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The Spitzer Survey of the Small Magellanic Cloud: S3MC Imaging and Photometry in the Mid- and Far-Infrared Wave Bands
We present the initial results from the Spitzer Survey of the SmallMagellanic Cloud (S3MC), which imaged the star-forming bodyof the SMC in all seven MIPS and IRAC wave bands. We find that theF8/F24 ratio (an estimate of PAH abundance) haslarge spatial variations and takes a wide range of values that areunrelated to metallicity but anticorrelated with 24 μm brightness andF24/F70 ratio. This suggests that photodestructionis primarily responsible for the low abundance of PAHs observed instar-forming low-metallicity galaxies. We use the S3MC imagesto compile a photometric catalog of ~400,000 mid- and far-infrared pointsources in the SMC. The sources detected at the longest wavelengths fallinto four main categories: (1) bright 5.8 μm sources with very faintoptical counterparts and very red mid-infrared colors([5.8]-[8.0]>1.2), which we identify as YSOs; (2) bright mid-infraredsources with mildly red colors (0.16<~[5.8]-[8.0]<0.6), identifiedas carbon stars; (3) bright mid-infrared sources with neutral colors andbright optical counterparts, corresponding to oxygen-rich evolved stars;and (4) unreddened early B stars (B3-O9) with a large 24 μm excess.This excess is reminiscent of debris disks and is detected in only asmall fraction of these stars (<~5%). The majority of the brightestinfrared point sources in the SMC fall into groups 1-3. We use thisphotometric information to produce a catalog of 282 bright YSOs in theSMC with a very low level of contamination (~7%).
| Age distribution of young clusters and field stars in the Small Magellanic Cloud
Aims.In this paper we discuss the cluster and field star formation inthe central part of the Small Magellanic Cloud. The main goal is tostudy the correlation between young objects and their interstellarenvironment. Methods: . The ages of about 164 associations and 311clusters younger than 1 Gyr are determined using isochrone fitting. Thespatial distribution of the clusters is compared with the HI maps, withthe HI velocity dispersion field, with the location of the CO clouds andwith the distribution of young field stars. Results: .The clusterage distribution supports the idea that clusters formed in the last 1Gyr of the SMC history in a roughly continuous way with periods ofenhancements. The two super-shells 37A and 304A detected in the HIdistribution are clearly visible in the age distribution of theclusters: an enhancement in the cluster formation rate has taken placefrom the epoch of the shell formation. A tight correlation between youngclusters and the HI intensity is found. The degree of correlation isdecreasing with the age of the clusters. Clusters older than 300 Myr arelocated away from the HI peaks. Clusters and associations younger than10 Myr are related to the CO clouds in the SW region of the SMC disk. Apositive correlation between the location of the young clusters and thevelocity dispersion field of the atomic gas is derived only for theshell 304A, suggesting that the cloud-cloud collision is probably notthe most important mechanism of cluster formation. Evidence ofgravitational triggered episode due to the most recent close interactionbetween SMC and LMC is found both in the cluster andfield star distribution.
| An ATCA radio-continuum study of the Small Magellanic Cloud - III. Supernova remnants and their environments
A total of 717 sources from the Australia Telescope Compact Array (ATCA)catalogue of the Small Magellanic Cloud (SMC) have been classified inPaper II (of this series) by Payne et al. Here, we present a statisticalanalysis of all 16 confirmed supernova remnants (SNRs) and five newcandidate remnants. Included is a detailed discussion of the latter andthree other sources that have some SNR characteristics. We have alsofound a new microquasar candidate (ATCA J005523-721055) in addition toATCA J004718-723947 reported in Paper II. Source diameter comparisonssuggest that SNRs as a group are of similar size in radio, optical andX-ray with surface brightness values in the range of Galactic remnants.Remnant spectral indices, α[defined asSν~να, with Sν (fluxdensity) and ν (frequency)], have a mean of -0.63 [standard deviation(s.d.) = 0.43] and ROSAT X-ray hardness ratios confirm them to be softX-ray sources compared to background objects. We could not find anymeaningful correlation between SNR surface brightness and diameter; wealso discuss the number-diameter relation. A Venn diagram summarizesthat most SNRs emit radiation in all three of the radio, optical andX-ray domains. HII region diameter comparisons between radio and opticalsources show them to be a very diverse group that defies any simplerelationship, preventing any meaningful calculation of flux density orspectral index. To better understand environments containing SNRs, wehave scaled Hα images of four SMC regions and subtracted theirflux from the ATCA 2.37-GHz radio image. These 2.37 GHz-Hαsubtraction (or difference) images reveal some new sources withpredominantly non-thermal emission, exposing SNRs confused with HIIregions.
| Results of the ESO-SEST Key Programme on CO in the Magellanic Clouds. X. CO emission from star formation regions in LMC and SMC
We present J=1-0 and J=2-1 12CO maps of several star-formingregions in both the Large and the Small Magellanic Cloud, and brieflydiscuss their structure. Many of the detected molecular clouds arerelatively isolated and quite small with dimensions of typically 20 pc.Some larger complexes have been detected, but in all cases the extent ofthe molecular clouds sampled by CO emission is significantly less thanthe extent of the ionized gas of the star-formation region. Very littlediffuse extended CO emission was seen; diffuse CO in between orsurrounding the detected discrete clouds is either very weak or absent.The majority of all LMC lines of sight detected in 13CO hasan isotopic emission ratio I( 12CO)/I( 13CO) ofabout 10, i.e. twice higher than found in Galactic star-formingcomplexes. At the lowest 12CO intensities, the spread ofisotopic emission ratios rapidly increases, low ratios representingrelatively dense and cold molecular gas and high ratios marking COphoto-dissociation at cloud edges.
| The Small Magellanic Cloud in the far infrared. I. ISO's 170 mu m map and revisit of the IRAS 12-100 mu m data
The ISOPHOT experiment onboard the ISO satellite generated a completeview of the Small Magellanic Cloud (SMC) at 170 mu m with 1.5 arcminresolution. The map is analysed using an automated photometry programenabling accurate photometric characterization of the far infrared (FIR)emitting regions. An integrated FIR luminosity of 8.5x 107Lsun is obtained, leading to a star formation rate ofSFRFIR=0.015 Msun/yr. With an average dusttemperature of , the total dustmass follows to MD=3.7x105 Msun. Inthis paper, the sources detected at 170 mu m are compared with thoseobtainable from the IRAS satellite data. For this purpose, the 12 mu m,25 mu m, 60 mu m, and 100 mu m IRAS high resolution (HiRes) maps of theSMC are re-examined using the same method. In contrast to formerstudies, this provides an all-band ISO/IRAS source catalog which is nolonger based on eyeball classification, but relies on an algorithm whichis capable of automated, repeatable photometry, even for irregularsources. In the mid infrared IRAS bands numerous bright FIR emittingregions in the SMC are detected and classified: 73 sources are found at12 mu m, 135 at 25 mu m (most of them with Fnu <1.0 Jy).All three FIR bands at 170 mu m, 100 mu m, and 60 mu m reproduce theoverall morphological structure of the SMC similarly well, in contrastto the 12 mu m and 25 mu m maps which only contain a limited number ofextended sources and do not trace the main body of the SMC. 243 sourcesare detected in the ISO 170 mu m map, 155 of them with Fnu>=2.0 Jy. Comparable numbers are found for the two FIR IRASmaps at 60 mu m (384) and 100 mu m (338) with fluxes up to 450 Jy. 70 ofthe 243 170 mu m sources are assigned a general SED type (``cold'',``warm'', i.e., <30 K, >30 K) for the first time. A comparisonwith earlier IRAS results suggests that many source flux densities inthose studies have been under- or overestimated because ofnon-standardized fitting methods. Many sources with flux densities up to40 Jy listed in former catalogs cannot be identified in our data.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.The tables in Appendices A to E are only 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/401/873}
| Review of Discrete X-Ray Sources in the Small Magellanic Cloud: Summary of the ASCA Results and Implication on the Recent Star-Forming Activity
We made 22 observations on the Small Magellanic Cloud (SMC) and coveredfull regions of the main body and the eastern wing by the end of theASCA mission. We detected 106 discrete sources with a criterion of S/N> 5 and performed systematic analyses on all of the sources. Wedetermined the source positions with an ˜ 40" error radius (90%confidence) for sources detected in the central 20' radius of the GIS.We detected coherent pulsations from 17 sources. Among them, eight werenewly discovered during this study. We classified most of these pulsarsas X-ray binary pulsars (XBPs) based on their properties, such as theflux variability and the existence of an optical counterpart. Wedetected X-ray emission from eight supernova remnants (SNRs). Amongthem, five SNRs showed emission lines in their spectra, hence weregarded the five as thermal SNRs. We found that XBPs and thermal SNRsin the SMC can be clearly separated by their spectral hardness ratio.Applying this emp irical law to faint (thus unclassified) sources, wefound 19 XBP candidates and four thermal SNR candidates. We also foundseveral tens of candidates for active galactic nuclei, both from thehardness ratio and the logN--logS relation of extragalactic sources.Based on these ASCA results and further information from ROSAT, SAX,RXTE, CGRO, Chandra, and XMM-Newton, we compiled comprehensivecatalogues of discrete X-ray sources in the Small Magellanic Cloud.Using the catalogues, we derived the spatial distributions of XBPs andSNRs. XBPs and SNRs were found to be concentrated in the main body andeastern wing, which resembles the distribution of young stars with agesof ˜ 2 × 107 yr. By comparing the sourcepopulations in the SMC and our Galaxy, we suggest that the star-formingrate (per unit mass) in the SMC was much higher than the Galaxy ˜107 yr ago. We also discuss the recent change of thestar-forming rate in the SMC.
| 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.
| Detection and study of the compact HII region N26A-B in the Small Magellanic Cloud
This paper presents new imagery and spectrophotometric results for theN26 HII region in the Small Magellanic Cloud. The observations usingmonochromatic images and low-resolution spectra (3700-10 000 Å)reveal a compact and complex nebula composed of two cores A and B whereA in the region of Hβ is brighter than B by a factor ~ 5 anddistance of 2arcsec . The core A of FWHM ~ 2farcs 1 or 0.6 pc presents ahigh excitation [O III] lambda lambda5007 +4959/Hβ up to ~ 8 and ahigh reddening E(B-V) <= 0.6, while the core B is less excited buthas a higher reddening >=0.8. Each core contains one exciting source;the brighter one should be responsible for the high excitation of A. Theapparent spectral type of the two cores ranges from O7 to O9 V and thegas electron density and temperature were derived from the absorptionand emission-line intensities. The total mass of the ionized gas isevaluated at 13 Msun. The chemical abundances of He, O, N,Ne, S, and Ar were computed. These abundances seem consistent withaverage abundances for SMC HII regions, except N that appears slightlyoverabundant. N26A-B is comparable to the objects previously observed inthe LMC and SMC that we have called ``blobs''.
| Ultraviolet Imaging Telescope Observations of the Magellanic Clouds
We present wide-field far-ultraviolet (FUV; 1300-1800 Å) images ofthe Large and Small Magellanic Clouds (LMC, SMC). These data wereobtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1(1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the imagesprovide an extensive FUV mosaic of the SMC and contain numerous regionsin the LMC, covering a wide range of stellar densities and current starformation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OBassociations are completely or partially included in the observedfields. FUV data can identify the hottest OB stars more easily than canoptical photometry, and these stars dominate the ionizing flux, which iscorrelated to the observed Hα flux of the associated H ii regions.Of the H ii regions in the catalog of Davies, Elliott, & Meaburn(DEM), the UIT fields completely or partially include 102 DEM regions inthe LMC and 74 DEM regions in the SMC. We present a catalog of FUVmagnitudes derived from point-spread function photometry for 37,333stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMCwere presented recently by Cornett et al.), with a completeness limit ofm_UV ~ 15 mag and a detection limit of m_UV ~ 17.5. The averageuncertainty in the photometry is ~0.1 mag. The full catalog withastrometric positions, photometry, and other information is alsoavailable from publicly accessible astronomical data archives. We dividethe catalog into field stars and stars that are in DEM regions. Weanalyze each of these two sets of stars independently, comparing thecomposite UV luminosity function of our data with UV magnitudes derivedfrom stellar evolution and atmosphere models in order to derive theunderlying stellar formation parameters. We find a most probable initialmass function (IMF) slope for the LMC field stars of Gamma = -1.80 +/-0.09. The statistical significance of this single slope for the LMCfield stars is extremely high, though we also find some evidence for afield star IMF slope of Gamma ~ -1.4, roughly equal to the Salpeterslope. However, in the case of the stars in the DEM regions (the starsin all the regions were analyzed together as a single group), we findthree IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and-2.0. No typical age for the field stars is found in our data for timeperiods up to a continuous star formation age of 500 Myr, which is themaximum age consistent with the completeness limit magnitude of thecatalog's luminosity function. The best age for the collection ofcluster stars was found to be t_0 = 3.4 +/- 1.9 Myr; this is consistentwith the age expected for a collection of OB stars from many differentclusters.
| Molecular Cloud Structure in the Magellanic Clouds: Effect of Metallicity
The chemical structure of neutral clouds in low-metallicity environmentsis examined, with particular emphasis on the H to H2 and C+ to COtransitions. We observed near-IR H2 (1, 0) S(1), (2, 1) S(1), and (5, 3)O(3) lines and the 12CO J = 1 --> 0 line from 30 Doradus andN159/N160 in the Large Magellanic Cloud and from DEM S 16, DEM S 37, andLI-SMC 36 in the Small Magellanic Cloud. We find that the H2 emission isUV-excited and that (weak) CO emission always exists (in our surveyedregions) toward positions where H2 and [C II] emission have beendetected. Using a PDR code and a radiative transfer code, we simulatethe emission of line radiation from spherical clouds and from largeplanar clouds. Because [C II] emission and H2 emission arise on thesurface of the cloud and because the lines are optically thin, theselines are not affected by changes in the relative sizes of the neutralcloud and the CO-bearing core, while the optically thick CO emission canbe strongly affected. The sizes of clouds are estimated by measuring thedeviation of CO emission strength from that predicted by a planar cloudmodel of a given size. The average cloud column density, and thereforeits size, increases as the metallicity decreases. Our result agrees withthe photoionization-regulated star formation theory of McKee.
| 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.
| 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.
| Results of the ESO / SEST Key Programme - Co/ in the Magellanic Clouds - Part Two - Co/ in the Southwest Region of the Small Magellanic Cloud
We present the results of fully-sampled observations of the ^12^CO(1-0)line in the SW "Bar" region of the Small Magellanic Cloud (SMC), madewith the SEST telescope at 45" resolution. Previously, this line hadonly been observed in the SMC at the resolution of 8.8' (Rubio et al.1991). We have also observed the ^12^CO(2-1), ^13^CO(1-0) and^13^CO(2-1) lines at a number of positions. In this region of the SMC wehave detected CO only in the lower-velocity (i.e. nearer) component ofthis galaxy. The main CO features correspond to dark clouds seenoptically; some are associated with H II regions and with far-infraredsources. The spatial and velocity distribution of CO is complex withstructures on all scales and large-scale velocity gradients. This is anintrinsic property of the observed clouds: it is not due to confusionalong the line of sight. For two resolved CO clouds associated with H IIregions comparable to the Orion nebula, all four CO lines have beenobserved at a similar angular resolution. The ^12^CO(1-0)/^13^CO(1-0)line intensity ratios are between 10 and 16, the ^12^CO(2-1)/^12^CO(1-0)are slightly larger than 1 and the ^13^CO(2-1)/^13^CO(1-0) even larger.
| A comparison of far infrared and H-alpha emission of H-II regions in the Magellanic Clouds
From a comparison of the IRAS and smoothed H-alpha maps of theMagellanic Clouds, it was found that H-II regions with core-halostructure usually have higher F(60 microns)/F(H-alpha) ratios andprobably emit more in the far infrared than do extended low-density H-IIregions. This is consistent with the idea that the far infrared emissionis mainly produced by dust within H-II regions.
| A Comparison of Far Infrared and Hα Emission of HII Regions in the Magellanic Clouds
Not Available
| 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.
| 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.
| H II regions and star formation in the Magellanic Clouds
Photoelectrically calibrated maps of the H-alpha emission in theMagellanic Clouds have been used to measure integrated fluxes forseveral hundred H II regions and to study the properties of the H IIregion populations in the galaxies. The H II regions span a range of10,000 in luminosity, from objects on the scale of the Orion Nebula tothe 30 Doradus complex. The H-alpha luminosity function is wellrepresented over this entire range by a power law function, indicatingthat there is no characteristic luminosity scale for the H II regions.The distributions of nebular diameters, on the other hand, are fittedwell by exponential functions, with a scale length of 80 pc. Approximatefluxes for several of the extended filamentary networks in the galaxieshave also been measured. This extended component probability contributes15-25 percent of the total H-alpha luminosity of the galaxies.
| 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.
| The nebular complexes of the large and small Magellanic Clouds
Long exposures of the complexes of ionized hydrogen in both the LMC andSMC have been taken with the 48-in. SRC Schmidt camera through a H-alpha+ forbidden NII interference filter of 100-A bandwidth. These plates andidentifying charts are presented in a form in which little informationis lost. A catalog of many individual emission regions in both thesegalaxies is also compiled. The relationships between the nebulositiesand OB associations as well as between 21-cm neutral hydrogen emissionand continuum radio emission are discussed, and a number ofsupernova-remnant candidates are listed for further study.
| Catalogues of Hα-EMISSION Stars and Nebulae in the Magellanic Clouds.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1956ApJS....2..315H&db_key=AST
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Membre dels grups següents:
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Dades d'Observació i Astrometria
Catàlegs i designacions:
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