New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules out interpretation of the radio emission as beamed emission from a relativistic jet. In addition, the radio spectral index of the counterpart is well determined from three frequencies and found to be alpha=-0.8 pm 0.2. The radio emission is likely to be optically-thin synchrotron emission from a nebula surrounding the X-ray source. The radio luminosity of the counterpart is 3.8 x 10^34 erg/s and the minimum energy required to power the nebula is ~1 x 10^49 erg. These values are two orders of magnitude larger than in any Galactic nebula powered by an accreting compact object.
I propose that some of the most luminous planetary nebulae (PNs) are actually proto-PNs, where a companion white dwarf (WD) accretes mass at a relatively high rate from the post-asymptotic giant branch star that blew the nebula. The WD sustains a continuous nuclear burning and ionizes the nebula. The WD is luminous enough to make the dense nebula luminous in the [O III]5007 line, In young stellar populations these WD accreting systems account for a small fraction of [O III]-luminous PNs, but in old stellar populations these binaries might account for most, or even all, of the [OIII]-luminous P…
We report the first observation of Ly$α$ pumped molecular hydrogen emission lines in planetary nebulae. The H$_{2}$ emission observed in the ultraviolet spectra of NGC 6853 and NGC 3132 can be explained by excitation of vibrationally hot H$_{2}$ by Ly$α$ photons. Constraints are placed on the nebular Ly$α$ emission profile, as well as the molecular hydrogen temperature, column density and turbulent motion. These parameters are similar for the two nebulae, pointing to similar physical conditions in these objects. The ro-vibrational cascade following Ly$α$ pumping is predicted to have low surfac…
We report the detection of an Halpha emission-line structure in the upstream side of the planetary nebula NGC 40, which is predicted by numerical simulations, and which is attributed to Rayleigh-Taylor instability. Such a Rayleigh-Taylor instability is expected to occur at early stages of the interaction process between the interstellar medium (ISM) and a fast moving planetary nebula, as is the case for NGC 40. We resolved the Rayleigh-Taylor instability `tongues', as well as the flatness of the nebula around the `tongues', which results from the deceleration by the ISM.
We have obtained X-ray observations of the bipolar planetary nebulae (PNe) NGC 2346 and NGC 7026 with XMM-Newton. These observations detected diffuse X-ray emission from NGC 7026 but not from NGC 2346. The X-ray emission from NGC 7026 appears to be confined within the bipolar lobes of the PN and has spectral properties suggesting a thermal plasma emitting at a temperature of 1.1 +0.5/-0.2 times 10^6 K. The X-ray spectrum of NGC 7026 is modeled using nebular and stellar abundances to assess whether a significant amount of nebular material has been mixed into the shocked-wind, but the results of…
An early analysis of Chandra observations of the planetary nebula NGC 6543 suggested that the location and the derived temperature for the X-ray emitting region is inconsistent with the abundances measured for this object (Chu et al. 2001). We revisit observations of this source (following significant updates to both the reprocessing and extraction software) in order to propose a resolution to this apparent anomaly. Our re-analysis using abundances found from observations in the infrared, optical, and ultraviolet suggests that the location and temperature of the X-ray emission from NGC 6543 is…
NGC 1360 is a large planetary nebula (PN) without an obvious shell morphology. We have analyzed H-alpha images and high-dispersion echelle spectra of NGC 1360 in order to construct spatio-kinematic models and to determine its density distribution. The best-fit model indicates that NGC 1360 is a prolate ellipsoidal shell whose major axis is twice as long as its minor axis and is tilted by 60 arcdeg with respect to the line of sight. The large kinematic age of the shell, ~10,000 yr, and the low density of the nebula, <130 H-atom cm^-3, imply that NGC 1360 is an evolved PN and has begun to merge …
We have examined a stream-source model for the production of the cometary tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately supersonic stream of ionized gas overruns a source of ionized gas. Hydrodynamic calculations reveal velocity structures which are in good agreement with the observational data on tail velocities and are consistent with observations of the nebular structure. The results also are indicative of a stellar atmosphere origin for the cometary globules. Tail remnants persist for timescales long enough for their identification with faint striations vis…
We present the results of mapping the planetary nebula NGC6369 using multiple long slit spectra taken with the CTIO 1.5m telescope. We create two dimensional emission line images from our spectra, and use these to derive fluxes for 17 lines, the Halpha/Hbeta extinction map, the [SII] line ratio density map, and the [NII] temperature map of the nebula. We use our photoionization code constrained by these data to determine the distance, the ionizing star characteristics, and show that a clumpy hour-glass shape is the most likely three-dimensional structure for NGC6369. Note that our knowledge of…
XMM-Newton EPIC observations of the planetary nebula (PN) NGC 7009, the Saturn Nebula, have detected extended X-ray emission in its central cavity. The diffuse X-ray emission must originate in the shocked fast stellar wind. The spectra show that the temperature of the hot gas is 1.8*10^6 K. The rms density derived from the volume emission measure is a few tens H-atom/cm^3. The hot gas does not appear over-pressurized with respect to the nebular shell. NGC 7009 represents an evolutionary stage at which the influence of the hot gas in the PN interior starts to decline due to the diminishing stre…
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2) emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the physical properties of the molecular material in these reflection nebulae. Our observations of NGC 2023 and NGC 7023 and the physical parameters we derive for these nebulae are in good agreement with previous studies. Both NGC 1333 and NGC 2068 have no previously-published analysis of near-infrared spectra. Our study reveals that the rotational-vibrational states of molecular hydrogen in NGC 1333 are populated quite differently from NGC 2023 and …
We present a study of the structure and kinematics of the triple-shell planetary nebula NGC 7662 based on long-slit echelle spectroscopic observations and HST archival narrow-band images. The structure of NGC 7662 main nebula consists of a central cavity surrounded by two concentric shells. The bright inner shell has a non-negligible thickness and the highest density in the nebula. The outer shell is filled with nebular material whose density profile shows a propto 1/r drop-off in the inner regions of the shell and flattens in the outermost regions. Both the inner and the outer shells can be d…
For the planetary nebula NGC 7009, we present a comparison of two unpublished Very Large Array archive data sets taken with a time separation of 8.09 years to confirm the proper motions of its ansae observed in the optical. We determine values of 23$\pm$6 and 34$\pm$10 mas yr$^{-1}$ for the eastern and western ansae, respectively. There is marginal evidence suggesting that the flux densities of the jets that connect the ansae with the main body of the nebula diminished in about 30% over the period between the two observations. We also set an upper limit to the expansion of the main body of the…
We use a 3D photoionization modeling tool to study the morpho-kinematic properties of the Planetary Nebula (PN) NGC 3132. We show that it is possible to reproduce the low resolution observations (spectra and images) with an ellipsoidal shell. However, high resolution observations, as those showing a density variation along the nebula and the [O III]5007 velocity profiles, definitively rule out this description. We show that a bipolar Diabolo shape with a 40 degres rotation of the symmetry axis relative to the line of sight successfully reproduces the observed images, as well as the high resolu…
We present here the 7.0-8.7 micron spectrum of the bright reflection nebula NGC 7023. Our observations are made with the Short Wavelength Spectrometer (SWS) on the European satellite Infrared Space Observatory (ISO). The vibrational bands of the ionized fullerene C60+ are expected at 7.11 and 7.51 microns, while those of the neutral fullerene C60 are expected at 7.0 and 8.45 microns. We estimate an upper limit in NGC 7023 for the C60+ abundance of <0.26% of the interstellar abundance of carbon, while C60 contains <0.27% of interstellar carbon.
We report the detection of 21-cm line emission from H I in the planetary nebula NGC 7293 (the Helix). The observations, made with the Very Large Array, show the presence of a ring of atomic hydrogen that is associated with the outer portion of the ionized nebula. This ring is most probably gas ejected in the AGB phase that has been subsequently photodissociated by radiation from the central star. The H I emission spreads over about 50 km/s in radial velocity. The mass in H I is approximately 0.07 solar masses, about three times larger than the mass in molecular hydrogen and comparable with the…
We present Hubble Space Telescope WFPC2 images of a portion of the bright northeast rim of NGC 6888, the nebular shell physically associated with the Wolf-Rayet star HD 192163. The exposures are taken in the light of Halpha 6563, [O III] 5007, and [S II] 6717,6731. The images are used to constrain models of the ionization structure of nebular features. From these models we infer physical conditions within features, and estimate elemental abundances within the nebula. The results of our analysis, together with the degree of small scale inhomogeneity apparent in the images, call into question th…
We report the discovery of X-ray emission from NGC 7027, a prototypical object for the study of the formation and evolution of planetary nebulae (PNs). Observations with the Advanced CCD Imaging Spectrometer (ACIS) aboard the Chandra X-ray Observatory show that the X-ray emission from NGC 7027 is extended and is bipolar in morphology. The ACIS spectrum displays strong emission from highly ionized Ne and weaker emission features which we attribute to O, Mg, and Si. Model fits to this spectrum suggest a characteristic temperature T_x ~ 3x10^6 K and an intrinsic (unabsorbed) X-ray luminosity of L…
We present new radial velocity measurements of 586 stars in a one-degree field centered on the open cluster M46, and the planetary nebula NGC 2438 located within a nuclear radius of the cluster. The data are based on medium-resolution optical and near-infrared spectra taken with the AAOmega spectrograph on the Anglo-Australian Telescope. We find a velocity difference of about 30 km/s between the cluster and the nebula, thus removing all ambiguities about the cluster membership of the planetary nebula caused by contradicting results in the literature. The line-of-sight velocity dispersion of th…
NGC 6543 is one of the few planetary nebulae (PNe) whose X-ray emission has been shown to be extended and originate from hot interior gas. Using FUSE observations we have now detected nebular O VI emission from NGC 6543. Its central star, with an effective temperature of ~50,000 K, is too cool to photoionize O V, so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell. We modeled the O VI emission incorporating thermal conduction, but find that simplistic assumptions for the AGB and fast wind mass loss rates overpr…
We find that a non-axisymmetric bow shock model, with an appropriate choice of parameters, could fit the line splitting velocity field of the cone-like H$α$ nebula in NGC 4945 better than a canonical cone model. Meanwhile, the bow shock model could also reproduce the morphology of the H$α$ nebula. The bow shock results from the interaction of the galactic superwinds with a giant HII region. It is implied that the starburst ring or disk around the galactic nucleus should be generating strong winds, and the bright H$α$ knot northwest of the nucleus be suffering an anisotropic mass loss process.
We use line-of-sight velocity information on the filamentary emission-line nebula of NGC 1275 to infer a dynamical model of the nebula's flow through the surrounding intracluster gas. We detect outflowing gas and flow patterns that match simulations of buoyantly rising bubbles from which we deduce that some of the nebula filaments have been drawn out of NGC 1275. We find a radial gradient of the ratio [NII]6584/H-alpha which may be due to a variation in metallicity, interactions with the surrounding intracluster medium or a hardening of the excitation mechanism. We find no preferred spatial co…
Direct imagery and long-slit, spatially resolved echellograms of the high excitation planetary nebula NGC 1501 allowed us to study in detail the expansion velocity field, the physical conditions (electron temperature, electron density, ionization) and the spatial distribution of the nebular gas. An electron temperature of 11500 K and a turbulence of 18 km/s are derived by comparing the Halpha and [OIII] emission line profiles, but large, small scale fluctuations of both these quantities are present in the ionized gas. The radial density distribution shows external peaks up to 1400 cm-3; they h…
Spitzer images of the luminous blue variable (LBV) candidate HD168625 reveal the existence of a bipolar nebula several times larger than its previously-known equatorial dust torus. The outer nebula of HD168625 has a full extent of about 80 arcsec or 0.85 pc, and one of the lobes has a well-defined polar ring. The nebula is a near twin of the triple-ring system around SN1987A. Because of these polar rings, and accounting for stellar/progenitor luminosity, HD168625 is an even closer twin of SN1987A than the B supergiant Sher 25 in NGC3603. HD168625's nebula was probably ejected during a giant LB…
We present high spatial resolution (0$\farcs$3) polarimetric images in the $H$ and $K$ bands and direct images in the $L'$ and $M'$ bands of the NGC 6334 V infrared nebulae. The images show complex structures including the multi-shells and various knots in the nebulae. The appearances and colors of the eastern and western nebulae differ considerably. Our polarization images also show differences between the illuminating sources of the nebulae: the eastern nebula is illuminated by a deeply embedded mid-infrared source, KDJ 4, and the western nebula by our newly detected near-infrared source, WN…
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…
Continuing our series of papers on the three-dimensional (3-D) structures of and accurate distances to Planetary Nebulae (PNe), we present our study of the planetary nebula NGC6781. For this object we construct a 3-D photoionization model and, using the constraints provided by observational data from the literature we determine the detailed 3-D structure of the nebula, the physical parameters of the ionizing source and the first precise distance. The procedure consists in simultaneously fitting all the observed emission line morphologies, integrated intensities and the 2-D density map from the…
We revisit Chandra observations of the planetary nebulae NGC 7027 and BD +30 3639 in order to address the question of abundance anomalies in the X-ray emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30 3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30 3639, we speculate that the high…
As the first step to determine disk mass-to-light ratios for normal spiral galaxies, we present the results of an imaging survey for planetary nebulae (PNe) in six nearby, face-on systems: IC 342, M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), NGC 5068, and NGC 6946. Using Blanco/Mosaic II and WIYN/OPTIC, we identify 165, 153, 241, 150, 19, and 71 PN candidates, respectively, and use the Planetary Nebula Luminosity Function (PNLF) to obtain distances. For M74 and NGC 5068, our distances of 8.6 +/- 0.3 Mpc and 5.4 +0.2/-0.4 Mpc are the first reliable estimates to these objects; for IC 342 (3.5 …
New, very long (25'), cuts of spatially resolved profiles of the Halpha and [N II] optical emission lines have been obtained over the face of the Helix planetary nebula, NGC 7293. These directions were chosen to supplement previous similar, though shorter, cuts as well as crossing interesting phenomena in this nebular envelope. In particular one new cut crosses the extremes of the proposed CO J=2-1 emitting outer "torus" shown by Huggins and his co-workers to be nearly orthogonal to its inner counterpart. The second new cut crosses the extensive outer filamentary arcs on either side of the bri…
The ouflowing proper motions of fifteen 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 unambiguous distance to NGC 6302 of 1.17 +/- 0.14 kpc is now determined. Also all the v…
Long-slit echellograms of the high excitation planetary nebula NGC1501, reduced according to the methodology developed by Sabbadin et al. (2000a, b), allowed us to obtain the ``true'' distribution of the ionized gas in the eight nebular slices covered by the spectroscopic slit. A 3-D rendering procedure is described and applied, which assembles the tomographic maps and rebuilds the spatial structure. The images of NGC 1501, as seen in 12 directions separated by 15 deg, form a series of stereoscopic pairs giving surprising 3-D views in as many directions. The main nebula consists of an almost o…
We present narrow-band images and long-slit echelle spectra of the planetary nebula (PN) NGC 6778. The data show this PN as bipolar, with a very prominent low-excitation equatorial toroid, high-excitation lobes and two pairs of collimated outflows. Morphologically, the pairs of outflows are different from each other; one is linear and oriented along the bipolar axis, the other presents an S-shape with changing orientations. Besides the different morphology, both pairs of collimated outflows present radial velocities increasing with distance from the central star and share a common origin in br…
We briefly review and extend our discussion of the ROSAT detection of the extraordinarily luminous (> 10^{42} erg/s) partly extended (> 30 kpc diameter) X-ray emission from the ultraluminous infrared galaxy NGC 6240. The `standard'-model of starburst outflow is contrasted with alternatives and a comparison with the X-ray properties of ellipticals is performed.
We present interferometric observations of $^{12}$CO and $^{13}$CO $J$=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. The torus exhibits very complex and fragmentated structure; (2) high velocity molecular knots moving at high velocity, higher than 20 \kms, and located in the opt…
V651 Mon is the binary central star of the bipolar planetary nebula NGC 2346. The star showed the second-ever deep fading in 1996-1997, which was presumably caused by obscuration by a dust cloud in the planetary nebula, as was proposed to explain the 1981-1985 event. The entire duration of the 1996-1997 event was \~400 d, remarkably shorter than the 1981-1985 event, suggesting that the obscuring body was smaller or had a larger tangential velocity. The most remarkable feature in this event was the presence of a sharply defined transient clearing (brightening). From the time-scale of the variat…
Echelle spectrophotometry of the planetary nebula NGC 5307 is presented. The data consists of VLT UVES observations in the 3100 to 10360 A range. Electron temperatures and densities have been determined using different line intensity ratios. We determine the H, He, C, and O abundances based on recombination lines, these abundances are almost independent of the temperature structure of the nebula. We also determine the N, O, Ne, S, Cl, and Ar abundances based on collisionally excited lines, the ratios of these abundances relative to that of H depend strongly on the temperature structure of the …
We investigate the electron temperature of the inner halo and nebular core regions of NGC 6543, using archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) images taken through narrow band [O III] filters. Balick et al. (2001) showed that the inner halo consists of a number of spherical shells. We find the temperature of this inner halo to be much higher (~15000 K) than that of the bright core nebula (~8500 K). Photo-ionization models indicate that hardening of the UV radiation from the central star cannot be the main source of the higher temperature in the halo region. U…
Spatially resolved, long-slit echellograms at different position angles of the bright, low excitation planetary nebula NGC 40 indicate that, the higher is the gas excitation, the faster is the radial motion, thus confirming the overturn of the Wilson law already suggested by Sabbadin and Hamzaoglu (1982). New reduction procedures, giving the radial trends of the electron density and of the ionic and chemical abundances, were applied to NGC 40; they show that: - the radial matter distribution has a sharp ``bell'' profile with peaks up to 4000 cm-3; - the ionization structure is peculiar, indica…
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] im…
High resolution, 1.1, 1.6, and 2.2 micron imaging of the luminous infrared galaxy NGC 6090 obtained with NICMOS of the Hubble Space Telescope are presented. These new observations are centered on the two nuclei of the merger, and reveal the spiral structure of the eastern galaxy and the amorphous nature of the western galaxy. The nuclear separation of 3.2 kpc (H_0 = 75 km/s/Mpc) indicates that NGC 6090 is at an intermediate stage of merging. Bright knots/clusters are also visible in the region overlapping the merging galaxies; four of these knots appear bluer than the underlying galaxies and h…
We report the results of an [O III] lambda 5007 survey for planetary nebulae (PN) in five galaxies that were hosts of well-observed Type Ia supernovae: NGC 524, NGC 1316, NGC 1380, NGC 1448 and NGC 4526. The goals of this survey are to better quantify the zero-point of the maximum magnitude versus decline rate relation for supernovae Type Ia and to validate the insensitivity of Type Ia luminosity to parent stellar population using the host galaxy Hubble type as a surrogate. We detected a total of 45 planetary nebulae candidates in NGC 1316, 44 candidates in NGC 1380, and 94 candidates in NGC 4…
We report the discovery of a dozen Herbig-Haro jets illuminated by the Lyman continuum and/or softer far-ultraviolet radiation fields of nearby high mass stars. Five irradiated outflows lie in the outer parts of the Orion Nebula and seven lie near the reflection nebula NGC 1333 in the Perseus molecular cloud. We propose that the UV radiation field has eroded residual material left over from the formation of these young source stars. Many of the irradiated jets exhibit unusual C-shaped bending. In the outskirts of the Orion Nebula, most irradiated jets bend away from the core of the nebula. On …
We report the results of XMM-Newton observations for an especially luminous supersoft X-ray source (SSS) with bolometric luminosity of 10^39 erg/s in the spiral galaxy NGC 300. The source was detected as a SSS in 1992 and disappeared in subsequent X-ray observations. The source was active again during recent XMM-Newton observations. It appeared to be very soft (kT~60 eV) and very luminous (~10^38 - 10^39 erg/s). The two XMM-Newton observations also reveal that the source went from a ``high'' state to a ``low'' state in 6 days. We also found a 5.4-hr periodicity during the ``low'' state. We con…
We have observed the S(0), S(1), S(2), S(3), S(4) and S(5) rotational lines of molecular hydrogen (H2) towards the peak of the photodissociation region (PDR) associated with the reflection nebula NGC 7023. The observed H2 line ratios show that they arise in warm gas with kinetic temperatures ~300 - 700 K. However, the data cannot be fitted by an ortho- to para- (OTP) ratio of 3. An OTP ratio in the range ~1.5 - 2 is necessary to explain our observations. This is the first detection of a non-equilibrium OTP ratio measured from the H2 pure-rotational lines in a PDR. The existence of a dynamical …
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 dete…
Narrow-band H$α$ and [N {\sc ii}] images and high-dispersion spatially-resolved echelle spectroscopy of the planetary nebula NGC 6891 are presented. These observations show a great wealth of structures. The bright central nebula is surrounded by an attached shell and a detached outer halo. Both the inner and intermediate shells can be described as ellipsoids with similar major to minor axial ratios, but different spatial orientations. The kinematical ages of the intermediate shell and halo are 4,800 and 28,000 years, respectively. The inter-shell time lapse is in good agreement with the evolut…
We present an X-ray spectral study of the prototype far-infrared galaxy NGC6240 from ASCA. The soft X-ray spectrum (below 2 keV) shows clear signatures of thermal emission well described with a multi-temperature optically-thin plasma, which probably originates in a powerful starburst. Strong hard X-ray emission is also detected with ASCA and its spectrum above 3 keV is extremely flat with a prominent iron K line complex, very similar to that seen in the Seyfert 2 galaxy NGC1068 but about an order of magnitude more luminous [L(3-10keV)=1.4E42 erg/s]. The hard X-ray spectrum indicates that only …
The H2 and optical (Ha, [NII]) morphology of NGC 6881 are very different. Here we present a preliminary report of the analysis of new optical (Ha and [NII]) and near-IR (Brg and H2) images and intermediate resolution JHK spectra of this nebula. Our observations confirm the association of the H2 bipolar lobes to NGC 6881 and reveal that H2 is predominantly shock excited in this nebula. We conclude that NGC 6881 has multiple bipolar lobes that formed at different phases of the nebular evolution and that the collimation conditions or even the collimating agent changed from one ejection to the oth…
Images obtained with the ESO VLT and FORS1 in [O III] 5007 on- and off-band as well as r_Gunn filters are analyzed to search for Planetary Nebula (PN) candidates in the dwarf irregular NGC 3109. In the continuum-subtracted [O III] 5007 on-band images, a large number of emission-line regions were detected. We describe the criteria employed in order to distinguish PN candidates from compact HII regions, finding that the sizes and the properties of the ionizing stars are the most unambiguous discriminators for separating the two classes. Based upon these criteria, we have found 20 PN candidates f…
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2) emission from molecular filaments in the reflection nebula NGC 7023. We derive the relative column densities of H_2 rotational-vibrational states from the measured line emission and compare these results with several model photodissociation regions covering a range of densities, incident UV-fields, and excitation mechanisms. Our best-fit models for one filament suggest, but do not require, either a combination of different densities, suggesting clumps of 10^6 cm^{-3} in a 10^4 - 10^5 cm^{-3} filament, or a combinatio…
We infer the gas kinematics, diagnostics and ionic radial profiles, distance and central star parameters, nebular photo- ionization model, spatial structure and evolutionary phase of the PN NGC 6741 by means of long-slit high-resolution spectra at nine position angles. NGC 6741 (distance ~2.0 kpc, age ~1400 yr, ionized mass Mion ~0.06 Mo) is a dense (electron density up to 12,000 cm^(-3)), high-excitation, almost- prolate ellipsoid, surrounded by a sharp low-excitation skin (the ionization front), and embedded into a spherical (radius ~ 0.080 pc), almost-neutral, high-density (n(HI) ~7 x 10^3 …
We report the optical identifications of two X-ray luminous point sources in the spiral galaxy NGC4565 based on archive data of Chandra and the Hubble Space Telescope. The central X-ray point source, RXJ1236.3+2559, is found to be the nucleus of NGC4565 with an X-ray luminosity of Lx= 4.3 10^{39} erg. We show that its multi-waveband properties are consistent with it being a low-luminosity active galactic nucleus. A faint optical counterpart with B=25.1 and I=24.0 was identified for the off-nucleus X-ray point source, RXJ1236.2+2558. Its extinction corrected B magnitude is estimated to be 24.5.…
NGC 3576 (G291.28-0.71; l=291.3o, b=-0.7o) and NGC 3603 (G291.58-0.43; l=291.6o, b=-0.5o) are optically visible, luminous HII regions located at distances of 3.0 kpc and 6.1 kpc, respectively. We present 3.4 cm Australian Telescope Compact Array (ATCA) observations of these two sources in the continuum and the H90a, He90a, C90a and H113b recombination lines with an angular resolution of 7" and a velocity resolution of 2.6 km/s. All four recombination lines are detected in the integrated profiles of the two sources. Broad radio recombination lines are detected in both NGC 3576 (DV_{FWHM}>= 50 k…
We report the discovery of a new open cluster (OC) in the Galaxy at $\ell=167.0^\circ$ and $b=-1.0^\circ$. Its field includes the planetary nebula (PN) PK 167-0.1. We study the possible associations of the PN/OC pairs NGC 2818/NGC 2818A, NGC 2438/M 46 (NGC 2437), PK 6+2.5/NGC 6469, as well as of the PN PK 167-0.1 with New Cluster 1. The analyses are based on near-infrared colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs). NGC 6469 is located in a heavily contaminated bulge field. The CMD morphology, especially for the latter two cases, is defined with a field star dec…
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 spe…
We have detected CO+ toward the photon-dominated region (PDR) associated with the reflection nebula NGC 7023. This is the first detection of CO+ in the vicinity of a Be star. A CO+ column density of ~ 3E11 cm-2 has been derived toward the PDR peak. We have, however, not detected CO+ in a well shielded clump of the adjacent molecular cloud, where the CO+/HCO+ abundance ratio is at least 100 times lower than in the PDR. Our results show, for the first time, that CO^+ column densities as large as ~ 3E11 cm-2 can be produced in regions with incident UV fields of just a few 1E3 (in units of Habing …
We present ROSAT (HRI and PSPC) and ASCA observations of the two luminous (L_x ~10^{41-42} erg s^{-1}) star-forming galaxies NGC3310 and NGC3690. The HRI shows clearly that the sources are extended with the X-ray emission in NGC3690 coming from at least three regions. The combined 0.1-10 keV spectrum of NGC3310 can be described by two components, a Raymond-Smith plasma with temperature kT=0.81^{+0.09}_{-0.12} keV and a hard power-law, Gamma=1.44^{+0.20}_{-0.11}, (or alternatively a harder Raymond-Smith plasma with kT ~15 keV), while there is no substantial excess absorption above the Galactic.…
We explain the Ha emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star.We compare these models with the Ha emission obtained from the HST archival images of the Helix Nebula. From a comparison of the Ha emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5e45 s^-1, which is consistent with estimates based on the total Hb flux of the nebula. We also…
Chandra X-ray Observatory (CXO) images have revealed that the X-ray emitting regions of the molecule-rich young planetary nebulae (PNs) BD+30 3639 (BD+30) and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD+30, NGC 7027, and another planetary nebula resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by sub-pixel repositioning of individual X-ray events. The resulting subarcsecond-resolution images reveal that the soft (E < 0.7 keV) X-ray emiss…
We present results from Planetary Nebula Spectrograph (PN.S) observations of the elliptical galaxy NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae of which 191 are ascribed to NGC 3379, and 23 to the companion galaxy NGC 3384. Comparison with data from the literature show that the PN.S velocities have an internal error of <20km/s and a possible offset of similar magnitude. We present the results of kinematic modelling and show that the PN kinematics are consistent with absorption-line data in the region where they overlap. The resulting combined kin…
