey, we select 298,036 K giant candidates. The metallicities of the sample are also estimated with uncertainty of $0.13\sim0.29$\,dex based on the equivalent widths of Mg$_{\rm b}$ and iron lines. A Bayesian method is then developed to estimate the posterior probability of the distance for the K giant stars, based on the estimated metallicity and 2MASS photometry. The synthetic isochrone-based distance estimates have been calibrated using 7 globular clusters with a wide range of metallicities. The uncertainty of the estimated distance modulus at $K=11$\,mag, which is the median brightness of the K giant sample, is about 0.6\,mag, corresponding to $\sim30$% in distance. As a scientific verification case, the trailing arm of the Sagittarius stream is clearly identified with the selected K giant sample. Moreover, at about 80\,kpc from the Sun, we use our K giant stars to confirm a detection of stream members near the apo-center of the trailing tail. These rediscoveries of the features of the Sagittarius stream illustrate the potential of the LAMOST survey for detecting substructures in the halo of the Milky Way.

astro-ph.GA↗

arXiv2014

Pulse-Doppler Signal Processing with Quadrature Compressive Sampling

Quadrature compressive sampling (QuadCS) is a newly introduced sub-Nyquist sampling for acquiring inphase and quadrature (I/Q) components of radio-frequency signals. For applications to pulse-Doppler radars, the QuadCS outputs can be arranged in 2-dimensional data similar to that by Nyquist sampling. This paper develops a compressive sampling pulse-Doppler (CoSaPD) processing scheme from the sub-Nyquist samples. The CoSaPD scheme follows Doppler estimation/detection and range estimation and is conducted on the sub-Nyquist samples without recovering the Nyquist samples. The Doppler estimation is realized through spectrum analyzer as in classic processing. The detection is done on the Doppler bin data. The range estimation is performed through sparse recovery algorithms on the detected targets and thus the computational load is reduced. The detection threshold can be set at a low value for improving detection probability and then the introduced false targets are removed in the range estimation stage through inherent detection characteristic in the recovery algorithms. Simulation results confirm our findings. The CoSaPD scheme with the data at one eighth the Nyquist rate and for SNR above -25dB can achieve performance of the classic processing with Nyquist samples.

cs.IT↗

arXiv2014

The First Hypervelocity Star from the LAMOST Survey

We report the first hypervelocity star (HVS) discovered from the LAMOST spectroscopic survey. It is a B-type star with a heliocentric radial velocity about 620 km/s, which projects to a Galactocentric radial velocity component of ~477 km/s. With a heliocentric distance of ~13 kpc and an apparent magnitude of ~13 mag, it is the nearest bright HVS currently known. With a mass of ~9Msun, it is one of the three most massive HVSs discovered so far. The star is clustered on the sky with many other known HVSs, with the position suggesting a possible connection to Galactic center structures. With the current poorly-determined proper motion, a Galactic center origin of this HVS remains consistent with the data at the 1sigma level, while a disk run-away origin cannot be excluded. We discuss the potential of the LAMOST survey to discover a large statistical sample of HVSs of different types.

astro-ph.GA↗

arXiv2014

Exploring the total Galactic extinction with SDSS BHB stars

Aims: We used 12,530 photometrically-selected blue horizontal branch (BHB) stars from the Sloan Digital Sky Survey (SDSS) to estimate the total extinction of the Milky Way at the high Galactic latitudes, $R_V$ and $A_V$ in each line of sight. Methods: A Bayesian method was developed to estimate the reddening values in the given lines of sight. Based on the most likely values of reddening in multiple colors, we were able to derive the values of $R_V$ and $A_V$. Results: We selected 94 zero-reddened BHB stars from seven globular clusters as the template. The reddening in the four SDSS colors for the northern Galactic cap were estimated by comparing the field BHB stars with the template stars. The accuracy of this estimation is around 0.01\,mag for most lines of sight. We also obtained $ $ to be around 2.40$\pm1.05$ and $A_V$ map within an uncertainty of 0.1\,mag. The results, including reddening values in the four SDSS colors, $A_V$, and $R_V$ in each line of sight, are released on line. In this work, we employ an up-to-date parallel technique on GPU card to overcome time-consuming computations. We plan to release online the C++ CUDA code used for this analysis. Conclusions: The extinction map derived from BHB stars is highly consistent with that from Schlegel, Finkbeiner & Davis(1998). The derived $R_V$ is around 2.40$\pm1.05$. The contamination probably makes the $R_V$ be larger.

astro-ph.GA↗

arXiv2013

Same Initial States Attack in Yang et al.'s Quantum Private Comparison Protocol and the Improvement

In Yang et al.'s literatures (J. Phys. A: Math. 42, 055305, 2009; J. Phys. A:Math. 43, 209801, 2010), a quantum private comparison protocol based on Bell states and hash function is proposed, which aims to securely compare the equality of two participants' information with the help of a dishonest third party (TP). However, this study will point out their protocol cannot resist a special kind of attack, TP's same initial states attack, which is presented in this paper. That is, the dishonest TP can disturb the comparison result without being detected through preparing the same initial states. Finally, a simple improvement is given to avoid the attack.

quant-ph↗

arXiv2013

Secure Quantum Private Comparison of Equality Based on Asymmetric W State

Recently, Liu et al. [Opt. Commun. 284, 3160, 2011] proposed a protocol for quantum private comparison of equality (QPCE) based on sym- metric W state. However, Li et al. [Eur. Phys. J. D. 66, 110, 2012] pointed out that there is a flaw of information leak, and they proposed a new protocol based on EPR pairs. While examining these two protocols, we find that there exists a same flaw: the third party (TP) can know the comparison result. In this paper, through introducing and constructing a special class of asymmet- ric W state, a secure QPCE protocol based on this asymmetric W state is presented. Analysis shows the present protocol can not only effectively avoid the information leak found by Li et al, but also ensure TP would not get any information about the comparison result.

quant-ph↗

arXiv2013

Quantum Private Comparison: A Review

As an important branch of quantum secure multiparty computation, quantum private comparison (QPC) has attracted more and more attention recently. In this paper, according to the quantum implementation mechanism that these protocols used, we divide these protocols into three categories: The quantum cryptography QPC, the superdense coding QPC, and the entanglement swapping QPC. And then, a more in-depth analysis on the research progress, design idea, and substantive characteristics of corresponding QPC categories is carried out, respectively. Finally, the applications of QPC and quantum secure multi-party computation issues are discussed and, in addition, three possible research mainstream directions are pointed out.

cs.CR↗

arXiv2013

Substructure in bulk velocities of Milky Way disk stars

We find that Galactic disk stars near the anticenter exhibit velocity asymmetries in both the Galactocentric radial and vertical components across the mid-plane as well as azimuthally. These findings are based on LAMOST spectroscopic velocities for a sample of ~400,000 F-type stars, combined with proper motions from the PPMXL catalog for which we have derived corrections to the zero points based in part on spectroscopically discovered galaxies and QSOs from LAMOST. In the region within 2 kpc outside the Sun's radius and +/-2 kpc from the Galactic midplane, we show that stars above the plane exhibit net outward radial motions with downward vertical velocities, while stars below the plane have roughly the opposite behavior. We discuss this in the context of other recent findings, and conclude that we are likely seeing the signature of vertical disturbances to the disk due to an external perturbation.

astro-ph.GA↗

arXiv2013

DA white dwarfs observed in LAMOST pilot survey

A total of $\sim640,000$ objects from LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs by fitting Sérsic profiles to the Balmer H$β$, H$γ$ and H$δ$ lines of the spectra, and calculating the equivalent width of the CaII K line. 2964 candidates are selected by constraining the fitting parameters and the equivalent width of CaII K line. All the spectra of candidates are visually inspected. We identify 230 (59 of them are already in Villanova and SDSS WD catalog) DA white dwarfs, 20 of which are DA white dwarfs with non-degenerate companions. In addition, 128 candidates are classified as DA white dwarf/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DA white dwarf number estimated based on the LEGUE target selection algorithm.

astro-ph.SR↗