Dust Formation and Mass Loss in Evolved Stars

The problem of dust formation in the circumstellar envelopes of Asymptotic Giant Branch stars is addressed. We summarize the basic thermodynamic prerequisites necessary to enable the formation and growth of solid particles from the gas phase and draw some conclusions on the evolution of the emergent dust component. In a circumstellar environment the dust grains interact with the stellar radiation field, which leads to a strong coupling among the local thermodynamic conditions and the dust formation process itself. By a consistent treatment of the physics describing the dust forming circumstellar shells of evolved stars we demonstrate, that the non-linear interaction among the dust formation process and the hydrodynamic and thermodynamic conditions of the dust forming system leads to a complex dynamical structure of these shells. Some observable consequences resulting from corresponding model calculations are given. This revised version was published online in August 2006 with corrections to the Cover Date.     

Shock Waves in Outflows from Young Stars

This review focuses on physics of the cooling zones behind radiative shocks and the emission line diagnostics that can be used to infer physical conditions and mass loss rates in jets from young stars. Spatial separations of the cooling zones from the shock fronts, now resolvable with HST, and recent evidence for C-shocks have greatly increased our understanding of how shocks in outflows interact with the surrounding medium and with other material within the flow. By combining multiple epoch HST images, one can create `movies' of flows like those produced from numerical codes, and learn what kinds of instabilities develop within these systems.     

Multi-Wavelength Studies on H2O Maser Host Galaxies

H₂O maser emissions have been found in external galaxies for more than 30 years. Main sciences associated with extragalactic H₂O masers can be summarized roughly into three parts: maser emission itself, AGN sciences and cosmology exploration. Our work in this field focusses on two projects: X-ray data analysis of individual maser source using X-ray penetrability to explore maser host obscured AGN; multi-wavelength statistical properties of the whole published H₂O maser sample. Here their nuclear radio properties were investigated in detail, based on their 6-cm and 20-cm radio observation data. Comparing the radio properties between maser-detected sources and non-detected sources at similar distance scale, we find that: (1) maser host galaxies tend to have higher nuclear radio luminosity; (2) the spectral index of both samples is comparable (～0.6), within the error ranges. In addition, for AGN-maser sources, the isotropic maser luminosity tends to increase with rising radio luminosity. Thus we propose the nuclear radio luminosity as one good indicator for searching AGN-masers in the future.     

The Eddington Ratio of H 2 O Maser Host AGN

The Eddington ratio λ was derived for the entire maser host AGN sample, based on the intrinsic X-ray luminosity, the X-ray bolometric correction C _X and the mass of central black hole. Further the [O III] bolometric correction C _{[O III]} was estimated for our sample. Possible relations were also investigated between the maser luminosity and the bolometric luminosity – the Eddington ratio.     

Interactive Winds in Low-Mass Evolved Stars

Hypersonic flows occur in the late evolutionary stages of low to intermediate mass stars (M _ZAMS < 8M⊙). We present, from an observational point of view, the story of the matter ejected and shaped by different stellar winds. From the end of the Asymptotic Giant Branch to the Planetary Nebulae phases, the temperature and density both of the star and of the gas and dust envelope show drastic variations over a few thousand of years. We present multiwavelength (X-ray to radio) signatures of the outflows, observed through spectra and images of the consecutive phases, and discuss some open questions (bipolar morphology, microstructures,...). This revised version was published online in July 2006 with corrections to the Cover Date.     

Models of Circumstellar Maser in Late-Type Variable Stars

The database of observations of H₂O maser variability in a sample of late-type variable stars (oxygen-rich Miras and semiregulars) is analyzed. Some regularities of the H₂O variability are traced: in most of the stars studied, the H₂O flux peaks soon after visual light maxima with a phase delay Δ_ϕ ranging between 0.1 and 0.3 P. The models considered involve the effects of pulsation-induced shock waves: direct impact of a shock on the masering layer in the circumstellar envelope or amplification of the radio continuum, produced by the postshock ionized gas near light maxima, by pumped H₂O molecules. This revised version was published online in September 2006 with corrections to the Cover Date.     

An Infrared Photometric Study of Galaxies with Extragalactic H2O Maser Sources

All galaxies with extragalactic H_2O maser sources observed so far are collected. With the 2MASS and the IRAS photometric data an infrared study is performed on those galaxies.By a comparison between the H_2O maser detected sources and non-detected sources in the infrared it is indicated that infrared properties in the IRAS 12-25μm and 60-100μm are important for producing H_2O masers in galaxies.It is also found that the H_2O maser galaxies with different nuclear activity types have rather different infrared properties mainly in the IRAS 12-60μm region.     

Distribution of SiO and OH Maser Stars in the Galactic Plane

The observational results of the Nobeyama 45-m SiO maser survey and the Arecibo 305-m OH maser survey are assembled for an analysis of the distribution and kinematics of late-type stars in the Galactic plane.It is found that neither SiO maser stars nor OH maser stars show any concentration to the spiral arms,which imply that they do not belong to the arm population and quite possibly they are low-mass stars in late stage of evolution.A rotational curve is also derived for these objects and a few features which may be real are discussed and compared with those derived from planetary nebulae and AGB stars.     

H2 Diagnostics of Magnetic Molecular Shocks in Bipolar Outflows

The shock waves associated with molecular outflows may be of continuous (C) type or jump (J) type, depending on conditions in the preshock gas, notably the magnetic field strength and the degree of ionisation. Intermediate situations also exist, in which a J-discontinuity terminates or is embedded in a C-type flow. We show that proper allowance for the departure of the chemistry from equilibrium (particularly the dissociation/reformation of H₂) and for the inertia of charged dust grains, is crucial for an accurate treatment of the C to J transition. We illustrate the use of H₂ population diagrams and H₂ line profiles, in conjunction with our detailed shock model, to constrain conditions in shocks propagating in molecular outflows. We show that H₂ pure rotational lines yield evidence for C-type precursors in bipolar outflows from young stars, with transverse magnetic field strengths B (μG) ? 1–10 × $\sqrt {n_{H/{\text{cm}}^{ - 3} } } $ similar to those inferred from Zeeman splitting and from the dispersion of dust polarization vectors in dense clouds.     

The Methanol Maser Emission in W51

The European VLBI Network (EVN) has been used to make phase referenced, wide-field (several arcminute) spectral line observations of the 6.7-GHz methanol maser emission towards W51. In the W51 Main region, the bulk of the methanol is offset from an UCHii region. This probably indicates the methanol emission arises at the interface of the expanding UCHii region and not from an edge-on circumstellar disc, as previously suggested. Near the W51 IRS2 region, the methanol emission is associated with a very compact, extremely embedded source supporting the hypothesis that methanol masers trace the earliest stages of massive star formation. As well as these two well-studied sites of star formation, many previously unknown regions star formation are detected, demonstrating that methanol masers are powerful means of detecting young massive stars.     

Analysis of X-Ray Emission from OB Stars. IV. About X-Ray Emission from Inhomogeneous Winds of OB Stars

We study the origin of X-ray emission from OB stars due to collisions of stellar winds and/or inhomogeneities in the winds. The low-resolution X-ray spectra of a big sample of OB stars were fitted by both the stationary APEC/MEKAL models and by this model with an additional PSHOCK component describing the nonstationary X-ray emission. These spectra were also described by two-temperature PSHOCK models. More than ～50% of considered spectra can be described by the above-mentioned model combinations...     

演化晚期恒星星周包层的21μm特征

21μm特征是指在一些富碳的原行星状星云的红外光谱中波长约20 μm处观测到的一个较宽的发射谱带特征,它是目前星周包层物质研究的热点问题。到目前为止,一共有12个原行星状星云被证认为21μm特征源。此外,还有两个中心是沃尔夫拉叶星的行星状星云也有可能是21 μm特征源。这12个原行星状星云源的21μm特征都有相似的谱线轮廓,峰值都位于20．1μm。这些源的中心星都是富碳的F或G型超巨星,贫金属(却有丰富的慢中子过程元素),有很强的红外色余。在过去10年中,人们提出了大量21 μm特征载体的候选物质,主要有:氢化足球烯 (C60Hm,m=0-60)、多环芳香烃(PAH)、氢化无定形碳(HAC)、纳米金刚石颗粒、合成含碳大分子、氨基化合物(特别是尿素)、铁的氧化物(如:μ-Fe2O3、Fe3O4和FeO)、SiS2、纳米TiC团簇、掺杂的SiC颗粒、表面覆盖SiO2幔的SiC颗粒等。但所有这些候选物质都还没有得到确认。概述了21 μm特征的发现及其后续的相关研究,介绍了21μm特征源的共同特征,比较详细地探讨了人们已提出的该特征载体的候选物质。     

An Accessible Model of Dynamics and Cycle for Interstellar OH and H2O Maser Associations

An accessible model for interstellar OH/H₂O maser associations is presented. It can be classified into radiative pumping model. It can close the dynamical cycle of H₂O and OH species, and can give an interpretation on interstellar OH/H₂O associations. A reasonable scheme for both regeneration and destruction of interstellar H₂O and OH molecules is argued. Our model has overcome the defects of former radiative models, and is compatible with astronomical conditions. It is shown that the rotational population of H₂O and OH in these regions is much less affected by collisions than by radiation. Some experiments have confirmed our proposal.     

Extragalactic H2O Megamaser Sources： Central Black Holes, Nuclear X-ray and Maser Emissions

Extragalactic H2O megamasers are typically found within the innermost few parsecs of active galaxy nuclei (AGN) and the maser emission is considered to be excited most likely by the X-ray irradiation of the AGN.We investigate a comprehensive sample of extragalactic H2O masers in a sample of 38 maser host AGN to check potential correlations of the megamaser emission with parameters of the AGN,such as X-ray luminosity and black hole (BH) masses.We find a relation between the maser luminosities and BH masses,LH2O∝ M3.64-0.4 BH,which supports basically the theoretical prediction.The relation between the maser emission and X-ray emission is also confirmed.     

Variability of Circumstellar Emission from Dust Envelopes Around Carbon Stars

The main constituents of the dust, produced around late-type carbon stars, are thought to be carbon and silicon carbide (SiC), although their exact nature is not yet well established. This subject has been addressed by several authors and good fits of a large number of IRAS sources (chosen among the carbon stars exhibiting the SiC feature at about 11.3 μm) were obtained. In this work we use the same procedure on a limited number of objects, of the same type, taking into account the variability of such sources and the changes induced in their IR spectra. For this purpose, the chosen stars have been observed with a spectral resolution higher than that available for the IRAS data (λ/Δλ ≅ 50), using the CGS3 instrument of the UKIRT telescope, both in the low (λ/Δλ ≅ 160) and in the high (λ/Δλ ≅ 500) resolution configuration. The results are discussed and some preliminary conclusions are drawn. This revised version was published online in September 2006 with corrections to the Cover Date.     

陕西天文台守时用氢原子钟

本文介绍了陕西天文台守时用氢原子钟（Ｈ２）的改进、分项误差、准确度及在国际时间局（ＢＩＰＭ）公报中取权情况．Ｈ２自１９９１年开始连续运转至今已两年多了。１９９１年３月开始每月向ＢＩＰＭ报送数据，１９９１年９月开始取权１．Ｈ２是我国唯一的一台在ＢＩＰＭ公报上取权的氢钟．     

Variability of Circumstellar Emission from Dust Envelopes Around Carbon Stars

The main constituents of the dust, produced around late-type carbon stars, are thought to be carbon and silicon carbide (SiC), although their exact nature is not yet well established. This subject has been addressed by several authors and good fits of a large number of IRAS sources (chosen among the carbon stars exhibiting the SiC feature at about 11.3 m) were obtained. In this work we use the same procedure on a limited number of objects, of the same type, taking into account the variability of such sources and the changes induced in their IR spectra. For this purpose, the chosen stars have been observed with a spectral resolution higher than that available for the IRAS data (/ 50), using the CGS3 instrument of the UKIRT telescope, both in the low ( / 160) and in the high ( / 500) resolution configuration. The results are discussed and some preliminary conclusions are drawn.