Spectroscopic research with the Hartebeesthoek radio telescope

Research into star-forming regions, evolved HII regions, late-type stars with circumstellar dust shells and comet Halley using spectroscopic observations made with the Hartebeesthoek radio telescope is reviewed.     

Herbig Ae/Be Stars with the IRAS Low-Resolution Spectra

IRAS low-resolution spectra are presented for 36 HAEBE stars. It is found that silicate dust is common material in the circumstellar disks or/and shells of HAEBE stars. It is also found that the PAH feature often appears as well. In addition, the correlation between the dust features and spectral type, and infrared excesses can be found for most HAEBE stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared low-resolution spectra of symbiotic stars

The IRAS low-resolution spectra (LRS) of 29 symbiotic stars are presented in this paper. It is found that silicate dust is common material in the circumstellar shells of symbiotic stars. From the IRAS low-resolution spectra, IRAS colors, symbiotic types and spectral types, some physical properties of symbiotic stars with the LRS spectra are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Circumstellar dust and circumstellar extinction curves

We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it.     

Circumstellar dust and variability of carbon stars

A sample of 35 variable carbon stars has been considered. The stellar temperatures, radii, absolute luminosities, masses, and mass loss rates have been estimated. Then the spherical circumstellar dust shells around the analysed stars are discussed and the differences in the infrared properties between semi-regular, irregular, and long-period variables are indicated.     

Terminal Outflow Velocities from the Mass-Losing AGB Stars

We have calculated terminal gas outflow velocities from the circumstellar shells of the AGB stars. Our results confirm that, in case of the stationary dust driven winds, the dynamics of the outflow is governed essentially by the dust properties and stellar luminosities, while only marginally by other stellar parameters. Although being far from comprehensive, our calculations also indicate that the dust driven winds may only occur in cool, luminous, low mass objects. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation and evolution of planetary systems: the impact of high-angular resolution optical techniques

The direct images of giant extrasolar planets recently obtained around several main sequence stars represent a major step in the study of planetary systems. These high-dynamic range images are among the most striking results obtained by the current generation of high-angular resolution instruments which will be superseded by a new generation of instruments in the coming years. It is, therefore, an appropriate time to review the contributions of high-angular resolution visible/infrared techniques to the rapidly growing field of extrasolar planetary science. During the last 20 years, the advent of the Hubble Space Telescope, of adaptive optics on 4- to 10-m class ground-based telescopes, and of long-baseline infrared stellar interferometry, has opened a new viewpoint on the formation and evolution of planetary systems. By spatially resolving the optically thick circumstellar discs of gas and dust where planets are forming, these instruments have considerably improved our models of early circumstellar environments and have thereby provided new constraints on planet formation theories. High-angular resolution techniques are also directly tracing the mechanisms governing the early evolution of planetary embryos and the dispersal of optically thick material around young stars. Finally, mature planetary systems are being studied with an unprecedented accuracy thanks to single-pupil imaging and interferometry, precisely locating dust populations and putting into light a whole new family of long-period giant extrasolar planets.     

晚型星红外谱拟合的线性化方法

本文针对晚型星红外辐射的一些模型，讨论相应的拟合方法。对有拱星壳层的晚型星的双黑体模型，给出了具体的拟合方法──线性化方法。最后我们就拟合结果及拟合程序编制方面讨论了线性化方法的优缺点。     

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.     

星周尘埃壳层的消光值

本文首次提出了从观测得到的具有星周尘埃壳层的恒星的能谱分布求取星周尘埃云的消光,并由此可通过改正星周消光改正后的星际消光法求得恒星距离的方法。     

Long baseline interferometric observations of long period variable stars

Recent observations of long-period variable stars at spatial resolutions from approximately 1 arcsec to several milli-arcsecs have provided new insights into pulsation, dust formation, and mass-loss of AGB stars. These insights have come from long baseline interferometric observations obtained across a wide range of wavelengths, from the optical, through the infrared, to wavelengths as long as several millimeters. The present status and recent results from long baseline interferometry, particularly at optical and infrared wavelengths, are discussed. Such results include diameters and limb-darkening, surface features, mode of pulsation, location of SiO masers, inner radii of dust shells, physical conditions in the dust formation zone and of the inner regions of the dust shells. The results are interpreted in terms of present models of dust formation and mass-loss.     

The structure of circumstellar envelopes

Copious mass loss on the Asymptotic Giant Branch dominates the late stages of stellar evolution. Maps of extended circumstellar envelopes provide a history of mass loss and trace out anisotropic mass loss. This review concentrates on observations of millimeter wavelength molecular line emission, on high resolution maps of maser emission and on observations of submillimeter, millimeter and radio wavelength continuum emission. Radio continuum observations show that AGB stars are larger at radio than at optical wavelengths. The extended chromospheres indicated by these observations extend to distances from the star large enough for dust to form, thereby initiating mass loss. Molecular line maps have found time-variable mass loss for some stars, including detached shells indicating interrupted mass loss and evidence for a rapid increase in the mass loss rate at the end of the AGB phase. Maps of circumstellar envelopes show evidence of flattening, bipolar outflow and angular variations in both the mass loss rate and the outflow velocity. As stars evolve away from the AGB and planetary nebula formation begins, these structures become more pronounced, and fast bipolar molecular winds are observed. The time scales derived from the dynamical times of these winds and from the expansion rates of the central planetary nebulae are very rapid in some cases, about 100 years, in agreement with the predictions of stellar evolution theory.     

Submm observations of herbig ae/be systems

Studies of circumstellar dust and gas in pre-main-sequence systems are in part motivated by a desire to probe possible sites of future or on-going planet formation and, should accretion onto young stars be taking place, to determine whether the observed remnants of embryonic envelopes might yet contribute significantly to the final masses of stars as they evolve towards the main sequence. New 0.35- to 1.3-mm flux measurements have been made for 13 intermediate-mass pre-MS systems in order to estimate total dust+gas masses and to investigate the spatial distribution of circumstellar material. At least two sources may be dominated by compact circumstellar disks; emission from extended envelopes appears to be important for the majority of sources. HD 163296 is an unusual object and warrants further examination.     

Hydrocarbon molecules in carbon stars

It is shown that the 10–13 μ band observed in the spectra of some carbon stars could arise from simple hydrocarbon molecules, especially of the type C _n H _2n . These molecules could be present in circumstellar dust shells which are responsible for the 1000 K black-body excess observed for these stars, and the emission band is produced by collisional excitation.     

Binaries and multiple systems among the young stars

Binary formation is the dominant mode of star formation in the nearby Taurus and Ophiuchus star forming regions. I describe very briefly our present understanding of multiplicity among the youngest stars, the evolution and structure of their circumstellar disks, and their potential as sites of planetary system formation.     

Theoretical studies of the infrared emission from circumstellar dust shells: the infrared characteristics of circumstellar silicates and the mass-loss rate of oxygen-rich late-type giants

We have modeled the infrared emission of spherically symmetric, circumstellar dust shells with the aim of deriving the infrared absorption properties of circumstellar silicate grains and the mass-loss rates of the central stars. As a basis for our numerical studies, a simple semianalytical formula has been derived that illustrates the essential characteristics of the infrared emission of such dust shells. A numerical radiative transfer program has been developed and applied to dust shells around oxygen-rich late-type giants. Free parameters in such models include the absorption properties and density distribution of the dust. An approximate, analytical expression is derived for the density distribution of circumstellar dust driven outward by radiation pressure from a central source. A large grid of models has been calculated to study the influence of the free parameters on the emergent spectrum. These results form the basis for a comparison with near-infrared observations. Observational studies have revealed a correlation between the near-infrared color temperature, Tc, and the strength of the 10 micrometers emission or absorption feature, A10. This relationship, which essentially measures the near-infrared optical depth in terms of the 10 micrometers optical depth, is discussed. Theoretical A10-Tc relations have been calculated and compared to the observations. The results show that this relation is a sensitive way to determine the ratio of the near-infrared to 10 micrometers absorption efficiency of circumstellar silicates. These results as well as previous studies show that the near-infrared absorption efficiency of circumstellar silicate grains is much higher than expected from terrestrial minerals. We suggest that this enhanced absorption is due to the presence of ferrous iron (Fe2+) color centers dissolved in the circumstellar silicates. By using the derived value for the ratio of the near-infrared to 10 micrometers absorption efficiency, the observed A10-Tc relation can be calibrated in terms of the total dust column density of the circumstellar shell and thus the mass-loss rate of late-type giants can easily be derived. Detailed models have been made of the infrared emission of three well-studied Miras: R Cas, IRC 10011, and OH 26.5+0.6, with the emphasis on the shape of the 10 micrometers emission or absorption feature. The results show that the intrinsic shape of the 10 micrometers resonance varies from a very broad feature in R Cas to a relatively narrower feature in OH 26.5+0.6, with IRC 10011 somewhere in between. Possible origins of this variation are discussed. The mass-loss rates from these objects are calculated to be 3 x 10(-7), 2 x 10(-5), and 2 x 10(-4) M Sun yr-1 for R Cas, IRC 10011, and OH 26.5+0.6, respectively. These results are compared to other determinations in the literature.     

Dust metamorphosis in the galaxy

Summary Cosmic dust grains play an important role for the thermal, dynamical, and chemical structure of the interstellar medium. This is especially true for the star formation process and the late stages of stellar evolution. Dust grains determine the spectral appearance of protostars, very young stellar objects with disk-like structures as well as of evolved stars with circumstellar envelopes.In this review, we will demonstrate that solid particles in interstellar space are both agent and subject of galactic evolution. We will especially discuss the different dust populations in circumstellar envelopes, the diffuse interstellar medium, and the molecular clouds with strong emphasis on the evolutionary aspects and the metamorphosis of these populations.     

Mass Loss and Circumstellar Dust Shells at T Tauri Stars

The mass loss of T Tauri stars leads to the production of dust in circumstellar space. The total amount of lost mass (and therefore of produced dust) is observed to be positiv correlated with the intensity of the H and Call emission lines of the objects. This fact is used in the present paper to explain quantitatively another correlation, namely the observation, that the interstellar extinction which is found by star counts in the wider surroundings of a T Tauri star is roughly proportional to the intensity of its Hα emission. By this, the outflow of circumstellar matter into the interstellar space seems to be observable directly.     

The physical properties of the Hα-emission line stars: CD-42°11721, KK Oph and XX Oph

By use of the data found in the literature, observed at the ESO and taken from the IRAS point source catalogue, the physical properties of the H-emission stars: CD-42°11721, KK Oph and XX Oph were studied photometrically and spectroscopically. The results can be summarized as follows: (a) Feii and [Feii] emission lines show that these stars have a gaseous envelope of which part is of low density. (b) P Cygni profiles in the spectral lines of XX Oph indicate the existence of rapid gaseous motions in its atmosphere. The presence of P Cygni profiles in the spectra of the other two stars is not certain. (c) In the outer atmosphere of CD-42°11721 and KK Oph at least two circumstellar concentrations of dust grains seems to be present; the temperatures of these dust shells are, for both stars, about 1200 and 250 K. (d) XX Oph has an M6III companion causing the infrared excess. Furthermore, we have studied the physical properties of the dust shells. The distances to the central star, the chemical composition and the masses of each shell were estimated. It is recommended to study above stars further with high dispersion spectroscopy and simultaneous long-term photometry.Based partly on data obtained at the ESO, La Silla, Chile and with the Infrared Astronomical Satellite (IRAS).     

Large-scale photometric activity of UX Ori type stars

Radical readjustments in the photometric activity of young stars owing to sudden changes in the circumstellar extinction are discussed using the light curves of two UX Ori type stars, CQ Tau and V1184 Tau, as examples. Changes of this sort can be caused by large deviations from axial symmetry in the distribution of circumstellar dust, as well as by large variations in the mass accretion rate in circumstellar disks. A large amount of dust may also appear in the vicinity of a young star owing to collisions of planetesimals. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 5–13 (February 2008).