It is not really known how low and intermediate mass stars eject mass to form PNs. We present preliminary results from a programme of near–IR imaging, in which we study a sequence of objects, from extreme AGB stars through proto–planetaries to young, compact PNs. We aim to study the sequence of morphologies, to see where the onset of bipolar shaping occurs, and to use the IR molecular hydrogen lines to map neutral regions around ionized nebulae.

We report 10 and 20μm spectroscopic observations of four C–rich post–AGB objects which exhibit the unidentified emission feature at 21μm. The observations were carried out in October 1990 and May 1991 using CGS3 on UKIRT. The spectral resolutions were 70 for the wavelength range of 7.4–13.3μm and 80 for the region between 15.4–24.1μm. Three of the sources reported here are from the list of Kwok, Volk & Hrivnak (1989), i.e., IRAS 04296+3429; IRAS 07134+1005 and IRAS 22272+5435. Figure 1 shows the full spectrum of IRAS 04296+3429 and IRAS 22272+5435. The 10μm spectra of these objects exhibit UIR bands whose peaks all fall longwards of the usual peak wavelengths associated with such features. This may be related to the fact that they are the lowest excitation objects so far found to exhibit UIR emission bands. We also found narrow emission features superimposed on the long wavelength wing of the 21μm emission bands of IRAS 04296+3429 and IRAS 22272+5435. The fourth object we observed, SAO 163075, was found to also exhibit a (weak) 21μm emission feature. However, there is no PAH features in the 10μm region, apart from the plateau at 12μm.

An unidentified emission feature at 21 μm has been detected in the IRAS Low Resolution Spectra (LRS) of 5 IRAS sources (Kwok, Volk, and Hrivnak 1989, Hrivnak and Kwok 1991). The sources are generally found to be F and G supergiants with cool, detached dust shells. We have searched for additional 21 μm sources in the LRS database and have obtained ground-based UKIRT spectra at 10 and 20 μm in an attempt to confirm the LRS feature.

We present mid-IR (8–13μm) images of dust in seven proto-planetary nebulae (PPN), GL2343, HD 161796, 89 Her, OH 0739–1435, CRL2688, IRAS 22272+5435, and CRL618. The images were taken at UKIRT and the IRTF with the Berkeley mid-IR camera which was developed at the Space Sci. Lab. in UC Berkeley and is supported by IGPP and LEA, LLNL. The results presented here are part of an on-going mid-IR imaging project to study the morphological development of a star as it evolves from the Asymptotic Giant Branch (AGB) to the planetary nebula (PN) stage. In particular, we aim to establish when non-spherical symmetry which is evident in so many PN arises. Four of the objects are oxygen rich. Of these, GL2343 (Fig. 1) and HD 161796 are found to have spherical dust shells in the mid-IR with diameters of 6.″ 7 and 3″, respectively. OH 0739 is marginally resolved, but has a hint of elongation that is aligned with the bipolar nebula evident in the near-IR. 89 Her is unresolved at 1.″ 1 resolution. 89 Her, HD 161796 and GL2343, all high latitude supergiants, appear to form an evolutionary sequence as evidenced by the size of their circumstellar dust shells. Three of the objects are carbon rich. CRL2688 and IRAS 22272 +5435 are found to have elongated structures which suggest a bipolar morphology. CRL 618 is unresolved at 1.″ 7, but is known to be bipolar from optical studies. Hence, with the exception of OH 0739, a known binary, the oxygen rich PPN have spherical dust shells while the carbon rich have bipolar dust shells. Our small number of observations do not provide solid statistics; however, this trend suggests that nebula bipolarity is linked to carbon-rich chemistry.

The carbon-rich PPN M 2–9 and IRAS 21282+5050 are known to exhibit UIR feature emission at 7.7, 8.6, and 11.22 μm. The two nebulae have been imaged in the mid-IR with the UA/SAO/NRL Mid-Infrared Array Camera (MIRAC) in bandpasses which include UIR feature emission (8.8 and 11.22 μm). Near-infrared images of the nebulae have also been taken with the NICMOS 3 Hg:Cd:Te array camera at J, H, and K for M 2–9 and at K for IRAS 21282+5050.

A detailed analysis of the visual extinction and physical conditions of the extreme bipolar nebula M2-9 has been made from optical spectra acquired with the Intermediate Dispersion Spectrograph of the 2.5m Isaac Newton Telescope (Observatorio del Roque de los Muchachos, La Palma) in combination with a 235mm camera and the IPCS detector covering the spectral range 3500-7500 Å with spectral resolution of 4 Å.

I have made an extensive series of numerical simulations of aspherical PNs. This interacting-winds model consists of a point source of fast tenuous gas embedded in a flattened cloud of dense slow gas which is two-dimensional and cylindrically symmetric. I used a hydrocode specially designed to handle the extremely large gradients between the winds to second order accuracy. The outer shock shapes correspond very well to my analytic predictions. This shock may form cusps which compress the gas to form two rings on opposite sides of the equatorial plane.

Using the IRAM interferometer, we have mapped with a 2″.4 = 3″.4 resolution the J = 1 → 0 HCN line emission in the proto–planetary nebula CRL 618. Our maps resolve the 200 kms−1 molecular outflow (Cernicharo et al. 1989), as well as the slowly expanding circumstellar envelope (Bujarrabal et al. 1988), allowing a very precise positioning (≤ 0″.1) of these components with respect to the central HII region. 70% of the HCN envelope emission comes from a very compact, spherically symmetric core of size ≃ 3″.2. The core surrounds the high velocity gas which appears localized in a number of small ‘clumps’ (≤ 0″.5) – see figure. The large range of velocities observed in the ‘clumps’ suggests that we are not observing a decelerating molecular flow, but the impacts of a bipolar outflow on the slowly moving core, close to the HII region. The collision of a neutral gas outflow with high density regions (the ‘clumps’) results in the generation of dissociative shock-waves pushing and tearing the inner surface of the envelope. CRL 618 appears to have reached the stage where the stellar winds begin to disrupt and to scrape through the massive envelope, shortly before it evolves towards a Planetary Nebula.

IRAS 17423-1755 is a new transition object between the post-AGB phase and the planetary nebula (PN) stage included in an observational program of IRAS sources with infrared colours similar to those of PNe. We have taken B, V, R, I, Z, Hα and [OIII] CCD images at the 1m JKT (Roque de los Muchachos, La Palma), where a clearly marked bipolar structure with a total extension of about 10″ can be seen, and long slit (low and high resolution) optical spectra at the 4.2m WHT, 2.5m INT (RM, La Palma) and ESO 1.5m (La Silla) telescopes.

High-resolution optical spectra were taken of the carbon star V Hydrae at 10 different epochs spanning two stellar periods using the Hamilton Echelle Spectrograph at Lick Observatory. Velocities were determined at each epoch by performing a cross-correlation analysis against the spectra of standard stars with previously-determined velocities. The velocities of individual atomic absorption and emission lines, and their variations with phase, were also determined. The rising and falling of the photosphere is clearly in evidence, with an amplitude about the mean of ± 5 km s−1 and a phase consistent with the expectation that maximum light occurs at minimum radius. In addition, the spectra were subjected to a rotational broadening analysis, in which we determined V sin(i), where V is the presumed equatorial rotation velocity, by minimizing the differences between the spectra of V Hya and a number of artificially broadened comparison stars showing no evidence for broadening. Mechanisms other than stellar rotation, including both turbulence and opacity, were found to be unlikely contributors to the broadening. The very unusual rotation velocity (for a red giant) was found to vary between about 10 and 18 km s−1, with an average of ≈ 14 km s−1. The relative phase of the rotation velocity curve is consistent with the hypothesis that the pulsation leads to a periodically varying moment of inertia. The assumption of angular momentum conservation, coupled with the radial velocity and V sin(i) curves, permits a deduction of the equilibrium stellar radius: The similar magnitudes of the rotation and pulsation velocities imply that they are dynamically coupled, leading to a latitude–dependent circulation which probably affects the mass loss. This interaction should also have a strong effect on the convective structure of the envelope. The angular momentum implied for the V Hya atmosphere leads us to the conclusion that it is a common-envelope binary, and that it is a precursor of a binary nucleus in a bipolar planetary nebula.

Modelling of evolution of dusty shells around post-AGB star of 0.598 M⊙ mass has been performed by means of the Yorke's hydrodynamical code. In this code dust and gas are treated as two separate components coupled by friction, and the spectral appearance of the model is computed by solving the angle and frequency dependent continuum radiation transfer problem. Calculations were carried out for dust composed of silicate grains. The emergent spectrum of the model has been convolved with the IRAS photometric band profiles to construct theoretical tracks on the IRAS two-color diagram.

Following Kahn & West (1985), we investigate the formation of PNe in the slow and fast wind interaction scheme by assuming the slow wind axially symmetric. We have further assumed that the mass loss rate for the slow wind is not steady. It is found that the final morphology of the nebula depends not only upon the initial degrees of the seed asymmetry in the slow wind, but also upon the time variations of the mass loss rate. As an example, we show in some detail the case where the central star first blows an axially symmetric slow wind during its red giant stage, this wind is followed by a superwind while the star is on the AGB, these slow wind is later overtaken and shaped by the fast wind during the post-AGB branch. It is found that a small initial asymmetry can be amplified and reproduces the various observed morphologies of the PNe.

The F-type supergiant HD 56126 (F5I) is an IRAS source with detached cold dust shell with characteristics similar to the dust shells around planetary nebulae. From an analysis of high resolution and high signal to noise ratio spectra metal and CNO abundances have been determined. It is found that in HD 56126 [C/H] =−0.01, [N/H]=+0.17, [O/H] = −0.02, [S/H]=+0.01 and [Fe/H]< −1.0. The C/N/O abundance ratios and CNO abundances relative to Fe of HD 56126 and related post AGB stars suggest that they have on their photospheres the material processed by triple alpha, CN and ON cycles.

UBVRI polarization measurements of 25 post AGB stars with circumstellar dust shells (CDSs) were made. Most of them show A, F, G, K supergiant type spectra. IRAS data of these stars show evidence for the presence cool detached circumstellar dust envelopes. Some of these stars also have warm dust shells. Many show significant polarization. The observed polarization in UBVRI is most likely due to scattering of the central star radiation by CDSs. Some of these stars show variation in polarization and position angle. The polarization data suggest that several may have aspherical or bipolar CDSs.

We are engaged in a program to monitor radial velocity variability in proto-planetary nebulae (PPN). Observations are being made with the radial velocity spectrometer at the DAO, with a precision of ±0.5 km s−1. Radial velocity variability can arise from binary motion and/or pulsation in these post-AGB stars. The demonstration of a binary nature for some of these objects can have important implications for the understanding of their physical properties, and for the shaping of their circumstellar shells.

The high-dispersion Hα line profiles of the candidate stars for proto-planetary nebulae(PPNs) are presented. Samples include mostly F supergiant stars. Volk and Kwok(1989) summarized four classes of stars which had been described in various literature as PPNs. They selected nearly one hundred new candidates of PPNs due to IRAS data. Our goal is to make clear the mass-losing phenomena and to establish the adequate models for the explaination of observed data from such stars. F supergiant stars also attract our attention to the spectroscopic variability in connection to their extended, rarefied and therefore complicated atmosphere. Observations were made at the Okayama Astrophysical Observatory with the 188-cm reflector and its coudé spectrograph. We used the intensified Reticon as the detector which has one dimensional array of 1024 pixels. The size of one pixel is 25 μm and corresponds to 0.13 Åand 5.9 km/sec at Hα. Our stellar samples are HD 46703, HD 56126, HD 112374, R CrB, HD 161796, 89 Her, IRAS 18095+2704, AFGL 2343, HD 187885, and UU Her. We obtained the Hα profiles and found a couple of absorption lines like the FeIλ6569, FeIIλ6516, and CIλ6587 among them. An express report and a short discussion about a part of these samples was already presented by Tamura and Takeuti(1991). Subsequent reports appeared and will appear in the articles of Tamura, Takeuti, and Zalewski(1992); Tamura and Takeuti (1992). We show various kinds of Hα profiles which consist of absorption and emission components and show clear evidence of activity in the extended atmosphere. We classify them into (i)P Cyg type profile, e.g. 89 Her, (ii)inverse P Cyg type profile, e.g. HD 187885, (iii)profiles with the central reversal at the bottom of absorption, e.g. HD 46703, (iv)profile with shoulder emission components, e.g. HD 56126, (v) asymmetric absorption profile, e.g. AFGL 2343. Concerning HD 161796 the Hα profile seems to have only an absorption component. It should be attributed to time variations, because the profile observed by Luck et al.(1990) is similar to (iv). In addition to a wide variety of the Hα profiles we have noticed their time variations ranging over different time spans, month, day, even in several ten minutes. Arellano Ferro(1985) noticed the change of Hα profiles of luminous yellow supergiants in various time scales too. We obtained radial velocities and compared with the expected values by the galactic rotation. We can recognize our sample stars are far from the galactic kinematics of disk members.

We report on multi-wavelength observations of a young halo planetary nebula, LSIV −12° 111. This object was previously classified as an emission-line young B-type star but a model atmosphere abundance analysis of high resolution optical spectra revealed it to be an evolved object, probably in the post-asymptotic giant branch (post-AGB) evolutionary phase. The presence of an infrared excess and low excitation nebular emission lines implies that the central star may have just started to photoionize the remnant (AGB) circumstellar material. Here we discuss the nebular and dust properties of LSIV −12° 111 and re-determine some metal abundances for the central star. These results are used to constrain the evolutionary status of this unique halo planetary nebula.

During a time interval of only 2 years, a sudden change has been detected in the emission line spectrum of a young planetary nebula IRAS 0656 2-0337, whose nature has been recently discussed by several authors, was firstly observed in December 1987 at La Silla (Chile) and in October 1988 at La Palma (Spain), showing only the Balmer lines in emission. A third spectrum, obtained at La Silla during February 1990, shows a quite different appearance, with forbidden emission lines typical of a planetary nebula. The electronic density is very high, and there is a high infrared excess too. An expansion velocity of 45 km s−1 has been estimated. It is shown that the effective temperature of the central star has significantly increased over this period of time. This is consistent with the evolution of a massive progenitor in the post-AGB phase, with L = 7000L ⊙ at D = 4 kpc, in which the ionization of the neutral envelope, detected in CO, is now taking place.

Some criteria based on the model calculations and observational data were used to choose preplanetaries and young planetary nebulae. Observational results for the object TH4–4 are presented. The spectral and photometric variations of TH4–4 are discussed.

Infrared photometry suggests that IRAS 17516-2525 is in transition from the AGB to a young PN (Van der Veen et al. 1989a, b, Manchado et al. 1989). IR spectra (van der Veen et al. 1989b, Käufl et al. 1992) revealed that the object has an ionized core. Optical observations show a weak Hα emitting unresolved object (Käufl et al. 1992) at the coordinates (α1950 : 17h 51m 37.8s , δ1950 : −25° 25′ 58″) suggested by van der Veen et al. (1989a). The spectrum between 2.0 and 2.4μm shows e.g. Brγ, 2.0875μm (unidentified) or the NaI -doublet but no molecular Hydrogen (present in other compact PNs). The ratio of Pfβ to Brα (hardly affected by extinction, insensitive to electron density and temperature) has been studied in IRAS 17516-2525 and Hen1044. For Hen1044 we find this ratio to be in accordance with theoretical calculations (Hummer and Storey, 1987). For IRAS 17516-2525, however, Pfβ is approximately twice as strong. Hence presumably Hen1044 is optically thin whereas IRAS 17516-2525 is optically thick at λ ≈4–5μm. Line fluxes appear to be constant over a time scale of 4 years. If the visual extinction AV in the object is ≥ 2mag than the infrared luminosity equals the total luminosity (i.e. L = 1400L ⊙ ∗ (distance/kpc)2). A main-sequence object accidentally extincted by an intervening cloud can be excluded because the observed line-width of Brα (≈ 40km/s, Van der Veen et al. 1989a) is too small as compared to that of O-stars (≈1000km/s, Käufl, 1992). Assuming an expansion-velocity of ≈ 20km/s the stellar appearance constrains dynamic age and size (age ≤ 360y ∗ distance/kpc, size ≤ 1500AU ∗ distance/kpc). A pre-main sequence object can also be excluded. The max. luminosity (LPMS ≤ 5000L ⊙) then requires the object to be closer than 2 kpc where it should be resolvable. In conclusion the observational evidence strongly suggests that IRAS 17516-2525 represents the searched ‘missing link’ between AGB stars and young compact Planetary Nebulae.