Ring Nebula Research Articles

A unique Galactic planetary nebula with a [WN] central star

We report the discovery of the first probable Galactic [WN] central star of a planetary nebula (CSPN). The planetary nebula candidate was found during our systematic scans of the AAO/UKST Ha Survey of the Milky Way. Subsequent confirmatory spectroscopy of the nebula and central star reveals the remarkable nature of this object. The nebular spectrum shows emission lines with large expansion velocities exceeding 150 km s - 1 , suggesting that perhaps the object is not a conventional planetary nebula. The central star itself is very red and is identified as being of the [WN] class, which makes it unique in the Galaxy. A large body of supplementary observational data supports the hypothesis that this object is indeed a planetary nebula and not a Population I Wolf-Rayet star with a ring nebula.

Dusty ring nebulae around new candidate Luminous Blue Variables

We report on the discovery of a further two ring nebulae in the Midcourse Space Experiment (MSX) Galactic Plane Survey; G24.73+0.69 and G26.47+0.02. Morphologically, both appear similar to the nebulae found around the Luminous Blue Variable (LBV) candidates G79.29+0.46 and Wra 17-96. A central, unresolved point source was identified in both cases - positional coincidence with the star StRS 237 was found for G26.47+0.02, while no optical counterpart could be identified for G24.73+0.69. However, subsequent near IR broadband imaging of the G24.73+0.69 field identified a very red - (J − K) ∼ 2 mag - stellar counterpart to the central object. Near-IR spectroscopy of both objects reveal rich emission line spectra dom- inated by H  ,H e and low excitation metals, suggesting classification as luminous B supergiants and revealing a striking superficial similarity to the other MSX ring sources and known LBVs. We utilised a NLTE model atomsphere code to model the K band spectra and near-IR spectral energy distributions of the central stars in order to determine their physical parameters. Adopting a distance, d = 5.2 kpc to G24.73+0.69 yields a temperature, T = 12 kK, luminosity, log (L/L� ) = 5.6 and mass loss rate, u M = 1 × 10 −5 Myr −1 . G26.47+0.02 appears to be a more extreme object; adopting d = 6.5 kpc results in T = 17 kK, log (L/L� ) = 6.0 and u M = 9 × 10 −5 Myr −1 , placing it at the Humphreys-Davidson limit for massive stellar objects. Analysis of the spatially resolved mid-IR fluxes of both objects reveal extended periods of enhanced mass loss, resulting in comparatively low mass nebulae, with chemistries dominated by O-rich dust (with a population of small Fe grains existing co-spatially with the silicate dust). Comparison to the other MSX ring nebulae sources reveals a homogeneous group of objects, with both stellar and nebular properties consistent with known LBVs. With both spectroscopic and/or photometric variability observed for those sources with multiepoch observations, we propose a close affinity between both classes of object and suggest that long term monitoring of the MSX sources will reveal them to be bona fide LBVs.

WR bubbles and He II emission

We present the very first high quality images of the Heiiλ4686 emission in three high excitation nebulae of the Magellanic Clouds. A fourth high excitation nebula, situated around the WR star BAT99-2, was analysed in a previous letter. Using VLT FORS data, we investigate the morphology of the ring nebulae around the early-type WN stars BAT99-49 & AB7. We derive the total Heii fluxes for each object and compare them with the most recent th eoretical WR models. Whilst the ionization of the nebula around BAT99-49 can be explained by a WN star of temperature 90-100 kK, we find that the Heii emission measure of the nebula associated with AB7 requires an He + ionizing flux larger than predicted for the hottest WN model a vailable. Using Hα, [Oiii] and Heiλ5876 images along with long-slit spectroscopy, we investigate the physical properties of these ring nebulae and find only moderate chemical enrichment. We also surveyed seven other LMC WR stars but we failed to detect any Heii emission. This holds also true for BAT99-9 which had been proposed to excite an Heii nebula. Four of these surveyed stars are surrounded by a ring nebula, and we use the FORS data to derive their chemical composition: the nebula around BAT99-11 shows a N/O ratio and an oxygen abundance slightly lower than the LMC values, while the nebula around BAT99-134 presents moderate chemical enrichment similar to the one seen near BAT99-2, 49 and AB7. Comparing the WR stars of the LMC, BAT99-2 and 49 appear rather unique since similar stars do not reveal high excitation features. The third high excitation nebula presented in this paper, N44C, does not harbor stars hotter than mid-O main sequence stars. It was suggested to be a fossil X-ray nebula ionized by the transient LMC X-5. Our observations of N44C reveal no substantial changes in the excitation compared to previous results reported in the literature. Therefore, we conclude that either t he recombination timescale of the X-ray nebula has been underestimated or that the excitation of the nebula is produced by another, yet unknown, mechanism.

The Mysterious Ring in the Open Cluster NGC 3572: Planetary Nebula or Photoevaporating Globule?

ABSTRACTWe discuss optical and infrared emission from the putative planetary nebula in the young open cluster NGC 3572. Velocity images of [N ii] λ6583 obtained with the Rutgers/CTIO Fabry‐Perot interferometer reveal that most gas in the nebula is expanding at velocities ≲5 km s -1, with marginal evidence for bipolar expansion. A few outer condensations are seen at faster redshifted velocities, but their origin is uncertain. Optical spectra reveal a spatial excitation gradient, with higher excitation in a diffuse outer halo and low excitation in the bright inner nebula, suggesting that the nebula is externally ionized by hot stars in the open cluster and that the nebula and cluster are therefore equidistant. The nebula coincides with an infrared source detected by the MSX and IRAS satellites and has a spectral energy distribution implying a total mass of 5–10 M⊙. MSX also reveals diffuse infrared emission associated with the cluster, and its morphology implies a connection with the ring nebula. We discuss two very different interpretations of this object—it is either a strange planetary nebula or (more probably) a young photoevaporating globule left over from the molecular cloud that formed the cluster.

Molecular Hydrogen in the Ring Nebula: Clumpy Photodissociation Regions

ABSTRACTWe present a 0.″65 resolution H2 1–0 S(1) 2.122 μm image of the Ring Nebula (NGC 6720), which was taken with the Near Infrared Imager at the WIYN 3.5 m telescope on Kitt Peak. The high resolution of the H2 observation is sufficient to reveal the finer structure of the molecular material in this nebula. The morphology of the molecular emission is compared to that of the ionized emission from the Ring Nebula as seen by the Hubble Space Telescope (HST; Heii, [Oiii], and [Nii]), and it is clear that the dark clumps seen by HST match the locations of clumpy H2 emission, suggesting that these clumps are similar to the cometary knots seen in the Helix Nebula. As with the Helix, the clumpy H2 emission from the main ring of the Ring Nebula is contained within the optically bright ionized nebula, implying that the molecular gas is shielded inside dense condensations. Comparison of the observed H2 average surface brightnesses for the Ring Nebula [(1.5 ± 0.5) × 10-4 ergs cm−2 s−1 sr−1] with time‐dependent models of the expected H2 emission from planetary nebulae (PNe) shows that it is consistent with H2 excitation in photodissociation regions (PDRs), confirming previous suggestions. Comparison of the Ring Nebula H2 emission with a younger PN, NGC 2346, and an older PN, the Helix Nebula, suggests an evolution in H2 surface brightness consistent with the time‐dependent PDR models. Moreover, the knots of molecular gas appear to become more isolated as the PN evolves, consistent with optical studies of knots in PNe.

High resolution molecular hydrogen imaging of the Ring Nebula

The relative morphologies and structures of molecular and ionized gas emission from planetary nebulae (PNe) allow a better understanding of the nature and evolution of these objects. The classical paradigm for the structure of PNe is that of an ionized gas bubble bounded by neutral gas and molecules. However, it has been shown that molecular gas exists within ionized regions, leading to a re-evaluation of the classic structure. In the Helix Nebula (NGC 7293) dense condensations known as cometary knots are known to exist in the main ionized nebula. The molecules in these knots are shielded from the ionizing radiation and thus survive within the ionized zone. Another PN in which H2 emission is seen to originate from within the ionized nebula in NGC 6720 (the Ring Nebula).

New WR Central Stars of PNe identified on the AAO/UKST Hα Survey

Candidate central stars can be seen in some (≤10%) of the many (~1000) new planetary nebulae found during scans of the AAO/UKST Hα survey of the Milky Way (Parker et al. 2002). Spectroscopic observations have identified seven with Wolf-Rayet emission features, thereby adding significantly to the 56 [WR] central stars already known (Jeffery et al. 1996). Two of these are late-type stars – [WC8] and [WC9-10], and four are early-type around [WC4] or possibly [WO4]. Two of the early-type stars have unusual strong emission features near 4610Å which may be attributed to [N V] and could be indicative of enhanced nitrogen in their atmospheres and unusual stellar evolution. The seventh star is of the [WN] sequence and, if confirmed, would be the only known [WN] central star in the Galaxy. Its surrounding circular nebula has emission lines with expansion velocities exceeding 150 km/s suggesting that the object may not a conventional planetary nebula. The possibility that it is a Population I star with a ring nebula, although looking increasingly less likely, cannot be finally ruled out at this stage. Full details of the [WC] stars have been given by Morgan, Parker, & Russeil (2001) and Parker & Morgan (2002). The candidate [WN] star is described by Morgan & Parker (2002).

Ring nebulae around massive stars throughout the Hertzsprung-Russell diagram

Massive stars evolve across the H-R diagram, losing mass along the way and forming a variety of ring nebulae. During the main sequence stage, the fast stellar wind sweeps up the ambient interstellar medium to form an interstellar bubble. After a massive star evolves into a red giant or a luminous blue variable, it loses mass copiously to form a circumstellar nebula. As it evolves further into a WR star, the fast WR wind sweeps up the previous mass loss and forms a circumstellar bubble. Observations of ring nebulae around massive stars not only are fascinating, but also are useful in providing templates to diagnose the progenitors of supernovae from their circumstellar nebulae. In this review, I will summarize the characteristics of ring nebulae around massive stars throughout the H-R diagram, show recent advances in X-ray observations of bubble interiors, and compare supernovae's circumstellar nebulae with known types of ring nebulae around massive stars.

Molecular gas around the Wolf-Rayet star WR 18 (WN4)

Optically observed ring nebulae and H i cavities around Wolf-Rayet stars have enabled us to obtain information on the history of mass-loss associated with these massive evolved stars. However, such studies have left a number of unanswered questions regarding the amount of mass-loss and the conditions of the stars during a sequence of mass-loss phases. Here we discuss the molecular gas environments of the WR star WR 18, which has an associated optical ring nebula NGC 3199. Our observations show that significant amounts of molecular gas appear close to and associated with the star. Mapping of molecular CO near the star shows that molecular materials appear to substantially avoid areas of optical emission and, instead, form a distorted clumpy shell interior to NGC 3199. Molecular emission lines are broader than lines seen in the interstellar medium and suggest the shell is composed of ejecta. This is further corroborated by the enhanced abundances of molecules containing C, N and O. Implications of the observations for the evolution of WR 18 are discussed.

Radio observations of interstellar bubbles surrounding massive stars

We show radio continuum observations of the WR ring nebulae around WR 101 and WR 113 obtained using the VLA and H i 21 cm line data of the interstellar bubble around the O type stars BD +24° 3866 and BD+25° 3952 obtained with the DRAO Synthesis Telescope. We review previous radio continuum and H i line results toward WR and O-type stars.

CFHT Fabry-Perot 2D spectroscopy in Hα of the ejected Wolf-Rayet ring nebula M1-67: universal multifractal analysis and turbulent status

Using CFHT-sis Fabry-Perot interferograms of the Wolf-Rayet ring nebula M1-67, we present an investigation of the statistical properties of fluctuating gas motions using structure functions (SFs) traced by Hα emission-line centroid velocities. We consider the SFs 〈|Δv(r)|p〉 of order p, i.e., the spatially averaged moments of order p of the spatial velocity increments at projected spatial scale r of M1-67's velocity field: we test for (i) SF scaling, 〈|Δv(r)|p〉 ∝ rζ(p), and (ii) nonlinearity of the observed scaling exponents ζ(p)s, as expected for intermittent flows. We find that there is a clear correlation at scales 0.02-0.22 pc between the mean quadratic differences of radial velocities and distance over the surface of M1-67. The first and second order SFs are found to scale as 〈|Δv(r)|〉 ∝ r0.5 and 〈|Δv(r)|2〉 ∝ r0.9 (Grosdidier et al. 2001). The former scaling law strongly suggests that supersonic turbulence is at play in M1-67, on the other hand, the latter scaling law agrees very well with Larson-type laws for velocity turbulence. Additionally, we can discuss the nature of the turbulence in terms of Universal Multifractals (UM), a continuous-scale limit of multiplicative cascades (Schertzer & Lovejoy 1987) and derive the level of intermittency in the nebula.

CFHT Fabry-Perot 2D Spectroscopy in Hα of the Planetary Nebula NGC 40: Universal Multifractal Analysis and Turbulent Status

Using CFHT/SIS Fabry-Perot interferograms of the planetary nebula NGC 40, we present an investigation of the statistical properties of fluctuating gas motions using structure functions traced by Hα emission-line centroid velocities (Grosdidier et al. 2002). We consider the structure functions 〈|Δv(r)|p〉 of order p, i.e. the spatially averaged moments of order p of the spatial velocity increments at projected spatial scale r of NGC 40's velocity field. In this poster, we test for i) Structure-function scaling related to turbulence in the nebula, 〈|Δv(r) |p〉 ~ rχ(p),ϛ and ii) Nonlinearity of the observed scaling exponents χ(p)s, as expected for intermittent turbulent flows. The first order structure function is indeed found to scale at the smallest scales with χ(1) = H ≈0.68. This value is larger than the value expected in the case of incompressible or compressible turbulence, for which one would obtain H = 1/3 or 1/2, respectively. This result suggests that the Hα layer is thick compared to the projected spatial separations (O'Dell & Castañeda 1987, and references therein). We can give a more quantitative description of the turbulent status of the PN NGC 40 through the examination of the structure function for different orders. Additionally we can discuss the nature of the turbulence in terms of Universal Multifractals, a continuous-scale limit of multiplicative cascades (Schertzer & Lovejoy 1987) and derive the level of intermittency in the nebula. The function χ(p) is indeed found to be nonlinear and well reproduced with the following Universal Multifractal parameters: α ≈ 1.90–2 (the field is highly multifractal with a gaussian generator) and C1 ≈ 0.08 (the field suffers significant intermittency, which is incompatible with monofractal additive stochastic models usually introduced in similar studies): ϛ(p) = p × H − C1(pα - p)/(α - 1). Fig. 1 shows the hierarchy of exponents χ(p) along with its best Universal Multifractal fit. Our results on the Wolf-Rayet ring nebula M 1–67 (Grosdidier et al. 2001) led to the same level of multifractality (α ≈ 1.9) but with smaller intermittency (C1 ≈ 0.04). Preliminary results on the Orion nebula (Grosdidier 2002) reveal a turbulent status relatively similar to that of the PN NGC 40. On the whole, such a study provides a way to quantify turbulence in PN, and more generally in other HII regions (with potential implications for the estimation of temperature fluctuations) and give insight on astrophysical turbulence for its own sake.

Structure and Dynamics of Candidate O Star Bubbles in N44

Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard, adiabatic model for wind-blown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars, to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44F, N44J, and N44M, in the outskirts of the HII complex N44 in the Large Magellanic Cloud. We have obtained ground-based and HST emission-line images and high dispersion echelle spectra for these objects. From the Halpha luminosities and the [OIII]/Halpha ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7V, O9.5V and O9.5V for N44F, N44J, and N44M, respectively. We find that the observed expansion velocity of 12 km/s for N44F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44J and N44M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44F of strongly-defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models.

Stellar wind bubbles around WR and [WR] stars

We study the dynamics of stellar wind bubbles around hydrogen-deficient stars using numerical simulations with time- and ion dependent cooling. We consider two types of hydrogen-deficient stars, massive WR stars, producing Ring Nebulae, and low mass (WR) stars, producing Planetary Nebulae. We show that for the Planetary Nebulae, the dierent cooling properties of the hydrogen-deficient wind lead to a later transition from momentum- to energy-driven flow, which could explain the observed turbulence of these nebulae. We find that Ring Nebulae should all be energy-driven, and show how comparing the bubble's momentum and kinetic energy to the input wind momentum and kinetic energy, can give misleading information about the dynamics of the bubble.

X-ray emission from wind blown interstellar bubbles

The interaction of fast stellar winds with the surrounding interstellar medium produces large amounts of hot X-ray emitting gas. Scarce previous observations indicated substantial differences between simple one-dimensional models and the data. We have obtained ROSAT images of several wind blown interstellar bubbles. In this first paper we present an X-ray map of the prototype ring nebula NGC 6888. Although we confirm previous total flux determinations and the possibility to reconcile these values with theory by introducing ionization non-equilibrium, our map demonstrates that the assumed geometry (a thick shell) is far from reality

An Infrared Ring Nebula around MSX5C G358.5391+00.1305: The True Nature of Suspected Planetary Nebula Wray 17‐96 Determined via Direct Imaging and Spectroscopy

The Midcourse Space Experiment (MSX) Galactic plane survey discovered a nearly perfectly circular ring nebula around the suspected planetary nebula Wray 17-96. Using near-IR spectral typing and modeling of the mid-IR nebula, we find that Wray 17-96 is more likely a candidate to be a luminous blue variable (LBV) surrounded by a large spherical ejecta shell. It is very similar to the G79.29+0.46 LBV candidate in Cygnus and the Pistol Star. The K-band spectrum and the mid-IR data indicate a stellar temperature of 13,000 K. The most likely distance to the source is 4.5 kpc, leading to a luminosity of 1.8 × 106 L☉. We suggest that the nebula consists of multiple shells and that an evolution from oxygen-rich to carbon-rich chemistry may be indicated.

First Detections of Molecular Gas Associated with the Wolf‐Rayet Ring Nebula NGC 3199

This paper presents the first observations of molecular gas associated with the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. This includes first observations of the molecules HCN, HCO+, CN, and HNC seen in any Wolf-Rayet ring nebula. Our observations immediately suggest the presence of high-density molecular gas (>104 cm-3) in the nebula with significant amounts of associated molecular gas, which is in the form of clumpy ejecta and/or interstellar material. Molecular CO gas was mapped across the optically bright portion of the nebula and out into the diffuse ionized component using the 12CO J = 1 → 0 line. CO gas is not seen within the optically bright rim of NGC 3199 but adjacent to it. The optical emission rim therefore appears to mark regions of photodissociation. Velocity components in the CO data are consistent with those seen in high-resolution optical spectra of the Hα line but extend beyond the visible emission. A prior suggestion of the formation of the nebula via a bow shock appears unlikely since Hipparcos measurements show the proper motion of WR 18 is almost at right angles to the direction required for the bow shock model. Instead, line splitting toward the north of the nebula suggests that a possible blowout of the Wolf-Rayet wind through surrounding ejecta may be responsible for some of the velocity features observed. Preliminary estimates of molecular abundances in the nebula seen toward the central star are significantly higher than for the interstellar medium and are similar to those in planetary nebulae, although CN is distinctly underabundant in comparison to the very high values found in many planetary nebulae. The abundances found are consistent with the idea that at least a portion of the molecular material is associated with ejecta from the central star.

Laboratory simulation of the collision of supernova 1987A with its circumstellar ring nebula

The collision of the remnant of supernova 1987A with its circumstellar ring is laboratory-simulated by using the Gekko XII laser fusion facility at the Institute of Laser Engineering, Osaka University. The pure hydrodynamic approximation was applied using the invariance of the Euler equations. The basic properties of the formation of a young supernova remnant and its collision with the ring are illustrated. Shock wave propagation and plasma flow with triple wavy, vortex ringlike structures were observed using gated X-ray shadowgraphy. The observed hydrodynamic behaviors compare favorably with one-and two-dimensional numerical simulations and further illustrate important qualitative trends. Our experiments suggest the shock reverberation between the ring and the contact discontinuity in the supernova remnant 1987A and resultant oscillation and enhancement of X-ray emission, as well as the emergence of downstream vortex structures.

Spatially Resolved OiiRecombination Line Observations of the Ring Nebula, NGC 6720

We present new long-slit CCD spectra of O II permitted lines and [O III] forbidden lines in the Ring Nebula NGC 6720. These observations provide spatially resolved information on both O II and [O III] over the 70'' diameter of the main shell. We find significant differences in the spatial distribution of the O II lines and [O III] λ4959. The [O III] emission follows the Hβ emission measure, peaking slightly radially inward from the Hβ peak. The O II emission peaks inside the [O III] emission. This suggests that radiative recombination may not be the primary mechanism for producing the O II lines. O+2 abundances derived from O II lines are 5-10 times larger than those derived from [O III] in the region within 20'' of the central star. Outside of this region, however, the O II-derived and [O III]-derived abundances agree to within 0.2-0.3 dex. The electron temperature derived from [O III] lines rises smoothly from about 10,000 K in the outer shell to about 12,000 K in the center; we see no evidence for a temperature jump that would be associated with a shock. If temperature fluctuations are responsible for the discrepancy in O+2 abundances, the average temperature would have to be approximately 6500 K in the He+2 zone and about 9000 K in the outer shell in order to force the [O III]-derived abundance to equal that derived from O II. This would conflict with ionization models for planetary nebulae, which predict that the temperature is higher in the He+2 region close to the ionizing star. We therefore argue that temperature fluctuations cannot explain the abundance discrepancy. A comparison of the spatial distribution of O II emission with the location of dusty knots shows that the O II recombination lines do not peak where the dense knots are located, creating difficulties for models that explain the recombination line/forbidden line discrepancy by density fluctuations. We examine the possibility that high-temperature dielectronic recombination in a central hot bubble enhances the recombination line strengths in the central part of the nebula. However, comparison of recombination rates with collisional excitation rates shows that the increase in recombination emission due to dielectronic recombination at T ≈ 105 K is not sufficient to overcome the increase in collisonally excited emission. We are unable to find a completely satisfactory model to explain the discrepancy between recombination line and forbidden line abundances.

Interstellar Bubbles in Two Young H [CSC]ii[/CSC] Regions

Massive stars are expected to produce wind-blown bubbles in the interstellar medium; however, ring nebulae, suggesting the existence of bubbles, are rarely seen around main-sequence O stars. To search for wind-blown bubbles around main-sequence O stars, we have obtained high-resolution Hubble Space Telescope WFPC2 images and high-dispersion echelle spectra of two pristine H II regions, N11B and N180B, in the Large Magellanic Cloud. These H II regions are ionized by OB associations that still contain O3 stars, suggesting that the H II regions are young and have not hosted any supernova explosions. Our observations show that wind-blown bubbles in these H II regions can be detected kinematically, but not morphologically, because their expansion velocities are comparable to or only slightly higher than the isothermal sound velocity in the H II regions. Bubbles are detected around concentrations of massive stars, individual O stars, and even an evolved red supergiant (a fossil bubble). Comparisons between the observed bubble dynamics and model predictions show a large discrepancy (1–2 orders of magnitude) between the stellar wind luminosity derived from bubble observations and models and that derived from observations of stellar winds. The number and distribution of bubbles in N11B differ from those in N180B, which can be explained by the difference in the richness of stellar content between these two H II regions. Most of the bubbles observed in N11B and N180B show a blister structure, indicating that the stars were formed on the surfaces of dense clouds. Numerous small dust clouds, similar to Bok globules or elephant trunks, are detected in these H II regions, and at least one of them hosts on-going star formation.