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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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| 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}
| 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.
| Dust and molecules in the Local Group galaxy NGC 6822. III. The first-ranked HII region complex Hubble V
We present maps of the first-ranked HII region complex Hubble V in themetal-poor Local Group dwarf galaxy NGC 6822 in the first fourtransitions of 12CO, the 158 mu m transition ofC+, the 21-cm line of HI, the Pabeta line of HII, and thecontinuum at 21 cm and 2.2 mu m wavelengths. We have also determinedvarious integrated intensities, notably of HCO+ and near-IRH2 emission. Although the second-ranked HII region Hubble Xis located in a region of relatively strong HI emission, our mappingfailed to reveal any significant CO emission from it. The relativelysmall CO cloud complex associated with Hubble V is comparable in size tothe ionized HII region. The CO clouds are hot (Tkin = 150 K)and have high molecular gas densities (n( H2) ~104 cm-3). Molecular hydrogen probably extendswell beyond the CO boundaries. C+ column densities are morethan an order of magnitude higher than those of CO. The total mass ofthe complex is about 106 Msun and molecular gasaccounts for more than half of this. The complex is excited by luminousstars reddened or obscured at visual, but apparent at near-infraredwavelengths. The total embedded stellar mass may account for about 10%of the total mass, and the mass of ionized gas for half of that. HubbleV illustrates that modest star formation efficiencies may be associatedwith high CO destruction efficiencies in low-metallicity objects. Theanalysis of the Hubble V photon-dominated region (PDR) confirms in anindependent manner the high value of the CO-to- H2 conversionfactor X found earlier, characteristic of starforming low-metallicityregions.
| First Results of a CO Survey of the Small Magellanic Cloud with NANTEN
We present the first results from a new CO12 (J = 1 -- 0)survey of the Small Magellanic Cloud (SMC) with the NANTENmillimeter-wave telescope. The survey covered the northeast andsouthwest regions of the main Bar, and the HII regions N 84, N 88 in theWing at a linear resolution of ~ 50 pc. Twenty-one giant molecularclouds (GMCs), whose masses are ~ 104 -- 106Msolar, were identified. Significant CO emission,T*R ~ 0.36 K, was first detected toward the N 84region in the Wing, which is comparable to those of the prominent COclouds associated with infrared sources LIRS 36, LIRS 49 in the Bar. TheGMCs exhibit a good spatial correlation with the HII regions and youngclusters, indicating that cluster formation is on-going in these GMCs.On the other hand, they show little correlation with older clusters orwith supernova remnants, suggesting rapid dissipation of CO.
| 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.
| 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.
| The Optical Gravitational Lensing Experiment. Cepheids in Star Clusters from the Magellanic Clouds
We present Cepheids located in the close neighborhood of star clustersfrom the Magellanic Clouds. 204 and 132 such stars were found in the LMCand SMC, respectively. The lists of objects were constructed based oncatalogs of Cepheids and star clusters, recently published by theOGLE-II collaboration. Location of selected Cepheids on the skyindicates that many of them are very likely cluster members. Photometricdata of Cepheids and clusters are available from the OGLE Internetarchive.
| The Optical Gravitational Lensing Experiment. Variable Stars in Star Clusters of the Magellanic Clouds. I.Eclipsing Systems in the Clusters of the SMC
The list of 127 eclipsing stars in optical coincidence with starclusters from the SMC is presented. It was prepared using the catalogsof eclipsing systems and star clusters from the SMC based onobservations collected during the OGLE-II microlensing project. Locationof 12 eclipsing stars in the color-magnitude diagram of clusters allowsto exclude their membership. Photometric data of 73 systems supporttheir membership. The remaining 42 objects were found in loose, faintclusters and therefore no conclusive statement about their membershipcan be made. All presented data are available from the OGLE archive.
| Molecular abundance variations in the Magellanic Clouds
We have observationally studied the effect of metallicity andfar-ultraviolet (FUV) radiation on the physical conditions and themolecular abundances in interstellar clouds in the Small and the LargeMagellanic Clouds (SMC and LMC, respectively). Spectral line emissionfrom a number of molecules was observed in a sequence of clouds withpositions in and between the 30 Doradus (30Dor) and the southern part ofthe N159 region in the LMC, and in one cloud (N27, also denoted LIRS 49)located in the SMC bar. Physical conditions and molecular abundanceswere estimated from the observational data by excitation and radiativetransfer calculations. A comparison of the molecular abundances inclouds in the SMC, the LMC, and the Galaxy is presented. We also reportthe first detection of hydrogen sulphide (ortho-H2S) in anextragalactic source, detections of methanol (CH3OH) inthermal emission and methyl acetylene (CH3CCH), and atentative detection of thio-formaldehyde (H2CS) in N159W. Theabundances (relative to H_2) of molecular species (except CO) in the LMCsources and in N27 are estimated to be typically 5x10(-10) , and1x10(-10) , respectively. These values apply to the gas volume definedby the CO line emission. Relative to Galactic clouds, the abundances inN159W (our reference cloud) are five to twenty times lower. In two ofthe clouds: N27 and the centremost cloud in 30Dor (30Dor-10), thederived abundances deviate significantly from those in the other cloudsin our sample, by being on the average six and eight times lower,respectively. In N27, the most likely explanation is the lowermetallicity in the SMC, whereas the underabundance in 30Dor-10 isprobably mainly caused by a more rapid photodissociation due to the moreintense FUV radiation in this area. An alternative explanation for theunderabundances in both N27 and 30Dor-10 would be a higher H/H_2 ratioinside these molecular clouds. The ethynyl radical (C_2H), with anestimated average abundance of 5x10(-9) in seven clouds in the LMC and3x10(-9) in two clouds in the SMC, is the most abundant observed tracemolecule after the CO isotopomers. Qualitatively, the high C_2Habundance can be explained as reflecting the C(+) -rich and FUVphoton-rich environment, i.e., a chemistry characteristic forphoton-dominated regions. For N27 we have, using HCO(+) and H(13) CO(+)data, estimated the gas-phase (12) C/(13) C ratio to be 40-90, a rangethat encompasses the values found in N159W and in Galactic disc clouds.In all clouds in our sample, the number density estimates from anexcitation analysis of CS, SO, HCO(+) , HCN and H_2CO are in the range(1-100)*E(4) cm(-3) . CO data gives a lower limit of a few*E(3) cm(-3) .However, the average densities (estimated from the virial mass) aresignificantly lower, typically a fewx10(2) cm(-3) , suggesting that theclouds (as probed by trace molecules) are very clumpy withvolume-filling factors <<1. In N159W, where our data-base is byfar most extensive, the number density and the kinetic temperature ofmolecular hydrogen in the dense part of the gas are estimated to be(1-10)*E(5) cm(-3) and 25+/-10 K, respectively. The correspondingnumbers in N27 are, although based on less data than in N159W,(5-50)x10(4) cm(-3) and 15+/-5 K. Thus, the metallicity differencebetween the LMC and the SMC does not seem to affect the density and thetemperature of the gas dramatically. In the SMC, the CO(J=1-0)/HCO(+)(J=1-0) line intensity ratio follows the same trend with respect to thestar-formation activity as in the LMC: a lower ratio is found in cloudswith a more vigorous star-formation activity. A similar trend is alsoexhibited by the CO(J=1-0)/C_2H(N=1-0) line intensity ratio in the LMC.Based on observations using the Swedish-ESO Submillimetre Telescope(SEST) at the European Southern Observatory (ESO), La Silla, Chile.Figs. 3-14 are only available in the electronic version of this paper.
| The Detection of Extragalactic ^15N: Consequences for NitrogenNucleosynthesis and Chemical Evolution
Detections of extragalactic ^15N are reported from observations of therare hydrogen cyanide isotope HC^15N toward the Large Magellanic Cloud(LMC) and the core of the (post-)starburst galaxy NGC 4945. Accountingfor optical depth effects, the LMC data from the massive star-formingregion N113 infer an ^14N/^15N ratio of 111+/-17, which is about twicethe ^12C/^13C value. For the LMC star-forming region N159HW and for thecentral region of NGC 4945, ^14N/^15N ratios are also ~100. The^14N/^15N ratios are smaller than all interstellar nitrogen isotoperatios measured in the disk and center of the Milky Way, stronglysupporting the idea that ^15N is synthesized by massive stars. Althoughthis appears to be in contradiction with standard stellar evolution andnucleosynthesis calculations, it supports recent findings of abundant^15N production due to rotationally induced mixing of protons into thehelium-burning shells of massive stars.
| 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.
| Optical Gravitational Lensing Experiment. The Catalog of Clusters in the Small Magellanic Cloud
We present the catalog of clusters found in the area of approx 2.4square degree in the central regions of the Small Magellanic Cloud. Thecatalog contains data for 238 clusters, 72 of them are new objects. Foreach cluster equatorial coordinates, radii, approximate number ofmembers, cross-identification, finding chart and color magnitudediagrams: V-(B-V) and V-(V-I) are provided. Photometric data for allclusters presented in the catalog are available from the OGLE Internetarchive.
| Molecular abundances in the Magellanic Clouds. III. LIRS36, a star-forming region in the Small Magellanic Cloud
Detections of CO, CS, SO, CCH, HCOp, HCN, HNC, FORM, and CYCP arereported from LIRS36, a star-forming region in the Small MagellanicCloud. CeiO, NO, METH, and most notably CN have not been detected, whilethe rare isotopes thCO and, tentatively, CfoS are seen. This is so farthe most extensive molecular multiline study of an interstellar mediumwith a heavy element depletion exceeding a factor of four. The X =N(H_2)/I_CO conversion factor is ~ 4.8 x 10(21) cmsq(Kkms)(-1), slightlylarger than the local Galactic disk value. The CO (1-0) beam averagedcolumn density then becomes N(MOLH) ~ 3.7 x 10(21) cmsq and the densitynumd ~ 100percc. A comparison with X-values from Rubio et al. (1993a)shows that on small scales (R ~ 10pc) X-values are more similar toGalactic disk values than previously anticipated, favoring a neutralinterstellar medium of predominantly molecular nature in the cores. TheI(thCO)/I(CeiO) line intensity ratio indicates an underabundance of (12)C(18) O relative to (13) C(16) O w.r.t. Galactic clouds. I(HCOp)/I(HCN)and I(HCN)/I(HNC) line intensity ratios are > 1 and trace a warm(Tkin > 10K) molecular gas exposed to a high ionizing flux.Detections of the CS J = 2-1, 3-2, and 5-4 lines imply the presence of ahigh density core with numd = 10(5) - 10(7) percc. In contrast tostar-forming regions in the LMC, the CN 1-0 line is substantially weakerthan the corresponding ground rotational transitions of HCN, HNC, andCS. CO, CS, HCOp, and FORM fractional abundances are a factor ~10smaller than corresponding values in Galactic disk clouds. Fractionalabundances of HCN, HNC, and likely CN are even two orders of magnitudebelow their `normal', Galactic disk values. The CN/CS abundance ratio isla 1. Based on chemical model calculations, we suggest that this isbecause of the small metallicity of the SMC, which affects thedestruction of CN but not CS, and because of the high molecular coredensity which also favors CN destruction. Based on observations with theSwedish-ESO Submillimeter Telescope (SEST) at the European SouthernObservatory (ESO), La Silla, Chile
| Cool gas in southern galaxies.
Not Available
| Obscured AGB stars in the Magellanic Clouds. I. IRAS candidates
We have selected 198 IRAS sources in the Large Magellanic Cloud, and 11in the Small Magellanic Cloud, which are the best candidates to bemass--loosing AGB stars (or possibly post--AGB stars). We used thecatalogues of \cite[Schwering \& Israel (1990)]{ref42} and\cite[Reid et al. (1990)]{ref36}. They are based on the IRAS pointedobservations and have lower detection limits than the Point SourceCatalogue. We also made cross-identifications between IRAS sources andoptical catalogues. Our resulting catalogue is divided in 7 tables.Table \ref{tab1} lists optically known red supergiants and AGB stars forwhich we found an IRAS counterpart (7 and 52 stars in the SMC and LMC,respectively). Table \ref{tab2} lists ``obscured'' (or ``cocoon'') AGBstars or late-type supergiants which have been identified as such inprevious works through their IRAS counterpart and JHKLM photometry (2SMC and 34 LMC sources; no optical counterparts). Table \ref{tab3} listsknown planetary nebulae with an IRAS counterpart (4 SMC and 19 LMC PNe).Table \ref{tab4} lists unidentified IRAS sources that we believe to begood AGB or post--AGB or PNe candidates (11 SMC and 198 LMC sources).Table~\ref{tab5} lists unidentified IRAS sources which could be any typeof object (23 SMC and 121 LMC sources). Table \ref{tab6} lists IRASsources associated with foreground stars (29 SMC and 135 LMC stars).Table \ref{tab7} lists ruled out IRAS sources associated with HIIregions, hot stars, etc... We show that the sample of IRAS AGB stars inthe Magellanic Clouds is very incomplete. Only AGB stars more luminousthan typically 10^4 L_\odot and with a mass-loss rate larger thantypically 5 10^{-6} M_\odot/yr could be detected by the IRAS satellite.As a consequence, one expects to find very few carbon stars in the IRASsample. We also expect that most AGB stars with intermediate mass--lossrates have not been discovered yet, neither in optical surveys, nor inthe IRAS survey. Tables 1 to 8 are also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://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
| Molecular abundances in the Magellanic Clouds. I. A multiline study of five cloud cores.
Nine Hii regions of the LMC were mapped in ^13^CO(1-0) and three in^12^CO(1-0) to study the physical properties of the interstellar mediumin the Magellanic Clouds. For N113 the molecular core is found to have apeak position which differs from that of the associated Hii region by20". Toward this molecular core the ^12^CO and ^13^CO peak T_MB_ linetemperatures of 7.3K and 1.2K are the highest so far found in theMagellanic Clouds. The molecular concentrations associated with N113,N44BC, N159HW, and N214DE in the LMC and LIRS36 in the SMC wereinvestigated in a variety of molecular species to study the chemicalproperties of the interstellar medium. I(HCO^+^)/I(HCN) andI(HCN)/I(HNC) intensity ratios as well as lower limits to theI(^13^CO)/I(C^18^O) ratio were derived for the rotational 1-0transitions. Generally, HCO^+^ is stronger than HCN, and HCN is strongerthan HNC. The high relative HCO^+^ intensities are consistent with ahigh ionization flux from supernovae remnants and young stars, possiblycoupled with a large extent of the HCO^+^ emission region. The bulk ofthe HCN arises from relatively compact dense cloud cores. Warm orshocked gas enhances HCN relative to HNC. From chemical modelcalculations it is predicted that I(HCN)/I(HNC) close to one should beobtained with higher angular resolution (<~30") toward the cloudcores. Comparing virial masses with those obtained from the integratedCO intensity provides an H_2_ mass-to-CO luminosity conversion factor of1.8x10^20^mol/cm^2^(K.km/s)^-1^ for N113 and2.4x10^20^mol/cm^2^(K.km/s)^-1^ for N44BC. This is consistent withvalues derived for the Galactic disk.
| Results of the ESO-SEST Key Programme: CO in the Magellanic Clouds. V. Further CO observations of the Small Magellanic Cloud.
We present further results of fully-sampled observations in theJ=1->0 and J=2-> 1 rotational transitions of ^12^CO and ^13^CO,obtained with the SEST telescope at 43" or 22" resolution, toward theSmall Magellanic Cloud. These observations concern six molecular cloudswith a variety of physical conditions. Maps in the ^12^CO(1->0),^12^CO(2-> 1), and in some cases of ^13^CO(1->0) lines arepresented, as well as profiles of the four lines at the peak positionsconvolved to the same angular resolution of 43". The line ratio resultsare interpreted in Paper IV of this series (Lequeux et al. 1994).
| 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.
| Results of the ESO-SEST Key Program: CO in the Magellanic Clouds. 4: Physical properties of molecular clouds in the Small Magellanic Cloud
We discuss recent molecular observations of CO in the Small MagellanicCloud (SMC), made with the Swedish ESO submillimeter telescope (SEST)telescope. For 6 resolved CO clouds the intensities of the (12)CO(1-0),(12)CO(2-1), (13)CO(1-0) and (13)CO(2-1) lines have been measured at thesame angular resolution of 45 arc seconds. The line ratios are comparedwith the predictions of a one-side illuminated plane-parallel modelwhich treats consistently radiative, chemical and excitation processesof CO and its isotopes. A satisfactory agreement is achieved for amoderately high-density gas whose properties can be understood asresulting mainly from a higher rate of photodissociation and of heatingof CO through the combined effects of a higher UV radiation field andcosmic-ray flux, of a lower dust/gas ratio and of a lower abundance ofcarbon (compared to our Galaxy). This confirms the qualitative resultsof a previous paper (Rubio et al. 1993). In the Magellanic Clouds,contrary to the Galaxy, CO molecules can only survive in dense parts('clumps') of interstellar clouds at densities which depend on theradiation fields, while they are photodissociated in the less dense('interclump') medium. We also predict that hydrogen in thislower-density medium is mainly atomic rather than molecular. In twoclouds submitted to a particularly high UV radiation field, the linewidths are significantly larger for (12)CO than for (13)CO, perhapsindicating an evaporation flow. We also discuss the emission offar-infrared fine-structure lines by the clouds, and discuss inparticular the line of C(+) at 158 micrometers in some of them.
| 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.
| Results of the ESO / SEST Key Programme - Co/ in the Magellanic Clouds - Part Three - Molecular Gas in the Small Magellanic Cloud
We discuss some of the results of observations of CO in the SW "Bar"region of the Small Magellanic Cloud (SMC), made with the SEST telescopeat 45" resolution. These observations are reported in a companion paper(Rubio et al. 1992; Paper II). For two resolved CO clouds associatedwith H II regions comparable to the Orion nebula, the intensities in thefour observed CO lines imply that they are clumpy with a surface fillingfactor lower than for Galactic clouds, and that their temperature iscomparatively high. For a variety of CO structures with a wide range ofradii R (10 to 200 pc), the ^12^CO (1-0) line width {DELTA}V isproportional to R^1/2^, as for Galactic molecular clouds and complexes.However, the relations between the CO luminosity L_CO_ and {DELTA}V orthe virial mass M_vir_ are different in the SMC and in the Galaxy. Atthe smallest scales that we can resolve (R ~ 10 pc) the SMC structuresare less luminous in CO than Galactic molecular structures of the samesize by a factor of only a few (about 1.5 when compared to clouds in theouter Galaxy), while at large scales (R ~ 200 pc) they are much lessluminous, by a factor of 10-20. This property as well as the propertiesof two clouds discussed in details, can be understood as resulting froma higher rate of photodissociation of CO in the SMC through the combinedeffects of a higher UV radiation field, of a lower dust/gas ratio and ofa lower abundance of carbon. In the SMC, contrary to the Galaxy, COmolecules can only survive in dense clumps, while they arephotodissociated in the diffuse medium and even in the interclump mediumof individual clouds, if any. As a result, the largest interstellarentities in the SMC, which are dominated by diffuse gas, contain littleCO: CO is found only in dense regions. Even these denser cores (the"molecular clouds') are permeated by UV radiation so that CO exists onlyin the densest clumps. Presumably most of the interstellar hydrogen inthe SMC is atomic rather than molecular except in these dense regions.Assuming that the interstellar structures are in virial equilibrium atall observed scales, we derive a preliminary estimate for thecalibration coefficient for determining the total column density of gasN(H_2_ + 2H) from the ^12^CO (1-0) line intensity I(CO). Thiscoefficient X_SMC_ is larger than the canonical galactic value X_gal_ =2.3 10^20^ mol cm^-2^ (K km s^-1^)' and depends on the scale R: X_SMC_ ~9 10^20^ (R/10 pc)^0.7^ mol cm^-2^(K km s^-1^)^-1^. This explains why ithas been so difficult to detect CO in irregular galaxies at low linearresolutions, while its detection is easier at higher linear resolutions.
| 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.
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