类太阳色球活动恒星的高色散光谱观测和锂丰度

基于对9颗类太阳色球活动恒星高信噪比的高色散光谱观测, 测量了这些恒星锂线(入 = 6707.8 A'')的等值宽度, 计算了这些恒星表层锂元素丰度. 通过研究这些类太阳色球活动恒星锂丰度和X射线光度之间的关系, 发现X射线光度 强的类太阳色球活动恒星锂丰度值大于X射线较弱的恒星. 也就是说活动性较强的类太阳色球活动恒星其锂丰度较高, 活动性较弱的类太阳色球活动恒星其锂丰度较低. 考虑到主序的类太阳恒星锂元素和恒星自转速度随着恒星年龄的增加逐渐减少, 以及随着类太阳色球活动恒星自转速度的减小, 色球活动又逐渐变弱. 因此类似于锂丰度, 类太阳色球活动恒星自转速度的大小和恒星的 活动水平也同样可以表明恒星的年龄.     

恒星黑子的观测

本文对恒星黑子的观测方法和结果作了综合评述。介绍了恒星黑子温度的多色测光测定方法；黑子大小，温度和分布情况的测光畸变波黑子模型解，和高信噪比谱线轮廓多普勒成像方法；以及黑子寿命观测统计方法等的基本原理和研究进展。对已取得的有关恒星黑子的观测分析结果作了概括，与太阳典型黑子情况作了对比，并对恒星黑子的巨黑子，极区黑子以及长寿命黑子特殊性作了讨论，对需要进一步观测研究的问题作了总结。     

恒星球形刚体自转与谱线半强度宽

本文考虑了一个有临边昏暗的球形刚体自转恒星模型,并考虑仪器轮廓是高斯曲线,计算了一系列转致轮廓与仪器轮廓的卷积。由计算结果得到了转致轮廓半强度宽PHW与卷积后轮廓半强度宽CHW的关系图,以及仪器轮廓半强度宽GHW与卷积后轮廓半强度宽CHW的关系图。表明,当RHW/GHW>2时,就半强度宽(FWHM)而言,卷积的结果使其变窄(而不是加宽);但是,这一变化不大于RHW的3％。这一结果使得在某些情况下对不同仪器上进行的恒星自转速度测量的归算变得简单。理论计算还指出,有可能利用实测的V·sin i—FWHM关系得到临边昏暗系数ε;并利用文献中的观测资料初步得到早型恒星O9.5—B9的临边昏暗系数为0.44—0.99。     

红亚矮星研究进展

红亚矮星是甚小质量恒星中的贫金属成员,质量从约0.5M⊙(M⊙为太阳质量)到H燃烧的最小质量(0:075M⊙0:085M⊙,取决于金属丰度),其寿命长于哈勃年龄,是银河系结构和化学增丰史的重要示踪体。与银盘上数量最多的恒星成员红矮星不同,红亚矮星在太阳附近非常稀少,并且其运动学特征与盘矮星有较大差异,属于年老银河系星族,即为年老盘星族、厚盘星族或晕星族。观测上,红亚矮星可以根据其不同于红矮星的自行、测光和光谱特征被识别和证认。由于其恒星表面大气温度很低,并且颜色比同质量的矮星更蓝,因此红亚矮星在赫罗图上位于主序带末端的下方,介于矮星与白矮星之间。红亚矮星的光学波段光谱由金属氧化物(如TiO和VO)和氢化物(如CaH和H2O)的分子吸收带占主导。红亚矮星可按其光谱形态和分子带特征分成不同的光谱型和金属丰度等级,其中晚M型到早L型的亚矮星既可能是小质量的恒星,也可能是较大质量的年轻褐矮星。介绍了对红亚矮星研究的历史背景和前沿动态,详细阐述了光谱分析方法在研究亚矮星中的重要性,以及根据光谱特征对亚矮星进行分类的方法。最后,总结了甚小质量恒星大气模型的发展过程,并探讨了如何利用模型对亚矮星的大气参数进行估算等热点问题。     

星系际弥散恒星成分研究进展

对于星系际弥散恒星的研究是分别从观测、数值模拟和半解析模型这三个方面进行的.现在已经在邻近星系团及中低红移处观测到弥散恒星,甚至在Virgo及Coma星系团中观测到了单个的弥散恒星.观测数据的积累使得人们能够从统计上了解星系际弥散恒星的性质.研究表明星系际弥散恒星围绕着星系团势阱中心呈椭球状对称分布,其在星系团恒星总质...     

视宁度测量中的一个常数

大气视宁严谨严重影响了地面望远镜观测的成像分辩率。对于直径ｒＬ的单孔镜，Ｆｒｉｅｄ给出恒星光波波前到达角的均方差为〈ａ＾２〉＝０．３５８（λ／ｒＬ）＾１／３（λ／ｒ０）＾５／３这一公式被很多文献所采用，并推广应用于双瞳视宁度仪测量，等等。事实上，Ｙｕｒａ和Ｔａｖｉｓ（１９８５）指出，恒星光波波前的瞬间倾角一像重心在焦平面的角位置并不完全相关，波前倾角的方差与焦平面重心的方差也不是一致的。实际测量中     

恒星半径的测量

恒星半径自身是一个基本物理量，而且与许多其他基本物理量有关。本文综述了恒性半径测量的几种重要方法：测量恒星角径而得出其半径的月掩星，光斑干涉，干涉仪方法；直接测出恒星半径的食用双星法；通过物理关系测出其他物理量来求出恒星半径的表面这度法，绝对法和热流量法，以及专用脉动变星的Ｂａａｄｅ－Ｗｅｓｓｅｌｉｎｋ方法等。并对恒星半径的测量精度及其产生误差的原因进行了讨论。     

The Cannon:一种基于光谱数据的恒星参数测量方法

新一代大规模光谱巡天项目产生了近千万条低分辨率恒星光谱,基于这些光谱数据,介绍一种名为The Cannon的机器学习方法。该方法完全基于已知恒星大气参数(有效温度、表面重力加速度和金属丰度等)的光谱数据,通过数据驱动来构建特征向量,建立光谱流量特征和恒星参数的函数对应关系,进而应用到观测光谱数据中,实现对恒星光谱的大气参数求解。The Cannon的主要优势为不直接基于任何恒星物理模型,适用性更广;由于使用了全谱信息,即便对于低信噪比光谱也能得到较高可信度的参数结果,该算法在大规模恒星光谱的数据处理和参数求解方面具有明显的优势。此外,还利用The Cannon得到LAMOST光谱数据中K巨星和M巨星的恒星参数。     

AGB星辐射s-过程核合成模型中子辐照量分布的计算方法和特点

考虑到模型参量随脉冲数的变化,推导出13C辐射燃烧的低质量AGB星s-过程核合成模型中子辐照量分布的计算方法,该方法具有普适性和简洁性.利用该方法,计算了3 M☉、太阳金属丰度恒星模型的中子辐照量分布.结果表明,若合理假设13C薄层内中子数密度均匀分布,则辐射核合成模型最终的中子辐照量分布趋近于指数分布.对于初始质量一定、金属丰度一定的恒星模型,平均中子辐照量τ0与每个脉冲的中子辐照量△τ存在确定的关系:τ0=0.434λ(q1,q2…qmmax+1,r1,r2…rmmax+1)△τ,式中mmax是带挖掘的脉冲总数,比例系数λ(q1,q2…qmmax+1,r1,r2…rmmax+1)可通过对最终中子辐照量分布的指数拟合得到.该式从中子辐照量分布角度定量地将经典模型和辐射s-过程核合成模型统一起来,使经典模型能为恒星模型核合成数值计算提供指导和约束.     

太阳邻域银河系薄盘标高研究

利用Hipparcos卫星精确天体测量观测数据,尝试研究由不同样本恒星所反映的太阳邻域银河系薄盘的标高.根据Tycho测光系统的色指数,并考虑观测样本的完备性,分别在主序段和水平支上选取了几类恒星样本,以考察银河系薄盘标高的演化特征.分析结果发现,在比较完备的样本空间上,由O-B型主序星定义的银盘标高为103.1±3.0 pc,太阳位于其平均平面以上的15.2±7.3 pc处;水平支恒星定义的银盘标高为144.0±10.0 pc,太阳位于其平均平面以上3.5±5.4pc处.A、F、G、K、M型的主序星由于观测样本完备性的限制而无法进行可信的标高研究.     

Measurement of the limb darkening coefficient of stars

This paper introduces a new method of describing the limb darkening phenomenon of stellar atmospheres in the 1st and 2nd order approximations. The limb darkening coefficients are given by direct measurements of relevant physical quantities or measurements of flux and a supplementary quantity, namely, the star's surface temperature. The comparison either with solar observations or with the method of numerical simulation of the atmosphere shows that the new method can very accurately describe the star's limb darkening and determine its relevant coefficients. The accuracy of the new direct method is assessed using 176 observational data points of the Sun, and the mean fractional differences is found to be 0.38% for the 1st-, and 0.26% for the 2nd-order approximations. The mean differences of the indirect method are close to, and slightly larger than these values. When compared with numerical models of the atmosphere, the differences are in the range from under 1% to under 3%.     

The standard systems of stellar rotation measurements (II)

The limb darkening effect on the measurement of stellar rotation is discussed in this paper. It is shown that this effect plays an important role in the measurement of V_e sin i. In the extreme case with the limb darkening coefficient of 1.0, it may cause a difference up to 17%. As a sequel to paper [1], this work presents the following new explanation for the systematic differences between Slettebak's new and old systems. The main causes of the systematic differences are: (1) The old system made an inadequate twice repeated correction for the limb darkening. (2) Owing to the historical reason, the old system used too large limb darkening coefficients.     

Limb darkening effect included in a simple formula for solar energy flux

It is proposed a formula for the solar energy flux including the limb darkening effect and beeing valid at any distance from the Sun.     

Reflection effect in close binaries. IV. Limb darkening of the reflected radiation incident from an extended surface of the secondary

The law of limb darkening has been calculated when the atmosphere of the primary component is illuminated by the extended surface of the secondary component in a binary system. The specific intensities calculated at infinity show marked changes when the plane-parallel approximation is replaced by the assumption of spherical symmetry. The middle portions of the illuminated surface reflect maximum radiation while the innermost and outermost layers show lesser amount of reflected radiation.     

Cepheid Limb Darkening Models for the VLTI

We present a new method to compute wavelength- and phase-dependent limb darkening corrections for classical Cepheids. These corrections are derived from hydrodynamic simulations and radiative transfer modeling with a full set of atomic and molecular opacities. Comparison with hydrostatic models having the same stellar parameters show a larger limb darkening for most phases in our models, and temporal variations related to the hydrodynamics of the stellar pulsation. Weassess the implications of our results with respect to the geometric Baade-Wesselink method, which uses interferometric measurements of Cepheid angular diameters to determine their distances and radii. The relevance of the hydrodynamic effects predicted by our model on the limb darkening of pulsating Cepheids is finally discussed in terms of the predicted capabilities of the VLTI. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hubble Space Telescope observations of SV Cam – II. First derivative light-curve modelling using phoenix and atlas model atmospheres

The variation of the specific intensity across the stellar disc is an essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting star-spots. We use phoenix and atlas model atmospheres to model light curves derived from high precision (signal-to-noise ratio ≃ 5000) Hubble Space Telescope ( HST ) data of the eclipsing binary SV Cam (F9V+K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system light curve. For the first time, we use χ² comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (<1σ) between the χ² comparison fits of the two model atmospheres to the HST light curve at all wavelengths. The usefulness of the second derivative of the light curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.     

Spotted star light curve numerical modeling technique and its application to HII 1883 surface imaging

We developed a code for imaging the surfaces of spotted stars by a set of circular spots with a uniform temperature distribution. The flux from the spotted surface is computed by partitioning the spots into elementary areas. The code takes into account the passing of spots behind the visible stellar limb, limb darkening, and overlapping of spots. Modeling of light curves includes the use of recent results of the theory of stellar atmospheres needed to take into account the temperature dependence of flux intensity and limb darkening coefficients. The search for spot parameters is based on the analysis of several light curves obtained in different photometric bands. We test our technique by applying it to HII 1883.     

Center-to-limb variation of the continuum intensity and linear polarization of stars with transiting exoplanets

The limb darkening and center-to-limb variation of the continuum polarization is calculated for a grid of one-dimensional stellar model atmospheres and for a wavelength range between 300 and 950 nm. Model parameters match those of the transiting stars taken from the NASA exoplanet archive. The limb darkening of the continuum radiation for these stars is shown to decrease with the rise in their effective temperature. For the λ = 370 nm wavelength, which corresponds to the maximum of the Johnson–Cousins UX filter, the limb darkening values of the planet transiting stars lie in a range between 0.03 and 0.3. The continuum linear polarization depends not only on the effective temperature of the star but also on its gravity and metallicity. Its value decreases for increasing values of these parameters. In the UX band, the maximum linear polarization of stars with transiting planets amounts to 4%, while the minimum value is approximately 0.3%. The continuum limb darkening and the linear polarization decrease rapidly with wavelength. At the R band maximum (λ = 700 nm), the linear polarization close to the limb is in fact two orders of magnitude smaller than in the UX band. The center- to-limb variation of the continuum intensity and the linear polarization of the stars with transiting planets can be approximated, respectively, by polynomials of the fourth and the sixth degree. The coefficients of the polynomials, as well as the IDL procedures for reading them, are available in electronic form. It is shown that there are two classes of stars with high linear polarization at the limb. The first one consists of cold dwarfs. Their typical representatives are HATS-6, Kepler-45, as well as all the stars with similar parameters. The second class of stars includes hotter giants and subgiants. Among them we have CoRoT-28, Kepler-91, and the group of stars with effective temperatures and gravities of approximately 5000 K and 3.5, respectively.     

Multiple scattering in planetary atmospheres

Methods for solving radiative transfer problems within the extended visible spectrum in planetary atmospheres are reviewed for use by the nonspecialist. Emphasis is placed on rapid, approximate procedures for the determination of such quantities as the plane and spherical (Bond) albedo, surface illumination, absorbed energy, limb darkening, phase curve, and spectra. Precise numerical methods and analytical results are also discussed. Recent approaches to such complications as atmospheric inhomogeneity and reflection from a porous regolith are described briefly.     

Formation of P Cygni profiles with limb darkening of the stellar core

In the framework of the Sobolev approximation, we investigate the effects of limb darkening of the stellar core onto the formation of line profiles in rapidly expanding envelopes.For the case of outward-accelerating envelopes, it is shown that the P Cygni profiles, calculated with and without limb darkening, do appear almost identical as long as the size of the envelope remains large with respect to the radius of the star. As the extent of the envelope decreases, the resulting profiles are found to differ appreciably close to the line center.When considering outward-decelerating (or equivalently inward-accelerating) envelopes, calculations with the limb darkening effects reveal that the P Cygni profiles are more strongly affected than in the previous case. These effects are specially important in the blue wing and near the center of the resulting line profiles, irrespective of the size of the envelope.Chargé de Recherches au Fonds National de la Recherche Scientifique, Belgium.