Millimeter continuum measurements of circumstellar dust around very young low-mass stars

We investigate the question of disk formation during the protostar phase. We build on the results of Keene and Masson (1990) whose analysis of L1551 showed the millimeter continuum emission comes from both an unresolved circumstellar component, i.e., a disk and an extended cloud core. We model the dust continuum emission from the cloud core and show how it is important at 1.3 mm but negligible at 2.7 mm. Combining new 2.7 mm Owens Valley Interferometer data of IRAS-Dense cores with data from the literature we conclude that massive disks are also seen toward a number of other sources. However, 1.3 mm data from the IRAM 30 m telescope for a larger sample shows that massive disks are relatively rare, occurring around perhaps 5% of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. At 1.3 mm the median flux of IRAS-Dense cores is nearly the same as T Tauri stars in the sample of Beckwithet al. (1990). We conclude that the typical disk mass during the embedded phase is nearly the same or less than the typical disk mass during the T Tauri phase.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Circumstellar dust shells around very young and massive stars

In this paper we investigate the properties of dust in circumstellar shells around very young massive compact IR sources (Becklin-Neugebauer objects).We found no correlation between the optical depth in the centre of the 10-m band and the 3.1-m ice band. An inverse correlation between the strength of the silicate feature and the colour temperature for the 8–13 m interval was detected. Our sample of BN objects extends this kind of relation already known for Mira stars and OH/IR stars to higher optical depths.We present a radiative transfer model for BN objects and discuss its main properties. Using this model, the interpretation of the observations led to the conclusion that the type of silicates present in the dust shells of very young stellar objects is different from that type around oxygen-rich giants and supergiants. These different silicates may be tentatively identified with pyroxenes and olivines, respectively.We studied the influence of the adopted dust model in deriving source parameters of BN objects. The object W3-IRS5 was discussed in some detail.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.     

Infrared appearance of dynamical models for circumstellar dust shells around long-period variables

Synthetic brightness profiles resulting from consistent dynamical models for circumstellar dust shells around long-period variables are presented and discussed with respect to a corresponding observation of IRC +10216.     

Molecules and dust in circumstellar shells

Circumstellar shells provide a unique environment for the study of dust formation and the relation of dust composition to specific atomic and molecular components. As a specific example, the formation of carbonaceous dust is discussed in relation to the presence of polycyclic aromatic hydrocarbon molecules and their survival in the interstellar medium. Some conclusions will be drawn concerning the composition of carbonaceous dust in circumstellar sources and that in the diffuse interstellar medium.     

Parameters of very young and massive stars with dust shells

A radiative transfer model for newly formed hot stars surrounded by optically thick dust shells is applied to a number of compact infrared sources. The model parameters are derived by fitting the synthetic spectra to the observed one between 8 and 13 m and used to derive additional properties of the shells and the embedded stars.     

Extended dust shells around carbon stars resolved by “HIRAS”

We have examined forty-two carbon stars which show excess emission at 60 and/or 100µm by applying maximum-entropy image reconstruction techniques to the IRAS 60µm survey data. Thirteen stars are found to be extended in the reconstructed images. Four of them show a detached ring centered on the stellar position. In particular, U Ant may have a double detached dust shell. The implications of our results are discussed concerning the variation of mass loss on the AGB evolution.     

The evidence of circumstellar shells for K type stars

Photometric data between 0.4 and 20 are discussed for K stars. Evidence of IR excess in the 11 region has been found for five stars in a sample of fourteen. The hypothesis of the presence of circumstellar matter in K stars seems to be confirmed.

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Sono stati analizzati dati fotometrici per stelle K tra 0.4 e 20 . Si è riscontrata la presenza di un eccesso infrarosso nella regione intorno a 11 per 5 stelle su 14. Sembra così trovare conferma la ipotesi della presenza di materia circumstellare anche in stelle K.

     

The effect of magnetic fields on the formation of circumstellar discs around young stars

We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation.     

Cold dust around chamaeleon stars

Here we present the results of 1.3 millimetre continuum measurements for intermediatemass stars in the Chamaeleon system of dark clouds. The detected millimetre radiation is most probably thermal emission from cold circumstellar dust grains. The measured millimetre fluxes are combined with infrared observations to model the broad-band spectral energy distribution (SED). In this way the parameters of the emission regions are determined.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

A study of some stars with circumstellar dust envelopes. I

We present the results of a study of circumstellar dust envelopes of 36 stars of early(O-B-A) types in the directions of the associations Cas OB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640 Å, the linear radius of the dust envelopes, the mean value of the coefficient k, and the masses of the envelopes. They differ significantly from one another.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

Dust shells of distant young stars

TheIRAS colours of some B stars with anomalous UV extinction confirm that they have circumstellar dust.     

Chemical evolution of circumstellar matter around young stellar objects

Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models.     

Infrared spectra of evolved stars with unusual dust shells

New mid-infrared spectra are presented of a number of oxygen-rich evolved stars which have IRAS LRS (Low Resolution Spectrometer) spectra that were classified as showing SiC emission. Two of the sources, IRC−20445 and IRC−20461, show the unidentified infrared (UIR) bands superposed on silicate emission features. Both objects have been classified as M supergiants. Several other sources show three-component spectra, with peaks at 10, 11 and 13 μm. The 13-μm source FI Lyr shows a narrow emission feature at 19 μm. Emission by oxide grains may be responsible for the 11-, 13- and 19-μm features. One object, IRC−20455, shows a self-absorbed silicate feature. There is no clear evidence for SiC emission in any of the spectra: the LRS spectra were erroneously classified as showing SiC emission because of the relatively strong 11-μm emission.     

The nature of the 10 and 20 micrometer features in circumstellar dust shells

Photographic reduction techniques are compared using an iris photometer, GALAXY and PDS for measurement of two plates (B andV) of the globular cluster M15. The comparison shows that satisfactory results can be obtained by any of these methods in the outer areas of globular clusters, but in inner regions, due to crowding of stellar images and to the influence of the background density, the most appropriate technique is the procedure applied to PDS scans of the plates.     

Dust shells around certain early-Type stars with emission lines

The results of an investigation of circwnstellar dust shells around 22 early- type stars with emission lines are given. The absorption at 1640 Å, the linear sizes, and the masses of the dust shells were determined. They differ from one another (see Table 1). In determining the mass of a shell, the radius of the H II zone was taken into account, where the average electron density in the gaseous shell was taken to be n_e = 2.5·10⁵ cm^-3.