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VLT/NACO near-infrared imaging and spectroscopy of N159A in the LMC HII complex N159
We present near-infrared imaging and spectrocopic observations of theHII region N159A (~10 pc) in the giant star-forming region N159 (50 pc)in the LMC. N159A was observed in the J and Ks bands at high spatialresolution ~0.2 arcsec using the ESO Very Large Telescope UT4 (VLT),equipped with the NAOS adaptative optics system. Our data reveal themorphology of this region in unprecedented detail. The protostar P2, oneof the first YSOs of Class I identified in the LMC is now resolved intwo YSO candidates. The ultracompact HII region LI-LMC 1501W is found tobe a tight cluster embedded in a compact HII region ionised by a late Osource. A new multiple system composed of a tight star cluster and anYSO candidate, all embedded in a compact nebular region (0.4 pc) is alsodetected at the north-east edge of N159A. The stellar population of thewhole N159A region appears composed of two main stellar populations, onewith an age ≤3 Myr and the other one with a large range of age (300Myr-10 Gyr). Using spectroscopy, one of the two exciting O stars in theHII region N159A is classified O5-O6.
| Hunting Massive Stars Around the Tarantula
We have studied the N159A region which is located aproximately atα = 5^{h}40^{m}07^{s} and δ = -69°47'47''. This regionbelongs to a major complex called N159 located in the LMC at thesouthern edge of the 30 Dor Nebula. All the complex is ˜50 pcdiameter long and in its interior there are at least six HII regions(N159A, E, F, G, H, K). N159 is an extremely young complex and showscharacteristic features of active stellar fomation. The interest instudying N159 is based in the fact that it is an extragalactic (thoughnear) star forming region, in a low metallicity environment and locatedspatially close to an enormous complex of star formation like 30 Dor.These conditions make N159 an excellent place to study and learn about asubject as sequential star formation, IMF of stars at low metallicities,peculiar objects, etc. In the present work we made a spectrophotometricanalysis of a large number of N159 objects. The images used forspectroscopy were taken with the 2.5-m telescope at Las CampanasObservatory (Chile) during the nights from 26th to 28th November 2003.The images, 25 arc minutes wide, were taken with the Wide FieldReimaging CCD Camera, using masks for multiobjet spectroscopy withmedium spectral resolution.In this study aproximately 150 stars were classificated as a result ofthe analysis of 5 masks. We have found 50 O-type stars, 70 early B typestars and 30 stars of spectral type later than A (which most probablyare field stars) in a region where no spectral classification had beenobtained before.
| An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.
| High spatial resolution radio continuum observations of compact H {II} regions in the Magellanic Clouds
We present high spatial resolution observations of the 6 cm continuumemission of compact H II regions in well-known sites of massive starformation located in the Small and Large Magellanic Clouds. Theobservations include N81 in the SMC, and N4A, N83B, N11A, N160A andN159-5 in the LMC. Some of the compact H II regions are isolated, whileothers are embedded in more diffuse ionised regions. A description ofthe radio morphology of the sources, together with comparisons withother observations, is given in detail. The regions cover a wide rangein size (from ˜ 0.1 to 7 pc), rms electron density (from ˜200 to 6500 cm-3), emission measure (from~3×105 to 2×107 pc cm-6),ionised gas mass (from ˜ 0.2 to 750 Mȯ) and rateof Lyman continuum photons (from ~ 3× 1047 to5×1049 s-1). The spectral types determinedfrom the Lyman continuum fluxes are consistent with opticaldeterminations. We have compared these Magellanic Cloud H II regionswith their Galactic counterparts in terms of size, rms electron densityand Lyman continuum flux. This comparison shows that their propertiesrelate to each other in the same way as those in Galactic H II regions.
| Spitzer IRAC Observations of Star Formation in N159 in the Large Magellanic Cloud
We present observations of the giant H II region complex N159 in the LMCusing IRAC on the Spitzer Space Telescope. One of the two objectspreviously identified as protostars in N159 has a spectral energydistribution (SED) consistent with classification as a Class I youngstellar object (YSO), and the other is probably a Class I YSO as well,making these two stars the youngest stars known outside the Milky Way.We identify two other sources that may also be Class I YSOs. Onecomponent, N159AN, is completely hidden at optical wavelengths but isvery prominent in the infrared. The integrated luminosity of the entirecomplex is L~9×106 Lsolar, consistent withthe observed radio emission, assuming a normal Galactic initial massfunction (IMF). There is no evidence for a red supergiant populationindicative of an older burst of star formation. The N159 complex is 50pc in diameter, larger in physical size than typical H II regions in theMilky Way with comparable luminosity. We argue that all of theindividual components are related in their star formation history. Themorphology of the region is consistent with a wind-blown bubble ~1-2 Myrold that has initiated star formation now taking place at the rim. Otherthan its large physical size, star formation in N159 appears to beindistinguishable from star formation in the Milky Way.
| Near-Infrared Imaging Observations of the N159/N160 Complex in the Large Magellanic Cloud: Large Clusters of Herbig Ae/Be Stars and Sequential Cluster Formation
We have carried out deep near-infrared imaging observations of theN159/N160 star-forming region in the Large Magellanic Cloud. We observedan area of ~380 arcmin2 (~80,000 pc2 at thedistance of the LMC) in the J, H, and Ks bands. Theobservations are deep enough to detect Herbig Ae/Be stars down to ~3Msolar in the LMC. We discovered a total of 338 and 464candidate Herbig Ae/Be and OB stars, respectively, based on thenear-infrared colors and magnitudes. The Herbig Ae/Be candidatescomprise 10 clusters, the OB star candidates 13. We discovered anembedded Herbig Ae/Be cluster in the N159 East giant molecular cloud(GMC) and a Herbig Ae/Be cluster at the northeast tip of the N159 SouthGMC. Together with two neighboring H II regions, the Herbig Ae/Becluster at the tip of the N159S GMC provides a hint of the beginning ofsequential cluster formation in N159S. The spatial distributions of theHerbig Ae/Be and OB clusters, in conjunction with previously knownoptical clusters and embedded massive stars, indicate (1) sequentialcluster formation within each of the N159 and N160 star-forming regionsand (2) large-scale sequential cluster formation over the entireobserved region from N160 to N159S. Possible triggers for thelarge-scale cluster formation are the supergiant shell SGS 19 and anexpanding superbubble. Some of the Herbig Ae/Be clusters in theN159/N160 complex are significantly larger in spatial extent thanpre-main-sequence clusters of similar age in the Milky Way. Highlyturbulent gas motion in the LMC is probably responsible for forming thelarge young clusters.
| 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.
| 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.
| A statistical study of binary and multiple clusters in the LMC
Based on the Bica et al. (\cite{bica}) catalogue, we studied the starcluster system of the LMC and provide a new catalogue of all binary andmultiple cluster candidates found. As a selection criterion we used amaximum separation of 1farcm4 corresponding to 20 pc (assuming adistance modulus of 18.5 mag). We performed Monte Carlo simulations andproduced artificial cluster distributions that we compared with the realone in order to check how many of the found cluster pairs and groups canbe expected statistically due to chance superposition on the plane ofthe sky. We found that, depending on the cluster density, between 56%(bar region) and 12% (outer LMC) of the detected pairs can be explainedstatistically. We studied in detail the properties of the multiplecluster candidates. The binary cluster candidates seem to show atendency to form with components of similar size. When possible, westudied the age structure of the cluster groups and found that themultiple clusters are predominantly young with only a few cluster groupsolder than 300 Myr. The spatial distribution of the cluster pairs andgroups coincides with the distribution of clusters in general; however,old groups or groups with large internal age differences are mainlylocated in the densely populated bar region. Thus, they can easily beexplained as chance superpositions. Our findings show that a formationscenario through tidal capture is not only unlikely due to the lowprobability of close encounters of star clusters, and thus the evenlower probability of tidal capture, but the few groups with largeinternal age differences can easily be explained with projectioneffects. We favour a formation scenario as suggested by Fujimoto &Kumai (\cite{fk}) in which the components of a binary cluster formedtogether and thus should be coeval or have small age differencescompatible with cluster formation time scales. Table 6 is only availablein electronic form at the 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/391/547
| A CO Survey of the LMC with NANTEN: III. Formation of Stellar Clusters and Evolution of Molecular Clouds
In order to elucidate star formation in the LMC, we made a completestudy of CO clouds with NANTEN. In the present paper, we compare 55giant molecular clouds (GMCs), whose physical quantities were welldetermined, with young objects, such as young stellar clusters and HIIregions. We find that the GMCs are actively forming stars and clusters;23 and 40 are found to be associated with the clusters and the HIIregions, respectively. The clusters associated with the GMCs aresignificantly young; ~ 85% of them are younger than ~ 10 Myr. Inaddition, compact groups of the young clusters are often found at thepeak position of the GMCs, e.g., N 159 and N 44, while much loosergroups are away from the GMCs. This suggests that the clusters areformed in groups and disperse as they become old. The distributions ofthe CO, [CII], and UV indicate that the GMCs are likely to be rapidlydissipated within several Myr due to UV photons from the clusters. Wealso estimate the evolutionary time scale of the GMCs; they form starsin a few Myr after their birth, and form clusters during the next fewMyr, and are dissipated in the subsequent few Myr.
| 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''.
| An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.
| Extinction of H II regions in the Large Magellanic Cloud
The extinction properties of H II regions in the Large Magellanic Cloudare investigated using radio continuum data obtained from the MolongloObservatory Synthesis Telescope, digitized and calibrated H-alpha data,and published Balmer decrement measurements. The resultingextinction-color excess diagram suggests that (1) most H II regions inthe Magellanic Clouds have similar extinction properties to the Galacticones, (2) all imaginable gas/dust configurations are possible, and (3)the extinction of some highly reddened H II region cores originatesexternally in cocoon shells. The puzzle of different extinction-colorexcess ratios of Galactic and extragalactic H II regions is explained asbeing due to the different populations of observed samples rather thanany intrinsic differences. The extinction of the observed Galactic H IIregions produced by foreground dust overwhelms the internal extinction,while the situation in the observed extragalactic H II regions is justthe opposite.
| Scaleheights of 486 southern spiral galaxies and some statistical correlation
Based on Peng's method (1988), we obtain scaleheights of 486 southernspiral galaxies, the images of which are taken from the Digitized SkySurvey at Xinglong Station of Beijing Astronomical Observatory. Thefitted spiral arms of 70 galaxies are compared with their images to gettheir optimum inclinations. The scaleheights of other 416 ones arelisted in Table A1 in Appendix. After compiling and analyzing the data,we find some statistical correlations. The most interesting results arethat a flatter galaxy is bluer and looks brighter, and galaxies becomeflatter along the Hubble sequence Sab -- Scd. Based on photographic dataof the National Geographic Society -- Palomar Observatory Sky Survey(NGS-POSS) obtained using the Oschin Telescope Palomar Mountain. TheNGS-POSS was funded by a grant from the National Geographic Society tothe California Institute of Technology. The plates were processed intothe present compressed digital form with their permission. The DigitizedSky Survey was produced at the Space Telescope Science Institute underUS Government grant NAG W-2166. Table A1 is available in electronic fromonly, via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Gamma-Ray Line Signals from 26Al and 60Fe in the Galaxies of the Local Group
Steady state gamma -line fluxes from the decay of 26Al and 60Fe situatedin the Local Group galaxies LMC, SMC, NGC 6822, IC 1613, M31, and M33are estimated with standard supernova yields and supernova rates basedon blue luminosities and other indicators of massive star formation. Thelargest 26Al flux, 2.0 x 10-6 gamma s-1 cm-2, comes from the LMC, whilethe SMC is estimated to provide an 26Al flux of 3.6 x 10-7 gamma s-1cm-2. Andromeda is surprisingly weak, only about 1.6 x 10-8 gamma s-1cm-2 because of both its distance and its current low rate of starformation. Expected 60Fe fluxes are about 16% of the 26Al flux.Detection of these fluxes is feasible but will probably requiresensitivities greater than those of the International Gamma-RayAstrophysical Laboratory mission. The chief utility of such observationsis the new insight they provide into the mechanisms of supernovaexplosions, the distribution and nature of massive-star--forming regionsin irregular and spiral galaxies, and the nucleosynthetic history ofLocal Group members.
| Integrated UBV Photometry of 624 Star Clusters and Associations in the Large Magellanic Cloud
We present a catalog of integrated UBV photometry of 504 star clustersand 120 stellar associations in the LMC, part of them still embedded inemitting gas. We study age groups in terms of equivalent SWB typesderived from the (U-B) X (B-V) diagram. The size of the spatialdistributions increases steadily with age (SWB types), whereas adifference of axial ratio exists between the groups younger than 30 Myrand those older, which implies a nearly face-on orientation for theformer and a tilt of ~45^deg^ for the latter groups. Asymmetries arepresent in the spatial distributions, which, together with thenoncoincidence of the centroids for different age groups, suggest thatthe LMC disk was severely perturbed in the past.
| A Catalog of the Youngest YSOs and Candidate Protostars
We have compiled a catalog of candidate protostars from the majorastronomical journals up to the end of 1993. The Beichman-Ichikawa colorcriterion was used as the main test of an author's claim that a sourceshould be deemed a candidate protostar. Names, positions (1950 and2000), LSR velocities, information on whether there are associatedoutflows, and references are provided. This catalog is meant to updatean earlier compilation by Wynn-Williams (1982). (SECTION: Stars)
| A radio continuum study of the Magellanic Clouds. IV. Catalogues of radio sources in the Large Magellanic Cloud at 1.40, 2.45, 4.75, 4.85 and 8.55 GHz.
From observations with the Parkes radio telescope, we present cataloguesof radio sources in the Large Magellanic Cloud at four frequencies:1.40, 2.45, 4.75 and 8.55GHz, and an additional catalogue from a sourceanalysis of the Parkes-MIT-NRAO survey at 4.85GHz. A total of 469sources have been detected at least one of these frequencies, 132 ofwhich are reported here for the first time as radio sources.
| A Search for Methanol Masers in the Magellanic Clouds
We report the discovery of a second methanol maser in the LargeMagellanic Cloud and we present the results of synthesis observations ofthis and the methanol maser detected previously. The second discoverywas made using the Australia Telescope National Facility's 64-m Parkesradio telescope during an extensive maser search for 6.6-GHz maseremission from the 5_1_-6_0_ A^+^ transition in both Magellanic Clouds.Spectra were obtained towards 35 HII regions in the Large MagellanicCloud and 13 regions in the Small Magellanic Cloud, and also on a3-arcmin grid over an area 0.3^deg^ square, south of the 30 Doradusnebula. Parkes observations at 12.2 GHz towards the two maser sitesyielded no detectable emission from the 2_0_-3_-1_, E methanoltransition. The results suggest that methanol masers are less abundantin the Magellanic Clouds than in our Galaxy. Observations of the twomasers with the Australia Telescope Compact Array showed one to belocated near the continuum emission peak of the H II region MC18 (N11),while the other appeared to be centred near OH emission on thesouth-eastern boundary of MC23 (N105a).
| The massive star content of the blue dwarf galaxy IZw 36 from Faint Object Camera observations
We have observed the blue dwarf galaxy IZw 36 with the f/96 relay of theFaint Object Camera and have for the first time resolved massive stars,using the broad band filters F175W, F342W, F430W and F480LP. We havemeasured the fluxes of 143 of these objects and studied theircharacteristics in color-magnitude diagrams. A few stars may be redsupergiants but their contribution to the integrated light is less than5% in the F430W filter. The F175W-F430W color of the integrated stellarpopulation is redder than expected from the current burst of starformation, suggesting therefore the presence of an older and unresolvedunderlying population. The ultraviolet measurements combined withsynthetic photometry calculations allow us to place the massive stars ina bolometric magnitude vs. temperature diagram. In this diagram, thestars are compared to evolutionary tracks for different stellar masses.The current burst probably has an age less than 12 Myr. We infer anInitial Mass Function, with a power-law slope in the range -1.7 to -2.6for masses M>20Msun_. This is consistent with most of thevalues reported for sites of star formation in the Galaxy and theMagellanic Clouds and does not support the view of an IMF flattening atlow metallicity.
| Molecular hydrogen emission in galaxies: Determination of excitation mechanism
Infrared H2 emission is a mixture of thermal and nonthermal components.We have found the excitation temperature of the thermal component to be2000 K for supernova remnants and less than or equal to 1000 K forphotodissociation regions. Hence the thermal H2 temperature is adiscriminant between shock excitation by supernovae and UV excitation byOB stars. Utilizing this result, we discuss the origin of H2 emission inSeyfert and starburst galaxies, which has long been controversial.
| The stellar content of the Large Magellanic Cloud II region N 59 A
We present UBV photometry of the stellar cluster associated with N59A, adusty H II region in the LMC. N59A's main detected source of ionizationis an O5V (or possibly earlier type) star with a visual extinction of1.2 mag. N59A also contains fifteen O-B3 stars that may contribute tothe ionization; these stars are affected by greatly differing amounts ofextinction. However, the observed stellar content of N59A cannotcompletely account for the ionization of the gas and the heating of theassociated dust. Some early massive star(s) still probably lieundetected in the core of (or behind) the central absorbing cloud. Inaddition to this young population associated with the H II region, wedetect an underlying older population of giant stars. We have alsodetected one Galactic star, and a few supergiant candidates. Theseresults are discussed in terms of the initial mass function.
| Photometry of the LMC HII region N159A and of its stellar content. I - Stellar observations and reductions
The paper presents UBV RI photometry, based on CCD frames, of stellarcluster associated with the LMC HII region N159A, as well as of thesurrounding stellar field, with particular attention given to theobservations and to reduction procedures. In the reduction procedures,the DAOPHOT photometry program was adapted to observations of stars inclusters embedded in bright filamentary nebulae. The results presentedinclude the magnitudes for each star, obtained in five wavelength bands,both in the instrumental UBVRI system and after transformation toLandolt's (1983) standard system.
| Bar star clusters in the LMC - Formation history from UBV integrated photometry
The sample of star clusters in the LMC Bar region with integrated UBVphotometry was enlarged by approximately a factor four, totaling 129objects. The (B-V) histogram gap between blue and red clustersdisappears with this deeper sample. Age groups in terms of equivalentSWB types were derived and their spatial distribution studied. Clustersyounger than t about 200 Myr are not homogeneously distributed throughthe bar. In particular a strong star forming event at t about 100 Myrwas detected in the eastern part of the Bar, consisting of a compactgrouping of seven coeval clusters around NGC 2058 and NGC 2065. Also, 11close pairs and two trios are analyzed, and the colors indicate thatonly four pairs are clearly not coeval.
| Photometry of the LMC H II region N 159 A and of its stellar content. II - Young stars, gas, and dust
This paper presents an analysis of the stellar and nebular photometricobservations of the LMC H II region N 159 A and its surroundings. Wehave identified the probable exciting stars of N 159 A and of itssmaller neighbor H II regions. Fourteen O-B2 stars are present in thisfield of 1.6 arcmin x 2.5 arcsec. N 159 A itself is ionized by a closepair of stars of probable spectral types O5 to O6 V, and O7 to O8 V;these stars are separated by 1.0 arcmin and are affected by a visualextinction of 1.2 to 1.4 mag. The protostar of Jones et al. (1986),lying at the border of N 159 A, corresponds to an H-alpha emissionobject which is possibly a very compact H II region or a Herbig Ae/Bestar. The young stellar population (associated with the H II regions) issuperposed on an older population of giants, aged 10 exp 9 yr or more.The observed stellar content of N 159 A successfully accounts for thelevel of ionization of the gas as well as for this region's radiocontinuum and Balmer line emission.
| Near-infrared spectra and classification diagnostics of Seyfert galaxies
Observational results of a previous spectroscopic survey of Seyfertgalaxies in the near-IR are presented, and the potential for usingemission-line ratios in this spectral region as a classificationdiagnostic tool is examined. Near-IR CCD spectra, which cover thewavelength range of 7000-10,000 at a nominal resolution of about 12 A,of 15 additional Seyferts and two starburst galaxies are obtained.Relative emission-line intensities from these observations, incombination with measurements from previous studies and measurements ofnew, signal-to-noise ratio optical spectra of many of these objects, areused to study the diagnostic diagrams involving forbidden S III 9069,9531/H-alpha, forbidden O II 7320, 7330/H-alpha, forbidden S II 6716,6731/H-alpha, and forbidden O III 5007/H-beta. Comparisons are made inthese diagrams between observational data from the active galaxies andpublished measurements of H II regionlike objects, as well as withpredictions from simple one-component models calculated for the twotypes of objects.
| DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud
This paper investigates the Large Magellanic Cloud star DD 13, which islikely the major ionizing source of the nebula N159A. New opticalspectroscopy and new estimates of the broadband photometric propertiesof DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, andM(V) = -6.93 is found. The spectral type cannot be more preciselydefined due to contamination of the spectral data by nebular emission,obliterating the important He I classification lines. These results,plus a published estimate of the Lyman continuum photon injection rateinto N159A, suggest that DD 13 actually consists of about 2-4 young,early O stars still enshrouded by their natal dust cloud. The star DD 13may be a younger example of the type of tight cluster represented by theLMC 'star' Sk-66 deg 41, recently revealed to be composed of six or morecomponents.
| Dust in emission nebulae of the LMC derived from photometric reddening of stars
VBLUW photometric observations of stars in emission nebulae of the LMCare reported. The luminosities and extinctions of the stars are derived.Agreement is found between the average photometric extinctions of thenebulae and the extinctions derived from the Balmer line decrementmeasured by Caplan and Deharveng (1985 and 1986). The photometricextinctions are shown in the CO map of the LMC (Cohen et al., 1988).
| Molecular hydrogen in H II regions in the Magellanic Clouds
Near-infrared spectrophotometry of three objects in the SMC and seven inthe LMC, all listed as compact H-alpha emission line objects, areobtained, and a position in one of the two main ionization fronts of 30Doradus is observed. Three objects did not show Brackett-gamma emission,and thus do not appear to be H II regions. The 30 Doradus positionshowed He I and Br-gamma emission, but no H2 emission. In six of theremaining objects, H2 emission was also searched for. It was detected inthree cases, and possibly in two more. Analysis of the observationsshows that the molecular hydrogen emission may be caused either by shockexcitation due to stars embedded in a molecular cloud or by fluorescenceof molecular material in the ultraviolet radiation field of the OB starsexciting the H II region. The molecular hydrogen associated with N88 isat least in part shock-excited.
| VBLUW photometry of emission nebulae
Observed VBLUW colors of emission nebulae of the SMC, LMC, and the Orionnebula are reconstructed from published emission line ratios andtheoretical H and 2-photon continua. Color corrections for internalreddening, scattered star light, and stellar backgrounds are discussed.The calculations reproduce the observed colors in the (B-U)/(B-L)diagram and indicate a correlation of (B-U) with the forbidden line O II3727 A/H-beta line ratios. After a more detailed empirical confirmation,this correlation may eventually be used as a diagnostic indicator ofemission nebulae.
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