目录
图像
上传图像
DSS Images Other Images
相关文章
Yellow and Red Supergiants in the Large Magellanic Cloud
Due to their transitionary nature, yellow supergiants (YSGs) provide acritical challenge for evolutionary modeling. Previous studies withinM31 and the Small Magellanic Cloud show that the Geneva evolutionarymodels do a poor job at predicting the lifetimes of these short-livedstars. Here, we extend this study to the Large Magellanic Cloud (LMC)while also investigating the galaxy's red supergiant (RSG) content. Thistask is complicated by contamination by Galactic foreground stars thatcolor and magnitude criteria alone cannot weed out. Therefore, we useproper-motions and the LMC's large systemic radial velocity (~278 kms-1) to separate out these foreground dwarfs. After observingnearly 2000 stars, we identified 317 probable YSGs, 6 possible YSGs, and505 probable RSGs. Foreground contamination of our YSG sample was ~80%,while that of the RSG sample was only 3%. By placing the YSGs on theHertzsprung-Russell diagram and comparing them against the evolutionarytracks, we find that new Geneva evolutionary models do an exemplary jobat predicting both the locations and the lifetimes of these transitoryobjects.
| A Compilation of Interstellar Column Densities
We have collated absorption line data toward 3008 stars in order tocreate a unified database of interstellar column densities. These datahave been taken from a number of different published sources and includemany different species and ionizations. The preliminary results from ouranalysis show a tight relation [N(H)/E(B - V) = 6.12 ×1021] between N(H) and E(B - V). Similar plots havebeen obtained with many different species, and their correlations alongwith the correlation coefficients are presented.
| 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).
| A selection of hot subluminous stars in the GALEX survey - I. Correlation with the Guide Star Catalog
We assembled a catalogue of bright, hot subdwarf and white dwarf starsextracted from a joint ultraviolet, optical and infrared source list.The selection is secured using colour criteria that correlate well witheffective temperatures Teff? 12 000 K. We built aNUV- V versus V - J diagram for ?60 000 bright sourcesusing the Galaxy Evolution Explorer (GALEX) NUV magnitude(NUV < 14), the associated Guide Star Catalog (GSC2.3.2)photographic quick-V magnitude and the Two-Micron All-Sky Survey J and Hmagnitudes. This distillation process delivered a catalogue of ?700sources with NUV- V < 0.5 comprising ˜160 known hotsubdwarf stars and another ˜60 known white dwarf stars. A reducedproper-motion diagram built using the proper-motion measurementsextracted from the Naval Observatory Merged Astrometric Dataset allowedus to identify an additional ˜120 new hot subdwarf candidates and˜10 hot white dwarf candidates. We present a spectroscopic studyof a subset of 52 subdwarfs, 48 of them analysed here for the firsttime, and with nine objects brighter than V ˜ 12. Our sample ofspectroscopically confirmed hot subdwarfs comprises 10 sdO-type starsand 42 sdB-type stars suitable for pulsation and binary studies. We alsopresent a study of 50 known white dwarfs selected in the GALEX surveyand six new white dwarfs from our catalogue of subluminous candidates.Ultraviolet, optical and infrared synthetic magnitudes employed in theselection and analysis of white dwarf stars are listed in appendix.Based on observations made with ESO telescopes at the La Silla ParanalObservatory under programmes 82.D-0750, 83.D-0540 and085.D-0866.Visiting Astronomer, Kitt Peak National Observatory, NationalOptical Astronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA) under cooperativeagreement with the National Science Foundation.
| 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)
| Spitzer SAGE Infrared Photometry of Massive Stars in the Large Magellanic Cloud
We present a catalog of 1750 massive stars in the Large Magellanic Cloud(LMC), with accurate spectral types compiled from the literature, and aphotometric catalog for a subset of 1268 of these stars, with the goalof exploring their infrared properties. The photometric catalog consistsof stars with infrared counterparts in the Spitzer SAGE survey database,for which we present uniform photometry from 0.3 to 24 μm in theUBVIJHKs +IRAC+MIPS24 bands. The resulting infraredcolor-magnitude diagrams illustrate that the supergiant B[e], redsupergiant, and luminous blue variable (LBV) stars are among thebrightest infrared point sources in the LMC, due to their intrinsicbrightness, and at longer wavelengths, due to dust. We detect infraredexcesses due to free-free emission among ~900 OB stars, which correlatewith luminosity class. We confirm the presence of dust around 10supergiant B[e] stars, finding the shape of their spectral energydistributions (SEDs) to be very similar, in contrast to the variety ofSED shapes among the spectrally variable LBVs. The similar luminositiesof B[e] supergiants (log L/L sun >= 4) and the rare, dustyprogenitors of the new class of optical transients (e.g., SN 2008S andNGC 300 OT), plus the fact that dust is present in both types ofobjects, suggests a common origin for them. We find the infrared colorsfor Wolf-Rayet stars to be independent of spectral type and their SEDsto be flatter than what models predict. The results of this studyprovide the first comprehensive roadmap for interpreting luminous,massive, resolved stellar populations in nearby galaxies at infraredwavelengths.
| 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.
| VLT UVES Observations of Interstellar Molecules and Diffuse Bands in the Magellanic Clouds
We discuss the abundances of interstellar CH, CH+, and CN inthe Magellanic Clouds, derived from spectra of seven SMC and 13 LMCstars obtained (mostly) with the VLT UVES. CH and/or CH+ havenow been detected toward three SMC and nine LMC stars; CN is detectedtoward Sk 143 (SMC) and Sk -67 2 (LMC). These data represent nearly allthe optical detections of these molecular species in interstellar mediabeyond the Milky Way. In the LMC, the CH/H2 ratio iscomparable to that found for diffuse Galactic molecular clouds in foursight lines but is lower by factors of 2.5-4.0 in two others. In theSMC, the CH/H2 ratio is comparable to the local Galacticvalue in one sight line but is lower by factors of 10-15 in two others.The abundance of CH in the Magellanic Clouds thus appears to depend onlocal physical conditions and not just on metallicity. In both the SMCand the LMC, the observed relationships between the column density of CHand those of CN, CH+, Na I, and K I are generally consistentwith the trends observed in our Galaxy.Using existing data for the rotational populations of H2 inthese sight lines, we estimate temperatures, radiation field strengths,and local hydrogen densities for the diffuse molecular gas. The inferredtemperatures range from about 45 to 90 K, the radiation fields rangefrom about 1 to 900 times the typical local Galactic field, and thedensities (in most cases) lie between 100 and 600 cm-3.Densities estimated from the observed N(CH), under the assumption thatCH is produced via steady state gas-phase reactions, are considerablyhigher than those derived from H2. Much better agreement isfound by assuming that the CH is made via the (still undetermined)process(es) responsible for the observed CH+. A significantfraction of the CH and CH+ in diffuse molecular material inthe SMC and LMC may be produced in photon-dominated regions. Theexcitation temperature obtained from the populations of the two lowestCN rotational levels toward Sk -67 2 is quite consistent with thetemperature of the cosmic microwave background radiation measured withCOBE.Toward most of our targets, the UVES spectra also reveal absorption atvelocities corresponding to the Magellanic Clouds ISM from several ofthe strongest of the diffuse interstellar bands (DIBs; at 5780, 5797,and 6284 Å). On average, the three DIBs are weaker by factors of7-9 (LMC) and about 20 (SMC), compared to those typically observed inGalactic sight lines with similar N(H I), presumably due to the lowermetallicities and stronger radiation fields in the LMC and SMC. Thethree DIBs are also weaker (on average, but with some exceptions), byfactors of order 2-6, relative to E(B-V), N(Na I), and N(K I) in theMagellanic Clouds. The detection of several of the so-calledC2 DIBs toward Sk 143 and Sk -67 2 with strengths similar tothose in comparable Galactic sight lines, however, indicates that nosingle, uniform scaling factor (e.g., one related to metallicity)applies to all DIBs (or for all sight lines) in the Magellanic Clouds.Based on observations collected at the European Southern Observatory,Chile, under programs 67.C-0281, 70.D-0164, 72.C-0064, 72.C-0682, and74.D-0109.
| The Large Magellanic Cloud: diffuse interstellar bands, atomic lines and the local environmental conditions
The Large Magellanic Cloud (LMC) offers a unique laboratory to study thediffuse interstellar bands (DIBs) under conditions that are profoundlydifferent from those in the Galaxy. DIB carrier abundances depend onseveral environmental factors, in particular the local UV radiationfield. In this paper we present measurements of twelve DIBs in fivelines of sight to early-type stars in the LMC, including the 30Doradus region. From the high resolution spectra obtained withVLT/UVES we also derive environmental parameters that characterise thelocal interstellar medium (ISM) in the probed LMC clouds. These includethe column density components (including total column density) for theatomic resonance lines of Na I, Ca II, Ti II, K I. In addition, wederive the H I column density from 21 cm line profiles, thetotal-to-selective visual extinction RV and the gas-to-dustratio N(H I)/A_V. Furthermore, from atomic line ratios we derive theionisation balance and relative UV field strength in these environments.We discuss the properties of the LMC ISM in the context of DIB carrierformation. The behaviour of DIBs in the LMC is compared to that of DIBsin different local environmental conditions in the Milky Way. A keyresult is that in most cases the diffuse band strengths are weak (up tofactor 5) with respect to Galactic lines of sight of comparablereddening, EB-V. In the line of sight towards Sk -69223 the 5780 and 5797 Å DIBs are very similar instrength and profile to those observed towards HD144217, which is typical of an environment exposed to a strongUV field. From the velocity analysis we find that DIB carriers (towardsSk -69 243) are better correlated with the ionisedspecies like Ca II than with neutrals (like Na I and CO). The mostsignificant parameter that governs the behaviour of the DIB carrier isthe strength of the UV field.
| FUSE Measurements of Far-Ultraviolet Extinction. II. Magellanic Cloud Sight Lines
We present an extinction analysis of nine reddened/comparison star pairsin the Large and Small Magellanic Clouds (LMC and SMC) based onFar-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date,just two LMC sight lines have probed dust grain composition and sizedistributions in the Magellanic Clouds using spectral data forwavelengths as short as 950 Å. We supplement these two with datafrom four regions distinguished by their IR through UV extinction curvesand grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinctcharacters of extinction in the Clouds and Milky Way, our results aregenerally analogous to those found for Galactic curves-namely, that theFUSE portions of each extinction curve are described reasonably well byFitzpatrick & Massa curves fitted only to longer wavelength data andlack any dramatic new extinction features, and any deviations from theCardelli, Clayton, & Mathis (CCM) formalism continue into FUVwavelengths. A maximum entropy method analysis of all of these curvessuggests that LMCAvg and SMC wing sight lines, whose extinctionparameters more closely resemble those for Galactic paths, require moresilicon and/or carbon in dust than current abundance measurements wouldindicate are available. The requirements for LMC2 and SMC bar sightlines do not fully tax the available reservoirs, in part because largegrains contribute less to the extinction in these directions. Anintermediate product of this extinction analysis is the measurement ofnew H2 abundances in the Magellanic Clouds. Collectivelyconsidering Cloud sight lines that possess significant H2column densities, E(B-V)/N(H I) ratios are reduced by significantfactors relative to the Galactic mean, whereas the correspondingE(B-V)/N(H2) values more closely resemble their Galacticcounterpart. These trends reflect the fact that among these sight linesf(H2) values are lower than those common in the Milky Way forpaths with similar degrees of reddening.Based on observations with the NASA-CNES-CSA Far-UltravioletSpectroscopic Explorer, which is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS-32985.
| Diffuse Interstellar Bands in NGC 1448
We present spectroscopic VLT/UVES observations of two emergingsupernovae, the Type Ia SN 2001el and the Type IISN 2003hn, in the spiral galaxy NGC1448. Our high resolution and high signal-to-noise spectradisplay atomic lines of Ca II, Na I, Ti II and K I in the host galaxy.In the line of sight towards SN 2001el, we alsodetect over a dozen diffuse interstellar bands (DIBs) withinNGC 1448. These DIBs have strengths comparable to lowreddening galactic lines of sight, albeit with some variations. Inparticular, a good match is found with the line of sight towards theσ type diffuse cloud (HD 144217). The DIBstowards SN 2003hn are significantly weaker, and thisline of sight has also lower sodium column density. The DIB centralvelocities show that the DIBs towards SN 2001el areclosely related to the strongest interstellar Ca II and Na I components,indicating that the DIBs are preferentially produced in the same cloud.The ratio of the λ 5797 and λ 5780 DIB strengths (r 0.14) suggests a rather strong UV field in the DIB environment towardsSN 2001el. We also note that the extinction estimatesobtained from the sodium lines using multiple line fitting agree withreddening estimates based on the colors of the Type Ia SN2001el.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO Programmes 67.D-0227 and 71.D-0033).Table \ref{tb:ISfit} and Figs. \ref{fig:IS_MW} and \ref{fig:6284} areonly available in electronic form at http://www.edpsciences.org
| A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves
We present an exhaustive quantitative comparison of all the knownextinction curves in the Small and Large Magellanic Clouds (SMC and LMC)with our understanding of the general behavior of Milky Way extinctioncurves. The RV-dependent CCM relationship of Cardelli,Clayton, and Mathis and the sample of extinction curves used to derivethis relationship are used to describe the general behavior of Milky Wayextinction curves. The ultraviolet portion of the SMC and LMC extinctioncurves are derived from archival IUE data, except for one new SMCextinction curve, which was measured using Hubble Space Telescope SpaceTelescope Imaging Spectrograph observations. The optical extinctioncurves are derived from new (for the SMC) and literature UBVRIphotometry (for the LMC). The near-infrared extinction curves arecalculated mainly from 2MASS photometry supplemented with DENIS and newJHK photometry. For each extinction curve, we giveRV=A(V)/E(B-V) and N(H I) values that probe the same dustcolumn as the extinction curve. We compare the properties of the SMC andLMC extinction curves with the CCM relationship three different ways:each curve by itself, the behavior of extinction at differentwavelengths with RV, and the behavior of the extinction curveFitzpatrick and Massa fit parameters with RV. As has beenfound previously, we find that a small number of LMC extinction curvesare consistent with the CCM relationship, but the majority of the LMCand all the SMC curves do not follow the CCM relationship. For the firsttime, we find that the CCM relationship seems to form a bound on theproperties of all the LMC and SMC extinction curves. This resultstrengthens the picture dust extinction curves exhibit of a continuum ofproperties between those found in the Milky Way and the SMC bar.Tentative evidence based on the behavior of the extinction curves withdust-to-gas ratio suggests that the continuum of dust extinction curvesis possibly caused by the environmental stresses of nearby starformation activity.Partially based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal 8198.
| Detection of Diffuse Interstellar Bands in the Magellanic Clouds
With the Ultraviolet Visual Echelle Spectrograph mounted at the VeryLarge Telescope, we have observed at unprecedented spectral resolutionthe absorption spectrum toward reddened stars in the Magellanic Cloudsover the wavelength range of 3500-10500 Å. This range covers thestrong transitions associated with neutral and charged large carbonmolecules of varying sizes and structures. We report the first detectionof diffuse interstellar bands (DIBs) at 5780 and 5797 Å in theSmall Magellanic Cloud and the variation of the 6284 Å DIB towardseveral targets in the Large Magellanic Cloud. The variation of DIBs inthe Magellanic Clouds compared with Galactic targets may be governed bya combination of the different chemical processes prevailing inlow-metallicity regions and the local environmental conditions. Based onobservations made at the European Southern Observatory, Paranal, Chile(ESO program 67.C-0281).
| The PAH emission spectra of Large Magellanic Cloud H II regions
A set of ISOPHOT spectra from a sample of H Ii regions in the LargeMagellanic Cloud (LMC) is presented. In all the spectra, emission bandsarising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearlypresent. These features are observed to vary considerably in relativestrength to each other from source to source and even within 30 Doradus.The LMC spectra have been compared with ISO-SWS spectra from Galactic HIi regions and with the ISOCAM observation towards a quiescent molecularcloud in the SMC (Reach et al. \cite{Reach}). A correlation is foundbetween the I7.7/I11.2 versusI6.2/I11.2 and theI8.6/I11.2 versus I6.2/I11.2ratios. A segregation between the sources in the different types ofenvironment (Milky Way - LMC - SMC) is present. Furthermore, within theLMC observations, a clear distinction between 30 Doradus and non-30Doradus pointings is found. We discuss the variations in the relativestrength of the PAH features in view of the different physicalenvironments and highlight the relation with the PAH/dust ratio and theextinction curve. We conclude that 1) the same conditions responsiblefor the observed trends in the relative PAH-feature strengths alsoaffect the carrier of the 2175 Å bump leading to the differencesin strength of the latter, and 2) the molecular structure is the majorcause of the observed variations in the relative strength of the PAHfeatures. In the SMC and 30 Doradus compact PAH species dominate, whilePAHs with an open, uneven structure are the dominant ones in Galactic HIi regions and the non-30 Dor LMC sources. Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.
| UBV photometry of Galactic foreground and LMC member stars - III. LMC member stars - a new data base
New UBV photometry for 878 luminous member stars of the Large MagellanicCloud (LMC) and 13 stars of uncertain membership is presented. The datawill be available at Centre de Données astronomiques deStrasbourg. Including former observations now UBV data are available foraltogether 2470 luminous LMC stars and 2106 foreground stars plus 65stars of uncertain membership. The observations have been used alreadyfor several investigations dealing e.g. with interstellar reddeninglines and intrinsic colours, the dust distribution and the calibrationof charge-coupled device exposures.
| Ultraviolet Spectral Classification of O and B Stars in the Large Magellanic Cloud
In this extension of our previous ultraviolet classification study ofSmall Magellanic Cloud stars, we construct an ultraviolet classificationsystem for stars of the Large Magellanic Cloud, a galaxy with ametallicity intermediate to those of the Milky Way and the SMC.Employing the same method as was used for the SMC, we compile a set ofLMC spectra drawn from the International Ultraviolet Explorer archivesand use these spectra to construct a framework of reliable UV spectralclassification criteria. We classify 143 LMC IUE spectra and identifythe LMC standard stars that define our UV classification system.Representative spectra are presented to illustrate the patterns that theUV spectral classification features exhibit. The LMC and SMC UVclassification systems are intended to be applicable to UV spectra ofstars in external galaxies for which no MK standards have been defined.Such UV spectra are expected to be collected from spaceborne instrumentssuch as the Space Telescope Imaging Spectrograph, aboard the HubbleSpace Telescope.
| A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud
We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV)extinction using data from the IUE final archive. Our new analysis takesadvantage of the improved signal-to-noise ratio of the IUE NEWSIPSreduction, the exclusion of stars with very low reddening, the carefulselection of well-matched comparison stars, and an analysis of theeffects of Galactic foreground dust. Differences between the averageextinction curves of the 30 Dor region and the rest of the LMC arereduced compared with previous studies. We find that there is a group ofstars with very weak 2175 Å bumps that lie in or near the regionoccupied by the supergiant shell, LMC 2, on the southeast side of 30Dor. The average extinction curves inside and outside LMC 2 show a verysignificant difference in 2175 Å bump strength, but their far-UVextinctions are similar. While it is unclear whether or not theextinction outside the LMC 2 region can be fitted with the relation ofCardelli, Clayton, & Mathis (CCM), sight lines near LMC 2 cannot befitted with CCM because of their weak 2175 Å bumps. While theextinction properties seen in the LMC lie within the range of propertiesseen in the Galaxy, the correlations of UV extinction properties withenvironment seen in the Galaxy do not appear to hold in the LMC.
| 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.
| Redshifts of high-luminosity stars - The K effect, the Trumpler effect and mass-loss corrections
The Trumpler effect is demonstrated in B and A supergiants in h + ChiPersei, as well as in other associations of young luminous stars. TheK-Trumpler effect is also shown in O, B, and A supergiants in theMagellanic Clouds, as well as in nearby galaxies such as NGC 1569 and2777 and in blue irregular variables in M31 and M33. Mass outflow inluminous stars is shown to require an average correction of about 20km/s and to increase the excess redshifts of the stars in the MagellanicClouds to a significance level of 6 sigma. Completely empirical andindependent measurements show that mass-loss corrections of this sizeare required on average for supergiants in both the SMC and LMC and alsoin the Milky Way.
| A spectroscopic survey of B supergiants in the Large Magellanic Cloud
The results of a low-dispersion digital optical spectral survey of about100 B-type supergiants in the Large Magellanic Cloud are presented. TheMK spectral classification framework for B supergiants has beentransferred to the metal-weak LMC stars, and recommended classificationstandards have been designated. Variations among the metal linestrengths are examined. The most extreme variations are found for thenitrogen lines, for which a range of a factor of 3 or more may be seenin the equivalent widths within some spectral subclasses. It issuggested that these variations indicate a range of nitrogen surfaceabundances among the B supergiants, resulting from contamination of someof the stellar surfaces by processed material from the originalH-burning core.
| Interstellar extinction variations in the Large Magellanic Cloud
The properties of ultraviolet interstellar extinction in the LargeMagellanic Cloud (LMC) are investigated, with particular emphasis on theissue of regional variability. New IUE observations of reddened andstandard stars are combined with existing data from the IUE archives.The author finds essentially identical extinction properties toward 12stars located within about 500 pc projected distance of the core of the30 Dor nebula. The extinction curve toward these stars possesses a weak2175 Å extinction bump and a steep far-ultraviolet rise. Sixreddened stars located outside the 30 Dor region yield extinction curveswhich are significantly weaker in the far-ultraviolet than the 30 Dorcurve. These six curves all have similar far-ultraviolet strengths butshow a range of 2175 Å bump strengths. The results suggest that"typical" LMC extinction curves may not differ from typical Milky Waycurves as much as has been inferred from past studies which weredominated by stars near 30 Dor.
| Interstellar dust in the Large Magellanic Cloud
The results of observations from the IUE satellite and Las Campanas60-cm telescope of the physical characteristics of interstellar dust inthe Large Magellanic Cloud (LMC) are presented. Extinction andpolarization analysis of 12 reddened stars in the LMC showed dustcharacteristics in the optical and infrared regions similar to those ofGalactic dust. The derived wavelength dependences of polarization andpolarization efficiency were found to be comparable to Galactic values.The dust-to-gas ratios of the NH and CNO in the LMC were several timeslower than Galactic values. The relationship between the ultravioletextinction properties and heavy element abundances in the Galaxy, LMC,and the Small Magellanic Cloud (SMC) are discussed.
| Spectrophotometry of early type supergiants in the Large Magellanic Cloud
A sample of LMC supergiants ranging in type from O5 to A1 has beensubjected to equivalent width measurements of major stellar features inthe visible and UV spectral regions, using methods that are entirelynumerical and to the greatest possible degree free from subjectiveestimates. LMC member results are compared with those obtained for theirgalactic counterparts. It is noted that O and early B supergiants of theLMC have smaller equivalent widths for Si IV 1400 and C IV 1550 thangalactic supergiants of similar spectral type, and that middle and lateB supergiant members of the LMC have He I line strengths that are lowerthan those of the corresponding classes in the Galaxy.
| The wavelength dependence of interstellar polarization in the Large Magellanic Cloud
The wavelength dependence of interstellar polarization in the LargeMagellanic Cloud has been studied. It was found that data were fittedwell by the empirical Serkowski relation used for Galactic stars. Therange in the wavelength of maximum polarization is also like that foundin the Galaxy. The grains in both galaxies have similar opticalpolarization properties.
| On the photometric differences between luminous OBA type stars in the LMC with and without P Cygni characteristics
A comparison is presented of the photometric characteristics of O to A0stars of high luminosity in the Large Magellanic Cloud which exhibit ordo not exhibit P Cygni characteristics. VBLUW observations were made of29 OBA stars and 15 OBA stars with P Cygni characteristics (OBA/PC/stars). Examination of the three two-color diagrams in the systemindicates that the OBA(PC) stars are slightly bluer than the OBA stars,which may be explained by a higher luminosity for the OBA(PC) stars at agiven temperature. Computations of the free-free emission from theextended envelopes of OBA(PC) stars show the contribution of suchemission to the spectral energy distribution to be negligible in theabsence of extreme conditions in the shell. Comparison of observed withtheoretical colors reveals the reddening in OBA(PC) stars to be on theaverage 0.07 magnitudes greater than in OBA stars. Results suggest thatOBA supergiants start to exhibit P Cygni characteristics when the massloss rate becomes greater than or equal to 2 x 10 to the -6th solarmass/year.
| Effective temperatures and radii of Large Magellanic Cloud supergiants
The absolute fluxes and flux distributions of 25 Large Magellanic Cloudsupergiants are compared to those of similar galactic stars. Theeffective temperatures and radii of the LMC stars are found bycomparison with models. No significant difference between the LMC andour Galaxy is found.
| Infrared interstellar extinction in the LMC
J(1.25 micron), H(1.65 micron) and K(2.20 microns) magnitudes arepresented for early-type supergiants in the Large Magellanic Cloud, theobservations being obtained with the AAO Infrared Photometer on the3.9-m Anglo-Australian telescope. A value of 3.21 + or - 0.13 for theratio of total to selective extinction is derived for the LMC from thecolor excess ratio. This value is the same as the mean value of theextinction ratio for the Galaxy within the errors of observations. Theemission line stars show infrared excesses which are consistent withemission from electrons in shells around the stars.
| Radial velocities from objective-prism plates in the direction of the Large Magellanic Cloud
A catalog is presented of 711 Large Magellanic Cloud (LMC) stars, withattention to the radial velocities of 418 of these. Also given are theradial velocities of 1127 galactic stars in the direction of the LMC, aswell as discussions of the precision of these measurements and of radialvelocity dispersion in different fields.
| Interstellar extinction in the Large Magellanic Cloud
Interstellar extinction in the ultraviolet as a function of position inthe Large Magellanic Cloud (LMC) is investigated using an enlargedsample of reddened and comparison stars. With the exception ofSK-68-108, the extinction curves of the LMC stars do not showsignificant variation. The mean extinction curve increases towards 2200A, with some maxima near 2200 A and 1900 A. Differences in the meangalactic law include the weak 2200 A feature and a significantly higherLMC extinction in the ultraviolet. The strength of the 1920 A feature isdetermined as a function of luminosity and spectral type, and extinctioncurves are corrected for the mismatch of this feature. Intrinsic colorsof luminous cloud members are also derived. The possibility of localvariations associated with circumstellar clouds is not ruled out due tothe heavily reddened star SK-69-108, and it is suggested that the LMCextinction curves are due to stellar Fe III lines being stronger in somereddened stars than in the comparison stars.
| Circumstellar absorption and intrinsic colours of massive stars
The reality of significant amounts of circumstellar obscuration formassive stars is discussed. At the same time, the question of thepossible incorrectness of currently adopted data for intrinsic colorindices of O stars is examined. The work is based primarily on recentdata for stars in the Large Magellanic Cloud and on new observationalresults for the galactic stellar aggregate surrounding HD 101205. Forthe high-luminosity stars in the Large Magellanic Cloud only marginaleffects of possible circumstellar obscuration may be traced, whereas forthe galactic stellar aggregate no such effects are noted. There is noconvincing overall indication that currently adopted intrinsic colorsfor O stars should be significantly in error.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
INHABITED SKY
