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Yellow Supergiants in the Small Magellanic Cloud: Putting Current Evolutionary Theory to the Test
The yellow supergiant content of nearby galaxies provides a criticaltest of massive star evolutionary theory. While these stars are thebrightest in a galaxy, they are difficult to identify because a largenumber of foreground Milky Way stars have similar colors and magnitudes.We previously conducted a census of yellow supergiants within M31 andfound that the evolutionary tracks predict a yellow supergiant durationan order of magnitude longer than we observed. Here we turn ourattention to the Small Magellanic Cloud (SMC), where the metallicity is10× lower than that of M31, which is important as metallicitystrongly affects massive star evolution. The SMC's large radial velocity(~160 km s-1) allows us to separate members from foregroundstars. Observations of ~500 candidates yielded 176 near-certain SMCsupergiants, 16 possible SMC supergiants, along with 306 foregroundstars, and provide good relative numbers of yellow supergiants down to12 M sun. Of the 176 near-certain SMC supergiants, thekinematics predicted by the Besançon model of the Milky Waysuggest a foreground contamination of <=4%. After placing the SMCsupergiants on the Hertzsprung-Russell diagram (HRD) and comparing ourresults to the Geneva evolutionary tracks, we find results similar tothose of the M31 study: while the locations of the stars on the HRDmatch the locations of evolutionary tracks well, the models overpredictthe yellow supergiant lifetime by a factor of 10. Uncertainties aboutthe mass-loss rates on the main sequence thus cannot be the primaryproblem with the models.
| 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.
| Light variations of alpha Cygni variables in the Magellanic Clouds
We present time-series monitoring of 19 Magellanic Cloud super- andhypergiants, among which 13 alpha Cygni variables, viz.: S18 =AzV154, HDE268835 = R66, HD37974 = R126, HDE268757 = R59, HDE268822 =GV505, HDE269355 = GV258, HDE269612 = GV322, HDE270025 = GV439, AzV121,HD5277 = AzV136 = R10, AzV197, AzV310 = R26, and AzV369; the LMC starsHD32034 = GV80 = R62, HDE268819 = GV91, HDE269661 = GV346 = R111,HDE269697 = GV352, HDE269953 = GV423 = R150 and HDE270111 = GV460.
| A photometric study of 11 massive stars in the Magellanic Clouds
We present and discuss VBLUW photometry of eleven massive stars in theMagellanic Clouds: the SMC stars AzV 121, AzV 136 = HD 5277 = R 10, AzV197, AzV 310 = R 26 and AzV 369; the LMC stars GV 80 = HD 32034 = R 62,GV 91 = HDE 268 819, GV 346 = HDE 269661 = R 111, GV 352 = HDE 269697,GV 423 = HDE 269953 = R 150 and GV 460 = HDE 270111. Only one G0 Ia SMCsupergiant is found to be variable, whereas all members of the LMCsample show definite variability. We find that roughly aboveM/M\sun = 25, supergiants become photometrically unstable.The reddening-independent metal-index [B-L] is used to investigate themetallicity of the late-type supergiants in both galaxies relative tosimilar supergiants in the solar neighbourhood.
| Characteristics and classification of A-type supergiants in the Small Magellanic Cloud
We address the relationship between spectral type and physicalproperties for A-type supergiants in the Small Magellanic Cloud (SMC).First, we construct a self-consistent classification scheme for Asupergiants, employing the calcium K to Hɛ line ratio as atemperature-sequence discriminant. Following the precepts of the `MKprocess', the same morphological criteria are applied to Galactic andSMC spectra, with the understanding that there may not be acorrespondence in physical properties between spectral counterparts indifferent environments. Then we discuss the temperature scale,concluding that A supergiants in the SMC are systematically cooler thantheir Galactic counterparts at the same spectral type, by up to ~10 percent. Considering the relative line strengths of Hγ and the CH Gband, we extend our study to F- and early G-type supergiants, for whichsimilar effects are found. We note the implications for analyses ofluminous extragalactic supergiants, for the flux-weightedgravity-luminosity relationship and for population synthesis studies inunresolved stellar systems.
| Long period variables detected by ISO in the Small Magellanic Cloud
This article presents the study of the light-curves extracted from theMACHO database of a sample of stars observed by the Infrared SpaceObservatory in the Small Magellanic Cloud. These stars belong to theISO-Mini-Survey catalogue of the Magellanic Clouds (ISO-MCMS, Loup etal. {in preparation}). Most of them are in the asymptotic giant branch(AGB) and supergiant phases. The dominant period and amplitude ofpulsation have been derived and the stars have been classified as Miraor Semi-Regular pulsators. Furthermore, the cross-identification withnear-infrared DENIS and 2MASS magnitudes available within the ISO-MCMSallowed us: (i) to investigate the properties of these stars in thecombined near- and mid-infrared colour-magnitude diagrams, (ii) toderive the bolometric magnitude by integrating the spectral energydistribution and (iii) to estimate the mass-loss rate. The stars havebeen divided into carbon- (C-) and oxygen-rich (O-rich) using the(J-KS, KS) colour-magnitude diagram and theirperiod and amplitude distributions have been compared. C-rich AGB starshave a sharp peak in their period distribution at about 250 days andhave on average a larger amplitude than O-rich AGB stars. This effect,not previously detected from the study of similar stars in the LargeMagellanic Cloud and in the Baade's window, might be closely related tothe metallicity of the environment in which the stars have formed.Complete Tables 1 and 4 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/406/51
| A-Type Supergiant Abundances in the Small Magellanic Cloud: Probes of Evolution
New abundances of N, O, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Sr, Zr, and Baare presented for 10 A-type supergiants in the Small Magellanic Cloud(SMC), plus upper limits for C. In interpreting the CNO results forconstraints on stellar evolution theories, careful attention has beenpaid to the comparison abundances, i.e., the present day abundances ofSMC nebulae and B dwarf stars. These new results are also compared topublished results from F-K supergiant analyses and found to be in goodagreement when both sets of data are carefully examined as differential(SMC minus Galactic standard) abundances. With the exception ofnitrogen, very small star-to-star abundance variations are found for allelements in this analysis. The N variations are not predicted bystandard stellar evolution models. Instead, the results support the newpredictions reported from rotating stellar models, where the range innitrogen is the result of partial mixing of CN-cycled gas from thestellar interior due to main-sequence rotation at different rates. Theoverall overabundance of nitrogen in the sampled stars also implies thatthese stars have undergone the first dredge-up in addition to havingbeen mixed while on the main sequence. The alpha elements (O, Mg, Si,Ca, Ti) have similar underabundances to Fe, which is not the same asseen in metal-poor stars in the solar neighborhood of the Galaxy. Inaddition, certain light s-process elements (Zr, Ba) are slightly moreunderabundant than Fe, which is predicted by the bursting chemicalevolution model presented by Pagel & Tautvaišien? forthe SMC.
| 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.
| Luminosities of yellow supergiants from near-infrared spectra - Calibration through Magellanic Cloud stars
The possibility of using medium resolution spectrograms in the nearinfrared region to determine luminosities of A-G supergiants has beenexplored. A sample of 49 of these stars has been observed in the twoMagellanic Clouds, and using the intensities of the O I 7774 triplet andan index (CP), which is a combination of the Ca II triplet and Paschenlines intensities, a preliminary luminosity calibration, based on LMCstars, has been obtained. Such a calibration predicts reliableluminosities for Galactic supergiants, and offers the advantage of beingcompletely reddening independent. The reddening free CP index combinedwith BVRI color indices has also been used to estimate the interstellarreddenings of Magellanic Cloud stars.
| 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
| Photometric Studies of Magellanic Cloud Supergiants. II. Variability
Abstract image available at:http://adsabs.harvard.edu/abs/1986ApJS...62..451G
| Photometric studies of Magellanic Cloud supergiants. I - Mean magnitudes and reddenings. II - Variability
In the first part of this paper, a combination of spectral types for 81LMC and 46 SMC supergiants from the literature with new BVRI photometryyields reddenings and intrinsic colors for these stars. While reddeningsup to E(B-V) of 0.3 are found for the LMC sample, the figure for the SMCappears to be no more than 0.2. The dust/gas ratio in both galaxies isfound to be lower than in the Milky Way. In the second part, individualBVRI photoelectric observations for 88 LMC and 46 SMC intermediatespectral type supergiants are analyzed for variability. It is noted that45 percent of the stars exhibit significant variations, with theincidence of variability decreasing with decreasing luminosity.
| Mass loss in A and B supergiants and the extragalactic distance scale
Samples of B5 and A0 stars in the Large Magellanic Cloud (LMC)demonstrate the existence of tight correlations between luminosity andequivalent widths in the H-alpha and H-beta lines. The H-alpha line isin emission for stars brighter than M(v) = -7, and this easilyidentifiable feature should be detectable at the distances of nearbygroups of galaxies. The correlations imply that mass loss in A and Bsupergiants is strongly dependent on luminosity and therefore on stellarmass. Similar samples of stars in the Small Magellanic Cloud (SMC) showssystematically smaller H-alpha emission and more scatter in therelationships between luminosity and H-alpha line strengths than werefound for the LMC stars. There is independent evidence that mass-lossrates are smaller in the SMC than in the LMC, and this fact probablyaccounts for the lower emission at H-alpha in the SMC stars. Thedifferences between the samples in the two clouds may be caused bydifferences in stellar chemical composition.
| The brightest stars as extragalactic distance indicators
The luminosities of the brightest blue and red supergiants werecalibrated by the spectra and photometry of such stars in six LocalGroup galaxies. It is found that the brightest blue supergiants are allA type supergiants, their luminosities depending strongly on theluminosity of the parent galaxy. The same relationship obtains for themost luminous (bolometric) stars in the galaxies. It is also found thatthere is a tight upper limit to the visual luminosities of the Msupergiants at -8.0 + or - 0.2 mag in spiral and irregular type galaxiescovering almost 6 mag of galactic luminosity from -20.5 to -14.8 mag. Itis concluded that the M supergiants are excellent extragalactic distanceindicators for spiral and Magellanic type irregular galaxies andprobably also for the less luminous dwarf irregular galaxies.
| 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.
| The Galactic Foreground Reddening in the Direction of the Magellanic Couds
Abstract image available at:http://adsabs.harvard.edu/abs/1980PASP...92..587M
| 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.
| SI II equivalent widths in SMC A-type supergiants.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976MNRAS.174P...9F&db_key=AST
| List of 506 stars, probable Small Magellanic Cloud members
Abstract image available at:http://adsabs.harvard.edu/abs/1975A&AS...22..285A
| The brightest stars in the Small Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/abs/1973ApJ...181..327O
| Error analysis of the photoelectric catalogue
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973A&AS....9..297F&db_key=AST
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