The study of nebular continuum emission is important for several reasons (Pottasch 1984, Planetary Nebulae, Dordrecht: Reidel). First of all, it can provide information about the temperature and the density of the nebula, when the object is large enough, or when the central star is weak enough, so that the nebular continuum is easily observed without interference from the stellar continuum. On the other hand, for small planetary nebulae, both the central star and the nebula contribute to the observed continuum. In this latter case, in order to obtain the stellar continuum the theoretical nebular emission must be used. Thus, studies of the evolution of planetary nebula nuclei through the HR diagram rely on a good calculation of the theoretical nebular continuum.

A method of Radiation Transfer treatment (Global Shell Model (GSM)) has been developed, different from the On the Spot Approximation (OSA), taking into account all diffuse reemission from radiative recombination of hydrogen and helium in the nebula itself. For optically thick and optically thin nebulae the method automatically reproduces the On the Spot Approximation and the models without any Radiation Transfer, respectively.

Previous attempts to model the integrated line ratio [O III] 5007 A/[O II] 3727 A for high excitation planetary nebulae (Che and Köppen 1983) have suggested that the charge exchange coefficient (k) for the reaction could be 10 times smaller than the predicted theoretical value. The influence of other factors that may contribute to differences in the O III/O II ratio, such as the ionizing spectrum and the nebular gas density distribution, seem to be relatively small in the high excitation nebulae.

The purpose of this review is to examine the fundamental observational parameters of the central stars of planetary nebulae, namely their apparent magnitudes and gross spectral characteristics, and how these relate to the derivation or estimation of effective temperatures.

Fast winds have been detected in the spectra of many nuclei of planetary nebulae (PNNi). The wind velocities range from about 600 km/s to roughly 4000 km/s. While these winds add little to the mass of the nebular shells they may significantly effect their internal kinematics.

By studying the emission from the faint outer envelopes of PNe one can infer also the wind properties of the progenitors of the current nuclei. This will in the end allow to test quantitatively current models of the origin of PNe.

As a result of photometric-monitoring studies, 7 planetary-nebula nuclei are now known to be binaries with orbital periods less than one day. These systems were probably produced via a common-envelope interaction, during which a wide pair was converted to a close binary surrounded by an ejected red-giant envelope. The frequency of occurrence of such close binaries among PNNs is about 10–15%, showing that binary-star interactions are a significant production mechanism for planetary nebulae. The descendants of close-binary PNNs are probably the cataclysmic variables. Two CVs surrounded by nebulae resembling old planetaries, 0623+71 and GK Per, may provide the most direct evidence for the origin of CVs through PN ejection. The observed birth rate for close-binary PNNs is more than an order of magnitude higher than for CVs, possibly indicating that our census of the CV population is very incomplete. The nucleus of K 1-16 is a member of the GW Vir class of extremely hot pulsating pre-white dwarfs, and the only one known to be surrounded by a PN. These objects offer exciting opportunities for direct measurement of evolutionary timescales and for seismological investigations of the interiors of PNNs and their immediate descendants.

1. Introduction. 2. A list of binary and multiple CSPN. 3. A radial velocity study of CSPN at high spectral resolution. 4. Spectroscopic binaries or intrinsic variables? 5. Concluding remarks.

It is a good tradition in IAU Symposia about PN to have a paper on model atmospheres. However, this is always a difficult task for the authors, because the majority of the PN researchers still believe that the best model atmosphere for a Central Star is a black body. Of course, this puts a theorist in stellar atmospheres into a somewhat desperate position. However, Central Stars of Planetary Nebulae (hereafter CSPN) - as all other stars - show spectral lines. And we will try to use the opportunity of this paper to convince that - as for all other stars - the quantitative analysis of these lines on basis of model atmospheres yields extremely valuable information about the physical nature of the stars.

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.

We have detected the central star of NGC 7027 by imaging the nebula through a narrow band ‘continuum’ filter onto the IPCS detector at the 2.5m Isaac Newton Telscope. We obtain an apparent visual magnitude for the central star of mv = 17.7 ± 0.5 mags.

Assuming that the central star radiates approximately as a blackbody, which is reasonable for the case of a hot star, then Zanstra temperatures for the central star can be calculated. We find TZ(H) = 3.9 × 105K and Tz(HeII) = 2.6 × 105K. Using the correction due to Stasinska & Tylenda (1986) we estimate the central star of NGC 7027 to have a temperature, Teff = 3.1 × 105K.

The luminosity and radii are found assuming a distance of d=1.2 kpc., giving L = 12,600 L⊙ and R = 0.039 R⊙. Placing the central star on the Log L - Log T diagram and comparing with evolutionary tracks for central stars with various masses from Wood & Faulkner (1986), indicates that the central star of NGC 7027 must have a mass, M ≥ 0.8 M⊙.

Radio observations of NGC 7027 have been taken using the Westerbork Radio Synthesis Telesocpe at 21cm. Self calibration techniques have been employed to give a radio continuum map of high dynamic range. These observations are being compared with a deep optical Hβ map to study the nature of the faint halo seen around NGC 7027 (Atherton et al. 1979)

Magnitudes have been measured for 44 faint central stars of planetary nebulae by imaging the star and nebula of a CCD detector. The nebula is suppressed by using a continuum filter. The remaining nebular continuum is then subtracted as background as long as the star can be clearly seen. This is true in 41 of the 44 cases observed. Zanstra temperatures are calculated from the observed magnitudes, and discussed.

An attempt is made to deconvolve the contribution of the central star from that of nebular emission, in small and large Planetary Nebulae. The use of medium-band Strömgren photometry is possibly a more powerful tool than that of broad-band photometries to achieve meaningful stellar parameters, specially by means of a four-channel photometer. The various methods used to subtract the nebular contribution are analysed and the results of the first ubvy and HΒ observations are presented.

As the first step toward assessing the high-mass end of the PNN (and hence, WD) mass distributions, we report here on new PNN found in extended, low surface brightness PN. Our ultimate goal is to obtain luminosities and temperatures for these stars and to compare them with evolutionary calculations to derive stellar masses. Based on studies of a local sample of PN, Schönberner (1981, 1983) and Schönberner and Weidmann (1983) infer that the PNN mass distribution is sharply peaked near 0.6 M⊙, and that 80% of PNN have masses < 0.61 M⊙. Their sample was necessarily chosen for the existence of photometric data for both the nebulae and their central stars, criteria that excluded small bright PN whose central stars are masked by nebular emission, and large faint PN for which there is little photometric data for the nebulae and/or the central stars. Acknowledging these selection effects, those authors have urged further investigation, especially of low luminosity PNN.

Central stars of nineteen planetary nebulae were observed for X-ray emission using the Einstein Observatory and four of them were detected. High resolution observations with the Einstein Observatory indicates that the X-ray source in NGC 246 is a point source. These planetary nebulae with positive observations turn out to be the nearest, have the least extinction and also have the largest size of the nebulae around them. It is possible that X-ray emission is observed from these planetary nebulae with larger ages because of the smaller extinction by the nebulae and also due to the settling of heavy elements in the central star which otherwise prevents escape of X-rays by providing opacity.

The temperature of a planetary nebula central star (CPN) may be determined by observing the nebular flux in an H I or He II recombination line and the stellar flux in a continuum band. The former measures the integrated UV stellar continuum blueward of the ionization edge of the recombined ion. By assuming a continuum shape (usually a blackbody), the ratio of these two fluxes yields an effective temperature for the CPN. This particular method, first introduced by Zanstra (1931), has an advantage over others in that the observables are relatively straightforward to obtain. However, this method also carries a troublesome ambiguity with it: CPN temperatures determined using He II recombination lines. This Zanstra discrepancy is reviewed by Kaler (1985) and Henry and Shipman (1986). Examples of He II and H I temperatures for numerous CPNs are given in Pottasch (1984), where it is shown that the He II Zanstra temperature often exceeds the H I temperature by several times 104K.

The classical Zanstra method compares a H Balmer line or He II λ4686 with flux from a limited region of the planetary nuclear (PNN) spectrum to obtain the central star temperature T(PNN). Long ago it was found that generally T(He II) > T(H I), a result attributed to differing optical depths at the Lyman limits of He II and H I; T(He II) often was regarded as the “real” temperature. Much attention has been paid to discordances between Zanstra temperatures and those found by other means, such as the energy-balance (EB) method. (see e.g., Preite-Martinez and Pottasch 1984).

The location of Planetary Nebulae Nuclei (PNNi) on the H-R diagram is a very important clue to understand the evolution of these objects.

We have analyzed the flux distributions of 13 planetary nuclei (NPN) spanning the full range of spectral classes known among central stars, except white dwarfs (see Table 1). We combined low-dispersion spectra from the IUE archives with absolute spectrophotometry scans taken by Dr. P. Massey at Kitt Peak with the Intensified Reticon Scanner (IRS) to obtain flux distributions covering the wavelength range λλ1150–7200 at ∼ 7 A resolution. In order to get the intrinsic stellar energy distributions, we first corrected the observed fluxes for interstellar extinction, and then subtracted out the nebular fluxes, which we estimated from the Hβ flux on the IRS observations and from the nebular temperature, density and helium ionic abundances. Finally, we fitted blackbody curves to the stellar continua.

Photometric and spectroscopic observations of some bright central stars of planetary nebulae (PN) have been conducted between 1984 and 1987 with the following tools: differential photometer P7 (70-cm swiss telescope, La Silla c/o ESO); radial velocity scanner CORAVEL (1-m swiss telescope, Observatoire de Haute-Provence = OHP); spectrograph CARELEC with CCD detector (193-cm telescope, OHP).

HFG1 (136+5°1) is a large, low-surface-brightness planetary nebula that was discovered by Heckathorn, Fesen, and Gull (Astrpn. Astronphys., 114, 414, 1982). in the autumn of 1986, photoelectric photometry by A.D.G. and H.E.B. showed that the 14th-mag central star of HFG1 is a large-amplitude variable. Subsequent CCD photometry by R.C. and H.E.B. reveals a sinusoidal variation with a period of 13.96 hr and an amplitude of 1.1 mag in the B band.