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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 chemical composition of the Galactic H II regions M8 and M17. A revision based on deep VLT echelle spectrophotometry
We present new echelle spectrophotometry of the Galactic H II regions M8and M17. The data have been taken with the VLT UVES echelle spectrographin the 3100 to 10400 Å range. We have measured the intensities of375 and 260 emission lines in M8 and M17 respectively, increasingsignificatively the number of emission lines measured in previousspectrophotometric studies of these nebulae. Most of the detected linesare permitted lines. Electron temperatures and densities have beendetermined using different diagnostics. We have derived He^+,C++, O^+ and O^{++} ionic abundances from pure recombinationlines. We have also derived abundances from collisionally excited linesfor a large number of ions of different elements. Highly consistentestimations of t^2 have been obtained by using different independentindicators; the values are moderate and very similar to those obtainedin other Galactic H II regions. We report the detection of deuteriumBalmer emission lines, up to Dɛ, in M8 and show that theirintensities are consistent with continuum fluorescence as their mainexcitation mechanism.

The Young Stellar Population in M17 Revealed by Chandra
We report here results from a Chandra ACIS observation of the stellarpopulations in and around the M17 H II region. The field reveals 886sources with observed X-ray luminosities (uncorrected for absorption)between ˜29.3 ergs s-1< log LX<32.8ergs s-1, 771 of which have stellar counterparts in infraredimages. In addition to comprehensive tables of X-ray source properties,several results are presented:1. The X-ray luminosity function is calibrated to that of the OrionNebula Cluster population to infer a total population of roughly8000-10,000 stars in M17, one-third lying in the central NGC 6618cluster.2. About 40% of the ACIS sources are heavily obscured withAV>10 mag. Some are concentrated around well-studiedstar-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, andM17-North -- but most are distributed across the field. As previouslyshown, star formation appears to be widely distributed in the molecularclouds. X-ray emission is detected from 64 of the hundreds of Class Iprotostar candidates that can be identified by near- and mid-infraredcolors. These constitute the most likely protostar candidates known inM17.3. The spatial distribution of X-ray stars is complex: in addition tothe central NGC 6618 cluster and well-known embedded groups, we find anew embedded cluster (designated M17-X), a 2 pc long arc of young starsalong the southwest edge of the M17 H II region, and 0.1 pc substructurewithin various populations. These structures may indicate that thepopulations are dynamically young.4. All (14/14) of the known O stars but only about half (19/34) of theknown B0-B3 stars in the M17 field are detected. These stars exhibitthe long-reported correlation between X-ray and bolometric luminositiesof LX˜10-7Lbol. While many O andearly-B stars show the soft X-ray emission expected from microshocks intheir winds or moderately hard emission that could be caused bymagnetically channeled wind shocks, six of these stars exhibit very hardthermal plasma components (kT>4 keV) that may be due to collidingwind binaries. More than 100 candidate new OB stars are found, including28 X-ray detected intermediate- and high-mass protostar candidates withinfrared excesses.5. Only a small fraction (perhaps 10%) of X-ray selected high- andintermediate-mass stars exhibit K-band-emitting protoplanetary disks,providing further evidence that inner disks evolve very rapidly aroundmore massive stars.

s-Process Abundances in Planetary Nebulae
The s-process should occur in all but the lower mass progenitor stars ofplanetary nebulae, and this should be reflected in the chemicalcomposition of the gas that is expelled to create the current planetarynebula shell. Weak forbidden emission lines are expected from severals-process elements in these shells and have been searched for and insome cases detected in previous investigations. Here we extend thesestudies by combining very high signal-to-noise ratio echelle spectra ofa sample of PNe with a critical analysis of the identification of theemission lines of Z>30 ions. Emission lines of Br, Kr, Xe, Rb, Ba,and Pb are detected with a reasonable degree of certainty in at leastsome of the objects studied here, and we also tentatively identify linesfrom Te and I, each in one object. The strengths of these lines indicateenhancement of s-process elements in the central star progenitors, andwe determine the abundances of Br, Kr, and Xe, elements for which atomicdata relevant for abundance determination have recently becomeavailable. As representative elements of the ``light'' and ``heavy''s-process peaks, Kr and Xe exhibit similar enhancements over solarvalues, suggesting that PN progenitors experience substantial neutronexposure.

The Galactic distribution of magnetic fields in molecular clouds and HII regions
Aims.Magnetic fields exist on all scales in our Galaxy. There is acontroversy about whether the magnetic fields in molecular clouds arepreserved from the permeated magnetic fields in the interstellar medium(ISM) during cloud formation. We investigate this controversy usingavailable data in the light of the newly revealed magnetic fieldstructure of the Galactic disk obtained from pulsar rotation measures(RMs). Methods: We collected measurements of the magnetic fields inmolecular clouds, including Zeeman splitting data of OH masers in cloudsand OH or HI absorption or emission lines of clouds themselves. Results: The Zeeman data show structures in the sign distribution of theline-of-sight component of the magnetic field. Compared to thelarge-scale Galactic magnetic fields derived from pulsar RMs, the signdistribution of the Zeeman data shows similar large-scale fieldreversals. Previous such examinations were flawed in the over-simplifiedglobal model used for the large-scale magnetic fields in the Galacticdisk. Conclusions: .We conclude that the magnetic fields in the cloudsmay still "remember" the directions of magnetic fields in the GalacticISM to some extent, and could be used as complementary tracers of thelarge-scale magnetic structure. More Zeeman data of OH masers in widelydistributed clouds are required.Tables 1 and 2 are only available in electronic form athttp://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).

VLT K-band spectroscopy of massive young stellar objects in (ultra-)compact HII regions
High-quality K-band spectra of strongly reddened point sources, deeplyembedded in (ultra-)compact H II, have revealed a population of 20 youngmassive stars showing no photospheric absorption lines, but sometimesstrong Brγ emission. The Brγ equivalent widths occupy a widerange (from about 1 to over 100 Å); the line widths of 100-200 kms-1 indicate a circumstellar rather than a nebular origin.The K-band spectra exhibit one or more features commonly associated withmassive young stellar objects (YSOs) surrounded by circumstellarmaterial: a very red colour (J-K)  2, CO bandhead emission,hydrogen emission lines (sometimes doubly peaked), and Fe II and/or MgII emission lines. The large number of objects in our sample allows amore detailed definition and thorough investigation of the properties ofthe massive YSOs. In the (K, J-K) colour-magnitude diagram (CMD) themassive YSO candidates are located in a region delimited by the OBzero-age main sequence, Be stars, Herbig Ae and Be stars, and B[e]supergiants. The massive YSO distribution in the CMD suggests that themajority of the objects are of similar spectral type as the Herbig Bestars, but some of them are young O stars. The spectral properties ofthe observed objects do not correlate with the location in the CMD. TheCO emission must come from a relatively dense (˜ 1010cm-3) and hot (T˜ 2000-5000 K) region, sufficientlyshielded from the intense UV radiation field of the young massive star.The hydrogen emission is produced in an ionised medium exposed to UVradiation. The best geometrical solution is a dense and neutralcircumstellar disk causing the CO bandhead emission, and an ionisedupper layer where the hydrogen lines are produced. We present argumentsthat the circumstellar disk is more likely a remnant of the accretionprocess than the result of rapid rotation and mass loss such as inBe/B[e] stars.

Balmer and Paschen Jump Temperature Determinations in Low-Metallicity Emission-Line Galaxies
We have used the Balmer and Paschen jumps to determine the temperaturesof the H+ zones of a total sample of 47 H II regions. TheBalmer jump was used on MMT spectrophotometric data of 22low-metallicity H II regions in 18 blue compact dwarf (BCD) galaxies andof one H II region in the spiral galaxy M101. The Paschen jump was usedon spectra of 24 H II emission-line galaxies selected from the DataRelease 3 of the Sloan Digital Sky Survey (SDSS). To derive thetemperatures, we have used a Monte Carlo technique varying the electrontemperature in the H+ zone, the extinction of the ionized gasand that of the stellar population, the relative contribution of theionized gas to the total emission, and the star formation history to fitthe spectral energy distribution of the galaxies. For the MMT spectra,the fit was done in the wavelength range 3200-5200 Å, whichincludes the Balmer discontinuity, and for the SDSS spectra, in thewavelength range 3900-9200 Å, which includes the Paschendiscontinuity. We find for our sample of H II regions that thetemperatures of the O2+ zones determined from thenebular-to-auroral line intensity ratio of doubly ionized oxygen [O III]λλ(4959+5007)/λ4363 do not differ, in a statisticalsense, from the temperatures of the H+ zones determined fromfitting the Balmer and Paschen jumps and the spectral energydistributions (SEDs). We cannot rule out small temperature differencesof the order of 3%-5%.

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.

L-band (3.5 μm) IR-excess in massive star formation. II. RCW 57/NGC 3576
Context: .We present a JHKsL survey of the massive star forming regionRCW 57 (NGC 3576) based on L-band data at 3.5 μm taken with SPIREX(South Pole Infrared Explorer), and 2MASS JHKs data at 1.25-2.2 μm.This is the second of two papers, the first one concerning a similarJHKsL survey of 30 Doradus. Aims: .Colour-colour andcolour-magnitude diagrams are used to detect sources with infraredexcess. This excess emission is interpreted as coming from circumstellardisks, and hence gives the cluster disk fraction (CDF). Based on the CDFand the age of RCW 57, it is possible to draw conclusions on theformation and early evolution of massive stars. Methods: .Theinfrared excess is detected by comparing the locations of sources inJHKsL colour-colour and L vs. (K_s-L) colour-magnitude diagrams to thereddening band due to interstellar extinction. Results: .A totalof 251 sources were detected. More than 50% of the 209 sources includedin the diagrams have an infrared excess. Conclusions: .Comparisonwith other JHKsL surveys, including the results on 30 Doradus from thefirst paper, support a very high initial disk fraction (>80%) evenfor massive stars, although there is an indication of a possible fasterevolution of circumstellar disks around high mass stars. 33 sources onlyfound in the L-band indicate the presence of heavily embedded, massiveClass I protostars. We also report the detection of diffuse PAHsemission throughout the RCW 57 region.

An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.

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.

Faint C and O recombination lines in H II regions and large telescopes: The case of S311
We present preliminary results on the analysis of very deep echelle UVES(8m VLT) spectra of the Galactic H II region S311 (NGC 2467) locatedoutside the Solar circle. The data cover from 3100 to 10450 Å witha resolution R ~ 8800. We have detected and measured more than 300emission lines, some of them are faint recombination lines ofheavy-element ions. We have derived the O abundances from OI and OIIrecombination lines and the C abundance from CII lines and an IonizationCorrection Factor (ICF). This kind of observations will permit us toderive the Galactic abundance gradient of C and O from nebularrecombination lines, which are almost independent on the realtemperature structure of the nebula.

Chemical Composition of Two H II Regions in NGC 6822 Based on VLT Spectroscopy
We present long-slit spectrophotometry of regions V and X of the LocalGroup irregular galaxy NGC 6822. The data consist of VLT FORSobservations in the 3450-7500 Å range. We have obtained electrontemperatures and densities using different line intensity ratios. Wehave derived the He, C, and O abundances relative to H based onrecombination lines; the abundance ratios among these elements arealmost independent of the temperature structure of the nebulae. We havealso determined the N, O, Ne, S, Cl, and Ar abundances based oncollisionally excited lines; the ratios of these abundances relative tothat of H depend strongly on the temperature structure of the nebulae.The chemical composition of NGC 6822 V is compared with those of theSun, the Orion Nebula, NGC 346 in the SMC, and 30 Doradus in the LMC.The value of O/H derived from recombination lines is in good agreementwith that derived by Venn and coworkers from two A-type supergiants inNGC 6822.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 69.C-0203(A).

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.

Two-dimensional Monte Carlo simulations of HI line formation in massive young stellar object disc winds
Massive young stellar objects (YSOs) are powerful infrared HI lineemitters. It has been suggested that these lines form in an outflow froma disc surrounding the YSO. Here, new two-dimensional Monte Carloradiative transfer calculations are described which test thishypothesis. Infrared spectra are synthesized for a YSO disc wind modelbased on earlier hydrodynamical calculations. The model spectra are inqualitative agreement with the observed spectra from massive YSOs, andtherefore provide support for a disc wind explanation for the HI lines.However, there are some significant differences: the models tend tooverpredict the Brα/Brγ ratio of equivalent widths andproduce line profiles which are slightly too broad and, in contrast totypical observations, are double-peaked. The interpretation of thesedifferences within the context of the disc wind picture and suggestionsfor their resolution via modifications to the assumed disc and outflowstructure are discussed.

Two 2MASS-selected Young Stellar Clusters: Photometry, Spectroscopy, and the Initial Mass Function
We present near-IR (NIR) J, H, and Ks images and K-bandspectroscopy of two newly discovered stellar clusters at differentstages of evolution. Our spectra suggest the presence of massive youngstellar objects in the heavily embedded cluster in the star-formingregion near radio source G353.4-0.4 and an O5-O6 V star in the clusternear radio source G305+00.2. We determine a K-band luminosity function(KLF) for both clusters and an initial mass function (IMF) for thecluster near G305+00.2. The derived IMF slope is Γ=-1.5 if the KLFis used to derive the IMF and is Γ=-0.98 if the color-magnitudediagram (CMD) and spectra are used. The more reliable CMD-based slope isflatter than the Salpeter value usually found for stellar clusters. Wefind that using the KLF alone to derive an IMF is likely to produce anoverly steep slope in stellar clusters subject to variable extinction.

Deep echelle spectrophotometry of S 311, a Galactic HII region located outside the solar circle
We present echelle spectrophotometry of the Galactic HII region S 311.The data have been taken with the Very Large TelescopeUltraviolet-Visual Echelle Spectrograph in the 3100-10400 Årange.We have measured the intensities of 263 emission lines; 178 arepermitted lines of H0, D0 (deuterium),He0, C0, C+, N0,N+, O0, O+, S+,Si0, Si+, Ar0 and Fe0; someof them are produced by recombination and others mainly by fluorescence.Physical conditions have been derived using different continuum- andline-intensity ratios. We have derived He+, C++and O++ ionic abundances from pure recombination lines aswell as abundances from collisionally excited lines for a large numberof ions of different elements. We have obtained consistent estimationsof t2 applying different methods. We have found that thetemperature fluctuations paradigm is consistent with theTe(HeI) versus Te(HI) relation for HII regions, incontrast with what has been found for planetary nebulae. We report thedetection of deuterium Balmer lines up to Dδ in the blue wings ofthe hydrogen lines, whose excitation mechanism seems to be continuumfluorescence.

Millimetric observations of southern H II regions
We report on millimetric continuum observations of two bright compact HII regions, which have been observed for the first time in thisfrequency range. For the two observed regions(G291.6-0.5 and G291.3-0.7), wederive the flux densities at the two observed wavelengths (1.25 and 2mm) as well as the spectral index and the temperature of the surroundingdust by fitting a modified blackbody curve to our results combined withIR values obtained from the literature. We also estimate the dust massand the bolometric luminosity of the two regions.

The [Fe IV] Discrepancy: Constraining the Iron Abundances in Nebulae
We study the current discrepancy between the model-predicted andmeasured concentrations of Fe++ and Fe+3 inionized nebulae. We calculate a set of photoionization models, updatedwith the atomic data relevant to the problem, and compare their resultswith those derived for the available nebulae where both [Fe III] and [FeIV] lines have been measured. Our new model results are closer to themeasured values than the results of previous calculations, but adiscrepancy remains. This discrepancy translates into an uncertainty inthe derived Fe abundances of a factor of up to ~4. We explore thepossible causes of this discrepancy and find that errors in the Featomic data may be the most likely explanation. The discrepancy can befully accounted for by any of the following changes: (1) an increase bya factor of ~10 in the recombination rate (radiative plus dielectronic,or charge transfer) for Fe+3, (2) an increase by a factor of2-3 in the effective collision strengths for Fe++, or (3) adecrease by a factor of 2-3 in the effective collision strengths forFe+3. We derive the Fe abundances implied by these threeexplanations and use the results to constrain the degree of depletion ofFe in our sample nebulae. The Galactic H II regions and planetarynebulae are found to have high depletion factors, with less than 5% oftheir Fe atoms in the gas phase. The extragalactic H II regions (LMC 30Doradus, SMC N88A, and SBS 0335-052) have somewhat lower depletions. Themetal-deficient blue compact galaxy SBS 0335-052 could have from 13% to40% of Fe in the gas phase. The depletions derived for the differentobjects define a trend of increasing depletion at higher metallicities.

The near-infrared extinction law in regions of high AV
We present a spectroscopic study of the shape of the dust-extinction lawbetween 1.0 and 2.2μm towards a set of nine ultracompact HII regionswith AV>~ 15mag. We find some evidence that the reddeningcurve may tend to flatten at higher extinctions, but just over half ofthe sample has extinction consistent with or close to the average forthe interstellar medium (ISM). There is no evidence of extinction curvessignificantly steeper than the standard law, even where water ice ispresent. Comparing the results to the predictions of a simple extinctionmodel, we suggest that a standard extinction law implies a robust upperlimit to the grain-size distribution at around 0.1-0.3μm. Flattercurves are most likely the result of changes in this upper limit,although the effects of flattening as a result of unresolved clumpyextinction cannot be ruled out.

Composite Dust Grains: Modeling of Infrared Absorption Bands
We investigate the influence of small-scale asphericity of the surfacesof dust grains on the characteristics of the two deepest absorptionbands observed in the spectra of protostellar objects and stars (the 3.1μm water-ice and 9.7 μm silicate bands). The model used hascomposite particles in the form of radially inhomogeneous spheres withintermediate layers in which the index of refraction changes. Theobserved band widths and the ratios of the optical depths at the bandcenters can be explained if the grains are composed of small particlesconsisting of silicate cores with thin ice mantles and rough surfaces.The grain surface roughness considerably broadens the profile of thesilicate band.

Helium recombination spectra as temperature diagnostics for planetary nebulae
Electron temperatures derived from the HeI recombination line ratios,designated Te(HeI), are presented for 48 planetary nebulae(PNe). We study the effect that temperature fluctuations inside nebulaehave on the Te(HeI) value. We show that a comparison betweenTe(HeI) and the electron temperature derived from the Balmerjump of the HI recombination spectrum, designated Te(HI),provides an opportunity to discriminate between the paradigms of achemically homogeneous plasma with temperature and density variations,and a two-abundance nebular model with hydrogen-deficient materialembedded in diffuse gas of a `normal' chemical composition (i.e.~solar), as the possible causes of the dichotomy between the abundancesthat are deduced from collisionally excited lines and those deduced fromrecombination lines. We find that Te(HeI) values aresignificantly lower than Te(HI) values, with an averagedifference of = 4000 K. Theresult is consistent with the expectation of the two-abundance nebularmodel but is opposite to the prediction of the scenarios of temperaturefluctuations and/or density inhomogeneities. From the observeddifference between Te(HeI) and Te(HI), we estimatethat the filling factor of hydrogen-deficient components has a typicalvalue of 10-4. In spite of its small mass, the existence ofhydrogen-deficient inclusions may potentially have a profound effect inenhancing the intensities of HeI recombination lines and thereby lead toapparently overestimated helium abundances for PNe.

Carbon, Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem
Eleven models of Galactic chemical evolution, differing in the carbon,nitrogen, and oxygen yields adopted, have been computed to reproduce theGalactic O/H values obtained from H II regions. All the models fit theoxygen gradient, but only two models also fit the carbon gradient, thosebased on carbon yields that increase with metallicity owing to stellarwinds in massive stars (MSs) and decrease with metallicity owing tostellar winds in low- and intermediate-mass stars (LIMSs). Thesuccessful models also fit the C/O versus O/H evolution history of thesolar vicinity obtained from stellar observations. We also compare thepresent-day N/H gradient and the N/O versus O/H and the C/Fe, N/Fe, O/Feversus Fe/H evolution histories of the solar vicinity predicted by ourtwo best models with those derived from H II regions and from stellarobservations. While our two best models fit the C/H and O/H gradients,as well as the C/O versus O/H history, only model 1 fits well the N/Hgradient and the N/O values for metal-poor stars but fails to fit theN/H values for metal-rich stars. Therefore, we conclude that our twobest models solve the C enrichment problem but that further work needsto be done on the N enrichment problem. By adding the C and O productionsince the Sun was formed predicted by models 1 and 2 to the observedsolar values, we find an excellent agreement with the O/H and C/H valuesof the solar vicinity derived from H II region O and C recombinationlines. Our results are based on an initial mass function (IMF) steeperthan Salpeter's a Salpeter-like IMF predicts C/H, N/H, and O/H ratioshigher than observed. One of the most important results of this paper isthat the fraction of carbon due to MSs and LIMSs in the interstellarmedium is strongly dependent on time and on the galactocentric distance;at present about half of the carbon in the interstellar medium of thesolar vicinity has been produced by MSs and half by LIMSs.

The Stellar Content of Obscured Galactic Giant H II Regions. V. G333.1-0.4
We present high angular resolution, near-infrared images of the obscuredGalactic giant H II region G333.1-0.4, in which we detect an OB starcluster. For G333.1-0.4 we find OB stars and other massive objects invery early evolutionary stages, possibly still accreting. We obtainedK-band spectra of three stars; two show O-type photospheric features,while the third has no photospheric features but does show CO 2.3 μmband-head emission. This object is at least as hot as an early B-typestar, based on its intrinsic luminosity, and is surrounded by acircumstellar disk or envelope that produces near-infrared excessemission. A number of other relatively bright cluster members alsodisplay excess emission in the K band, indicative of disks or envelopesaround young massive stars. Based on the O star photometry andspectroscopy, the distance to the cluster is 2.6+/-0.4 kpc, similar to arecently derived kinematic (near side) value. The slope of the K-bandluminosity function is similar to those found in other young clusters.The mass function slope is more uncertain, and we find-1.3+/-0.2<Γ<-1.1+/-0.2 for stars with M>5Msolar, where the upper and lower limits are calculatedindependently for different assumptions regarding the excess emission ofthe individual massive stars. The number of Lyman continuum photonsderived from the contribution of all massive stars in the cluster is0.2×1050 s-1 < NLyc <1.9×1050 s-1. The integrated cluster mass is1.0×103 Msolar < Mcluster <1.3×103 Msolar.

Massive star formation in the W49 giant molecular cloud: Implications for the formation of massive star clusters
We present results from JHKs imaging of the densest region of the W49molecular cloud. In a recent paper (\cite{AH03}, ApJL, 589, L45), wereported the detection of (previously unknown) massive stellar clustersin the well-known giant radio HII region W49A, and here we continue ouranalysis. We use the extensive line-of-sight extinction to isolate apopulation of objects associated with W49A. We constrain the slope ofthe stellar luminosity function by constructing an extinction-limitedluminosity function, and use this to obtain a mass function. We find noevidence for a top-heavy MF, and the slope of the derived mass functionis -1.6 ± 0.3. We identify candidate massive stars from ourcolor-magnitude diagram, and we use these to estimate the current totalstellar mass of 5-7×104 Mȯ in theregion of the W49 molecular cloud covered by our survey. Candidateionizing stars for several ultra-compact HII regions are detected, withmany having multipe candidate sources. On the global molecular cloudscale in W49, massive star formation apparently did not proceed in asingle concentrated burst, but in small groups, or subclusters. This maybe an essential physical description for star formation in what willlater be termed a ``massive star cluster''.

Carbon and Oxygen Galactic Gradients: Observational Values from H II Region Recombination Lines
We present results of deep echelle spectrophotometry of eight Galactic HII regions located at Galactocentric distances between 6.3 and 10.4 kpc.The data have been taken with the Very Large Telescope UltravioletEchelle Spectrograph in the 3100-10360 Å range. We have derivedC++ and O++ abundances from recombination linesfor all the objects as well as O+ abundances from this kindof line for three of the nebulae. The intensity of recombination linesis almost independent of the assumed electron temperature as well as ofthe possible presence of spatial temperature variations or fluctuationsinside the nebulae. These data allow the determination of the gas-phaseC and O abundance gradients of the Galactic disk, of paramountimportance for chemical evolution models. This is the first time the Cgradient is derived from such a large number of H II regions distributedin such a wide range of Galactocentric distances. Abundance gradientsare found of the form Δlog(O/H)=-0.044+/-0.010 dexkpc-1, Δlog(C/H)=-0.103+/-0.018 dex kpc-1,and Δlog(C/O)=-0.058+/-0.018 dex kpc-1.Based on observations collected at the European Southern Observatory,Chile; proposals ESO 68.C-0149(A) and ESO 70.C-0008(A).

MSX mid-infrared imaging of massive star birth environments - II. Giant HII regions
We conduct a Galactic census of giant HII (GHII) regions, based on theall-sky 6-cm data set of Kuchar & Clark, plus the kinematicdistances obtained by Russeil. From an inspection of mid-infrared (MIR)Mid-course Space Experiment (MSX) and far-IR IRAS Sky Survey Atlasimages, we identify a total of 56 GHII regions in the Milky Way, ofwhich 15 per cent (65 per cent) can be seen at optical (near-IR)wavelengths. The mid to far-IR fluxes from each GHII region aremeasured, and sample the thermal emission from the ubiquitous dustpresent within the exciting clusters of OB stars, arising from theintegrated luminosity of the hot stars heating the cluster dust, forwhich we obtain log L(IR) = 5.5-7.3Lsolar. The MIR 21-μmspatial morphology is presented for each GHII region, and oftenindicates multiple emission sources, suggesting complicated clusterformation. IR colour-colour diagrams are presented, providinginformation concerning the temperature distribution and the opticaldepth of the dust. For the clusters of our study, the dust is notoptically thick to all stellar radiation, thus the measured infraredluminosity is lower than Lbol. As the dust environment of acluster begins to dissipate, the thermal emission and its optical depthought to decrease even before the stars evolve appreciably. We seeevidence of this in our empirical relationship between the integrated IRand Lyman continuum luminosities.

A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry
We present Very Large Telescope (VLT) UVES echelle spectrophotometry ofthe Orion nebula in the 3100-10400 Årange. We have measured theintensity of 555 emission lines, many of them corresponding to permittedlines of different heavy-element ions. This is the largest set ofspectral emission lines ever obtained for a Galactic or extragalacticHII region. We have derived He+, C2+,O+, O2+ and Ne2+ abundances from purerecombination lines. This is the first time that O+ andNe2+ abundances have been obtained from these kinds of linesin the nebula. We have also derived abundances from collisionallyexcited lines for a large number of ions of different elements. In allcases, ionic abundances obtained from recombination lines are largerthan those derived from collisionally excited lines. We have obtainedremarkably consistent independent estimations of the temperaturefluctuation parameter, t2, from different methods, which arealso similar to other estimates from the literature. This resultstrongly suggests that moderate temperature fluctuations (t2between 0.02 and 0.03) are present in the Orion nebula. We have comparedthe chemical composition of the nebula with those of the Sun and otherrepresentative objects. The heavy-element abundances in the Orion nebulaare only slightly higher than the solar ones, a difference that can beexplained by the chemical evolution of the solar neighbourhood.

Chemical abundances of planetary nebulae from optical recombination lines - II. Abundances derived from collisionally excited lines and optical recombination lines
In Paper I, we presented spectrophotometric measurements of emissionlines from the ultraviolet (UV) to the far-infrared for 12 Galacticplanetary nebulae (PNe) and derived nebular thermal and densitystructures using a variety of plasma diagnostics. The measurements andplasma diagnostic results are used in the current paper to determineelemental abundances in these nebulae. Abundance analyses are carriedout using both strong collisionally excited lines (CELs) and weakoptical recombination lines (ORLs) from heavy element ions.Assuming electron temperatures and densities derived from HIrecombination spectra (line and continuum), we are able to determine theORL C abundance relative to hydrogen for all the PNe in our sample, Nand O abundances for 11 of them and Ne abundances for nine of them. Inall cases, ORL abundances are found to be systematically higher than thecorresponding values deduced from CELs. In NGC 40, the discrepancybetween the abundances derived from the two types of emission linereaches a factor of 17 for oxygen. For the other 10 PNe, thediscrepancies for oxygen vary from 1.6 to 3.1. In general, collisionallyexcited infrared fine-structure lines, which have excitation energiesless than 103 K and consequently emissivities that areinsensitive to electron temperature and temperature fluctuations, yieldionic abundances comparable to those derived from optical/UV CELs. For agiven nebula, the discrepancies between the ORL and CEL abundances areof similar magnitude for different elements. In other words, relativeabundance ratios such as C/O, N/O and Ne/O deduced from the traditionalmethod based on strong CELs are comparable to those yielded by ORLs, fora wide range of ORL to CEL oxygen abundance ratios, varying from nearunity to over a factor of 20.We have also determined ORL abundances relative to hydrogen for thethird-row element magnesium for 11 nebulae in our sample. In strongcontrast to the cases for second-row elements, Mg abundances derivedfrom the MgII 3d-4f λ4481 ORL are nearly constant for all the PNeanalysed so far and agree within the uncertainties with the solarphotospheric value.In accordance with results from previous studies, the ORL to CELabundance ratio is correlated with the difference between the electrontemperatures derived from the [OIII] forbidden-line ratio, on the onehand, and from the hydrogen recombination Balmer discontinuity, on theother. We find that the discrepancy between the ORL and CEL abundancesis correlated with nebular absolute diameter, surface brightness, theelectron density derived from [SII] CELs, and excitation class. Theresults confirm that the dichotomy of temperatures and heavy elementalabundances determined from the two types of emission line, which hasbeen widely observed in PNe, is a strong function of nebular evolution,as first pointed out by Garnett and Dinerstein.Our analyses show that temperature fluctuations and/or densityinhomogeneities are incapable of explaining the large discrepanciesbetween the heavy elemental abundances and electron temperaturesdetermined from the two types of emission line. Our analyses support thebi-abundance model of Liu et al., who have proposed that PNe containanother previously unseen component of ionized gas which, highlyenriched in heavy elements, has an electron temperature of<~103 K and emits strongly in recombination lines but notin CELs. Our determinations of low average emission temperatures fromthe observed line intensity ratios of HeI and OII ORLs lend furthersupport to this scenario.

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Right ascension:11h11m49.80s
Apparent magnitude:99.9

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NGC 2000.0NGC 3576

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