G. Silvestro

Mid‐infrared imaging of AGB star envelopes

The recent development of bi‐dimensional detectors operating in the mid‐infrared has opened new possibilities for the observation and analysis of the high mass loss processes in the last stages of stellar evolution. Radio (CO) and maser (OH) observations show that an important role is played by intermediate and low mass stars (1–8 M⊙) in the TP‐AGB phase, and by their circumstellar envelopes of gas and dust. We analyze the problem of characterizing the chemical nature of the AGB envelopes (C‐rich or O‐rich) through mid‐infrared observations by means of a suitable photometric system.

Theoretical modelling of dust shells around AGB stars

A numerical code is developed, which integrates the equation of non‐grey radiative transfer through a spherically symmetric dust shell around an evolved AGB star. The code computes, by an iterative method, a self‐consistent thermal structure of the envelope, for multiple grain components of carbon‐ and oxygen‐rich material. The emergent radial brightness distribution at different wavelengths is calculated taking into account the effect of non‐isotropic scattering, absorption, and thermal reemission by grains. Our model results, for different sets of stellar and nebular parameters, are compared with observational data on oxygen‐ and carbon‐rich AGB stars with envelopes of different optical depths.

NEAR-IR CHARACTERIZATION OF AGB SOURCES

AGB stars are surrounded by cool circumstellar envelopes (CSEs) of gas and dust that are strong sources of IR thermal radiation.

Dust Envelopes and IR Excesses in a Sample of RS CVn-Type Binaries

In recent years, we have been studying the energy distribution of RS CVn binaries from UV to far IR wave lengths, in order to derive general properties of the systems and to better understand their evolutionary scenario, including the history of mass loss (Busso et al., 1987, 1988). In this framework, we have so far analyzed, at different level of accuracy, a total of about 60 sources, among which 30 have been the object of detailed studies, on the basis of broad band photomertic observations from 0.35 to 60 µm, using ground-based telescopes (mainly at La Silla, ESO) and IRAS-PSC information. For these 30 binaries, the quasi-periodic variations due to the presence of photospheric spots have been carefully subtracted, to derive the energy distributions of the unperturbed systems.

IRAS and Near Infrared Observations of Peculiar Nebulosities

We used the IRAS survey to search for IR emission from 95 peculiar nebulosities with CO emission. Fifty-two IRAS sources are associated with them. We discuss the nature of the sources on the basis of their IRAS color-color diagram and of near-IR observations.

Evidence of Thin Dust Shells in Some RS CVN Stars

UBVRIJHK light curves of four active RS CVn binaries together with coordinated spectrophotometric observations in the range 1.4–2.5 μm are presented. From the reconstruction of the whole spectral distributions (from U to K) we may infer the presence of infrared excesses which cannot be simply explained by a combination of stellar spectra (even changing the accepted spectral types) and of spot-related photospheric perturbations. Our analysis shows that thermal radiation from a thin dust shell surrounding the system may explain the data.

A Thin- Shell Model for Molecular Outflows

The morphology and emissivity of shell-shaped bipolar outflows, driven by a spherically-symmetric stellar wind which interacts with a non-isotropically distributed cir-cumstellar gas within a dense molecular cloud, was estimated previously (Silvestro and Robberto, 1987) in the framework of the shell model of collimated outflows suggested by Barrai and Canto (1981). Our computation allowed to obtain a spatial map of the profiles of the molecular lines emitted in the region of interaction between the wind and the circumstellar material. The position-velocity map of the rotational CO lines from the source Mon R2 turns out to be in very good agreement with the assumed thin-shell model for the outflowing lobe.

An Outflow Model for Bipolar Planetary Nebulae and the Case of NGC 6302

NGC 6302 is a bright, bipolar planetary nebula (PN) that exhibits a wide range of excitation and ionization conditions, together with complex structure and kinematics. High-velocity (> 300 Km s−1) flows were detected in the nebula, which are believed to originate as a wind from the central star (Elliot and Meaburn, 1977). A very high effective temperature ~2x105K is estimated (Rodriguez et al., 1985) for the central star, which is heavily obscured from view, being located within a neutral filament in the nebular core.