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Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

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Chinese Journal of Astronomy and Astrophysics 2007年第1期7卷 91-100页

作者： Xu Zhang Ye Lu yong-heng zhao National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 Department of Physics Shandong University Jinan 250100 Department of Physics The University of Hong Kong Pokfulam Road Hong Kong

We suggest a two-dimensional time dependent analytic model to describe the accretion of matter onto a neutron star moving at a high speed across the ejecta left in the aftermath of a supernova explosion. The formation of a strange star resulting from the accretion is also addressed. The newborn neutron star is assumed to move outward at a kick velocity of Vns- 10^3 kin s^-1, and the accretion flow is treated as a dust flow. When the neutron star travels across the ejecta with high speed, it sweeps up material, and when the accreted mass has reached a critical value, the neutron star will undergo a phase transition, for instance, to become a strange star. Our results show that the accretion rate decreases in a complicated way in time, not just a power law dependence： it drops much faster than the power law derived by Colpi et al. We also found that the total accreted mass and the phase transition of the neutron star depend sensitively on the velocity of supernova ejecta.

关键词： pulsars general-pulsars-stars neutron-X-rays stars-accretion disk-instability

Possible Streams of the Globular Clusters in the Galaxy

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Chinese Journal of Astronomy and Astrophysics 2007年第1期7卷 111-119页

作者： Shuang Gao Bi-Wei Jiang yong-heng zhao Department of Astronomy Beijing Normal University Beijing 100875 National Astronomical Observatories Chinese Academy of Sciences Beijing 100012

We aim to retrieve ghost streams under the assumption that all the globular clusters in our Galaxy were formed in very early merge events. Our results are based on two speculations： that the specific energy and angular momentum of the globular clusters after merge are not changed in the course of evolution and that the globular clusters with a common origin would stay in the same orbit plane as the parent galaxy. After taking into account the apogalacticum distance of the orbits, we suggest with some Confidence five possible streams. The number of streams is consistent with the previous results. Three of the four well established members of the Sagittadus stream were found to be in one of our streams. Several other globular clusters in our result were also thought to come from accretion by previous researchers. The orbital parameters of the streams are derived, which provide a way to test whether these streams are true with the help of more accurate measurement of proper motions of the globular clusters.

关键词： globular clusters general-Galaxy formation-Galaxy halo

Random forest algorithm for classification of multiwavelength data

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Chinese Journal of Astronomy and Astrophysics 2009年第2期9卷 220-226页

作者： Dan Gao Yan-Xia Zhang yong-heng zhao National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 China Graduate University of Chinese Academy of Sciences Beijing 100049 China

We introduced a decision tree method called Random Forests for multiwavelength data classification. The data were adopted from different databases, including the Sloan Digital Sky Survey （SDSS） Data Release five, USNO, FIRST and ROSAT. We then studied the discrimination of quasars from stars and the classification of quasars, stars and galaxies with the sample from optical and radio bands and with that from optical and X-ray bands. Moreover, feature selection and feature weighting based on Random Forests were investigated. The performances based on different input patterns were compared. The experimental results show that the random forest method is an effective method for astronomical object classification and can be applied to other classification problems faced in astronomy. In addition, Random Forests will show its superiorities due to its own merits, e.g. classification, feature selection, feature weighting as well as outlier detection.

关键词： classification-- astronomical databases miscellaneous -- catalogs -- meth- ods data analysis -- methods statistical

Simulation of the emission lines radiated from cataclysmic variables

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Research in Astronomy and Astrophysics 2013年第10期13卷 1213-1218页

作者： Lin He Guo-Liang Lü yong-heng zhao College of Physical Science and Technology Xinjiang University National Astronomical Observatories Chinese Academy of Sciences

Cataclysmic variables are special celestial bodies because they have particular light curves and *** mechanisms for generating emission lines radiated from dwarf novae in their quiescent phases are *** assume that the incident radiation field which is emitted by a hot source（white dwarf and boundary layer）irradiates the gaseous layer evaporated from the accretion disk,and the emission lines are radiated from the *** model the fluxes of emission lines by using the photoionization code *** this method,we input some reasonable parameters and get a series of simulated *** order to find a simulated spectrum which is the best fit to an observed spectrum,we use a cross-correlation method to match *** the calculation,we use the approximation that the parameters of the simulated spectrum can simulate the observed ***,we learn more about the physical conditions of the system.

关键词： stars:cataclysmic variables accretion disk:emission lines emission lines:photoionization

Automated estimation of stellar fundamental parameters from low resolution spectra: the PLS method

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Research in Astronomy and Astrophysics 2009年第6期9卷 712-724页

作者： Jian-Nan Zhang A-Li Luo yong-heng zhao National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 China

PLS （Partial Least Squares regression） is introduced into an automatic estimation of fundamental stellar spectral parameters. It extracts the most correlative spectral component to the parameters （Teff, log g and [Fe/H]）, and sets up a linear regression function from spectra to the corresponding parameters. Considering the properties of stellar spectra and the PLS algorithm, we present a piecewise PLS regression method for estimation of stellar parameters, which is composed of one PLS model for Teff, and seven PLS models for log g and [Fe/H] estimation. Its performance is investigated by large experiments on flux calibrated spectra and continuum normalized spectra at different signal-to-noise ratios （SNRs） and resolutions. The results show that the piecewise PLS method is robust for spectra at the medium resolution of 0.23 nm. For low resolution 0.5 nm and 1 nm spectra, it achieves competitive results at higher SNR. Experiments using ELODIE spectra of 0.23 nm resolution illustrate that our piecewise PLS models trained with MILES spectra are efficient for O ～ G stars： for flux calibrated spectra, the systematic offsets are 3.8%, 0.14 dex, and -0.09 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.44 dex and 0.38 dex, respectively; for continuum normalized spectra, the systematic offsets are 3.8%, 0.12dex, and -0.13 dex for Teff, log g and [Fe/H], with error scatters of 5.2%, 0.49 dex and 0.41 dex, respectively. The PLS method is rapid, easy to use and does not rely as strongly on the tightness of a parameter grid of templates to reach high precision as Artificial Neural Networks or minimum distance methods do.

关键词： methods data analysis -- methods statistical -- stars fundamental param- eters (classification, temperatures, metallicity) -- techniques spectroscopic -- surveys

Variability of Soft X-ray Spectral Shape in Blazars Observed by ROSAT

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Chinese Journal of Astronomy and Astrophysics 2002年第2期2卷 117-126页

作者： Lin-Peng Cheng yong-heng zhao Jian-Yan Wei National Astronomical Observatories Chinese Academy of SciencesBeijing 100012

In previous paper we have shown that the soft X-ray spectra of two types of Seyfert 1 galaxies behave statistically differently with increasing intensity. In order to see how the spectrum of blazars changes, we made plots of Hardness Ratio 1 versus Count Rates (HR1-Cts) for 18 blazars observed by ROSAT/PSPC. According to our criteria, ten showed a positive HR1-Cts correlation, two a negative correlation, and the remaining six, no clear correlation. Thus, most blazars of our sample show a hardening spectrum during overall flux increase, though some vary randomly. By investigating the photon index of these objects and different radiation theories, we argue that relative dominance between the synchrotron and inverse Compton emission in the soft X-ray band can well account for the differing spectral behaviours and that the different spectral variations might represent a sequence of synchrotron peaked frequency.

关键词： galaxies active-galaxies X-ray-blazar spectrum

Two Novel Approaches for Photometric Redshift Estimation based on SDSS and 2MASS

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Chinese Journal of Astronomy and Astrophysics 2008年第1期8卷 119-126页

作者： Dan Wang Yan-Xia Zhang Chao Liu yong-heng zhao National Astronomical Observatories Chinese Academy of Sciences Beijing 100012 Graduate School of Chinese Academy of Sciences Beijing 100049

We investigate two training-set methods; support vector machines （SVMs） and Kernel Regression （KR） for photometric redshift estimation with the data from the databases of Sloan Digital Sky Survey Data Release 5 and Two Micron All Sky Survey. We probe the performances of SVMs and KR for different input patterns. Our experiments show that with more parameters considered, the accuracy does not always increase, and only when appropriate parameters are chosen, the accuracy can improve. For different approaches, the best input pattern is different. With different parameters as input, the optimal bandwidth is dissimilar for KR. The rms errors of photometric redshifts based on SVM and KR methods are less than 0.03 and 0.02, respectively. Strengths and weaknesses of the two approaches are summarized. Compared to other methods of estimating photometric redshifts, they show their superiorities, especially KR, in terms of accuracy.

关键词： galaxies distances and redshifts - galaxies general - methods data analysis - techniques photometric

A new strategy for estimating photometric redshifts of quasars

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Research in Astronomy and Astrophysics 2019年第12期19卷 223-234页

作者： Yan-Xia Zhang Jing-Yi Zhang Xin Jin yong-heng zhao Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of SciencesBeijing 100101China University of Chinese Academy of Sciences Beijing 100049China

Based on the SDSS and SDSS-WISE quasar datasets, we put forward two schemes to estimate the photometric redshifts of quasars. Our schemes are based on the idea that the samples are firstly classified into subsamples by a classifier and then a photometric redshift estimation of different subsamples is performed by a regressor. Random Forest is adopted as the core algorithm of the classifiers, while Random Forest and k NN are applied as the key algorithms of regressors. The samples are divided into two subsamples and four subsamples, depending on the redshift distribution. The performances based on different samples, different algorithms and different schemes are compared. The experimental results indicate that the accuracy of photometric redshift estimation for the two schemes generally improves to some extent compared to the original scheme in terms of the percents in |△z|1+zi< 0.1 and |△z|1+zi<0.2 and mean absolute error. Only given the running speed, k NN shows its superiority to Random Forest. The performance of Random Forest is a little better than or comparable to that of k NN with the two datasets. The accuracy based on the SDSS-WISE sample outperforms that based on the SDSS sample no matter by k NN or by Random Rorest. More information from more bands is considered and helpful to improve the accuracy of photometric redshift estimation. Evidently, it can be found that our strategy to estimate photometric redshift is applicable and may be applied to other datasets or other kinds of objects. Only talking about the percent in |△z|1+zi<0.3, there is still large room for further improvement in the photometric redshift estimation.

关键词： astronomical databases:catalogs (galaxies:)quasars:general methods:statistical techniques:miscellaneous

The relative calibration of radial velocity for LAMOST medium resolution stellar spectra

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Research in Astronomy and Astrophysics 2021年第10期21卷 239-249页

作者： Jian-Ping Xiong Bo Zhang Chao Liu Jiao Li yong-heng zhao yong-Hui Hou Key Laboratory of Optical Astronomy National Astronomical ObservatoriesChinese Academy of SciencesBeijing 100101China University of Chinese Academy of Sciences Beijing 100049China Department of Astronomy Beijing Normal UniversityBeijing 100875China Key Laboratory of Space Astronomy and Technology National Astronomical ObservatoriesChinese Academy of SciencesBeijing 100101 China Yunnan Observatories Chinese Academy of SciencesKunming 650011China Nanjing Institute of Astronomical Optics&Technology National Astronomical ObservatoriesChinese Academy of SciencesNanjing 210042China School of Astronomy and Space Science University of Chinese Academy of SciencesBeijing 100049China

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST)started a medianresolution spectroscopic(MRS,R~7500)survey since October *** main scientific goals of MRS,including binary stars,pulsators and other variable stars,were launched with a time-domain spectroscopic ***,the systematic errors,including the bias induced from wavelength calibration and the systematic difference between different spectrographs,have to be carefully considered during radial velocity *** this work,we provide a technique to correct the systematics in the wavelength calibration based on the relative radial velocity measurements from LAMOST MRS *** show that,for the stars with multi-epoch spectra,the systematic bias which is induced from the exposures on different nights can be corrected well for LAMOST MRS in each *** addition,the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km s^(-1).As a by-product,we also give the constant star candidates^(**),which can be the secondary radial-velocity standard star candidates of LAMOST MRS time-domain surveys.

关键词： methods:data analysis techniques:radial velocities stars:statistics catalogs surveys