At Queen's University, we have been undertaking a spectroscopic programme to elucidate the nature of faint blue stars at high galactic latitudes. We have identified approximately 50 that appear to spectroscopically identical (even at high resolution and signal-to-noise) to normal young Population I B-type stars in the galactic disc. However, we have also found seven faint objects () that were previously classified as Population I on the basis of photometry and/or low resolution spectroscopy; careful model atmosphere analyses of high resolution spectra now indicate that they have non-Population I compositions. Their derived atmospheric parameters are coincident with theoretical post-AGB evolutionary tracks and thus together with their peculiar composition, they would appear to be hot evolved post-AGB objects.

A set of thermal pulse models was computed, for initial stellar masses extending from low (M=1.5, 3 M⊙) to intermediate (M=5, 7 M⊙), using the FRANEC evolutionary code and assuming standard mass loss and solar metallicity. The main features are: i) the third dredge–up is naturally found, even for core masses below 0.7–0.8 M⊙; ii) before the dredge–up occurrence, the main characteristics of the models (convective shell mass, interpulse duration, overlapping between adjacent pulses) are determined solely by the core mass MH , well reproducing a behaviour which is typical in the current literature (see e.g. Schonberner, 1979): in particular, the shell mass is a decreasing function of MH ; iii) after dredge–up is started, the evolutionary track is modified and the strength of the pulses is enhanced; iv) the amount of dredge–up increases in time, from ≃ 10−4 M⊙ to ≃ 10−3 M⊙.

We discuss the evolutionary properties of shell burning star models which, due to the erosion of the external hydrogen envelope, leave the Hayashi track and evolve to the blue of the HR diagram. We analyze the properties of star models which evolve off the Asymptotic Giant Branch (AGB), the Early-AGB or the Red Giant Branch (RGB).

We study the spectral appearance of an evolving post-AGB object by means of two-fluid radiation hydrodynamics (gas and dust) in spherical symmetry. The stellar parameters as well as mass-loss rates are assumed to vary consistently with the stellar model calculations of Blöcker (1989, Diplom Thesis, University of Kiel) for a 3 M⊙ sequence, resulting in a 0.605 M⊙ remnant. The expelled mass is assumed to consist of spherical astronomical silicates with grain radii of 0.1 μm and an initial dust-to-gas mass ratio of 7.5·10−3. For further details on the computational method we refer to Szczerba & Marten (1992, in: Proc. of the ESO/CTIO Workshop, Mass Loss on the AGB and Beyond, La Serena 1992, in press).

A new numerical method allows an acurate calculation of the radiation hydrodynamics of time dependent stellar winds including also the radiation pressure on newly formed dust grains. The numerical procedure is based on an adaptive grid which distributes the grid points at locations of large gradients. All equations are written in conservation form and a monotonic 2. order transport scheme is used to advect the physical variables through the cell boundaries. We are able to resolve the shock waves running through the stellar atmospheres. These waves are generated by a pulsating star which is simulated by a moving piston. The following plots show the radial velocity and temperature structure of an extended atmosphere and several shock waves are clearly seen. Note that the innermost shock waves is a so-called supercritical shock where the radiative cooling zone behind the wave is clearly visible. The outer waves are almost isothermal because the material is optically thin in this region. The stellar parameters of this example are given by M = 1.2 M ⊙, L = 5315L ⊙ and R = 270R ⊙ and the period of the moving piston is fixed at 350 days yielding a massive and slow wind with a mass loss rate of · = 1.24 10−6M ⊙ yr−1 and a final velocity of v = 7.7 km s−1.

We wish to explore the hypothesis that the shells and halos of planetary nebulae are formed during the thermal pulsing of progenitor AGB stars. Using published data and model results we compare the AGB interpulse time ΔtT and the time between shell ejections ΔtD for a sample of PNe with halos. ΔtT is derived from the Paczynski relation using the PNe central star luminosity. ΔtD is calculated using the radii and velocity of PNe shells and halos.

There may exist stars during and just after the asymptotic giant branch (AGB) stage. V441 (89) is a candidate star. Fernie and Sasselov (1989) have discussed the evolutionary changes of the pulsation periods based on those of the effective temperature, to compare those with observational data. They used Worell's theorical results (Worell 1986) and have found that the observational changes of the period and the colors are too small compared with the theorical values. To examine the effect of opacities on the pulsations periods, we calculate the periods based on new opacities (OPAL, Iglesias and Rogers 1991), and compare them with the data reported previously.

Using the OPAL opacity, we have calculated the post-AGB evolutions of low mass stars. It is newly found that optically thick wind occurs for M core > 0.55M ⊙ and the evolutionary time scale is drastically shortened.

A simple analytical model can explain the large angular momentum loss which a star suffers in its post-main-sequence evolution up to the white dwarf stage.

The formation of planetary nebulae from the mass loss remnants of previous phases forms an interesting problem in both hydrodynamics and astrophysics. Not in the least because of the asphericity that most PNe show. The origin of this asphericity is still largely a mystery. Binarity is the most favoured explanation at the moment (Morris 1987)

We carried out the empirical investigation of the evolution of gas density distribution in the envelopes of planetary nebulae (PN). For this purpose we analysed the isophotal maps of 10 PN in the lines H alpha, H beta or in the optically thin radio continuum. To obtain the spatial radial distribution of gas density n(r) (where n = n(H)+n(He)) we used Abell's integral equation in the simplest, spherically-symmetric case. We found that n(r) for all PN envelopes in our sample can be described by an approximative expression:

An axisymmetric density distribution likely to be representative for many Planetary Nebulae (PN). We suggest that the axial symmetry of PN results from a predominant ejection of matter in the equatorial direction due to the duplicity of the central star (Livio et al.,1979, Ap. J.,188,1). We present an illustrative example of the formation of the bipolar PN Soutern Crab (He 2–104). In this model high velocity matter ejected by hot central star interacts with an outer oblate envelope located around a symbiotic binary star. The binary consists of a Mira variable, ejected matter forming a thick disk, and a hot component (Lutz et al.,1989, PASP 101,966). Accretion of some disk matter onto the hot component (dwarf) may lead to recurrent thermal shell flashes (Igumenshchev et al., 1990, Astrofizika,30,282). which result in the double shell nebula, observed in the Southern Crab (Burgarella et al., 1991, A&A,249,199). The phase of forming a single shell stucture of the Southern Crab was simulated numerically. The results of a 2D hydrodynamical nonadiabatic calculation are illustrated in the Figure. The density contours with logarithmic spacing of 0.25 and the velocity field (arrows) of the model are shown at time t ≃ 360 yrs after the central star outburst with energy 1044erg. The dense shell may be observed as a Crab-like nebula (“hour-glass” type). In this model the mass of the envelope is 7 · 10−4 M ⊙ and the maximal velocity is over of 200 km/s.

We present first results of the deprojection of some optical and radio images of planetary nebulae using an iterative technique based on the Lucy method, as discussed in Leahy (1991). This approach allows a greater range of possible geometries to be investigated and is less sensitive to the effects of noise compared with the direct matrix-inversion method which can be used for the spherical case. We assume that the emissivity function can be represented as

In order to investigate the early evolution of planetary nebulae (PNe) we solved numerically the hydrodynamical equations in cylindrical coordinates (r, z) assuming azimutal symmetry. The numerical method used is described in detail by Mair et al. (1988). Our simulations model the interaction of a fast, tenuous, spherical symmetrical central star wind with a slow, dense, aspherical Red Giant Envelope (RGE) expelled from the progenitor star. For the aspherical RGE with a polar/equatorial density contrast we used the initial model given by Mellema et al. (1991) in cylindrical coordinates. We have investigated the influence of each initial model parameter upon the evolution of PNe. Thereby we confirm that the polar/equatorial density contrast in the RGE and the thickness of the RGE-disk play an important role for the morphology of PNe. In agreement with the results from Mellema et al. (1991). The polar/equatorial density contrast in the RGE influences the ratio of the distances of the bright inner rim to the central star in z- and r-direction. This ratio increases with decreasing polar/equatorial density contrast. We find the thickness of the RGE-disk to be a key parameter for getting an elliptical or a butterfly PN: thin RGE-disks produce the first type of nebulae, thick disks the latter. We thank G. Mair, E. Müller and W. Hillebrandt for making available to us a copy of the SADIE code.

High resolution two-dimensional long slit spectra of NGC 7009, NGC 3242, IC 2448 and TC 1 in the Hα(6565 Å), OIII(5007 Å) and NII(6584 Å) emission lines have been used as input to determine radial velocity gradients in PNe by fitting calculated high resolution spatially resolved spectral structures of single emission lines. Contributions to the Doppler structure of emission lines taken into account are the global expansion of the PN shell and thermal line broadening due to temperatures of some 104 K. This model is based on two parametrized empirically chosen spherical symmetric functions:

1. 1. A empirical two parameter density function ρ(r).

2. 2. A linear radial velocity component function ν(r).

NGC 6905 appears to be an unusual source in displaying conical outflow lobes superimposed upon a spheroidal inner shell.

Recently written model of photoionization structure of planetary nebula has been used to calculate the emissivities of different nebular lines. With these values the line profiles have been modelled. The profiles can be obtained for different size and position of the spectrograph slit on the nebula image. The prepared computer code allows for estimation of the expansion velocities (Vexp ) in the way consistent with the methods used by observers, i.e. defined as the half-width of the line profile (when nebula is not resolved, “whole neb.” in the Figure) or defined as the half of the emission peaks separation (when the nebula can be resolved — “rectangle” in the Figure — and the line can be observed in the central part of the nebula). The real matter velocity at the radius where the observed surface brightness of Hβ falls below 20 % of the maximal value is shown for comparison (“model” in the Figure).

The faint, giant haloes observed in many Planetary Nebulae (PNe) are believed to be the remnants of the superwinds ejected by the progenitor asymptotic giant branch stars. Spatially resolved, high spectral resolution observations of [OIII]5007Å line profiles from the haloes of NGC 6543 and NGC 6826 have been obtained with the Manchester echelle spectrometer at the Isaac Newton Telescope. The widths of the observed profiles from NGC 6826 decrease towards the outer edge of the halo which implies a definite radial expansion of this halo. The expansion velocities vexp of the two haloes are derived using both thick and thin shell models.

TAURUS, the imaging Fabry-Perot spectrometer, using a CCD detector, has been commissioned on the 4.2m William Herschel Telescope, creating a powerful tool for studying the kinematics of emission line objects such as planetary nebulae. The mode of operation when using a CCD is different to that with IPCS (where rapid scanning is employed), so that account has to be taken of changes in atmospheric transmission between CCD readouts. The large dynamic range of the CCD (>200) allows the simultaneous study of the line profiles from the right and faint parts of the nebulae.

We made high-dispersion spectroscopic observations of IC 351 = PK 159 − 15°1, a compact high-excitation planetary nebula. We found the expansion velocity from Hα, [OIII] and HeII line, of 45.1kms −1, 38.7kms −1, and 33.3kms −1, respectively. Moreover, for the Hα line we detected an additional faint blue-shifted component escaping from the center of expansion with high velocity of 120kms −1. This value is larger than the usual expansion velocity (< 50kms −1) but considerably smaller than that of stellar wind. This faint blue-shifted component of Hα is also coincidently identified as the helium line, HeIIλ6560 (Pickering 6). Using the emission coefficients of recombination lines (Case B, T= 10,000K; Osterbrock 1989), and the intensity ratio, I(HeIIλ4686)/I(Hβ)=0.56±0.05 (Aller&Czyzak 1979), we can estimate the expected line intensity ratio, I(HeIIλ6560)/I(Hα)=0.026±0.002. On the other hand, the intensity ratio of faint component to Hα in the present observations is estimated as Ifaintcomp ./I(Hα)=0.064±0.013, based up the Hα profile in 1988. Comparing these two estimated intensity ratios it is clearly unable to emit the all intensity of the observed faint component by HeIIλ6560 line only. Therefore, even if this component is partly polluted by HeIIλ6560 line, we can consider the rest part as an emission from high velocity component of Hα. We can also recognize the bumped feature of [OIII]λ5007 line around the same velocity to the blue-shifted component of Hα. This fact supports the idea that these components can be attributed to the high velocity flow. We discuss the three ideas for the interpretation of such component; (a) colliding winds, (b) unresolved bipolar flow, and (c) secondary formation of an expanding shell. We have investigated whether the colliding winds model gives such high velocity or not. As the results of the calculation, if we use the usual mass-loss rate, the model can not give such high velocity. Therefore, it is doubtful that we can take this model as an explanation of the high velocity components. The high velocity component of IC 351 is not uncommon; such large internal motions are reported for other planetaries by Weinberger(1989) and compiled by Grewing(1989). The article on the results of IC 351 is submitted to PASP (Yadoumaru & Tamura 1992). We also report the other analysis on NGC 3242.