Formation of Dust Grains in Circumstellar Envelopes of Oxygen-Rich AGB Stars

We discuss dust formation in steady state dust driven winds around oxygen-rich AGB stars, including not only homogeneous Al₂O₃ and silicate grains but also heterogeneous grains consisting of an Al₂O₃ core and a silicate mantle. In the inner subsonic region, Al₂O₃ grains with radii of ∼ 0.15 μm condense first, then condensation of silicate on Al₂O₃ starts slightly inside the sonic point, which accelerates the gas flow into the supersonic region. Also small silicate grains, whose radii are a few tens of ?ngstroms form beyond the sonic point. The carrier of 13 μm feature observed towards oxygen-rich AGB stars is considered to be the core-mantle grains consisting of an α-Al₂O₃ core and a silicate mantle from the radiation transfer calculations based on the results of dust formation calculations. This revised version was published online in September 2006 with corrections to the Cover Date.     

Time-Series Monitoring of Variable Stars in the Field of NGC7789

Through the photometric observations of NGC7789 with the Schmidt telescopefor four consecutive nights, sixteen variable stars were discovered. It was found that three are Algol (EA) type, seven are W UMa (EW) orβ Lyrae (EB) type and one is δ Scuti type. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Dust on the Propagation of Shock Waves in Circumstellar Envelopes of Late-Type Variable Stars

Physical processes affecting propagation of moderate-strength shocks in the inner parts of circumstellar envelopes of Miras are considered. In particular, the influence of dust, which is heated later than heavy gas particles, is critical for pumping of circumstellar H₂O masers. Strong shocks with Mach numbers M exceeding ∼ 10 destroy dust and molecules and partly ionize the gas. Weaker shocks with 3 < M < 10 spend their energy mainly to dissociate molecules. Shocks with 1 < M < 3 lose their energy via heating of dust and may stimulate formation of dust. Some implications are discussed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Near Infrared Observation of Carbon and S Stars with Silicate Feature

We apply the concept of wave quantum theory in an astronomical scale and discuss the periodic distribution of the galaxies red shift. A generalization of the commutation relation is used to write down the wave equation and the redshift spectrum is calculated. This revised version was published online in July 2006 with corrections to the Cover Date.     

Discovery of Four Peculiar Variable Stars

Four stars, GSC 1258-0143, GSC 1986-1665, GSC 3045-0892 and GSC 2983-1597 were found to be new variables during a campaign of CCD photometric monitoring of short-period eclipsing binary stars. The variabilities of these new variables are reported. The main characteristics and probable classifications of the variables are discussed through a preliminary analysis on their light curves. Among these stars, GSC 1258-0143 is classified as a RR Lyr star of sub-type a. Its pulsation period is determined to be about 0.5206 days. The other three stars, are definite variables, but their periods and types of light variations remain unknown.     

Observations of the Dust Around Evolved Stars

ISOPHOT has been used to obtain low resolution spectra from 2.5μm to 5μm and 5.8μm to 11.6μm and multi-aperture photometry at 60μm of several evolved stars; oxygen-rich and carbon-rich (including the peculiar carbon-rich stars R CrB and RY Sgr). R CrB was observed early in the ISO mission, 3 weeks after it had been at minimum light. Another spectrum was obtained several months later. The second spectrum shows that the broad plateau (from around 6μm to 8μm) is still present but the flux density has declined from 60Jy to 50Jy. The spectrum for RY Sgr shows the same type of plateau. The multi-aperture data suggest that the dust shells are resolved around R CrB, RY Sgr, Y CVn and RS Lib. This revised version was published online in August 2006 with corrections to the Cover Date.     

脉动变星数据库的建立

为了方便国内学者对脉动变星进行研究,建立了一个数据库,目前包含了时间长达10 yr左右的巡天项目MAssive Compact Halo Objects(大质量致密银晕天体,MACHO)和Optical Gravitational Lensing Experiment(光学引力透镜实验,OGLE)发现的脉动变星,一共容纳了来自银河系核球与大小麦哲伦云中的共近23万颗变星.采用的软件是LAMP,即Linux+Apache+MySQL+PHP.数据库的使用通过网页的简单搜索界面实现,搜索参数主要是天体的赤经、赤纬和半径.鉴于本数据库的灵活性,将来很方便加入其他的变星数据.     

Construction of the Database for Pulsating Variable Stars

A database for pulsating variable stars is constructed to favor the study of variable stars in China. The database includes about 230,000 variable stars in the Galactic bulge, LMC and SMC observed in an about 10 yr period by the MACHO(MAssive Compact Halo Objects) and OGLE(Optical Gravitational Lensing Experiment) projects. The software used for the construction is LAMP, i.e., Linux+Apache+MySQL+PHP. A web page is provided for searching the photometric data and light curves in the database through the right ascension and declination of an object. Because of the flexibility of this database, more up-to-date data of variable stars can be incorporated into the database conveniently.     

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.     

Radiative Transfer Models of Dust Shells Around Post-AGB Stars

We present a radiative transfer analysis of circumstellar dust shells around the Post-AGB stars HD 179821, HD 56126, HD 101584 and early R star HD 100764, using the code DUSTY. Parameters like mass-loss, shell inner radius, dust temperature, outflow velocity etc., are derived for HD 179821and HD 56126 whose observed SED could be reproduced by our models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxygen-Rich Cool Stars in the Cepheus Region. New Spectroscopic Observations of Selected Objects from the KP2001 Catalog

Astrophysics - Spectra taken with the 2.6-m telescope at the Byurakan Astrophysical Observatory (BAO) in the λλ4000-7250Å range for eight M giants from the KP2001 catalog confirm...     

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.     

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.     

Formation of Al2O3 Grains and the 13µM Feature in Circumstellar Envelopes of Oxygen-Rich AGB Stars

The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13μm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13μm feature when the mass loss rate is less than 2 × 10-5M·/yr. This revised version was published online in August 2006 with corrections to the Cover Date.     

Discovery of Three Variable Stars in Direction toward NGC 4565

We present the results of a variable star search in a field next to the edge-on galaxies NGC 4565 from the archive data of Beijing-Arizona-Taibei-Connecticut Multicolor Sky Survey. Three new variable stars were discovered. Based on spectra obtained from the 2.16 m NAOC telescope, we identify two of these as RR Lyr stars and one as a special T Tauri star.     

Searching for Variable Stars in the Field of NGC 7789

We present the results of a time-series CCD photometric survey of variable stars in the field of the open cluster NGC 7789. In a field of about one degree centering on the cluster, a total of 28 new variable stars are discovered (14 W UMa systems, nine EA-type eclipsing binaries, one RR Lyr star, and four unclassified). In addition, we recovered 11 old variables previously discovered by other authors. Preliminary parameters are given for some of these variables.     

Model Atmospheres of AGB Stars: Dynamics, Molecules and Dust

The atmospheres and circumstellar envelopes of AGB stars are characterized by complex physical phenomena like shock waves caused by stellar pulsation or formation of molecules and dust which often lead to a heavy mass loss and have a strong influence on IR properties as observed by ISO. To allow a physical interpretation of various observations we have constructed improved dynamical model atmospheres of long-period variables. In this contribution we mainly investigate the dependence of the atmospheric structure and its variability on stellar pulsation, molecular opacities and time-dependent dust formation. IR spectra resulting from our models are discussed in detail by Loidl et al. (1997b) and compared to ISO-SWS spectra obtained by Hron et al. (1997). This revised version was published online in August 2006 with corrections to the Cover Date.     

基于SDSS的河内恒星DIBs自动识别方法

星际弥散带(Diffuse Interstellar Bands,DIBs)自发现以来已经经历了近百年的研究,但是至今仍然是天体光谱学上的一个未解之谜.针对SDSS DR7的光谱数据提出了一种星际弥散带特征自动识别方法.该方法基于谱线特征匹配,通过光谱流量限制的方法进行星际弥散带特征的自动识别.利用它可对相对定标的巡天光谱进行广泛的星际弥散带候选天体搜索,在海量光谱数据中获取更多具有星际弥散带特征的河内恒星.通过对SDSS DR7中位置相对合适的超过300个盘的天体光谱的遍历,已经得到了一系列具有星际弥散带特征的候选河内恒星,并且证明了该方法简单有效且具有鲁棒性.这为载体证认等工作提供了大量辅助数据,极大地推进了星际弥散带的研究.