The ney-allen nebula: an unambiguous example of circumstellar dust emission

Diffraction-limited array images of the Trapezium/Ney Allen infrared nebula have been obtained at six wavelengths between 7.8 and 12.4 microns, including the 9.7 micron silicate feature. Extended emission from warm dust shows significant differences in structure around each of the four Trapezium stars. The most dramatic infrared source is associated with ¹ Orionis D, where the bright mid-infrared emission is found to be a distinct crescent-shaped ridge or shell, concentric with the O star. This unambiguous relationship between a known type stellar luminosity source and a distinct circumstellar dust cloud of known distance and dimensions provides a unique opportunity to test the predictions of dust grain emission models for circumstellar infrared sources.     

BN天体尘壳性质的探讨

本文对ＢＮ天体的红外连续辐射多波段测光资料进行理论分析研究，采用光学薄的球对称尘壳模型，推算出ＢＮ天体尘壳的物理参数：光学厚度及红外连续发射率的波长分布，密度与温度的空间分布，尘壳总质量。     

Cloudy circumstellar dust shells around young variable stars

A number of variable stars of the Orion population has been identified with IRAS point sources by us. This finding supports the conclusion that the prominent Algol-like minima in the lightcurves of these stars originate from obscurations by dust clouds in a circumstellar shell. The discussion of the existingUBVR data leads to the remarkable conclusion that the extinction properties of the grain populations contained in individual dust clouds moving in one and the same circumstellar shell are quite different.From the multicolour photometric data of the different Algol-like minima we derived individual values of the reddening parameterR = A _v /E(B - V). It covers a remarkable wide range of values from that one typical of the interstellar extinction law up to 7. In the case of SV Cep one of the grain populations produces a virtually neutral extinction. The large values ofR speak in favour of larger than normal (interstellar) dust grains, which may have grown by coagulation processes. The cloudy circumstellar dust shell provides a natural explanation for the observed infrared excess. The properties derived from the optical light variations are fully compatible with the properties deduced from the infrared radiation. The irregularity of the light variations indicates that many clouds are involved and may sometimes superimpose themselves.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

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。     

Near Infrared Observations of Sakurai's Object during 1998–99

We present new near-infrared observations of Sakurai's Object obtainedduring 1998–99 when this final helium shell flash object was in the dustcondensation phase. The infrared colours have reddened compared to earlierepochs, indicating increased dust condensation. The infrared spectrareveal all the features of a carbon star superposed on a dust continuum.     

Scattered light models of the dust shell around HD 179821

The dust shell around the evolved star HD 179821 has been detected in scattered light in near-IR imaging polarimetry observations. Here, we subtract the contribution of the unpolarized stellar light to obtain an intrinsic linear polarization of between 30 and 40 per cent in the shell that seems to increase with radial offset from the star. The J - and K -band data are modelled using a scattering code to determine the shell parameters and dust properties. We find that the observations are well described by a spherically-symmetric distribution of dust with an r ⁻² density law, indicating that when mass-loss was occurring, the mass-loss rate was constant. The models predict that the detached nature of a spherically-symmetric, optically-thin dust shell, with a distinct inner boundary, will only be apparent in polarized flux. This is in accordance with the observations of this and other optically-thin circumstellar shells, such as IRAS 17436+5003. By fitting the shell brightness we derive an optical depth to the star that is consistent with V -band observations and that, assuming a distance of 6 kpc, gives an inner-shell radius of     , a dust number density of     at r _in and a dust mass of     . We have explored axisymmetric shell models but conclude that any deviations from spherical symmetry in the shell must be slight, with an equator-to-pole density contrast of less than 2:1. We have not been able to fit simultaneously the high linear polarizations and the small     colour excess of the shell and we attribute this to the unusual scattering properties of the dust. We suggest that the dust grains around HD 179821 either are highly elongated or consist of aggregates of smaller particles.     

Infrared spectroscopy of Nova Cassiopeiae 1993 – IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared (IR) spectral energy distribution (SED) with the dusty code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of 2:1 by number (  ∼1.26:1  by mass) and a relatively flat size distribution. The 9.7- and 18-μm silicate features are consistent with freshly condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions  ∼0.06 μm  ; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova unidentified infrared (UIR) features.     

Infrared emission from 6.7-GHz methanol maser sources

Near-infrared photometry was performed on 56 southern 6.7-GHz methanol maser sources. A simple spherically symmetric model of the radiative transfer through a dust shell was developed and used to study the conditions in the dust cloud in which the masers are produced. The parameters investigated were the size of the cloud, the spectral type of the embedded star, the optical depth of the dust cloud and the dust density distribution. It was found that the infrared colours of the models have a complex dependence on the parameters and that no unique combination of parameter values explains the spectral energy distribution of any particular source. The model effectively reproduces the far-infrared ( IRAS ) colours but cannot simultaneously explain the near-infrared colours for any of the observed sources.     

Massive star formation in galaxies: radiative transfer models of the UV to millimetre emission of starburst galaxies

We present illustrative models for the UV to millimetre emission of starburst galaxies which are treated as an ensemble of optically thick giant molecular clouds (GMCs) centrally illuminated by recently formed stars. The models follow the evolution of the GMCs owing to the ionization-induced expansion of the H  ii regions and the evolution of the stellar population within the GMC according to the Bruzual & Charlot stellar population synthesis models. The effect of transiently heated dust grains/PAHs on the radiative transfer, as well as multiple scattering, is taken into account.
The expansion of the H  ii regions and the formation of a narrow neutral shell naturally explain why the emission from PAHs dominates over that from hot dust in the near- to mid-infrared, an emerging characteristic of the infrared spectra of starburst galaxies.
The models allow us to relate the observed properties of a galaxy to its age and star formation history. We find that exponentially decaying 10⁷–10⁸ yr old bursts can explain the IRAS colours of starburst galaxies. The models are also shown to account satisfactorily for the multiwavelength data on the prototypical starburst galaxy M82 and NGC 6090, a starburst galaxy recently observed by ISO . In M82 we find evidence for two bursts separated by 10⁷ yr. In NGC 6090 we find that at least part of the far-infrared excess may be due to the age of the burst (6.4×10⁷ yr). We also make predictions about the evolution of the luminosity of starbursts at different wavelengths which indicate that far-infrared surveys may preferentially detect older starbursts than mid-infrared surveys.     

On the uncertainty of the transmission function of the optically thick AGB dust shells

The uncertainty of the spectral transmission function due to the nonsphericity of cosmic dust particles is analysed for optically thick C-rich and O-rich dust shells. The transmission function directly prescribes the intensity of radiation that passes through a dust shell. It is shown that nonspherical particles affect the stellar spectra in a different way than a system of equally sized spherical particles. Discrepancies in the stellar spectra for both morphological models (spheres and irregular targets) grow with optical thickness of the dust shell and the spectral behaviour of the optical thickness depends on the particle size distribution function. In particular, two most frequently used distrubutions, a power law and Dirac’s delta function, are considered in the presented numerical runs. Light transmission through C-rich dust shells is strongly influenced by absorption which dominates in case of carbonaceous particles. Irregularly shaped carbonaceous particles reduce the intensity of stellar spectra more efficiently than spherical particles of the same composition (the difference is about 10–30%). MgO particles which may be present in O-rich dust shells are almost pure scatterers, thus the ratio of transmission functions for irregularly shaped particles and spherical ones show specific resonant features (especially at wavelengths below 0.4 μm).     

V838 Mon: light echo evolution and distance estimate

Following its 2002 February eruption, V838 Mon developed a light echo that continues to expand and evolve as light from the outburst scatters off progressively more distant circumstellar and/or interstellar material. Multifilter images of the light echo, obtained with the South African Astronomical Observatory (SAAO) 1.0-m telescope between 2002 May and 2004 December, are analysed and made available electronically. The expansion of the light echo is measured from the images and the data compared with models for scattering by a thin sheet and a thin shell of dust. From these model results we infer that the dust is probably in the form of a thin sheet distant from the star, suggesting that the material is of interstellar origin, rather than being from earlier stages in the evolution of the star. Although the fit is uncertain, we derive a stellar distance of ∼9 kpc and a star–dust distance of ∼5 pc, in good agreement with recent results reported from other methods. We also present JHKL  and Cousins UBVRI  photometry obtained at the SAAO during the post-outburst second, third and fourth observing seasons of the star. These data show complex infrared colour behaviour while V838 Mon is slowly brightening in the optical.     

The surface brightness of IRAS galaxies

Using IRAS measurements, we find positive correlations between both the infrared to optical flux ratio (L _FIR/L _B) and infrared colour temperature (L ₆₀/L ₁₀₀) with optical surface brightness. These correlations can be understood by high surface brightness galaxies having (i) a high star formation rate, or (ii) a high-space density of stars and dust.In an attempt to interpret (ii) above, we have produced radiative transfer models for the dust absorption in a galactic disc. These models indicate that the highest surface brightness galaxies may be the most dust obscured (i.e., optically thick) and that the total luminosity (and, hence, mass) of these galaxies may be considerably underestimated.     

Observations of the infrared source in s 140 and the heating of the associated gas

Near infrared observation of the infrared source in S140 was made using the 1.26 m infrared telescope of Beijing Observatory, yielding fluxes in the J, H and K bands. Combining with data from satellite and ground-based IR and submillimetre observations, we found the spectral gradient, IR luminosity and the shell structure of the IR source.

We also combined observations of CO and NH₃ in an analysis of the heating of the gas associated with the source. Besides through dust, the central source may also input energy into the gas through shock waves. External sources of heat and heating by photoelectric process and cosmic rays are also discussed.     

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.     

Baryonic fraction in the cold plus hot dark matter universe

The observational infrared spectra of a number of Wolf–Rayet stars of WC8–9 spectral classes are shown to be quite satisfactorily explained by making use of the detailed theoretical model of a dust shell made up of spherical amorphous carbon grains, the dynamics, growth–destruction, thermal and electrical charge balance of which are taken into account. The dust grains acquire mainly positive electrical charge, move with suprathermal drift velocities and may grow up to 100–200 Å as a result of implantation of impinging carbon ions. For most of the stars the fraction of condensed carbon does not exceed 1 per cent. While the nature of the grain nucleation remains unknown, the condensation distances and the grain seed production can be estimated by fitting the observational spectra with theoretical ones.     

Comparison of dust‐to‐gas ratios in luminous,ultraluminous, and hyperluminous infrared galaxies

The dust‐to‐gas ratios in three different samples of luminous, ultraluminous, and hyperluminous infrared galaxies are calculated by modelling their radio to soft X‐ray spectral energy distributions (SED) using composite models which account for the photoionizing radiation from H II regions, starbursts, or AGNs, and for shocks. The models are limited to a set which broadly reproduces the mid‐IR fine structure line ratios of local, IR bright, starburst galaxies. The results show that two types of clouds contribute to the IR emission. Those characterized by low shock velocities and low preshock densities explain the far‐IR dust emission, while those with higher velocities and densities contribute to the mid‐IR dust emission. Clouds with shock velocities of 500 km s^–1 prevail in hyperluminous infrared galaxies. An AGN is found in nearly all of the ultraluminous infrared galaxies and in half of the luminous infrared galaxies of the sample. High IR luminosities depend on dust‐to‐gas ratios as high as ∼0.1 by mass, however most hyperluminous IR galaxies show dustto‐gas ratios much lower than those calculated for the luminous and ultraluminous IR galaxies. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Observations and Modeling of B[e] Stars

We give a short definition of B[e] star and we show spectrocopicvariabilities obtained for the peculiar star HD 45677 especialy for the strong He I line at 5876 Å. We present also some photometric observations for this type of stars that show a important excess in thenear and mid infrared due to the presence of circumstellar dust. The modelingof these stars shows that a simple model with a spherical dust shell fits well the observations. High spatial resolution gives an additional constraint on the modeling.     

An analysis of the energy distribution of some Be stars in the Northern hemisphere

All the available published fluxes and photometry, from the ultraviolet to the infrared of 17 bright Be stars, have been compiled for energy distribution investigation. Most of these stars were found to have intrinsic colour excesses of order of 0^m.14. The possible interpretation of this may be viewed as a result of dust formation in a shell around the star or the binary system. No traces of cool stellar components of these stars have been found in the energy distributions, even for those known to be binaries. For the Be stars which exhibit pulsation, we did not find any remarkable difference from the other Be stars in the energy-distribution curves.     

S140红外源的近红外观测——成协气体的供热

对S140中的红外源用北京天文台1.26米红外望远镜进行了近红外测光,获得了J.H.K三个波段的流量值,并利用红外天文卫星及地面红外和亚毫米波观测资料作了光谱综合分析,得出光谱斜率、红外光度和红外源的壳层结构。 本文还结合CO及NH_3等观测结果分析了红外源成协气体的供热,除了通过尘埃的作用,中心源还可能通过激波对气体输入能量;我们还讨论了外部热源及光电过程和宇宙线的加热作用。     

Size scale of the R Coronae Borealis variables

The radius of R CrB is estimated to 90–100 radii of the Sun. The dust layer, which is the cause of the deep minimum in the visible light of R Coronae Borealis-type stars, has a radius of about 6 radii of the star. This follows from the delay of the infrared light increase after the visual minimum and also from the analogy with the dust formation in novae. The emission lines appearing in the minimum have an origin analogous to the metal emissions in the comets of 1882 II and 1965 VIII in which they were observed at a distance of 20–30 R⊙. The permanent dust shell has a radius about of 26 stellar radii.