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On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars and Population III Stars
We suggest that the mass lost during the evolution of very massive starsmay be dominated by optically thick, continuum-driven outbursts orexplosions, instead of by steady line-driven winds. In order for amassive star to become a Wolf-Rayet star, it must shed its hydrogenenvelope, but new estimates of the effects of clumping in winds fromO-type stars indicate that line driving is vastly insufficient. Wediscuss massive stars above roughly 40-50 Msolar, which donot become red supergiants and for which the best alternative is massloss during brief eruptions of luminous blue variables (LBVs). Ourclearest example of this phenomenon is the 19th century outburst ofη Carinae, when the star shed 12-20 Msolar or more inless than a decade. Other examples are circumstellar nebulae of LBVs andLBV candidates, extragalactic η Car analogs (the so-called supernovaimpostors), and massive shells around supernovae and gamma-ray bursters.We do not yet fully understand what triggers LBV outbursts or whatsupplies their energy, but they occur nonetheless, and they present afundamental mystery in stellar astrophysics. Since line opacity frommetals becomes too saturated, the extreme mass loss probably arises froma continuum-driven wind or a hydrodynamic explosion, both of which areinsensitive to metallicity. As such, eruptive mass loss could haveplayed a pivotal role in the evolution and ultimate fate of massivemetal-poor stars in the early universe. If they occur in thesePopulation III stars, such eruptions would also profoundly affect thechemical yield and types of remnants from early supernovae andhypernovae thought to be the origin of long gamma-ray bursts.
| On the Determination of N and O Abundances in Low-Metallicity Systems
We show that in order to minimize the uncertainties in the N and Oabundances of low-mass, low-metallicity (O/H<=1/5 solar)emission-line galaxies, it is necessary to employ separateparameterizations for inferring Te(N+) andTe(O+) from Te(O+2). Inaddition, we show that for the above systems, the ionization correctionfactor (ICF) for obtaining N/O from N+/O+, wherethe latter is derived from optical emission-line flux ratios, is=1.08+/-0.09. These findings are based on state-of-the-art single-star HII region simulations, employing our own modeled stellar spectra asinput. Our models offer the advantage of having matching stellar andnebular abundances. In addition, they have O/H as low as 1/50 solar(lower than any past work), as well as log(N/O) and log(C/O) fixed atcharacteristic values of -1.46 and -0.7, respectively. The above resultswere used to rederive N and O abundances for a sample of 68 systems with12+log(O/H)<=8.1, whose dereddened emission-line strengths werecollected from the literature. The analysis of the log(N/O) versus12+log(O/H) diagram of the above systems shows that (1) the largestgroup of objects forms the well-known N/O plateau with a value for themean (and its statistical error) of-1.43+0.0084-0.0085, (2) the objects aredistributed within a range in log(N/O) of -1.54 to -1.27 in Gaussianfashion around the mean with a standard deviation ofσ=+0.071-0.084, and (3) a χ2analysis suggests that only a small amount of the observed scatter inlog(N/O) is intrinsic.
| 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%.
| 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.
| C [III] imagery of planetary nebulae and H II regions.
Not Available
| 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.
| A comparative analysis of empirical calibrators for nebular metallicity
We present a new analysis of the main empirical calibrators of oxygenabundance for ionized gas nebulae. With that aim we have compiled anextensive sample of objects with emission-line data including thenear-infrared [SIII] lines and the weak auroral lines which allow forthe determination of the gas electron temperature. For all the objectsthe oxygen abundances have been derived in a homogenous way, using themost recent sets of atomic coefficients and taking into the account theeffect of particle density on the temperature of O+. Theresiduals between directly and empirically derived abundances as afunction of abundance have been studied. A grid of photoionizationmodels, covering the range of physical properties of the gas, has beenused to explain the origin of the uncertainties affecting each abundancecalibrator. The range of validity for each abundance parameter has beenidentified and its average uncertainty has been quantified.
| Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies
We compare the properties of young star clusters with and withoutWolf-Rayet (W-R) stars in W-R galaxies using optical, near-infraredimagery and optical spectroscopy. Our work identifies the clusters withW-R stars in these galaxies for the first time. With this information,comparisons of clusters with and without W-R stars are now possible,enabling us to understand the chemical and morphological impact ofmassive stars on their environment and to constrain the parameters formodeling these systems. We find that clusters with W-R stars (W-Rclusters) are systematically younger, bluer clusters. Knowing this agedifference between the two cluster sets, we use an evolutionary scenarioto interpret their other properties. Young clusters, typically W-Rclusters, have a Strömgren sphere-like gas configuration. They alsotend to have H-K colors redder than those of theoretical models. Weinterpret the H-K excess as a combination of thermal emission from hotdust, nebular emission, and molecular emission. Older clusters,typically clusters without W-R stars, have ionized gas in a superbubbleconfiguration caused by the prolonged influence of stellar winds andsupernovae. The H-K excess is generally absent for these clusters. Thenitrogen-to-oxygen abundance ratio (N/O) does not appear to increase asa function of age over the first 10 Myr. Systems without W-R stars doappear to have a significant, elevated N/O over systems with W-R starsin the metallicity range 12+log(O/H)=7.7-7.9. For the entire metallicityrange in our sample, this finding is only marginally significant. Weconcur with previous studies, which find no correlation between thesulfur-to-oxygen abundance ratio and metallicity.
| A HST Study of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometarydwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2images obtained with the Hubble Space Telescope. The resultingcolor-magnitude diagram reaches down to I~26.0 mag. It reveals not onlya young population of blue main-sequence stars (age <~30 Myr) butalso an intermediate-age population of blue and red supergiants (20 Myr<~ age <~100 Myr) and older evolved populations of asymptoticgiant branch (AGB) stars (age >~100 Myr) and red giant branch (RGB)stars (age >~1 Gyr). The measured magnitude I=23.65+/-0.10 mag of theRGB tip results in a distance modulus m-M=27.67+/-0.10, whichcorresponds to a distance of 3.42+/-0.15 Mpc, in agreement with previousdistance determinations. The youngest stars are associated with thebright complex of H II regions NGC 2363 (=Mrk 71) in the southwestextremity of the galaxy. As a consequence of the diffusion andrelaxation processes of stellar ensembles, the older the stellarpopulation is, the smoother and more extended is its spatialdistribution. An underlying population of older stars is foundthroughout the body of NGC 2366. The most notable feature of this olderpopulation is the presence of numerous relatively bright AGB stars. Thenumber ratio of AGB to RGB stars and the average absolute brightness ofAGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403,indicating a younger age of the AGB stars in NGC 2366. In addition tothe present burst of age <~100 Myr, there has been strong starformation activity in the past of NGC 2366, from ~100 Myr to <~3 Gyrago.Based on observations obtained with the NASA/ESA Hubble Space Telescopethrough the Space Telescope Science Institute, which is operated byAURA, Inc., under NASA contract NAS5-26555.
| DDO 43: A Prototypical Dwarf Irregular Galaxy?
We present sensitive and high-resolution 21 cm observations of the dwarfirregular (Im) galaxy DDO 43, in conjunction with optical broadband andnarrowband images in U, B, V, and Hα. The observations are used toexamine the relationship of its H I morphology and kinematics to pastand present star formation. Optically, it is a small (R25=990pc), faint (MB of -14.0) dwarf Im with a slightly boxy shape.In H I, DDO 43 has an extended (RHI/RH=2.8) gasenvelope. There is a high-density ridge associated with the optical bodyof the galaxy containing several higher density knots and lower densityholes. The largest hole is ~850×530 pc. No expansion is detected,so it must be relatively old. The largest and potentially oldest (7-70Myr) of the six identified star clusters is located at the western edgeof the hole. Four of the other clusters are located near high-densitypeaks. There are several H II regions, most (but not all) of which areassociated with peaks in the H I surface density. The overall starformation rate is average for its type. In many ways, DDO 43 is a verytypical dwarf Im galaxy. Its H I morphology is consistent with a historyof episodes of localized star formation that create holes and shells inthe interstellar medium, some of which can overlap. These features arelocated within the area of solid-body rotation in the galaxy; the lackof shear in these small systems allows such structures to persist forlong periods of time.
| DDO 88: A Galaxy-sized Hole in the Interstellar Medium
We present an H I and optical study of the gas-rich dwarf irregulargalaxy DDO 88. Although the global optical and H I parameters of DDO 88are normal for its morphological type, it hosts a large (3 kpc diameter)and unusually complete ring of enhanced H I emission. The normalappearance of this galaxy in the optical and the outer regions of the HI give no hint of the presence of the striking H I ring in the innerregions. The gas ring is located at approximately one-third of the totalH I radius and one-half the optically defined Holmberg radius, andcontains 30% of the total H I of the galaxy. The ring surrounds acentral depression in the H I distribution. If the H I ring and centraldepression in the gas were formed by the energy input from winds andsupernova explosions of massive stars formed in a starburst, as isthought often to be the case, the star-forming event would have formed0.1%-1% of the total stellar mass of the galaxy. However, the UBV colorsin the H I hole are not bluer than the rest of the galaxy, as would beexpected if an unusual star-forming event had taken place thererecently, although there is an old (~1-3 Gyr), red cluster near thecenter of the hole that is massive enough to have produced the hole inthe H I. An age estimate for the ring is uncertain, however, because itis not observed to be expanding. An expansion model produces a lowerestimate of 0.5 Gyr, but the presence of faint star formation regionsassociated with the ring indicates a much younger age. We also estimatethat the ring could have dispersed by now if it is older than 0.5 Gyr.This implies that the ring is younger than 0.5 Gyr. A younger age wouldindicate that the red cluster did not produce the hole and ring.Therefore, uncertainties prevent us from concluding that the cluster andthe H I hole are definitely related. If this ring and the depression inthe gas that it surrounds were not formed by stellar winds andsupernovae, this would indicate that some other, currently unidentified,mechanism is operating.
| What is the temperature structure in the giant HII region NGC 588?
We present the results of an exhaustive study of the ionized gas inNGC 588, a giant Hii region in the nearby spiralgalaxy M 33. This analysis uses a high number ofdiagnostics in the optical and infrared ranges. Four temperaturediagnostics obtained with optical lines agree with a gas temperature of11 000 K, while the [Oiii] λ5007/λ88 μm ratio yields amuch lower temperature of ≈8000 K. This discrepancy suggests thepresence of large temperature inhomogeneities in the nebula. Weinvestigated the cause of this discrepancy by constructingphotoionization models of increasing complexity. In particular, we usedthe constraints from the Hα and Hβ surface brightnessdistributions and state-of-the-art models of the stellar ionizingspectrum. None of the successive attempts was able to reproduce thediscrepancy between the temperature diagnostics, so the thermal balanceof NGC 588 remains unexplained. We give an estimateof the effect of this failure on the O/H and Ne/O estimates and showthat O/H is known to within ±0.2 dex.
| 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.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - III. 15 new velocity fields and study of 46 rotation curves
We present Fabry-Pérot observations obtained in the frame of theGHASP survey (Gassendi Hα survey of SPirals). We have derived theHα maps, the velocity fields and the rotation curves for a set of15 galaxies. The data presented in this paper are combined with the datapublished in our two previous papers in order to make a preliminaryanalysis of the rotation curves obtained for 46 galaxies. We check theconsistency of our data with the Tully-Fisher relationship and concludethat our Hα rotation curves reach the maximum velocity in most ofthe cases, even with solid-body rotating galaxies. We find that ourrotation curves, on average, almost reach the isophotal radiusR25. We confirm the trend, already mentioned by Rubin,Waterman & Kenney and Márquez et al., that the maximumextension of the Hα rotation curves increases with the type of thespiral galaxy, up to t~ 7-8 and we find that it decreases for magellanicand irregular galaxies. We also confirm the trend seen by Márquezet al. that later types tend to have lower values of the internal slopeof the rotation curve, in agreement with Rubin et al.
| A Keck/HIRES Study of Kinematics of the Cold Interstellar Medium in Dwarf Starburst Galaxies
We have obtained high-resolution Echelle spectra (R=30,000-50,000) ofthe Na D absorption doublet (λλ5890, 5896) for six dwarfstarburst galaxies and two more luminous starbursts: M82 and NGC 1614.The absorption features were separated into multiple components andseparated into stellar and interstellar parts on the basis ofkinematics. We find that three of the dwarfs show outflows, with anaverage blueshift of 27 km s-1. This is small compared withthe highest velocity components in NGC 1614 and M82 (blueshifted by 150and 91 km s-1, respectively); these two brighter galaxiesalso show more complex absorption profiles than the dwarfs. None of theoutflow speeds clearly exceed the escape velocity of the host galaxy.Sight lines in NGC 2363 and NGC 4214 apparently intersect expandingshells. We compare the shocked gas velocity (vNaD) with theionized gas velocity (vHα) and interpret the velocitydifference as either a trapped ionization front (NGC 4214) or a leaky HII region (NGC 2363). The dwarfs showNNaD=1011.8-1013.7cm-2,while the Na D columns in M82 and NGC 1614 are 1013.7 and1014.0 cm-2, respectively. The mass of expelledgas is highly sensitive to outflow geometry, dust depletion, andionization fraction, but with a simple shell model we estimate neutraloutflow gas masses from ~106 to ~1010Msolar.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof 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 H I Kinematics and Distribution of Four Blue Compact Dwarf Galaxies
We present VLA H I observations of the blue compact dwarf (BCD) galaxiesNGC 2366, NGC 4861, VII Zw 403, and Haro 2. These galaxies span a rangeof BCD morphological types. The cometary-like BCDs NGC 2366 and NGC 4861have regular rotational kinematics with a V/σ of 8.7 and 6.4,respectively. On the other hand, the velocity fields of the iE BCD VIIZw 403 and of the nE BCD Haro 2 lack regularity, and their rotationalmotion is around the major, not the minor, axis. The H I distribution iscentrally peaked in VII Zw 403 and Haro 2, a general feature of all iEand nE-type BCDs, the most common ones. In contrast, cometary-type BCDshave multiple H I peaks that are scattered over the disk. The activeregions of star formation are associated with regions of high H I columndensities, with slight displacements between the H I and stellar peaks.NGC 2366 shows many H I minima, resulting from the disruptive influenceof massive star formation and supernovae on the interstellar medium(ISM). In NGC 2366 and NGC 4861, there is a tendency for H I gas with ahigher velocity dispersion to be associated with regions of lower H Icolumn density. This anticorrelation can be understood in the context ofa two-phase model of the ISM. In all BCDs, the radio continuum emissionis associated with the star-forming regions and is predominantly thermalin nature. H I clouds with no optical counterparts have been found inthe vicinity of NGC 4861 and Haro 2.Based on observations obtained at the National Radio AstronomyObservatory, a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe 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/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/
| The Impact of Starbursts in the Halos of Blue Compact Dwarf Galaxies (A Proposal for OSIRIS)
We propose to perform deep narrow band imaging in several emission linesof interest for a sample of blue compact dwarf galaxies at the GTC, withthe OSIRIS facility in tunable filter mode. From the emission line maps,we will determine the physical properties of the diffuse ionized gas anddisentangle the nature of the ionizing source.
| 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.
| 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.
| Luminous Blue Variables, cool hypergiants and some impostors in the H-R diagram
Current observations of the S Dor/LBVs and candidates and theimplications for their important role in massive star evolution arereviewed. Recent observations of the cool hypergiants are altering ourideas about their evolutionary state, their atmospheres and winds, andthe possible mechanisms for their asymmetric high mass loss episodeswhich may involve surface activity and magnetic fields. Recent resultsfor IRC+10 420, ρ Cas and VY CMa are highlighted. S Dor/LBVs ineruption, and the cool hypergiants in their high mass loss phases withtheir optically thick winds are not what their apparent spectra andtemperatures imply; they are then `impostors' on the H-R diagram. Theimportance of the very most massive stars, like η Carinae and the`supernovae impostors' are also discussed.
| The Effect of Collisional Enhancement of Balmer Lines on the Determination of the Primordial Helium Abundance
This paper describes a new determination of the primordial heliumabundance (YP), based on the abundance analysis of fivemetal-poor extragalactic H II regions. For three regions of the sample(SBS 0335-052, I Zw 18, and H29) we present tailored photoionizationmodels based on improved calculations with respect to previous models.In particular, we use the photoionization models to study quantitativelythe effect of collisional excitation of Balmer lines on thedetermination of the helium abundance (Y) in the individual regions.This effect is twofold: first, the intensities of the Balmer lines areenhanced with respect to the pure recombination value, mimicking ahigher hydrogen abundance; second, the observed reddening is larger thanthe true extinction, as a result of the differential effect ofcollisions on different Balmer lines. In addition to these effects, ouranalysis takes into account the following features of H II regions: (1)the temperature structure, (2) the density structure, (3) the presenceof neutral helium, (4) the collisional excitation of the He I lines,(5) the underlying absorption of the He I lines, and (6) the opticalthickness of the He I lines. The object that shows the highest increasein Y after the inclusion of collisional effects in the analysis is SBS0335-052, whose helium abundance has been revised by ΔY=+0.0107.The revised Y-values for the five objects in our sample yield anincrease of +0.0035 in YP, givingYP=0.2391+/-0.0020.
| [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.
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| Temperature Structure and Chemical Abundances in Gaseous Nebulae
In this summary I review some of the results presented in this symposiumrelating to planetary nebulae, H II regions, chemical evolution ofgalaxies, and the determination of the primordial helium abundance. Toget a more general perspective of this symposium I encourage you to readall the contributions to these proceedings.
| Thirty Years of Extragalactic H II Region Studies
I review a small part of the past thirty years of studies ofextragalactic H II regions. Comparing a review of available results in1975 to what we know today, we see a enormous increase in our knowledgeof physical conditions and abundances in extragalactic H II regions,chemical evolution of galaxies, and the primordial helium fraction.Manuel Peimbert and Silvia Torres-Peimbert have made pioneeringcontributions to this field. Here I outline the progress inunderstanding extragalactic H II regions and highlight the Peimberts'contributions.
| Are Temperature Fluctuations Out There?
The effect of temperature fluctuations in the spectra of ionized nebulaewas firstly explored by Peimbert (1967). Since then, the problem oftheir existence has remained an open question. In fact, there areobservations and models that argue both in favor and against suchfluctuations and these are reviewed in this paper. We also discuss themechanisms that could produce such fluctuations and their possiblepresence and effects in extragalactic objects.
| Chemical Abundances of NGC 5461 and NGC 5471 Derived from Echelle Spectrophotometry
We present high resolution spectroscopic data of the two giantextragalactic H II regions NGC 5461 and NGC 5471 in M101, which havebeen obtained with the 2.1-m telescope of the Observatorio AstronómicoNacional in San Pedro Mártir, Baja California. We measured theintensities of several H and He recombination lines, and of forbiddenlines of a large number of ions. We calculate the physical conditions inthe two nebulae with a large number of diagnostics and determine theirchemical abundances by applying ionization correction factors (icf's) to the observed ionic abundances. For NGC 5461, theicf's are based on a tailored photoionization model of the region(Luridiana & Peimbert 2001), while for NGC 5471 they are computedfrom those predicted by a photoionization model of NGC 2363 (Luridiana,Peimbert, & Leitherer 1999), a region which is similar to NGC 5471in the ionization structure. For both regions, the icf's arecompared to those computed following the prescriptions by Mathis &Rosa (1991). Such comparison shows large discrepancies for severalelements, 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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