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| 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.
| Radio Recombination Lines in Galactic H II Regions
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.
| 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.
| Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.
| 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.
| 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).
| 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.
| Determination of temperature of the ionizing stars of H II regions
The determination of temperature (T_eff) of the ionizing stars of H Iiregions was considered. In this work we used photoionization models forH Ii regions ionized by a single star to show that the index R=log ([OIi]lambda lambda 3726+3729/[O Iii]lambda 5007) can be used to estimateT_eff. The relation R vs. T_eff proved to be rather independent of thechemical abundances, but strongly dependent on the ionization parameterof the nebula. In order to check the reliability of using R fortemperature determination, we compared the values of T_eff obtained viathe index R for a sample of H Ii regions with data available in theliterature with independent estimations.
| On the oxygen abundance in our Galaxy
The compilation of published spectra of Galactic H II regions withavailable diagnostic [OIII]lambda 4363 line information has been carriedout. Our list contains 71 individual measurements of 13 H II regions inthe range of galactocentric distances from 6.6 to 14.8 kpc. The oxygenabundances in all the H II regions were recomputed in the same way,using the classic T_e-method. The oxygen abundance at the solargalactocentric distance traced by those H II regions is in agreementwith the oxygen abundance in the interstellar medium in the solarvicinity derived with high precision from the interstellar absorptionlines towards stars. The derived radial oxygen abundance distributionwas compared with that for H II regions from the Shaver et al.(\cite{Shaver83}) sample which is the basis of many models for thechemical evolution of our Galaxy. It was found that the original Shaveret al.'s oxygen abundances are overestimated by 0.2-0.3 dex. Oxygenabundances in H II regions from the Shaver et al. sample have beenredetermined with the recently suggested P-method. The radialdistribution of oxygen abundances from the Shaver et al. sampleredetermined with the P-method is in agreement with our radialdistribution of (O/H)T_e abundances.
| 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.
| Complex molecules in Wolf-Rayet ring nebula NGC 2359
We report the detection and analysis of several molecules arising fromthe WR nebula NGC 2359 around the WN4 star WR 7 (HD 56925). We havedetected 3mm and 1mm rotational lines from 13CO,C18O, HCO+, CS, CN and HCN, and determined lowerlimits for the SiO. The survey was performed in the southeastern partof the optical nebula, where we previously inferred the presence ofdense (104 cm-3) material.
| 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.
| VLA Radio Continuum and IRAS Observations of the Ring Nebulae around WR 101 and WR 113
We report radio continuum observations at 1465 MHz obtained with theVery Large Array (VLA) in the DnC configuration toward the ring nebulaeassociated with the stars WR 101 and WR 113, with resolutions of ~38"and 30", respectively. IRAS images of the nebulae with resolutions ofabout 2' (90 Msolar, 40 cm-3) are also analyzed. Aremarkable resemblance among the optical, infrared, and radio images ofthese ring nebulae is observed. The VLA data indicate that Anon. WR 101is thermal in nature. An ionized mass of ~230+/-40 Msolar andelectron densities in the range ~40-55 cm-3 were estimatedfor Anon. WR 101. The derived ionized masses and electron densities inthe inner and outer shells of the nebula related to WR 113 are ~20+/-10Msolar, 180-500 cm-3 and ~90 Msolar 40cm-3, respectively. Based on infrared data at 60 and 100μm, the derived masses and temperatures for the dust component in thering nebula around WR 101 are 0.3-1 Msolar and ~40 K. Theassociated masses suggest that the ring nebula related to WR 101 and theouter arc associated with WR 113 consist of swept-up interstellarmatter, while the relatively low ionized mass associated with the innershell of the nebula around WR 113 may contain a nonnegligiblecontribution of expelled ejecta material. The derived electron densitiesfor the nebula around WR 101 and the inner shell around WR 113 arecomparable to electron densities for other W-R ring nebulae. Low fillingfactors are inferred for both nebulae. The nebulae probably originatedduring the current W-R phase of the stars.
| On the oxygen abundances in H II regions of the Galaxy
Oxygen abundances in H II regions of the Galaxy were derived with theclassical Te-method within the framework of the two-zone H IIregion model using published spectro-photometric data (69 spectra of 11H II regions in the range of galactocentric distances from 6.6 to 14.8kpc). The derived radial distribution of the oxygen abundance wascompared with that from Shaver et al. (1983), which is widely used inconstructing the model of the chemical evolution of the Galaxy. It wasfound that the oxygen abundances given by Shaver et al. areoverestimated by 0.2-0.3 dex.
| The cosmological density of baryons from observations of 3He+ in the Milky Way
Primordial nucleosynthesis after the Big Bang can be constrained by theabundances of the light elements and isotopes 2H,3He, 4He and 7Li (ref. 1). The standardtheory of stellar evolution predicts that 3He is alsoproduced by solar-type stars, so its abundance is of interest not onlyfor cosmology, but also for understanding stellar evolution and thechemical evolution of the Galaxy. The 3He abundance instar-forming (HII) regions agrees with the present value for the localinterstellar medium, but seems to be incompatible with the stellarproduction rates inferred from observations of planetary nebulae, whichprovide a direct test of stellar evolution theory. Here we develop ourearlier observations, which, when combined with recent theoreticaldevelopments in our understanding of light-element synthesis anddestruction in stars, allow us to determine an upper limit for theprimordial abundance of 3He relative to hydrogen:3He/H = (1.1 +/- 0.2) × 10-5. The primordialdensity of all baryons determined from the 3He data is inexcellent agreement with the densities calculated from othercosmological probes. The previous conflict is resolved because mostsolar-mass stars do not produce enough 3He to enrich theinterstellar medium significantly.
| First Detections of Molecular Gas Associated with the Wolf-Rayet Ring Nebula NGC 3199
This paper presents the first observations of molecular gas associatedwith the Wolf-Rayet ring nebula NGC 3199 around the WR star WR 18. Thisincludes first observations of the molecules HCN, HCO+, CN,and HNC seen in any Wolf-Rayet ring nebula. Our observations immediatelysuggest the presence of high-density molecular gas (>104cm-3) in the nebula with significant amounts of associatedmolecular gas, which is in the form of clumpy ejecta and/or interstellarmaterial. Molecular CO gas was mapped across the optically brightportion of the nebula and out into the diffuse ionized component usingthe 12CO J=1-->0 line. CO gas is not seen within theoptically bright rim of NGC 3199 but adjacent to it. The opticalemission rim therefore appears to mark regions of photodissociation.Velocity components in the CO data are consistent with those seen inhigh-resolution optical spectra of the Hα line but extend beyondthe visible emission. A prior suggestion of the formation of the nebulavia a bow shock appears unlikely since Hipparcos measurements show theproper motion of WR 18 is almost at right angles to the directionrequired for the bow shock model. Instead, line splitting toward thenorth of the nebula suggests that a possible blowout of the Wolf-Rayetwind through surrounding ejecta may be responsible for some of thevelocity features observed. Preliminary estimates of molecularabundances in the nebula seen toward the central star are significantlyhigher than for the interstellar medium and are similar to those inplanetary nebulae, although CN is distinctly underabundant in comparisonto the very high values found in many planetary nebulae. The abundancesfound are consistent with the idea that at least a portion of themolecular material is associated with ejecta from the central star.Based on observations collected at the Swedish-ESO SubmillimetreTelescope (SEST) at the European Southern Observatory, La Silla, Chile.The Swedish-ESO Submillimetre Telescope is operated jointly by theEuropean Southern Observatory (ESO) and the Swedish National Facilityfor Radio Astronomy, Onsala Space Observatory, at Chalmers University ofTechnology.
| Observations of neutral and ionized gas of the WR ring nebula NGC 2359
We investigated the distribution of neutral and ionized material in theregion of NGC 2359, the optical nebular ring around the Wolf-Rayet starHD56925. We used 21cm line and 20cm continuum radio observations withthe Very Large Array. The radio continuum shows very good correlationwith the optical line emission. The HI data reveal for the first timethe presence of neutral hydrogen clearly associated tp NGC 2359. Wedetermine the main physical parameters of the HII region.
| Shocked Ammonia in the Wolf-Rayet Nebula NGC 2359
We report the detection of the (1, 1) and (2, 2) metastable lines ofammonia (NH3) in the molecular cloud associated with theWolf-Rayet (W-R) nebula NGC 2359. Besides the CO and H2, thisis the first molecule detected in the environs of a W-R star. Width(ΔV1/2=3 km s-1) and radial velocity(VLSR~54 km s-1) indicate that the NH3lines arise from the molecular cloud that is interacting with the W-Rstar. The rotational temperature derived from the (1, 1) and (2, 2) lineintensity ratios is about 30 K, significantly larger than the typicalkinetic temperature of the ambient gas of ~10 K. The derivedNH3 abundance is ~10-8. Line width, abundance, andkinetic temperature can be explained if NH3 is released fromdust grain mantles to the gas phase by shocks produced by the expansionof the bubble created by the W-R stellar wind. We briefly discuss theimplications of the detection of warm NH3 associated with aW-R star in connection to the hot NH3 emission detected inthe Galactic center and in the nuclei of external galaxies.
| CO Observations of NGC 2359: The Molecular Clouds Revisited
Based on CO (2-1) observations obtained with the Swedish-ESOSubmillimeter Telescope, the distribution of molecular materialassociated with the Wolf-Rayet ring nebula NGC 2359 has been determined.The angular resolution is 22", and the velocity resolution is 0.33 kms-1. Three molecular components are detected in the directionof the nebula. The bulk of the molecular gas is observed at 54 kms-1 and follows the southeastern border of the nebula.Adopting a distance d=5 kpc for NGC 2359, an H2 mass of about1200+/-500 Msolar appears to be related to the southern partof the nebula, while 140+/-50 Msolar are linked to thefilamentary windblown bubble. The volume density of the molecular gasrelated to the southern bar is >=103 H2molecules cm-3, while lower densities were estimated for thematerial associated with the filamentary windblown bubble. From thepresent data, it is not clear if the molecular gas at 37 kms-1 (~=380+/-120 Msolar, d=5 kpc) is associatedwith the nebula, but the molecular material observed at 67 kms-1 (~=70+/-25 Msolar, d=5 kpc) seems to beunconnected. The comparison between ionized, H I, and moleculardistributions indicates that the H I filament detected with the VeryLarge Array is located at the interface between the ionized andmolecular material, and that the H I filament at 54 km s-1has originated in the photodissociation of the H2. Most ofthe molecular gas associated with the filamentary bubble seems to beinterstellar in origin. The dynamics of the nebula is reanalyzed basedon these new molecular results. It is consistent with either momentumconservation or an intermediate stage between energy and momentumconservation.
| On the oxygen abundance determination in HII regions. High-metallicity regions
This is our second paper devoted to the problem of line intensity -oxygen abundance calibration starting from the idea of McGaugh(\cite{mcg91}) that the strong oxygen lines ([OII] lambda lambda 3727,3729 and [OIII] lambda lambda 4959, 5007) contain the necessaryinformation to determine accurate abundances in HII regions. In theprevious study (Pilyugin 2000) the corresponding relations were obtainedfor the low-metallicity HII regions (12+log O/H <= 7.95, the lowerbranch of the O/H - R23 diagram). The high-metallicity HIIregions (12+log O/H >= 8.2, the upper branch of the O/H -R23 diagram) are considered in the present study. A relationof the type O/H=f(P, R23) between oxygen abundance and thevalue of abundance index R23, introduced by Pagel et al.(\cite{pag79}), and the excitation parameter P (which is defined here asthe contribution of the radiation in [OIII] lambda lambda 4959, 5007lines to the ``total" oxygen radiation) has been derived empiricallyusing the available oxygen abundances determined via measurement of atemperature-sensitive line ratio [OIII]4959,5007/[OIII]4363(Te-method). By comparing oxygen abundances inhigh-metallicity HII regions derived with the Te-method andthose derived with the suggested relations (P-method), it was found thatthe precision of oxygen abundance determination with the P-method isaround 0.1 dex (the mean difference for the 38 HII regions considered is~ 0.08 dex) and is comparable to that of the Te-method. Arelation of the type Te=f(P, R23) between electrontemperature and the values of abundance index R23 and theexcitation parameter P was derived empirically using the availableelectron temperatures determined via measurement oftemperature-sensitive line ratios. The maximum value of differencesbetween electron temperatures determined via measurement oftemperature-sensitive line ratios and those derived with the suggestedrelation is around 1000 K for HII regions considered here, the meanvalue of differences for 38 HII regions is ~ 500 K, which is the sameorder of magnitude as the uncertainties of electron temperaturedeterminations in high-metallicity HII regions via measuredtemperature-sensitive line ratios.
| On the history of the interplay between HD 56925 and NGC 2359
NGC 2359 is an optical nebula excited by the powerful wind and theradiation of the Wolf-Rayet star HD 56925. We have investigated theinteraction between this massive star and the surrounding neutral gas byanalyzing the large-scale 21 cm-Hi emission and by mapping the nebula inthe J= 1 -> 0 and the J= Hig\ lines of CO. We found a conspicuous (70x 37 pc) Hi shell, expanding at 12 km s-1, likely producedduring the main-sequence phase of the star. The molecular gas towardsNGC 2359 shows three velocity components. Two of these components, A1and A2, have narrow linewidths (1-2 km s-1) and radialvelocities of 35-38 and 64-68 km s-1, respectively. The thirdcomponent is detected at radial velocities between 50 and 58 kms-1 and has a broader profile (up to 5.5 km s-1).Furthermore, this component is morphologicaly related with the nebulaand has a velocity gradient of a few km s-1. We have alsoestimated the physical parameters of the molecular gas by means of a LVGmodelling of the CO emission. The gas projected onto the southern Hii\region of the nebula has low CO column density and is rather hot,probably up to 80 K. Several profiles of the 13CO J= 1 ->0 line near the peak of the emission, together with a weak emissionbridge between the broad and one of the narrow components (componentA2), suggest the presence of a shock front acting in the southern partof the nebula. This shock was likely produced in a previous RSG/LBVphase of HD 56925.
| The HE II Emitting Nebula N44C in the Large Magellanic Cloud: Optical/Ultraviolet Spectroscopy of the Nebula and Its Ionizing Star
We present HST spectroscopy and imaging, along with new ground-basedspectroscopy and ROSAT HRI imaging, of the He II emitting nebula N44Cand its ionizing star. A GHRS spectrogram of the ionizing star yields aspectral type of about O7 for the star. The lack of P Cygni profiles forSi IV and C IV indicates that the star is not a supergiant. The nebularabundances in the ionized gas are consistent with average abundances forLMC H II regions, with the possible exception that nitrogen may beenhanced. Enrichment by a former evolved companion star is not evident.A long-slit echelle spectrogram in Hα+[N II] shows no evidence forhigh-velocity gas in N44C. This rules out high-velocity shocks as thesource of the nebular He II emission. A 108 ks ROSAT HRI image of N44Cshows no X-ray point source to a 3 σ upper limitLX<1034 ergs s-1 in the 0.1-2.0 keVband. Based on new measurements of the electron density in the He IIemitting region, we derive recombination timescales of ~20 yr forHe+2 and ~4 yr for Ne+4. If N44C is a fossil X-rayionized nebula, this places severe constraints on when the putativeX-ray source could have turned off. The presence of strong [Ne IV]emission in the nebula is puzzling if the ionizing source has turnedoff. It is possible the system is related to the Be X-ray binaries,although the O star in N44C does not show Be characteristics at thepresent time. Monitoring of X-rays and He II emission from the nebula,as well as a radial velocity study of the ionizing star, are needed tofully understand the emission line spectrum of N44C. Based in part onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| A Morphological Diagnostic for Dynamical Evolution of Wolf-Rayet Bubbles
We have observed Hα and [O III] emission from eight of the mostwell-defined Wolf-Rayet ring nebulae in the Galaxy. We find that in manycases the outermost edge of the [O III] emission leads the Hαemission. We suggest that these offsets, when present, are due to theshock from the Wolf-Rayet bubble expanding into the circumstellarenvelope. Thus, the details of the WR bubble morphology at Hα and[O III] can then be used to understand better the physical condition andevolutionary stage of the nebulae around Wolf-Rayet stars, as well as toplace constraints on the nature of the stellar progenitor and itsmass-loss history.
| An Empirical Test and Calibration of H II Region Diagnostics
We present spectrophotometry in the 3600-9700 Å region for asample of 39 H II regions in the Galaxy and Magellanic Clouds, for whichindependent information is available on the spectral types and effectivetemperatures of the ionizing stars. The spectra have been used toevaluate nebular diagnostics of stellar temperature, metal abundance,and ionization parameter, and to compare the observed behavior of theline indices with predictions of nebular photoionization models. Weobserve a strong degeneracy between forbidden-line sequences produced bychanges in stellar Teff and metal abundance, which severelycomplicates the application of many forbidden-line diagnostics toextragalactic H II regions. Our data confirm however that the Edmunds& Pagel [O II]+[O III] abundance index and the Vílchez &Pagel η' index provide more robust diagnostics of metalabundance and stellar effective temperature, respectively. A comparisonof the fractional helium ionization of the H II regions with stellartemperature confirms the reliability of the spectral type versusTeff calibration for the relevant temperature rangeTeff<=38,000 K. We use empirical relations between thenebular hardness indices and Teff to reinvestigate the casefor systematic variations in the stellar effective temperatures and theupper initial mass functions of massive stars in extragalactic H IIregions. The data are consistent with a significant softening of theionizing spectra (consistent with cooler stellar temperatures) withincreasing metal abundance, especially for Z<=Zsolar.However, unresolved degeneracies between Z and Teff stillcomplicate the interpretation of this result.
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