EURD: The Mission and the Stellar Absolute Fluxes of B-Type Stars

We present here stellar spectra of B stars obtained with the EURD spectrograph, one of the three instruments on board MINISAT-01. EURD is a spectrograph specially designed to detect diffuse radiation in thewavelength range between 350 and 1100 Å with 5 Å spectral resolution. EURD main scientific targets are: the spectrum of interstellar medium,atmospheric airglow, decaying neutrinos, Moon and early type stars.     

Infrared Low-Resolution Spectra of HAEBE Stars

1　ＩｎｔｒｏｄｕｃｔｉｏｎＨＡＥＢＥｓｔａｒｓａｒｅｔｈｏｕｇｈｔｔｏｂｅｎｅｗｌｙｆｏｒｍｅｄｙｏｕｎｇｓｔａｒｓｗｉｔｈｉｎｔｅｒｍｅｄｉａｔｅｍａｓｓｉｎｔｈｅｐｒｅ-ｍａｉｎ -ｓｅｑｕｅｎｃｅ (ＰＭＳ) .Ｔｈｅｙａｒｅｕｓｕａｌｌｙｆｏｕｎｄｉｎｏｂｓｃｕｒｅｄｒｅｇｉｏｎｓａｎｄｉｌｌｕｍｉｎａｔｅｆａｉｒｌｙｂｒｉｇｈｔｎｅｂｕｌｏｓｉｔｙｉｎｔｈｅｉｒｉｍｍｅｄｉａｔｅｖｉｃｉｎｉｔｉｅｓ (Ｈｅｒｂｉｇ ,1 96 0 ;Ｔｈｅｅｔａｌ.,1 994) .Ｉｔｉｓｎｏｓｕｒ ｐｒｉｓｅｔｈａｔＨＡＥＢＥ…     

具有大红外色余的早型发射线星

本文给出由Wackerling早型发射线星星表与IRAS点源表交叉认证得到的V-[25]≥8的早型发射线星星表。对它们空间分布的研究表明:这些星属于极端星族Ⅰ,从而为进一步研究主序前和主序后中等和大质量恒星的演化提供较为完备的观测对象。     

Dust Formation around Wolf-Rayet Stars

We discuss the observational evidence in favour of dust formation around massive, evolved carbon-rich Wolf-Rayet stars. Large dust shells containing (amorphous carbon) grains are observed in persistent dust makers, such as the pinwheel system WR104 (WC9+OB) and periodic/episodic dust makers, such asWR140 (WC7+O) close to periastron. Problems with the nucleation and growth of dust in such harsh environments are discussed. It is probable that mixing of carbon-rich material from the WC star plus hydrogen from an OB companion, together with efficient shielding via the shocked region of their colliding winds, provides the necessary ingredients. Many problems with the nucleation and growth remain to be addressed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of Expanding Envelopes in Wolf-Rayet Stars

Some features of the envelopes of WR stars have not yet been explained in terms of the standard stellar wind. One of these features is the presence of gas condensations (clumps) in the envelopes. In this paper the possible reasons for clump formation are examined along with their role in the structure of envelopes. Clumps can be formed in the transitional zone between a star and its envelope because of instabilities in the gas. This zone lies much deeper than the "optical radus" of the star, so it cannot be observed. A clump expands as it moves under the action of radiation pressure from the star and its density decreases at the same time. The clump mixes with the surrounding gas if its mass is low. Large clumps can reach the visible region of the envelope.     

Extreme Infrared Stars Discovered in Magellanic Cloud Globular Clusters

We report preliminary results of our systematic survey for infrared stars in the globular clusters of the Magellanic Clouds. In the course of an ISOCAM survey for AGB stars in the intermediate-age clusters, we have discovered extremely red AGB stars in NGC 419 and NGC 1978. From their colours and luminosities, they are thought to be experiencing intense mass-loss and to be in the final or superwind phase of the AGB evolution. However, they seem to be of somewhat lower luminosity than the corresponding visible AGB stars when only the mid-infrared data are taken into account. This suggests that hitherto unobserved infrared excesses may exist at longer wavelengths. This revised version was published online in August 2006 with corrections to the Cover Date.     

早期中子星和夸克物质

夸克禁闭的解除与夸克物质的存在一直是物理学家极感兴趣的问题。尽管理论上已指出在超高温或超高密的条件下可以有夸克物质存在,但是由于地面实验室的条件所限,目前还不能通过实验证实这一点.宇宙中被观测到的中子星(例如crab和Vela脉冲星)的中心密度大于4倍的核物质密度,其中心温度也可以达到10~8—10~9K,于是人们希     

20颗依巴谷碳星的近红外测光

对20颗依巴谷(Hipparcos)卫星所观测的碳星作了近红外JHK测光，由近红外观测结果估算了其在K波段的热改正BCK和视热星等mbpl以及有效温度Te，结合依巴谷卫星所得视差，得到其中一些星的绝对热星等Mbol。     

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.     

Long-Term Evolution of FU Ori-Type Stars at Infrared Wavelengths

We investigated the brightness evolution of 5 FU Ori systems in the 1–200 μm wavelength range using observations from the Infrared Space Observatory, 2MASS and MSX data. The SEDs were compared with earlier ones derived from the IRAS photometry and ground-based observations around the epoch 1983. In three cases no difference was seen, in one object the near- and mid-infrared part of the spectrum became fainter by a factor of 2, and in the fifth case a marginal fading was observed. We study in detail the case of V1057 Cyg.     

Polarimetric Studies of Stars with an Infrared Emission Excess

The results of polarimetric and IR (IRAS) observations of 24 B-A-F stars are given. Intrinsic polarization of the light from 11 of the 24 stars is observed. The degree of polarization for the other 13 stars is within the measurement errors. Two-color diagrams are also constructed. From a comparison of the degree of polarization with the color index on the two-color diagrams it is seen that 8 of these 13 stars probably are of the Vega type, while 5 are stars with gas—dust shells and/or disk—shells. It is shown that 6 of the aforementioned 11 stars with intrinsic polarization evidently are stars with gas—dust shells and/or disk—shells, while 5 of them (also including No. 24) are of the Vega type. It is also shown that the IR emission from 10 of the stars corresponds to a power-law distribution F . This fact may be explained both by free—free transitions of electrons and by thermal emission from dust grains in circumstellar gas—dust shells (disks).     

具有特殊红外色余的Be星及其分类

本文利用列于IRAS Point Source Catalog上的Be星资料,讨论了利用IRAS的三个流量(12μm、25μm、60μm)区分红外色余是自由-自由跃迁还是由星周尘埃再发射所产生。前者被认为是经典的Be星,而后者则可能是处于主序前的Herbig Ae/Be星。但是有16个一般认为是经典Be星的,却也显示出部分红外色余来自尘埃的再发射。它们的物理状态显然值得进一步研究。     

Formation of Massive Stars with High Accretion

We make models of the formation of massive stars by an accretion which is growing with the mass already accreted. These models are supported by several observations. They have implications for the IMF and the maximum stellar mass. This revised version was published online in September 2006 with corrections to the Cover Date.     

贫金属富碳天琴RR变星的形成

贫金属富碳恒星(Carbon-Enhanced Metal-Poor, CEMP)是研究宇宙早期恒星性质和化学演化的极佳样本,通常认为来自双星.目前发现的贫金属富碳星中有9颗天琴RR变星(RR Lyrae star, RRL),其中至少7颗未表现出任何双星特征.传统双星物质转移模型不足以充分解释贫金属富碳天琴RR变星(CEMP-RR Lyrae)单星的形成.之前研究表明氦白矮星和赫氏空隙星(HG)的并合模型可以解释部分富碳红巨星单星的碳增丰现象,因此贫金属富碳星单星也可能来自氦白矮星和赫氏空隙星的并合模型渠道.通过详细计算的氦白矮星和赫氏空隙星并合模型来检验这一演化渠道,结果表明:该并合模型在后续的演化过程中,其重力加速度、温度、表面碳丰度均能与观测符合较好.由此,氦白矮星和赫氏空隙星并合模型极有可能是贫金属富碳天琴RR变星的形成渠道之一.     

On the Formation of Massive Primordial Stars

We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 M _⊙. The high accretion rate used in the models (M _⊙ = 4.4 x 10^-3 M_⊙ yr^-1) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the in falling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M _⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 M _⊙. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation of Carbon-Enhanced Metal-Poor RR Lyrae Stars

Carbon-enhanced metal-poor (CEMP) stars are considered to be related to the first generation of stars, and responsible for the chemical evolution of the early Galaxy. More than half of them are in binaries, and could be explained by the binary evolution, but the formation channel of them is still not fully understood. Among the hundreds of CEMP stars, there are nine CEMP RR Lyrae stars identified, and at least seven of which are very likely not binaries. The usual binary star evolution channel is difficult to produce such a single star, particularly that of carbon enrichment. One way in which such a single star might be produced is the merger of a helium white dwarf with a Hertzsprung gap (HG) star. We use a stellar evolution program to calculate the models of the merger remnants, and find that the models can reproduce the observed distribution of these CEMP single RR Lyrae stars in terms of surface temperature, gravity, and carbon abundance. Hence, it is extremely possible that the helium white dwarf and HG star merger model is one of the formation channels of the metal-poor carbon-rich RR Lyrae stars.