Nebula NGC Research Articles

Non–Local Thermodynamic Equilibrium Model of NGC 6543’s Central Star and Its Relation to the Surrounding Planetary Nebula

We analyze the chemical composition of the central star of the planetary nebula NGC 6543 based on a detailed NLTE model of its stellar wind. The logarithmic abundances by number are H = 12.00, He = 11.00, C = 9.03, N = 8.36, O = 9.02, Si = 8.19, P = 5.53, S = 7.57, and Fe = 7.24. Compared with the solar abundances, most of the elements have solar composition with respect to hydrogen, except C, which is overabundant by 0.28 dex, and Fe, which is depleted by ~0.2 dex. Contrary to most previous work, we find that the star is not H-poor and has a normal He composition. These abundances are compared with those found in the diffuse X-ray plasma and the nebular gas. Compared to the plasma emitting in diffuse X-rays, the stellar wind is much less depleted in iron. Since the iron depletions in the nebular gas and X-ray plasma are similar, we conclude that the plasma emitting diffuse X-rays is derived from the nebular gas rather than the stellar wind. Excellent agreement is obtained between the abundances in the stellar wind and the nebular recombination line abundances for He, C, and O relative to H. On the other hand, the derived stellar N abundance is smaller than the nebular N abundance derived from recombination lines and agrees with the abundance found from collisionally excited lines. The mean temperature variation determined by five different methods indicates that the difference in the nebular abundances between the recombination lines and collisionally excited lines can be explained as due to the temperature variations in a chemically homogeneous medium.

A Spectral Line Survey of NGC 7027 at Millimeter Wavelengths

We report on a recent spectral line survey of the planetary nebula (PN) NGC 7027 using the Arizona Radio Observatory (ARO) 12 m telescope and the Heinrich Hertz Submillimeter Telescope (SMT) at millimeter wavelengths. The spectra covering the frequency ranges 71-111, 157-161, and 218-267 GHz were obtained with a typical sensitivity of -->rms < 8 mK. A total of 67 spectral lines are detected, 21 of which are identified with 8 molecular species, 32 with recombination lines from hydrogen and helium, and 14 remain unidentified. As the widths of emission lines from CO, other neutral molecules, molecular ions, and recombination of H+ and He+ are found to be different from each other, the line strengths and profiles are used to investigate the physical conditions and chemical processes of the neutral envelope of NGC 7027. The column densities and fractional abundances relative to the H2 of the observed molecular species are calculated and compared with predictions from chemical models. We found evidence for overabundance of N2H+ and underabundance of CS and HNC in NGC 7027, suggesting that X-ray emission and shock waves may play an important role in the chemistry of the hot molecular envelope of the young PN.

Abundances in planetary nebulae: NGC 6826

Aims. We determine the chemical abundances and other parameters of the nebula NGC 6826 and its central star.

A Search for Temperature and Density Variations in NGC 7027 with theHubble Space Telescope

We observed the young planetary nebula NGC 7027 with the HST Imaging Spectrograph (STIS) in long-slit mode at five slit positions along the major axis of the nebula, over the wavelength range from 3000 to ~10,000 A. We used dereddened line ratios to determine line-of-sight average values for (1) He^(++) and He^+ ionization fractions from He II λ4686 and He I λ5876; (2) electron density from both [S II] (λ6716/λ6731) and [Ar IV] (λ4711/λ4740); and (3) temperature from [O III] (λ4959 + λ5007)/λ4363. The He^(++)/H^+ ratio varies across the nebula, with values up to 0.06 over the central parts, falling off to 0.03 at offsets of 5-6 from the central star. The line-of-sight mean densities vary by factors of ~5 on angular scales as small as ~1. The average electron temperatures are generally consistent with a constant = 13,800 ± 2000 K within the uncertainties, except for ~2% of measured pixels where T_e > 18,000 K. The variations of density along the slits are not obviously correlated with variations in extinction. The ionization structure of the nebula varies on both large and small scales. [O I] λ6300, Hα, and He II λ4686 intensity profiles along each slit appear to trace ionization gradients at Hα peaks, with the highest ionization on the side toward the central star and lowest ionization on the far side. These structures may result from effects of stellar radiation, shocks from a fast stellar wind, or some combination of the two processes.

THE UNUSUAL DISTRIBUTIONS OF IONIZED MATERIAL AND MOLECULAR HYDROGEN IN NGC 6881: SIGNPOSTS OF MULTIPLE EVENTS OF BIPOLAR EJECTION IN A PLANETARY NEBULA

The planetary nebula NGC 6881 displays in the optical a quadrupolar morphology consisting of two pairs of highly collimated bipolar lobes aligned along different directions. An additional bipolar ejection is revealed by the hydrogen molecular emission, but its wide hourglass morphology is very different from that of the ionized material. To investigate in detail the spatial distribution of molecular hydrogen and ionized material within NGC 6881, and to determine the prevalent excitation mechanism of the H2 emission, we have obtained new near-IR Br-gamma and H2 and optical H-alpha and [N II] images, as well as intermediate resolution JHK spectra. These observations confirm the association of the H2 bipolar lobes to NGC 6881 and find that the prevalent excitation mechanism is collisional. The detailed morphology and very different collimation degree of the H2 and ionized bipolar lobes of NGC 6881 not only imply that multiple bipolar ejections have occurred in this nebula, but also that the dominant shaping agent is different for each bipolar ejection: a bipolar stellar wind most likely produced the H2 lobes, while highly collimated outflows are carving out the ionized lobes into the thick circumstellar envelope. The asymmetry between the southeast and northwest H2 bipolar lobes suggests the interaction of the nebula with an inhomogeneous interstellar medium. We find evidence that places NGC 6881 in the H II region Sh 2-109 along the Orion local spiral arm.

Abundances of planetary nebula NGC 2392

The spectra of the planetary nebula NGC 2392 is reanalysed using spectral measurements made in the mid-infrared with the Spitzer Space Telescope. The aim is to determine the chemical composition of this object. We also make use of IUE and ground based spectra. Abundances determined from the mid-infrared lines, which are insensitive to electron temperature, are used as the basis for the determination of the composition, which are found to differ somewhat from earlier results. The abundances found, especially the low value of helium and oxygen, indicate that the central star was originally of rather low mass. Abundances of phosphorus, iron, silicon and chlorine have been determined for the first time in this nebula. The variation of electron temperature in this nebula is very clear, reaching quite high values close to the center. The temperature of the central star is discussed in the light of the observed high stages of ionization. The nebular information indicates that the spectrum of the star deviates considerably from a blackbody.

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.

Hubble-type outflows of the high-excitation poly-polar planetary nebula NGC 6302 - from expansion proper motions

The outflowing proper motions of 15 knots in the dominant northwestern lobe of the high-excitation poly-polar planetary nebula NGC 6302 have been determined by comparing their positions relative to those of faint stars in an image taken at the San Pedro Martir Observatory in 2007 to those in a South African Astronomical Observatory archival plate obtained by Evans in 1956. The Hubble-type expansion of this lobe is now directly confirmed in a model-independent way from these measurements. Furthermore, an unambigous distance to NGC 6302 of 1.17 ± 0.14 kpc is now determined. Also, all the velocity vectors of the 15 knots (and two others) point back to the central source. An eruptive event from within the central torus ≈2200 yr previously must have created the high-speed lobes of NGC 6302.

The Detection of a Molecular Bipolar Flow in the Multipolar Planetary Nebula NGC 2440

We report the detection of a molecular bipolar outflow in the CO J = 3–2 line in the optical multipolar planetary nebula NGC 2440. The observed CO J = 3–2 emission shows two major velocity components which are blue- and redshifted by 18 km s−1 relative to the systemic velocity of VLSR = 43 km s−1. Both components are detected only near the central region and have a size of about 24''. In addition to this bulk motion, weak CO emissions in the central region are detected at higher velocities of ±20 to ±40 km s−1. The presence of the fast CO components are in line with the picture that the neutral dense torus is being torn apart by very fast outflowing winds. The spatial distribution of CO emission shows two emission peaks at the systemic velocity that are located 30'' to the northeast and 30'' to the southwest from the center in point symmetry. The positions of the two CO emission peaks coincide well with the emission knots in the optical, suggesting that the CO outflow corresponds to one pair of the multiple optical bipolar lobes of the nebula. This particular pair of bipolar lobes seem distinctly rich in heavy molecules compared with the other optical bipolar lobes. The extent of CO emission in NGC 2440 is well constrained. The total amount of CO molecules in NGC 2440 is 3.5 × 1051 if the distance to NGC 2440 is 2.19 kpc and the excitation temperature is 25 K.

SerendipitousChandraX-Ray Detection of a Hot Bubble within the Planetary Nebula NGC 5315

We report the serendipitous detection of the planetary nebula NGC 5315 by the Chandra X-ray Observatory. The Chandra imaging spectroscopy results indicate that the X-rays from this PN, which harbors a Wolf-Rayet (WR) central star, emanate from a T_X ~ 2.5x10^6 K plasma generated via the same wind-wind collisions that have cleared a compact (~8000 AU radius) central cavity within the nebula. The inferred X-ray luminosity of NGC 5315 is ~2.5x10^{32} erg s^{-1} (0.3-2.0 keV), placing this object among the most luminous such ``hot bubble'' X-ray sources yet detected within PNe. With the X-ray detection of NGC 5315, objects with WR-type central stars now constitute a clear majority of known examples of diffuse X-ray sources among PNe; all such ``hot bubble'' PN X-ray sources display well-defined, quasi-continuous optical rims. We therefore assert that X-ray-luminous hot bubbles are characteristic of young PNe with large central star wind kinetic energies and closed bubble morphologies. However, the evidence at hand also suggests that processes such as wind and bubble temporal evolution, as well as heat conduction and/or mixing of hot bubble and nebular gas, ultimately govern the luminosity and temperature of superheated plasma within PNe.

The Planetary Nebula NGC 1360: A Test Case of Magnetic Collimation and Evolution after the Fast Wind

The central star of this nebula has an observed intense magnetic field and the fast wind is no longer present, indicating that a back flow process has probably developed. Long-slit, spatially resolved echelle spectra have been obtained across the main body of NGC 1360 and over its system of bipolar jets. Deep images of the knotty structures of the jets have also been obtained. The data allow a detailed study of the structure and kinematics of this object and the results are modeled considering the effects of a magnetic collimation process in the development of the nebula and then switching off the fast stellar wind to follow its evolution to its current state. The model is able to successfully reproduce many key features of NGC 1360 under these premises.

행성상 성운 NGC 6881의 운동학적 특성과 기하학적 구조

행성상성운 NGC 6881은 4극자형태를 이루고 있고 JET이 영상에 나타난다. Lick 천문대에 있는 Hamilton Echelle Spectrograph(HES)을 사용하여 관측한 가시광 영역의 방출선 스펙트럼의 선윤곽을 연구하였다. 우리가 연구하는 HES 분광 자료는 직경이 4초각인 영역의 안쪽에서 관측된 것이다. IRAF와 StarLink/Dipso분석프로그램을 사용하여 HI, HeI, HeII, [OIII], [NII], [ArIII], [SII], [SIII] 등의 강한 선들의 선윤곽을 통해 팽창속도를 얻었다. HI선의 경우 단일 정상을 보이는 반면, He과 다른 금지선들은 두 개의 정상 분포가 겹치는 특성을 보였다. 가스 유출 속도는 중심별의 복사압에 의해 바깥쪽으로 갈수록 가속되는 것으로 분석되었다. 우리는 팽창속도자료를 통해, HST 영상에서 보여진 중심부분의 세 개의 고리는 양극콘(HI 선등에 나타남)과 고리(He, [SIII]선에 특성이 보임)의 복합 구조가 투영된 것으로 결론지었다. The Planetary nebula NGC 6881 displays quadrupole morphology and it also has a jet feature in its image. We investigated the line profiles of the optical region spectral emission lines, using the Hamilton Echelle Spectrograph (HES) at the Lick observatory. The HES data obtained in this study was the radiation coming from the inner region within the diameter of 4 second of arc. Expansion velocity was obtained, based on the strong emission line profiles of e.g. H, Hel, Hell, [OIII], [NII], [ArIII], [SII], and [SIII}, using the IRAF and StarLink/Dipso reduction packages. The HI recombination lines showed one single peak profile, while the He and forbidden strong lines displayed double peaks. The results of this study show that the outflow velocity of gas increases radially outwards due to the central stellar radiation pressure. It was concluded that three central rings appeared in the HST image are the result of a combined structure of bipolar cones (seen in e.g. HI lines) and a ring (seen in He, [SIII] lines) in projection.

Structure and clumping in the fast wind of NGC 6543

Far-ultraviolet spectroscopy from the FUSE satellite is analysed to uniquely probe spatial structure and clumping in the fast wind of the central star of the H- rich planetary nebula NGC 6543 (HD 164963). Time-series data of the unsaturated Pv lambda lambda 1118, 1128 resonance line P Cygni profiles provide a very sensitive diagnostic of variable wind conditions in the outflow. We report on the discovery of episodic and recurrent optical depth enhancements in the Pv absorption troughs, with some evidence for a similar to 0.17-d modulation time-scale. Empirical line-synthesis modelling is used to derive physical properties, including the optical depth evolution of individual 'events'. The characteristics of these features are essentially identical to the ` discrete absorption components' (DACs) commonly seen in the ultraviolet lines of massive OB stars. We have also employed the unified model atmosphere code CMFGEN to explore spectroscopic signatures of clumping, and report, in particular, on the clear sensitivity of the Pv lines to the clump volume filling factor. The results presented here have implications for the downward revision of mass-loss rates in planetary nebula central stars. We conclude that the temporal structures seen in the Pv lines of NGC 6543 likely have a physical origin that is similar to that operating in massive, luminous stars, and may be related to near-surface perturbations caused by stellar pulsation and/or magnetic fields.

The High‐Excitation Planetary Nebula NGC 246. II.FUSEandChandraObservations

This paper presents Far-Ultraviolet Spectroscopic Explorer (FUSE) medium-resolution slit and Chandra X-Ray Observatory ACIS-S observations of the high-excitation planetary nebula NGC 246. The Ne V λ1146 and [Ne V] λ3426 emission lines, the latter obtained in a previous investigation, indicate that the nebula is photoionized by the intense UV radiation of the central object and has a temperature of ~20,000 K. The O VI λ1032 and λ1037 emission lines are moderately optically thick and probably contain contributions from both collisional excitation and radiative scattering of the UV continuum of the central star. The Chandra observation did not detect any X-ray emission from the nebula, but did detect the central star. The absence of nebular X-ray emission and the photoionized nature of the nebula support the hypothesis that the nebula contains a biconical cavity, which extends to the edge of the nebula. In this geometry the stellar wind does not shock the nebular material, resulting only in a cool photoionized plasma. The X-ray spectrum of the central object is consistent with NLTE models for PG 1159-type stars.

Massive Expanding Torus and Fast Outflow in Planetary Nebula NGC 6302

We present interferometric observations of 12CO and 13CO J = 2–1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula into two distinct kinematic components: (1) a massive expanding torus seen almost edge-on and oriented in the north-south direction (roughly perpendicular to the optical nebula axis), which exhibits very complex and fragmented structure; and (2) high-velocity molecular knots moving at more than 20 km s−1 and located in the optical bipolar lobes. These knots show a linear position-velocity gradient (Hubble-like flow), which is characteristic of fast molecular outflow in young planetary nebulae. From the low but variable 12CO/13CO J = 2–1 line intensity ratio, we conclude that the 12CO J = 2–1 emission is optically thick over much of the nebula. Using the optically thinner line 13CO J = 2–1, we estimate a total molecular gas mass of ~0.1 M☉, comparable to the ionized gas mass; the total gas mass of the NGC 6302 nebula, including the massive ionized gas from the photon dominated region, is found to be ~0.5 M☉. From radiative transfer modeling, we infer that the torus is seen at an inclination angle of 75° with respect to the plane of the sky, and is expanding at a velocity of 15 km s−1. Comparison with recent observations of molecular gas in NGC 6302 is also discussed.

VLT/near-infrared integral field spectrometer observations of molecular hydrogen lines in the knots of the planetary nebula NGC 7293 (the Helix Nebula)

Knots are commonly found in nearby planetary nebulae (PNe) and star forming regions. Within PNe, knots are often found to be associated with the brightest parts of the nebulae and understanding the physics involved in knots may reveal the processes dominating in PNe. As one of the closest PNe, the Helix Nebula (NGC 7293) is an ideal target to study such small-scale (~300 AU) structures. We have obtained infrared integral spectroscopy of a comet-shaped knot in the Helix Nebula using SINFONI on the Very Large Telescope at high spatial resolution (50-125 mas). With spatially resolved 2 micron spectra, we find that the H2 rotational temperature within the cometary knots is uniform. The rotational-vibrational temperature of the cometary knot (situated in the innermost region of the nebula, 2.5 arcmin away from the central star), is 1800 K, higher than the temperature seen in the outer regions (5-6 arcmin from the central star) of the nebula (900 K), showing that the excitation temperature varies across the nebula. The obtained intensities are reasonably well fitted with 27 km s-1 C-type shock model. This ambient gas velocity is slightly higher than the observed [HeII] wind velocity of 13 km s-1. The gas excitation can also be reproduced with a PDR (photo dominant region) model, but this requires an order of magnitude higher UV radiation. Both models have limitations, highlighting the need for models that treats both hydrodynamical physics and the PDR.

Infrared photometry of eight planetary nebulae

We discuss the infrared (IR) (1.25–5 µm) photometry of eight planetary nebulae performed in 1999–2006. For all of the nebulae under study, we have firmly established IR brightness and color variations on time scales shorter than one year and up to 6–8 years. The greatest IR brightness variations were observed in IC 2149, IC 4997, and NGC 7662. Their J magnitudes varied within 0 . 2–0 . 25. In the remaining objects, the J magnitude variations did not exceed 0 . 15. All of the planetary nebulae under study exhibited IR color variations. Based on the IR photometry, we have classified the central regions of the planetary nebula NGC 1514 and of the northern part of NGC 7635 seen through a 12″ aperture as a B(3–7) main-sequence star (NGC 1514) and a ∼O9.5 upper-main-sequence star (NGC 7635). The nebulae IC 4997 and NGC 7027 exhibited an excess emission (with respect to the emission from a hot source) at λ > 2.5 µm.

The evolution of BD+60°2522: with mass loss and overshooting

The ionizing star BD+60°2522 is known as the central star of Bubble Nebulae NGC 7635—wind-blown bubble created by the interaction of the stellar wind of BD+60°2522 (O6.5 IIIef, V=8.7 mag, mass loss rate 10−5.76 M ⊙/year) with the ambient interstellar medium. From the evolutionary calculations for the star with mass loss and overshooting, we find that the initial mass of the star is 60M ⊙, its present age is 2.5×106 years, and the present mass is 45M ⊙.

The massive expanding molecular torus in the planetary nebula NGC 6302

Aims.We measure the mass and kinematics of the massive molecular torus in the the proto-typical butterfly planetary nebula NGC 6302. Determining the mass-loss history of the source is an important step in understanding the origin and formation of the wing-like morphology. Methods. Using the SMA interferometer we have imaged both the continuum emission and the J=2-1 transitions of 12CO and 13CO at arcsecond resolution. These data are analysed in combination with observations of both the J=2-1 and J=3-2 transitions of 12CO and 13CO made with JCMT. Results. The 12CO and 13CO emission match the dark lane seen in absorption in the H${\alpha}$ image of the object and trace an expanding torus of material. The CO indicates a mass of the torus of ~2 $M_\odot \pm 1$ $M_\odot$. The torus is expanding with a velocity of ~8 km s-1, centred at $V_{\rm lsr}=-31.5\rm\,km\,s^{-1}$. The size and expansion velocity of the torus indicates that it was ejected from ~7500 yr to 2900 yr ago, with a mass-loss rate of $5\times10^{-4}\,M_\odot\,\rm yr^{-1}$. In addition we detect a ballistic component in the CO images which has a velocity gradient of 140 km s-1 pc-1. Conclusions.The derived mass-loss history of the torus favours binary interaction as the cause of the ejection of the torus and we predict the existence of a companion with an orbital period $P\lesssim 1\,$month.

Integrated spectrum of the planetary nebula NGC 7027

Integrated spectrum of the planetary nebula NGC 7027