Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Ultraviolet extinction toward a quiescent molecular cloud in the Small Magellanic Cloud
Context. The mean UV extinction law for the Small Magellanic Cloud (SMC)is usually taken as a template for low-metallicity galaxies. However,its current derivation is based on only five stars, which renders itsuniversality questionable. More targets with measured extinction laws inthe SMC are necessary to determine its possible dependence on parameterssuch as metallicity and star-forming activity. Aims: We aim tomeasure the UV extinction law for several stars in the quiescentmolecular cloud SMC B1-1. Methods: We obtained HST/STIS slitlessUV spectroscopy of a 25'' × 25'' field of view and combined itwith ground-based NIR and visible photometry of the stars in the field.The results were processed using the Bayesian photometric packageCHORIZOS to derive the visible-NIR extinction values for each star. Theunextinguished spectral energy distributions (SEDs) obtained in this waywere then used to derive the UV extinction law for the four mostextinguished stars. We also recalculated the visible-NIR extinction forthe five SMC stars with preexisting UV extinction laws. Results:The UV extinction law for four SMC B1-1 stars that are situated withinseveral pc of each other differs significantly from star to star. The2175 Å bump is moderately strong in one, weak in two, and absentin the fourth.
| Interstellar H I and H2 in the Magellanic Clouds: An Expanded Sample Based on Ultraviolet Absorption-line Data
We have determined column densities of H I and/or H2 forsight lines in the Magellanic Clouds from archival Hubble SpaceTelescope and Far-Ultraviolet Spectroscopic Explorer spectra of H ILy? and H2 Lyman-band absorption. Together with somesimilar data from the literature, we now have absorption-based N(H I)and/or N(H2) for 285 Large Magellanic Cloud (LMC) and SmallMagellanic Cloud (SMC) sight lines (114 with a detection or limit forboth species)—enabling more extensive, direct, and accuratedeterminations of molecular fractions, gas-to-dust ratios, and elementaldepletions in these two nearby, low-metallicity galaxies. For sightlines where the N(H I) estimated from 21 cm emission is significantlyhigher than the value derived from Ly? absorption (presumably dueto emission from gas beyond the target stars), integration of the 21 cmprofile only over the velocity range seen in Na I or H2absorption generally yields much better agreement. Conversely, N(21 cm)can be lower than N(Ly?) by factors of 2-3 in some LMC sightlines—suggestive of small-scale structure within the 21 cm beam(s)and/or some saturation in the emission. The mean gas-to-dust ratiosobtained from N(Htot)/E(B - V) are larger than in ourGalaxy, by factors of 2.8-2.9 in the LMC and 4.1-5.2 in theSMC—i.e., factors similar to the differences in metallicity. TheN(H2)/E(B - V) ratios are more similar in the threegalaxies, but with considerable scatter within each galaxy. These datamay be used to test models of the atomic-to-molecular transition at lowmetallicities and predictions of N(H2) based on comparisonsof 21 cm emission and the IR emission from dust.), the MAST archive at STScI (FUSE data), and the University of Bonn(LAB and GASS 21 cm surveys).
| Interstellar TiII in the Milky Way and Magellanic Clouds
We discuss several sets of TiII absorption-line data, which probe avariety of interstellar environments in our Galaxy and in the MagellanicClouds. Comparisons of high-resolution [full width at half-maximum(FWHM) ~ 1.3-1.5kms-1] TiII spectra of Galactic targets withcorresponding high-resolution spectra of NaI, KI and CaII reveal bothsimilarities and differences in the detailed structure of theabsorption-line profiles - reflecting component-to-component differencesin the ionization and depletion behaviour of those species.Moderate-resolution (FWHM ~ 3.4-4.5kms-1) spectra of moreheavily reddened Galactic stars provide more extensive information onthe titanium depletion in colder, denser clouds - where more than 99.9per cent of the Ti may be in the dust phase. Moderate-resolution (FWHM ~4.5-8.7kms-1) spectra of stars in the Magellanic Cloudssuggest that the titanium depletion is generally much less severe in theLarge and Small Magellanic Clouds than in our Galaxy [for a givenN(Htot), E(B - V), or molecular fraction f(H2)] -providing additional evidence for differences in depletion patterns inthose two lower-metallicity galaxies. We briefly discuss possibleimplications of these results for the interpretation of gas-phaseabundances in quasi-stellar object absorption-line systems and ofvariations in the D/H ratio in the local Galactic interstellar medium.This paper is based in part on observations collected at the EuropeanSouthern Observatory, Chile, under programmes 65.I-0526, 67.C-0281,67.D-0238, 70.D-0164, 72.C-0682 and 74.D-0109. Based in part onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Science Institute. STScI isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract NAS5-26555.Visiting observer, European Southern Observatory and Kitt Peak NationalObservatory. ‡E-mail: dwelty@astro.illinois.edu (DEW); Paul.crowther@sheffield.ac.uk(PAC)
| StarCAT: A Catalog of Space Telescope Imaging Spectrograph Ultraviolet Echelle Spectra of Stars
StarCAT is a catalog of high resolution ultraviolet spectra of objectsclassified as "stars," recorded by Space Telescope Imaging Spectrograph(STIS) during its initial seven years of operations (1997-2004). StarCATis based on 3184 echelle observations of 545 distinct targets, with atotal exposure duration of 5.2 Ms. For many of the objects, broadultraviolet coverage has been achieved by splicing echellegrams taken intwo or more FUV (1150-1700 Å) and/or NUV (1600-3100 Å)settings. In cases of multiple pointings on conspicuously variablesources, spectra were separated into independent epochs. Otherwise,different epochs were combined to enhance the signal-to-noise ratio(S/N). A post-facto correction to the {\sf calstis} pipeline data setscompensated for subtle wavelength distortions identified in a previousstudy of the STIS calibration lamps. An internal "fluxing" procedureyielded coherent spectral energy distributions (SEDs) for objects withbroadly overlapping wavelength coverage. The best StarCAT materialachieves 300 m s-1 internal velocity precision;absolute accuracy at the 1 km s-1 level; photometricaccuracy of order 4%; and relative flux precision several times better(limited mainly by knowledge of SEDs of UV standard stars). WhileStarCAT represents a milestone in the large-scale post-processing ofSTIS echellegrams, a number of potential improvements in the underlying"final" pipeline are identified.
| A FUSE Survey of the Rotation Rates of Very Massive Stars in the Small and Large Magellanic Clouds
We present projected rotational velocity values for 97 Galactic, 55 SMC,and 106 LMC O-B type stars from archival FUSE observations. The evolvedand unevolved samples from each environment are compared through theKolmogorov-Smirnov test to determine if the distribution of equatorialrotational velocities is metallicity dependent for these massiveobjects. Stellar interior models predict that massive stars with SMCmetallicity will have significantly reduced angular momentum loss on themain sequence compared to their Galactic counterparts. Our results findsome support for this prediction but also show that even at Galacticmetallicity, evolved and unevolved massive stars have fairly similarfractions of stars with large Vsin i values. Macroturbulent broadeningthat is present in the spectral features of Galactic evolved massivestars is lower in the LMC and SMC samples. This suggests the processesthat lead to macroturbulence are dependent upon metallicity.
| The Far Ultraviolet Spectroscopic Explorer Legacy in the Magellanic Clouds: An Online Stellar Sight Line Atlas
We present an atlas of stellar sight line data from the Far UltravioletSpectroscopic Explorer (FUSE) satellite for 287 stars in the MagellanicClouds, obtained from eight years of satellite operations. The intent ofour project is to make this rich ensemble data set accessible to a broadcommunity of researchers in a standardized format that will enable easyidentification of subsets of these data that are appropriate forpursuing specific science programs. We present the data in astandardized manner, showing key interstellar lines on a velocity scale,optical (MCELS) and infrared (Spitzer) sight line context images, andoverview plots of the spectral region containing the O VI??1032, 1038 doublet, and the entire 905-1187 Åspectral range observed with FUSE. Objects with multiple observationshave had their data summed to directly provide the highestsignal-to-noise ratio available. These data are accessible online as aHigh Level Science Product through the Multimission Archive at SpaceTelescope (MAST). In this article, we describe the data sets andprocessing, the atlas materials, and the MAST interface in detail, andalso provide examples of how to use these materials.
| 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.
| Interstellar gas, dust and diffuse bands in the SMC
Aims.In order to gain new insight into the unidentified identity of thediffuse interstellar band (DIB) carriers, this paper describes researchinto possible links between the shape of the interstellar extinctioncurve (including the 2175 Å bump and far-UV rise), the presence orabsence of DIBs, and physical and chemical conditions of the diffuseinterstellar medium (gas and dust) in the Small Magellanic Cloud (SMC). Methods: We searched for DIB absorption features in VLT/UVES spectra ofearly-type stars in the SMC whose reddened lines-of-sight probe thediffuse interstellar medium of the SMC. Apparent column density profilesof interstellar atomic species (Na i, K i, Ca ii and Ti ii) areconstructed to provide information on the distribution and conditions ofthe interstellar gas. Results: The characteristics of eight DIBsdetected toward the SMC wing target AzV 456 arestudied and upper limits are derived for the DIB equivalent widthstoward the SMC stars AzV 398, AzV214, AzV 18, AzV 65 andSk 191. The amount of reddening is derived for theseSMC sightlines, and, using RV and the H i column density,converted into a gas-to-dust ratio. From the atomic column densityratios we infer an indication of the strength of the interstellarradiation field, the titanium depletion level and a relative measure ofturbulence/quiescence. The presence or absence of DIBs appears to berelated to the shape of the extinction curve, in particular with respectto the presence or absence of the 2175 Å feature. Our measurementsindicate that the DIB characteristics depend on the local physicalconditions and chemical composition of the interstellar medium of theSMC, which apparently determine the rate of formation (and/or)destruction of the DIB carriers. The UV radiation field (viaphotoionisation and photo-destruction) and the metallicity (i.e. carbonabundance) are important factors in determining diffuse band strengthswhich can differ greatly both between and within galaxies.Based on observations collected with VLT/UVES at the European SouthernObservatory, Paranal, Chile (ESO programs 67.C-0281 and 71.C-0679).Tables [see full text]-[see full text] and Fig. [see full text]are only available in electronic form at http://www.aanda.org
| H? Emission from the Magellanic Bridge
We present here a preliminary report and commentary of recentlyprocessed observations of H? emission towards the MagellanicBridge. These data have been analysed in an attempt to quantify theextent to which the stellar population is capable of reshaping the localISM. We find that the H? emission regions are small, weak andsparsely distributed, consistent with a relatively quiescent andinactive ISM where radiative and collisional ionisation is inefficientand sporadic. This suggests that energetic processes at the small scale(i.e. ~tens of pc) do not dominate the energy balance within the ISM ofthe Bridge, which therefore hosts a pristine turbulent structure,otherwise inaccessible within our own Galaxy. We find H? emissionthat is well correlated with detected 12CO(1-0) line emission(a proxy for molecular hydrogen), as well as other easily identifiedring-like Hi features.
| Models for Massive Stellar Populations with Rotation
We present and discuss evolutionary synthesis models for massive stellarpopulations generated with the Starburst99 code in combination with anew set of stellar evolution models accounting for rotation. The newstellar evolution models were compiled from several data releases of theGeneva group and cover heavy-element abundances ranging from twice solarto one-fifth solar. The evolution models were computed for rotationvelocities on the zero-age main sequence of 0 and 300 km s-1and with the latest revision of stellar mass-loss rates. Since the masscoverage is incomplete, in particular at nonsolar chemical composition,our parameter study is still preliminary and must be viewed asexploratory. Stellar population properties computed with Starburst99 andthe new evolution models show some marked differences in comparison withmodels obtained using earlier tracks. Since individual stars now tend tobe more luminous and bluer when on the blue side of theHertzsprung-Russell diagram, the populations mirror this trend. Forinstance, increases by factors of 2 or more are found for thelight-to-mass ratios at ultraviolet to near-infrared wavelengths, aswell as for the output of hydrogen-ionizing photons. If these resultsare confirmed once the evolution models have matured, recalibrations ofcertain star formation and initial mass function indicators will berequired.
| B-type supergiants in the Small Magellanic Cloud: rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants havebeen analysed to estimate the contributions of both macroturbulence androtation to the broadening of their metal lines. Two differentmethodologies are considered, viz. goodness-of-fit comparisons betweenobserved and theoretical line profiles and identifying zeros in theFourier transforms of the observed profiles. The advantages andlimitations of the two methods are briefly discussed with the lattertechniques being adopted for estimating projected rotational velocities(v sin i) but the former being used to estimate macroturbulentvelocities. The projected rotational velocity estimates range fromapproximately 20 to 60 km s-1, apart from one SMC supergiant,Sk 191, with a v sin i ≃ 90 km s-1. Apart from Sk 191,the distribution of projected rotational velocities as a function ofspectral type are similar in both our Galactic and SMC samples withlarger values being found at earlier spectral types. There is marginalevidence for the projected rotational velocities in the SMC being higherthan those in the Galactic targets but any differences are only of theorder of 5-10 km s-1, whilst evolutionary models predictdifferences in this effective temperature range of typically 20 to 70 kms-1. The combined sample is consistent with a linearvariation of projected rotational velocity with effective temperature,which would imply rotational velocities for supergiants of 70 kms-1 at an effective temperature of 28 000 K (approximately B0spectral type) decreasing to 32 km s-1 at 12 000 K (B8spectral type). For all targets, the macroturbulent broadening wouldappear to be consistent with a Gaussian distribution (although otherdistributions cannot be discounted) with an 1/e half-width varying fromapproximately 20 km s-1 at B8 to 60 km s-1 at B0spectral types.
| B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models
High-resolution UCLES/AAT spectra are presented for nine B-typesupergiants in the SMC, chosen on the basis that they may show varyingamounts of nuclear-synthetically processed material mixed to theirsurface. These spectra have been analysed using a new grid ofapproximately 12 000 non-LTE line blanketed tlusty model atmospheres toestimate atmospheric parameters and chemical composition. The abundanceestimates for O, Mg and Si are in excellent agreement with those deducedfrom other studies, whilst the low estimate for C may reflect the use ofthe C II doublet at 4267 Å. The N estimates are approximately anorder of magnitude greater than those found in unevolved B-type stars orH II regions but are consistent with the other estimates in AB-typesupergiants. These results have been combined with results from aunified model atmosphere analysis of UVES/VLT spectra of B-typesupergiants (Trundle et al. 2004, A&A, 417, 217) to discuss theevolutionary status of these objects. For two stars that are in commonwith those discussed by Trundle et al., we have undertaken a carefulcomparison in order to try to understand the relative importance of thedifferent uncertainties present in such analyses, includingobservational errors and the use of static or unified models. We findthat even for these relatively luminous supergiants, tlusty models yieldatmospheric parameters and chemical compositions similar to thosededuced from the unified code fastwind.
| Understanding B-type supergiants in the low metallicity environment of the SMC II
Despite a resurgence of effort over the last decade in the area ofmassive stars there is still ambiguity over their evolutionary path,contamination of their surface abundances and the behaviour of theirstellar winds. Here 10 SMC B-type supergiants are analysed applying aunified model atmosphere code fastwind to intermediate resolutionspectra from the ESO Multi Mode Instrument (emmi) on the NTT telescope.Combined with the 8 targets analysed in Paper I (Trundle et al. 2004),this work provides observational results on the properties of the windsand chemical compositions of B-type supergiants in the SMC. This paperemphasizes and substantiates the implications for stellar evolution fromPaper I; that current theoretical models need to produce larger degreesof surface nitrogen enhancements at lower rotational velocities. Inaddition a significant discrepancy between theoretical and observedmass-loss rates is discussed which will have important implications forthe rotational velocities obtained from stellar evolution calculations.Furthermore, an initial calibration of the wind-momentum luminosityrelationship for B-type supergiants in a low metallicity environment (Z= 0.004) is presented.
| The Ultraviolet and Optical Spectra of Luminous B-Type Stars in the Small Magellanic Cloud
We present ultraviolet spectra from the Space Telescope ImagingSpectrograph (STIS) of 12 early B-type stars in the Small MagellanicCloud (SMC), composed of nine supergiants and three giants. Amorphological comparison with Galactic analogs is made using archivaldata from the International Ultraviolet Explorer (IUE). In general, theintensity of the P Cygni emission in the UV resonance lines is greaterand is seen to later spectral types in the Galactic spectra than intheir metal-deficient SMC counterparts. We attribute these effects asmost likely arising from weaker stellar winds in the SMC targets, aspredicted by radiatively driven wind theory. We also include unpublishedSTIS observations of two late O-type stars in the SMC. In combinationwith published O-type STIS data, we now have an extensive ultravioletspectral library of metal-deficient stars to use in the study ofunresolved starbursts and high-redshift star-forming galaxies. In thiscontext, we present empirical measurements for the B-type spectra of thenew ``1978 index'' suggested by Rix et al. as a probe of metallicity insuch systems.
| Terminal Velocities of Luminous, Early-Type Stars in the Small Magellanic Cloud
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS)are used to determine terminal velocities for 11 O and B-type giants andsupergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IVresonance lines. Using archival data from observations with the GoddardHigh-Resolution Spectrograph and the International Ultraviolet Explorertelescope, terminal velocities are obtained for a further five B-typesupergiants. We discuss the metallicity dependence of stellar terminalvelocities for supergiants, finding no evidence for a significantscaling between Galactic and SMC metallicities forTeff<30,000 K, consistent with the predictions ofradiation-driven wind theory. A comparison of thev&infy;/vesc ratio between the SMC and Galacticsamples, while consistent with the above statement, emphasizes that theuncertainties in the distances to galactic OB-type stars are a seriousobstacle to a detailed comparison with theory. For the SMC sample thereis considerable scatter in v&infy;/vesc at agiven effective temperature, perhaps indicative of uncertainties instellar masses.
| Understanding B-type supergiants in the low metallicity environment of the SMC
Spectroscopic analyses of 7 SMC B-type supergiants and 1 giant have beenundertaken using high resolution optical data obtained on the VLT withUVES. FASTWIND, a non-LTE, spherical, line-blanketed model atmospherecode was used to derive atmospheric and wind parameters of these starsas well as their absolute abundances. Mass-loss rates, derived fromH? profiles, are in poor agreement with metallicitydependent theoretical predictions. Indeed the wind-momenta of the SMCstars appear to be in good agreement with the wind-momentum luminosityrelationship (WLR) of Galactic B-type stars, a puzzling result giventhat line-driven wind theory predicts a metallicity dependence. Howeverthe galactic stars were analysed using unblanketed model atmosphereswhich may mask any dependence on metallicity. A mean nitrogenenhancement of a factor of 14 is observed in the supergiants whilst onlyan enrichment of a factor of 4 is present in the giant, AV216. Similarexcesses in nitrogen are observed in O-type dwarfs and supergiants inthe same mass range, suggesting that the additional nitrogen is producedwhile the stars are still on the main-sequence. These nitrogenenrichments can be reproduced by current stellar evolution models, whichinclude rotationally induced mixing, only if large initial rotationalvelocities of 300 km s-1 are invoked. Such large rotationalvelocities appear to be inconsistent with observed v sin i distributionsfor O-type stars and B-type supergiants. Hence it is suggested that thecurrently available stellar evolution models require more efficientmixing for lower rotational velocities.
| Characterizing the Stellar Population in NGC 1705-1
We observed the brightest super-star cluster NGC 1705-1 in the nearbydwarf galaxy NGC 1705 with the Space Telescope Imaging Spectrograph inthe echelle mode between 1200 and 3100 Å. The data allow a studyof the young stellar population at hitherto unprecedented spectralresolution and signal-to-noise ratios. A comprehensive list of strongand weak stellar and interstellar absorption lines is given, togetherwith the measured line parameters. Four distinct velocity systems areidentified: stellar lines at the measured H I velocity, blueshiftedinterstellar lines from outflowing gas, Milky Way foreground absorption,and a high-velocity cloud. Comparison with stellar template spectraindicates an equivalent spectral type of B0 to B1, with mostly dwarf andgiant stars contributing. When placed on a theoreticalHertzsprung-Russell diagram, these stars constrain the age of NGC 1705-1to 12+/-31 Myr. Since this age is derived purelyfrom spectroscopy, it is independent of reddening corrections. Acomparison of the observed and theoretical mass-to-light ratio for thederived age was performed. We find no significant evidence for ananomalous initial mass function at the low-mass end, contrary tosuggestions found in the literature. The stellar population of NGC1705-1 is similar to that in other massive clusters, such as 30 Doradusor NGC 1569-A, after taking into account age differences and modeluncertainties. We discuss the difficulty of relating observed andtheoretical mass-to-light ratios because of the unknown gas massfraction lost by the cluster and the uncertain mass-loss rates ofasymptotic giant branch stars in population synthesis models.
| 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.
| Starburst-like Dust Extinction in the Small Magellanic Cloud
The recent discovery that the ultraviolet (UV) dust extinction instarburst galaxies is similar to that found in the Small MagellanicCloud (SMC) motivated us to reinvestigate the UV extinction found in theSMC. We have been able to improve significantly on previous studies bycarefully choosing pairs of well-matched reddened and unreddened stars.In addition, we benefited from the improved signal-to-noise ratio of theNEWSIPS International Ultraviolet Explorer (IUE) data and the largersample of SMC stars now available. Searching the IUE Final Archive, wefound only four suitable early-type stars that were significantlyreddened and had well-matched comparison stars. The extinction for threeof these stars is remarkably similar. The curves are roughly linear withlambda -1 and have no measurable 2175 Angstroms bump. The fourth starhas an extinction curve with a significant 2175 Angstroms bump andweaker far-UV extinction. The dust along all four sight lines is thoughtto be local to the SMC. There is no significant Galactic foregroundcomponent. The first three stars lie in the SMC bar, and the line ofsight for each of them passes through regions of recent star formation.The fourth star belongs to the SMC wing, and its line of sight passesthrough a much more quiescent region. Thus, the behavior of the dustextinction in the SMC supports a dependence of dust properties on starformation activity. However, other environmental factors (such asgalactic metallicity) must also be important. Dust in the 30 Dor regionof the LMC, where much more active star formation is present, does notshare the extreme extinction properties seen in SMC dust.
| 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.
| 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.
| Spectroscopic observations of AB supergiants in M 33.
We have observed 10 luminous blue stars in M 33 at intermediatedispersion and have determined spectral types and luminosity classesfrom the blue and red spectrograms by comparing with galactic and SMCstars. Halpha_ profiles have proved to be very useful indisentangling luminosity and metallicity effects, and constitute avaluable diagnostic when comparing spectral classifications in galaxieswith different metallicities. We derive qualitative estimates of thestellar metallicities which range from solar to SMC-like and are broadlyconsistent with abundance gradient results obtained for M 33 from Hiiregion studies. Comparison of the new data for B 324 with previous workshows that there is evidence for significant spectral variabilitystrongly suggesting an LBV nature for this star.
| FAUST far ultraviolet observations of Shapley's wing in the SMC-LMC bridge.
The Small Magellanic Cloud-Large Magellanic Cloud (SMC-LMC) bridge hasbeen the subject of a limited number of studies. It is the closestbridge among a number of known intergalactic bridges situated in groupsof galaxies. Because of tidal interactions, star formation is likely tobe very active in these bridges, and the details of these interactionscan be investigated through the study of hot stars; UV wide-fieldtelescopes are especially suited to this class of work. A new suchobservation of the SMC in the far UV (1400-1800A) was obtained with theFAUST telescope (SSL, Berkeley; LAS, Marseille) flown aboard the spaceshuttle in March, 1992 as part of the ATLAS-1 mission. A previousdetection by the Very-Wide-Field Camera (VWFC; LAS, Marseille) aboardSpacelab-1 revealed the stellar UV contribution of the Shapley Wing ofthe SMC in the direction of the LMC. The much deeper (4.6 magnitudes)FAUST observations, with a resolution thrice better than that of theVWFC, confirm this extension as part of the bridge and allow a clearidentification of 29 UV-bright stars with known opticalspectrophotometry and radial velocities.
| A catalog of far-ultraviolet point sources detected with the fast FAUST Telescope on ATLAS-1
We list the photometric measurements of point sources made by the FarUltraviolet Space Telescope (FAUST) when it flew on the ATLAS-1 spaceshuttle mission. The list contains 4698 Galactic and extragalacticobjects detected in 22 wide-field images of the sky. At the locationssurveyed, this catalog reaches a limiting magnitude approximately afactor of 10 fainter than the previous UV all-sky survey, TDl. Thecatalog limit is approximately 1 x 10-14 ergs A sq cm/s,although it is not complete to this level. We list for each object theposition, FUV flux, the error in flux, and where possible anidentification from catalogs of nearby stars and galaxies. Thesecatalogs include the Michigan HD (MHD) and HD, SAO, the HIPPARCOS InputCatalog, the Position and Proper Motion Catalog, the TD1 Catalog, theMcCook and Sion Catalog of white dwarfs, and the RC3 Catalog ofGalaxies. We identify 2239 FAUST sources with objects in the stellarcatalogs and 172 with galaxies in the RC3 catalog. We estimate thenumber of sources with incorrect identifications to be less than 2%.
| CCD spectroscopy of SMC supergiants
The equivalent widths of the stellar features of a sample of SMCsupergiants have been measured from CCD spectra obtained in the bluewavelength region with the ESO 3.6-m telescope at a dispersion of 29A/mm. A comparison of these measurements with those for their galacticcounterparts shows that the SMC members have generally weak metal lines.No weakness is detected in the SMC members' helium lines. The presentobservations confirm that the diffuse interstellar 4430 A band ispresent in the SMC. Its strength relative to E(B-V) is significantlyweaker than in the Galaxy.
| Structure of the interstellar medium in the Magellanic Clouds
Using optical and UV observations, the internal gas structure ofMagellanic Clouds (MCs) was examined for clues to the recent evolutionof the Magellanic System and further signs of this collision event.Results indicate that the intermediate velocity clouds (IVCs) havedifferent origins. Thus, the '60-km/s' IVC is clearly of Galacticorigin. The '130-km/s' IVC is probably also Galactic and is not directlyassociated with the MCs. The '170 km/s' IVC is quite clearly associatedwith the LMC and seems to be the front of the LMC. The overall gasstructure in the MCs indicates that there is a distance-velocityassociation except in two regions: one to the south-west of the SMC (andis enigmatic), and the other to the south of 30 Doradus and appears tobe smoothly linked with the H I bridge, indicating that both featureshave a common overall cause such as the recent tidal collision.
| 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.
| Effective temperatures and radii of Small Magellanic Cloud supergiants
The effective temperatures and radii of Small Magellanic Cloudsupergiants are found by comparison with models. The methods usedparallel those used previously by the authors for the Large Cloud and byUnderhill and co-workers for the Galaxy. No significant difference isfound among these galaxies in this respect. A new analysis of the SMCextinction law is presented.
| Dust in external galaxies - Diffuse bands and the albedo
The importance of detecting the 4430 Å diffuse absorption band inthe Magellanic Clouds is discussed. Recent observations of the band inthe spectra of reddened SMC stars are described. The albedo of theinterstellar dust near 30 Doradûs is estimated by comparingultraviolet extinction laws constructed from small- and large-apertureobservations.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Южная Гидра |
| Прямое восхождение: | 01h41m42.05s |
| Склонение: | -73°50'38.2" |
| Видимая звёздная величина: | 12.025 |
| Собственное движение RA: | -1.6 |
| Собственное движение Dec: | 2.6 |
| B-T magnitude: | 11.835 |
| V-T magnitude: | 12.01 |
Каталоги и обозначения:
|
