Denise R. Goncalves

Low-Ionization Structures in Planetary Nebulae: Confronting Models with Observations

Around 50 planetary nebulae (PNs) are presently known to possess small-scale low-ionization structures (LISs) located inside or outside their main nebular bodies. We consider here the different kinds of LISs (jets, jetlike systems, symmetrical and nonsymmetrical knots) and present a detailed comparison of the existing model predictions with the observational morphological and kinematical properties. We find that nebulae with LISs appear indistinctly spread among all morphological classes of PNs, indicating that the processes leading to the formation of LISs are not necessarily related to those responsible for the asphericity of the large-scale morphological components of PNs. We show that both the observed velocities and locations of most nonsymmetrical systems of LISs can be reasonably well reproduced assuming either fossil condensations originated in the asymptotic giant branch (AGB) wind or in situ instabilities. The jet models proposed to date (hydrodynamical and magnetohydrodynamical interacting winds or accretion disk collimated winds) appear unable to account simultaneously for several key characteristics of the observed high-velocity jets, such as their kinematical ages and the angle between the jet and the symmetry axes of the nebulae. The linear increase in velocity observed in several jets favors magnetohydrodynamical confinement compared to pure hydrodynamical interacting wind models. On the other hand, we find that the formation of jetlike systems characterized by relatively low expansion velocities (similar to those of the main shells of PNs) cannot be explained by any of the existing models. Finally, the knots that appear in symmetrical and opposite pairs of low velocity could be understood as the survival of fossil (symmetrical) condensations formed during the AGB phase or as structures that have experienced substantial slowing down by the ambient medium.

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.

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.

Knots in the outer shells of the planetary nebulae ic 2553 and ngc 5882

We present images and high-resolution spectra of the planetary nebulae IC 2553 and NGC 5882. Spatiokinematic modeling of the nebulae shows that they are composed of a markedly elongated inner shell and of a less aspherical outer shell expanding at a considerably higher velocity than the inner one. Embedded in the outer shells of both nebulae are found several low-ionization knots. In IC 2553, the knots show a point-symmetric distribution with respect to the central star: one possible explanation for their formation is that they are the survivors of preexisting point-symmetric condensations in the asymptotic giant branch wind, a fact that would imply a quite peculiar mass-loss geometry from the giant progenitor. In the case of NGC 5882, the lack of symmetry in the distribution of the observed low-ionization structures makes it possible that they are the result of in situ instabilities.

K 4−47: a planetary nebula excited by photons and shocks

K 4-47 is an unusual planetary nebula composed of a compact high-ionization core and a pair of low-ionization knots. Long-slit medium-resolution spectra of the knots and core are analyzed in this paper. Assuming photoionization from the central star, we have derived physical parameters for all the nebular components, and the (icf) chemical abundances of the core, which appear similar to Type-I PNe for He and N/O but significantly deficient in oxygen. The nebula has been further modelled using both photoionization (CLOUDY) and shock (MAPPINGS) codes. From the photoionization modelling of the core, we find that both the strong auroral [O iii] 4363u and [N ii] 5755u emission lines observed and the optical size of the core cannot be accounted for if a homogeneus density is adopted. We suggest that a strong density stratification, matching the high-density core detected at radio wavelengths and the much lower density of the optical core, might solve the problem. From the bow-shock modelling of the knots, on the other hand, we find that knots’ chemistry is also represented by Type-I PN abundances, and that they would move with velocities of 250 - 300kms 1 .

The Possibility of Thermal Instability in Early-Type Stars Due to Alfvén Waves

The importance of Alfven waves to explain the winds of Wolf-Rayet stars was shown by dos Santos and coworkers. We investigate here the possible importance of Alfven waves in the creation of inhomogeneities in the winds of early-type stars. The observed infrared emission (at the base of the wind) of early-type stars is often larger than expected. The clumping explains this characteristic in the wind, increasing the mean density and hence the emission measure, making it possible to understand the observed infrared, as well as the observed enhancement in the blue wing of the Hα line. In this study, we investigate the formation of these clumps via thermal instability. The heat-loss function used, H(T, n), includes physical processes such as emission of (continuous and line) recombination radiation, resonance line emission excited by electron collisions, thermal bremsstrahlung, Compton heating and cooling, and damping of Alfven waves. As a result of this heat-loss function we show the existence of two stable equilibrium regions. At high temperature the stable equilibrium region is the diffuse medium, and at low temperature it is the clumps. Using this reasonable heat-loss function, we show that the two stable equilibrium regions can coexist over a narrow range of pressures describing the diffuse medium and the clumps.

Evolution of active galactic nuclei broad-line region clouds: low- and high-ionization lines

The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfven heating has been discussed by Goncalves, Jatenco-Pereira & Opher. In particular, these studies showed the relevance of Alfven heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 107-K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering the Alfven heating as the non-radiative mechanism.

Physical parameters of low-ionization knots and jets in PNE: NGC 7009, K 4-47, and NGC 6543

In addition to the large-scale outflows, which form their round, elliptical, and bipolar shells, planetary nebulae (PNe) also have, usually on smaller scales, pairs of highly collimated outflows, or jets. These jets, as well as the pairs of knots that appear at their tips (very prominent in the low-ionization emission lines), are the subject of the present study. We show our results on the temperatures and densities of jets and knots, compare these physical parameters with those of the main shells of PNe, and compare them with theoretical model predictions. We note particularly that the knots at the tips of the jets are not denser than the jets, and that neither is their emission collisionally excited, as one would expect if they were by-products of the associated supersonic jets.

Magnetic support and Alfvén heating in quasar broad-line region clouds

Goncalves et al. (1996) showed the relevance of Alfven heating in order to establish the stability of quasar broad-line region clouds in the intercloud medium due to thermal instability. Although, photoionization by the continuum radiation from the central source is the most important mechanism to heat and ionize the broad-line region, Alfvenic heating being a second order process. In this article we show the results of the time-dependent calculations to follow the evolution of the forming clouds from the 107 K intercloud medium, calculating the UV and optical line emission associated with the clouds in order to compare them with observations.

Are Microstructures in the Outer Shells of PNe Fossil Condensations of the AGB Wind

Some planetary nebulae (PNe) show microstructures of low ionization located in their outer regions, i.e. external to the bright rim defined by the fast vs. slow wind interaction region. Morphological and kinematic analysis of some of these PNe show low ionization knots which clearly share the expansion of the outer shell of the host nebulae (e.g. IC 2553). Sometimes, these knots possess an accentuated degree of symmetry. Given that in situ instabilities are not thought to form symmetrical features, we explore the idea that these symmetric pairs of knots are fossil condensations of the AGB wind being modified by interaction with the expanding ionization front. This very appealing possibility would imply that the mass loss of the pre-PN stages has episodes of highly collimated and symmetric ejections. In other PNe knots of low ionization in the outer shells are not symmetric (e.g. NGC 5882) and could be formed by either simple in situ instabilities and/or fossil condensations.