We present the results of an examination of low-resolution IUE spectra of Galactic planetary nebulae in an effort to determine the occurence of stellar winds in their central stars. Our analysis confirms the results obtained in previous studies.

The formation and evolution of a planetary nebula is based on the occurrence of a strong AGB wind and the rapid evolution of the central star with corresponding changes of its ionizing flux and wind power. We have studied the influence of different mass-loss histories in combination with various central-star properties.

The Local Group Census is a narrowband survey of all the galaxies of the Local Group (LG) with Dec ≥ −30°, being carried out as part of the Isaac Newton Group's Wide Field Survey programme. Observations are being obtained with the Wide Field Camera at the 2.5m Isaac Newton telescope, equipped with a mosaic of four 2k x 4k EEV CCDs covering a field of view of 34′ x 34′.

Hubble Space Telescope narrowband images of four PNe resolve the microstructures in four PNe known to contain FLIERs (e.g. Balick et al 1994). FLIERs are seen to fall into two morphological groups: very low ionization clumps of small knots with cometary-like tails, and sinewy transverse filaments with fainter extensions pointing outward from the nucleus. Generally speaking, FLIERs exhibit [NII]λ6584/Hα > 1. They are marginally detectable in Hα above the bright background of nebular emission, and all but invisible in [O III]λ5007A. In every case the star-facing side of the structure is on the order of a pixel in width (about 1015 cm). A decreasing ionization gradient is seen [O I]λ6300A, [S II]λ6717 + 31A, and [N II]λ6584A images.

The variability of P Cygni profiles is important because of its connection with the mechanism of wind production and with the behaviour of the associated mass loss rates.

The interacting stellar winds (ISW) theory (Kwok, S., Purton, C. R., Fitzgerald, P. M., 1978, ApJL, 219, L125) is nowadays widely accepted in the physics of Planetary Nebulae (PNe). It received much support from the observed fast winds in the central stars of PNe (CSPN), recognized to be a quite common phenomenon (e.g., Perinotto, M., 1993, in IAU Symp. No. 155, Planetary Nebulae, eds. R. Weinberger and A. Acker, Kluwer, Dordrecht, 57). Thus, the existance of a hot bubble in the PNe structure is a cornerstone of the ISW model. The high velocities (600–3500 km s–1) of the CSPN winds, are, according to the ISW model, directly responsible for an high gas temperature in the hot bubble, which is then expected to be the source of an extended X-ray and extreme ultraviolet (EUV) radiation. The PNe should also emit infrared coronal lines (IRCL) of highly ionized species since the high temperature plasma of the hot bubble is in contact with the much colder outer shell (optical PN) and the thermal conduction will produce a region of intermediate temperatures (5 × 105–106 K). A model considering the structure of the hot bubble in PNe with taking into account the thermal conductivity effects was present by Zhekov and Perinotto (1996, A&A, 309, 648).

We have solved the radiative transfer equation taking into account both absorption and scattering into the line of sight to the observer to model the disc of spiral galaxies.

A dust model has been adopted, suitable for the diffuse interstellar medium of our Galaxy, to obtain, in aU the considered spectral range (1000 ÷ 10000 Å), consistent quantities to describe both the absorption and the scattering properties of the dust.

In the Introduction we recall the mass loss history of a progenitor of a planetary nebula (PN). Then we concentrate on the status of knowledge of fast winds in central stars of planetary nebulae (CSPN) : the detection and statistics, the observed edge velocities, relationships of the edge velocities with other stellar or nebular parameters. We then summarize the methods used to derive the mass loss rates associated to the fast winds, and review the determinations of the “observed” mass loss rates. The comparison with predictions from the radiation driven theory (RDT) is then discussed as well as possible lines for future improvements.

The high resolution IUE images SWP 16742 and SWP 42675 have been studied to derive the fast wind properties of the central stars of the two low temperature planetary nebulae IC 2149 and Tc 1. The first image is from the IUE archive, while the second one has been taken by us.

We have studied with the SEI method (Sobolev approximation plus Exact Integration of the transfer equation) seven hot stars, five belonging to the Large Magellanic Cloud (LMC) and two to the Small Magellanic Cloud (SMC), which have been observed with IUE in high resolution. We present preliminary results of the work, i.e. the terminal velocities and the optical depth of the P Cygni profile. An evaluation of qiM has been done, as well as a comparison with previous work.

Central stars of planetary nebulae (PNCS) frequently exhibit fast winds (cf. Cerruti-Sola and Perinotto, 1985; C.P. and Grewing, this volume). They may be important for the structure of the whole nebula as well as for the evolution of the central star. Their speed is typically two orders of magnitude higher than that of the classical optically visible nebula, which in turn expands a few times faster than the most external winds detected in few cases in the radio domain.

I review here the status of art in the determination of the mass loss rates (M) associated with these fast winds. I restrict myself to the ‘observational’ determinations. Only at the end I will say something about the predictions of multi-scattering line radiation wind driven theory in connection with one best studied object: N6C 6543. This allows one to conclude that this theory may be the right explanation also for these winds.

The structure of a stellar wind and its associated mass loss can be derived from the analysis of ñ Cygni-like line profiles. These do better occur in the UV range and have been extensively observed with the Copernicus (900–1450 A) and IUE Satellites (1150–3200 A).

We studied the interstellar UV extinction towards 115 stars, selected from IUE data bank, belonging to a variety of different galactic areas, including well-known dense clouds.

The flux originating from the central stars of 27 planetary nebulae in the spectral range 1200-2000 Å has been deduced from the analysis of a large number of released IUE low resolution spectra.

Stellar winds have been revealed in a large fraction of central stars of planetary nebulae from P Cygni profiles observed with the IUE satellite. The relevant lines are essentially the resonance lines NV λ 1240, Si IV λ 1397, CIV λ 1549 and the subordinate lines OIV∗ λ 1342, 0V∗ λ 1371, NIV∗ λ 1579. Edge velocities are of the order of 1000-3000 km s−1, similar to the case of population I O stars. Detailed determinations of the mass loss rate have been performed for NGC 6543, NGC 2371, IC 2149 and IC 3568 with values between 4.10−9 to 7. 10−7 Mo yr−1. The accuracy of these determinations is not well known. It is however clear from the variety of observed profiles in these and in several other objects that properties of the winds (ionization structure, etc.) varies considerably from object to object and that very likely the mass loss rate will span over a large interval. Some possible consequences of these winds are discussed.

The problem of determining the distance scale for planetary nebulae (PN) is approached through the kinematics of this subpopulation in the galaxy. To this end, we have compiled a catalogue of all known radial velocities for 457 galactic PN with standardized error statistics. External (calibration?) errors of the same magnitude as internal spectral line deviations are noted.

This paper presents a preliminary study of the IUE UV spectrum of the planetary nebula NGC 40. Details of the observations are given in Table 1 which were obtained in the low resolution (Δλ ∼ 6å) spectrophotometric mode using both SWP (λλ1150–2000) and LWR (λλ1850–3250) spectrographs. All data were acquired using the large (10′x20′) apertures. SWP3076 was taken with the centre of the aperture offset by 7′.5N of the central star: all other images were obtained with the slot centred on the nucleus.

The fate of the Hell Lya photons, produced during recombinations of the He++ ions and electrons, is important in considerations of the ionization and thermal structure of high excitation nebulae (Flower, 1968, IAU Symp. No. 34, Planetary Nebulae, p. 77). Hummer and Seaton (1964, M.N.R.A.S. 127, 217) effectively supposed that these photons are absorbed by H° in the central (He++) zone, whereas detailed line transfer calculations (Weymann and Williams, 1969, Ap. J. 157, 1201; Harrington, 1972, Ap. J. 176, 127) suggest that a large fraction of the photons escapes from the central zone. We consider the effects of the absorption of these photons on the ionization and thermal structure of the outer (He+, H+) zone of a nebula which is optically thick in the HI Lyman continuum. (Paper will appear in Astronomy and Astrophysics.)