Dust properties of [WCL] CSPNS

The rather rare class of central stars of planetary nebulae thatshow Wolf-Rayet spectra have been a subject of great interest,particularly in the infrared, since their discovery in the late1960s. I will focus on further peculiarities found within thepast 1-2 years with the advent of infrared spectroscopy fromISO. Notably, these stars simultaneously betray the presenceof regions of carbon-rich and oxygen-rich dust chemistry. Icompare and contrast complete ISO spectra from 2 to 200 micronsof a small sample of [WC9] to [WC11] central stars.     

The impact of binary-star yields on the spectra of galaxies

One of the complexities in modelling integrated spectra of stellar populations is the effect of interacting binary stars besides Type Ia supernovae (SNeIa). These include common envelope systems, cataclysmic variables, novae, and are usually ignored in models predicting the chemistry and spectral absorption line strengths in galaxies. In this paper, predictions of chemical yields from populations of single and binary stars are incorporated into a galactic chemical evolution model to explore the significance of the effects of these other binary yields. Effects on spectral line strengths from different progenitor channels of SNeIa are also explored. Small systematic effects are found when the yields from binaries, other than SNeIa, are included, for a given star formation history. These effects are, at present, within the observational uncertainties on the line strengths. More serious differences can arise in considering different types of SNIa models, their rates and contributions.     

The dual dust chemistries of planetary nebulae with [WCL] central stars

The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO . Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system.     

Observations of chemical processing in the circumstellar environment

High resolution interferometer and single-dish observations of young, deeply embedded stellar systems reveal a complex chemistry in the circumstellar environments of low to intermediate mass stars. Depletions of gas-phase molecules, grain mantle evaporation, and shock interactions actively drive chemical processes in different regions around young stars. We present results for two systems, IRAS 05338-0624 and NGC 1333 IRAS 4, to illustrate the behavior found and to examine the physical processes at work.     

The shapes of light curves of Mira‐type variables

Using a sample of 454 Mira light curves from the ASAS survey we study the shape of the light variations in this kind of variable stars. Opposite to earlier studies, we choose a general approach to identify any deviation from a sinusoidal light change. We find that about 30% of the studied light curves show a significant deviation from the sinusoidal reference shape. Among these stars two characteristic light curve shapes of comparable frequency could be identified. Some hint for a connection between atmospheric chemistry and light curve shape was found, but beside that no or only very weak relations between light curve shape and other stellar parameters seem to exist (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The infrared [WC] stars

A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for10⁴ yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M_⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars.     

Time travel and chemical evolution: a look at the outer solar system

Many space scientists think that the chemical conditions today on planets and moons of the outer solar system are similar to conditions on Earth soon after it formed. If so, we can learn much about the chemistry that led to life on this planet. We can also speculate about exotic habitats that might have given rise to other types of life. And if we are able to discern the chemical reactions now occurring in the outer solar system, we may be able to extrapolate these rules to other solar systems, and so define the habitable zones around other stars where the potential for life is high.     

Element spots in Ap and Hg‐Mn stars from current‐driven diffusion

The stars of the middle main sequence often have spot‐like chemical structures at their surfaces. We consider the diffusion process caused by electric currents that can lead to the formation of such chemical spots. Diffusion is considered using the partial momentum equations derived by the Chapman‐Enskog method. We argue that diffusion caused by electric currents can substantially change the surface chemistry of stars and form spotted chemical structures even in a relatively weak magnetic field. The considered mechanism can be responsible for a formation of element spots in Hg‐Mn and Ap‐stars. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Discovery of high-velocity EHB stars in the globular cluster ωCentauri(NGC 5139)

We report the discovery of 45 high-velocity extreme horizontal branch(EHB) stars in the globular cluster Omega Centauri(NGC 5139).The tangential velocities of these EHB stars are determined to be in the range 93~313 km s~(-1),with an average uncertainty of ~27 km s~(-1).The central escape velocity of the cluster is determined to be in the range 60~105 km s~(-1).These EHB stars are significantly more concentrated toward the cluster core compared with other cluster members.The formation mechanisms of these EHB stars are discussed.Our conclusions can be summarized as follows:(1) A comparison of the tangential velocities of these EHB stars to the centra]escape velocity of the cluster shows that most if not all of these EHB stars are unbound to the cluster;(2) These EHB stars obtained high velocities in the central cluster region no longer than ~1 Myr ago and may be subsequently ejected from the cluster in the next ~1 Myr;(3) If the progenitors of these EHB stars were single stars,then they may have experienced a fast mass-loss process.If the progenitors were in close binaries,then they may have formed through disruptions by the intermediate-mass black hole in the cluster center.     

Spectrum variability of some selected bright CP stars on the main sequence

The initial findings of a survey for spectrum variations in selected CP stars is reported. It is found that almost all observed mercury-manganese stars are spectrum variables. The implications of chemical anomalies in these stars is briefly discussed. In particular, arguments are advanced against the general assumption that CP3 stars are non-magnetic. The observed low rotational velocities of the CP3 stars is interpreted as rotational braking by magnetic fields mostly in the pre-Main-Sequence stage of evolution of these objects. We advance the hypothesis that high resolution spectroscopic observations of such stars would exhibit Zeeman-broadening in lines that vary in strength and are also sensitive to the magnetic field.     

J-type carbon stars in the Large Magellanic Cloud

A sample of 1497 carbon stars in the Large Magellanic Cloud (LMC) has been observed in the red part of the spectrum with the 2dF facility on the Anglo-Australian Telescope. Of these, 156 have been identified as J-type (i.e. ¹³C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple two-dimensional classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c and j indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the   K − ( J − K )  colour–magnitude diagram, parallel to and 0.6 mag fainter than the N-star sequence. A subset of the J stars (about 13 per cent) are brighter than this J-star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature.     

Chemically peculiar stars in the orion OB1 association. I. occurrence frequency, spatial distribution, and kinematics

A total of 85 CP stars of various types are identified among 814 members of the OriOB1 association. The fraction of CP stars decreases with age for different cluster subgroups: from 15.1% in the youngest subgroup (b) to 7.7% in the oldest one (a). Individual comments are provided for each of the 85 stars, where we analyze the physical parameters and distance of the objects. All the 23 Am stars identified as a result of this study are found to have heliocentric distances between 100 and 300 pc and appear not to be members of the OrionOB1 association, but foreground objects. We identified 59 Bp stars, which account for 13.4% of the total number of B-type stars in the association. The fraction of peculiar B-type stars in the OriOB1 association is found to be twice higher than that of peculiar A-type stars. The same is true for field stars. The association contains 22 magnetic stars, out of which 21 are Bp stars and only one is an Ap star. Seventeen of these stars are objects with anomalous helium lines. Magnetic stars show a well-defined tendency to concentrate in the central region of the association (in Orion’s Belt), which contains most of these objects. No significant differences are found between the field strengths in the B-type stars of the association and Bp-type field stars, although there is a noticeable trend for He-rich stars to have stronger fields compared to He-weak stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied, which is the standard ratio for CP stars. Except for one HgMn star (HD35548), the radial velocities and proper motions of our identified objects are consistent with the corresponding parameters of normal B-type stars.     

Low-Mass Quark Stars

More and more observational hints of quark stars are proposed these years though pulsars are considered conventionally to be normal neutron stars. The existence of low-mass quark stars is a direct consequence of the possibility that pulsar-like stars are actually quark stars, because of the ability that quark matter can confine itself by color interaction. After a brief introduction to the study of quark stars, the various astrophysical implications of low-mass quark stars are investigated. It is addressed that some of the transient unidentified γ -ray sources are probably merging quark stars. The observability of low-mass quark stars is discussed.     

East-west asymmetry of Saturn's rings

The energy distribution curves of eleven Ap stars, three Am stars, four normal A stars and one F0 V magnetic star have been obtained between 478 nm and 680 nm. For four of the Ap stars, two Am stars and all the four normal A stars, the effective temperatures are believed to have been estimated for the first time. For the rest, these estimates are expected to be an improvement over previously available values.It is concluded that the Ap and Am stars are not much different from the normal A stars in so far as their temperatures are concerned.     

High-dispersion Spectroscopic Observations of the T Tauri-type Stars with Weak Emission Lines

By using the 2.16 m telescope of Xinglong Observing Station of National Astronomical Observatories and its high-dispersion spectrograph, the high-dispersion spectroscopic observations of six T Tauri-type stars with weak emission lines (i.e., weak-line T Tauri-type stars, abbreviated as WTTS) were carried out. The lithium abundances of these weak-line T Tauri-type stars are calculated and the relationships of the lithium abundances with the rotation periods as well as the amplitudes of light variations of these stars are discussed. It is found by this study that the lithium abundance for the weak-line T Tauri-type stars with fast rotations tends to be less than that of those with slow rotations. However, for all these weak-line T Tauri-type stars, the lithium abundances have no conspicuous correlation with the amplitudes of light variations of these stars in the V waveband.     

Dust Grains and Circumstellar Motion: The Problem of the Henand the Egg

In the last ten years, the number of models of circumstellar envelopes has exponentially increased, both concerning evolved stars and forming stars with their planetary systems. The main progress was intoduction of chemistry and coupling with dynamics. However, paradoxically, the nucleation theories did not evolve at the same rate; moreover, dynamics and nucleation are rarely analysed at the same degree of accuracy: one is of them is always mostly drawing attention. At the same time observations showed the presence of dust grains at a lot of sites where it could not be expected from usual quasi equilibrium theories. Self consistent systems are omnipresent. Thus, it is important to distinguish causes and effects, even if the phenomena are are strongly coupled. This revised version was published online in July 2006 with corrections to the Cover Date.     

金牛T型星的红外光谱

给出２６ 个金牛Ｔ 型星的ＩＲＡＳ低分辨率红外光谱．根据这些光谱讨论了它们有关的性质和所处环境．分析发现硅酸盐尘埃在金牛Ｔ 型星拱星壳层中是常见的物质     

Properties of atmospheres and winds of H-deficient central stars and related objects

According to latest results presented at this workshop, it appears that thesurface chemistry of hydrogen-deficient post-AGB stars can now be explained byevolutionary calculations involving a late He-shell flash. A quantitativecomparison of theoretical predictions with observed element abundances caneventually constrain details of this evolutionary scenario. I will emphasizehere the diverse chemical compositions observed in H-deficient post-AGB stars,which call for an explanation. The abundance patterns vary from almost purehelium atmospheres to hydrogen- and helium-free atmospheres, with a richchoice of compositions in between, involving rather different mixtures of H,He, and CNO.     

T Tauri Stars

The observable characteristics of young solar-mass stars (T Tauri stars) are reviewed. Modern theoretical concepts and models of these stars are discussed briefly.     

A search for magnetic fields in cool sdB stars

Hot cluster horizontal branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibit abundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars because they are slowly rotating. In particular, the slow rotation of the hot HB stars (T_eff > 11000 K), which show abundance anomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with those of red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown. In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of such fields in sdB stars with T_eff < 30 000 K, whose temperatures overlap with those of the hot HB stars. We conclude that large‐scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, the possibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field in SB 290; further observations are needed to confirm it (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)