Submillimetre photometry of post-asymptotic giant branch stars

Stars in the post-asymptotic giant branch (post-AGB) phase of evolution are surrounded by detached circumstellar envelopes containing dust which emits thermally in the mid- and far-infrared. Here we present 850-μm SCUBA photometry of nine candidate post-AGB stars. All targets are detected at 850 μm and we use these fluxes to estimate the envelope dust masses and, by comparison with the 100-μm IRAS fluxes, the dust emissivity index.     

Axisymmetry in protoplanetary nebulae – II. A near-infrared imaging polarimetric survey

In an imaging polarimetry survey of candidate post-AGB stars, scattering envelopes are detected around 20 objects. These envelopes represent the final mass-loss phases at the end of the AGB. In all cases, evidence for axisymmetry in the dust density is seen, suggesting that the presence of an axisymmetric outflow may be a ubiquitous phenomenon of the AGB to post-AGB transition. We use the polarized flux images to classify the objects into detached shell, bipolar and unresolved targets. Modelling based on a simple axisymmetric shell geometry supports the contention that post-AGB objects fall into one of two classes that differ in the amount of dust in the circumstellar environment: the detached shells correspond to stars with an optically thin expanding circumstellar envelope (CSE) whereas the bipolar and unresolved targets have optically thick dust structures, probably in the form of discs, which remain bound to the star, rather than partaking in the expansion of the AGB CSE. It is suggested that this bifurcation in morphology is rooted in the presence or absence of a binary companion, which determines whether or not a disc forms. Because the detached shell objects also appear axisymmetric, an additional mechanism for generating the axisymmetry, such as a magnetically shaped outflow, is needed if they do indeed have single star progenitors.     

UBV observations of post-AGB supergiants and peculiarities of their photometric variability

The systematic UBV observations of six variable post-AGB supergiants in 1991–1999 are presented. Their variability is analyzed. The coolest stars V1027 Cyg and V354 Lac exhibit bimodal pulsations with variable amplitudes. Apart from pulsations, the hotter stars V887 Her and IRAS 19386+0155 show light variations associated with a stellar wind. A variable stellar wind appears to be mainly responsible for the photometric variations in the still hotter stars SAO 163075 and IRAS 20572+4919. Distinct trends in the yearly mean brightness have been found in three of the six supergiants studied, with the trend amplitude being independent of the spectral range. They are interpreted as the result of dust envelopes composed of large grains with R=A _V /E(B?V)≥7 becoming optically thin.     

The properties of a large sample of OH/IR stars,S stars and C-rich AGB stars

We collected infrared and radio data on 110 OH/IR stars, 65 S stars and 184 C-rich stars from the literature. We analyzed their spectral energy distributions, bolometric magnitudes, infrared colors and mass-loss rates. Our study confirms that OH/IR stars and C-rich stars reach similarly high mass-loss rates at similar luminosities, supporting the idea that mass-loss rates are determined by internal stellar activities such as pulsation rather than the properties of their envelopes. The mass-loss rates of OH/IR stars and C-rich stars are strongly correlated with the color indices K-[A], K-[C], K-[D] and K-[E], and the two populations can be described with one formula. Our study also reveals that only those stars with color indices K-[A]<4 or K-[C]<5 have SEDs that peak in the near-infrared waveband.     

The UV enigma of post-starburst galaxies

We have studied the panchromatic broad-band properties from the FUV to the MIR of a sample of 808 post-starburst galaxies. We find that in the optical and near-IR bands post-starburst galaxies (PSGs) form a remarkably uniform class of objects and that, on average, simple populations synthesis models (SSP) reproduce very well the SEDs of PSGs over a broad wavelength range, but not in the UV. We also find that, while the photometric variance in the optical and near-IR properties of the sample is small and comparable to the observational errors, both in the UV and the mid-IR the observed variance is much larger than the errors. We find a strong correlation between the UV fluxes and those in the mid-IR, indicating that the large variance in UV properties of PSGs could be related to a non-uniform distribution of dust covering the intermediate age populations. The disagreement between models and observations in the UV could be due to inadequate modelling; to the contribution of AGB and post-AGB stars; or to a non-uniform distribution of dust; possibly all three. Further progress in understanding this important class of galaxies, therefore, requires at the same time better modelling and better observations in the UV and mid-IR.     

Acceleration and Transport of Energetic Charged Particles in Space

Recent advances in constructing stellar evolution models of hydrogen-deficient post-asymptotic giant branch (AGB) stars are presented. Hydrogen-deficiency can originate from mixing and subsequent convective burning of protons in the deeper layers during a thermal pulse on the post-AGB (VLTP). Dredge-up alone may also be responsible for hydrogen-deficiency of post-AGB stars. Models of the last thermal pulse on the AGB with very small envelope masses have shown efficient third dredge-up. The hydrogen content of the envelope is diluted sufficiently to produce H-deficient post-AGB stars (AFTP). Moreover, dredge-up alone may also cause H-deficiency during the Born-again phase (LTP). During the second AGB phase a convective envelope develops. A previously unknown lithium enrichment at the surface of Born-again stellar models may be used to distinguish between objects with different post-AGB evolution. The observed abundance ratios of C, O and He can be reproduced by all scenarios if an AGB starting model with inclusion of overshoot is used for the post-AGB model sequence. An appendix is devoted to the numerical methods for models of proton capture nucleosynthesis in the He-flash convection zone during a thermal pulse.     

Be/X射线双星包层性质的研究

Be单星和Be/X射线双星作为一类特殊早型天体和特殊的大质量X射线双星 ,在各个波段都有与其它相同光谱型的B型天体显著不同的特征 ,因此长期以来引起中外天文学者的关注。首先在可见光波段发射线的存在 ,就是对仅产生吸收线的经典大气的挑战 ;其次Be星作为一类早型带有包层天体 ,研究Be星包层的性质 ,对研究原恒星包层性质和进一步了解早型星其它光谱型的性质是非常重要的 ;再者可以研究Be星的存在与星际磁场或湍流的星际介质是否有关 ;最后研究Be/X射线双星 ,对双星的演化模型也有很重要的作用。随着天文观测手段的不断完善和理论模型的发展 ,我们对Be星的现象有了更深的了解 ,并且产生了一些模型。研究内容包括Be星包层和星风的性质 ,以及包层形成机制 ,Be/X射线双星物质相互作用等。本文共分五章 ,第一章主要概要介绍Be和Be/X射线双星历史和目前已经取得的成就 ;第二章介绍Be单星多波段观测结果 ;第三章介绍Be/X射线双星的观测结果 ;第四章介绍目前主要的Be单星和Be/X射线双星模型 ;第五章给出Be/X射线双星XPer/ 4U0 352 30的分光观测结果 ,并结合单臂振动盘模型给出定性解释。     

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.     

Binary (Post) AGB evolution

In this contribution the observational characteristicsof F to G type post-AGB stars, showing both O-rich andC-rich circumstellar chemistry, are reviewed. It turnsout that binarity and the presence of a stablecircumbinary dusty disc are fundamental properties of these objects. The possible common origin of the mixed chemistry observed in these systems and in IR-bright [WC] stars is discussed.     

Detection of detached dust envelopes around oxygen-rich AGB stars

Extended emission components are clearly found in the IRAS scan data of optically visible oxygen-rich AGB stars which show no 10µm silicate band feature in the IRAS LRS spectra but a strong infrared excess in the IRAS photometric data. It is most likely that these stars really have their circumstellar dust envelopes, which are detached from the central stars, indicating a halting of mass loss for a significant period.     

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.     

Extrasolar planets and the rotation and axisymmetric mass-loss of evolved stars

I examine the implications of the recently found extrasolar planets on the planet-induced axisymmetric mass-loss model for the formation of elliptical planetary nebulae (PNe). This model attributes the low departure from spherical mass-loss of upper asymptotic giant branch (AGB) stars to envelope rotation which results from deposition of orbital angular momentum of the planets. Since about half of all PNe are elliptical, i.e., have low equatorial to polar density contrast, it was predicted that about 50 per cent of all Sun-like stars have Jupiter-like planets around them, i.e., a mass about equal to that of Jupiter, M _J, or more massive. In the light of the new findings that only 5 per cent of Sun-like stars have such planets, and a newly proposed mechanism for axisymmetric mass-loss, the cool magnetic spots model, I revise this prediction. I predict that indeed ∼50 per cent of PN progenitors do have close planets around them, but the planets can have much lower masses, as low as ∼0.01 M _J, in order to spin-up the envelopes of AGB stars efficiently. To support this claim, I follow the angular momentum evolution of single stars with main-sequence mass in the range of 1.3–2.4 M_⊙ , as they evolve to the post-AGB phase. I find that single stars rotate much too slowly to possess any significant non-spherical mass-loss as they reach the upper AGB. It seems, therefore, that planets, in some cases even Earth-like planets, are sufficient to spin-up the envelope of these AGB stars for them to form elliptical PNe. The prediction that on average several such planets orbit each star, as in the Solar system, still holds.     

Fitting the integrated spectral energy distributions of galaxies

Fitting the spectral energy distributions (SEDs) of galaxies is an almost universally used technique that has matured significantly in the last decade. Model predictions and fitting procedures have improved significantly over this time, attempting to keep up with the vastly increased volume and quality of available data. We review here the field of SED fitting, describing the modelling of ultraviolet to infrared galaxy SEDs, the creation of multiwavelength data sets, and the methods used to fit model SEDs to observed galaxy data sets. We touch upon the achievements and challenges in the major ingredients of SED fitting, with a special emphasis on describing the interplay between the quality of the available data, the quality of the available models, and the best fitting technique to use in order to obtain a realistic measurement as well as realistic uncertainties. We conclude that SED fitting can be used effectively to derive a range of physical properties of galaxies, such as redshift, stellar masses, star formation rates, dust masses, and metallicities, with care taken not to over-interpret the available data. Yet there still exist many issues such as estimating the age of the oldest stars in a galaxy, finer details of dust properties and dust-star geometry, and the influences of poorly understood, luminous stellar types and phases. The challenge for the coming years will be to improve both the models and the observational data sets to resolve these uncertainties. The present review will be made available on an interactive, moderated web page (), where the community can access and change the text. The intention is to expand the text and keep it up to date over the coming years.     

Chemical Evolution from the AGB to the Planetary Nebula Phase

An overview is given on the wealth of data recently provided by large mm-wave radiotelescopes on AGB stars, planetary nebulae (PNe), and transition objects. The observations reveal that there is an observable chemical evolution in the neutral gas as a star evolves beyond the AGB, through the proto-PN and PN phases. Significant changes in the abundances of some key molecules (such as CS, CN, HCO+, HCN, and HC3N) take place during the fast evolution of the envelopes. Chemistry can thus be used as a rough clock to date the evolutionary stage of post-AGB envelopes and proto-PN objects. However, once the PN is formed, the observed abundances in the molecular clumps of the envelope remain relatively constant. The chemical evolution of the molecular envelopes likely occurs through the development of photon-dominated regions produced by the ultraviolet field of the central star. The main chemical processes which likely control the evolution are also reviewed. This revised version was published online in September 2006 with corrections to the Cover Date.     

Classification of Stars by the Reddening-Free Paraneters Q and D

Relationships between two interstellar reddening-free quantities, the photometric parameter Q and the Balmer jump D, are derived for main sequence O9-G0 stars. We compare our average values of the Balmer jumps for main sequence stars with a theoretical model of the Balmer jump as a function of the effective temperature for main sequence stars. The theoretical and observational data, as well as our calculations, are shown to correlate well.     

Study of star formation in RCW 106 using far-infrared observations

High-resolution far-infrared observations of a large area of the star-forming complex RCW 106 obtained using the TIFR 1-m balloon-borne telescope are presented. Intensity maps have been obtained simultaneously in two bands centred around 150 and 210 μm. Intensity maps have also been obtained in the four IRAS bands using HIRES-processed IRAS data. From the 150- and 210-μm maps, reliable maps of dust temperature and optical depth have been generated. The star formation in this complex has occurred in five linear sub-clumps. Using the map at 210 μm, which has a spatial resolution superior to that of IRAS at 100 μm, 23 sources have been identified. The spectral energy distribution (SED) and luminosity of these sources have been determined using the associations with the IRAS maps. The luminosity distribution of these sources has been obtained. Assuming these embedded sources to be zero-age main-sequence stars and using the mass–luminosity relation for these, the power-law slope of the initial mass function is found to be −1.73±0.5 . This index for this very young complex is about the same as that for more evolved complexes and clusters. Radiation transfer calculations in spherically symmetric geometry have been undertaken to fit the SEDs of 13 sources with fluxes in both the TIFR and the IRAS bands. From this, the r ⁻² density distribution in the envelopes is ruled out. Finally, a correlation is seen between the luminosity of embedded sources and the computed dust masses of the envelopes.     

Observational support for radiative pumping of SiO masers in evolved stars

SiO maser emission in AGB stars is found to be systematically distributed in ring structures, displaying tangential linear polarization. Using the VLBA, we have performed observations of such SiO maser emission at 43 GHz in the circumstellar envelopes of several late-type stars. We simultaneously mapped the v=1 and v=2, J=1–0 transitions. Our new maps show, for most spots, a systematic spatial shift between both maser lines. We discuss the implication of these results on the possible pumping mechanisms, concluding that the existing data favour radiative pumping schemes.     

On the Red Edge of the δ Scuti Instability Strip

The S Scuti star catalogue is used to derive the observational locations of such stars on the HR diagram. The theoretical and observational instability strips are compared to check the theoretical red edge obtained by considering nonlocal time-dependent convection theory. The observational instability strip almost overlaps with the theoretical one, but the observed blue and red envelopes are hotter than the theoretical edges. The distribution of S Scuti stars in the pulsation strip is not uniform.     

Long-term behaviour of three well-known flare stars of the UV Ceti type

Long-term changes in two flare stars, EV Lac and BY Dra, have been detected by Mavridiset al. (1982). These long cyclic periods have been registered in theB-band at their quiescent-state luminosity as well as at their flare activity. Those observational data of the two previously mentioned flare stars as well as another observational data of the flare star, UV Cet, at its flare activity were submitted to our computer programme for periodicity detection excluding cyclic periods which have amplitudes less than tenth magnitude for great significance. Some of our results coincided with the long cyclic periods detected by Mavridiset al. (1982) while the remainder of our results have had no complete coincidence with those registered by Mavridiset al. (1982).     

Spectral energy distribution of OH megamaser galaxies

The properties of OH megamaser galaxies are analyzed. Using the linear dependence of L _OH on L _FIR and the luminosity function for IR galaxies, the expected number of megamasers is estimated. From observational data at many frequencies (from radio waves to x rays), the spectral energy distributions (SEDs) of 16 objects have been plotted and studied. The SEDs of megamasers have the same form from the radio to the x ray range. Considerable emission is observed in the 3 μm range, which evidently indicates the presence of red giants and supergiants in these objects. Possible reasons for the departure of the slope of the dependence of L _OH on L _FIR from unity are considered. The assumption that collisional pumping may play an important role in the inversion of ground levels of the OH molecule and the saturated gain mechanism in megamasers is confirmed. Translated from Astrofizika, Vol. 42, No. 1, pp. 23–36, January–March, 1999.