The kinematics of NGC 4361, a Population II planetary nebula with a bipolar outflow

High-resolution, spatially-resolved profiles of H α , He  ii λ 6560 and [O  iii ] λ 5007 and deep narrow-band CCD images in the H α and [O  iii ] λ 5007 emission lines have been obtained of the planetary nebula (PN) NGC 4361. In addition, VLA-DnC λ 3.6-cm continuum observations are presented. This material allows one to explore in unprecedented detail the morphology and kinematics of this PN. The morphology of this object is complex given the highly filamentary structure of the envelope, which is confirmed to possess a low mass. The halo has a high expansion velocity that yields incompatible kinematic and evolutionary ages, unless previous acceleration of the nebular expansion is considered. However, the most remarkable result from the present observations is the detection of a bipolar outflow in NGC 4361, which is unexpected in a PN with a Population II low-mass-core progenitor. It is shown that shocks resulting from the interaction of the bipolar outflow with the outer shell are able to provide an additional heating source in this nebula.     

Morphological and kinematic signatures of a binary central star in the planetary nebula Hu 2-1

We present H α , [N  ii ] and [O  iii ] ground-based and HST archive images, VLA–A 3.6-cm continuum and H92 α emission-line data and high-resolution long-slit [N  ii ] spectra of the planetary nebula Hu 2-1. A large number of structural components are identified in the nebula: an outer bipolar and an inner shell, two pairs of collimated bipolar structures at different directions, monopolar bow-shock-like structures, and an extended equatorial structure within a halo. The formation of Hu 2-1 appears to be dominated by anisotropic mass ejection during the late-AGB stage of the progenitor and by variable, 'precessing' collimated bipolar outflows during the protoplanetary nebula and/or early planetary nebula phases. Different observational results strongly support the existence of a binary central star in Hu 2-1, among them (1) the observed point-symmetry of the bipolar lobes and inner shell, and the departures from axial symmetry of the bipolar lobes, (2) the off-centre position of the central star, (3) the detection of mass ejection towards the equatorial plane, and (4) the presence of 'precessing' collimated outflows. In addition, (5) an analysis of the kinematics shows that the systemic velocity of the bipolar outflows does not coincide with the systemic velocity of the bipolar shell. We propose that this velocity difference is a direct evidence of orbital motion of the ejection source in a binary system. From a deduced orbital velocity of ∼10 km s⁻¹, a semimajor axis of ∼ 9–27 au and period of ∼ 25–80 yr are obtained, assuming a reasonable range of masses. These parameters are used to analyse the formation of Hu 2-1 within current scenarios of planetary nebulae with binary central stars.     

High-speed knots in the hourglass-shaped planetary nebula Hubble 12

We present a detailed kinematical analysis of the young compact hourglass-shaped planetary nebula Hb 12. We performed optical imaging and long-slit spectroscopy of Hb 12 using the Manchester echelle spectrometer with the 2.1-m San Pedro Mártir telescope. We reveal, for the first time, the presence of end caps (or knots) aligned with the bipolar lobes of the planetary nebula shell in a deep [N  ii ]λ6584 image of Hb 12. We measured from our spectroscopy radial velocities of  ∼120 km s⁻¹  for these knots.
We have derived the inclination angle of the hourglass-shaped nebular shell to be ∼65° to the line of sight. It has been suggested that Hb 12's central star system is an eclipsing binary which would imply a binary inclination of at least 80°. However, if the central binary has been the major shaping influence on the nebula, then both nebula and binary would be expected to share a common inclination angle.
Finally, we report the discovery of high-velocity knots with Hubble-type velocities, close to the core of Hb 12, observed in Hα and oriented in the same direction as the end caps. Very different velocities and kinematical ages were calculated for the outer and inner knots showing that they may originate from different outburst events.     

L 43: the late stages of a molecular outflow

Our new 21-arcsec resolution CO J  = 2 → 1 map of the L 43 dark cloud shows a poorly collimated molecular outflow, with little evidence for wings at velocities 10 km s⁻¹. The outflow appears not to be currently driven by a jet: its structure can instead be modelled as a slowly expanding shell. The shell may be compressed either by a wide-angled wind catching up with an existing shell (as in the case of planetary nebulæ), or by the thermal pressure of a hot low-emissivity medium interior to the shell. The outflow is most probably in a late stage of evolution, and appears to be in the process of blowing away its molecular cloud. We also present a 45-arcsec resolution CO J  = 1 → 0 map of the whole molecular cloud, showing that the outflow structure is clearly visible even in the integrated intensity of this low excitation line, and suggesting that rapid mapping may prove useful as a way of finding regions of outflow activity. We also examine the immediate surroundings of the driving source with 450 μm imaging: this confirms that the outflow has already evacuated a bay in the vicinity of the young stellar object.     

Kinematics of the Gum nebula region

To investigate the kinematics of the neutral material around the Gum nebula, emission from hydroxyl at 1667 MHz was observed at many positions over the region. Fitting models of expanding shells to these data together with previously published molecular line data shows that the diffuse molecular clouds and cometary globules form a single expanding shell centred on G261−2.5. The mean angular radius is 10.5° and its maximum radius is 14°. The models show that the distance range to the expansion centre is from 200 pc to 500 pc.
The path of the runaway O-star ζ Puppis passed within <0.5° of the expansion centre of the neutral shell ∼1.5 Myr ago. The supernova of the erstwhile binary companion of ζ Puppis is the probable origin of the Gum nebula and the swept up expanding neutral shell. The 500-pc distance to the supernova is adopted as the distance to the expansion centre of the neutral shell. At this distance the energy required to produce the observed expansion could have been met with a single supernova. The radii of the front and back faces of the shell are 130 and 70 pc respectively. The front face is expanding faster than the back face, at 14 and 8.5 km s⁻¹ respectively.
The extent of the neutral shell matches the radio continuum and H α emission of the Gum nebula well. The photoionized gas in the nebula is probably primarily ionized by ζ Puppis, which is still within the neutral shell. No evidence was found for the IRAS -Vela shell as a separate expanding shell.     

The nature and structure of the emission line nebula K 3-35: a very young planetary nebula with precessing bipolar jet-like outflows?

We present Hα, [N  II ]6583 and 6-cm continuum images of the emission line nebula K 3-35. The optical images reveal an extended nebula (size ≃ 11 × 9 arcsec² in [N  II ]) in which most of the emission originates in a very narrow (width 0.7–1.3 arcsec) S-shaped region which extends almost all along the nebula (≃ 7 arcsec). The 6-cm continuum emission also arises in this narrow region, which is characterized by an exceedingly high point-symmetry and systematic and continuous changes of the orientation with respect to the nebular centre. The properties of the narrow region suggest that it represents a system of precessing bipolar jet-like components. Two low-excitation, compact bipolar knots near the tips of the jet-like components are observed in the deduced [N  II ]/Hα image ratio. These knots may be generated by the interaction of the collimated outflows with surrounding material. A comparison of the optical and radio images shows the existence of differential extinction within the nebula. Maximum extinction is observed in a disc-like region which traces the equator of the elliptical shell previously observed at 20-cm continuum. All available data strongly suggest that K 3-35 is a very young planetary nebula in which we could be observing the first stages of the formation of collimated outflows and point-symmetric structures typically observed in planetary nebulae. The properties of the jet-like components in K 3-35 are in good agreement with models of binary central stars in which highly collimated outflows originate either from a precessing accretion disc or via magnetic collimation in a precessing star.     

Precessing collimated outflows in the planetary nebula IC 4846

We present [N  ii ] and H α images and high-resolution long-slit spectra of the planetary nebula IC 4846, which reveal, for the first time, its complex structure and the existence of collimated outflows. The object consists of a moderately elongated shell, two (and probably three) pairs of collimated bipolar outflows at different orientations, and an attached circular shell. One of the collimated pairs is constituted by two curved, extended filaments the properties of which indicate a high-velocity, bipolar precessing jet. A difference of ≃10 km s⁻¹ is found between the systemic velocity of the precessing jets and the centroid velocity of the nebula, as recently reported for Hu 2-1. We propose that this difference is as a result of orbital motion of the ejection source in a binary central star. The orbital separation of 30 au and period 100 yr estimated for the binary are similar to those in Hu 2-1, linking the central stars of both planetary nebulae to interacting binaries. Extraordinary similarities also exist between IC 4846 and the bewildering planetary nebula NGC 6543, suggesting a similar formation history for both objects.     

Hydrogen recombination lines from the Gum nebula

Hydrogen recombination lines in the H156 α and H139 α transitions have been detected at four widely separated positions in the Gum nebula. This confirms that the radio continuum emission seen in parts of the nebula is predominantly bremsstrahlung rather than synchrotron emission.
The derived electron temperatures and emission measures are in the range 4200 to 8500 K and 220 to 470 pc cm⁻⁶ respectively. This is consistent with the presence of a low-density, photoionized plasma. The linewidth observed at the position away from the edge of the nebula is significantly larger than those near the edge of the nebula. Together with the negative line velocity observed at this position, this suggests there is systematic expansion of the near side of the nebula.     

Hydrostatic equilibrium in a magnetized, warped Galactic disc

Hydrostatic equilibrium of the multiphase interstellar medium in the solar vicinity is reconsidered, with the regular and turbulent magnetic fields treated separately. The regular magnetic field strength required to support the gas is consistent with independent estimates, provided that energy equipartition is maintained between turbulence and random magnetic fields. Our results indicate that a mid-plane value of B ₀=4 μG for the regular magnetic field near the Sun leads to more attractive models than B ₀=2 μG . The vertical profiles of both the regular and random magnetic fields contain disc and halo components, the parameters of which we have determined. The layer at 1≲| z |≲4 kpc can be overpressured and an outflow at a speed of about 50 km s⁻¹ may occur there, presumably associated with a Galactic fountain flow, if B ₀≃2 μG .
We show that hydrostatic equilibrium in a warped disc must produce asymmetric density distributions in z , in rough agreement with H  i observations in the outer Galaxy. This asymmetry may be a useful diagnostic of the details of the warping mechanism in the Milky Way and other galaxies. We find indications that gas and magnetic field pressures are different above and below the warped midplane in the outer Galaxy, and quantify the difference in terms of turbulent velocity and/or magnetic field strength.     

An H i interstellar bubble surrounding WR 85 and RCW 118

We analyse the distribution of the interstellar matter in the environs of the Wolf-Rayet star LSS 3982 (= WR 85, WN6+OB?) linked to the optical ring nebula RCW 118. Our study is based on neutral hydrogen 21-cm line data belonging to the Southern Galactic Plane Survey (SGPS).
The analysis of the H  i data allowed the identification of a neutral hydrogen interstellar bubble related to WR 85 and the 25-arcmin-diameter ring nebula RCW 118. The H  i bubble was detected at a systemic velocity of −21.5 km s⁻¹, corresponding to a kinematical distance of 2.8 ± 1.1 kpc, compatible with the stellar distance. The neutral structure is about 25 arcmin in radius or 21 ± 8 pc, and is expanding at 9 ± 2 km s⁻¹. The associated ionized and neutral masses amount to  3000 M_⊙  . The carbon monoxide (CO) emission distribution depicts a region lacking CO coincident in position and velocity with the H  i structure. The 9.3-arcmin-diameter inner optical nebula appears to be related to the approaching part of the neutral atomic shell. The H  i void and shell are the neutral gas counterparts of the optical bubble and have very probably originated in the action of the strong stellar wind of the central star during the O-type and WR phases on the surrounding interstellar medium. The H  i bubble appears to be in the momentum conserving stage.     

A local void and the accelerating Universe

RCW 114 is a filamentary nebula of about 250 arcmin diameter. Based on its large diameter-to-filament-width ratio, the expansion velocity, distance and size of the shell, it has been suggested that RCW 114 is a supernova remnant in its momentum-conserving phase. Confirmation of this identification is important, as the large angular size and extensive optical emission of this object will allow for detailed study to improve our knowledge of supernova remnants and their interaction with the interstellar medium.
We have used the FLAIR instrument on the UK Schmidt Telescope to obtain optical spectra of several filaments in RCW 114. These confirm that the emission is being produced by the interaction of the shock wave of a supernova remnant with the surrounding interstellar medium. We also obtained narrow-band H α +[N  ii ] and [S  ii ] images to examine the spatial variation in ionization structure.     

A new Wolf–Rayet star and its ring nebula: PCG 11

In a search for new Galactic planetary nebulae from our systematic scans of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) Hα Survey of the Southern Galactic Plane, we have identified a Population I Wolf–Rayet star of type WN7h associated with an unusual ring nebula that has a fractured rim. We present imagery in Hα, the 843-MHz continuum from the Molonglo Observatory Synthesis Telescope (MOST), the mid-infrared from the Midcourse Space Experiment ( MSX ), and confirmatory optical spectroscopy of the character of the nebula and of its central star. The inner edge of the Hα shell shows gravitational instabilities with a well-defined wavelength around its complete circumference.     

The emission and kinematic structures of the bipolar planetary nebula M 1-8

We have undertaken echelle spectroscopy and narrow-band line imaging of the bipolar planetary nebula M 1-8. This has permitted us to map the outflow in [N  ii ]λλ 6548+6583 Å, Hα, and in the v = 1–0 S(1) transition of H₂ at λ 2.122 μm. It has also permitted us to acquire high-resolution spectra for [N  ii ]λ 6583 Å, Hα and He  ii λ 6560 Å. Our observations support the results of a previous 2MASS analysis by two of the authors (J. P. Phillips and G. Ramos-Larios), and confirm that there is strong H₂ emission outside of the ionized zone, as well as along the major axis of the outflow. Finally, we have investigated the spatial structure of the outflow in low and high excitation lines, and noted evidence for strong ionization stratification within the envelope of the source. We also note that major axis spectra show asymmetries attributable to outflow along the lobes, oriented at an angle i ∼ 35°–40° to the line of sight. Asymmetries along the minor axis, by contrast, appear to be associated with the central collimating disc, and may be interpretable in terms of asymmetries in disc structure, or rotation at an angular velocity of Ω∼ 1.4 10⁻¹² rad s⁻¹. If the disc arises due to common-envelope evolution, then it seems that angular momentum constraints must be relatively tight, and can only be satisfied given fairly extreme physical assumptions (such as low disc mass, high primary star mass, a low distance to the source and so forth).     

Possible detection of an old bipolar shell associated with η Carinae

Continuum-subtracted dereddened images in the light of several atomic lines show the presence of an extended bipolar nebula surrounding η Carinae with size ∼100×45 arcsec² (1.3×0.5 pc²). This feature is best delineated in [O  iii ] 5007. The geometrical disposition and mass of the shell suggest that it was formed by mass ejections from η Carinae. The dynamic age of the nebula is ∼13 000/ V ₇ yr, where V ₇ is the mean expansion velocity in 100 km s⁻¹, and its mass is between 5 and 10 M_⊙. The nebula is photoionized and composed of unprocessed material. The major axes of the nebula and of the Homunculus are nearly perpendicular. We also report the discovery of elongated emission knots prominent in [N  ii ] located 64 to 100 arcsec away from η Carinae, which implies that they were ejected either centuries ago or at a more recent date but with extremely large velocities.     

Radially aligned clumps and tails in planetary nebulae

We study the formation of radially aligned condensations and tails through the compression of material inside ionization shadows at early ionization phases of planetary nebulae. A dense clump, formed before ionization starts, forms an ionization shadow behind it. The surroundings, which are ionized before the shadow, have a higher temperature, and as a result compress the material in the shadow, forming a compressed tail. If the compressed tail crosses a dense shell, a dense condensation (clump) is formed there. At later stages this condensation is ionized and observed as a bright knot, radially aligned with the inner clump. We find that for the shadow to be effective, the clump should be already present as the ionization by the central star starts, and its density enhancement should be by a factor of ≳ 5. We propose this mechanism as an explanation for the radially aligned condensations recently found in the planetary nebula IC 4593.     

The Manchester occulting mask imager (MOMI): first results on the environment of P Cygni

The design and first use of the Manchester occulting mask imager (MOMI) is described. This device, when combined with the Cassegrain or Ritchey–Chretien foci of large telescopes, is dedicated to the imagery of faint-line emission regions around bright central sources. Initial observations, with MOMI on the Nordic Optical Telescope (NOT), of the V =4.8 mag P Cygni environment have revealed a ≥5 arcmin long [N  II ] λ6584-Å emitting filament projecting from the outer nebular shell of this luminous blue variable (LBV) star. The presence of a monopolar lobe older than both the inner (22 arcsec diameter) and outer (1.6 arcmin diameter) shells is suggested.     

Morphology, kinematics and modelling of the elliptical planetary nebula Sa 2-21

The little studied PN, Sa 2-21 has been observed using the Manchester echelle spectrometer at the Anglo-Australian telescope. Narrow band, long-slit spectra were obtained at six positions over two perpendicular position angles in both the [N ii ]λ6584  Å and [O iii ]λ5007  Å emission lines. An [O iii ] halo has been detected for the first time. A morphological modelling program was used to help determine the geometry, structure and kinematics of this ellipsoidal PN. It is proposed that the structure includes a pair of mid-latitude rings of [N ii ] emission, not previously seen in elliptical PNe. Radial spokes of [O iii ] emission have been detected in the main nebular shell indicating the presence of dynamical instabilities.     

G309.2–00.6 and jets in supernova remnants

We present Australia Telescope Compact Array observations of the supernova remnant (SNR) G309.2–00.6. In a 1.3-GHz continuum image the remnant appears as a near-circular shell, but with two brightened and distorted arcs of emission on opposite sides. H  i absorption against the SNR yields a distance in the range 5.4 to 14.1 kpc, corresponding to an age (1−20) × 10³ yr. On the basis of the morphology of the SNR we argue that it is a younger analogue of the W 50/SS 433 system, and that its unusual appearance is a result of opposed jets or outflows from a central source. A jet-like feature and breaks in the shell can both be seen along the axis of proposed outflow, providing further support for this interpretation; the central source itself is not detected. The SNR may be interacting with the adjacent H  ii region RCW 80 through an extension of the proposed outflow beyond its shell. This would put the SNR at the lower limit of its distance range and would imply an age 4000 yr. We consider other SNRs similar to G309.2–00.6, and propose remnants whose shells are affected by jets as one of several classes of SNR from which the presence of a central source can be inferred.     

Time evolution of galactic warps in prolate haloes

A recent observation with the Hipparcos satellite and some numerical simulations imply that the interaction between an oblate halo and a disc is inappropriate for the persistence of galactic warps. Following on from this , we have compared the time evolution of galactic warps in a prolate halo with that in an oblate halo. The haloes were approximated as fixed potentials, while the discs were represented by N -body particles. We have found that the warping in the oblate halo continues to wind up, and finally disappears. On the other hand, for the prolate halo model, the precession rate of the outer disc increases when the precession of the outer disc recedes from that of the inner disc, and vice versa. Consequently, the warping in the prolate halo persisted to the end of the simulation by retaining the alignment of the line of nodes of the warped disc. Therefore, our results suggest that prolate haloes could sustain galactic warps. The physical mechanism of the persistence of warp is discussed on the basis of the torque between a halo and a disc and that between the inner and outer regions of the disc.     

OH maser mapping of the evolved star HD 179821: evidence for interacting outflows

The evolved star HD 179821 continues to be the subject of much debate as to whether it is a nearby     post-asymptotic giant branch (post-AGB) star or a distant     high initial mass     post-red supergiant. We have mapped the OH maser emission around HD 179821 in the 1612- and 1667-MHz lines with the MERLIN interferometer array at a resolution of 0.4 arcsec and 0.35 km s⁻¹. The OH emission lies in a thick shell with inner and outer radii of 1.3 and         and expansion velocity of 30 km s⁻¹. Although we find some evidence for acceleration and for deviations from spherical symmetry, the bulk of the maser emission is consistent with a constant-velocity spherical shell. The extent of the shell agrees with H₂O and OH dissociation models and supports a distance estimate of 6 kpc. However, the shell is incomplete and appears to have been disrupted by more recent collimated outflow activity within the last 1500 yr. We suggest that this activity is also responsible for the active envelope chemistry (in particular the presence of HCO⁺) and for the apparent offset of the star from the centre of the shell. The luminous yellow hypergiant star IRC +10420 also shows signs of recent outflows, and HD 179821 may be at a similar, perhaps slightly earlier, phase of evolution. We suggest that the SiO thermal emission arises from the same detached envelope as the OH maser emission as in IRC +10420. If so then this would strengthen the connection between these two stars and probably rule out a post-AGB status for HD 179821.