Emanuel Vassiliadis

A theoretical calibration of the planetary nebular cosmic distance scale

An extensive grid of optically thick model planetary nebulae (PN) was computed in order to determine the extent to which the emission-line fluxes used in the planetary nebular distance scale are affected by the stellar effective temperature and the nebular and stellar metallicity. It is concluded that the nebular flux in the Hβ line is closely related to the luminosity of the central star, but that the more commonly used flux in the [O III] line at 5007 A can also be calibrated to give a reliable estimate of this quantity

Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. VII. Cycle 3 and Archive Narrowband [O III] 500.7 nanometer Imaging

We present the continuation of our analysis of narrowband [O III] images, obtained using the Hubble Space Telescope (HST), of planetary nebulae (PNs) in the Magellanic Clouds. Six objects, five in the Large Magellanic Cloud, are observed as part of the General Observer program. An additional 15 objects are retrieved from the HST archive. The majority of the images are obtained prior to the first maintanence and refurbishment mission and are dominated by nebular [O III] 500.7 nm emission: the PN central stars are generally not seen. The raw images are deconvolved using 100 iterations of the Richardson-Lucy image restoration algorithm. The nebular dimensions and morphologies are determined. The nebular dynamical ages are derived using [O III] 500.7 nm expansion velocities from the literature. Using the techniques presented in Paper IV, the measured nebular ages are compared to theoretical evolutionary ages for H-burning and He-burning PN nuclei. Approximately 50% of the PN central stars are He burners in both the Large and Small Magellanic Clouds.

Hubble Space Telescope observations of planetary nebulae in the magellanic clouds. 2: SMP 85, a young planetary

We have obtained Hubble Space Telescope Planetary Camera images in the (O III) lambda 5007 A emission line, and Faint Object Spectrograph (FOS) UV spectrophotometry of the low-excitation planetary nebula SMP 85 in the Large Magellanic Cloud. By combining these results with existing optical spectrophotometry, absolute flux measurements, and dynamical and density information, we have been able to construct a fully self-consistent nebular model. This proves that SMP 85 is a dense, young, carbon-rich object which started to be ionized about 500-1000 years ago, and which contains a substantial inner reservoir of atomic or molecular gas, probably in the form of many small cloudlets. These cloudlets have been ejected at a velocity not exceeding 6 km/s, a result which, together with the morphology is an important clue to mass loss during late asymptotic giant branch (AGB) evolution. We have directly detected the central star through its UV continuum emission, and from both Zanstra techniques and nebular modeling derive a stellar temperature of 46000 +/- 2000 K, a stellar luminosity of 7300 +/- 700 solar luminosity, and a core mass of 0.63-0.67 solar mass. The nebular analysis also demonstrates that there is severe depletion of the nebular gases onto dust grains, most likely of the calcium magnesium silicate variety; a surprising result in view of the carbon-rich nature of the ionized nebula.

Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. VI. Cycles 4 and 5 Ultraviolet Spectroscopy Using the Faint Object Spectrograph

The Faint Object Spectrograph on board the Hubble Space Telescope (HST) is used to obtain spectra in the wavelength range λλ1150-4800 of three planetary nebulae in the Small Magellanic Cloud and 10 planetary nebulae in the Large Magellanic Cloud. This sample extends the sample of 12 objects previously observed with HST and reported in the third paper of this series. Observed and dereddened emission-line fluxes are presented. Reddening estimates from the He II λ1640/λ4686 flux ratio are generally up to 0.2 dex lower than the reddening derived from the Balmer decrement. Nebular temperatures are estimated from the N+ λλ2138, 2142/λλ6548, 6583 flux ratio. Nebular densities are calculated from the O IV] complex at λ1400, the N IV] λλ1483, 1487 doublet, and the Si III] λλ1883, 1892 doublet. Densities calculated using the oxygen lines are comparable to those determined from the optical lines. Densities calculated from the nitrogen lines show a scatter of over 3 dex, which is relatively large compared to the optically derived densities. Three of the five densities derived from the silicon lines are greater than 10,000 cm-3. The C+2/O+2, Si+2/C+2, and N+2/O+2 ionic abundance ratios are calculated using the available ultraviolet emission features. The C/O and N/O ratios are anticorrelated, which supports the premise that third dredge-up has taken place during the asymptotic giant branch phase. In contrast to the third paper in this series, type I classification does not imply the presence of Si III] emission. Three objects show P Cygni-like line profiles at C IV λλ1548, 1551, indicative of stellar winds.

Hot Evolved Objects in Different Parent Galaxies: The Stellar Winds of Three Planetary Nebula Nuclei in the Large Magellanic Cloud

Three planetary nebulae in the Large Magellanic Cloud (LMC) were observed with the Faint Object Spectrograph on board the Hubble Space Telescope. Their central stars display P Cygni-like features in the ultraviolet. We modeled these profiles to derive the stellar wind velocities. The wind velocities of the LMC planetary nebula nuclei (PNNs) are comparable to those of some Galactic counterparts, but other Galactic objects of similar temperatures exist with higher wind velocities. One object, LMC-SMP 61, has a Wolf-Rayet (WR) type central star. For this nucleus we derived a wind velocity of -1600 km s-1 and an upper limit to the mass-loss rate of ≤ 7 × 10-7 M☉ yr-1 from the He and C lines. The observed continuum emission in the UV and optical range is modeled with a combination of stellar and nebular contributions, yielding independent determinations of stellar and nebular quantities. The sample is numerically too limited for a significant comparison between LMC and Galactic PNN populations, but represents the first exploration of mass-loss properties of evolved stellar objects beyond the Milky Way.

The kinematics of planetary nebulae in the outer fields of the Large Magellanic Cloud

We present echelle observations of 16 planetary nebulae (PNs) in the [O III] λ5007 emission line, originally detected in a UK-Schmidt objective prism survey of the outer fields of the Large Magellanic Cloud (LMC). Of these 16 objects, 11 have had no velocity information published previously. Expansion and radial velocities have been derived by a simple Gaussian fitting technique

Mass Loss Processes in Hot Evolved Stars: Planetary Nebula Nuclei in the Large Magellanic Cloud

As part of a broad study of stellar winds in different galaxies, to establish observational constraints on the influence of metallicity on the mass loss process, we investigate the winds of planetary nebula (PN) nuclei in the Large Magellanic Cloud. Theoretically, the radiation pressure mechanism implies a strong dependence of the wind acceleration on the element abundances. Observationally, model predictions have been tested using Pop. I stars in the Magellanic Clouds (Puls et al. 1996, AA Bianchi et al. 1996, in prep.). We now extend this investigation to low luminosity, evolved objects.