ISO SWS Spectroscopy of Proto-Planetary Nebulae

We are obtaining low- (SWS01) and medium-resolution (SWS06) spectra of PPNe to investigate spectral features arising in these transitional objects. Three sources with the 21 μm feature have been confirmed, one new source found, and evidence of some substructure seen. This revised version was published online in August 2006 with corrections to the Cover Date.     

红团簇巨星元素丰度

基于高分辨率、高信噪比光谱观测资料,分析了１７颗红团簇巨星的５种α元素（Ｍｇ、Ｓｉ、Ｓ、Ｃａ、Ｔｉ）的化学丰度．结果表明,按金属丰度的不同可以将红团簇巨星分为两类：一类为富金属红团簇巨星（［Ｆｅ／Ｈ］≥０．０）;另一类为贫金属丰度红团簇巨星（［Ｆｅ／Ｈ］≤－０．３）．这两类红团簇巨星中的５种α元素均具有随金属丰度变大而增丰的趋势．同时,分析还表明,这两类红团簇巨星的α元素与铁元素的增丰历史可能是同步的．     

Actinium Abundance in the Atmospheres of Three Red Supergiants in the Magellanic Clouds

The actinium abundance in the atmospheres of red supergiants PMMR23 and PMMR144 in the Small Magellanic Cloud and RM_1-667 in the Large Magellanic Cloud was estimated. The results of spectral observations with the ESO 3.6-m telescope with resolving power R = 30000 were used. Since actinium was not found in the atmospheres of PMMR23 and PMMR144, only the upper limits were set on its abundance: logN(Ac/H) <–15.1 and–15.0, respectively. The estimated abundance of actinium in the atmosphere of RM_1-667 varied with parameters of the atmospheric model from–14.1 to–13.3. The lines of ionized actinium λ 616.475 nm and λ 581.085 nm were used in this analysis.     

Fred Hoyle, Red Giants and beyond

The impact of Fred Hoyle's work on the structure and evolution of red giants, particularly his breakthrough contribution with Martin Schwartzschild (1955), is described and assessed. Working with his students in the early 1960s, Hoyle presented new physical ways of understanding some of the approximations used, and results obtained, in that seminal paper. His initial viewpoint on the critical role of the outer surface boundary condition was replaced by a more subtle, if related one, which emphasized the peculiar difficulty of storing much mass outside a dense stellar core. That viewpoint that – low-mass red giants are essentially white dwarfs with a serious mass-storage problem – is still extremely fruitful. Recently, I have extended Hoyle's approach to explain not only many of the structural properties of red giants themselves, but also to link and unify the structures of low-mass stars from the main sequence through both the red giant and horizontal branch phases of evolution. Many aspects of these stars that had remained mysterious for decades have now fallen into place, and some questions have been answered that were not even posed before. With red giants as the simplest example, this recent work emphasizes that stars, in general, may have at least two distinct but very important centres: (i) a geometrical centre, and (ii) a separate nuclear centre, residing in a shell outside a zero-luminosity dense core for example. This two-centre perspective leads to an explicit, analytic, asymptotic theory of low-mass red giant structure. In this theory, there arises a naturally important in situ measure of central compactness: the parameter . That parameter, like others, is derived self-consistently and explicitly, and can be used to show how close a given model's properties are to ultimate asymptotic relationships. The results obtained also imply that the problem of understanding why such stars become red giants is one of anticipating a remarkable yet natural structural bifurcation which occurs in them. In the resulting theory, both the ratio and products like prove to be important, self-consistently derived quantities. Two striking theorems involving such quantities express between them the very essence of red giant behaviour, proving analytically for the first time that stars with dense cores are necessarily (i) extremely luminous, and (ii) very large. Perhaps the most astonishingly unexpected single result is that for the very value Nature provides for the relevant nuclear energy-generating temperature exponent (CNO's η=15), ρ_sh and behave in a well-defined, precisely inverse manner. This emphasizes that the internal behaviour of such stars is definitely anti-homologous rather than homologous, thus showing how very unfortunate the term `shell homology' is. Finally, I sketch a viewpoint which (i) links the structural and evolutionary behaviour of stars from the main-sequence through horizontal branch phases of evolution, and also (ii) has implications for post-main-sequence developments in more massive stars. This revised version was published online in August 2006 with corrections to the Cover Date.     

PAH features in the ISO SWS01 spectra of [WR] planetary nebulae

We present Infrared Space Observatory (ISO)Short Wavelength Spectrometer (SWS) observations for 16Wolf–Rayet ([WR]) planetary nebulae (PNe) in the range from 2.4 to16.5 m with the aim of analyzing the dust features present inthis group of objects. We have found that Policyclic AromaticHydrocarbon (PAH) molecular bands are present in most of the observed[WR] planetary nebulae with clear exception for K 2–16 among latetype [WC] stars.     

Molecules in AGB Stars Observed with ISO

I present in this review the recent results obtained with ISO concerning gas phase molecules in evolved stars. The near, medium and far-infrared spectrum of C-rich and O-rich stars can provide a very important information on the molecular content of these objects. Detailed models for the envelopes of AGB stars are also presented. I also report the detection of triatomic carbon in the far-infrared and I analyze the contribution of the ro-vibrational lines of the low energy bending modes of polyatomic molecules in the far-infrared spectrum of C-rich evolved stars. This revised version was published online in August 2006 with corrections to the Cover Date.     

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory （ISO） Short-Wavelength Spectrometer （SWS） spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Brα/HI（14-6） line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.     

Stellar Winds and Mass Loss in A-Type Supergiants

We examine the relationship between the surface escape velocities and wind terminal velocities of A-type supergiants according to the radiation-driven wind theory. This revised version was published online in July 2006 with corrections to the Cover Date.     

红团簇巨星的光谱分析及其作为I、K波段标准烛光的研究

基于高分辨率、高信噪比光谱观测资料，确定了19颗贫金属红团簇巨星的恒星大气参数，得到样本星4种α族元素(O、Mg、Ca、Si)的化学丰度．讨论了铁丰度与恒星大气参数的相关性以及α族元素丰度随金属丰度的变化，计算了共58颗红团簇巨星在I、K波段的绝对星等，讨论了它们与恒星铁丰度之间的关系．结果表明，在分析铁丰度范围内相对于I波段，K波段的绝对星等与铁丰度的相关性更弱，并且与利用理论模型得到的红团簇巨星I、K波段绝对星等与铁丰度的关系进行了比较与验证。     

红巨星中长副周期变星的起源

长副周期变星是一类特殊的半规则变星,大量存在于银河系和大小麦哲伦云中,约占渐近巨星分支的25%～30%。该类变星有两个光变周期,短的主周期为30～200 d,由径向脉动的低阶模式导致;长的副周期长达400～1500 d。在长周期变星的周光关系中,长副周期变星处在D序列,平行于径向脉动的A、B和C序列,且与属于双星系统的E序列部分重叠。长副周期现象的起源一直是个谜,现存的解释模型有:径向脉动、非径向脉动、双星、磁活动、带有一个巨大黑子的转动红巨星、转动的椭球红巨星、κ机制等,但这些模型没有一种能完美地解释长副周期变星的起源。     

The Red Rain Phenomenon of Kerala and its Possible Extraterrestrial Origin

A red rain phenomenon occurred in Kerala, India starting from 25th July 2001, in which the rainwater appeared coloured in various localized places that are spread over a few hundred kilometers in Kerala. Maximum cases were reported during the first 10 days and isolated cases were found to occur for about 2 months. The striking red colouration of the rainwater was found to be due to the suspension of microscopic red particles having the appearance of biological cells. These particles have no similarity with usual desert dust. An estimated minimum quantity of 50,000 kg of red particles has fallen from the sky through red rain. An analysis of this strange phenomenon further shows that the conventional atmospheric transport processes like dust storms etc. cannot explain this phenomenon. The electron microscopic study of the red particles shows fine cell structure indicating their biological cell like nature. EDAX analysis shows that the major elements present in these cell like particles are carbon and oxygen. Strangely, a test for DNA using Ethidium Bromide dye fluorescence technique indicates absence of DNA in these cells. In the context of a suspected link between a meteor airburst event and the red rain, the possibility for the extraterrestrial origin of these particles from cometary fragments is discussed.     

Sodium Enrichment in Yellow Supergiants： a Perspective from the Uncertainties of Reaction Rates

Sodium overabundance in yellow supergiants has stumped people for more than 20 years. The purpose of this paper is to explore this problem from the perspective of nuclear physics. We investigate carefully the CNO and NeNa cycles that are responsible for sodium production. We investigate some key reactions in the appropriate network. We show whether and how the sodium output can be affected by the rate uncertainties in these reactions. In this way, we evaluate if a reaction is important enough to deserve a better determination of its rate in terrestrial laboratories.     

The Red Giants EK Eri and OU And as Probable Descendents of Ap-stars: A Search for Abundance Anomalies

Astrophysics - The red giants EK Eri and OU And, with magnetic fields Bmax=99 and 41 G, respectively, which presumably are descendents of magnetic Ap-stars, are studied. Their fundamental...     

Non-Spherical Dust Grains in the Envelopes of Late-Type Giants

Effects of the grain shape on circumstellar dust dynamics and polarization of stellar radiation are analyzed. The grains are modeled by rotating prolate and oblate spheroids. It is shown that an asymmetry of the geometry of light scattering by non-spherical particles results in a component of the radiation pressure force perpendicular to the wave-vector of incident light. For silicate spheroids, this component can exceed 20 % of . For small metallic grains, the radiation pressure force for a spheroid can be 5–10 times greater than that for a sphere of the same volume. A simple light scattering consideration demonstrates that the distinction in the scattering geometry of aligned non-spherical grains can explain the observed wavelength variations of the positional angle of polarization. This revised version was published online in September 2006 with corrections to the Cover Date.     

Spectroscopy of Giants of the Sagittarius Dwarf Galaxy

In this paper we present the first results of the analysis of intermediate resolution (Δλ ～ 3.5 Å) spectra of giants of the Sagittarius dwarf galaxy acquired using the ESO NTT telescope. From the deep CCD photometry of Marconi et al. (1998a) we have selected a sample of giants representative of the metallicity spread suggested by the comparison of the colour-magnitude diagram of Sagittarius with those of galactic globular clusters. The spectra have been used to measure radial velocities, to confirm the membership to Sagittarius, and to provide a metallicity estimate by using spectral synthesis techniques. The analyzed stars show a spread in metallicities in the range ?1.0≤ [Fe/H]≤ +0.7, some 0.5 dex more metal-rich than the photometric estimates.     

一些M型巨星的红外观测研究

本文给出一批M型巨星JHK测光结果,在此基础上讨论了M巨星在(J—H)～(H—K)双色图上的分布特点及其有效温度及总辐射能的近似求法,并依据IRAS资料讨论了它们在中远红外双色图上的分布特征。