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The Calibration of the O/H Abundance Indicators for Extragalactic H II Regions based on O II Recombination Lines
Based on O II recombination lines we present a new calibration (called OII_{RL}) of Pagel's O_{23} indicator to determine the O/H abundanceratio in extragalactic H II regions and emission line galaxies. The OII_{RL} calibration produces O/H abundances about a factor of two higherthan those derived from the T(4363) method with t^2 = 0.00. The OII_{RL} calibration has implications for the study of differentproperties of emission line galaxies such as their metallicity, starformation rate, and initial mass function. The O II_{RL} calibrationalso affects the abundance determinations based on other O/H indicators,that include collisionally excited lines, like those known as O_3N_2,N_2, S_{23], Ar_3O_3, and S_3O_3. We argue that the controversy betweenthe T(4363) method and the photoionization models method to derive O/Hvalues is mainly due to temperature variations inside the observed H IIregions.
| The VLT-FLAMES survey of massive stars: wind properties and evolution of hot massive stars in the Large Magellanic Cloud
We have studied the optical spectra of a sample of 28 O- and earlyB-type stars in the Large Magellanic Cloud, 22 of which are associatedwith the young star forming region N11. Our observations sample thecentral associations of LH9 and LH10, and the surrounding regions.Stellar parameters are determined using an automated fitting method(Mokiem et al. 2005), which combines the stellar atmosphere codefastwind (Puls et al. 2005) with the genetic algorithm basedoptimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively.The age difference and relative distance of the associations areconsistent with a sequential star formation scenario in which stellaractivity in LH9 triggered the formation of LH10. Our sample containsfour stars of spectral type O2. From helium and hydrogen line fitting wefind the hottest three of these stars to be 49{-}54 kK (compared to45{-}46 kK for O3 stars). Detailed determination of the helium massfraction reveals that the masses of helium enriched dwarfs and giantsderived in our spectroscopic analysis are systematically lower thanthose implied by non-rotating evolutionary tracks. We interpret this asevidence for efficient rotationally enhanced mixing leading to thesurfacing of primary helium and to an increase of the stellarluminosity. This result is consistent with findings for SMC stars byMokiem et al. (2006). For bright giants and supergiants no such massdiscrepancy is found; these stars therefore appear to follow tracks ofmodestly or non-rotating objects. The set of programme stars wassufficiently large to establish the mass loss rates of OB stars in thisZ ˜ 1/2 Zȯ environment sufficiently accurate toallow for a quantitative comparison with similar objects in the Galaxyand the SMC. The mass loss properties are found to be intermediate tomassive stars in the Galaxy and SMC. Comparing the derived modified windmomenta D_mom as a function of luminosity with predictions for LMCmetallicities by Vink et al. (2001) yields good agreement in the entireluminosity range that was investigated, i.e. 5.0 < logL/Lȯ< 6.1.Appendix A is only available in electronic form at http://www.aanda.org
| The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry
We present echelle spectrophotometry of the blue compact dwarf galaxyNGC 5253 obtained with the VLT UVES. We have measured the intensities ofa large number of permitted and forbidden emission lines in four zonesof the central part of the galaxy. We detect faint C II and O IIrecombination lines, the first time that these are unambiguouslydetected in a dwarf starburst galaxy. The physical conditions of theionized gas have been derived using a large number of different lineintensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fehave been determined following standard methods. C++ andO++ abundances have been derived from pure recombinationlines and are larger than those obtained from collisionally excitedlines (from 0.30 to 0.40 dex for C++ and from 0.19 to 0.28dex for O++). This result is consistent with a temperaturefluctuation parameter (t2) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized Nenrichment in certain zones of NGC 5253 and detect a possible slight Heoverabundance in the same zones. The enrichment pattern agrees with thatexpected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. Theamount of enriched material needed to produce the observed overabundanceis consistent with the mass lost by the number of W-R stars estimated inthe starbursts. We discuss the possible origin of the difference betweenabundances derived from recombination and collisionally excited lines(the so-called abundance discrepancy problem) in H II regions, findingthat a recent hypothesis based on the delayed enrichment by SN ejectainclusions seems not to explain the observed features.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 70.C-0008(A).
| Near-Infrared Observations of N11 in the Large Magellanic Cloud: Triggered Star Formation around the Periphery of LH 9
Near-infrared observations have been carried out to survey young stellarobjects in the second-largest H II region in the Large Magellanic Cloud,N11. A total area of about 700 arcmin2 is covered in the J,H, and KS bands. We selected a total of 559 OB and 127 HerbigAe/Be star candidates out of the detected sources based on theirnear-infrared colors and magnitudes. The existence of these youngstellar objects indicates that star formation activity is underway inthe whole N11 region. Many Herbig Ae/Be star candidates are distributedaround the periphery of the OB association LH 9. Spatial correlations ofthe OB and Herbig Ae/Be star candidates with the objects observed atother wavelengths (optical, radio continuum, Hα, CO, and X-ray)suggest that the birth of the young stellar populations in peripheralmolecular clouds was triggered originally by LH 9. It is likely that thetrigger for this star formation was an expanding supershell blown by theOB association. In N11 a new generation of stars would have been formedin the clouds developed from swept-up interstellar medium.
| Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| The temperature and ionization structure of the emitting gas in HII galaxies: implications for the accuracy of abundance determinations
We propose a methodology to perform a self-consistent analysis of thephysical properties of the emitting gas of HII galaxies adequate to thedata that can be obtained with the 21st century technology. Thismethodology requires the production and calibration of empiricalrelations between the different line temperatures that should supersedecurrently used ones based on very simple, and poorly tested,photoionization model sequences.As a first step to reach these goals, we have obtained simultaneous blueto far red long-slit spectra with the William Herschel Telescope (WHT)of three compact HII galaxies selected from the Sloan Digital Sky Survey(SDSS) Data Release 2 (DR2) spectral catalogue using the INAOE VirtualObservatory superserver. Our spectra cover the range from 3200 to10500Å, including the Balmer jump, the [OII]λλ3727,29Å lines, the [SIII]λλ9069, 9532Å doublet aswell as various weak auroral lines such as [OIII]λ4363Å and[SIII]λ6312Å.For the three objects, we have measured at least four line temperatures,T([OIII]), T([SIII]), T([OII]) and T([SII]), and the Balmer continuumtemperature T(Bac). These measurements and a careful and realistictreatment of the observational errors yield total oxygen abundances withaccuracies between 5 and 9 per cent. These accuracies are expected toimprove as better calibrations based on more precise measurements, bothon electron temperatures and densities, are produced.We have compared our obtained spectra with those downloaded from theSDSS DR3 finding a satisfactory agreement. The analysis of these spectrayields values of line temperatures and elemental ionic and totalabundances which are in general agreement with those derived from theWHT spectra, although for most quantities they can only be taken asestimates since, due to the lack of direct measurements of the requiredlines, theoretical models had to be used whose uncertainties areimpossible to quantify.The ionization structure found for the observed objects from theO+/O2+ and S+/S2+ ratiospoints to high values of the ionizing radiation, as traced by the valuesof the `softness parameter' η which is less than 1 for the threeobjects. The use of line temperatures derived from T([OIII]) based oncurrent photoionization models yields for the two highest excitationobjects, much higher values of η which would imply lower ionizingtemperatures. This is, however, inconsistent with the ionizationstructure as probed by the measured emission-line intensities.Finally, we have measured the T(Bac) for the three observed objects andderived temperature fluctuations. Only for one of the objects, thetemperature fluctuation is significant and could lead to higher oxygenabundances by about 0.20dex.
| The VLT-FLAMES survey of massive stars: observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large ArrayMulti-Element Spectrograph (FLAMES) instrument in fields centered on theclusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), andNGC 2004 and the N11 region in the Large Magellanic Cloud (LMC). Afurther 14 stars were observed in the N11 and NGC 330 fields using theUltraviolet and Visual Echelle Spectrograph (UVES) for a separateprogramme. Spectral classifications and stellar radial velocities aregiven for each target, with careful attention to checks for binarity. Inparticular, we have investigated previously unexplored regions aroundthe central LH9/LH10 complex of N11, finding ~25 new O-type stars fromour spectroscopy. We have observed a relatively large number of Be-typestars that display permitted Fe II emission lines. These are primarilynot in the cluster cores and appear to be associated with classicalBe-type stars, rather than pre main-sequence objects. The presence ofthe Fe II emission, as compared to the equivalent width of Hα, isnot obviously dependent on metallicity. We have also explored therelative fraction of Be- to normal B-type stars in the field-regionsnear to NGC 330 and NGC 2004, finding no strong evidence of a trend withmetallicity when compared to Galactic results. A consequence of serviceobservations is that we have reasonable time-sampling in three of ourFLAMES fields. We find lower limits to the binary fraction of O- andearly B-type stars of 23 to 36%. One of our targets (NGC 346-013) isespecially interesting with a massive, apparently hotter, less luminoussecondary component.
| IR Study of N11 in the LMC
N11 is a large complex in the LMC with many regions of star formation atdifferent evolution stages. It is characterized by a huge cavity (80×60pc) with a 5 Myr central cluster. It is surrounded by severalionized clouds where young where the youngest O star population isdeveloping (Walborn & Parker 1992, ApJ, 399, 87; Barbá et al.2003, AJ, 125, 1940). Current star formation is taking place in N11A andN11B. New millimeter data (CO and continuum) show that dust isassociated ``with the young stellar population, while the central cavityis relatively clean''. In this work we present a morphological study ofthe gas in this region obtained with observation of the IR emission gaslines Br γ and Paschen β. These images are compared to Hα and molecular gas images of N11. The main goal of this studywill consist in a study of the extinction in this region usingmultiwavelength imaging of the gas and dust.
| Faint emission lines in the Galactic HII regions M16, M20 and NGC 3603*
We present deep echelle spectrophotometry of the Galactic HII regionsM16, M20 and NGC 3603. The data have been taken with the Very LargeTelescope Ultraviolet-Visual Echelle Spectrograph in the 3100-10400Å range. We have detected more than 200 emission lines in eachregion. Physical conditions have been derived using different continuumand line intensity ratios. We have derived He+,C++ and O++ abundances from pure recombinationlines as well as collisionally excited lines (CELs) for a large numberof ions of different elements. We have obtained consistent estimationsof the temperature fluctuation parameter, t2, using differentmethods. We also report the detection of deuterium Balmer lines up toDδ (M16) and to Dγ (M20) in the blue wings of the hydrogenlines, which excitation mechanism seems to be continuum fluorescence.The temperature fluctuation paradigm agrees with the results obtainedfrom optical CELs, and the more uncertain ones from far-infraredfine-structure CELs in NGC 3603, although, more observations coveringthe same volume of the nebula are necessary to obtain solid conclusions.
| The VLT-FLAMES survey of massive stars.
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| 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 VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611
We introduce a new survey of massive stars in the Galaxy and theMagellanic Clouds using the Fibre Large Array Multi-Element Spectrograph(FLAMES) instrument at the Very Large Telescope (VLT). Here we presentobservations of 269 Galactic stars with the FLAMES-Giraffe Spectrograph(R ≃ 25 000), in fields centered on the open clusters NGC 3293,NGC 4755 and NGC 6611. These data are supplemented by a further 50targets observed with the Fibre-Fed Extended Range Optical Spectrograph(FEROS, R = 48 000). Following a description of our scientificmotivations and target selection criteria, the data reduction methodsare described; of critical importance the FLAMES reduction pipeline isfound to yield spectra that are in excellent agreement with lessautomated methods. Spectral classifications and radial velocitymeasurements are presented for each star, with particular attention paidto morphological peculiarities and evidence of binarity. Theseobservations represent a significant increase in the known spectralcontent of NGC 3293 and NGC 4755, and will serve as standards againstwhich our subsequent FLAMES observations in the Magellanic Clouds willbe compared.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| Statistical Confirmation of a Stellar Upper Mass Limit
We derive the expectation value for the maximum stellar mass(mmax) in an ensemble of N stars, as a function of theinitial mass function (IMF) upper mass cutoff (mup) and N. Westatistically demonstrate that the upper IMF of the local massive starcensus observed thus far in the Milky Way and Magellanic Clouds clearlyexhibits a universal upper mass cutoff around 120-200 Msolarfor a Salpeter IMF, although the result is more ambiguous for a steeperIMF.
| Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| A deep survey of heavy element lines in planetary nebulae - II. Recombination-line abundances and evidence for cold plasma
In our Paper I, we presented deep optical observations of the spectra of12 Galactic planetary nebulae (PNe) and three Magellanic Cloud PNe,carrying out an abundance analysis using the collisionally excitedforbidden lines. Here, we analyse the relative intensities of faintoptical recombination lines (ORLs) from ions of carbon, nitrogen andoxygen in order to derive the abundances of these ions relative tohydrogen. The relative intensities of four high-l CII recombinationlines with respect to the well-known 3d-4f λ4267 line are foundto be in excellent agreement with the predictions of recombinationtheory, removing uncertainties about whether the high C2+abundances derived from the λ4267 line could be due tonon-recombination enhancements of its intensity.We define an abundance discrepancy factor (ADF) as the ratio of theabundance derived for a heavy element ion from its recombination linesto that derived for the same ion from its ultraviolet, optical orinfrared collisionally excited lines (CELs). All of the PNe in oursample are found to have ADFs that exceed unity. Two of the PNe, NGC2022 and LMC N66, have O2+ ADFs of 16 and 11, respectively,while the remaining 13 PNe have a mean O2+ ADF of 2.6, withthe smallest value being 1.8.Garnett and Dinerstein found that for a sample of about 12 PNe themagnitude of the O2+ ADF was inversely correlated with thenebular Balmer line surface brightness. We have investigated this for alarger sample of 20 PNe, finding weak correlations with decreasingsurface brightness for the ADFs of O2+ and C2+.The C2+ ADFs are well correlated with the absolute radii ofthe nebulae, although no correlation is present for the O2+ADFs. We also find both the C2+ and O2+ ADFs to bestrongly correlated with the magnitude of the difference between thenebular [OIII] and Balmer jump electron temperatures (ΔT),corroborating a result of Liu et al. for the O2+ ADF.ΔT is found to be weakly correlated with decreasing nebularsurface brightness and increasing absolute nebular radius.There is no dependence of the magnitude of the ADF upon the excitationenergy of the ultraviolet, optical or infrared CEL transition used,indicating that classical nebular temperature fluctuations - i.e. in achemically homogeneous medium - are not the cause of the observedabundance discrepancies. Instead, we conclude that the main cause of thediscrepancy is enhanced ORL emission from cold ionized gas located inhydrogen-deficient clumps inside the main body of the nebulae, as firstpostulated by Liu et al. for the high-ADF PN, NGC 6153. We havedeveloped a new electron temperature diagnostic, based upon the relativeintensities of the OII 4f-3d λ4089 and 3p-3s λ4649recombination transitions. For six out of eight PNe for which bothtransitions are detected, we derive O2+ ORL electrontemperatures of <=300 K, very much less than the O2+forbidden-line and H+ Balmer jump temperatures derived forthe same nebulae. These results provide direct observational evidencefor the presence of cold plasma regions within the nebulae, consistentwith gas cooled largely by infrared fine-structure transitions; at suchlow temperatures, recombination transition intensities will besignificantly enhanced due to their inverse power-law temperaturedependence, while ultraviolet and optical CELs will be significantlysuppressed.
| XMM-Newton observations of the giant H II region N 11 in the LMC
Using the sensitive XMM-Newton observatory, we have observed the giant HII region N 11 in the LMC for ˜30 ks. We have detected severallarge areas of soft diffuse X-ray emission along with 37 point sources.One of the most interesting results is the possible association of afaint X-ray source with BSDL 188, a small extended object of uncertainnature.The OB associations in the field-of-view (LH9, LH10 and LH13) are alldetected with XMM-Newton, but they appear very different from oneanother. The diffuse soft X-ray emission associated with LH9 peaks nearHD 32228, a dense cluster of massive stars. The combined emission of allindividual massive stars of LH9 and of the superbubble they have createdis not sufficient to explain the high level of emission observed: hiddenSNRs, colliding-wind binaries and the numerous pre-main sequence starsof the cluster are most likely the cause of this discrepancy. Thesuperbubble may also be leaking some hot gas in the ISM since faint,soft emission can be observed to the south of the cluster. The X-rayemission from LH10 consists of three pointlike sources and a softextended emission of low intensity. The two brightest point sources areclearly associated with the fastest expanding bubbles blown by hot starsin the SW part of the cluster. The total X-ray emission from LH10 israther soft, although it presents a higher temperature than the othersoft emissions of the field. The discrepancy between the combinedemission of the stars and the observed luminosity is here less severethan for LH9 and could be explained in terms of hot gas filling thewind-blown bubbles. On the other hand, the case of LH13 is different: itdoes not harbour any extended emission and its X-ray emission could mostprobably be explained by the Sk -66°41 cluster alone.Finally, our XMM-Newton observation included simultaneous observationswith the OM camera that provide us with unique UV photometry of morethan 6000 sources and enable the discovery of the UV emission from theSNR N11L.Based on observations collected with XMM-Newton, an ESA Science Missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA).Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr /cgi-bin/qcat?J/A+A/418/841
| A deep survey of heavy element lines in planetary nebulae - I. Observations and forbidden-line densities, temperatures and abundances
We present deep optical spectrophotometry of 12 Galactic planetarynebulae (PNe) and three Magellanic Cloud PNe. Nine of the Galactic PNewere observed by scanning the slit of the spectrograph across thenebula, yielding relative line intensities for the entire nebula thatare suitable for comparison with integrated nebular fluxes measured inother wavelength regions. In this paper we use the fluxes ofcollisionally excited lines (CELs) from the nebulae to derive electrondensities and temperatures, and ionic abundances. We find that thenebular electron densities derived from optical CEL ratios aresystematically higher than those derived from the ratios of the infrared(IR) fine-structure (FS) lines of [OIII]. The latter have lower criticaldensities than the typical nebular electron densities derived fromoptical CELs, indicating the presence of significant density variationswithin the nebulae, with the IR CELs being biased towards lower densityregions.We find that for several nebulae the electron temperatures obtained from[OII] and [NII] optical CELs are significantly affected by recombinationexcitation of one or more of the CELs. When allowance is made forrecombination excitation, much better agreement is obtained with theelectron temperatures obtained from optical [OIII] lines. We alsocompare electron temperatures obtained from the ratio of optical nebularto auroral [OIII] lines with temperatures obtained from the ratio of[OIII] optical lines to [OIII] IR FS lines. We find that when the latterare derived using electron densities based on the [OIII]52 μm/88μm line ratio, they yield values that are significantly higher thanthe optical [OIII] electron temperatures. In contrast to this, [OIII]optical/IR temperatures derived using the higher electron densitiesobtained from optical [ClIII]λ5517/λ5537 ratios show muchcloser agreement with optical [OIII] electron temperatures, implyingthat the observed [OIII] optical/IR ratios are significantly weighted bydensities in excess of the critical densities of both [OIII] FS lines.Consistent with this, ionic abundances derived from [OIII] and [NIII] FSlines using electron densities from optical CELs show much betteragreement with abundances derived for the same ions from optical andultraviolet CELs than do abundances derived from the FS lines using thelower electron densities obtained from the observed [OIII]52 μm/88μm ratios. The behaviour of these electron temperatures, obtainedmaking use of the temperature-insensitive [OIII] IR FS lines, providesno support for significant temperature fluctuations within the nebulaebeing responsible for derived Balmer jump electron temperatures that arelower than temperatures obtained from the much more temperaturesensitive [OIII] optical lines.
| Three-dimensional photoionization modelling of the hydrogen-deficient knots in the planetary nebula Abell 30
We have constructed a photoionization model, using the three-dimensionalMonte Carlo code MOCASSIN, for one of the hydrogen-deficient knots (J3)of the born-again planetary nebula Abell 30. The model consists ofspherical knots, comprising a cold, dense, hydrogen-deficient core withvery high metal abundances. The inner core, occupying 9.1 per cent ofthe total volume of the knot, is surrounded by a less densehydrogen-deficient and metal-enriched gas envelope, with less extremeabundances. The envelope of the knot might have been formed by themixing of the knot material with the surrounding nebular gas.This bi-chemistry, bi-density model did not produce enough heating tomatch the fluxes of the collisionally excited emission lines (CELs) andof the optical recombination lines (ORLs) observed in the spectrum ofthe knot. We therefore included heating by photoelectric emission fromdust grains in the thermal equilibrium calculations, and found thatdust-to-gas ratios of 0.077 and 0.107 by mass for the central core andthe envelope of the knot, respectively, are sufficient to fit thespectrum. Surprisingly, photoelectric emission from grains is thedominant source of heating in the hot envelope of the knot, whileheating by photoionization of helium and heavy elements dominates in thecold core.We obtain a good fit with the observations for most of the significantemission lines treated in our model. The two major discrepanciesoccurred for the [O II] 3727,29-Å doublet and the [N II]6548,6584-Å lines, which are severely underestimated in our model.Recombination contributions could be significant and we included themfor the O II transitions. However, this was not sufficient to resolvethe discrepancy, due to the high collisional de-excitation rates in thedense core, where most of the recombination lines would be produced.This possibly highlights a weakness in using a discontinuous densitydistribution like ours, where in reality one might expect anintermediate phase to exist.The chemical abundances inferred from our modelling of the central coreregion and of the envelope of the knot are, at least qualitatively, inagreement with the abundances derived by the empirical analysis ofWesson, Liu & Barlow, although the discrepancies between the coreand the envelope abundances that we find are less dramatic than thoseimplied by the ORL and CEL empirical analysis. Our models also indicate,in agreement with the empirical analysis of Wesson et al., that the C/Oratio in the two regions of the knot is less than unity, contrary totheoretical predictions for born-again nebulae.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| 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.
| Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula
We present Hubble Space Telescope (HST)/STIS long-slit spectroscopy ofNGC 1976. Our goal is to measure the intrinsic line ratio [OIII]4364/5008 and thereby evaluate the electron temperature (Te)and the fractional mean-square Te variation(t2A)across the nebula. We also measure theintrinsic line ratio [NII] 5756/6585 in order to estimate Teand t2A in the N+ region. Theinterpretation of the [NII] data is not as clear cut as the [OIII] databecause of a higher sensitivity to knowledge of the electron density aswell as a possible contribution to the [NII] 5756 emission byrecombination (and cascading). We present results from binning the dataalong the various slits into tiles that are 0.5 arcsec square (matchingthe slit width). The average [OIII] temperature for our four HST/STISslits varies from 7678 K to 8358 K; t2A variesfrom 0.00682 to at most 0.0176. For our preferred solution, the average[NII] temperature for each of the four slits varies from 9133 to 10232K; t2A varies from 0.00584 to 0.0175. Themeasurements of Te reported here are an average along eachline of sight. Therefore, despite finding remarkably lowt2A, we cannot rule out significantly largertemperature fluctuations along the line of sight. The result that theaverage [NII]Te exceeds the average [OIII]Teconfirms what has been previously found for Orion and what is expectedon theoretical grounds. Observations of the proplyd P159-350 indicate:large local extinction associated; ionization stratification consistentwith external ionization by θ1 Ori C; and indirectly,evidence of high electron density.
| Active Star Formation in the N11B Nebula in the Large Magellanic Cloud: A Sequential Star Formation Scenario Confirmed
The second largest H II region in the Large Magellanic Cloud, N11B hasbeen surveyed in the near-IR. We present JHKs images of theN11B nebula. These images are combined with CO (1-->0) emission-linedata and with archival New Technology Telescope and Hubble SpaceTelescope WFPC2 optical images to address the star formation activity ofthe region. IR photometry of all the IR sources detected is given. Weconfirm that a second generation of stars is currently forming in theN11B region. Our IR images show the presence of several bright IRsources that appear to be located toward the molecular cloud as seenfrom the CO emission in the area. Several of these sources show IRcolors with young stellar object characteristics, and they are primecandidates to be intermediate-mass Herbig Ae/Be stars. For the firsttime, an extragalactic methanol maser is directly associated with IRsources embedded in a molecular core. Two IR sources are found at 2"(0.5 pc) of the methanol maser reported position. Additionally, wepresent the association of the N11A compact H II region to the moleculargas, where we find that the young massive O stars have eroded a cavityin the parental molecular cloud, typical of a champagne flow. The N11region turns out to be a very good laboratory for studying theinteraction of winds, UV radiation, and molecular gas. Severalphotodissociation regions are found.Based in part on observations with the NASA/ESA Hubble Space Telescopeobtained from the archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. IX. The giant LMC HII region complex N 11
The second-brightest star formation complex in the Large MagellanicCloud, N 11, was surveyed extensively in the J = 1-0 transition of12CO. In this paper we present maps and a cataloguecontaining the parameters of 29 individual molecular clouds in thecomplex, although more may be present. The distribution of molecular gasin the N 11 complex is highly structured. In the southwestern part of N11, molecular clouds occur in a ring or shell surrounding the major OBstar association LH 9. In the northeastern part, a chain of molecularclouds delineates the rim of one of the so-called supergiant shells inthe LMC. There appears to be very little diffuse molecular gasin-between the individual well-defined clouds, especially in thesouthwestern ring. Most of the clouds have dimensions only slightlylarger than those of the survey beam, i.e. diameters of 25 pc or less. Asubset of the clouds mapped in J= 1-0 12CO transition wasalso observed in the J= 2-1 12CO transition, and in thecorresponding transitions of 13CO. Clouds mapped in J= 2-112CO with a two times higher angular resolution show further,clear substructure. The elements of this substructure, however, havedimensions once again comparable to those of the mapping beam. For a fewclouds, sufficient information was available to warrant an attempt atmodelling their physical parameters. They contain fairly warm(Tkin = 60-150 K) and moderately dense (nH_2 =3000 cm-3) gas. The northeastern chain of CO clouds, althoughlacking in diffuse intercloud emission, is characteristic of the morequiescent regions of the LMC, and appears to have been subject torelatively little photo-processing. The clouds forming part of thesouthwestern shell or ring, however, are almost devoid of diffuseintercloud emission, and also exhibit other characteristics of anextreme photon-dominated region (PDR).
| 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.
| Bubble Nebulae around Ultraluminous X-Ray Sources
The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearbygalaxies continues to be an enigma, since their adopted isotropichigh-energy output would surpass the Eddington limit of even the mostmassive stellar black holes. Many ultraluminous X-ray sources aresurrounded by emission nebulae that show indications of both shockionization and X-ray ionization. Relatively compact X-ray ionizednebulae can be used to independently infer the luminosities, and thus toexclude possible beaming effects into our line of sight. Largerbubble-like nebulae reach several hundred parsec diameters and provideimportant information on the formation and/or mass loss history of ULX.We point out the close relationship to microquasars and the previouslyunique SS 433 system with its radio nebula W 50.
| Heavy elements in Galactic and Magellanic Cloud HII regions: recombination-line versus forbidden-line abundances
We have obtained deep optical, long-slit spectrophotometry of theGalactic HII regions M 17, NGC 3576 and of the Magellanic Cloud HIIregions 30 Doradus, LMC N11B and SMC N66, recording the opticalrecombination lines (ORLs) of CII, NII and OII. A spatial analysis of 30Doradus is performed, revealing that the forbidden-line [OIII] electrontemperature is remarkably constant across the nebula. The forbidden-lineO2+/H+ abundance mapped by the [OIII]λ4959collisionally excited line (CEL) is shown to be consistently lower thanthe recombination-line abundance mapped by the OII V1 multiplet at 4650Å. In addition, the spatial profile of theC2+/O2+ ratio derived purely from recombinationlines is presented for the first time for an extragalactic nebula.Temperature-insensitive ORL C2+/O2+ andN2+/O2+ ratios are obtained for all nebulae exceptSMC N66. The ORL C2+/O2+ ratios show remarkableagreement within each galactic system, while also being in agreementwith the corresponding CEL ratios. The disagreement found between theORL and CEL N2+/O2+ ratios for M 17 and NGC 3576can be attributed to the NII V3 and V5 ORLs that were used beingaffected by fluorescent excitation effects.For all five nebulae, the O2+/H+ abundance derivedfrom multiple OII ORLs is found to be higher than the correspondingvalue derived from the strong [OIII]λλ4959, 5007 CELs, byfactors of 1.8 to 2.7 for four of the nebulae. The LMC N11B nebulaexhibits a more extreme discrepancy factor for the O2+ ion,~5. Thus, these HII regions exhibit ORL/CEL abundance discrepancyfactors that are similar to those previously encountered amongstplanetary nebulae.Our optical CEL O2+/H+ abundances agree to within20-30 per cent with published O2+/H+ abundancesthat have been obtained from observations of infrared fine-structurelines. Since the low excitation energies of the latter make theminsensitive to variations about typical nebular temperatures,fluctuations in temperature are ruled out as the cause of the observedORL/CEL O2+ abundance discrepancies. We present evidence thatthe observed OII ORLs from these HII regions originate from gas of verysimilar density (<3500 cm-3) to that emitting the observedheavy-element optical and infrared CELs, ruling out models that employhigh-density ionized inclusions in order to explain the abundancediscrepancy. We consider a scenario whereby much of the heavy-elementORL emission originates from cold (<=500 K) metal-rich ionizedregions. These might constitute haloes that are being evaporated frommuch denser neutral cores. The origin of these metal-rich inclusions isnot clear - they may have been ejected into the nebula by evolved,massive Of and Wolf-Rayet stars, although the agreement found betweenheavy-element ion ratios derived from ORLs with the ratios derived fromCELs provides no evidence for nuclear-processed material in theORL-emitting regions.
| The relation between radio flux density and ionising ultra-violet flux for HII regions and supernova remnants in the Large Magellanic Cloud
We present a comparison between the Parkes radio surveys (Filipovic etal. 1995) and Vacuum Ultra-Violet (VUV) surveys (Smith et al. 1987) ofthe Large Magellanic Clouds (LMC). We have found 72 sources in common inthe LMC which are known HII regions (52) and supernova remnants (SNRs)(19). Some of these radio sources are associated with two or more UVstellar associations. A comparison of the radio flux densities andionising UV flux for HII regions shows a very good correlation, asexpected from theory. Many of the Magellanic Clouds (MCs) SNRs areembedded in HII regions, so there is also a relation between radio andUV which we attribute to the surrounding HII regions.
| 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.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/547
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