M. Perinotto

FLIERs and Other Microstructures in Planetary Nebulae. IV. Images of Elliptical PNs from the Hubble Space Telescope

?????We report new results from high spatial resolution Wide Field Planetary Camera 2 imaging studies of FLIERs and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucleus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density ?104 cm-3. Can neutral knots account for the properties of FLIERs? We compare two broad classes of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevaporated gas swept by winds into head-tail shapes. Both classes of models fail basic consistency tests. Hence an entirely new conceptual paradigm is needed to account for the phenomenology of FLIERs.

The evolution of planetary nebulae I. A radiation-hydrodynamics parameter study

We follow hydrodynamically the evolution of spherical model planetary nebulae subject to different initial conditions and with various central stars, investigating how combinations of central-star mass and asymptotic giant branch mass-loss rate determine the shape and kinematics of a planetary nebula. With this approach we aim at constituting a framework useful for the interpretation of the evolutionary status and previous mass-loss history of observed individual nebulae, making use of their kinematical properties and surface brightness characteristics. In particular, the models are compared with the observed morphologies and kinematics of double shell nebulae. The dynamical structure of all the models is characterized by a more or less complicated shock wave pattern set up by ionization and wind interaction whose combined action results in general in a typical double-shell structure. We have found that models with simple initial structures based on a constant AGB mass-loss rate fail to comply with observed shell morphologies and surface-brightness distributions. A reasonable agreement with the observations is only found for a model where the mass-loss rate is strongly increasing towards the end of the asymptotic giant-branch evolution. Depending on the central stars evolutionary speed and the density of the cool wind expelled along the asymptotic giant-branch, planetary nebulae may never get optically thin. This is primarily the case for the more massive central stars, and this fact offers a rather natural explanation for the long standing problem of the very existence of molecular hydrogen in the immediate vicinity of hot central stars. We also show that distances to planetary nebulae based on expansion parallaxes are systematically too small by a significant amount.

The evolution of planetary nebulae - II. Circumstellar environment and expansion properties

We investigate and discuss the expansion properties of planetary nebulae by means of 1D radiation-hydrodynamics models computed for different initial envelope configurations and central star evolutionary tracks. In particular, we study how the expansion depends on the initial density gradient of the circumstellar envelope and show that it is possible to derive in- formation on the very last mass-loss episodes during the stars final evolution along and off the asymptotic giant branch. To facilitate the comparison of the models with real objects, we have also computed observable quantities like surface brightness and emission-line profiles. With the help of newly acquired high-resolution emission-line profiles for a sample of planetary nebulae we show that models with initial envelopes based on the assumption of a stationary wind outflow fail to explain the observed expansion speeds of virtually all of the observed planetary nebulae. Instead it must be assumed that during the very last phase of evolution along the final asymptotic giant branch evolution the mass-loss rate increases in strength, resulting in a much steeper slope of the circumstellar radial density distribution. Under these conditions, the expansion properties of the nebular gas differ considerably from the self-similar solutions found for isothermal conditions. Furthermore, the mass loss must remain at a rather high level until the stellar remnant begins to evolve quickly towards the central star regime. Current theoretical computations of dust-driven mass-loss which are restricted to rather low temperatures cannot be applied during the stars departure from the asymptotic giant branch.

FLIERs and Other Microstructures in Planetary Nebulae. III.

Long-slit spectroscopic observations along the major axes of four planetary nebulae with interesting jets and FLIERs (Hb 4, IC 4634, NGC 6369, and NGC 7354) have been conducted with the Palomar 5 m telescope. Chemical abundances and physical conditions (n, T) in microstructures were derived along their structural axes. No evidence of conspicuous shock activity or N/O abundance anomalies is seen in most cases, unlike some earlier studies of similar features in other planetary nebulae. Microstructures seem to be a heterogeneous class of structures aside from their low ionization and generally supersonic motions.

The chemistry of planetary nebulae and HII regions in the dwarf galaxies Sextans A and B from deep VLT spectra

Spectroscopic observations obtained with the VLT of one planetary nebula (PN) in Sextans A and of five PNe in Sextans B and of several H  regions in these two dwarf irregular galaxies are presented. The extended spectral coverage, from 320.0 to 1000.0 nm, and the large telescope aperture allowed us to detect a number of emission lines, covering more than one ionization stage for several elements (He, O, S, Ar). The electron temperature diagnostic [O ] line at 436.3 nm was measured in all six PNe and in several H  regions allowing for an accurate determination of the ionic and total chemical abundances by means of the Ionization Correction Factors method. For the time being, these PNe are the farthest ones where such a direct measurement of the electron temperature is obtained. In addition, all PNe and H  regions were also modelled using the photoionization code CLOUDY (Ferland et al. 1998, PASP, 110, 761). The physico-chemical properties of PNe and H  regions are presented and discussed. A small dispersion in the oxygen abundance of H  regions was found in both galaxies: 12 + log (O/H) = 7.6 ± 0. 2i n Sextans A, and 7.8 ± 0.2 in Sextans B. For the five PNe of Sextans B, we find that 12 + log (O/H) = 8.0 ± 0.3, with a mean abundance consistent with that of H  regions. The only PN known in Sextans A appears to have been produced by a quite massive progenitor, and has a significant nitrogen overabundance. In addition, its oxygen abundance is 0.4 dex larger than the mean abundance of H  regions, possibly indicating an efficient third dredge-up for massive, low-metallicity PN progenitors. The metal enrichment of both galaxies is analyzed using these new data.

The Local Group Census: Planetary nebulae in IC 10, Leo A and Sextans A

In the framework of our narrow-band survey of the Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in the dwarf irregular galaxies IC 10, Leo A and Sextans A. Using the standard on-band/off-band technique, sixteen new candidate PNe have been discovered in the closest starburst galaxy, IC 10. The optical size of this galaxy is estimated to be much larger than previously thought, considering the location of the new PNe in an area of 3.6 kpc × 2.7 kpc. We also confirm the results of previous studies for the other two dwarf irregular galaxies, with the detection of one candidate PN in Leo A and another one in Sextans A. We review the number of planetary nebulae discovered in the Local Group to date and their behaviour with metallicity. We suggest a possible fall in the observed number of PNe when (Fe/H) � -1.0, which might indicate that below this point the formation rate of PNe is much lower than for stellar populations of near Solar abundances. We also find non-negligible metallicity effects on the (O ) luminosity of the brightest PN of a galaxy.

Episodic dust formation by HD 192641 (WR 137) – II

Observations of the dust and gas around embedded stellar clusters reveal some of the processes involved in their formation and evolution. Large scale mass infall with rates dM/dt=4e-4 solar masses/year is found to be disrupted on small scales by protostellar outflows. Observations of the size and velocity dispersion of clusters suggest that protostellar migration from their birthplace begins at very early times and is a potentially useful evolutionary indicator.

Jets, Knots, and Tails in Planetary Nebulae: NGC 3918, K1-2, and Wray 17-1

We analyze optical images and high-resolution, long-slit spectra of three planetary nebulae that possess collimated, low-ionization features. NGC 3918 is composed of an inner, spindle-shaped shell mildly inclined with respect to the plane of the sky. Departing from the polar regions of this shell, we find a two-sided jet expanding with velocities that increase linearly with distance from 50 to 100 km s-1. The jet is probably coeval with the inner shell (age ≈1000D yr, where D is the distance in kpc), suggesting that its formation should be ascribed to the same dynamical processes that also shaped the main nebula, and not to a more recent mass-loss episode. We discuss the formation of the aspherical shell and jet in the light of current hydrodynamical and magnetohydrodynamical theories. K1-2 is a planetary nebula with a close binary nucleus that shows a collimated string of knots embedded in a diffuse, elliptical shell. The knots expand with a velocity similar to that of the elliptical nebula (~25 km s-1), except for an extended tail located out of the main nebula, which linearly accelerates up to ~45 km s-1. We estimate an inclination on the line of the sight of ~40° for the string of knots; once the orientation of the orbit is also determined, this information will allow us to test the prediction of current theories of the occurrence of polar jets from close binary systems. Wray 17-1 has a complex morphology, showing two pairs of low-ionization structures located in almost perpendicular directions from the central star, and embedded in a large, diffuse nebula. The two pairs show notable similarities and differences, and their origin is very puzzling.

Chemical abundances of Planetary Nebulae in M 33

Using spectroscopic data presented in Magrini et al. (2003), we have analyzed with the photoionization code CLOUDY 94.00 (Ferland et al. 1998) 11 Planetary Nebulae belonging to the spiral galaxy M 33. Central star temperatures and nebular parameters have been determined. In particular the chemical abundances of He/H, O/H, N/H, Ar/H, and S/H have been measured and compared with values obtained via the Ionization Correction Factors (ICF) method, when available. Chemical abundance relationships have been investigated; in particular, a correlation between N/ Ha nd N/O similar to the Galactic one (Henry 1990), and a feeble anti-correlation between O/ Ha nd N/O have been found. A gradient in O/H across the disc of M 33 is consistent with the one found from H  regions in this galaxy (Vilchez et al. 1988). Further studies in the outer parts of M 33 are however needed to ascertain this point. The present result shows that oxygen and helium abundances (with lower accuracy also nitrogen, argon and sulphur) can be actually estimated from the brightest PNe of a galaxy, even if the electron temperature cannot be measured. We also found that the oxygen abundance is quite independent of the absolute magnitude of the PN and consequently the brightest PNe are representative of the whole PN population. This represents an important tool for measuring the metallicity of galaxies at the time of the formation of PNe progenitors.

The Physical Parameters, Excitation, and Chemistry of the Rim, Jets, and Knots of the Planetary Nebula NGC 7009*

We present long-slit optical spectra along the major axis of the planetary nebula NGC 7009. These data allow us to discuss the physical, excitation, and chemical properties of all the morphological components of the nebula, including its remarkable systems of knots and jets. The main results of this analysis are the following: (1) the electron temperature throughout the nebula is remarkably constant, Te[O ] = 10,200 K; (2) the bright inner rim and inner pair of knots have similar densities of Ne ~ 6000 cm-3, whereas a much lower density of Ne ~ 1500 cm-3 is derived for the outer knots as well as for the jets; (3) all the regions (rim, inner knots, jets, and outer knots) are mainly radiatively excited; and (4) there are no clear abundance changes across the nebula for He, O, Ne, or S. There is marginal evidence for an overabundance of nitrogen in the outer knots (ansae), but the inner ones (caps) and the rim have similar N/H values that are at variance with previous results. Our data are compared with the predictions of theoretical models, from which we conclude that the knots at the head of the jets are not matter accumulated during the jet expansion through the circumstellar medium; nor can their origin be explained by the proposed hydrodynamic or MHD interacting wind models for the formation of jets/ansae, since the densities, as well as the main excitation mechanisms of the knots, disagree with model predictions.