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A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud(SMC), carried out using radio Faraday rotation and optical starlightpolarization data. Consistent negative rotation measures (RMs) acrossthe SMC indicate that the line-of-sight magnetic field is directeduniformly away from us with a strength 0.19+/-0.06 μG. Applying theChandrasekhar-Fermi method to starlight polarization data yields anordered magnetic field in the plane of the sky of strength 1.6+/-0.4μG oriented at a position angle 4deg+/-12deg,measured counterclockwise from the great circle on the sky joining theSMC to the Large Magellanic Cloud (LMC). We construct athree-dimensional magnetic field model of the SMC, under the assumptionthat the RMs and starlight polarization probe the same underlyinglarge-scale field. The vector defining the overall orientation of theSMC magnetic field shows a potential alignment with the vector joiningthe center of the SMC to the center of the LMC, suggesting thepossibility of a ``pan-Magellanic'' magnetic field. A cosmic-ray-drivendynamo is the most viable explanation of the observed field geometry,but has difficulties accounting for the observed unidirectional fieldlines. A study of Faraday rotation through the Magellanic Bridge isneeded to further test the pan-Magellanic field hypothesis.
| An exploratory non-LTE analysis of B-type supergiants in the Small Magellanic Cloud
A preliminary differential non-LTE model atmosphere analysis of moderateresolution (R ~ 5 000) and signal-to-noise ratio spectra of 48 SmallMagellanic Cloud B-type supergiants is presented. Standard techniquesare adopted, viz. plane-parallel geometry and radiative and hydrostaticequilibrium. Spectroscopic atmospheric parameters (T_eff, log g andv_turb), luminosities and chemical abundances (He, C, N, O, Mg and Si)are estimated. These are compared with those deduced for a comparablesample of Galactic supergiants. The SMC targets appear to have similaratmospheric parameters, luminosities and helium abundances to theGalactic sample. Their magnesium and silicon underabundances arecompatible with those found for main sequence SMC objects and there isno evidence for any large variation in their oxygen abundances. Bycontrast both their carbon and nitrogen lines strengths are inconsistentwith single abundances, while their nitrogen to carbon abundance ratiosappear to vary by at least as much and probably more than that found inthe Galactic sample.
| UV Spectral Classification of O and B Stars in the Small Magellanic =
We present an ultraviolet classification system for 0 and B stars of theSmall Magellanic Cloud (SMC). This system is defined by a set ofstandard, low resolution, International Ultraviolet Explorer (JUE)spectra. This UV classification system results from the development ofspectral sequences demonstrating systematic patterus of UV spectralfeatures. These spectral sequences yield Uv classifications for 133 0and B stars of the SMC, which we also present here. Although independentof the MK System, our UV classifications show general agreement withthose deduced from visual data. This UV classification system isdesigned to be applicable to 0 and B stars in other nearby galaxies ofmetallicity comparable to the SMC. The classification technique itselfis applicable to any UV dataset of sufficient size and quality, and canbe used to extend our UV classification system to other galactic metalabundances. These UV classification systems will be essential foranalysis of datasets from new spaceborne instrumentation such as theSpace Telescope Imaging Spectrograph (STIS), which will be capable ofobserving stars in external galaxies for which no opticalclassifications exist. 0 1997 American Astronomical Society.[S0004-6256(97)0231 1-X]
| Dust in the Small Magellanic Cloud: Interstellar Polarization and Extinction
The typical extinction curve for the Small Magellanic Cloud (SMC), incontrast to that for the Galaxy, has no bump at 2175 Angstroms and asteeper rise into the far-ultraviolet. For the Galaxy, theinterpretation of the extinction, and therefore the dust content of theinterstellar medium, has been greatly assisted by measurements of thewavelength dependence of the polarization. Up to the present, no suchmeasurements existed for the SMC. Therefore, to further elucidate thedust properties in the SMC, we have measured, for the first time, linearpolarization in five colors in the optical region of the spectrum for asample of reddened stars. For two of these stars, for which there wereno existing UV spectrophotometric measurements, but for which wemeasured a relatively large polarization, we have also obtained datafrom the IUE in order to study the extinction. With the help ofparameterization, we attempted to correlate the SMC extinction andpolarization data. In addition, we performed dust model fits to bothextinction and polarization using silicate and graphite, or amorphouscarbon spheres and silicate cylinders. The size distribution for thecylinders is taken from a fit to the polarization, and we introduce thenotion of volume continuity between this and the silicate sphere sizedistribution. The main results are (1) the wavelength of maximumpolarization, lambda max, in the SMC is typically smaller than that inthe Galaxy; (2) however, AZV 456, which shows the UV extinction bump,has a lambda max typical of that in the Galaxy, its polarization curveis narrower, its bump is shifted to shorter wavelengths as compared withthe Galaxy, and its UV extinction does not conform to the Galacticanalytical interpolation curve based on the ratio of total to selectiveextinction; (3) the "typical," monotonic SMC extinction curve can bebest fitted with amorphous carbon and silicate grains; (4) theextinction toward AZV 456 may be explained only by assuming a largergas-to-dust ratio than the observed N(H I)/A(V) value, with a smallamount of the available carbon in graphite form; and (5) from ananalysis of both the extinction and polarization data and our modelfits, it appears that the SMC has typically smaller grains than those inthe Galaxy. The absence of the extinction bump in the SMC generally hasbeen thought to imply a lower amount or even an absence of carbon insolid form in the SMC compared with the Galaxy. Our results show thatthe size distribution, and not only the carbon abundance, is differentin the SMC as compared with the Galaxy. In addition, and contrary toprevious findings, another component besides silicates is indeed neededto provide a sizable part of the observed extinction toward the SMC.Using the SMC as a laboratory for studying the solid component of theinterstellar medium, we also discuss some of the implications of ourresults in view of proposed interstellar dust models.
| The HIPPARCOS proper motion of the Magellanic Clouds
The proper motion of the Large (LMC) and Small (SMC) Magellanic Cloudusing data acquired with the Hipparcos satellite is presented. Hipparcosmeasured 36 stars in the LMC and 11 stars in the SMC. A correctlyweighted mean of the data yields the presently available most accuratevalues, mu_alpha cos(delta) = 1.94 +/- 0.29 mas/yr, mu_delta = - 0.14+/- 0.36 mas/yr for the LMC. For the SMC, mu_alpha cos(delta) = 1.23 +/-0.84 mas/yr, mu_delta = - 1.21 +/- 0.75 mas/yr is obtained, whereby careis taken to exclude likely tidal motions induced by the LMC. Bothgalaxies are moving approximately parallel to each other on the sky,with the Magellanic Stream trailing behind. The Hipparcos proper motionsare in agreement with previous measurements using PPM catalogue data byKroupa et al. (1994), and by Jones et al. (1994) using backgroundgalaxies in a far-outlying field of the LMC. For the LMC the Hipparcosdata suggest a weak rotation signal in a clockwise direction on the sky.Comparison of the Hipparcos proper motion with the proper motion of thefield used by Jones et al. (1994), which is about 7.3 kpc distant fromthe center of the LMC, also suggests clockwise rotation. Combining thethree independent measurements of the proper motion of the LMC and thetwo independent measurements of the proper motion of the SMC improvesthe estimate of the proper motion of the LMC and SMC. The correspondinggalactocentric space motion vectors are computed. Within theuncertainties, the LMC and SMC are found to be on parallel trajectories.Recent theoretical work concerning the origin of the Magellanic Systemis briefly reviewed, but a unique model of the Magellanic Stream, forthe origin of the Magellanic Clouds, and for the mass distribution inthe Galaxy cannot yet be decided upon. Future astrometric space missionsare necessary to significantly improve our present knowledge of thespace motion of the two most conspicuous galactic neighbours of theMilky Way.
| 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.
| Revised spectral types for 64 B-supergiants in the Small Magellanic Cloud: metallicity effects.
The problem of the classification of metal poor stars, such as occur inthe Small Magellanic Cloud (SMC), is discussed with reference to theapplicability of the MK system in such an environment. An alternativemethod is presented here and applied to B-type supergiants in the SMC. Alocal reference system is first devised and then a transformation to MKspectral types is determined by comparing the trends of metal linestrengths in these two systems. For the determination of the luminosityclass, we emphasize the need to use the hydrogen Balmer line strengthsindependently of metal line-strength considerations. This method is usedto determine new spectral types for 64 supergiants in the SMC, 75% ofthe sample requiring classifications different from previous findings.These new types result in much improved line strength - spectral typecorrelations for He, C, N, O, Mg and Si. Corresponding changes in thedistribution of these stars in the Hertzsprung-Russell diagram of theSMC reveal more clearly than before the existence of a ridge which maybe the SMC analogue of a similar feature found for the LMC byFitzpatrick & Garmany (1990ApJ...363..119F). The group of veryluminous supergiants lying above this ridge includes the LBV AV415(R40), a property which this object has in common with LBVs in the LargeMagellanic Cloud. Also, for the first time, clear examples of BN/BCsupergiants are found in the SMC.
| 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.
| Small Magellanic Cloud: H-gamma-line equivalent widths and luminosity classes of the brightest blue star members
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...69..421A&db_key=AST
| Studies of massive stars in the Magellanic Clouds. II - New spectral classification of OB stars in the SMC
Abstract image available at:http://adsabs.harvard.edu/abs/1987AJ.....93.1070G
| Radial velocities of southern stars obtained with the photoelectric scanner CORAVEL. VI - 233 F to M type stars in and near the Small Magellanic Cloud - Comparison with 80 spectrographic radial velocities of O to K type stars in this galaxy
Abstract image available at:http://adsabs.harvard.edu/abs/1987A&AS...67..423M
| Studies of luminous stars in nearby galaxies. VIII - The Small Magellanic Cloud
The spectra, colors, and positions on the H-R diagrams of the SmallMagellanic (SMC) supergiants of all spectral types are examined andcompared with corresponding data for supergiant populations in the LMCand Milky Way for clues to the role of chemical composition (i.e., lowmetallicity) on their evolution. A comparison of the observed luminosityfunction shows that while the solar neighborhood and the LMC areessentially the same, the SMC is significantly different, especially atthe upper end. These differences are not due to chemical-compositionvariations. The relative numbers of supergiants of differentluminosities vary most closely with the mass of the galaxy. The blue tored supergiant ratio in the SMC confirms the trends observed in theGalaxy and LMC, and is probably affected by chemical abundancedifferences, especially in the lower luminosity intervals. In addition,it is found that a group of supergiants in the SMC, with spectral typesB8-A5, have anomalous colors and hydrogen lines too strong for theirluminosities.
| Catalogue of the Small Magellanic Cloud star members
This catalogue contains 524 Small Magellanic Cloud members locatedbetween 0.51 h and 1.22 h in right ascension for the epoch 1975.0.Equatorial coordinates, localization of stars with respect to thecomplexes of ionized hydrogen and information about possible closeoptical companions are given. Photoelectric photometric UBV data,spectral classifications and radial velocities published by variousauthors are listed as well in this catalogue. A master set and 17identification astrographic charts including one panelled chart forisolated members are provided.
| Equivalent width measurements in galactic supergiant and in Small Magellanic Cloud star spectra
Measurements of equivalent width are made in spectra of 40 galacticsupergiants and 21 Small Magellanic Cloud stars. These measurementsconfirm the results of spectral classification in the SMC (Dubois etal., 1977) and show a general weakness of the metallic lines in the SMCstar spectra. This weakness is not the same for all thy metals and somecases may be attributable to physical phenomena which occur in theatmospheres of these luminous stars.
| Structure and kinematics of the Small Magellanic Cloud as outlined by its brightest stars
Some basic properties of extreme Population I stars in the SmallMagellanic Cloud (SMC) are investigated on the basis of a systematicspectroscopic and photometric survey of the brightest supergiantsbelonging to the SMC. It is found that the main body and Wing of the SMCappear to have essentially the same mean radial velocity but that the K1region has a markedly different velocity. A differential distancemodulus of 0.50 + or - 0.07 is determined, along with an internal meanB-V color excess of 0.06 mag, a radial depth greater than that of theLarge Magellanic Cloud, and a disruption time of the order of 700million years.
| Additional radial velocities of supergiants in the Small Magellanic Cloud
Additional radial velocities of 28 SMC supergiants determined in theyears 1959-1969 at the Radcliffe Observatory are presented. These andother measures from ESO and elsewhere are intercompared. The meanRadcliffe velocities have an internal standard error of + or - 4.7 km/s,and a systematic error exceeding 4 km/s is regarded as unlikely. Eightstars in the SMC core have a corrected velocity dispersion of only 6.9km/s, similar to Feast's (1970) values for H II regions in the core. Butthe core H II regions have a velocity differential of -20 km/s relativeto these stars. The velocity dispersion for stars in other parts of theCloud is of the order of 15 km/s, as previously found. Two possiblyvariable-velocity stars are discussed without reaching a satisfactoryconclusion.
| Observations of supergiant stars in the Small Magellanic Cloud
Spectrographic and photometric data are presented for 91 supergiantstars belonging to the Small Magellanic Cloud and its Wing. The datainclude MK classes for 52 stars, radial-velocity data for 51 stars, andUBV photometry for 90 stars. In V the limiting magnitude for thespectrographic observations is 13.7 and for the photometricobservations, 13.9. Whenever possible, radial velocities forinterstellar Ca II and forbidden O II have been measured and listed.Notes are given for the individual stars concerning the data obtainedand comparisons with results of previous investigations.
| MK classification in the Small Magellanic cloud
This paper compares in detail the spectra of SMC supergiants with thoseof galactic supergiants. The helium and metallic lines are weaker in theformer, but not uniformly so for all metals. Also the Balmer jump issmall compared with standards, and some stars have quite large hydrogenprofiles. Other peculiar features are described.
| Etude DU Petit Nuage de Magellan par la technique DU prisme-objectif de Fehrenbach.
Not Available
| BVRI-Photometry of the Brightest Stars in the Magellanic Clouds
We have made photometric observations in the BVRI system of 100 objectsthat belong to the Magellanic Clouds. The observational data indicate atleast two remarkable differences between some of the Magellanic Cloudobjects and the supergiant stars in the Galaxy, namely, the former aremore luminous than the most luminous galactic supergiants and they alsohave a color excess in V-R unexplained by interstellar extinction alone.Two possible explanations are likely for the additional color excess;either the presence of a circumstellar dust envelope or a differentchemical atmospheric composition from galactic supergiant stars, orperhaps both of them.
| Polarization measurements of stars in the Magellanic Clouds.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970AJ.....75..778M&db_key=AST
| A finding list of proven or probable Small Magellanic Clouds members .
Abstract image available at:http://adsabs.harvard.edu/abs/1968AJ.....73..246S
| Equivalent widths of Hγ in stellar spectra of the Magellanic Clouds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966MNRAS.132..433H&db_key=AST
| New Members of the Small Magellanic Cloud
Not Available
| The brightest stars in the Magellanic Clouds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1960MNRAS.121..337F&db_key=AST
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Dades d'Observació i Astrometria
| Constel·lació: | Tucana |
| Ascensió Recta: | 01h01m17.00s |
| Declinació: | -72°17'31.1" |
| Magnitud Aparent: | 11.507 |
| Moviment propi RA: | -2.5 |
| Moviment propi Dec: | 1.5 |
| B-T magnitude: | 11.492 |
| V-T magnitude: | 11.506 |
Catàlegs i designacions:
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