An in-depth Exploration of LAMOST Unknown Spectra Based on Density Clustering

Large sky Area Multi-Object fiber Spectroscopic Telescope(LAMOST) has completed the observation of nearly 20 million celestial objects,including a class of spectra labeled “Unknown.” Besides low signal-to-noise ratio,these spectra often show some anomalous features that do not work well with current templates.In this paper,a total of 637,889 “Unknown” spectra from LAMOST DR5 are selected,and an unsupervised-based analytical framework of “Unknown” spectra named SA-Frame(Spectra Analysis-Frame) is provided to explore their origins from different perspectives.The SA-Frame is composed of three parts:NAPC-Spec clustering,characterization and origin analysis.First,NAPC-Spec(Nonparametric density clustering algorithm for spectra) characterizes different features in the “unknown” spectrum by adjusting the influence space and divergence distance to minimize the effects of noise and high dimensionality,resulting in 13 types.Second,characteristic extraction and representation of clustering results are carried out based on spectral lines and continuum,where these 13 types are characterized as regular spectra with low S/Ns,splicing problems,suspected galactic emission signals,contamination from city light and un-gregarious type respectively.Third,a preliminary analysis of their origins is made from the characteristics of the observational targets,contamination from the sky,and the working status of the instruments.These results would be valuable for improving the overall data quality of large-scale spectral surveys.

Observation of Small-amplitude Electromagnetic Cyclotron Waves in the Solar Wind

Our previous studies on low-frequency electromagnetic cyclotron waves(ECWs)with amplitudes larger than 0.1nT in the solar wind revealed that the left-handed(LH)polarized ECWs are the dominant waves,and these waves preferentially occur in plasma conditions of high proton speed(V_(p)),high proton temperature(T_(p)),low proton density(N_(p)).In the present study,using magnetic field and plasma data from the Wind mission between 2005 and2015,we perform a survey of small-amplitude ECWs with amplitudes smaller than 0.1 nT.It is revealed for the first time that the small-amplitude right-handed(RH)polarized ECWs tend to frequently occur in plasmas characterized by low V_(p),low T_(p),low N_(p),although the small-amplitude LH ECWs still preferentially occur in plasma conditions similar to the LH ECWs with amplitudes larger than 0.1 nT.Further investigation shows that the occurrences of small-amplitude RH ECWs and long-lasting radial interplanetary magnetic field(lrIMF)share the similar preferential plasma conditions of low T_(p)and low N_(p).During lrIMF events,in particular,the occurrence rates of RH and LH ECWs are comparable,with the occurrence rate of small-amplitude RH ECWs slightly larger than that of small-amplitude LH ECWs.The generation mechanism of the small-amplitude ECWs is discussed.

挤压镁合金薄壁方管的耐撞性能（英文）

对不同长度的完整AZ31B镁合金薄壁方管和A6063铝合金薄壁方管以及具有诱导特征的薄壁管进行不同应变率的轴向压缩试验研究。研究发现,压缩过程中镁合金薄壁管局部屈曲,进而断裂,其中在断裂阶段出现3种耐撞性能断裂模式,分别是水平断裂、斜向断裂和螺旋断裂。相比于铝合金方管,镁合金薄壁管的吸能效果较差。在上述研究基础上,进一步分析长宽比、应变率和诱导特征对薄壁管耐撞性能的影响。结果表明,随着长宽比从1增加到4,镁合金薄壁管的最大压缩载荷和整体吸能比以幂函数的规律减小,而铝合金薄壁管则基本保持不变。随着压缩速率从5×10-5增加到10m/s,镁合金薄壁管的主要耐撞性能参数以指数形式增加,而铝合金薄壁管则变化很小。此外,本文作者引入的圆孔和缝的诱导特征未明显提高镁合金薄壁管的吸能效果,因此,后期需要在薄壁管不同位置设计更多种类和尺寸的诱导特征来提高镁合金薄壁管的吸能比。

The first data release(DR1) of the LAMOST regular survey

The Large sky Area Multi-Object Fiber Spectroscopic Telescope（LAMOST） general survey is a spectroscopic survey that will eventually cover approximately half of the celestial sphere and collect 10 million spectra of stars, galaxies and QSOs. Objects in both the pilot survey and the first year regular survey are included in the LAMOST DR1. The pilot survey started in October 2011 and ended in June 2012, and the data have been released to the public as the LAMOST Pilot Data Release in August 2012. The regular survey started in September 2012, and completed its first year of operation in June 2013. The LAMOST DR1 includes a total of 1202 plates containing 2 955 336 spectra, of which 1 790 879 spectra have observed signalto-noise ratio（SNR） ≥ 10. All data with SNR ≥ 2 are formally released as LAMOST DR1 under the LAMOST data policy. This data release contains a total of 2 204 696 spectra, of which 1 944 329 are stellar spectra, 12 082 are galaxy spectra and 5017 are quasars. The DR1 not only includes spectra, but also three stellar catalogs with measured parameters： late A,FGK-type stars with high quality spectra（1 061 918 entries）, A-type stars（100 073 entries）, and M-type stars（121 522 entries）. This paper introduces the survey design, the observational and instrumental limitations, data reduction and analysis, and some caveats. A description of the FITS structure of spectral files and parameter catalogs is also provided.

A catalog of early-type emission-line stars and Hα line profiles from LAMOST DR2

We present a catalog including 11 204 spectra of 10436 early-type emission-line stars from LAMOST DR2, among which 9752 early-type emission-line spectra are newly discovered. For these early- type emissionqine stars, we discuss the morphological and physical properties of their low-resolution spectra. In this spectral sample, the Hα emission profiles display a wide variety of shapes. Based on the Hα line profiles, these spectra are categorized into five distinct classes： single-peak emission, single-peak emission in absorption, double-peak emission, double-peak emission in absorption, and P-Cygni profiles. To better understand what causes the Hc~ line profiles, we divide these objects into four types from the perspective of physical classification, which include classical Be stars, Herbig Ae/Be stars, close binaries and spectra contaminated by H n regions. The majority of Herbig Ae/Be stars and classical Be stars are identified and separated using a （H-K, K-W1） color-color diagram. We also discuss 31 binary systems that are listed in the SIMBAD on-line catalog and identify 3600 spectra contaminated by HⅡ regions after cross-matching with positions in the Dubout-Crillon catalog. A statistical analysis of line profiles versus classifications is then conducted in order to understand the distribution of Hc~ profiles for each type in our sample. Finally, we also provide a table of 172 spectra with Fen emission lines and roughly calculate stellar wind velocities for seven spectra with P-Cygni profiles.

Stellar spectra association rule mining method based on the weighted frequent pattern tree

Effective extraction of data association rules can provide a reliable basis for classification of stellar spectra. The concept of stellar spectrum weighted itemsets and stellar spectrum weighted association rules are introduced, and the weight of a single property in the stellar spectrum is determined by information entropy. On that basis, a method is presented to mine the association rules of a stellar spectrum based on the weighted frequent pattern tree. Important properties of the spectral line are highlighted using this method. At the same time, the waveform of the whole spectrum is taken into account. The experimental results show that the data association rules of a stellar spectrum mined with this method are consistent with the main features of stellar spectral types.

M Dwarf catalog of LAMOST general survey data release one

We present a spectroscopic catalog of 93 619 M dwarfs from the first data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope（LAMOST）general survey. During sample selection, M giant contamination was eliminated using2 MASS photometry and Ca H/TiO molecular indices. For each spectrum, the spectral subtype and values are provided including radial velocity, Hα equivalent width, a series of prominent molecular band indices, and the metal–sensitive parameter ζ, as well as distances and the space motions for high S/N objects. In addition, Hα emission lines are measured to examine the magnetic activity properties of M dwarfs and 7179 active ones are found. In particular, a subsample with significant variation in magnetic activity is revealed through observations from different epochs. Finally, statistical analysis for this sample is performed, including the metallicity classification, the distribution of molecular band indices and their errors.

A sample of galaxy pairs identified from the LAMOST spectral survey and the Sloan Digital Sky Survey

A small fraction （〈 10%） of the SDSS main galaxy （MG） sample has not been targeted with spectroscopy due to the effect of fiber collisions. These galaxies have been compiled into the input catalog of the LAMOST ExtraGAlactic Surveys and named the complementary galaxy sample. In this paper, we introduce this project and status of the spectroscopies associated with the complementary galaxies in the first two years of the LAMOST spectral survey （till Sep. of 2014）. Moreover, we present a sample of 1102 galaxy pairs identified from the LAMOST complementary galaxies and SDSS MGs, which are defined as two members that have a projected distance smaller than 100 h^-01kpc and a recessional velocity difference smaller than 500 km s-1. Compared with galaxy pairs that are only selected from SDSS, the LAMOST- SDSS pairs have the advantages of not being biased toward large separations and therefore act as a useful supplement in statistical studies of galaxy interaction and galaxy merging.

Nearby M subdwarfs from LAMOST data release 2

We identify 108 M subdwarfs（sd Ms） out of more than two hundred thousand M type spectra from the second data release（DR2） of the LAMOST regular survey. This sample, among which 58 members are identified for the first time, includes 33 extreme subdwarfs（esd Ms） and 11 ultra subdwarfs（usd Ms）.The selection is based on the usual ratio of absorption depth of Ca H2, Ca H3 and TiO 5 band systems.We also emphasize the use of the Ca H1 band. We provide estimates of spectral subtype（SPT）, L′epine metallicity index ζ, effective temperature and [Fe/H]. Both ζ–[Fe/H] and SPT–Teff figures show reasonable consistency; compared to PHOENIX model spectra, average rounded values of [Fe/H] for sd Ms, esd Ms and usd Ms are respectively –0.5, –1 and –1.5. The photometric distances are estimated, indicating that most sources are located within 500 pc of the Sun and 350 pc of the Galactic disk. Velocities and 3D Galactic motions are also briefly discussed. Among the 108 subdwarfs, seven stars appear to be active with a significant Hα emission line. The source LAMOST J104521.52＋482823.3 is a white dwarf- M subdwarf binary, while LAMOST J123045.52＋410943.8, also active, exhibits carbon features in red.

A Study on Low Frequency Electromagnetic Cyclotron Waves in the Solar Wind

According to Wind observations between 2005 and 2015,this paper investigates the dependences of the occurrence of low frequency electromagnetic cyclotron waves(ECWs)on the plasma parameters,the solar cycle,and the orientations of alpha-proton drift velocity(Vd)and the ambient magnetic field(B).The occurrence rates of ECWs with respect to six plasma parameters are calculated.Results show that the preferential conditions for generation of left-handed(LH)ECWs are higher proton temperature(Tp),higher proton velocity(Vp),lower proton density(Np),stronger proton temperature anisotropy(T⊥/T‖),higher normalized alpha-proton drift velocity(Vd/VA),and higher normalized alpha particle density(Nα/Np),where T⊥and T‖refer to proton temperatures perpendicular and parallel to B,and VAis the local Alfvén velocity.For right-handed(RH)ECWs,however,the dependences on these plasma parameters are not obvious.On the other hand,it is found that the occurrence rate of LH ECWs increases as the sunspot number decreases,and decreases as the sunspot number increases.Further investigation shows that the increased occurrence rate of LH ECWs is accompanied by an increase in the medians of Vp,Vd/VA,and Nα/Np.For RH ECWs,the occurrence rate appears to be nearly unrelated to the sunspot number,suggesting a negligible correlation with the solar cycle.In addition,a modified angle is introduced to include the factors of orientation of B(anti-sunward or sunward)and angle between Vdand B,simultaneously.It is found that the occurrence of LH ECWs has the strong preferential condition that Vdis anti-sunward,while a different situation arises for RH ECWs.These results are discussed in the context of the temperature-anisotropy-driven instabilities with the effect of alpha particles.

A sample of E+A galaxy candidates in the Second Data Release of LAMOST Survey

A sample of 70 E＋A galaxies is selected from 37 206 galaxies in the second data release of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope（LAMOST）. This sample is selected according to the criteria for E＋A galaxies defined by Goto, and each of these objects is further visually identified. In this sample, most objects are low redshift E＋A galaxies with z 〈 0.25, and are located in an area of the sky with high Galactic latitude and magnitude from 14 to 18 mag in the g, r and i bands. A stellar population analysis of the whole sample indicates that the E＋A galaxies are characterized by both young and old stellar populations（SPs）, and the metalrich SPs have relatively higher contributions than the metal-poor ones. Additionally, a morphological classification of these objects is performed based on images taken from the Sloan Digital Sky Survey.

Sediment supply mechanism and its influence on hydrocarbon accumulation in the Paleogene of the northern Huanghekou Sag,Bohai Bay Basin,China

Tectonic activity and sediment supply have significantly affected the rifted basin fillings and hydrocarbon accumulations.The Huanghekou Sag in the southeastern Bohai Bay Basin is a hydrocarbon rich sag and was bounded by the Bonan Uplift to the north in the Paleogene,which strongly affected sediment disribution and hydrocarbon accumulation in this region.The sediment supply mechanism of the Paleogene parent rocks in this area has not been revealed yet.Therefore,the Huanghekou Sag is a natural laboratory to reveal the relationship between the tectonic activties and hydrcoarbon accumulations in a rifted basin.In this study,the tectonic evolution characteristics of the Bonan Uplift in different geological periods are revealed during the main fault depression periods.Then,the direction of the provenance and distribution of the depositional system of the Bonan Uplift are analyzed based on the sedimentary system and mineralogy analysis.Finally,the sediment supply in the Bonan Uplift in different geological periods has been systematically studied.The results indicated that sediment supply is the dominant factor of hydrocarbon accumulation in the Paleogene in the Huanghekou Sag.During the Paleocene-Middle Eocene,extensive denudation occurred in the Bonan Uplift,sediments were mainly sourced from the Bonan Uplift in the northern margin of the Huanghekou Sag.During the Late Eocene to Early Oligocene,the Bonan Uplift had a large angle rotation and subsidence due to the strong rifting of the Bozhong Sag in the north,and the sand supply capacity was seriously reduced.Due to the different sediment supply in different geological periods,high quality reservoir and cap conditions were formed in the Paleogene in the central and northern parts of the Huanghekou Sag,which is conducive to the accumulation and preservation of oil and gas.