Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
A UBVR CCD Survey of the Magellanic Clouds
We present photometry and a preliminary interpretation of a UBVR surveyof the Large and Small Magellanic Clouds, which covers 14.5deg2 and 7.2 deg2, respectively. This study isaimed at obtaining well-calibrated data on the brighter, massive stars,complementing recent, deeper CCD surveys. Our catalog contains 179,655LMC and 84,995 SMC stars brighter than V~18.0, and is photometricallycomplete to U~B~V~15.7 and R~15.2, although stars in crowded regions areselectively missed. We compare our photometry to that of others, anddescribe the need for gravity-dependent corrections to our CCD U-bandphotometry. We provide a preliminary cross-reference between our catalogstars and the stars with existing spectroscopy. We discuss thepopulation of stars seen toward the two Clouds, identifying the featuresin the color-magnitude diagram, and using existing spectroscopy to helpconstruct H-R diagrams. We derive improved values for the blue-to-redstar ratios in the two Clouds, emphasizing the uncertainties involved inthis before additional spectroscopy. We compare the relative number ofred supergiants and Wolf-Rayet stars in the LMC and SMC with that ofother galaxies in the Local Group, demonstrating a very strong, tighttrend with metallicity, with the ratio changing by a factor of 160 fromthe SMC to M31. We also reinvestigate the initial mass function of themassive stars found outside of the OB associations. With the newer data,we find that the initial mass function (IMF) slope of this fieldpopulation is very steep, with Γ~-4+/-0.5, in agreement with ourearlier work. This is in sharp contrast to the IMF slope found for themassive stars with OB associations (Γ~-1.3). Although much morespectroscopy is needed to make this result firm, incompleteness can nolonger be invoked as an explanation.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| Spectroscopic binaries in the Large Magellanic Cloud.
Not Available
| HIPPARCOS photometry of 24 variable massive stars (alpha Cygni variables)
The photometric variability of 24 alpha Cyg variables, i.e. variablesuper-and hypergiants, observed by the Hipparcos satellite is described.Three of the selected stars are situated in the SMC, 12 in the LMC andthe remaining 9 in the Galactic plane. Four of them are hot S Dor-typevariables, or LBVs (luminous blue variables) and two are possiblemembers of this class. Light curves are presented for all stars. Forfive stars, among which one LBV, the variability was discovered from theHipparcos photometric data. Results of period searches are presentedand, when relevant, folded light curves are shown. The linear ephemerisof two variables was revised. For most of the program stars theHipparcos magnitude scale (Hp) differs from the V of the UBV system byla 0fm1 . For all variables temperatures and M_bol are given. Galacticforeground reddening for the objects in the Magellanic Clouds are givenbased on IRAS maps.
| 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.
| Massive stars in the field and associations of the magellanic clouds: The upper mass limit, the initial mass function, and a critical test of main-sequence stellar evolutionary theory
We investigate the massive star population of the Magellanic Clouds withan emphasis on the field population, which we define as stars locatedfurther from any OB association than massive stars are likely to travelduring their short lifetimes. The field stars must have been born aspart of more modest star-forming events than those that have populatedthe larger OB associations found throughout the Clouds. We begin byupdating existing catalogs of large magellanic cloud (LMC) and smallmagellanic cloud (SMC) members with our own new spectral types andderive H-R diagrams (HRDs) of 1584 LMC and 512 SMC stars. We use newphotometry and a spectroscopy of selected regions in order to determinethe incompleteness corrections of the catalogs as a function of mass andfind that we can reliably correct the number of stars in our HRDs downto 25 solar mass. Using these data, we derive distance modulli for theClouds via spectroscopic parallax, finding values of 18.4 +/- 0.1 and19.1 +/- 0.3 for the LMC and SMC. The average reddening of the fieldstars is small: E(B-V) = 0.13 (LMC) and 0.09 (SMC), with little spread.We find that the field does produce stars as massively as any found inassociations, with stars as massive as 85 solar mass present in the HRDeven when safeguards against the inclusion of runaway stars areincluded. However, such massive stars are much less likely to beproduced in the field (relative to lower mass stars) than in large OBcomplexes the slope of the IMF of the field stars is very steep, Gamma =-4.1 +/- 0.2 (LMC) and Gamma = -3.7 +/- 0.5 (SMC). We find that the lowmetallicity evolutionary tracks and isochrones do an excellent job ofreproducing the distribution of the stars in the HRD at higher masses,and in particular match the width of the main-sequence well. There mayor may not be an absence of massive stars with ages less than 2 Myr inthe Magellanic Clouds, as others have found for Galactic stars; ourreddening data renders unlikely the suggestion that such an absence (ifreal) would be due to the length of time it takes for a massive star toemerge. There is an increasing discrepancy between the theoretical zeroage main sequence (ZAMS) and the blue edge of the main-sequence at lowerluminosities; this may reflect a metallicity dependence for theintrinsic colors of stars of early B and later beyond that predicted bymodel atmospheres, or it may be that the low metallicity ZAMS ismisplaced to higher temperatures. Finally, we use the relative number offield main-sequence and Wolf-Rayet stars to provide a selection-freedetermination of what mass progenitors become WR stars in the MagellanicClouds. Our data suggest that stars with initial masses less than 30solar mass evolve to a WR phase in the LMC; while the statistics areconsiderably less certain for the SMC, they are consistent with thislimit being modestly higher there, possibly 50 solar mass, inqualitative agreement with modern evolutionary calculations.
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
| Evidence for processed material in the atmospheres of Large Magellanic Cloud B supergiants
We present a high-dispersion study of the optical spectra of 11 LMCB-type supergiants. These stars had been shown previously to have a widerange in nitrogen absorption line strengths, despite having very similartemperatures and luminosities. The current study shows that therelatively rare nitrogen-weak stars also have weaker photospheric heliumlines and stronger photospheric oxygen lines than the morespectroscopically typical supergiants. This pattern suggests that thevast majority of LMC B-type supergiants have had their surfacescontaminated by material from their original hydrogen-burning cores -with a resultant enhancement of surface nitrogen and helium and adepletion in oxygen - while the spectroscopically nitrogen-weak starsare more likely to have retained their main-sequence surface abundances.
| 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.
| 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.
| The properties of OB supergiants in the Large Magellanic Cloud. II - Spectral types and intrinsic colors
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1988ApJ...335..703F&db_key=AST
| An ultraviolet spectral atlas of interstellar lines toward SN 1987A
An UV spectral atlas of the interstellar spectrum toward SN 1987A in theLMC is presented, based on data obtained with the IUE satellite. Theatlas extends from 1250 to 3200 A and includes 200 lines from 12interstellar species. The results are interpreted in the context ofexisting optical and UV studies of interstellar gas toward the LMC.
| Gas in and toward the Magellanic Clouds
Attention is given to the Ca II K absorption spectra of 48 early-typesupergiants in the SMC and LMC, with a view to the Clouds' possiblefragmentary nature and the distance of the gas responsible for theabsorption lines at 60 and 120 km/sec. The comparison of the spectraobtained with H I measurements toward the Clouds indicates that in bothClouds, the velocity components in the neutral hydrogen distribution lieat different distances along the line of sight. Good agreement is foundbetween the radial velocity shown in absorbing material and the radialvelocity spread predicted by Murai and Fujimoto's (1980) dynamical modelof the formation of the Magellanic Stream by tidal stripping. It isspeculated that both the SMC and LMC are extended and fragmented alongthe line of sight.
| Neutral hydrogen in the galactic halo
Observations of 21 cm H I emission-line profiles have been made in 34directions from l = 172° through 0° to l = 97° at highgalactic latitudes and in 59 directions towards the LMC (b ?-33°) with the aim of detecting low-intensity halo features whichmay be compared with other element features of similar radial velocitiesdetected by ultraviolet or optical absorption lines. Data are presentedas spectra and tables. Using the H I column densities as references, theauthors have estimated depletions in the abundances of calcium andsodium in the halo, spiral arms and the LMC disk. In one directiontowards the LMC depletions in the halo features of carbon, nitrogen,oxygen, magnesium, aluminum, silicon and iron (from IUE data) have alsobeen estimated.
| Photoelectric UBV photometry of the Large Magellanic Cloud
Not Available
| 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.
| The Large Magellanic Cloud - The third list of stars that belong to the Large Magellanic Cloud and a list of galactic stars
Abstract image available at:http://adsabs.harvard.edu/abs/1981A&AS...46...13F
| A high-resolution optical survey of interstellar absorption lines toward globular clusters and extragalactic objects. II - Further data
A status report is given on an ongoing survey of narrow-line, visualabsorption components, which has been expanded to include lines of sightto the 3C 273 quasar, the nucleus of M87, and the NGC 1851, NGC 2808,and 47 Tuc globular cluster cores. The number density of components ofthe halo is calculated to lie between one-third and one per 100 kpc,though with substantial uncertainty. This estimate is held to beconsistent with the galactic halo interpretation of the origin of highcolumn density, quasar absorption-line systems.
| The velocity structure of gas in the lines of sight to the Magellanic Clouds
An attempt is made to form a coherent picture of the velocity structureof gas along the line of sight constituted by the Small and LargeMagellanic Clouds and foreground objects, by means of a survey of the CaK and Na D visual absorption lines and existing UV and visualabsorption-line data. It is confirmed that large sheets of interveningmaterial at intermediate velocities are present, but ambiguities remainin the interpretation of this gas as a hot but cooling galactic halowhose properties are closely constrained. The line of sight to theMagellanic Clouds may contain material at intermediate velocitiescorresponding to gas perturbed by the gravitational field of the galaxy,but marginally bound to the Magellanic Clouds. Two lines of sight,toward the 30 Doradus region and toward the SNR N206, are singled outfor special discussion because they show substantial velocity structure.
| 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.
| IUE ultraviolet spectroscopy of stars in the Magellanic Clouds
Low dispersion IUE spectra of seven hot supergiants in the LargeMagellanic Cloud and one in the Small Magellanic Cloud are presented.Stellar winds are present in all the stars, but the resonance lines haveweak or no emission and probably have lower terminal velocities thangalactic counterparts. Spectral energy distributions are used to deriveeffective temperature, luminosities, and interstellar extinction values.These are in reasonable agreement with ground-based estimates. Somepeculiarities are noted and compared with galactic stars. Line featuresin the spectra are discussed, including several unusual emission lines.The peculiar star R99 appears to be very luminous (bolometric magnitudeof about -11.4) and reddened.
| Stellar winds from hot stars in the Magellanic Clouds
Spectrographic data have been obtained of O and B supergiants in theMagellanic Clouds at dispersions 12 and 25 A/mm in the blue and 25 and50 A/mm in the red. Mass loss phenomena, line radial velocities, andline intensities have been measured on them and are compared withsimilar data on galactic stars. There appear to be some differencesbetween the SMC, LMC, and the Galaxy in all these respects. Mass loss isclearly occurring in the Magellanic Cloud stars, and the SMC stars haveparticularly discrepant line intensities.
| The interstellar lambda 4428 feature in the Large Magellanic Cloud
A search has been made for this interstellar absorption band in fourB-type supergiant members of the LMC. Upper limits are derived whichagree with predictions from the well-established galactic 4428-A/E(B-V)correlation. A previous report of anomalously strong 4428 A in thisdirection is shown to be in error, the overestimation probably beingcaused by stellar line interference.
| Rotation et masse DU grand nuage de Magellan.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973A&A....28..165P&db_key=AST
| Spectrographic and photometric observations of supergiants and foreground stars in the direction of the Large Magellanic Cloud
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&AS....6..249A&db_key=AST
| Nova Mensae 1970b in the Large Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/abs/1972A&A....16..404H
| BVRI-Photometry of the Brightest Stars in the Magellanic Clouds
We have made photometric observations in the BVRI system of 100 objectsthat belong to the Magellanic Clouds. The observational data indicate atleast two remarkable differences between some of the Magellanic Cloudobjects and the supergiant stars in the Galaxy, namely, the former aremore luminous than the most luminous galactic supergiants and they alsohave a color excess in V-R unexplained by interstellar extinction alone.Two possible explanations are likely for the additional color excess;either the presence of a circumstellar dust envelope or a differentchemical atmospheric composition from galactic supergiant stars, orperhaps both of them.
| A deep objective-prism survey for Large Magellanic Cloud members
Not Available
| Equivalent widths of Hγ in stellar spectra of the Magellanic Clouds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1966MNRAS.132..433H&db_key=AST
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Mensa |
| Ascensió Recta: | 05h30m47.79s |
| Declinació: | -71°04'02.3" |
| Magnitud Aparent: | 11.359 |
| Moviment propi RA: | 1.4 |
| Moviment propi Dec: | 0.1 |
| B-T magnitude: | 11.187 |
| V-T magnitude: | 11.345 |
Catàlegs i designacions:
|
