Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
A Radio and Optical Polarization Study of the Magnetic Field in the Small Magellanic Cloud
We present a study of the magnetic field of the Small Magellanic Cloud(SMC), carried out using radio Faraday rotation and optical starlightpolarization data. Consistent negative rotation measures (RMs) acrossthe SMC indicate that the line-of-sight magnetic field is directeduniformly away from us with a strength 0.19+/-0.06 μG. Applying theChandrasekhar-Fermi method to starlight polarization data yields anordered magnetic field in the plane of the sky of strength 1.6+/-0.4μG oriented at a position angle 4deg+/-12deg,measured counterclockwise from the great circle on the sky joining theSMC to the Large Magellanic Cloud (LMC). We construct athree-dimensional magnetic field model of the SMC, under the assumptionthat the RMs and starlight polarization probe the same underlyinglarge-scale field. The vector defining the overall orientation of theSMC magnetic field shows a potential alignment with the vector joiningthe center of the SMC to the center of the LMC, suggesting thepossibility of a ``pan-Magellanic'' magnetic field. A cosmic-ray-drivendynamo is the most viable explanation of the observed field geometry,but has difficulties accounting for the observed unidirectional fieldlines. A study of Faraday rotation through the Magellanic Bridge isneeded to further test the pan-Magellanic field hypothesis.
| An exploratory non-LTE analysis of B-type supergiants in the Small Magellanic Cloud
A preliminary differential non-LTE model atmosphere analysis of moderateresolution (R ~ 5 000) and signal-to-noise ratio spectra of 48 SmallMagellanic Cloud B-type supergiants is presented. Standard techniquesare adopted, viz. plane-parallel geometry and radiative and hydrostaticequilibrium. Spectroscopic atmospheric parameters (T_eff, log g andv_turb), luminosities and chemical abundances (He, C, N, O, Mg and Si)are estimated. These are compared with those deduced for a comparablesample of Galactic supergiants. The SMC targets appear to have similaratmospheric parameters, luminosities and helium abundances to theGalactic sample. Their magnesium and silicon underabundances arecompatible with those found for main sequence SMC objects and there isno evidence for any large variation in their oxygen abundances. Bycontrast both their carbon and nitrogen lines strengths are inconsistentwith single abundances, while their nitrogen to carbon abundance ratiosappear to vary by at least as much and probably more than that found inthe Galactic sample.
| 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.
| 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.
| The anomalous A-type supergiants in the Magellanic Clouds - Evidence for post-red supergiant evolution
Non-LTE model atmospheres are used to show that the strength of thehydrogen lines and the Balmer jump in a group of A-type supergiants inthe Large and Small Magellanic Clouds can best be explained by enhancedhelium in the atmospheres of these supergiants. On the HR diagram thesestars are found between M(Bol) is approximately equal to -6 to -8 mag,corresponding to an initial mass of 10 to 20 solar masses. Given theirlocation on the HR diagram, their spectral and color anomalies which canbe explained by enhanced helium, and the existence of spectroscopicallynormal A supergiants in the same luminosity range, it is suggested thatthese anomalous A-type supergiants are He-burning postred supergiantsand that they are excellent candidates for the predecessor of the kindof star that became SN 1987a.
| 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.
| Studies of massive stars in the Magellanic Clouds. II - New spectral classification of OB stars in the SMC
Abstract image available at:http://adsabs.harvard.edu/abs/1987AJ.....93.1070G
| Radial velocities of southern stars obtained with the photoelectric scanner CORAVEL. VI - 233 F to M type stars in and near the Small Magellanic Cloud - Comparison with 80 spectrographic radial velocities of O to K type stars in this galaxy
Abstract image available at:http://adsabs.harvard.edu/abs/1987A&AS...67..423M
| Studies of luminous stars in nearby galaxies. VIII - The Small Magellanic Cloud
The spectra, colors, and positions on the H-R diagrams of the SmallMagellanic (SMC) supergiants of all spectral types are examined andcompared with corresponding data for supergiant populations in the LMCand Milky Way for clues to the role of chemical composition (i.e., lowmetallicity) on their evolution. A comparison of the observed luminosityfunction shows that while the solar neighborhood and the LMC areessentially the same, the SMC is significantly different, especially atthe upper end. These differences are not due to chemical-compositionvariations. The relative numbers of supergiants of differentluminosities vary most closely with the mass of the galaxy. The blue tored supergiant ratio in the SMC confirms the trends observed in theGalaxy and LMC, and is probably affected by chemical abundancedifferences, especially in the lower luminosity intervals. In addition,it is found that a group of supergiants in the SMC, with spectral typesB8-A5, have anomalous colors and hydrogen lines too strong for theirluminosities.
| Structure and kinematics of the Small Magellanic Cloud as outlined by its brightest stars
Some basic properties of extreme Population I stars in the SmallMagellanic Cloud (SMC) are investigated on the basis of a systematicspectroscopic and photometric survey of the brightest supergiantsbelonging to the SMC. It is found that the main body and Wing of the SMCappear to have essentially the same mean radial velocity but that the K1region has a markedly different velocity. A differential distancemodulus of 0.50 + or - 0.07 is determined, along with an internal meanB-V color excess of 0.06 mag, a radial depth greater than that of theLarge Magellanic Cloud, and a disruption time of the order of 700million years.
| Photoelectric photometry in the Small Magellanic Cloud
Photoelectric photometry in the UBV system for stars in the main bodyand the wing of the Small Magellanic Cloud is presented. The objectsobserved include almost all stars in the main body from the catalog ofAzzopardi and Vigneau (1975) for which no previous photoelectricmeasurements were available as well as those supergiants which should bemembers of the wing according to the spectral survey of Sanduleak(1969). Identifications have been taken from the catalogs of Azzopardiand Vigneau (1975) and Sanduleak (1969), respectively. The mean externalerrors are on the average + or - 0.026 mag in V, + or - 0.016 mag in B-Vand + or - 0.021 mag in U-B.
| Observations of supergiant stars in the Small Magellanic Cloud
Spectrographic and photometric data are presented for 91 supergiantstars belonging to the Small Magellanic Cloud and its Wing. The datainclude MK classes for 52 stars, radial-velocity data for 51 stars, andUBV photometry for 90 stars. In V the limiting magnitude for thespectrographic observations is 13.7 and for the photometricobservations, 13.9. Whenever possible, radial velocities forinterstellar Ca II and forbidden O II have been measured and listed.Notes are given for the individual stars concerning the data obtainedand comparisons with results of previous investigations.
| Etoiles à grande vitesse dans l'aile DU petit nuage de Magellan
Abstract image available at:http://adsabs.harvard.edu/abs/1971A&AS....4..231C
| Polarization measurements of stars in the Magellanic Clouds.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970AJ.....75..778M&db_key=AST
| Proven and probable members in the wing of the Small Magellanic Cloud.
Abstract image available at:http://adsabs.harvard.edu/abs/1969AJ.....74..877S
| Supergiant stars in the wing of the Small Magellanic Cloud
Not Available
|
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: | Hydrus |
| Ascensió Recta: | 01h49m12.59s |
| Declinació: | -74°00'37.0" |
| Magnitud Aparent: | 12.332 |
| Moviment propi RA: | -1 |
| Moviment propi Dec: | 4.8 |
| B-T magnitude: | 12.01 |
| V-T magnitude: | 12.306 |
Catàlegs i designacions:
|
