Point-symmetric Planetary Nebulae Research Articles

Two evolved close binary stars: GALEX J015054.4+310745 and the central star of the planetary nebula Hen 2-84

ABSTRACT As part of a survey to find close binary systems among central stars of planetary nebula, we present two newly discovered binary systems. GALEX J015054.4+310745 is identified as the central star of the possible planetary nebula Fr 2-22. We find it to be a single-lined spectroscopic binary with an orbital period of 0.2554435(10) d. We support the previous identification of GALEX J015054.4+310745 as an sdB star and provide physical parameters for the star from spectral modelling. We identify its undetected companion as a likely He white dwarf. Based on this information, we find it unlikely that Fr 2-22 is a true planetary nebula. In addition, the central star of the true planetary nebula Hen 2-84 is found to be a photometric variable, likely due to the irradiation of a cool companion. The system has an orbital period of 0.485645(30) d. We discuss limits on binary parameters based on the available light-curve data. Hen 2-84 is a strongly shaped bipolar planetary nebula, which we now add to the growing list of axially or point-symmetric planetary nebulae with a close binary central star.

3D hydrodynamical models of point-symmetric planetary nebulae: the special case of H 1-67

We present 3D hydrodynamical simulations of a precessing jet with a time-dependent ejection velocity or a time-dependent ejection density, interacting with a circumstellar medium given by a dense, anisotropic, and slow AGB wind, forming a torus. We explore a set of configurations with different values for the precession angle and number of ejections. The temporal evolution of these models is analised at times up to 1500 or 1800 yr. From our hydrodynamical models, we obtain position-velocity diagrams (PV diagrams) in the [NII]{\lambda}6583 line to be compared with high resolution observations of the planetary nebula H1-67. From spectral data this object shows high-velocity jets and a point-symmetric morphology. With our synthetic PV diagrams we show that a precessing jet with a time-dependent ejection velocity or a time-dependent ejection density reproduce the point-symmetric morphological structure for this nebula if the precession cone angle is larger than 30$^{\circ}$. Our synthetic PV diagrams can be used to understand how the S-like morphology, also presented by other planetary nebulae, is formed. For H1-67 we found a heliocentric velocity of -8.05 km s^-1 and height below the galactic plane of -451.6 pc.

An Interacting Binary System Powers Precessing Outflows of an Evolved Star

Stars are generally spherical, yet their gaseous envelopes often appear nonspherical when ejected near the end of their lives. This quirk is most notable during the planetary nebula phase, when these envelopes become ionized. Interactions among stars in a binary system are suspected to cause the asymmetry. In particular, a precessing accretion disk around a companion is believed to launch point-symmetric jets, as seen in the prototype Fleming 1. Our finding of a post-common-envelope binary nucleus in Fleming 1 confirms that this scenario is highly favorable. Similar binary interactions are therefore likely to explain these kinds of outflows in a large variety of systems.

The dense and asymmetric central star wind of the young PN He 2−138

We present optical ESO time-series and UV archival (FUSE, HST, IUE) spectroscopy of the H-rich central star of He 2-138. Our study targets the central star wind in a very young planetary nebula, and explores physical conditions that may provide clues to the nature of the preceding post-AGB super-wind phases of the star. We provide evidence for a dense, slowly accelerating outflow that is variable on time-scales of hours. Line-synthesis modelling (SEI and CMFGEN) of low and high ionization UV and optical lines is interpreted in terms of an asymmetric, two-component outflow, where high-speed high-ionization gas forms mostly in the polar region. Slower, low ionization material is then confined primarily to a cooler equatorial component of the outflow. A dichotomy is also evident at photospheric levels. We also document temporal changes in the weak photospheric lines of He 2-138, with tentative evidence for a 0.36-day modulation in blue-to-red migrating features in the absorption lines. These structures may betray 'wave-leakage' of prograde non-radial pulsations of the central star. These multi-waveband results on the aspherical outflow of He 2-138 are discussed in the context of current interest in understanding the origin of axi- and point-symmetric planetary nebulae.

Multiple and Precessing Collimated Outflows in the Planetary Nebula IC 4634

With its remarkable double-S shape, IC 4634 is an archetype of point-symmetric planetary nebulae (PNe). In this paper, we present a detailed study of this PN using archival HST WFPC2 and ground-based narrowband images to investigate its morphology, and long-slit spectroscopic observations to determine its kinematics and to derive its physical conditions and excitation. The data reveal new structural components, including a distant string of knots distributed along an arclike feature 40 -->''-60 -->'' from the center of the nebula, a skin of enhanced [O III]/Hα ratio enveloping the inner shell and the double-S feature, and a triple-shell structure. The spatio-kinematical study also finds an equatorial component of the main nebula that is kinematically independent of the bright inner S-shaped arc. We have investigated in detail the bow shock-like features in IC 4634 and found that their morphological, kinematical, and emission properties are consistent with the interaction of a collimated outflow with surrounding material. Indeed, the morphology and kinematics of some of these features can be interpreted using a three-dimensional numerical simulation of a collimated outflow precessing at a moderate, time-dependent velocity. Apparently, IC 4634 has experienced several episodes of point-symmetric ejections oriented in different directions, with the outer S-shaped feature being related to an earlier point-symmetric ejection, and the outermost arclike string of knots being the relic of a still much earlier point-symmetric ejection. There is tantalizing evidence that the action of these collimated outflows has also taken part in the shaping of the innermost shell and inner S-shaped arc of IC 4634.

The physical structure of the point-symmetric and quadrupolar planetary nebula NGC 6309

We analyse the point-symmetric planetary nebula NGC 6309 in terms of its three-dimensional structure and of internal variations of the physical conditions to deduce the physical processes involved in its formation. We used VLA-D 3.6-cm continuum, ground-based, and HST-archive imaging as well as long slit high- and low-dispersion spectroscopy. The low-dispersion spectra indicate a high excitation nebula, with low to medium variations of its internal physical conditions. In the optical images, the point-symmetric knots show a lack of [NII] emission as compared with similar features previously known in other PNe. A rich internal structure of the central region is seen in the HST images, resembling a deformed torus. Long slit high-dispersion spectra reveal a complex kinematics in the central region. The spectral line profiles from the external regions of NGC 6309 indicate expanding lobes (~40 km/s) as those generally found in bipolar nebulae. Finally, we have found evidence for the presence of a faint halo, possibly related to the envelope of the AGB-star progenitor. Our data indicate that NGC 6309 is a quadrupolar nebula with two pairs of bipolar lobes whose axes are oriented PA=40 and PA=76. Equatorial and polar velocities for these two pairs of lobes are 29 and 86 km/s for the bipolar system at PA=40 and 25 and 75 km/s for the bipolar system at PA=76. There is also a central torus that is expanding at 25 km/s. Kinematical age for all these structures is around 3700 to 4000 yr. We conclude that NGC 6309 was formed by a set of well-collimated bipolar outflows (jets), which were ejected in the initial stages of its formation as a planetary nebula. These jets carved the bipolar lobes in the previous AGB wind and their remnants are now observed as the point-symmetric knots tracing the edges of the lobes.

IC 5217 as a double-shell, point-symmetric planetary nebula with a very narrow waist

Aims. Identification of the structural components and analysis of the internal kinematics in the planetary nebula IC 5217. Methods. Narrow-band images and high resolution long-slit spectra in the Hα ,[ Nii ]a nd [Oiii] emission lines, and VLA 6 cm continuum data. Results. IC 5217 is composed of a very bright equatorial ring, open bipolar lobes, off-axis point-symmetric features, on-axis distant faint regions, and an off-axis very elongated bipolar structure. The ring, open lobes, point-symmetric features and distant faint regions appear as elements of a single point-symmetric bipolar shell with a very narrow waist. This shell presents an axis ratio of ∼37 and an aspect ratio of ∼5. The 6 cm data show that the ring is an extremely flat disk with a central hole. Expansion velocity in the ring ranges from ≤10 km s −1 in He ii up to � 27 km s −1 in [N ii], whereas a velocity of ∼460 km s −1 is estimated for the polar regions of the bipolar shell. Strong acceleration of the outer regions of the ring is observed. The elongated bipolar structure probably represents a highly collimated (aspect ratio ∼12), high velocity cylindrical-like shell. A collimated agent (wind or jet) would account for the shaping of the bipolar shell if this agent has operated in the direction perpendicular to the equatorial disk. The point-symmetric features and cylindrical shell are probably related to collimated ejections but that occurred when the basic nebular shape had already been established.

Binary black holes at the core of galaxy clusters

In this paper, we push forward and exploit an analogy between the morphologies of the X-ray cavities observed in some galaxy clusters, and the optically deficient point-symmetric bubbles occurring in some planetary nebulae (PNe). Point-symmetric PNe are thought to be shaped by stellar binary interactions; namely, the presence of a companion to the PN’s progenitor star is required. We suggest that similar point-symmetric structures in the X-ray cavities of galaxy clusters might be associated with the presence of massive binary black holes. A systematic cataloguing of high-resolution images of the diffuse X-ray emission at the core of galaxy clusters might contribute to individuate massive binary black holes.

The remarkably high excitation planetary nebula GC 6537.

NGC 6537 is an unusually high excitation point symmetric planetary nebula with a rich spectrum. Its kinematical structures are of special interest. We are here primarily concerned with the high resolution spectrum as revealed by the Hamilton echelle Spectrograph at Lick Observatory (resolution approximately 0.2 A) and supplemented by UV and near-UV data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. The photoionization models that have been used successfully for many planetary nebulae are not entirely satisfactory here. The plasma electron temperature of a photoionization model cannot much exceed 20,000 K, but plasma diagnostics show that regions emitting radiation of highly ionized atoms such as [NeIV] and [NeV] are much hotter, showing that shock excitation must be important, as suggested by the remarkable kinematics of this object. Hence, instead of employing a strict photoionization model, we are guided by the nebular diagnostics, which reveal how electron temperature varies with ionization potential and accommodates density effects. The predictions of the photoionization model may be useful in estimating ionization correction factor. In effect, we have estimated the chemical composition by using both photoionization and shock considerations.

The Kinematics of Point-symmetric Planetary Nebulae

Four planetary nebulae (PNe) with conspicuous point-symmetric morphology are studied in this paper through high-quality imagery and long-slit echelle spectroscopy. Point symmetry is also found in the velocity space, and this is related to particular forms of bipolar collimated outflows. Morphology and kinematics together reveal the presence of collimated bipolar ejections in an episodic way with indications of rotation or displacement of the symmetry axis of the outflow. Point symmetry is currently known to occur in a wide variety of PNe and the convenience of a reevaluation of point-symmetric PNe as a main morphological class is pointed out.