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Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried H II Regions in the Magellanic Clouds
We present a systematic survey for ultracompact (UC) H II regions in theMagellanic Clouds. Understanding the physics of massive star formation(MSF) is a critical astrophysical problem. The study of MSF began in ourGalaxy with surveys of UC H II regions, but before now this has not beendone for other galaxies. We selected candidates on the basis of theirInfrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6cm with the Australia Telescope Compact Array. Nearly all of theobserved regions contain compact radio sources consistent with thermalemission. Many of the sources are related to optically visible H IIregions, and often the radio emission traces the youngest and densestpart of the H II region. The luminosity function and number distributionof Lyman continuum fluxes of the compact radio sources are consistentwith standard stellar and cluster initial mass functions. This type ofsystematic assessment of IRAS diagnostics is important for interpretingSpitzer Space Telescope data, which will probe similar physical scalesin nearby galaxies as IRAS did in the Magellanic Clouds.
| The Discovery of a 12th Wolf-Rayet Star in the Small Magellanic Cloud
We report the discovery of a relatively faint (V=15.5) early-type WNstar in the SMC. The line strength and width of He II λ4686emission are similar to those of the other SMC WN stars, and thepresence of N V λλ4603, 4619 emission (coupled with thelack of N III) suggests that this star is of spectral type WN3-4.5, andthus is similar in type to the other SMC WR stars. Also like the otherSMC WN stars, an early-type absorption spectrum is weakly present. Theabsolute magnitude is comparable to that of other (single) Galacticearly-type WN stars. The star is located in the Hodge 53 OB association,which is also the home of two other SMC WN stars. This star, which wedesignate SMC-WR12, was actually detected at a high significance levelin an earlier interference-filter survey, but the wrong star wasobserved as part of a spectroscopic follow-up, and this case of mistakenidentity resulted in its Wolf-Rayet nature not being recognized untilnow.
| On the origin of nitrogen
The problem of the origin of nitrogen is considered within the frameworkof an empirical approach. The oxygen abundances and nitrogen to oxygenabundances ratios are derived in H II regions of a number of spiralgalaxies through the recently suggested P-method using more than sixhundred published spectra. The N/O-O/H diagram for H II regions inirregular and spiral galaxies is constructed. It is found that the N/Ovalues in H II regions of spiral galaxies of early morphological typesare higher than those in H II regions with the same metallicity inspiral galaxies of late morphological types. This suggests along-time-delayed contribution to the nitrogen production. The N/O ratioof a galaxy can then be used as an indicator of the time that haselapsed since the bulk of star formation occurred, or in other words ofthe nominal ``age'' of the galaxy as suggested by Edmunds & Pagelmore than twenty years ago. The scatter in N/O values at a given O/H canbe naturally explained by differences in star formation histories ingalaxies. While low-metallicity dwarf galaxies with low N/O do notcontain an appreciable amount of old stars, low-metallicity dwarfgalaxies with an appreciable fraction of old stars have high N/O.Consideration of planetary nebulae in the Small Magellanic Cloud and inthe Milky Way Galaxy suggests that the contribution of low-mass stars tothe nitrogen production is significant, confirming the conclusion thatthere is a long-time-delayed contribution to the nitrogen production.
| Supernova Remnants in the Southwestern Part of the Small Magellanic Cloud
We have compared radio images from the Australia Telescope Compact Arrayand X-ray images from ROSAT of an area about 560 pc in diameter aroundthe very extended emission nebula N19 on the southwestern edge of thebright ridge of the Small Magellanic Cloud. These data have allowed usto identify tentatively seven supernova remnants in this region. Thisnumber is large enough to suggest that the area has recently undergonesome starburst activity. N19 itself appears to contain two overlappingsupernova remnants within the extended H II region.
| A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We report on a comprehensive search for Wolf-Rayet (W-R) stars in theSMC using interference filter imaging. Photometry of over 1.6 millionstellar images on multiple, overlapping fields covering 9.6deg2 found the previously known W-R stars at very highsignificance levels, two known Of-type stars, plus additionalcandidates, which we examined with slit spectroscopy. We discovered twonew Wolf-Rayet stars, both of type ``WN3+abs,'' bringing the totalnumber in the SMC to 11. We discuss their spectra, as well asreclassifying the previously known ones with our new data. Our surveyalso revealed four newly found Of-type stars, including one of the O5f?ptype, which is one of the earliest type stars known in the SMC. Anothernewly identified Of star is AV 398 (O8.5 If), a star often used inextinction studies under the assumption that it is of early B type. Werecover S18 (AV 154), a B[e] star whose spectrum currently lacks He IIλ4686 emission but which must have had strong emission a yearearlier; we compare this star to S Dor, suggesting that it is indeed aluminous blue variable. We also find a previously unknown symbiotic starwhose spectrum is nearly identical to the Galactic symbiotic AG Dra.More important, perhaps, than any of these discoveries is thedemonstration that there is not a significant number of W-R starswaiting to be discovered in the SMC. The number of W-R stars is a factorof 3 times lower in the SMC (per unit luminosity) than in the LMC. Thisstrongly suggests that at the low metallicity that characterizes the SMConly the most massive stars can evolve to W-R type.
| ROSAT HRI catalogue of X-ray sources in the SMC region
During the operational phase of the ROSAT satellite between 1990 and1998 the X-ray telescope pointed 71 times to the Small Magellanic Cloud(SMC) for observations with the High Resolution Imager (HRI), covering afield of 5 degr x 5 degr. From these data a catalogue of 121 discreteX-ray sources was derived. By cross-correlating the source cataloguewith the SIMBAD data base and the TYCHO catalogue, the systematicpositional error of the HRI source positions could be reduced. In totalthe X-ray position for 99 HRI sources was corrected yielding positionalerrors between 1'' and 16 arcsec. The HRI catalogue was alsocross-correlated with the catalogues derived from the ROSAT PositionSensitive Propotional Counter (PSPC) pointings (Kahabka et al.\cite{kah99}; Haberl et al. \cite{hab00}). For 75 HRI sources PSPCcounterparts were found and thus hardness ratios are given. With thehelp of the information obtained from the cross-correlations 56 HRIsources were identified with objects of known or proposed nature. Fourforeground stars, six supernova remnants, four supersoft sources, 12X-ray binaries, and one AGN were detected by the HRI. Based on theexistence of a likely optical counterpart or properties like hardnessratio and X-ray to optical flux ratio, further 15 HRI sources wereclassified into different source types.
| Updating the Census of Star Clusters in the Small Magellanic Cloud
Surveys using CCD detectors are retrieving bright and faint catalogedclusters and revealing new ones in the Magellanic Clouds. This paperdiscusses the contribution of the OGLE Survey to the overall census ofstar clusters in the Small Magellanic Cloud (SMC). A detailedcross-identification indicates that the new objects in the SMC OGLEcatalog are 46. The increase in the number of cataloged clusters is ~7%,the total sample being ~700. This updated census includes embeddedclusters in H II regions and a density range attaining loose systems.
| A ROSAT PSPC catalogue of X-ray sources in the SMC region
We present a catalogue of 517 discrete X-ray sources in a 6°x6°\field covering the Small Magellanic Cloud (SMC). The catalogue wasderived from the pointed ROSAT PSPC observations performed betweenOctober 1991 and May 1994 and is complementary to the Large MagellanicCloud (LMC) catalogue published by \cite[Haberl & Pietsch(1999).]{H99} We followed the same identification scheme and used, amongother information, X-ray hardness ratios and spatial extent to classifyunknown sources as candidates for active galactic nuclei (AGN),foreground stars, supernova remnants (SNRs), supersoft sources (SSSs)and X-ray binaries. For 158 sources a likely source type is given, fromwhich 46 sources are suggested as background AGN (including candidatesresulting from a comparison of X-ray and radio images). Nearly all ofthe X-ray binaries known in the SMC were detected in ROSAT PSPCobservations; most of them with luminosities below \ergs{36} suggestingthat the fraction of high luminosity X-ray binary systems in theMagellanic Clouds (MCs) is not significantly larger than in our galaxy.Seventeen X-ray sources are associated with SNRs found in earlier workand we suggest here two additional extended sources as SNR candidates.Three very soft sources are newly classified as SSSs from which one isidentified with the symbiotic star LIN 358 in the SMC. Table 2 is onlyavailable in electronic form at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A ROSAT PSPC X-ray survey of the Small Magellanic Cloud
We present the results of a systematic search for point-like andmoderately extended soft (0.1-2.4 keV) X-ray sources in a raster of ninepointings covering a field of 8.95\ deg(2) and performed with the ROSATPSPC between October 1991 and October 1993 in the direction of the SmallMagellanic Cloud (SMC). We detect 248 objects which we include in thefirst version of our SMC catalogue of soft X-ray sources. We set upseven source classes defined by selections in the count rate, hardnessratio and source extent. We find five high luminosity super-soft sources(1E 0035.4-7230, 1E 0056.8-7146, RX J0048.4-7332, RX J0058.6-7146 and RXJ0103-7254), one low-luminosity super-soft source RX J0059.6-7138correlating with the planetary nebula L357, 51 candidate hard X-raybinaries including eight bright hard X-ray binary candidates, 19supernova remnants (SNRs), 19 candidate foreground stars and 53candidate background active galactic nuclei (and quasars). We give alikely classification for ~ 60% of the catalogued sources. The totalcount rate of the detected point-like and moderately extended sources inour catalogue is 6.9 +/- 0.3 s(-1) , comparable to the backgroundsubtracted total rate from the integrated field of ~ 6.1+/-0.1 s(-1) .Table 1 is also available in electronic from at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A radio continuum study of the Magellanic Clouds. VIII. Discrete sources common to radio and infrared surveys of the Magellanic Clouds
We compare Parkes Telescope radio surveys with the IRAS Infrared (IR)surveys of the Magellanic Clouds (MCs). We find 130 discrete sources incommon towards the Large Magellanic Cloud (LMC) with both radio and IRemission. These 130 sources are mainly H Ii regions (89) and supernovaremnants (21). For 12 of the sources we have no identification and eightare background objects. We find 38 sources in common for the SmallMagellanic Cloud (SMC). Most of these sources are intrinsic (31) to theSMC, five sources are previously known background galaxies and twosources remain ambiguous. A flux density comparison of the radio and IRsources shows very good correlation and we note that the strongestsources at both radio and IR frequencies are H Ii regions. From theradio-IR comparison we propose that some 40 new sources in the LMC and10 in the SMC are H Ii regions or SNRs. All these new sources are alsoidentified in optical surveys.
| A radio continuum study of the Magellanic Clouds. VII. Discrete radio sources in the Magellanic Clouds
We present a study of discrete radio sources in the Magellanic Clouds(MCs) using the latest large-scale radio surveys made with the Parkesradio telescope between 1.4 and 8.55 GHz. These surveys achieved highersensitivity then previous surveys done with the Parkes telescope and sothe number of discrete radio sources detected towards the MCs hasincreased by factor of five. Also, we have obtained improved positions,flux densities and radio spectral indices for all of these sources. Atotal of 483 sources towards the Large Magellanic Cloud (LMC) and 224towards the Small Magellanic Cloud (SMC) have been detected at at leastone radio frequency. Most of the MC's sources have been classified inone of three groups: SNRs, H Ii regions or background sources accordingto classification criteria established here. In total, 209 discreteradio sources in the LMC and the 37 sources in the SMC are classifiedhere to be either H Ii regions or SNRs. We investigate their luminosityfunctions as well as the statistics of background sources behind theMCs. Also, we examine the distribution of SNRs and H Ii regions in theMCs. Tables 5 and 6 are only available electronically at the CDS via ftp130.79.128.5 or via http://cdsweb.u-strasbg.fr/Abstract.html
| A radio continuum study of the Magellanic Clouds. VI. Discrete sources common to radio and X-ray surveys of the Magellanic Clouds
By comparing Parkes telescope radio surveys with the X-ray ROSAT All-SkySurvey (RASS) we have found 71 discrete sources of both radio and X-rayemission in the Large Magellanic Cloud (LMC). These 71 sources aremainly supernova remnants (SNRs) and SNR candidates (36), and backgroundsources (27). For six of the sources we have no proposed identificationand the other two are Hii regions. A source-intensity comparison of theradio and X-ray sources shows very little correlation, but we note thatthe strongest SNRs at both radio and X-ray frequencies are young SNRsfrom Population I. Six new LMC SNR candidates are proposed. From theradio flux density of the SNRs we have estimated the SNR birth rate tobe one every 100 (+/-20) yr and the star-formation rate (SFR) to be 0.7(+/-0.2) M_ȯyr(-1) . A similar comparison was undertaken for theSmall Magellanic Cloud (SMC), but instead of the RASS we used a rosterof pointed observations made with the ROSAT Position SensitiveProportional Counter (PSPC). This comparison resulted in 27 sources incommon between the Parkes radio and ROSAT PSPC surveys. Two new SMCsources are proposed for SNR candidates. The SMC SNR birth rate wasestimated to be one every 350 (+/-70) yr and the SFR was estimated to be0.15 (+/-0.05) M_ȯyr(-1) . Tables 2 and 3 are also availableelectronically at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http: //cdsweb.u-strasbg.fr/Abstract.html
| UIT: Ultraviolet Observations of the Small Magellanic Cloud
A mosaic of four UIT far-UV (FUV) (lambda_ {eff} = 1620 Angstroms)images, with derived stellar and \hii\ region photometry, is presentedfor most of the Bar of the SMC. The UV morphology of the SMC's Bar showsthat recent star formation there has left striking features including:a) four concentrations of UV-bright stars spread from northeast tosouthwest at nearly equal ( ~ 30 arcmin=0.5 kpc) spacings; b) one of theconcentrations, near DEM 55, comprises a well-defined 8-arcmin diameterring surrounded by a larger \ha ring, suggestive of sequential starformation. FUV PSF photometry is obtained for 11,306 stars in the FUVimages, resulting in magnitudes \mbfive. We present a FUV luminosityfunction for the SMC bar, complete to \mbfive ~ 14.5. Detected objectsare well correlated with other SMC Population I material; of 711 \ha\emission-line stars and small nebulae within the UIT fields of view, 520are identified with FUV sources. The FUV photometry is compared withavailable ground-based catalogs of supergiants, yielding 191 detectionsof 195 supergiants with spectral type earlier than F0 in the UIT fields.The (\mbfive-V) color for supergiants is a sensitive measure of spectraltype. The bluest observed colors for each type agree well with colorscomputed from unreddened Galactic spectral atlas stars for types earlierthan about A0; for later spectral types the observed SMC stars rangesignificantly bluer, as predicted by comparison of low-metallicity andGalactic-composition models. Redder colors for some stars of allspectral types are attributed to the strong FUV extinction arising fromeven small amounts of SMC dust. Internal SMC reddenings are determinedfor all catalog stars. All stars with E(B-V)>0.15 are within regionsof visible \ha\ emission. FUV photometry for 42 \ha-selected \hii\regions in the SMC Bar is obtained for stars and for total emission (asmeasured in \hii-region-sized apertures). The flux-weighted averageratio of total to stellar FUV flux is 2.15; consideration of the stellarFUV luminosity function indicates that most of the excess total flux isdue to scattered FUV radiation, rather than stars fainter than\mbfive=14.5. Both stellar and total emission are well correlated with\ha\ fluxes measured by Kennicutt and Hodge (1986; hereafter KH),yielding FUV/\ha\ flux ratios that are consistent with models of SMCmetallicity, ages from 1-5 Myr, and moderate (E(B-V)=0.0-0.1 mag)internal SMC extinction.
| A radio continuum study of the Magellanic Clouds V. Catalogues of radio sources in the Small Magellanic Cloud at 1.42, 2.45, 4.75, 4.85 and 8.55 GHz
We present catalogues of radio sources in the Small Magellanic Cloudfrom observations with the Parkes radio telescope at 1.42, 2.45, 4.75and 8.55 GHz, and an additional catalogue from the Parkes-MIT-NRAOsurvey at 4.85 GHz. A total of 224 sources were detected at at least oneof these frequencies, 60 of which are reported here for the first timeas radio sources. We compare positions and flux densities of thesesources with previously published results and find no significantpositional displacement or flux discrepancies. Tables 2-7 are onlyavailable electronically at the CDS via ftp 130.79.128.5 or athttp://cdsweb.u-strasbg.fr/Abstract.html
| Extinction and reddening of HII regions in the Small Magellanic Cloud.
We present absolute Hα and Hβ fluxes, obtained with aFabry-Perot spectrophotometer, of 24 bright HII regions in the SmallMagellanic Cloud. The photographic Hα maps of Kennicutt &Hodge (1986) are re-calibrated using these new Hα fluxes; the newcalibration gives fluxes 25% smaller than those previously published.These photographic and photoelectric Hα data are used inconjunction with radio continuum observations at 843 MHz from theMolonglo Observatory Synthesis Telescope to study the dust associatedwith SMC HII regions. For most regions the derived reddenings andextinctions are compatible with the standard Galactic extinction law anduniform interstellar extinction. A few regions display relatively highreddening and extinction; these are bright compact sources, such asN13AB, N27 and the cores of N81 and N88, all of which probably haveclosely associated dust. Low resolution Hi observations do not detectthese high concentrations of dust.
| A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. I. Small Magellanic Cloud and Bridge
A survey of extended objects in the Magellanic System was carried out onthe ESO/SERC R and J Sky Survey Atlases. The present work is dedicatedto the Small Magellanic Cloud and to the inter-Magellanic Cloud region("Bridge") totaling 1188 objects, of which 554 are classified as starclusters, 343 are emissionless associations, and 291 are related toemission nebulae. The survey includes cross-identifications amongcatalogs, and we present 284 new objects. We provide accurate positions,classification, homogeneous sizes, and position angles, as well asinformation on cluster pairs and hierarchical relation for superimposedobjects. Two clumps of extended objects in the Bridge and one at theSmall Magellanic Cloud wing tip might be currently forming dwarfspheroidal galaxies.
| Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.
| The initial mass function for massive stars in the Magellanic Clouds. 2: Interstellar reddening toward 14 OB associations
We have used UBV CCD photometry to determine the interstellar reddeningtoward 14 OB associations in the Magellanic Clouds. The tworeddening-free indices available in the UBV system were used to obtainthe reddening estimates. The mean color excesses of the associationsrange from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC)and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). Wehave modeled the observed scatter in the color excesses of individualstars within the associations and find that statisically significantdifferential reddening exists in at least some of the associations.
| The initial mass function for massive stars in the Magellanic Clouds. 1: UBV photometry and color-magnitude diagrams for 14 OB associations
UBV charge coupled device (CCD) photometry has been obtained for 14 OBassociations in the Magellanic Clouds using the University of Toronto's0.6 m telescope and the Carnegie Institution of Washington's 1.0 mreflector, both on Las Campanas, Chile. The data are presented and usedto construct color-magnitude diagrams for the purposes of investigatingthe massive-star content of the associations.
| A New Catalogue of Hα Emission Line Stars and Small Nebulae in the Small Magellanic Cloud
An objective-prism survey of the Small Magellanic Cloud has beenperformed through an Hα + [N II] interference filter, using the0.90 m Curtis Schmidt telescope of Cerro Tololo. 1898 emission-lineobjects have been detected in the main body of this galaxy, almostquadrupling the number of those found, in the same region, by theprevious objective-prism surveys. Among these objects are newlydiscovered planetary nebulae, compact HII regions and late-type stars.Continuum intensity, as well as the shape and relative strength of theHα emission-line have been estimated; coordinates, cross-identifications for the listed objects and 2.25 arcmin square findingcharts for all the objects are provided.
| A new identification technique for OB associations - OB associations in the Small Magellanic Cloud
A new identification technique for OB associations in resolved galaxiesis proposed and compared with other techniques available in theliterature. This methodology of identification, for its intrinsicobjectivity, is particularly suitable for reliable comparisons ofstellar associations in galaxies. As a check, a new identification of OBassociations in the Small Magellanic Cloud is given and compared withprevious work. The correlation of the OB associations with the H-alphaemission in the SMC is very strong, even though roughly 70 percent ofthe H II regions do not contain any identified OB association.
| Physical properties of H II regions in the Small Magellanic Cloud
Some physical properties of the H II regions and their ionizing stellarassociations, such as the Lyman continuum photon flux, the rms electrondensity, and the mass of ionized gas, were derived for 30 H II regionsin the SMC. Bright H II regions are found to be large and diffuse,whereas faint ones are compact; i.e., small and dense. The decrease ofW(H-beta) with the increase of the nebular size and decrease of rmselectron density is consistent with a trend in the sense of an H IIregion expansion with aging. However, the rms electron density tends tobe a decreasing function of the nebular ionizing flux, which isinconsistent with a single evolutionary sequence for the H II regions.
| Infrared observations of the Magellanic Clouds. I - The Small Magellanic Cloud
Results of IRAS pointed observations in four infrared wavelength bands(12, 25, 60, and 100 microns) for the SMC are presented. Maps withorthogonal scan directions are shown, and a source list containing 219infrared sources is extracted from the data. Comparison with the IRASPoint Source Catalog (PSC) shows that only three entries in this catalogare spurious. All 13 entries in the IRAS Small Scale Structure Catalog(SSS) in the SMC are confirmed. Seventy-two new infrared sources, notincluded in either the PSC or in the SSS are found. The present SMCinfrared source list is compared to other object lists. Two blueglobular clusters, 28 SAO stars, and seven planetary nebulae areidentified. No SMC stars are found. In general there is a goodcorrelation of infrared emission with the distributiion of H II regionsand dark clouds.
| Global Photometric Observations and Physical Properties of HII Regions in the Small Magellanic Cloud
Integrated photoelectric measurements of the equivalent width WH,~, the[0 III]/Hfl ratio and the Hfl emission line flux were obtained for 30 Hii regions in the SMC. Physical properties of the H H regions and theirionizing stellar associations were derived. Some aspects of the recentstar formation in the SMC and the evolution of H ii regions arediscussed
| BVR photoelectric photometry of late-type stars and a compilation of other data in the Small Magellanic Cloud
The basic data used in a discussion of the structure and morphology ofthe SMC Martin et al., (1989) are presented. New BVR photoelectric dataacquired at ESO, 88 SMC K-M type supergiants and three foreground Mstars; for all these stars, high-accuracy Coravel radial velocities hadbeen obtained. Taking into account all available data, a list of mean Vmagnitudes is obtained for 307 stars in the direction of the SMC withknown radial velocities. Also established is a list of mean weightedradial velocities on the IAU standard system for the 307 stars (amongwhich only two are probably foreground Galactic stars).
| The structure of the Small Magellanic Cloud
The structure of the SMC is investigated using previous H-I data,accurate radial velocities of 307 young stars and 35 H-II regions, andhigh-spectral-resolution profiles of interstellar absorption lines. Itis found that 224 stars and 30 H-II regions of the main body of the SMCare associated with four H-I components, and that 54 of the objects arenot associated with H I. Two main complexes of gas, stars, and H-IIregions are found, one with a velocity of about -28 km/s and the otherwith a velocity of about +9 km/s. Most of the young stars are shown tolie within a depth smaller than 10 kpc, in agreement with recentMagellanic Cepheid data.
| Global photometric observations of 30 H II regions in the Small Magellanic Cloud
Integrated photoelectric measurements of the equivalent width, theforbidden O-III/H-beta ratio, and the H-beta emission line flux weremade of 30 H-II regions in the SMC. The photometric system employedconsists of a narrow H-beta filter with Delta lambda = 30 A, a wideH-beta filter with Delta lambda = 150 A, and a forbidden O-III filtercentered at 5000 A. Calibration of the system is discussed. The presentdata have been compared with previous observations.
| Age determination of extragalactic H II regions
The H II region evolution models of Copetti et al. (1984) were comparedwith observational data of H II regions in the Magellanic Clouds, M 33,M 101 and of 'isolated extragalactic H II regions'. IMF with chi = 3 or2.5 are inconsistent with a large number of H II regions. The moreuniform age distribution of isolated extragalactic H II regions obtainedthrough an IMF with chi = 2 suggests that this value is more realisticthan chi = 1 or 1.5. The H II region age estimates indicate a burst ofstar formation about 5.5 + or - 1.0 10 to the -6th yr ago in the LMC andabout 2.3 + or - 0.9 x 10 to the 6th yr ago in the SMC. The observedforbidden O III/H-beta gradient in M 33 and M 101 must be caused bycolor temperature variation of the radiation ionizing the H II regions.
| A survey of chemical compositions of H II regions in the Magellanic Clouds
The reported investigation had the objective to extend abundancedeterminations to a larger number of H II regions in the Small Cloud(and a few more in the Large Cloud) in connection with a study regardingthe possible occurrence of large-scale abundance gradients analogous tothose found in Sc galaxies and in the Galaxy, taking into account alsoquestions concerning the existence of a characteristic abundance patternrepresenting the entire young population of either cloud. It is foundthat the laws governing enrichment of the interstellar medium are verysimilar in the Magellanic Clouds to what they are in the outer parts ofSc galaxies, including the very massive system M101. There is no reasonto believe that any special process such as preferential escape of gashas operated to reduce the effective yield in the Magellanic Clouds.
| Abundances in 10 H II regions in the Small Magellanic Cloud
A photoelectric study of selected emission lines in the spectral regionfrom 3700 to 6800 A is reported for 12 positions in 10 of the mostprominent H II regions in the Small Magellanic Cloud (SMC). Electrontemperatures ranging from about 11,500 to 14,000 K are derived from theobserved forbidden O III line intensities, and concentrations relativeto H II are calculated for He II, N II, O II, O III, Ne III, and S II.An N(He)/N(H) ratio of approximately 0.081 is determined for theinterstellar gas in the SMC; this is in reasonable agreement withprevious results and rather similar to the value of approximately 0.084for the Large Magellanic Cloud (LMC). Comparison of the He II/H IIratios in the SMC and LMC suggests that the observed abundance in thetwo clouds may be identical if corrections for He I are similar for thenebulae in both. The composition of the interstellar medium in the SMCis shown to be extremely homogeneous, and a metal deficiency of theinterstellar gas is established which indicates that the LMC and SMC arerepresentative of an earlier epoch in chemical evolution as comparedwith the Galaxy. It is concluded that the N enrichment in the two cloudshas resulted mainly from primary nucleosynthesis processes.
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