测定天文大气折射和建立电磁波折射延迟实测模型(Ⅱ)-采用依巴谷星表作为工作星表

叙述了测定大气折射对工作星表的要求和星表系统误差的影响,并通过FK5星表相对于依巴谷星表的系统误差,来说明以往的各种基本星表都不适用的原因;文章说明了依巴谷星表所列星位置不含随天区而异的系统误差的特性,分析了该星表问世时的精度和十多年后的今天仍能达到的精度,说明它可以作为这种测定的工作星表,从而为直接测定天文大气折射值和折射率差提供了一个重要的条件;文章还介绍了依巴谷星表的星等分布,认为只需选用亮于6mag的依巴谷星,这一星等范围,为制定专用测量仪器终端的设计方案提供了依据。     

FK5星表和依巴谷星表的比较

由ＦＫ５星表和依巴谷星表在不同历元上的比较、检测和讨论了ＦＫ５星表的系统差．结果表明，ＦＫ５星表的自行系统差可能是影响ＦＫ５星表位置系统差的主要来源．在一些研究工作中，例如在依巴谷星表系统中研究地球自转的长期变化或以前的照相观测结果的重新归算，需要考虑依巴谷星表自行个别误差的影响     

FK5相对于依巴谷星表的系统差及其对EOP的影响

依巴谷星表在建立时已做到与以河外射电源定义的国际天球参考系（ICRS）相符合,而ICRS系统与由FK5星表实现的由J2000.0平赤道和平春分点定义的赤道坐标系统相一致。估计依巴谷星表的系统误差小于0.1mas,因此,依巴谷星表成为描述光学观测确定的地球定向参数（EOP）的一个更为均匀的参考系。本文主要讨论了FK5和依巴谷星表之间的星表系统差（FK5－Hip）,并用中国光电等高星表来检验（FK5－Hip）星表系统差的结果,并估算了星表系统差（FK5－Hip）对EOP的影响。     

依巴谷天文卫星及其输入星表数据库

本文简要介绍了欧洲空间局通过阿利亚那火箭发射的依巴谷天文卫星,也概述了为依巴谷卫星探测计划服务,并积累和编辑其观测资料的输入星表数据库。     

天体测量星表与巡天观测的进展

该文简述了巡天观测和星表编制的历史和意义.介绍了天体测量星表与巡天观测的最新进展,如位置星表URAT、Pan-STARSS、LSST;自行星表LSPM、PM2000、BDKP;视差星表USNO,CTOPI、L和T型星的视差测定等.简略介绍了天体物理星表,其中包括测光星表GCPD、UBV、GSPC;光谱星表MSS、BDSS,SDSS和视向速度星表GCRV、CORALIE、RAVE.特别描述了恒星名至今仍在使用的20世纪初编制的Durchmusterung、Lowell、HenryDraper星表.描述了近年来普遍受到人们关注的联合星表编制的发展,如SKYMAP、距离太阳10 pc内的近距星表RECONS、描述了局部天区的HDF、双星星表等.另外还介绍了其他波段的巡天观测,如射电的NVSS、FIRST;近红外的IRAS PSC和FSS;X波段的WGCAT星表等.最后,对今后开展巡天观测和编制星表提出了应注意的几个问题.     

观测射电星联系依巴谷星表和地面光学星表自行的精度估计

用蒙特卡罗方法对佘山40厘米望远镜射电星的观测模拟，由照相测定的自行来估计依巴谷星表和光学星表自行间联系的精度。结果表明由90颗星估计的联系精度为0″.16／百年。     

近三年来国际上天体测量研究的进展

本文简述近三年来国际上天体测量的研究进展 ,特别是依巴谷星表向暗星扩充、2 0世纪初照相观测改进自行的精度和巡天观测资料对特殊天体物理现象的检测等方面的研究动态。最后介绍了空间天体测量卫星DIVA、GAIA、SIM和FAME计划的进展。     

建立经纬度异常变化三角监测网的观测纲要和工作星表

为了在云南省及周边地区强地震前提供天文经、纬度异常变化的信息而建立三角监测网的需要,本文叙述了在各观测站都采用的两种观测纲要的编制方法.一是8个方位的观测纲要,通过一年观测数据的比较,拟合出各观测站的本地天文大气折射实用模型;二是长期使用的仅在子午方向观测的纲要.文章根据这种测量对工作星表的要求,在分析了依巴谷星表的亮星分布情况后,决定仅选用4.0～6.0mag的光谱型B、A、F、G、K型的依巴谷星.这一选星范围,也为测量仪器口径的选择提供了依据.     

大气反常折射对星表和参考系的影响

在过去的时间纬度工作中,对大气反常折射的研究证明,大气反常折射给时问纬度测量带来了严重的影响。而当时大多数星表的编制都是时间纬度测量的副产品,无疑大气反常折射也给这些星表带来严重的误差影响。过去我国的经典天体测量工作以测时、测纬为主,这一问题并没有得到重视。在星表和参考系工作已成为我国基本天休测量工作的一项主要内容的今天,对大气反常折射的测量与研究,尤其对有效的测量应得到足够的重视。作者根据自己的实际测量和研究指出,大气反常折射主要由观测室周围不良小气候的造成,其中以具有周年变化和周日变化的大气反常折射对初始星表系统的影响最为严重。并分别讨论了这种反常折射对子午星表、等高星表和参考系的影响,提出了今后开展这方面工作的建议。     

用地面测量建立高精度天图星表的初步方案—为适应小卫星精密定位和银河系研

以小卫星精密定位为例,叙述了今后一段时间内对天图星表的组成和精度要求,分析了目前现有天图星表尚不能满足这一要求的原因,提出了在依巴谷星表的基础上用地面测量方法建立所要求的天图星表的设想,拟定了分两步走的初步方案,阐述了采用新的误差理论和新技术后地面测量能达到这一精度的可能性。     

Star complexes and associations in the Andromeda galaxy

Large-scaleU andB plates obtained with the 2 m Ritchey-Chrétien telescope of the Rozhen Observatory (Bulgaria) were searched for new resolved star groups and for independent delineation of the boundaries of the known ones in M31. We detected 210 groups as real O-associations the mean diameter of which is 80 pc. Many of Hodge's open clusters are also reclassified as O-associations. The majority of van den Bergh's OB-associations were recognized as star complexes and their mean diameter is 650 pc. Almost all O-associations are located inside the star complexes. A dozen of new star complexes (mainly around the dark lanes between OB78 and OB22) and numerous groups presumably not containing O-stars were found out. The nature of these groups has to be object of further investigations. Young star groups closer than 3 kpc to the center of M31 were not identified.     

A multiwavelength study of the early evolution of the classical nova LMC 1988 1

We study star-formation-inducing mechanisms in galaxies through multiwavelength measurements of a sample of dwarf galaxies in the Virgo cluster described in Paper I. Our main goal is to test how star-formation-inducing mechanisms depend on several parameters of the galaxies, such as morphological type and hydrogen content. We derive the star formation rate and star formation histories of the galaxies, and check their dependence on other parameters.   Comparison of the sample galaxies with population synthesis models shows that these objects have significantly lower metallicity than the solar value. The colours can generally be explained as a combination of two different stellar populations: a young (3–20 Myr) metal-poor population which represents the stars currently forming presumably in a starburst, and an older (0.1–1 Gyr) population of previous stellar generations. There is evidence that the older stellar population was also formed in a starburst. This is consistent with the explanation that star formation in this type of objects takes place in short bursts followed by long quiescent periods.   No significant correlation is found between the star formation properties of the sample galaxies and their hydrogen content. Apparently, when star formation occurs in bursts, other parameters influence the star formation properties more significantly than the amount of atomic hydrogen. No correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies, suggesting that tidal interactions are not significant in triggering star formation in cluster dwarf galaxies.     

The masses, radii and luminosities of the components of U Geminorum

We present a phase-resolved spectroscopic study of the secondary star in the cataclysmic variable (CV) U Gem. We use our data to measure the radial velocity semi-amplitude, systemic velocity and rotational velocity of the secondary star. Combining this with literature data allows us to determine masses and radii for both the secondary star and white dwarf, which are independent of any assumptions about their structure. We use these to compare their properties to those of field stars and find that both components follow field mass–radius relationships. The secondary star has the mass, radius, luminosity and photometric temperature of an M2 star, but a spectroscopic temperature of M4. The latter may well be due to a high metallicity. There is a troubling inconsistency between the radius of the white dwarf inferred from its gravitational redshift and inclination and that inferred from its temperature, flux and astrometric distance.
We find that there are two fundamental limits to the accuracy of the parameters we can derive. First, the radial velocity curve of the secondary star deviates from a sinusoid, in part because of its asphericity (which can be modelled) and in part because the line flux is not evenly distributed over its surface. Secondly, we cannot be certain which spectral type is the best match for the lines of the secondary star, and the derived rotational velocity is a function of the spectral type of the template star used.     

WR 146 – observing the OB-type companion

We present new radio and optical observations of the colliding-wind system WR 146 aimed at understanding the nature of the companion to the Wolf–Rayet (WR) star and the collision of their winds. The radio observations reveal emission from three components: the WR stellar wind, the non-thermal wind–wind interaction region and, for the first time, the stellar wind of the OB companion. This provides the unique possibility of determining the mass-loss rate and terminal wind velocity ratios of the two winds, independent of distance. Respectively, these ratios are 0.20±0.06 and 0.56±0.17 for the OB-companion star relative to the WR star. A new optical spectrum indicates that the system is more luminous than had been believed previously. We deduce that the 'companion' cannot be a single, low-luminosity O8 star as suggested previously, but is either a high-luminosity O8 star, or possibly an O8+WC binary system.     

A simple model for the relationship between star formation and surface density

We investigate the relationship between the star formation rate per unit area and the surface density of the interstellar medium (ISM; the local Kennicutt–Schmitt law) using a simplified model of the ISM and a simple estimate of the star formation rate based on the mass of gas in bound clumps, the local dynamical time-scales of the clumps and an efficiency parameter of around  ε≈ 5  per cent. Despite the simplicity of the approach, we are able to reproduce the observed linear relation between star formation rate and surface density of dense (molecular) gas. We use a simple model for the dependence of H₂ fraction on total surface density to argue why neither total surface density nor the H  i surface density is a good local indicator of star formation rate. We also investigate the dependence of the star formation rate on the depth of the spiral potential. Our model indicates that the mean star formation rate does not depend significantly on the strength of the spiral potential, but that a stronger spiral potential, for a given mean surface density, does result in more of the star formation occurring close to the spiral arms. This agrees with the observation that grand design galaxies do not appear to show a larger degree of star formation compared to their flocculent counterparts.     

Star formation in galaxies along the Pisces-Cetus Supercluster filaments

We investigate the variation of current star formation in galaxies as a function of distance along three supercluster filaments, each joining pairs of rich clusters, in the Pisces-Cetus supercluster, which is part of the two-degree Field Galaxy Redshift Survey (2dFGRS). We find that even though there is a steady decline in the rate of star formation, as well as in the fraction of star-forming galaxies, as one approaches the core of a cluster at an extremity of such a filament, there is an increased activity of star formation in a narrow distance range between 3 and  4  h ⁻¹₇₀ Mpc  , which is 1.5–2 times the virial radius of the clusters involved. This peak in star formation is seen to be entirely due to the dwarf galaxies  (−20 < M _B ≤−17.5)  . The position of the peak does not seem to depend on the velocity dispersion of the nearest cluster, undermining the importance of the gravitational effect of the clusters involved. We find that this enhancement in star formation occurs at the same place for galaxies which belong to groups within these filaments, while group members elsewhere in the 2dFGRS do not show this effect. We conclude that the most likely mechanism for this enhanced star formation is galaxy–galaxy harassment, in the crowded infalling region of rich clusters at the extremities of filaments, which induces a burst of star formation in galaxies, before they have been stripped of their gas in the denser cores of clusters. The effects of strangulation in the cores of clusters, as well as excess star formation in the infalling regions along the filaments, are more pronounced in dwarfs since they more vulnerable to the effects of strangulation and harassment than giant galaxies.     

The Effect of AGN and SNe Feedback on Star Formation, Reionization and the Near Infrared Background

Feedback from supernovae (SNe) and from active galactic nuclei (AGN) accom-panies the history of star formation and galaxy evolution. We present an analytic model to explain how and when the SNe and AGN exert their feedback effects on the star formation and galaxy evolution processes. By using SNe and AGN kinetic feedback mechanisms based on the Lambda Cold Dark Matter (LCDM) model, we explore how these feedback mecha-nisms affect the star formation history (SFH), the Near-Infrared Background (NIRB) flux and the cosmological reionization. We find the values of the feedback strengths, ∈AGN =1.0+0.50.3and ∈SN=0.04+0.02-0.02, can provide a reasonable explanation of most of the observational re-suits, and that the AGN feedback effect on star formation history is quite different from the SNe feedback at high redshifts. Our conclusions manifest quantitatively that these feedback effects decrease star formation rate density (SFRD) and the NIRB flux (in 1.4 - 4.0μm), and postpone the time of completion of the cosmological reionization.     

Binaries and multiple systems among the young stars

Binary formation is the dominant mode of star formation in the nearby Taurus and Ophiuchus star forming regions. I describe very briefly our present understanding of multiplicity among the youngest stars, the evolution and structure of their circumstellar disks, and their potential as sites of planetary system formation.     

Star formation in the Small Magellanic Cloud: the youngest star clusters

We combined optical Hubble Space Telescope ACS images with mid-infrared Spitzer data of the two young star clusters NGC346 and NGC602 in the Small Magellanic Cloud, to study how local and global conditions may affect the process of star formation. We found that, while general conditions such as metallicity, or the mass or morphological type of the parent galaxy do not strongly affect the process of star formation, local conditions such as the gas and stellar density can affect both how star formation occurs and propagates, and also the evolution of a star cluster from early times.     

The PMS δ Scuti star PDS2

The Herbig Ae star PDS2 was found to pulsate as a Pre-Main-Sequence delta Scuti star on the basis of R.E.M. telescope photometry. From the frequency analysis we have found four significant pulsation frequencies. We discuss these results and their interpretation through the adoption of nonradial pulsation models, deriving the first information on the intrinsic properties of this young star.