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Central Stars of Planetary Nebulae in the Magellanic Clouds: A Detailed Spectroscopic Analysis
We observed five central stars of planetary nebulae (CSPN) in the LMCand three in the SMC with FUSE, in the range 905-1187 Å. Weperformed a model-based analysis of these spectra in conjunction withHST spectra in the UV and optical range to determine stellar and nebularparameters. The signature of hot (T>~2000 K) circumstellar molecularhydrogen is found in the FUSE spectra of most objects. We also findevidence of X-rays in the wind of SMP LMC 76.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer and archival data. FUSE is operated for NASA bythe Johns Hopkins University under NASA contract NAS5-32985.
| Envelope density pattern around wide binary AGB stars: a dynamical model
Context: Various morphologies such as multi-concentric shells andspiral-like patterns have been observed around Proto-Planetary Nebulaeand AGB stars. It is widely argued that the regular density patterns areproduced by binary systems. Aims: The goal is to build up a simpledynamical model for the out-flowing circumstellar envelope around AGBstars in a wide binary system to explore the parameter dependence of thegeometrical characteristics of column density patterns. Methods: For anAGB star in a wide binary system, the orbital motion of the star can beapproximated as a series of pistons that simultaneously push out dustand gas along radial directions, but work in different oscillationphases. The piston model can fast produce column density patterns withhigh enough spatial resolutions for parameter dependence exploration. Results: The formation of 3D quasi-spherical density structure isinduced by orbital motion of the AGB star. The column density patternonly depends on two parameters: eccentricity of the orbit e and theterminal outflow velocity to mean orbital velocity ratio γ. Whenviewed perpendicular to the orbital plane, spiral, broken spiral, andincomplete concentric shell patterns can be seen, while when viewedalong the orbital plane, alternative concentric half-shell, egg-shell,and half-shell half-gap patterns will develop. Non-zero eccentricitycauses asymmetry, while larger γ makes a weaker pattern and helpsbring out asymmetry. A spiral pattern may becomes broken when e >0.4. The spiral center is always less than 12% of spiral pitch away fromthe orbit center. One should have more chances (~80%) seeing spiral-likepatterns than seeing concentric shells (~20%) in the circumstellarenvelope of wide binary AGB stars.Figure 3 and Appendix are only available in electronic form athttp://www.aanda.org
| Multiple Coaxial Rings in the Bipolar Nebula Hubble 12
A series of two-dimensional rings along a common axis aligned with thebipolar lobes is found in the planetary nebula Hb 12. The rings haveseparations of about 0.3", which can be translated into physicalseparations of ~1000 AU, or time separations of the order of 50 yr. Wesuggest that the existence of the rings is a manifestation of theinteraction between a time-variable, collimated fast outflow with thecircumstellar envelope created by the stellar wind of an asymptoticgiant branch star. Comparison of Hb 12 with other bipolar nebulae withhourglass structures such as Hen 2-104, MyCn 18, and SN 1987A suggeststhat the nebula of Hb 12 may represent the inner nebula of a much largerbipolar structures, and we propose to search for such outer structureswith a wide-field infrared camera.
| White dwarf masses derived from planetary nebula modelling
Aims.We compare the mass distribution of central stars of planetarynebulae (CSPNe) with those of their progeny, white dwarfs (WD). Methods: We use a dynamical method to measure masses with an uncertaintyof 0.02 M_ȯ. Results: The CSPN mass distribution is sharplypeaked at 0.61~M_ȯ. The WD distribution peaks at lower masses(0.58~M_ȯ) and shows a much broader range of masses. Some of thedifference can be explained if the early post-AGB evolution is fasterthan predicted by the Blöcker tracks. Between 30 and 50 per cent ofWD may avoid the PN phase because they have too low a mass. However, thediscrepancy cannot be fully resolved and WD mass distributions may havebeen broadened by observational or model uncertainties.Data is only available in electronic form at http://www.aanda.org
| HST and VLT observations of the symbiotic star Hen 2-147. Its nebular dynamics, its Mira variable and its distance
Aims.We investigate the dynamics of the nebula around the symbiotic starHen 2-147, determine its expansion parallax, and compare it with thedistance obtained via the period-luminosity relation for its Miravariable. Methods: A combination of multi-epoch HST images and VLTintegral field high-resolution spectroscopy is used to study the nebulardynamics both along the line of sight and in the plane of the sky. Theseobservations allow us to build a 3D spatio-kinematical model of thenebula, which, together with the measurement of its apparent expansionin the plane of the sky over a period of 3 years, provides the expansionparallax for the nebula. Additionally, SAAO near-infrared photometryobtained over 25 years is used to determine the Mira pulsation periodand derive an independent distance estimation via the period-luminosityrelationship for Mira variables. Results: The geometry of the nebula isfound to be that of a knotty annulus of ionized gas inclined to theplane of sky and expanding with a velocity of ~90 km s-1. Astraightforward application of the expansion parallax method provides adistance of 1.5 ± 0.4 kpc, which is a factor of two lower thanthe distance of 3.0 ± 0.4 kpc obtained from the period-luminosityrelationship for the Mira (which has a pulsation period of 373 days).The discrepancy is removed if, instead of expanding matter, we areobserving the expansion of a shock front in the plane of the sky. Thisshock interpretation is further supported by the broadening of thenebular emission lines.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc. under NASAcontract No. NAS5-26555; on observations obtained at the 8 m VLTtelescope of the European Southern Observatory in Chile; and onobservations made at the South African Astronomical Observatory. Themovie (Fig. 3) is only available in electronic form athttp://www.aanda.org Table 2 is only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/481
| X-ray emission from planetary nebulae calculated by 1D spherical numerical simulations
We calculate the X-ray emission from both constant and time-evolvingshocked fast winds blown by the central stars of planetary nebulae (PNe)and compare our calculations with observations. Using sphericallysymmetric numerical simulations with radiative cooling, we calculate theflow structure and the X-ray temperature and luminosity of the hotbubble formed by the shocked fast wind. We find that a constant fastwind gives results that are very close to those obtained from theself-similar solution. We show that in order for a fast shocked wind toexplain the observed X-ray properties of PNe, rapid evolution of thewind is essential. More specifically, the mass-loss rate of the fastwind should be high early on when the speed is ~300-700 kms-1, and then it needs to drop drastically by the time the PNage reaches ~1000 yr. This implies that the central star has a veryshort pre-PN (post-asymptotic giant branch) phase.
| Chemical abundances in planetary nebulae and stellar evolution.
I will review basic aspects of galactic Planetary Nebulae focussing onstatus of art on their chemical abundances and relationship with thestellar evolution theory.
| XMM-Newton Observations of the Bipolar Planetary Nebulae NGC 2346 and NGC 7026
We have obtained X-ray observations of the bipolar planetary nebulae(PNe) NGC 2346 and NGC 7026 with XMM-Newton. These observations detecteddiffuse X-ray emission from NGC 7026 but not from NGC 2346. The X-rayemission from NGC 7026 appears to be confined within the bipolar lobesof the PN and has spectral properties suggesting a thermal plasmaemitting at a temperature of1.1+0.5-0.2×106 K. The X-rayspectrum of NGC 7026 is modeled using nebular and stellar abundances toassess whether a significant amount of nebular material has been mixedinto the shocked wind, but the results of this comparison are notconclusive owing to the small number of counts detected. Observations ofbipolar PNe indicate that diffuse X-ray emission is much less likelydetected in open-lobed nebulae than closed-lobed nebulae, possiblybecause open-lobed nebulae do not have strong fast winds or are unableto retain hot gas.
| Infrared Observations of the Helix Planetary Nebula
We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRACinstrument on the Spitzer Space Telescope. The Helix is one of theclosest bright PNs and therefore provides an opportunity to resolve thesmall-scale structure in the nebula. The emission from this PN in the5.8 and 8 μm IRAC bands is dominated by the pure rotational lines ofmolecular hydrogen, with a smaller contribution from forbidden lineemission such as [Ar III] in the ionized region. The IRAC images resolvethe ``cometary knots,'' which have been previously studied in this PN.The ``tails'' of the knots and the radial rays extending into the outerregions of the PN are seen in emission in the IRAC bands. IRS spectra onthe main ring and the emission in the IRAC bands are consistent withshock-excited H2 models, with a small (~10%) component fromphotodissociation regions. In the northeast arc, the H2emission is located in a shell outside the Hα emission.
| Planetary Nebula Abundances and Morphology: Probing the Chemical Evolution of the Milky Way
This paper presents a homogeneous study of abundances in a sample of 79northern Galactic planetary nebulae (PNe) whose morphological classeshave been uniformly determined. Ionic abundances and plasma diagnosticswere derived from selected optical line strengths in the literature, andelemental abundances were estimated with the ionization correctionfactor developed by Kingsbourgh & Barlow in 1994. We compare theelemental abundances to the final yields obtained from stellar evolutionmodels of low- and intermediate-mass stars, and we confirm that mostbipolar PNe have high nitrogen and helium abundance and are the likelyprogeny of stars with main-sequence mass greater than 3Msolar. We derive =0.27 and discuss the implication of such ahigh ratio in connection with the solar neon abundance. We determine theGalactic gradients of oxygen and neon and foundΔlog(O/H)/ΔR=-0.01 dex kpc-1 andΔlog(Ne/H)/ΔR=-0.01 dex kpc-1. These flat PNgradients are irreconcilable with Galactic metallicity gradientsflattening with time.
| Planetary nebulae abundances and stellar evolution
A summary is given of planetary nebulae abundances from ISOmeasurements. It is shown that these nebulae show abundance gradients(with galactocentric distance), which in the case of neon, argon, sulfurand oxygen (with four exceptions) are the same as HII regions and earlytype star abundance gradients. The abundance of these elements predictedfrom these gradients at the distance of the Sun from the center areexactly the solar abundance. Sulfur is the exception to this; the reasonfor this is discussed. The higher solar neon abundance is confirmed;this is discussed in terms of the results of helioseismology. Evidenceis presented for oxygen destruction via ON cycling having occurred inthe progenitors of four planetary nebulae with bilobal structure. Theseprogenitor stars had a high mass, probably greater than 5 Mȯ. Thisis deduced from the high values of He/H and N/H found in these nebulae.Formation of nitrogen, helium and carbon are discussed. The high massprogenitors which showed oxygen destruction are shown to have probablydestroyed carbon as well. This is probably the result of hot bottomburning.
| Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. I. Theoretical model - Mass-loss history unravelled in VY CMa
Context: .Mass loss plays a dominant role in the evolution of low massstars while they are on the Asymptotic Giant Branch (AGB). The gas anddust ejected during this phase are a major source in the mass budget ofthe interstellar medium. Recent studies have pointed towards theimportance of variations in the mass-loss history of such objects.Aims: .By modelling the full line profile of low excitation CO linesemitted in the circumstellar envelope, we can study the mass-losshistory of AGB stars. Methods: .We have developed a non-LTEradiative transfer code, which calculates the velocity structure and gaskinetic temperature of the envelope in a self-consistent way. Theresulting structure of the envelope provides the input for the molecularline radiative calculations which are evaluated in the comoving frame.The code allows for the implementation of modulations in the mass-lossrate. This code has been benchmarked against other radiative transfercodes and is shown to perform well and efficiently. Results: .Weillustrate the effects of varying mass-loss rates in case of a superwindphase. The model is applied to the well-studied case of VYCMa. We show that both the observed integrated line strengthsas the spectral structure present in the observed line profiles,unambiguously demonstrate that this source underwent a phase of highmass loss (~3.2 × 10-4 Mȯyr-1) some 1000 yr ago. This phase took place for some 100yr, and was preceded by a low mass-loss phase (~1 ×10-6 Mȯ yr-1) taking some 800 yr.The current mass-loss rate is estimated to be in the order of 8 ×10-5 Mȯ yr-1. Conclusions:.In this paper, we demonstrate that both the relative strength of the COrotational line profiles and the (non)-occurrence of spectral structurein the profile offer strong diagnostics to pinpoint the mass-losshistory.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy
We present the catalog of 477 spectra from the SerendipitousExtragalactic X-ray Source Identification (SEXSI) program, a surveydesigned to probe the dominant contributors to the 2-10 keV cosmic X-raybackground. Our survey covers 1 deg2 of sky to 2-10 keVfluxes of 1×10-14 ergs cm-2 s-1,and 2 deg2 for fluxes of 3×10-14 ergscm-2 s-1. Our spectra reach to R-band magnitudesof <~24 and have produced identifications and redshifts for 438 hardX-ray sources. Typical completeness levels in the 27 Chandra fieldsstudied are 40%-70%. The vast majority of the 2-10 keV selected sampleare active galactic nuclei (AGNs) with redshifts between 0.1 and 3; ourhighest redshift source lies at z=4.33. We find that few sources atz<1 have high X-ray luminosities, reflecting a dearth of high-mass,high-accretion-rate sources at low redshift, a result consistent withother recent wide-area surveys. We find that half of our sources showsignificant obscuration, with NH>1022cm-2, independent of unobscured luminosity. We classify 168sources as emission-line galaxies; all are X-ray-luminous(LX>1041 ergs s-1) objects withoptical spectra lacking both high-ionization lines and evidence of anonstellar continuum. The redshift distribution of these emission-linegalaxies peaks at a significantly lower redshift than does that of thesources we spectroscopically identify as AGNs. We conclude that few ofthese sources, even at the low-luminosity end, can be powered bystarburst activity. Stacking spectra for a subset of these sources in asimilar redshift range, we detect [Ne V] λ3426 emission, a clearsignature of AGN activity, confirming that the majority of these objectsare Seyfert 2 galaxies in which the high-ionization lines are diluted bystellar emission. We find a total of 33 objects lacking broad lines intheir optical spectra that have quasar X-ray luminosities(LX>1044 ergs s-1), the largestsample of such objects identified to date. In addition, we explore 17AGNs associated with galaxy clusters and find that the cluster-memberAGN sample has a lower fraction of broad-line AGNs than does thebackground sample.The majority of data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA. The Observatory was made possible by the generous financialsupport of the W. M. Keck Foundation.
| Observed Planetary Nebulae as Descendants of Interacting Binary Systems
We examine recent studies on the formation rate of planetary nebulae andfind this rate to be about one-third of the formation rate of whitedwarfs. This implies that only about one-third of all planetary nebulaethat evolve to form white dwarfs are actually bright enough to beobserved. This finding corresponds with the claim that it is necessaryfor a binary companion to interact with the asymptotic giant branchstellar progenitor for the descendant planetary nebulae to be brightenough to be detected. The finding about the formation rate alsostrengthens O. De Marco's conjecture that the majority of observedplanetary nebulae harbor binary systems. In other words, single starsalmost never form observed planetary nebulae.
| X-ray emission by a shocked fast wind from the central stars of planetary nebulae
We calculate the X-ray emission from the shocked fast wind blown by thecentral stars of planetary nebulae (PNe) and compare with observations.Using spherically symmetric self-similar solutions, we calculate theflow structure and X-ray temperature for a fast wind slamming into apreviously ejected slow wind. We find that the observed X-ray emissionof six PNe can be accounted for by shocked wind segments that wereexpelled during the early-PN phase, if the fast wind speed is moderate,v2~ 400-600kms-1, and the mass-loss rate is a fewtimes 10-7Msolaryr-1. We find, asproposed previously, that the morphology of the X-ray emission is in theform of a narrow ring inner to the optical bright part of the nebula.The bipolar X-ray morphology of several observed PNe, which indicates animportant role of jets, rather than a spherical fast wind, cannot beexplained by the flow studied here.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| Galactic Planetary Nebulae with Wolf-Rayet Nuclei III. Kinematical Analysis of a Large Sample of Nebulae
Expansion velocities (V_{exp}) of different ions and line widths at thebase of the lines are measured and analyzed for 24 PNe with [WC]-typenuclei (WRPNe), 9 PNe ionized by WELS (WLPNe) and 14 ordinary PNe. Acomparative study of the kinematical behavior of the sample clearlydemonstrates that WRPNe have on average 40-45% larger V_{exp}, andpossibly more turbulence than WLPNe and ordinary PNe. WLPNe havevelocity fields very much like the ones of ordinary PNe, rather than theones of WRPNe. All the samples (WRPNe, WLPNe and ordinary PNe) showexpansion velocities increasing with age indicators, for example is larger for low-density nebulae and also it is largerfor nebulae around high-temperature stars. This age effect is muchstronger for evolved WRPNe, suggesting that the [WC] winds have beenaccelerating the nebulae for a long time, while for non-WRPNe theacceleration seems to stop at some point when the star reaches atemperature of about 90,000 - 100,000. Non-WR nebulae reach a maximumV_{exp} ≤ 30 km s(-1) evolved WRPNe reach maximum V_{exp} about 40km s(-1) . For all kinds of objects (WRPNe and non-WRPNe) it is foundthat on average V_{exp}(N(+) ) is slightly larger than V_{exp}(O(++) ),indicating that the nebulae present acceleration of the external shells.
| Tentative Discovery of a New Supernova Remnant in Cepheus: Unveiling an Elusive Shell in the Spitzer Galactic First Look Survey
We have discovered an axially symmetric, well-defined shell of materialin the constellation of Cepheus, based on imaging acquired as part ofthe Galactic First Look Survey with the Spitzer Space Telescope. The86''×75'' object exhibits brightened limbson the minor axis and is clearly visible at 24 μm, but it is notdetected in the 3.6, 4.5, 5.8, 8.0, 70, or 160 μm images. Follow-up7.5-40 μm spectroscopy reveals that the shell is composed entirely ofionized gas and that the 24 μm imaging traces [O IV] 25.89 μmemission solely. The spectrum also exhibits weaker [Ne III] and [S III]emission, and very weak [Ne V] emission. No emission from warm dust isdetected. Spectral cuts through the center of the shell and at thenorthern limb are highly consistent with each other. The progenitor isnot readily identified, but with scaling arguments and comparison towell-known examples of evolved stellar objects, we find the observationsto be most straightforward to interpret in terms of a young supernovaremnant located at a distance of at least 10 kpc, some 400 pc above theGalactic disk. If confirmed, this would be the first supernova remnantdiscovered initially at infrared wavelengths.
| Photospheric opacity and over-expanded envelopes of asymptotic giant branch stars
I suggest that the behavior of the photospheric opacity in oxygen-rich(similar to solar abundance) upper asymptotic giant branch stars maycause these stars to substantially expand for a few thousand years. Iterm this process over-expansion. This may occur when the photospheric(effective) temperature drops to Tp 3000 K, and becausethe opacity sharply increases as temperature further decreases down toTp 2000 K. The much higher opacity implies a much lowerphotospheric density, which stabilizes the envelope structure. As massloss proceeds, the star eventually contracts to become a post-asymptoticgiant branch star. Some possible outcomes of the over-expanded phase arediscussed: (1) The over-expanded phase may be connected to the formationof semi-periodic concentric arcs (rings; shells); (2) the over-expandedphase may be related to the positive correlation between the mass lossrate and the transition to axisymmetric mass loss geometry; and (3) anover-expanded asymptotic giant branch star, which doubles its radius, issomewhat more likely to swallow a low mass companion.
| Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet
We present the discovery of strong mid-infrared emission lines ofmolecular hydrogen of apparently high-velocity dispersion (~870 kms-1) originating from a group-wide shock wave in Stephan'sQuintet. These Spitzer Space Telescope observations reveal emissionlines of molecular hydrogen and little else. This is the first time analmost pure H2 line spectrum has been seen in anextragalactic object. Along with the absence of PAH-dust features andvery low excitation ionized gas tracers, the spectra resemble shockedgas seen in Galactic supernova remnants, but on a vast scale. Themolecular emission extends over 24 kpc along the X-ray-emitting shockfront, but it has 10 times the surface luminosity as the soft X-rays andabout one-third the surface luminosity of the IR continuum. We suggestthat the powerful H2 emission is generated by the shock wavecaused when a high-velocity intruder galaxy collides with filaments ofgas in the galaxy group. Our observations suggest a close connectionbetween galaxy-scale shock waves and strong broad H2 emissionlines, like those seen in the spectra of ultraluminous infrared galaxieswhere high-speed collisions between galaxy disks are common.
| Iron Depletion in the Hot Bubbles in Planetary Nebulae
We have searched for the emission from [Fe X] λ6374 and [Fe XIV]λ5303 that is expected from the gas emitting in diffuse X-rays inBD +30°3639, NGC 6543, NGC 7009, and NGC 7027. Neither line wasdetected in any object. Models that fit the X-ray spectra of theseobjects indicate that the [Fe X] λ6374 emission should be belowour detection thresholds, but the predicted [Fe XIV] λ5303emission exceeds our observed upper limits (1 σ) by factors of atleast 3.5-12. The best explanation for the absence of [Fe XIV]λ5303 is that the X-ray plasma is depleted in iron. In principle,this result provides a clear chemical signature that may be used todetermine the origin of the X-ray gas in either the nebular gas or thestellar wind. At present, although various lines of evidence appear tofavor a nebular origin, the lack of atmospheric and nebular ironabundances in the objects studied here precludes a definitiveconclusion.Based on observations obtained at the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California, Mexico.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand NASA.
| On the nitrogen abundance of fast, low-ionization emission regions: the outer knots of the planetary nebula NGC 7009
We have constructed a 3D photoionization model of a planetary nebula(PN) similar in structure to NGC 7009 with its outer pair of knots (alsoknown as FLIERs - fast, low-ionization emission regions). The work ismotivated by the fact that the strong [NII]λ6583 line emissionfrom FLIERs in many PNe has been attributed to a significant localoverabundance of nitrogen. We explore the possibility that the apparentenhanced nitrogen abundance previously reported in the FLIERs may be dueto ionization effects. The model is constrained by the results obtainedby Gonçalves et al. from the analysis of both Hubble SpaceTelescope (HST) [OIII] and [NII] images, and long-slit spectra of NGC7009. Our model is indeed able to reproduce the main spectroscopic andimaging characteristics of the bright inner rim of NGC 7009 and itsouter pairs of knots, assuming homogeneous elemental abundancesthroughout the nebula, for nitrogen as well as all the other elementsincluded in the model.We also study the effects of a narrow slit on our non-sphericallysymmetric density distribution, via the convolution of the model resultswith the profile of the long slit used to obtain the spectroscopicobservations that constrained our model. This effect significantlyenhances the [NII]/Hβ emission, more in the FLIERs than in theinner rim.Because of the fact that the (N+/N)/(O+/O) ratiopredicted by our models is 0.60 for the rim and is 0.72 for the knots,so clearly in disagreement with the N+/N = O+/Oassumption of the ionization correction factor (icf) method, the icfswill be underestimated by the empirical scheme, in both components, rimand knots, but more so in the knots. This effect is partly responsiblefor the apparent inhomogeneous N abundance empirically derived. Thedifferences in the above ratio in these two components of the nebula maybe due to a number of effects including charge exchange - as pointed outpreviously by other authors - and the difference in the ionizationpotentials of the relevant species - which makes this ratio extremelysensitive to the shape of the local radiation field. Because of thelatter, a realistic density distribution is essential to the modellingof a non-spherical object, if useful information is to be extracted fromspatially resolved observations, as in the case of NGC 7009.
| Hubble's top 10.
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| Precatalysmic binary systems.
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| X-Ray Imaging of Planetary Nebulae with Wolf-Rayet-type Central Stars: Detection of the Hot Bubble in NGC 40
We present the results of Chandra X-Ray Observatory observations of theplanetary nebulae (PNs) NGC 40 and Hen 2-99. Both PNs feature late-typeWolf-Rayet central stars that are currently driving fast (~1000 kms-1), massive winds into denser, slow-moving (~10 kms-1) material ejected during recently terminated asymptoticgiant branch (AGB) evolutionary phases. Hence, these observationsprovide key tests of models of wind-wind interactions in PNs. In NGC 40,we detect faint, diffuse X-ray emission distributed within a partialannulus that lies nested within a ~40" diameter ring of nebulosityobserved in optical and near-infrared images. Hen 2-99 is not detected.The inferred X-ray temperature (TX~106 K) andluminosity (LX~2×1030 ergs s-1)of NGC 40 are the lowest measured thus far for any PN displaying diffuseX-ray emission. These results, combined with the ringlike morphology ofthe X-ray emission from NGC 40, suggest that its X-ray emission arisesfrom a ``hot bubble'' that is highly evolved and is generated by ashocked, quasi-spherical fast wind from the central star, as opposed toAGB or post-AGB jet activity. In contrast, the lack of detectable X-rayemission from Hen 2-99 suggests that this PN has yet to enter a phase ofstrong wind-wind shocks.
| Oxygen Recombination Line Abundances in Gaseous Nebulae
The determination of the heavy element abundances from giantextragalactic H II regions has been generally based on collisionallyexcited lines. We will discuss the reasons to study the characteristicsof recombination lines, and then use these lines to determine chemicalabundances. Of these lines the oxygen (specifically the O II) lines arethe most important; and, of them, the lines of multiplet 1 of O II arethe most accessible. It has often been assumed that by measuring theintensity of a single line within a multiplet the intensities of all thelines in the multiplet can be determined; in recent studies we havefound that the intensity ratios of lines within a multiplet can dependon density; we will present empirical density-intensity relationshipsfor multiplet 1 based on recent observations of H II regions andplanetary nebulae. From observations of H II regions we find that thecritical density for collisional redistribution of the multiplet 1 O IIrecombination lines amounts to 2800+/-500 cm-3. We point out that theO/H recombination abundances of H II regions in the solar vicinity arein excellent agreement with the O/H solar value, while the abundancesderived from collisionally excited lines are not. We present acalibration of Pagel's method in the 8.2 < 12 + log O/H < 8.8range based on O recombination lines.
| The kinematics of the large western knot in the halo of the young planetary nebula NGC 6543
A detailed analysis of the dominant ionized knot in the halo of theplanetary nebula NGC 6543 is presented. Observations were made at highspectral and spatial resolution of the [OIII]λ5007-Ålineusing the Manchester echelle spectrometer combined with the 2.1-m SanPedro Martir Telescope. A 20-element multislit was stepped across thefield to give almost complete spatial coverage of the large western knotand surrounding halo.The spectra reveal, for the first time, gas flows around thekinematically inert knot. The gas flows are found to have velocitiescomparable to the sound speed as gas is photoevaporated off an ionizedsurface. No evidence is found of fast wind interaction with the knot,and we find it likely that the fast wind is still contained in apressure-driven bubble in the core of the nebula. This rules out thepossibility of the knot having its origin in instabilities at theinterface of the fast and asymptotic giant branch (AGB) stellar winds.We suggest that the knot is embedded in the slowly expanding red giantwind, and that its surfaces are being continually photoionized by thecentral star.
| Polycyclic aromatic hydrocarbon emission bands in selected planetary nebulae: a study of the behaviour with gas phase C/O ratio
Airborne and space-based low-resolution spectroscopy in the 1980sdiscovered tantalizing quantitative relationships between the gas phaseC/O abundance ratio in planetary nebulae (PNe) and the fractions oftotal far-infrared (FIR) luminosity radiated by the 7.7- and 11.3-μmbands (the C = C stretch and C-H bend, respectively), of polycyclicaromatic hydrocarbons (PAHs). Only a very small sample of nebulae wasstudied in this context, limited by airborne observations of the7.7-μm band, or the existence of adequate IRAS Low ResolutionSpectrometer data for the 11.3-μm band. To investigate these trendsfurther, we have expanded the sample of planetaries available for thisstudy using Infrared Space Observatory (ISO) low-resolution spectrasecured with the Short Wavelength Spectrometer and the Long WavelengthSpectrometer. The new sample of 43 PNe, of which 17 are detected in PAHemission, addresses the range from C/O = 0.2-13 with the objective oftrying to delineate the pathways by which carbon dust grains might haveformed in planetaries. For the 7.7-μm and 11.3-μm bands, weconfirm that the ratio of band strength to total infrared (IR)luminosity is correlated with the nebular C/O ratio. Expressed inequivalent width terms, the cut-on C/O ratio for the 7.7-μm band isfound to be 0.6+0.2-0.4, in good accord with thatfound from sensitive ground-based measurements of the 3.3-μ band.
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