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Hα line profiles for a sample of supergiant HII regions. III. Model line profiles
We carried out a series of 1D hydrodynamical computations with the ZEUSfluid solver in order to reproduce the salient features in ourhigh-resolution Hα emission line profiles of a sample of HIIregions (see Rozas et al. 2006a,b). Four models were computed. In thefirst and second cases, an ionization-bounded HII region was modeledwith and without a shell produced by stellar winds. In the third andfourth cases, a density-bounded HII region was considered, both with andwithout a shell driven by stellar winds. The resulting line profileshave been modeled using the SHAPE rendering program. We find that ourobserved high-quality profiles are well-reproduced in thedensity-bounded model that includes an expanding shell formed as aresult of the stellar winds from the ionizing stars. Finally, weconsider the effects that the finite slit widths used for theobservations have on the physical conditions deduced from the lineprofile of the ionized bubble.

Free-free absorption in the gravitational lens JVAS B0218+357
We address the issue of anomalous image flux ratios seen in thedouble-image gravitational lens JVAS B0218+357. From the multi-frequencyobservations presented in a recent study (Mittal et al. 2006, A&A,447, 515) and several previous observations made by other authors, theanomaly is well-established in that the image flux-density ratio (A/B)decreases from 3.9 to 2.0 over the observed frequency range from 15 GHzto 1.65 GHz. In Mittral et al. (2206), the authors investigated whetheran interplay between a frequency-dependent structure of the backgroundradio-source and a gradient in the relative image-magnification canexplain away the anomaly. Insufficient shifts in the image centroidswith frequency led them to discard the above effect as the cause of theanomaly. In this paper, we first take this analysis further byevaluating the combined effect of the background source extension andmagnification gradients in the lens plane in more detail. This is doneby making a direct use of the observed VLBI flux-distributions for eachimage to estimate the image flux-density ratios at different frequenciesfrom a lens-model. As a result of this investigation, this mechanismdoes not account for the anomaly. Following this, we analyze the effectsof mechanisms which are non-gravitational in nature on the image fluxratios in B0218+357. These are free-free absorption and scattering, andare assumed to occur under the hypothesis of a molecular cloud residingin the lens galaxy along the line-of-sight to image A. We show thatfree-free absorption due to an Hii region covering the entire structureof image A at 1.65 GHz can explain the image flux ratio anomaly. We alsodiscuss whether Hii regions with physical parameters as derived from ouranalysis are consistent with those observed in Galactic andextragalactic Hii regions.

Temperature fluctuations in H II regions: t{^2} for the two-phase model
Aims.We investigate temperature fluctuations in H II regions in terms ofa two-phase model, which assumes that the nebular gas consists of a hotand a cold phase. Methods: We derive general formulae for T([O III]),the [O III] forbidden line temperature, and T(H I), the hydrogen Balmerjump temperature, in terms of the temperatures of the hot and coldphases, Th and T_c. Results: For large temperaturedifferences, the values of t2 required to account for theobserved difference between T([O III]) and T(H I) are much lower thanthose deduced using the classical formulae that assume random and smallamplitude temperature fluctuations. One should therefore be cautiouswhen using a two-phase model to account for empirically derivedt2 values. We present a correction of a recent work byGiammanco & Beckman, who use a two-phase model to estimate theionization rate of H II regions by cosmic rays. We show that a verysmall amount of cold gas is sufficient to account for t2values typically inferred for H ii regions.

The Oxygen Abundance in the Inner H II Regions of M101: Implications for the Calibration of Strong-Line Metallicity Indicators
I present deep spectroscopy of four H II regions in the inner,metal-rich zone of the spiral galaxy M101 obtained with the LRISspectrograph at the Keck telescope. From the analysis of thecollisionally excited lines in two of the target H II regions, H1013 andH493, I have obtained oxygen abundances 12+log(O/H)=8.52 and12+log(O/H)=8.74, respectively. These measurements extend thedetermination of the oxygen abundance gradient of M101 via the directmethod to only 3 kpc from the center. The intensity of the C IIλ4267 line in H1013 leads to a carbon abundance 12+log(C/H)=8.66,corresponding to nearly twice the solar value. From a comparison of thecontinuum temperature derived from the Balmer discontinuity, T(Bac)=5000K, and the line temperature derived from [O III]λ4363/λ5007, T[O III]=7700 K, an average temperatureT0=5500 K and a mean square temperature fluctuationt2=0.06 have been derived. Accounting for the spatialinhomogeneity in temperature raises the oxygen abundance obtained fromthe oxygen auroral lines to 12+log(O/H)=8.93. These findings arediscussed in the context of the calibration of strong-line metallicityindicators, in particular of the upper branch of R23. Thereis no evidence for the strong abundance biases arising from temperaturegradients predicted theoretically for metal-rich H II regions.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.

The oxygen abundances in HII regions of the spiral galaxy M101 from the Sloan Digital Sky Survey spectra.
Not Available

The massive star population in the giant HII region Tol89 in NGC5398
We present new high spectral resolution Very Large Telescope(VLT)/UV-Visual Echelle Spectrograph (UVES) spectroscopy and archivalHubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS)imaging and spectroscopy of the giant HII region Tol89 in NGC5398. Fromoptical and ultraviolet (UV) HST images, we find that the star-formingcomplex as a whole contains at least seven young compact massiveclusters. We resolve the two brightest optical knots, A and B, into fiveindividual young massive clusters along our slit, A1-4 and B1,respectively. From UV spectral modelling using the STARBURST99 code ofLeitherer et al., and nebular Hβ equivalent widths in the optical,we derive ages that are consistent with the formation of two separateburst events, of ~4 +/- 1 Myr and <3 Myr for knots A (A1-4) and B(B1), respectively. A Large Magellanic Cloud (LMC) metallicity ismeasured for both knots from a nebular line analysis, while nebular HeII4686 is observed in knot B and perhaps in knot A. We detect underlyingbroad wings on the strongest nebular emission lines indicatingvelocities up to 600 km s-1. From UV and opticalspectroscopy, we estimate that there are ~95 early WN stars and ~35early WC stars in Tol89-A, using empirical template spectra of LMCWolf-Rayet (WR) stars from Crowther & Hadfield, with the WCpopulation confined to cluster A2. Remarkably, we also detect a smallnumber of approximately three mid WNs in the smallest (mass) cluster inTol89-A, A4, whose spectral energy output in the UV is entirelydominated by the WN stars. From the strength of nebular Hβ, weobtain N(O) ~ 690 and 2800 for knots A and B, respectively, whichimplies N(WR)/N(O) ~ 0.2 for knot A. We also employ a complementaryapproach using STARBURST99 models, in which the O star content isinferred from the stellar continuum, and the WR population is obtainedfrom spectral synthesis of optical WR features using the grids fromSmith et al. We find reasonable agreement between the two methods forthe O star content and the N(WR)/N(O) ratio but find that the WR subtypedistribution is in error in the STARBURST99 models, with far too few WNstars being predicted. We attribute this failure to the neglect ofrotational mixing in evolutionary models. Our various modellingapproaches allow us to measure the cluster masses. We identify A1 as asuper star cluster (SSC) candidate with a mass of ~1-2 ×105 Msolar. A total mass of ~6 ×105 Msolar is inferred for the ionizing sourceswithin Tol89-B.Based on observations collected at the European Southern Observatory,Chile, proposal ESO 73.B-0238(A) and with the NASA/ESA HST, obtainedfrom the ESO/ST-ECF Science Archive Facility.E-mail: fs@star.ucl.ac.uk

An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.

Hα line profiles for a sample of supergiant HII regions. II. Broad, low intensity components
We analyze the broad, low intensity, high velocity components that areseen in the Hα line profiles for a sample of HII regions. TheseHII regions are chosen from among the brightest and most isolated in asample of spiral galaxies for which we have photometric andspectroscopic data: NGC 157, NGC 3631, NGC 6764, NGC 3344, NGC 4321, NGC5364, NGC 5055, NGC 5985, and NGC 7479. We confirm that the lineprofiles of most of these bright, giant extragalactic HII regionscontain broad kinematic components of low intensity, but high velocity,that we denote as wings. We analyze these components, deriving emissionmeasures, central velocities, and velocity dispersions of the blue andred features, which are similar. We interpret these components asexpanding shells within the HII regions and produced by the stellarwinds from the ionizing stars. We compare the kinetic energies of theseexpanding shells with the kinetic energy available from the stellarwinds. If we allow for the hypothesis that the brightest HII regions aredensity bounded, we show that, for these HII regions, the stellar windmechanism can explain the observed shell kinetic energies.

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%.

A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova Remnants
We present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed.

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.

Massive Stellar Content of Giant H II Regions in M33 and M101
Far-ultraviolet (900-1200 Å) spectral synthesis of nine giantextragalactic H II regions in M33 and M101 is performed to study theirmassive stellar content. Several parameters are quantified, predicted,and compared to the literature: age, stellar mass, initial mass function(IMF) slope, number of O-type and Wolf-Rayet stars, and Hα and5500 Å continuum fluxes. The results of this particular techniqueare consistent with other methods and observations. This work shows thata total stellar mass of a few 103 Msolar is neededto populate the IMF bins well enough at high masses to obtain accurateresults from the spectral synthesis technique in the far-ultraviolet. Aflat IMF slope seems to characterize better the stellar line profiles ofthese objects, which is likely the first sign of a small numberstatistics effect on the IMF. Finally, the H II region NGC 5461 isidentified as a good candidate for hosting a second generation of stars,not yet seen at far-ultraviolet wavelengths.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.

High-Ionization Emission in Metal-deficient Blue Compact Dwarf Galaxies
Primordial stars are expected to be very massive and hot, producingcopious amounts of hard ionizing radiation. The best place to study hardionizing radiation in the local universe is in very metal-deficient bluecompact dwarf (BCD) galaxies. We have carried out a MMT spectroscopicsearch for [Ne V] λ3426 (ionization potential of 7.1 ryd), [Fe V]λ4227 (ionization potential of 4 ryd), and He II λ4686(ionization potential of 4 ryd) emission in a sample of 18 BCDs. We haveadded data from previous work and from the Data Release 3 of the SloanDigital Sky Survey. In total, we have assembled a BCD high-ionizationsample with [Ne V] emission in four galaxies, [Fe V] emission in 15galaxies, and He II emission in 465 galaxies. With this large sample, wehave reached the following conclusions. There is a general trend ofhigher [Ne V], [Fe V], and He II emission at lower metallicities.However, metallicity is not the only factor that controls the hardnessof the radiation. High-mass X-ray binaries and main-sequence stars areprobably excluded as the main sources of the very hard ionizingradiation responsible for [Ne V] emission. The most likely source of [NeV] emission is probably fast radiative shocks moving with velocities>~450 km s-1 through a dense interstellar medium with anelectron number density of several hundreds cm-3 andassociated with supernova explosions of the most massive stars. Thesehave masses of ~50-100 Msolar and are formed in very compactsuper-star clusters (SSCs). The softer ionizing radiation required forHe II emission is likely associated with less massive evolved starsand/or radiative shocks moving through a less dense interstellar medium.The observations reported here were obtained at the MMT Observatory, ajoint facility of the Smithsonian Institution and the University ofArizona.

Temperature fluctuations in H II regions: Ionization by cosmic rays as a key mechanism
We present a detailed model capable of explaining quantitatively thetemperature fluctuations observed in luminous, large H II regions. Themodel is based on two assumptions which we justify on the basis ofobservations: that the major fraction of the hydrogen in the clouds thatform the H II regions is not photoionized and is essentially H I, thisH I is lightly ionized by fluxes of low energy cosmic rays (CR) producedby processes originating in the hot stars which illuminate the regions.

Abundance gradients in a sample of barred spiral galaxies
We used photoionization models in order to reproduce the observedgradients of emission-line ratios for H II regions located in the normalspiral galaxy M 101 and in three barred spiralgalaxies, namely NGC 1365, NGC925, and NGC 1073. The behavior of thefollowing nebular parameters across the disk of these galaxies wasdetermined: temperature of the ionizing star (T_eff), ionizationparameter (U), and the abundance ratios O/H, N/O, and S/O. Our O/Hpredictions were found to be consistent with some empirical abundancedeterminations, but are overestimated by a factor of 0.1-0.4 dex whencompared to the direct abundance determinations. NGC1073 seems to be overabundant in nitrogen compared to otherspiral galaxies. No gradient of S/O was found in the studied galaxies,and we derived a positive T_eff gradient of triangle T_eff/triangle R =(400 ± 112) K kpc-1 and U ranging from -3.0 to -2.3.The N/O vs. O/H diagram in general is consistent with chemical evolutionmodels that assume that the nitrogen synthesis has both a primary and asecondary component compared to oxygen. However, a very strong N/Odependence on the O/H at high abundance was found.

Clusters in the Luminous Giant H II Regions in M101
We have obtained HST WFPC2 observations of three very luminous butmorphologically different giant H II regions (GHRs) in M101, NGC 5461,NGC 5462, and NGC 5471, in order to study cluster formation in GHRs.Images obtained in the F547M and F675W bands are used to identifycluster candidates and for photometric measurements, and images in theF656N band are used to show ionized interstellar gas. The measuredcolors and magnitudes are compared with the evolutionary tracksgenerated by the Starburst99 and Bruzual & Charlot populationsynthesis models to determine the ages and masses of the clustercandidates that are more luminous than MF547M=-9.0. Thebrightest clusters detected in the PC images are measured and found tohave effective radii of 0.7-2.9 pc. NGC 5461 is dominated by a veryluminous core and has been suggested to host a super-star cluster (SSC).Our observations show that it contains three R136-class clusterssuperposed on a bright stellar background in a small region. This tightgroup of clusters may dynamically evolve into an SSC in the future, andmay appear unresolved and be identified as an SSC at large distances,but at present NGC 5461 contains no SSCs. NGC 5462 consists of looselydistributed H II regions and clusters without a prominent core. It hasthe largest number of cluster candidates among the three GHRs studied,but most of them are faint and older than 10 Myr. NGC 5471 has multiplebright H II regions and contains a large number of faint clustersyounger than 5 Myr. Two of the clusters in NGC 5471 are older than R136,but just as luminous; they may be the most massive clusters in the threeGHRs studied. The fraction of stars formed in massive clusters has beenestimated from the clusters' contribution to the total stellar continuumemission and from a comparison between the ionizing power of theclusters and the ionizing requirement of the associated H II regions.Both estimates show that <~50% of massive stars are formed in massiveclusters; consequently, the Hα luminosity of an H II region doesnot provide a sufficient condition for the existence of SSCs. Thecluster luminosity functions (LFs) of the three GHRs show differentslopes. NGC 5462 has the steepest cluster LF and the most looselydistributed interstellar gas, qualitatively consistent with thehypothesis that massive clusters are formed in high-pressureinterstellar environments. The combined cluster LF of the three GHRs hasa slope similar to the universal cluster LFs seen in starburst galaxiesand nonstarburst spiral galaxies.

Massive star populations in Wolf-Rayet galaxies
We analyse long-slit spectral observations of 14 Wolf-Rayet (WR)galaxies from the sample of Schaerer, Contini & Pindao. All 14galaxies show broad WR emission in the blue region of the spectrum,consisting of a blend of NIIIλ4640, CIIIλ4650,CIVλ4658 and HeIIλ4686 emission lines, which is a spectralcharacteristic of WN stars. Broad CIVλ5808 emission, termed thered bump, is detected in nine galaxies and CIIIλ5996 is detectedin six galaxies. These emission features are due to WC stars. We derivethe numbers of late WN and early WC stars from the luminosity of theblue and red bumps, respectively. The number of O stars is estimatedfrom the luminosity of the Hβ emission line, after subtracting thecontribution of WR stars. The Schaerer & Vacca models predict thatthe number of WR stars relative to O stars,NWR/NO, increases with metallicity. Forlow-metallicity galaxies, the results agree with predictions ofevolutionary synthesis models for galaxies with a burst of starformation, and indicate an initial mass function (IMF) slope -2<~Γ<~- 2.35 in the low-metallicity regime. Forhigh-metallicity galaxies our observations suggest a Salpeter IMF(Γ=-2.35) and an extended short burst. The main possible sourcesof error are the adopted luminosities for single WCE and WNL stars. Wealso report, for the first time, on NGC 450 as a galaxy with WRcharacteristics. For NGC 450, we estimate the number of WN and WC stars.The number ratio NWR/NO, and the equivalent widthsof the blue bump, EWλ4686, and of the red bump,EWλ5808, in NGC 450 are also in good agreement withthe instantaneous burst model prediction for WR galaxies.

Clusters in the Luminous Giant HII Regions in M101
Giant HII regions (GHRs), with high concentrations of massive stars, arean excellent laboratory to study modes of massive star formation and apotential host of super-star clusters (SSCs). We have used HST WFPC2images in the F547M and F675W bands to study cluster formation in threevery luminous but morphologically different GHRs in M101 - NGC5461,NGC5462, and NGC5471.NGC5461 has been suggested to host a SSC in its luminous core. Ourobservations show that it contains three R136-class clusters superposedon a small, bright stellar background. This tight group of clusters maydynamically evolve into an SSC in the future, and may appear unresolvedat large distances and be identified as an SSC. NGC5462 has looselydistributed HII regions and clusters without a prominent core. It hasthe largest number of clusters among the three GHRs, but most of themare faint and older than 10 Myr. NGC5471 has multiple bright HII regionsand many faint clusters younger than 5 Myr. It has two clusters olderthan R136, but just as luminous; they may be the most massive clustersin the three GHRs.The fraction of stars formed in massive clusters is estimated from theclusters' contribution to the total stellar continuum emission and acomparison of the clusters' ionizing power to the ionization requirementof the associated HII regions. Both estimates show that < ˜ 50%of stars are formed in massive clusters. The cluster luminosityfunctions (CLFs)of the three GHRs show different slopes. NGC5462 has thesteepest CLF and most loosely distributed interstellar gas,qualitatively consistent with the hypothesis that massive clusters areformed in high-pressure interstellar environments. The combined CLF ofthese three GHRs is similar to those in starburst galaxies and normalspiral galaxies.This research is supported by grants STScI GO-6829.01-95A andGO-9934.01-A.

XMM-Newton observations of the starburst merger galaxies NGC 3256 and NGC 3310
We present XMM-Newton EPIC observations of the two nearby starburstmerger galaxies NGC 3256 and NGC 3310. The broad-band (0.3-10 keV)integrated X-ray emission from both galaxies shows evidence ofmultiphase thermal plasmas plus an underlying hard non-thermal power-lawcontinuum. NGC 3256 is well fitted with a model comprising two MEKALcomponents (kT= 0.6/0.9 keV) plus a hard power law (Γ= 2), whileNGC 3310 has cooler MEKAL components (kT= 0.3/0.6 keV) and a harderpower-law tail (Γ= 1.8). Chandra observations of both galaxiesreveal the presence of numerous discrete sources embedded in the diffuseemission, which dominate the emission above ~2 keV and are likely to bethe source of the power-law emission. The thermal components show atrend of increasing absorption with higher temperature, suggesting thatthe hottest plasmas arise from supernova-heated gas within the discs ofthe galaxies, while the cooler components arise from outflowing galacticwinds interacting with the ambient interstellar medium. We find nostrong evidence for an active galactic nucleus in either galaxy.

An XMM-Newton view of M101 - I. The luminous X-ray source population
We present the first results of an XMM-Newton EPIC observation of theluminous X-ray source population in the face-on supergiant spiral galaxyM101. We have studied the spectral and temporal properties of the 14most luminous sources, all of which have intrinsic X-ray luminositiesexceeding the Eddington limit for a 1.4-Msolar neutron star,with a subset in the ultraluminous X-ray source (ULX) regime(LX>= 1039 erg s-1). Eleven sourcesshow evidence of short-term variability, and most vary by a factor of~2-4 over a baseline of 11-24 yr, providing strong evidence that thesesources are accreting X-ray binary (XRB) systems. Our resultsdemonstrate that these sources are a heterogeneous population, showing avariety of spectral shapes. Interestingly, there is no apparent spectraldistinction between those sources above and below the ULX luminositythreshold. Nine sources are well fitted with either simple absorbed discblackbody or power-law models. However, in three of the four sourcesbest fitted with power-law models, we cannot exclude the disc blackbodyfits and therefore conclude that, coupled with their high luminosities,eight out of nine single-component sources are possibly high-state XRBs.The nuclear source (XMM-10) has the only unambiguous power-law spectrum(Γ~ 2.3), which may be evidence for the presence of alow-luminosity active galactic nucleus (LLAGN). The remaining fivesources require at least two-component spectral fits, with an underlyinghard component that can be modelled by a power-law continuum or, inthree cases, a hot disc blackbody (Tin= 0.9-1.5 keV), plus asoft component modelled as a cool blackbody/disc blackbody/thermalplasma. We have compared the spectral shapes of nine sources covered byboth this observation and an archival 100-ks Chandra observation ofM101; eight show behaviour typical of Galactic XRBs (i.e. softening withincreasing luminosity), the only exception being a transient source(XMM-2) which shows little change in spectral hardness despite a factorof ~30 increase in luminosity. We find no definitive spectral signaturesto indicate that these sources contain neutron star primaries, andconclude that they are likely to be stellar-mass black hole XRBs(BHXBs), with black hole masses of ~2-23 Msolar if accretingat the Eddington limit.

Hot Interstellar Gas and Stellar Energy Feedback in the Antennae Galaxies
We have analyzed Chandra archival observations of the Antennae galaxiesto study the distribution and physical properties of its hotinterstellar gas. Eleven distinct diffuse X-ray emission regions areselected according to their underlying interstellar structures and starformation activity. The X-ray spectra of these regions are used todetermine their thermal energy content and cooling timescales. Youngstar clusters in these regions are also identified and their photometricmeasurements are compared to evolutionary stellar population synthesismodels to assess their masses and ages. The cluster properties are thenused to determine the stellar wind and supernova energies injected intothe interstellar medium (ISM). Comparisons between the thermal energy inthe hot ISM and the expected stellar energy input show that young starclusters are sufficient to power the X-ray-emitting gas in some, but notall, active star formation regions. Super star clusters, with masses>=1×105 Msolar, heat the ISM, but theyield of hot interstellar gas is not directly proportional to thecluster mass. Finally, there exist diffuse X-ray emission regions thatdo not show active star formation or massive young star clusters. Theseregions may be powered by field stars or low-mass clusters formed withinthe last ~100 Myr.

Highly Ionized Gas Surrounding High-Velocity Cloud Complex C
We present Far Ultraviolet Spectroscopic Explorer and Hubble SpaceTelescope observations of high-, intermediate-, and low-ion absorptionin high-velocity cloud (HVC) Complex C along the lines of sight towardfive active galaxies. Our purpose is to investigate the idea thatComplex C is surrounded by an envelope of highly ionized material,arising from the interaction between the cloud and a hot surroundingmedium. We measure column densities of high-velocity high-ion absorptionand compare the kinematics of low-, intermediate-, and high-ionizationgas along the five sight lines. We find that in all five cases, the H Iand O VI high-velocity components are centered within 20 kms-1 of one another, with an average displacement ofOVI-vHI>=3+/-12 km s-1. In thosedirections where the H I emission extends to more negative velocities(the so-called high-velocity ridge), so does the O VI absorption. Thekinematics of Si II is also similar to that of O VI, withOVI-vSiII>=0+/-15 km s-1. We compareour high-ion column density ratios to the predictions of various models,adjusted to account for both recent updates to the solar elementalabundances and relative elemental abundance ratios in Complex C. Alongthe PG 1259+593 sight line, we measure N(SiIV)/N(OVI)=0.10+/-0.02,N(CIV)/N(OVI)=0.35+0.05-0.06, andN(NV)/N(OVI)<0.07 (3 σ). These ratios are inconsistent withcollisional ionization equilibrium at one kinetic temperature.Photoionization by the extragalactic background is ruled out as thesource of the high ions since the path lengths required would make HVCsunreasonably large; photoionization by radiation from the disk of theGalaxy also appears unlikely since the emerging photons are notenergetic enough to produce O VI. By themselves, ionic ratios areinsufficient to discriminate between various ionization models, but byconsidering the absorption kinematics as well, we consider the mostlikely origin for the highly ionized high-velocity gas to be at theconductive or turbulent interfaces between the neutral/warm ionizedcomponents of Complex C and a surrounding hot medium. The presence ofinterfaces on the surface of HVCs provides indirect evidence for theexistence of a hot medium in which the HVCs are immersed. This mediumcould be a hot (T>~106 K) extended Galactic corona or hotgas in the Local Group.Based on observations from the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer mission, operated by Johns Hopkins University,supported by NASA contract NAS 5-32985, and from the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
We use high signal-to-noise ratio spectra of 20 H II regions in thegiant spiral galaxy M101 to derive electron temperatures for the H IIregions and robust metal abundances over radii R=0.19-1.25R0(6-41 kpc). We compare the consistency of electron temperatures measuredfrom the [O III] λ4363, [N II] λ5755, [S III]λ6312, and [O II] λ7325 auroral lines. Temperatures from[O III], [S III], and [N II] are correlated with relative offsets thatare consistent with expectations from nebular photoionization models.However, the temperatures derived from the [O II] λ7325 line showa large scatter and are nearly uncorrelated with temperatures derivedfrom other ions. We tentatively attribute this result to observationaland physical effects, which may introduce large random and systematicerrors into abundances derived solely from [O II] temperatures. Ourderived oxygen abundances are well fitted by an exponential distributionover six disk scale lengths, from approximately 1.3(O/H)solar in the center to 1/15 (O/H)solar in theoutermost region studied [for solar 12+log(O/H)=8.7]. We measuresignificant radial gradients in N/O and He/H abundance ratios, butrelatively constant S/O and Ar/O. Our results are in approximateagreement with previously published abundances studies of M101 based ontemperature measurements of a few H II regions. However, our abundancesare systematically lower by 0.2-0.5 dex than those derived from the mostwidely used strong-line ``empirical'' abundance indicators, againconsistent with previous studies based on smaller H II region samples.Independent measurements of the Galactic interstellar oxygen abundancefrom ultraviolet absorption lines are in good agreement with theTe-based nebular abundances. We suspect that most of thedisagreement with the strong-line abundances arises from uncertaintiesin the nebular models that are used to calibrate the ``empirical''scale, and that strong-line abundances derived for H II regions andemission-line galaxies are as much as a factor of 2 higher than theactual oxygen abundances. However, other explanations, such as theeffects of temperature fluctuations on the auroral line basedabundances, cannot be completely ruled out. These results point to theneed for direct abundance determinations of a larger sample ofextragalactic H II regions, especially for objects more metal-rich thansolar.

Searching for Embedded Super-Star Clusters in IC 4662, NGC 1705, and NGC 5398
Radio observations of IC 4662, NGC 5398, and NGC 1705 are used incombination with optical images to search for extremely young starclusters that are still embedded in their birth material. Radio datawere obtained at 3 and 6 cm using the Australia Telescope Compact Array,and optical data were retrieved from the Hubble Space Telescope (HST)archive. In IC 4662, three inverted spectrum thermal radio sources andone nonthermal source were detected. The thermal sources have Lymancontinuum fluxes ranging from ~20-200×1049s-1 (the equivalent of ~20-200 O7.5-type stars), modeled coreradii of r<~1 pc, and densitiesne>~103-104 cm-3. The HSTimages of this galaxy indicate a number of optical star clusters thatare <~10 Myr and have masses of ~103-104Msolar. The young optical clusters and radio clusters in IC4662 are spatially exclusive-supporting either large amounts ofextinction in the vicinity of the radio clusters or sequential starformation. No thermal sources are detected in NGC 1705 down to a 3σ level of ~5×1024 ergs s-1Hz-1 (0.17 mJy). At the distance of NGC 1705, the Galacticultracompact H II region complex W49A would have been a ~5 σdetection; therefore, the lack of detections of any thermal sources downto this detection threshold implies that star formation in NGC 1705 hasvery nearly ceased. NGC 5398 hosts an impressive thermal radio sourcethat is associated with the giant H II region Tol 89. This source has aLyman continuum flux of ~4500×1049 s-1 (theequivalent of ~4500 O7.5-type stars). At the distance to Tol 89 and theresolution of these radio data, we are only able to set an upper limiton its radius of r<~90 pc and a lower limit on its mean density ofne>50 cm-3.

[Fe IV] Emission in Ionized Nebulae
This paper presents an analysis of [Fe IV] emission based on newidentifications and previous measurements of [Fe IV] lines in 30Doradus, IC 4846, M42, SMC N88A, and SBS 0335-052. The Fe abundancesobtained by adding the abundances of the relevant Fe ions (mainlyFe++ and Fe3+) are found to be lower, by factorsin the range 2.6-5.9, than the Fe abundances implied by [Fe III]emission and an ionization correction factor derived from ionizationmodels. The most likely explanation of this discrepancy is that eitherthe collision strengths for [Fe IV] or the Fe ionization fractionspredicted by models are unreliable. The available data allow one neitherto distinguish between these two possibilities nor to exclude anotherpossible explanation: that the discrepancy implies the presence of agradient in the Fe abundance within the ionized gas. Furthermeasurements of [Fe IV] lines and checks on the Fe3+ atomicdata and ionization models are needed to reach a definitive conclusion.The discrepancy introduces an uncertainty in the determination of Feabundances in ionized nebulae. This uncertainty has implications for ourunderstanding of both the evolution of dust in ionized nebulae and thechemical history of low-metallicity galaxies.

The Wind of the B[e] Supergiant Henize S22 Viewed through a Reflection Nebula in DEM L106
Narrowband HST WFPC2 images reveal a bow-shock-like halo around the H IIregion N30B toward the B[e] supergiant Hen S22 located within the largerDEM L106 nebula in the Large Magellanic Cloud. High-dispersion spectraof N30B show a narrow Hα emission component from the ionized gas;the velocity variations indicate a gas flow of -5 to -10 kms-1 in the vicinity of the H II regions, which is resultantfrom interactions with Hen S22's stellar wind and responsible for thebow-shock morphology. Spectra of N30B's halo show broad Hαprofiles extending over more than 1000 km s-1, similar tothat of Hen S22, indicating that the halo is a reflection nebula of HenS22. Broadband morphologies of N30B's halo are also consistent with thereflection nebula interpretation. We use dust-scattering properties andthe observed brightnesses of the reflection nebula and Hen S22 toconstrain the reflection geometry. The reflected stellar Hαemission and absorption vary across the reflection nebula as a result ofviewing S22's anisotropic wind from different angles. This reflectionnebula, together with the edge-on orientation of Hen S22's disk,provides an invaluable opportunity to study the disk and polar winds ofa B[e] supergiant.

Planetary nebula carbon yields and the chemical evolution of the Galactic disc
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebulaabundances derived from CIIλ4267 and CIIIλλ1906 +1909 lines, respectively. These observational yields are assumed inchemical evolution models for the solar vicinity and the Galactic disc.C/O values observed in stars in the solar vicinity and Galactic HIIregions are compared with those predicted by chemical evolution modelsfor the Galaxy. It is concluded that the C yields derived from permittedlines are in better agreement with the observational constraints thanare those derived from forbidden lines.

Chemical Abundances of Extragalactic H II regions
The determination of the heavy element abundances from giantextragalactic H II regions is based on collisionally excited lines. Weargue that in the presence of temperature variations the abundancesdetermined are lower limits to the real heavy element abundances. Todetermine the real abundances it is necessary to take into account thetemperature variations present in these nebulae. We discuss therelevance of obtaining accurate line intensities of recombination linesof H, He, C, and O to determine the chemical composition ofextragalactic H II regions. We suggest that Pagel's method to derive theO/H ratio should be calibrated by using recombination lines instead ofphotoionization models or abundances derived from collisionally excitedlines.

Chemical abundances of the H II regions NGC 5461 and NGC 5471 in M 101, derived from echelle spectrophotometry
We present high resolution spectroscopic data of the two giantextragalactic HII regions NGC 5461 and NGC 5471 in M 101. We calculatethe physical conditions in the two nebulae with a large number ofdiagnostics, and determine their chemical abundances by applyingionization correction factors (ICFs) to the observed ionic abundances. The comparison of the ICFs based on photo-ionization models (Luridiana& Peimbert 2001; Luridiana et al. 1999) to those computed followingthe prescriptions by Mathis & Rosa (1991) shows large discrepanciesfor several elements, including nitrogen, neon and chlorine.

Optical Recombination Lines of Heavy Elements in Giant Extragalactic H II Regions
We present high-resolution observations of the giant extragalactic H IIregions NGC 604, NGC 2363, NGC 5461, and NGC 5471, based on observationstaken with the ISIS spectrograph on the William Herschel Telescope. Wehave detected, for the first time, C II and O II recombination lines inthese objects. We find that recombination lines give largerC++ and O++ abundances than collisionally excitedlines, suggesting that temperature variations may be present in theobjects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the mostconfident detection of optical lines of this kind in H II regions.Considering the temperature structure, we derive their H, He, C, N, O,Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461and NGC 5471, we determine the presence of C/H and O/H gradients inM101. We calculate the ΔY/ΔO and ΔY/ΔZ valuesconsidering the presence of temperature variations and under theassumption of constant temperature. We obtain a better agreement withmodels of galactic chemical evolution by considering the presence oftemperature variations than by assuming that the temperature is constantin these nebulae. Based on observations made with William HerschelTelescope operated on the island of La Palma by the Isaac Newton Groupof Telescopes in the Spanish Observatorio del Roque de Los Muchachos ofthe Instituto de Astrofísica de Canarias.

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Observation and Astrometry data

Constellation:Ursa Major
Right ascension:14h04m29.06s
Apparent magnitude:99.9

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NGC 2000.0NGC 5471

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