OH/IR stars with SiC circumstellar envelopes

From the LRS spectra of almost 2000 IRAS sources showing the 1612 MHz OH emission we found 9 candidates for OH/IR stars with SiC features in their envelopes. Further study indicates that 6 of these are definitely or very probably carbon star with the 11.3 μm SiC emission feature in addition of being OH/IR stars showing the 1612 MHz OH maser emission.     

The spatial evolution of stellar structures in the Large Magellanic Cloud

We present an analysis of the spatial distribution of various stellar populations within the Large Magellanic Cloud (LMC). We combine mid-infrared selected young stellar objects, optically selected samples with mean ages between ∼9 and ∼1000 Myr and existing stellar cluster catalogues to investigate how stellar structures form and evolve within the LMC. For the analysis we use Fractured Minimum Spanning Trees, the statistical Q parameter and the two-point correlation function. Restricting our analysis to young massive (OB) stars, we confirm our results obtained for M33, namely that the luminosity function of the groups is well described by a power law with index −2, and that there is no characteristic length-scale of star-forming regions. We find that stars in the LMC are born with a large amount of substructure, consistent with a two-dimensional fractal distribution with dimension     and evolve towards a uniform distribution on a time-scale of ∼175 Myr. This is comparable to the crossing time of the galaxy, and we suggest that stellar structure, regardless of spatial scale, will be eliminated in a crossing time. This may explain the smooth distribution of stars in massive/dense young clusters in the Galaxy, while other, less massive, clusters still display large amounts of structure at similar ages. By comparing the stellar and star cluster distributions and evolving time-scales, we show that infant mortality of clusters (or 'popping clusters') has a negligible influence on the galactic structure. Finally, we quantify the influence of the elongation, differential extinction and contamination of a population on the measured Q value.     

A High-accuracy, Small Field of View Star Guider with Application to SNAP

To accomplish its mission, the spaceborne observatory SNAP (SuperNova Acceleration Probe) requires a pointing stability of <0.03 arcseconds during exposures lasting up to 500 sec. A Monte Carlo simulation of the photoelectron statistics from the guiding star investigates geometrical (such as the pixel size of the detector or the plate scale) and physical parameters (such as the magnitude of the star). It is shown that simple centroiding calculations can lead to the desired accuracy with guide stars as faint as magnitude 16. Availability of these stars is verified thanks to the HST Guide Star Catalog complemented with a statistical model of the distribution of stars. Thus a through-the-lens sensor that uses stars as faint as magnitude 16 to provide the necessary guiding signals is feasible. This revised version was published online in July 2006 with corrections to the Cover Date.     

超新星在旋涡星系中的径向分布

本文研究了在1993年3月28日之前发现的899颗超新星（SNe）的样本．其中277颗SNe被用来研究超新星在其母星系中的径向分布．我们研究了四个星系样本中超新星在产生单个超新星（称为一般超新星）的星系及在产生多个超新星（称为多重超新星）的星系中的径向分布．这四个星系样本为：总旋涡星系样本，Sb～SbC星系样本，SC星系样本，Sc～Sd星系样本．研究的结果表明：一多重超新星比一般超新星在其母星系中具有更集中于星系核心（大多数恒星形成发生的地方）的倾向，说明星系中心的恒星形成活动会影响多重超新星事件的发生从而影响超新星的频率分布     

Wide Binaries from Few-Body Interactions

The orbital properties of wide binaries are modified by stellar encounters during their lifetimes as field stars. The few body process is particularly efficient right after the star formation in star clusters. In this article we review the orbital parameters which we expect from the few-body process. First, the solutions of the General Three-Body Problem are described, and then a detailed discussion of the theory relevant to binaries follows. Among other things, Öpik's law of the distribution of semi- major axes of the binaries is derived. In addition, we summarize recent results from the few-body problem of extended bodies which may be relevant to the early dynamical evolution of protostellar clouds.     

GD-1星流金属丰度与运动特性分析

星流在星系形成与演化过程中扮演了重要的角色,对银河系中星流的研究将有助于进一步探究银河系的合并历史.将LAMOST(Large Sky Area Multi-Object Fiber Spectroscopic Telescope)DR6光谱数据以及SDSS(Sloan Digital Sky Survey)DR12光谱数据分别与Gaia(Global Astrometric Interferometer for Astrophysics)DR2天体测量数据交叉匹配,获得恒星自行等数据.对GD-1星流在速度空间、几何空间和金属丰度上进行限制,从LAMOST DR6和SDSS DR12数据中共获得了157颗星流成员星.GD-1星流的平均金属丰度为[Fe/H]=-2.16±0.10 dex,延伸长度超过80°.收集前人给出的GD-1星流高概率成员星,组成较大的成员星样本进行对比分析,发现GD-1星流的金属丰度分布呈现内低外高的特点,沿着星流方向径向速度分布特点是两端大、中间小,?₁=-20°(?₁为GD-1星流坐标系横坐标)和?₁=-60°附近的间隙是因为成员星运动差异形成的.根据成员星分布及其速度分布特性,推测GD-1星流起源位置是在?₁=-40°附近.     

Generalized Lyapunov exponents indicators in Hamiltonian dynamics: An application to a double star system

The Lyapunov characteristic numbers (LCNs) which are defined as the mean value of the distribution of the local variations of the tangent vectors to the flow (=ln _k ⁱ ) (see Froeschlé, 1984) have been found to be sensitive indicators of stochasticity. So we computed the distribution of these local variations and determined the moments of higher order for the integrable and stochastic regions in a binary star system with =0.5.     

Analysis of the mass functions for three open star clusters

Based on our own observational data, we have constructed the mass functions for three young-and intermediate-age Galactic star clusters, Trumpler 2, NGC 7243, and NGC 1513. Their mean slopes Γ fall within the Scalo range: Γ = ?1.7 ± 0.5 for 1M _⊙ < M < 10M _⊙. For each of the clusters, we also analyze the mass distribution of their members as a function of the distance to the center. Noticeable mass segregation in the sense of massive-star concentration toward the cluster center has been detected in Trumpler 2 and NGC 7243. Deviations from rectilinearity in the same direction are observed at log M ≈ 0.41 in each of the three zones of Trumpler 2, which can be interpreted as a small dip in the mass function. We have found peculiarities in NGC 1513 that are difficult to explain by observational selection alone. Comparison of our results with similar studies reveals a similarity in radial Γ distribution for NGC 7243 with h Per and for Trumpler 2 with IC 1805.     

Kinematics of the Scorpius-Centaurus OB association

A fine structure related to the kinematic peculiarities of three components of the Scorpius-Centaurus association (LCC, UCL, and US) has been revealed in the UV-velocity distribution of Gould Belt stars. We have been able to identify the most likely members of these groups by applying the method of analyzing the two-dimensional probability density function of stellar UV velocities that we developed. A kinematic analysis of the identified structural components has shown that, in general, the center-of-mass motion of the LCC, UCL, and US groups follows the motion characteristic of the Gould Belt, notably its expansion. The entire Scorpius-Centaurus complex is shown to possess a proper expansion with an angular velocity parameter of 46 ± 8 km s^?1 kpc^?1 for the kinematic center with l ₀ = ?40° and R ₀ = 110 pc found. Based on this velocity, we have estimated the characteristic expansion time of the complex to be 21 ± 4 Myr. The proper rotation velocity of the Scorpius-Centaurus complex is lower in magnitude, is determined less reliably, and depends markedly on the data quality.     

RX J1856.5-3754 as a possible strange star candidate

RX J1856.5-3754 has been proposed as a strange star candidate due to its very small apparent radius measured from its X-ray thermal spectrum. However, its optical emission requires a much larger radius and thus most of the stellar surface must be cold and undetectable in X-rays. In the case the star is a neutron star such a surface temperature distribution can be explained by the presence of a strong toroidal field in the crust (Pérez-Azorín et al.: Astron. Astrophys. 451, 1009 (2006); Geppert et al.: Astron. Astrophys. 457, 937 (2006)) We consider a similar scenario for a strange star with a thin baryonic crust to determine if such a magnetic field induced effect is still possible. This work was partially supported by PAPIIT, UNAM, grant IN119306. J.A.H. studies at UNAM and travel to London are covered by fellowships from UNAM’s Dirección General de Estudios de Posgrado.     

On the shell star pleione (BU Tauri)

BU Tauri (Pleione) an interesting star in the Pleiades cluster, has been observed spectrophotometrically. The energy distribution curves of the star have been discussed vis a vis model atmospheres for normal stars in the appropriate range of temperature and effective gravity. The changes in the energy distribution curve noticed during our observations and previous observations taken from the literature have been pointed out. On the basis of the measured Há emission equivalent width, a rough estimate of the dimensions of the extended envelope of the star has been made.     

The ALFA Laser: Beam Relay and Control System

The ALFA laser subsystem uses a 4 watt continuous wave laser beam to produce an artificial guide star in the mesospheric sodium layer as a reference for wavefront sensing. In this article we describe the system design, focusing on the layout of the beam relay system. It consists of seven mirrors, four of which are motor-controlled in closed loop operation accounting for turbulences inside the dome and flexure of the main telescope.The control system features several computers which are located close to analysis and control units. The distribution of the tasks and their interaction is presented, as well as the graphical user interface used to operate the complete system. This is followed by a discussion of the aircraft detection system ALIENS. This system shuts off the laser beam when an object passes close to the outgoing laser.     

Compact stellar systems around NGC 1399

We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES (Very Large Telescope-Fibre Large Array Multi Element Spectrograph) fields around the Fornax cluster giant elliptical galaxy NGC 1399, identifying as cluster members 27 previously unknown faint     compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS.
By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 'unbound' systems that are either part of NGC  1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour–colour space; (ii) their projected distribution about NGC  1399 is anisotropic, following the Fornax cluster galaxy distribution, and there is weak evidence for group rotation about NGC  1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC  1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC  1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS  ( M _V < −11)  are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in   g '− i '  colour index with radial distance from NGC  1399.     

Cooling of neutron stars with strong toroidal magnetic fields

We present models of temperature distribution in the crust of a neutron star in the presence of a strong toroidal component superposed to the poloidal component of the magnetic field. The presence of such a toroidal field hinders heat flow toward the surface in a large part of the crust. As a result, the neutron star surface presents two warm regions surrounded by extended cold regions and has a thermal luminosity much lower than in the case the magnetic field is purely poloidal. We apply these models to calculate the thermal evolution of such neutron stars and show that the lowered photon luminosity naturally extends their life-time as detectable thermal X-ray sources. Work partially supported by UNAM-DGAPA grant #IN119306.     

射电脉冲星的逆康普顿散射（ICS）模型和偏振观测

近来我们在射电脉冲星的逆康普顿散射模型,致密星物理以及一些相关问题上做了下列几项工作：１） 对脉冲星物理中的一系列基本理论进行了较深入的研究,包括对裸奇异星比中子星更可能是射电脉冲星之起源的观点的论证。２） 进一步发展了射电脉冲星的逆康普顿散射（ＩＣＳ） 模型并完成了一系列相关工作。３） 进行了脉冲星的偏振分析和有关观测工作,并用其结果对银河系磁场和旋臂结构进行了分析。４） 研究了黑洞脉冲星双星系统的一些特性,为黑洞的搜寻工作作了初步准备。     

2D spectroscopy of galaxies with star formation regions. Study of SBS 1533+574

A preliminary analysis is given of 2D spectroscopic data on the galaxy SBS 1533+574(AB) obtained using the multipupil spectrographs on the 2.6-m telescope at the BAO (VAGR) and the 6-m telescope at the SAO (MPFS). The two components of the galaxy are star formation regions in different stages. The component SBS 1533+574B is known to be a BCDG. The plots of the intensity distribution of the radiation in the recombination lines of hydrogen and the forbidden lines of gases with a low degree of ionization obtained here make it possible to compare the basic characteristics of the HII-zones and the surrounding shell. The velocity distribution over the field of the galaxy is indicative of a common rotation of the system and of an intrinsic rotation of the components which is more distinct for component B. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 511–525 (November 2006).     

High-mass X-ray binaries and recent star formation history of the Small Magellanic Cloud

We study the relation between high-mass X-ray binary (HMXB) population and recent star formation history (SFH) for the Small Magellanic Cloud (SMC). Using archival optical SMC observations, we have approximated the color-magnitude diagrams of the stellar population by model stellar populations and, in this way, reconstructed the spatially resolved SFH of the galaxy over the past 100 Myr. We analyze the errors and stability of this method for determining the recent SFH and show that uncertainties in the models of massive stars at late evolutionary stages are the main factor that limits its accuracy. By combining the SFH with the spatial distribution of HMXBs obtained from XMM-Newton observations, we have derived the dependence of the HMXB number on the time elapsed since the star formation event. The number of young systems with ages ? 10 Myr is shown to be smaller than the prediction based on the type-II supernova rate. The HMXB number reaches its maximum ～20–50 Myr after the star formation event. This may be attributable, at least partly, to a low luminosity threshold in the population of X-ray sources studied, L _min ～ 10³⁴ erg s^?1. Be/X systems make a dominant contribution to this population, while the contribution from HMXBs with black holes is relatively small.     

Galaxy properties derived with spectral energy distribution fitting in the Hawaii-Hubble Deep Field-North

We compile multi-wavelength data from ultraviolet to infrared(IR) bands as well as redshift and source-type information, for a large sample of 178 341 sources in the Hawaii-Hubble Deep Field-North field. A total of 145 635 sources among the full sample are classified/treated as galaxies and have redshift information available. We derive physical properties for these sources utilizing the spectral energy distribution fitting code CIGALE that is based on Bayesian analysis. Through various consistency and robustness checks, we find that our stellar-mass and star-formation rate(SFR) estimates are reliable, which is mainly due to two facts. Firstly, we adopt the most up-to-date and accurate redshifts and point spread functionmatched photometry; and secondly, we make sensible parameter choices with the CIGALE code and take into account the influences of mid-IR/far-IR data, star-formation history models, and AGN contribution. We release our catalog of galaxy properties publicly(including, e.g., redshift, stellar mass, SFR, age, metallicity, dust attenuation). It is the largest of its kind in this field and should facilitate future relevant studies on the formation and evolution of galaxies.     

Evolution of The Spin Periods of Neutron Stars in Low Mass X-ray Binaries

We present a numerical analysis of the spin evolution of neutron stars in low-mass X-ray binaries, trying to explain the discrepancy in the spin period distribution between observations of millisecond pulsars and theoretical results. In our calculations, we take account of possible effects of radiation pressure and irradiation-induced instability on the structure of the disk, and the evolution of the mass transfer rate, respectively. We report the following results: (1) The radiation pressure in the accretion disk leads to a slight increase of spin periods, and the variation of mass transfer rate caused by the neutron star irradiation can shorten the spin-down phase of evolution. (2) The calculated results of the model combining radiation pressure and irradiation show that the accretion is strongly limited by the radiation pressure in the high mass transfer phase. (3) The accreted mass and fastness parameter can affect the number of systems in the equilibrium state.     

The Effect of Method of Constructing the Mass Distribution on Single Stellar Populations

We use two methods of constructing the initial mass distribution, the traditional way and Monte Carlo simulation, to obtain integrated U - B, B - V, V-R and V-I colours and absorption-line indices denned by the Lick Observatory image dissector scanner (referred to as Lick/IDS), for instantaneous burst solarmetallicity single stellar populations with ages in the range 1-15 Gyr. We find that the evolutionary curves of all colours obtained by the traditional method are smoother than those by Monte Carlo simulation, that the U - B and B - V colours obtained by the two methods agree with one another, while the V - R and V - I colours by the traditional method are bluer than those by Monte Carlo simulation. A comparison of the Lick/IDS absorption-line indices shows that the variations in all the indices by the traditional method are smoother than that for the Monte Carlo simulation, and that all the indices except for TiO1 and TiO2 are consistent with those for the Monte Carlo simulation.