Future solar systems

Widely regarded as pathological variable stars – with erratic photometric and spectroscopic behavior of unknown physical origin – 20 years ago, T Tauri stars (TTSs) turned out in the last decade to be promising laboratories for observing the formation of solar systems such as ours. This is because circumstellar, presumably protoplanetary disks were found to surround a large fraction of them. While evidence for disks was primarily indirect until 1995, recent high resolution imaging confirmed earlier claims that the infrared (IR) and ultraviolet (UV) excesses seen in the spectral energy distribution (SED) of these stars were due to disk emission. The activity displayed by young stellar objects at all wavelengths is due to the interaction between the circumstellar disk and the magnetized star and to non-stationary accretion/ejection phenomena. In this review, we briefly summarize properties of these young solar-type stars and describe their circumstellar disks in some detail, focusing on current optical, infrared and millimeter high angular resolution observations that now allow us to resolve the disks.     

IRAS observations of possible Herbig-Haro exciting sources in the Chamaleon dark cloud

Summary A sample of IRAS sources with colours typical of young stellar objects is investigated in the near-IR. They belong to the Chamaleon 1 association and lie in proximity of already known Herbig-Haro objects. On the basis of their total energy distributions, possible indications are suggested to identify candidates to excite the optical outflow (HH objects). Based on data collected at the European Southern Observatory, La Silla, Chile.     

OH/IR stars: Very long period variables with extreme mass loss rates on their nature and their distribution in the galaxy

Seldom are simple models sufficiently accurate to describe nature. The long period OH/IR stars with a well ordered, spherically symmetric, strong outflow of matter are such an exception. The variability of the central stars and the large dimension of the circumstellar dust shell enable a direct, geometrical determination of the distance to individual objects. Then most basic stellar quantities, such as mass, luminosity, mass loss rate, etc., may be derived. The values found indicate that OH/IR stars represent the last stage in the evolution of intermediate mass stars. In a relatively short ‘superwind’ phase a large fraction of the stellar mass is ejected and the objects are on the verge of becoming a planetary nebula or a white dwarf.The strength of the OH maser emission, and the accurate distance and velocity information, make the OH/IR stars particularly suited for the study of galactic structure. They are one of the few stellar populations that are detectable at large distances, even in the very centre of our Galaxy. The fact that they are found in the bulge of our Galaxy, seems to indicate that at least until recently (? 10⁹ years ago) star formation has been going on there.     

在SDSS DR8恒星光谱中自动搜寻稀有天体

SDSS DR8海量光谱中包含许多有研究价值的稀有天体,如特殊白矮星(DZ,DQ,DC)、碳星、白矮主序双星、激变变星等,如何在海量光谱中自动搜寻稀有天体有着极其重要的意义。提出一种基于核密度估计和K-近邻(K-nearest neighbor, KNN)相结合的方法在SDSS DR8 信噪比大于5的546 383个恒星光谱中搜寻稀有天体。首先对光谱进行高斯核密度估计,选取概率最小的5 000个光谱作为稀有类,概率最大的300 000个光谱作为普通类,然后进行KNN分类,同时也将5 000个稀有光谱的K个最近邻也作为稀有的天体,结果共有21 193条光谱。为了方便分析,对这些光谱聚类后进行人工检查。这些光谱主要包括由于数据缺失、红化、流量定标不准引起的问题光谱、行星状星云、没有物理联系的光谱双星、类星体、特殊白矮星(DZ,DQ,DC)、碳星、白矮主序双星、激变变星等。通过和SIMBAD,NED,ADS及一些主要的文献交叉验证,我们新发现了3个DZ白矮星、1个白矮主序双星、2个伴星为G型星的激变变星,3个激变变星的候选体、6个DC白矮星,1个DC白矮星候选体和1个 BL Lacertae(BL lac)候选体。还发现了1个有CaⅡ三重发射线和MgⅠ发射线的DA白矮星和1个光谱上表现出发射线的晚M恒星但测光图上像是一个星云或星系。     

Isolated Newtonian Dust Stars Are Unstable But Can Be Stabilized by Exterior Matter

The mathematical problem of the existence of solutions for Newtonian dust stars is formulated, and proofs for the non-existence of such stars as isolated objects are given. It is however shown that such stars can be stabilized by reasonable exterior (strained) matter. Comments are made on the problem of Einsteinian rotating dust stars.     

Possible evidence for the observation of noncompact (nonbaryonic) gravitational microlenses (“neutralino stars”)

The microlensing of distant stars by noncompact objects such as neutralino stars is considered. Recently, Gurevich and Zybin considered the objects as microlenses. Using a nonsingular density distribution, we analyze microlensing by noncompact objects. We obtain analytic solutions to the gravitational-lens equation and an analytic expression for the amplification factor of the gravitational lens. We show that, on the basis of a model of microlensing by noncompact objects, it is possible to interpret microlensing-event candidates having two typical maxima of light curves which are usually interpreted as binary microlenses.     

Modified astronomical speckle interferometry “speckle masking”

A simple method (speckle masking) for the reconstruction of real images from astronomical speckle photographs is described. This method is applicable to a restricted class of objects that includes double stars. Double stars are the object most frequently observed with speckle interferometry techniques. Certain other kinds of objects can also be investigated by the speckle masking method, which should work also at low light levels. Theory and laboratory experiments are presented.     

Non-Doppler redshift of some galactic objects

Redshift observations reported in several galactic objects, such as binary stars, star clusters, and in the K effect, are compared with the predicted non-Doppler redshift caused by the inelastic transmission of photons in gases. It is found that all the observations are compatible with the predicted redshift caused by a gas surrounding the star. The new mechanism explains, in a simple way and without requiring any of the previous ad hoc hypotheses, why so many early-type stars in the solar neighborhood appear to be generally redshifted. This mechanism has important consequences in cosmology     

The reflection of gravitational waves from compact stars

It is shown that gravitational waves reflect from sufficiently dense stellar objects. The condition of reflection is found and the reflection index, which reaches a value of 25% for neutron stars, is estimated.     

A method of separating blended images in space debris observation

Due to the relative movement between space debris and background stars, the blending of objects and stars is ineluctable through observation. It brings down position accuracy of objects and even makes the tracking break down in worse conditions. In view of the difference of geometry between stars and objects in space debris observation, a technique for separating blended objects based on mathematical morphology is presented. It’s sufficiently flexible to be applied in image processing, and the blending images can be separated effectively with a high degree of centroid precision.

     

漩涡星系M31和椭圆星系M32CaⅡ三重线的等值宽度

利用天文台２．１６米望远镜的卡焦摄谱仪,我们观测了著名的漩涡星系Ｍ３１核区与盘区以及椭圆星系Ｍ３２长轴方向与短轴方向的近红外光谱。对这些区域的重要光谱特征－－ＣａⅡ三重线进行了测量,得到它们的等值宽度。     

Search for carbon stars and DZ white dwarfs in SDSS spectra survey through machine learning

Carbon stars and DZ white dwarfs are two types of rare objects in the Galaxy.In this paper,we have applied the label propagation algorithm to search for these two types of stars from Data Release Eight(DR8)of the Sloan Digital Sky Survey(SDSS),which is verified to be efcient by calculating precision and recall.From nearly two million spectra including stars,galaxies and QSOs,we have found 260 new carbon stars in which 96 stars have been identified as dwarfs and 7 identified as giants,and 11 composition spectrum systems(each of them consists of a white dwarf and a carbon star).Similarly,using the label propagation method,we have obtained 29 new DZ white dwarfs from SDSS DR8.Compared with PCA reconstructed spectra,the 29 findings are typical DZ white dwarfs.We have also investigated their proper motions by comparing them with proper motion distribution of 9,374 white dwarfs,and found that they satisfy the current observed white dwarfs by SDSS generally have large proper motions.In addition,we have estimated their efective temperatures by fitting the polynomial relationship between efective temperature and g-r color of known DZ white dwarfs,and found 12 of the 29 new DZ white dwarfs are cool,in which nine are between 6,000 K and 6,600 K,and three are below 6,000 K.     

大质量恒星的形成:理论与观测

文章简要叙述了有关大质量恒星形成的理论以及相关观测证据。目前大质量恒星形成的理论主要有两种，即吸积说和并合说．吸积说认为，大质量星可能与小质量星形成于相似的过程；并合说主张大质量星可能是由小质量年轻星碰撞合并而成．解决这两种理论争论的关键在于在大质量星附近能否观测到吸积盘的存在，最新的观测表明大质量星更有可能是通过吸积增加自身的质量，但最终解决这一问题可能还需要更多的观测证据。文章还提出了一些本领域尚未解决的问题，为感兴趣的研究者提供参考。     

Gravitational Lensing of Fermion Stars

We discuss the fermion stars, the self-gravitating systems of Fermi gases, as possible gravitational lenses. It is supposed that the fermions interact with themselves and other particles only by gravity, so they are the candidates of dark matter. We calculate Einstein deflection angles, study the image configurations, and calculate the magnification factors for a number of fermion stars that range from strong relativistic configurations to nonrelativistic ones. We find that typically there are three images, one Einstein ring and one radial critical curve for both cases. Two of the images are within the Einstein ring, and the other is outside, which may be very far. All these lensing characteristics can help to identify fermion stars as potential lensing objects, thus might give direct evidence that dark fermion stars exist in the universe.     

Magnetic field decay in condensed objects the influence of space-time curvature

A generalization of Lamb's formula for the decay of a magnetic field due to ohmic dissipation is given which takes into account the influence of space-time curvature. It is found that for condensed objects such as heavy neutron stars the decay time for the magnetic field is substantially lengthened.     

Nearby stars as sources of gamma-ray bursts

Observations of ground-based telescopes and the Hubble space telescope made it possible to identify a part of gamma-ray bursts with far objects (redshift parameter Z ≥ 1).However, it remains unclear what are other bursts and what are their sources. The possibility of identifying other bursts with close sources known as small-mass flare stars is considered. The coordinates of space gamma-ray bursts (GRBs) for 2008–2013 and close stars (within the radius r < 25 pc) were compared by the correlation analysis method. Six coincidences were found with an accuracy of ～0.1°. The probability of accidental coincidence of GRBs with stars is 4 · 10^?8, which undoubtedly proves their stellar origin.     

On Charged Analogues of Matese and Whitman Interior Solutions in General Relativity

A set of three static fluid spheres once derived by Matese and Whitman (in Phys. Rev. D 22:1270, 1980) are charged by means of a particular electric intensity to obtain compact objects like white dwarfs, quark stars and neutron stars etc. The charged objects so obtain and their neutral counterparts are analysed on astrophysical grounds. This revealed that two of the charged spheres and one of the neutral seed solutions are well behaved. The corresponding red shift and adiabatic index together with other physical entities like pressure, density and velocity of sound through the star models are studied systematically.     

The stability of relativistic stars and the role of the adiabatic index

We study the stability of three analytical solutions of the Einstein’s field equations for spheres of fluid. These solutions are suitable to describe compact objects including white dwarfs, neutron stars and supermassive stars and they have been extensively employed in the literature. We re-examine the range of stability of the Tolman VII solution, we focus on the stability of the Buchdahl solution which is under contradiction in the literature and we examine the stability of the Nariai IV solution. We found that all the mentioned solutions are stable in an extensive range of the compactness parameter. We also concentrate on the effect of the adiabatic index on the instability condition. We found that the critical adiabatic index, depends linearly on the ratio of central pressure over central energy density \(P_c/{\mathcal{E}}_c\), up to high values of the compactness. Finally, we examine the possibility to impose constraints, via the adiabatic index, on realistic equations of state in order to ensure stable configurations of compact objects.     

Quasar microlensing

Quasar microlensing deals with the effect of compact objects along the line of sight on the apparent brightness of the background quasars. Due to the relative motion between quasar, lenses and observer, the microlensing magnification changes with time which results in uncorrelated brightness variations in the various images of multiple quasar systems. The amplitudes of the signal can be more than a magnitude with time scales of weeks to months to years. The effect is due to the “granular” nature of the gravitational microlenses—stars or other compact objects in the stellar mass range. Quasar microlensing allows to study the quasar accretion disk with a resolution of tens of microarcseconds, hence quasar microlensing can be used to explore an astrophysical field that is hardly accessible by any other means. Quasar microlensing can also be used to study the lensing objects in a statistical sense, their nature (compact or smoothly distributed, normal stars or dark matter) as well as transverse velocities. Quasar microlensing light curves are now being obtained from monitoring programs across the electromagnetic spectrum from the radio through the infrared and optical range to the X-ray regime. Recently, spectroscopic microlensing was successfully applied, it provides quantitative comparisons with quasar/accretion disk models. There are now more than a handful of systems with several-year long light curves and significant microlensing signal, lending to detailed analysis. This review summarizes the current state of the art of quasar microlensing and shows that at this point in time, observational monitoring programs and complementary intense simulations provide a scenario where some of the early promises of quasar microlensing can be quantitatively applied. It has been shown, e.g., that smaller sources display more violent microlensing variability, first quantitative comparison with accretion disk models has been achieved, and quasar microlensing has been used to determine the fraction of dark matter in a lensing galaxy for the first time. This is the quantitative beginning. The future of quasar microlensing is bright.