Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 6888 (Crescent nebula)



Upload your image

DSS Images   Other Images

Related articles

A Near-Infrared and X-Ray Study of W49 B: A Wind Cavity Explosion
We present near-infrared narrowband images of the supernova remnant W49B, taken with the WIRC instrument on the Hale 200 inch (5 m) telescopeon Mount Palomar. The 1.64 μm [Fe II] image reveals a barrel-shapedstructure with coaxial rings, which is suggestive of bipolar windstructures surrounding massive stars. The 2.12 μm shocked molecularhydrogen image extends 1.9 pc outside of the [Fe II] emission to thesoutheast. We also present archival Chandra data, which show an X-rayjetlike structure along the axis of the [Fe II] barrel, flaring at eachend. Fitting single-temperature X-ray emission models reveals anenhancement of heavy elements, with particularly high abundances of hotFe and Ni, and relatively metal-rich core and jet regions. We interpretthese findings as evidence that W49 B originated inside a wind-blownbubble (R~5 pc) inside a dense molecular cloud. This suggests that W49B's progenitor was a supermassive star that could significantly shapeits surrounding environment. We also suggest two interpretations for thejet morphology, abundance variations, and molecular hydrogen emission:(1) the explosion may have been jet driven, interacting with themolecular cavity (i.e., a gamma-ray burst); or (2) the explosion couldhave been a traditional supernova, with the jet structure being theresult of interactions between the shock and an enriched interstellarcloud.

Hard X-Rays from Ultracompact H II Regions in W49A
We report the Chandra detection of hard X-ray emission from the Welchring in W49A, an organized structure of ultracompact (UC) H II regionscontaining a dozen nascent early-type stars. Two UC H II regions areassociated with hard X-ray emission in a deep Advanced CCD ImagingSpectrometer (ACIS) image exposed for ~96.7 ks. One of the two X-raysources has no near-infrared counterpart and is extended by ~5", or ~0.3pc, at a distance of ~11.4 kpc, which is spatially aligned with thecometary radio continuum emission associated with the UC H II region.The X-ray spectrum of the emission, when fit with a thermal model,indicates a heavily absorbed plasma with extinction of~5×1023 cm-2, temperature ~7 keV, and X-rayluminosity in the 3.0-8.0 keV band of ~3×1033 ergss-1. Both the luminosity and the size of the emissionresemble the extended hard emission found in UC H II regions inSagittarius B2, yet they are smaller by an order of magnitude than theemission found in massive star clusters such as NGC 3603. Threepossibilities are discussed for the cause of the hard extended emissionin the Welch ring: an ensemble of unresolved point sources, shockedinteracting winds of the young O stars, and a wind-blown bubbleinteracting with ambient cold matter.

Forming a constant density medium close to long gamma-ray bursts
Aims.The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought tobe Wolf-Rayet stars, which generate a massive and energetic wind.Nevertheless, about 25 percent of all GRB afterglows light curvesindicate a constant density medium close to the exploding star. Weexplore various ways to produce this, by creating situations where thewind termination shock arrives very close to the star, as the shockedwind material has a nearly constant density. Methods: .Typically,the distance between a Wolf-Rayet star and the wind termination shock istoo large to allow afterglow formation in the shocked wind material.Here, we investigate possible causes allowing for a smaller distance: Ahigh density or a high pressure in the surrounding interstellar medium(ISM), a weak Wolf-Rayet star wind, the presence of a binary companion,and fast motion of the Wolf-Rayet star relative to the ISM.Results: .We find that all four scenarios are possible in a limitedparameter space, but that none of them is by itself likely to explainthe large fraction of constant density afterglows. Conclusions:.A low GRB progenitor metallicity, and a high GRB energy make theoccurrence of a GRB afterglow in a constant density medium more likely.This may be consistent with constant densities being preferentiallyfound for energetic, high redshift GRBs.

Cleaning Up η Carinae: Detection of Ammonia in the Homunculus Nebula
We report the first detection of ammonia in the Homunculus Nebula aroundη Carinae, which is also the first detection of emission from apolyatomic molecule in this or any other luminous blue variable (LBV)nebula. Observations of the NH3 (J,K)=(3,3) inversiontransition made with the Australia Telescope Compact Array revealemission at locations where infrared H2 emission had beendetected previously, near the strongest dust emission in the core of theHomunculus. We also detect ammonia emission from the so-called strontiumfilament in the equatorial disk. The presence of NH3 aroundη Car hints that molecular shells around some Wolf-Rayet stars couldhave originated in prior LBV eruptions, rather than in cool redsupergiant winds or the ambient interstellar medium. Combined with thelack of any CO detection, NH3 seems to suggest that theHomunculus is nitrogen-rich like the ionized ejecta around η Car. Italso indicates that the Homunculus is a unique laboratory in which tostudy unusual molecule and dust chemistry, as well as their rapidformation in a nitrogen-rich environment around a hot star. We encouragefuture observations of other transitions like NH 3 (1, 1) and(2, 2), related molecules like N2H+, and renewedattempts to detect CO.

Radio observations of Wolf-Rayet ring nebulae
The main physical characteristics of the optical ring nebulae around WRstars based on radio continuum, HI1 cm line and molecular observationsare summarized. The energetics of these structures is analyzed takinginto account predictions from evolutionary models of interstellarbubbles. The main open questions are also discussed.

Stratification of optical emission from NGC 6888 as a trace of the interaction between Wolf-Rayet stellar wind and the shell of a red supergiant
We suggest a model that explains the stratification peculiarities of the[O III] and Hα line emission from some of the ring nebulae aroundWolf-Rayet stars. These peculiarities lie in the fact that the [O III]line emission regions are farther from the central star than theHα regions, with the distance between them reaching several tenthsof a parsec. We show that the radiative shock produced by a Wolf-Rayetstellar wind and propagating with a velocity of ˜100 km s-1 cannotexplain such large distances between these regions due to the lowvelocity of the gas outflow from the shock front. The suggested modeltakes into account the fact that the shock produced by a Wolf-Rayetstellar wind propagates in a two-phase medium: a rarefied medium anddense compact clouds. The gas downstream of a fast shock traveling in ararefied gas compresses the clouds. Slow radiative shocks are generatedin the clouds; these shocks heat the latter to temperatures at whichions of doubly ionized oxygen are formed. The clouds cool down,radiating in the lines of this ion, to temperatures at which Balmer lineemission begins. The distance between the [O III] and Hα lineemission regions is determined by the cooling time of the cloudsdownstream of the slow shock and by the velocity of the fast shock.Using the ring nebula NGC 6888 as an example, we show that the gasdownstream of the fast shock must be at the phase of adiabatic expansionrather than deceleration with radiative cooling, as assumed previously.

Massive Stars and the Energy Balance of the Interstellar Medium. II. The 35 Msolar Star and a Solution to the ``Missing Wind Problem''
We continue our numerical analysis of the morphological and energeticinfluence of massive stars on their ambient interstellar medium for a 35Msolar star that evolves from the main-sequence through redsupergiant and Wolf-Rayet phases, until it ultimately explodes as asupernova. We find that structure formation in the circumstellar gasduring the early main-sequence evolution occurs as in the 60Msolar case but is much less pronounced because of the lowermechanical wind luminosity of the star. On the other hand, since theshell-like structure of the H II region is largely preserved, effectsthat rely on this symmetry become more important. At the end of thestellar lifetime 1% of the energy released as Lyman continuum radiationand stellar wind has been transferred to the circumstellar gas. Fromthis fraction 10% is kinetic energy of bulk motion, 36% is thermalenergy, and the remaining 54% is ionization energy of hydrogen. Thesweeping up of the slow red supergiant wind by the fast Wolf-Rayet windproduces remarkable morphological structures and emission signatures,which are compared with existing observations of the Wolf-Rayet bubbleS308, whose central star has probably evolved in a manner very similarto our model star. Our model reproduces the correct order of magnitudeof observed X-ray luminosity, the temperature of the emitting plasma,and the limb brightening of the intensity profile. This is remarkable,because current analytical and numerical models of Wolf-Rayet bubblesfail to consistently explain these features. A key result is that almostthe entire X-ray emission in this stage comes from the shell of redsupergiant wind swept up by the shocked Wolf-Rayet wind rather than fromthe shocked Wolf-Rayet wind itself as hitherto assumed and modeled. Thisoffers a possible solution to what is called the ``missing windproblem'' of Wolf-Rayet bubbles.

X-Ray Emission from Wind-blown Bubbles. III. ASCA SIS Observations of NGC 6888
We present ASCA SIS observations of the wind-blown bubble NGC 6888.Owing to the higher sensitivity of the SIS for higher energy photonscompared to the ROSAT PSPC, we are able to detect aT~8×106 K plasma component in addition to theT~1.3×106 K component previously detected in PSPCobservations. No significant temperature variations are detected withinNGC 6888. García-Segura & Mac Low's analytical models of WRbubbles constrained by the observed size, expansion velocity, and massof the nebular shell underpredict the stellar wind luminosity and cannotreproduce simultaneously the observed X-ray luminosity, spectrum,surface brightness profile, and SIS count rate of NGC 6888's bubbleinterior. The agreement between observations and expectations frommodels may be improved if one or more of the following ad hocassumptions are made: (1) the stellar wind luminosity was weaker in thepast, (2) the bubble is at a special evolutionary stage and the nebularshell has recently been decelerated to 1/2 of its previous expansionvelocity, and (3) the heat conduction between the hot interior and thecool nebular shell is suppressed. Chandra and XMM-Newton observationswith high spatial resolution and high sensitivity are needed toaccurately determine the physical conditions of NGC 6888's interior hotgas for critical comparisons with bubble models.

A new Wolf-Rayet star and its ring nebula: PCG11
In a search for new Galactic planetary nebulae from our systematic scansof the Anglo-Australian Observatory/United Kingdom Schmidt Telescope(AAO/UKST) Hα Survey of the Southern Galactic Plane, we haveidentified a Population I Wolf-Rayet star of type WN7h associated withan unusual ring nebula that has a fractured rim. We present imagery inHα, the 843-MHz continuum from the Molonglo Observatory SynthesisTelescope (MOST), the mid-infrared from the Midcourse Space Experiment(MSX), and confirmatory optical spectroscopy of the character of thenebula and of its central star. The inner edge of the Hα shellshows gravitational instabilities with a well-defined wavelength aroundits complete circumference.

H I bubbles surrounding southern optical ring nebulae: Anon (WR 23) and RCW 52
We analyze the interstellar medium in the environs of two hot andmassive stars, HD 92809 (=WR 23, WC6) and LSS 1887 (O8V), which ionizethe optical ring nebulae Anon (WR 23) and RCW 52, respectively. Ouranalysis is based on neutral hydrogen (Hi) 21 cm line data, which revealinterstellar bubbles surrounding the massive stars and their opticalring nebulae. The Hi bubble related to WR 23 is 13.3 pc in radius and isexpanding at 10 km s-1. The associated atomic neutral mass is830 M_ȯ. The Hi structure related to LSS 1887 is about 6.3 pc inradius, has an expansion velocity of 7 km s-1 and anassociated atomic neutral mass of 100 M_ȯ. These Hi features arethe neutral counterparts of the optical ring nebulae and were mainlycreated by the action of the stellar winds of the massive stars on theirenvirons. The dynamical age of the Hi bubble around WR 23(7×105 yr) suggests that it was created during the WRphase of stellar evolution. However, the large tangential motions of WR23 and LSS 1887 suggest that part of the observed optical and Histructures may be due to a bow shock. The analysis of the distributionof emission in the far infrared and in the CO(1-0) molecular line in theenvirons of WR 23 and LSS 1887 reveals that there are also infrared andmolecular counterparts of the detected Hi bubbles.

Sous le vent des étoiles massives
Not Available

An XMM-Newton look at the Wolf-Rayet star WR 40. The star itself, its nebula and its neighbours
We present the results of an XMM-Newton observation of the field of theWolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the highsensitivity of the satellite, the star itself is not detected: we thusderive an upper limit on its X-ray flux and luminosity. Joining thisresult to recent reports of a non-detection of some WC stars, we suggestthat the X-ray emission from single normal Wolf-Rayet stars could oftenbe insignificant despite remarkable instabilities in the wind. On thebasis of a simple modelling of the opacity of the Wolf-Rayet wind of WR40, we show that any X-ray emission generated in the particular zonewhere the shocks are supposed to be numerous will indeed have littlechance to emerge from the dense wind of the Wolf-Rayet star. We alsoreport the non-detection of the ejecta nebula RCW 58 surrounding WR 40.Concerning the field around these objects, we detected 33 X-ray sources,most of them previously unknown: we establish a catalog of these sourcesand cross-correlate it with catalogs of optical/infrared sources.Based on observations with XMM-Newton, an ESA Science Mission withinstruments and contributions directly funded by ESA Member States andthe USA (NASA).

2D Ionization Structure of Haloes of Wind Driven Nebulae .
The study of the nebular environments of stars provides clues to theirrecent evolution: the history of stellar winds and ejecta is written inthe extended nebulae around massive (Wolf-Rayet Ring Nebulae (WRNe)) andintermediate mass stars (Planetary Nebulae (PNe)). Fast winds canproduce shock excitation of the interstelar medium in the haloes of PNeand WR nebulae producing an extra source of heating to their thermalbalance. We have explored the impact on the interstellar medium of thiseffect by searching for shock-heated gas in the 2D ionization structureof haloes, derived from deep multi-filter imaging of a sample ofwind-driven nebulae. In this work, we present results for candidateregions where fotoionization does not appear to be the main ionizationmechanism.

Far Ultraviolet Spectroscopic Explorer Observations of Nebular O VI Emission from NGC 6543
NGC 6543 is one of the few planetary nebulae whose X-ray emission hasbeen shown to be extended and to originate from hot interior gas. UsingFar Ultraviolet Spectroscopic Explorer observations, we have nowdetected nebular O VI emission from NGC 6543. Its central star, with aneffective temperature of ~50,000 K, is too cool to photoionize O V, sothe O VI ions must have been produced by thermal collisions at theinterface between the hot interior gas and the cool nebular shell. Wemodeled the O VI emission incorporating thermal conduction but find thatsimplistic assumptions for the asymptotic giant branch and fast windmass-loss rates overproduce X-ray emission and O VI emission. We havetherefore adopted the pressure of the interior hot gas for the interfacelayer and find that expected O VI emission to be comparable to theobservations.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS 5-32985.

Observed Features of a Two-Phase Stellar Wind from WR 136 in the Vicinity of NGC 6888
A number of features are detected outside the nebula NGC 6888, within1.2° (30 pc) of the star WR 136, which can be explained in atwo-phase stellar-wind model. These include regions with finefilamentary gas structure that do not contain sources of stellar wind,extended radial “streams,” ultra-compact HII regions withhigh-velocity gas motions, and high-velocity gas motions outside theenvelope of NGC 6888. The two-phase wind consists of a rarefiedcomponent and dense compact condensations, or “bullets.” Thebullets generate cylindrical shocks in the interstellar gas, resultingin the presence of high-velocity gas up 20 30 pc from the star, outsidethe cavity formed by the rarified component of the wind.

The Diversity of Gamma-Ray Burst Afterglows and the Surroundings of Massive Stars
The finding of a Type Ic supernova connected with GRB 030329 showed amassive star origin for this burst, supporting evidence for thisassociation in previous bursts with light-curve bumps at the appropriatetime for a supernova. Here we explore the possibility that all longbursts have massive star progenitors, interacting with either the freelyexpanding wind of the progenitor or the shocked wind. We present modelsfor the afterglows of GRB 020405 and GRB 021211, which are a challengeto wind interaction models. Considering sources for which windinteraction models are acceptable, a range of wind densities isrequired, from values typical of Galactic Wolf-Rayet stars to values~102 times smaller. The reason for the low densities isunclear, but it may involve low progenitor masses and/or lowmetallicities. If mass is a factor, a low-density event should beassociated with a low-mass supernova. The interpretation of burstsapparently interacting with constant density media as interaction with ashocked wind requires both a range of mass-loss densities and a range ofexternal pressures. The highest pressures, p/k>~108cm-3 K, may be due to an extreme starburst environment, whichwould imply that the burst is superposed on an active star-formingregion. Although the range of observed events can be accommodated by theshocked-wind theory, special circumstances are necessary to bring thisabout. Finally, we consider the high-velocity, high-ionizationabsorption features observed in some afterglow spectra. If the featuresare circumstellar, the presence of the burst in a starburst region maybe important for the formation of clumps near the burst.

Energy Crisis in the Superbubble DEM L192 (N51D)
Superbubbles surrounding OB associations provide ideal laboratories inwhich to study the stellar energy feedback problem, because the stellarenergy input can be estimated from the observed stellar content of theOB associations, and the interstellar thermal and kinetic energies ofsuperbubbles are well defined and easy to observe. We have used DEML192, also known as N51D, to carry out a detailed case study of theenergy budget in a superbubble, and we find that the expected amount ofstellar mechanical energy injected into the interstellar medium,(18+/-5)×1051 ergs, exceeds the amount of thermal andkinetic energies stored in the superbubble,(6+/-2)×1051 ergs. Clearly, a significant fraction ofthe stellar mechanical energy must have been converted into other formsof energy. The X-ray spectrum of the diffuse emission from DEM L192requires a power-law component to explain the featureless emission at1.0-3.0 keV. The origin of this power-law component is unclear, but itmay be responsible for the discrepancy between the stellar energy inputand the observed interstellar energy in DEM L192.

Hot Gas in the Circumstellar Bubble S308
S308 is a circumstellar bubble blown by the WN4 star HD 50896. It is oneof the only two single-star bubbles that show detectable diffuse X-rayemission. We have obtained XMM-Newton EPIC observations of the northwestquadrant of S308. The diffuse X-ray emission shows a limb-brightenedmorphology, with a clear gap extending from the outer edge of thediffuse X-ray emission to the outer rim of the nebular shell. The X-rayspectrum of the diffuse emission is very soft and is well fitted by anoptically thin plasma model for an N-enriched plasma at temperatures of~1.1×106 K. A hotter gas component may exist but itstemperature is not well constrained since it contributes less than 6% ofthe observed X-ray flux. The total X-ray luminosity of S308,extrapolated from the bright northwest quadrant, is<=(1.2+/-0.5)×1034 ergs s-1. We have usedthe observed bubble dynamics and the physical parameters of the hotinterior gas of S308, in conjunction with the circumstellar bubble modelof García-Segura & Mac Low, to demonstrate that theX-ray-emitting gas must be dominated by mixed-in nebular material.

A Failed Gamma-Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the Gamma-Ray Burst-Supernova Connection from SN 2002ap
The Type Ic supernova (SN) 2002ap is an interesting event with verybroad spectral features like the famous energetic SN 1998bw associatedwith a gamma-ray burst (GRB) 980425. Here we examine the jet hypothesisfrom SN 2002ap recently proposed based on the redshifted polarizedcontinuum found in a spectropolarimetric observation. We show that jetsshould be moving at about 0.23c to a direction roughly perpendicular tous, and the degree of polarization requires a jet kinetic energy of atleast 5×1050 ergs, a similar energy scale to the GRBjets. The weak radio emission from SN 2002ap has been used to argueagainst the jet hypothesis, but we argue that this is not a problembecause the jet is expected to be freely expanding and unshocked.However, the jet cannot be kept ionized because of adiabatic coolingwithout external photoionization or a heating source. We exploredvarious ionization possibilities and found that only the radioactivityof 56Ni is a plausible source, indicating that the jet isformed and ejected from the central region of the core collapse, notfrom the outer envelope of the exploding star. Then we point out that,if the jet hypothesis is true, the jet will eventually sweep up enoughinterstellar medium and generate shocks in a few to 10 yr, producingstrong radio emission that can be spatially resolved, giving us a cleartest for the jet hypothesis. Discussions are also given on what the jetwould imply for the GRB-SN connection, when it is confirmed. We suggestthe existence of two distinct classes of GRBs from similar core-collapseevents but by completely different mechanisms. Cosmologically distantGRBs having an energy scale of ~1050-1051 ergs arecollimated jets generated by the central activity of core collapses,associated with 56Ni ejection along with the jets. SN 2002apcan be considered as a failed GRB of this type with large baryoncontamination. On the other hand, much less energetic ones including GRB980425 are rather isotropic, which may be produced by hydrodynamicalshock acceleration at the outer envelope. We propose that theradioactive ionization for the SN 2002ap jet may give a new explanationalso for the X-ray line features often observed in GRB afterglows.

Shocked gas layers surrounding the WR nebula NGC 2359
NGC 2359 is a Wolf-Rayet (W-R) nebula partially bound by a rather denseand warm molecular cloud. We present the results derived from CO and13CO fully sampled maps of the molecular material withangular resolutions up to 12arcsec . We have detected three differentvelocity components, and determined their spatial distribution andphysical properties. The kinematics, morphology, mass and density areclearly stratified with respect to the W-R star. These features allow usto learn about the recent evolutionary history of HD 56925, because themultiple layers could be associated to several energetic events whichhave acted upon the surrounding circumstellar medium. Hence, a carefulstudy of the different shockfronts contain clues in determining thepresent and past interaction of this evolved massive star with itssurroundings. From the analysis of the mass-loss history in massivestars like HD 56925, we suggest that the multiple layers of shockedmolecular gas are likely to be produced during the earlier LBV phaseand/or the actual W-R stage of HD 56925.

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor
The rapid localization of GRB 021004 by the HETE-2 satellite allowednearly continuous monitoring of its early optical afterglow decay, aswell as high-quality optical spectra that determined a redshift ofz3=2.328 for its host galaxy, an active starburst galaxy withstrong Lyα emission and several absorption lines. Spectralobservations show multiple absorbers at z3A=2.323,z3B=2.317, and z3C=2.293 blueshifted by ~450,~990, and ~3155 km s-1, respectively, relative to the hostgalaxy Lyα emission. We argue that these correspond to afragmented shell nebula that has been radiatively accelerated by thegamma-ray burst (GRB) afterglow at a distance >~0.3 pc from aWolf-Rayet star GRB progenitor. The chemical abundance ratios indicatethat the nebula is overabundant in carbon and silicon. The high level ofcarbon and silicon is consistent with a swept-up shell nebula graduallyenriched by a carbon-rich late-type Wolf-Rayet progenitor wind over thelifetime of the nebula prior to the GRB onset. The detection ofstatistically significant fluctuations and color changes about thejetlike optical decay further supports this interpretation, sincefluctuations must be present at some level as a result of irregularitiesin a clumpy stellar wind medium or if the progenitor has undergonemassive ejection prior to the GRB onset. This evidence suggests that themass-loss process in a Wolf-Rayet star might lead naturally to an ironcore collapse with sufficient angular momentum that could serve as asuitable GRB progenitor. Even though we cannot rule out definitely thealternatives of a dormant QSO, large-scale superwinds, or a severalhundred year old supernova remnant responsible for the blueshiftedabsorbers, these findings point to the likelihood of a signature for amassive-star GRB progenitor.Based in part on data obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and NASA, and was madepossible with the generous financial support of the W. M. KeckFoundation.

The Chaotic ISM of the POST(?)-STARBURST Galaxy NGC 1569
Not Available

New HST Observations of the Wolf-Rayet Shell Nebula NGC 6888
Not Available

Massive Stars and the Energy Balance of the Interstellar Medium. I. The Impact of an Isolated 60 Msolar Star
We present results of numerical simulations carried out with atwo-dimensional radiation hydrodynamics code in order to study theimpact of massive stars on their surrounding interstellar medium. Thisfirst paper deals with the evolution of the circumstellar gas around anisolated 60 Msolar star. The interaction of the photoionizedH II region with the stellar wind bubble forms a variety of interestingstructures like shells, clouds, fingers, and spokes. These resultsdemonstrate that complex structures found in H II regions are notnecessarily relics from the time before the gas became ionized but mayresult from dynamical processes during the course of the H II regionevolution. We have also analyzed the transfer and deposit of the stellarwind and radiation energy into the circumstellar medium until the starexplodes as a supernova. Although the total mechanical wind energysupplied by the star is negligible compared to the accumulated energy ofthe Lyman continuum photons, the kinetic energy imparted to thecircumstellar gas over the star's lifetime is 4 times higher than for acomparable windless simulation. Furthermore, the thermal energy of warmphotoionized gas is lower by some 55%. Our results document thenecessity to consider both ionizing radiation and stellar winds for anappropriate description of the interaction of OB stars with theircircumstellar environment.

Planetary Nebulae, Bubbles, and Superbubbles: What Can We Learn From Their Kinematics?
We present three examples on how the confrontation between theinformation we can derive from the kinematics of planetary nebulae,bubbles, and superbubbles and the theoretical models of evolution ofthese objects allows us to have a better understanding of the physicalmechanisms involved in the creation and evolution of these objects andto conceive better ways to observe them.

An Inside-Out View of Bubbles
Fast stellar winds can sweep up ambient media and form bubbles. Theevolution of a bubble is largely controlled by the content and physicalconditions of the shocked fast wind in its interior. This hot gas wasnot clearly observed until the recent advent of Chandra and XMM-NewtonX-ray observatories. To date, diffuse X-ray emission has beenunambiguously detected from two circumstellar bubbles blown by WR stars,four planetary nebulae, and two superbubbles blown by young clusters.Model fits to the X-ray spectra show that the circumstellar bubbles aredominated by hot gas with low temperatures ( < 3×10^6 K), whilethe interstellar bubbles contain significant fractions of hotter gas (< 5×10^6 K) . In all cases, large discrepancies in the X-rayluminosity are found between observations and conventional models ofbubbles. Future theoretical models of bubbles need to re-examine thevalidity of heat conduction and take into account realistic microscopicprocesses such as mass loading from dense clumps/knots and turbulentmixing. Chandra ACIS-S observation of NGC 6888 will shed light on theseastrophysical processes.

Shocked CO and 13CO around Wolf-Rayet ring nebulae
We show CO and 13CO maps toward the WR nebulae NGC 2359, NGC6888 and Anon (WR 134). We determine global parameters and discuss theprobable origin of this gas. Nowadays it is becoming clear thatmolecule formation and survival is be possible around massive evolvedstars.

Radio observations of interstellar bubbles surrounding massive stars}
We show radio continuum observations of the WR ring nebulae around WR101 and WR 113 obtained using the VLA and HI 21 cm line data of theinterstellar bubble around the O type stars BD +24 deg 3866 and BD+25deg 3952 obtained with the DRAO Synthesis Telescope. We review previousradio continuum and HI line results toward WR and O-type stars.

Ring nebulae around massive stars throughout the Hertzsprung-Russell diagram
Massive stars evolve across the H-R diagram, losing mass along the wayand forming a variety of ring nebulae. During the main sequence stage,the fast stellar wind sweeps up the ambient interstellar medium to forman interstellar bubble. After a massive star evolves into a red giantor a luminous blue variable, it loses mass copiously to form acircumstellar nebula. As it evolves further into a WR star, the fast WRwind sweeps up the previous mass loss and forms a circumstellar bubble.Observations of ring nebulae around massive stars not only arefascinating, but also are useful in providing templates to diagnose theprogenitors of supernovae from their circumstellar nebulae. In thisreview, I will summarize the characteristics of ring nebulae aroundmassive stars throughout the H-R diagram, show recent advances in X-rayobservations of bubble interiors, and compare supernovae's circumstellarnebulae with known types of ring nebulae around massive stars.

Neutral Hydrogen around the Oxygen-Sequence Wolf-Rayet Star WR 102 and the Nebula G2.4 + 1.4
Not Available

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:20h12m07.00s
Apparent magnitude:99.9

Catalogs and designations:
Proper NamesCrescent nebula
Crescent, Aleksandra   (Edit)
NGC 2000.0NGC 6888

→ Request more catalogs and designations from VizieR