S. R. Pottasch’s research while affiliated with University of Groningen and other places

Context. Spectra of planetary nebulae show numerous fine structure emission lines from ionic species, enabling us to study the overall abundances of the nebular material that is ejected into the interstellar medium. The abundances derived from planetary nebula emission show the presence of a metallicity gradient within the disk of the Milky Way up to Galactocentric distances of ~10 kpc, which are consistent with findings from studies of different types of sources, including H II regions and young B-type stars. The radial dependence of these abundances further from the Galactic centre is in dispute. Aims. We aim to derive the abundances of neon, sulphur and argon from a sample of planetary nebulae towards the Galactic anti-centre, which represent the abundances of the clouds from which they were formed, as they remain unchanged throughout the course of stellar evolution. We then aim to compare these values with similarly analysed data from elsewhere in the Milky Way in order to observe whether the abundance gradient continues in the outskirts of our Galaxy. Methods. We have observed 23 planetary nebulae at Galactocentric distances of 8–21 kpc with Spitzer IRS. The abundances were calculated from infrared emission lines, for which we observed the main ionisation states of neon, sulphur, and argon, which are little affected by extinction and uncertainties in temperature measurements or fluctuations within the planetary nebula. We have complemented these observations with others from optical studies in the literature, in order to reduce or avoid the need for ionisation correction factors in abundance calculations. Results. The overall abundances of our sample of planetary nebulae in the Galactic anti-centre are lower than those in the solar neighbourhood. The abundances of neon, sulphur, and argon from these stars are consistent with a metallicity gradient from the solar neighbourhood up to Galactocentric distances of ~20 kpc, albeit with varying degrees of dispersion within the data.

Most distances of planetary nebulae are known roughly within a factor of 2 or larger, except for some special objects for which the uncertainty can be as low as 50%. In the present work both IUE interstellar Lyman alpha profiles and 21 cm HI line surveys are used to infer the distances of four planetary nebulae.

Individual distances to planetary nebulae (PN) which are independent of any assumption of average nebular characteristics, can be found if one knows the relation between interstellar extinction (E(B-V)) and distance along the line-of-sight to the PN, together with the E(B-V) towards the PN itself (Lutz, 1973 and Acker, 1978). We used VBLUW-photometry (Lub and Pel, 1977) to derive accurate E(B-V)'s and distances of stars up to V-magnitude + 14, within 0°.3 from the PN. Table 1 lists the PN we studied. The E(B-V)'s of the PN are derived from:

The Very Large Array (VLA) in Socorro, New Mexico, has been used to measure the 6 cm continuum flux densities and the angular sizes of 42 planetary nebulae (PN) in the direction of the galactic centre (GC). These were all optically confirmed PN for which the radial velocities (and positions on the sky) make it very likely that they are close to the GC. With a detection limit of about 1 mJy, 34 PN were detected. Their flux densities range from 2 to 100 mJy. Initially we used a configuration of the VLA with an instrumental resolution of 1″. About 80% of the detected PN could be clearly resolved with this resolution. The unresolved PN were observed again with a configuration of the VLA that has a resolution of 0″.4. For all but one of the 34 detected PN we could determine reliable angular sizes. The inferred total ionized masses range from < 0.01 to ~ 0.5 M ⊙ , assuming a distance to the GC of 9 kpc. The results argue strongly against the use of the Shklovsky method for distance determinations. Previous measurements of PN at the GC showed that their luminosities were substantially higher than those for nearby PN (Pottasch, 1980). The luminosity distribution of the PN in our sample is broader towards lower luminosities (up to the detection limit of the observations), but the luminosities are still high compared with nearby PN. We interpret this as a selection effect: by studying only optically confirmed PN, the intrinsically brightest PN are selected.

HI observations at 21 cm have been made with the Westerbork Synthesis Radio Telescope in the direction of six planetary nebulae located in or near the galactic plane (N 7027, 2440, 6537, 6572, 7026, 7354).

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.

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.

UU Sge, the eclipsing binary central star (Bond et al, 1978) of the low-surface-brightness planetary nebula (PN) Abell 63, has been observed spectroscopically in the visible throughout its 11.2 hour period and especially during the minimum. A spectral determination of the binary system has been made. The primary hot central star is an ‘O’ type PN nucleus of temperature ≈40,000 K, consistent with the low excitation of the nebular spectrum (e.g. no He ii 4686Å nebular emission detected). From the spectrum at minimum light, the secondary star appears to be a cool dwarf star around G7. Measurement of the magnitude of the secondary during the eclipse of the primary enabled the distance to the PN to be directly determined as 3.6 kpc. For this distance the luminosity of the hot star is approximately 4 320 L ⊙ , in good agreement with evolutionary tracks for (single) PN nuclei. Deep CCD images of Abell 63 show it has a ‘butterfly’ morphology implying that the close binary central system may have had a strong effect on the nebula shaping. The paper describing this work has been submitted (Walton et al, 1992).

A new evolutionary interpretation of the sequence of colours observed in the IRAS two-colour diagram by AGB and post-AGB stars is given, which is capable of explaining the observational properties of both kind of objects. It is useful to define a parameter λ to define the position of a given star in this “infrared main sequence” ( IRMS ). Adopting and from the analysis of the expansion velocities, mass loss rates and luminosities observed in a selected sample of non-variable OH/IR stars with no optical counterpart in the Galactic bulge as a function of λ, we conclude that the position in the IRAS two-colour diagram at which a star leaves the IRMS (λ max ) only depends on the initial mass M z of the progenitor star, so that only massive objects can reach the upper end of this sequence. The relation found is: Expansion velocities increase with the initial mass while every point in the IRMS is found to be associated to a certain value of the mass loss rate. This model also predicts the evolution with time of the mass loss rate during the AGB as a function of the initial mass of the progenitor star, and confirms that most known planetary nebulae are the result of the evolution of considerably massive stars (between 2–3 solar masses) which means that the contribution of processed material to the interstellar medium is considerably higher than what theoretical models predict. Type I PNe are the result of the evolution of 3 — 5 M ⊙ progenitors while progenitors with M i ≤ 1.2 M ⊙ probably do not give PNe. The model is also in agreement with the narrow distribution of core masses found in central stars of PNe and white dwarfs and with the usual expansion velocities found in OH/IR stars.

PC 11 (HD 149427, PK 331-5 1) is classified as a young planetary nebula with strong OIII 4363Å and a Zanstra temperature of T Z = 27000K. It is also classified as (D′ — type) yellow symbiotic star with A — F type companion. It is an IRAS source with detached cold dust with far intrared (IRAS) colours similar to planetary nebulae. The IUE short wavelength (SWP) spectra show emission lines due to OIII] (1661/1666Å). NIII] (1746/1754Å) CIII] (1907/1909Å). The OIII] and NIII] emission lines show significant variation. Variation in the strength of CIII] is not very significant. The strength of OIII] has decreased and NIII] has increased. The long wavelength (LWP) spectrum shows stellar continuum (A-F) and absorption lines due Mg II 2800Å feature. It also show emission lines at 2772Å (?) 3133Å −3140Å (very strong) (OIII, [FeV], 3209Å (He II?) ([FEII]). The variation in the strength of emission line due OIII] and NIII] and the presence of stellar continuum (A-F) suggests that the central star of PC 11 is a binary.