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Spitzer Far-Infrared Detections of Cold Circumstellar Disks
Observations at 70 μm with the Spitzer Space Telescope have detectedseveral stellar systems within 65 pc of the Sun. Of 18 presumably youngsystems detected in this study, as many as 15 have 70 μm emission inexcess of that expected from their stellar photospheres. Five of thesystems with excesses are members of the Tucanae association. The 70μm excesses range from a factor of ~2 to nearly 30 times the expectedphotospheric emission from these stars. In contrast to the 70 μmproperties of these systems, there is evidence for an emission excess at24 μm for only HD 3003, confirming previous results for this star.The lack of a strong 24 μm excess in most of these systems suggeststhat the circumstellar dust producing the IR excesses is relatively cool(Tdust<~150 K) and that there is little IR-emittingmaterial within the inner few AU of the primary stars. Many of thesesystems lie close enough to Earth that the distribution of the dustproducing the IR excesses might be imaged in scattered light at opticaland near-IR wavelengths.
| An empirical temperature calibration for the Δ a photometric system . I. The B-type stars
We establish an empirical effective temperature calibration of mainsequence, luminosity class V to III B-type stars for the Δ aphotometric system which was originally developed to detect magneticchemically peculiar objects of the upper main sequence (early B-type toearly F-type) at 5200 Å. However, this system provides the index(g_1-y) which shows an excellent correlation with (B-V) as well as (b-y)and can be used as an indicator of the effective temperature. This issupplemented by a very accurate color-magnitude diagram, y or V versus(g_1-y), which can be used, for example, to determine the reddening,distance and age of an open cluster. This makes the Δ aphotometric system an excellent tool to investigate theHertzsprung-Russell-Diagram (HRD) in more detail. Using thereddening-free parameters and already established calibrations withinthe Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, apolynomial fit of third degree for the averaged effective temperaturesto the individual (g_1-y)0 values was derived. For thispurpose, data from the literature as well as new observations were takenresulting in 225 suitable bright normal B-type objects. The statisticalmean of the error for this sample is 238 K which is sufficient toinvestigate the HRD of distant galactic open clusters as well asextragalactic aggregates in the future.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| An infrared imaging search for low-mass companions to members of the young nearby β Pic and Tucana/Horologium associations
We present deep high dynamic range infrared images of young nearby starsin the Tucana/Horologium and β Pic associations, all 10 to 35Myrs young and at 10 to 60 pc distance. Such young nearby starsare well-suited for direct imaging searches for brown dwarf and evenplanetary companions, because young sub-stellar objects are stillself-luminous due to contraction and accretion. We performed ourobservations at the ESO 3.5m NTT with the normal infrared imagingdetector SofI and the MPE speckle camera Sharp-I. Three arc sec north ofGSC 8047-0232 in Horologium a promising brown dwarf companion candidateis detected, which needs to be confirmed by proper motion and/orspectroscopy. Several other faint companion candidates are alreadyrejected by second epoch imaging. Among 21 stars observed inTucana/Horologium, there are not more than one to five brown dwarfcompanions outside of 75 AU (1.5'' at 50 pc); most certainly only <=5% of the Tuc/HorA stars have brown dwarf companions (13 to 78 Jupitermasses) outside of 75 AU. For the first time, we can report an upperlimit for the frequency of massive planets ( 10 Mjup) atwide separations ( 100 AU) using a meaningfull and homogeneoussample: Of 11 stars observed sufficiently deep in β Pic (12 Myrs),not more than one has a massive planet outside of 100 AU, i.e.massive planets at large separations are rare (<= 9%).Based on observations obtained on La Silla, Chile, in ESO programs65.L-0144(B), 66.D-0135, 66.C-0310(A), 67.C-0209(B), 67.C-0213(A),68.C-0008(A), and 68.C-0009(A)} }
| Tucana Association
Among star clusters, only the sparse Ursa Major nucleus is closer toEarth than the recently identified Tucana association. Based on newphotometric VRI magnitudes, we construct a color-magnitude diagramcomposed of likely and possible Tucana members. The implied age of theTucana association stars, <~40 Myr, is consistent with the agedetermined from a previous analysis by Stelzer & Neuhäuser ofthe X-ray luminosities of plausible association members. Based primarilyon space motions and X-ray fluxes, we identify potential new members ofthe Tucana association not considered in earlier studies. Torres andcoworkers recently identified a group of post-T Tauri stars, theHorologium association, which is located near the Tucana association inright ascension and declination. Because the Horologium stars have thesame space motions, age, distance from Earth, volume density, and rangeof spectral types as Tucana stars, we suggest that, rather than beingcharacterized as a separate group, it would be appropriate andeconomical to subsume the Horologium association stars into the Tucanastream.
| CCD Speckle Observations of Binary Stars from the Southern Hemisphere. III. Differential Photometry
Two hundred seventy-two magnitude difference measures of 135 double starsystems are presented. The results are derived from speckle observationsusing the Bessel V and R passbands and a fast readout CCD camera.Observations were taken at two 60 cm telescopes, namely the Helen SawyerHogg Telescope, formerly at Las Campanas, Chile, and the Lowell-TololoTelescope at the Cerro Tololo Inter-American Observatory, Chile. Thedata analysis method is presented and, in comparing the results to thoseof Hipparcos as well as to recent results using adaptive optics, we findvery good agreement. Overall, the measurement precision appears to bedependent on seeing and other factors but is generally in the range of0.10-0.15 mag for single observations under favorable observingconditions. In four cases, multiple observations in both V and R allowedfor the derivation of component V-R colors with uncertainties of 0.11mag or less. Spectral types are assigned and preliminary effectivetemperatures are estimated in these cases.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Identification of a Nearby Stellar Association in theHipparcos Catalog: Implications for Recent, Local Star Formation
The TW Hydrae Association (~55 pc from Earth) is the nearest knownregion of recent star formation. Based primarily on the Hipparcoscatalog, we have identified a group of nine or 10 comoving star systemsat a common distance (~45 pc) from Earth that appear to compriseanother, somewhat older association (``the Tucanae Association'').Together with ages and motions recently determined for some nearby fieldstars, the existence of the Tucanae and TW Hydrae Associations suggeststhat the Sun is now close to a region that was the site of substantialstar formation only 10-50 Myr ago. The TW Hydrae Association representsa final chapter in the local star formation history.
| CCD Speckle Observations of Binary Stars from the Southern Hemisphere. II. Measures from the Lowell-Tololo Telescope during 1999
Speckle observations of 145 double stars and suspected double stars arepresented and discussed. On the basis of multiple observations, a totalof 280 position angle and separation measures are determined, as well as23 high-quality nondetections. All observations were taken with the(unintensified) Rochester Institute of Technology fast-readout CCDcamera mounted on the Lowell-Tololo 61 cm telescope at the Cerro TololoInter-American Observatory during 1999 October. We find that themeasures, when judged as a whole against ephemeris positions of binarieswith very well-known orbits, have root mean square deviations of1.8d+/-0.3d in position angle and 13+/-2 mas in separation. Elevendouble stars discovered by Hipparcos were also successfully observed,and the change in position angle and/or separation since the Hipparcosobservations was substantial in three cases.
| X-ray emission from young stars in the Tucanae association
We report on X-ray emission from members of the recently discoveredTucanae association, a group of stars with youth signatures and similarspace motion. The Tucanae association is the nearest known region ofrecent star formation ( ~ 45 pc) far from molecular clouds(\cite{Zuckerman00.1}). We have made use of the ROSAT Data Archive andsearched for X-rays from Tucanae stars in both ROSAT All-Sky Survey(RASS) and pointed observations. While the RASS provides completecoverage of the sky, only three potential Tucanae members have beenobserved during PSPC pointings. All three stars have been detected. Forthe RASS the percentage of detections is 59%. The comparison of theX-ray luminosity function of Tucanae to that of other star formingregions may provide clues to the uncertain age of the association. Wefind that the distribution of X-ray luminosities is very similar to theones derived for the TW Hya association, the Taurus-Auriga T TauriStars, and the IC 2602 cluster, but significantly brighter than theluminosity distribution of the Pleiades. We conclude that the stars inTucanae are most likely young, on the order of 10-30 Myr. Strongvariability of most stars emerges from the X-ray lightcurves whereseveral flares and irregular variations are observed.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Visible neutral helium lines in main sequence B-type stars: observations and NLTE calculations
Spectra in the visible range 410 - 710 nm have been obtained for asample of main sequence B-type stars to determine the behavior ofneutral helium lines with effective temperature and gravity. Equivalentwidths have been compared with new calculations which combine thecapabilities of LTE atmospheric models of including millions of linescontributing to the opacity and accurate NLTE treatment of the lineformation. The latter take advantage of accurate atomic cross-sections.We find a satisfactory agreement between theory and observations withdifferences probably due to the microturbulence. We also investigate theeffects of blending and find that the Hei412.1nm is severely blendedwith oxygen lines for spectral types earlier than B3.
| MSC - a catalogue of physical multiple stars
The MSC catalogue contains data on 612 physical multiple stars ofmultiplicity 3 to 7 which are hierarchical with few exceptions. Orbitalperiods, angular separations and mass ratios are estimated for eachsub-system. Orbital elements are given when available. The catalogue canbe accessed through CDS (Strasbourg). Half of the systems are within 100pc from the Sun. The comparison of the periods of close and widesub-systems reveals that there is no preferred period ratio and allpossible combinations of periods are found. The distribution of thelogarithms of short periods is bimodal, probably due to observationalselection. In 82\% of triple stars the close sub-system is related tothe primary of a wide pair. However, the analysis of mass ratiodistribution gives some support to the idea that component masses areindependently selected from the Salpeter mass function. Orbits of wideand close sub-systems are not always coplanar, although thecorresponding orbital angular momentum vectors do show a weak tendencyof alignment. Some observational programs based on the MSC aresuggested. Tables 2 and 3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Magnesium abundance in main sequence B-type and magnetic chemically peculiar stars.
The abundance of magnesium for a sample of 19 main sequence B-type and41 magnetic chemically peculiar stars has been derived by spectrumsynthesis analysis of the MgII448.1nm line under the LTE assumption. Thelogarithm of the average Mg abundance for the main sequence stars is log(N(MgII)/N(Tot))=-4.28+/-0.19. Comparing magnetic chemically peculiar(Cp) and main sequence stars with equal effective temperature andgravity, one finds that the magnesium abundance tends to be lower inpeculiar stars with the exception of helium rich stars where thiselement can be overabundant. In Cp stars with effective temperature ofabout 14000K, the magnesium abundance does not depend on gravity,microturbulent velocity or rotational period. There appears to exist acorrelation between the magnesium abundance and the surface magneticfield, with the stars poorest in magnesium presenting the strongestmagnetic fields. In accord with the theory of magnetically controlleddiffusion - which predicts a non-homogeneous distribution of magnesiumover the stellar surface and stratification in the photosphere - somepeculiar stars show evidence of spectral variability with the rotationalphase; the respective magnesium abundances of the HgMn stars HD49606 andHD78316 depend on optical depth.
| Transformations from Theoretical Hertzsprung-Russell Diagrams to Color-Magnitude Diagrams: Effective Temperatures, B-V Colors, and Bolometric Corrections
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...469..355F&db_key=AST
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| The late B-type stars: Refined MK classification, confrontation with stromgren photometry, and the effects of rotation
In the fourth and final of a series of papers on the late B to the earlyF type stars, we refine the Morgan Keenan (MK) spectral classificationsystem for the late B type stars and consider the effect of rotation onboth spectral classification and Stroemgren photometry of these stars.We extend the work of Morgan by establishing self-consistent sequencesof narrow and broadlined standards. We reclassify a number of Bp stars,compare these classifications with Stroemgren photometry and considerthe question of whether all Bp stars are main-sequence objects.
| Effective temperature and gravity from c(0) and beta indices for B-type stars
A sample of nonsupergiant B-type stars of solar chemical composition hasbeen analyzed for T(eff) and gravity differences due to the use of c(0)and beta indices from different photometric grids. The Moon andDworetsky (1985) grid, as well as an extension of the grid, are found toyield T(eff)s closer to those derived with other methods than the Lesteret al. (1986) grid; in addition, the former grid yields gravities thatare closer to values in the literature than the latter grid. A modifiedversion of the TEFFLOGG code of Moon (1985), which employs polynomialfunctions of the Stromgren indices, yields both T(eff) and gravity forthe present sample of B-type stars.
| Spectroscopic calibration of the magnitudes of B-stars of the Galactic disk
The present work is based on a catalog of the equivalent widths ofhydrogen and neutral helium lines in the spectra of 500 B-stars. Thecalibration relationship of absolute magnitude versus equivalent widthsis derived from spectral observations of B and A stars in open clusters.The zero point of this dependence is determined from the trigonometricparallaxes of the nearest A and B stars. By comparing values ofW(H-gamma) and the beta-indices of the Stromgren system, an M(V)(beta)calibration is obtained which differs substantially from the Crawfordcalibration.
| Accurate absolute luminosities, effective temperatures, radii, masses and surface gravities for a selected sample of field stars
Reliable estimates are presented of stellar effective temperatures,radii, and luminosities for individual field stars obtained fromspectrophotometry and trigonometric parallaxes. The average uncertaintyaffecting these quantities is on the order of 2 percent, 16 percent, and35 percent, respectively. Reasonably accurate mass and surface gravityvalues derived by assuming the soundness of predictions made by stellarevolution theories are given.
| Catalogue of i and w/w crit values for rotating early type stars
Not Available
| Effects of stellar rotation on the Geneva photometric system
The effects of stellar rotation on colors and parameters of the Genevaphotometric system are considered, using homogeneous material. Attentionis focused on these parameters useful for deriving physical propertiesof B- and A-type stars. Two major photometric planes in this respect,the (X, Y) plane and the (d, Delta) plane, are not discernibly affectedby rotation. The temperature parameter, B(2) - V(1) is reddened byrotation to an extent that is in agreement with model calculations foruniformly rotating stars.
| Spectroscopy of hot horizontal-branch stars in globular clusters. I - NGC 6397
Spectra have been obtained, with moderate signal-to-noise ratio, forfour, blue horizontal-branch (BHB) stars in the metal-poor globularcluster NGC 6397 using a photon-counting detector. These photometricallyselected stars have effective temperatures near 12,000 K. The observedH-delta profiles suggest values of log g near 2.8, putting these starsabove the main sequence of population I. At an apparent signal-to-noiseratio of 25:1 between 3900 A and 4200 A, the only identifiableabsorption lines other than the Balmer lines are weak He I 4026 A and CaII 3933 A (presumably interstellar). The scatter in the observedstrengths of He I 4026 A for several field stars makes it uncertainwhether or not He I is underabundant in these cluster BHB stars. Anexamination of the cores of the H-delta profiles suggests that thecluster HB stars are probably slow rotators.
| Observed and computed spectral flux distribution of non-supergiant O9-G8 stars. III - Determination of T(eff) for the stars in the Breger Catalogue
The effective temperatures and angular diameters of nonsupergiant O9-G8stars are determined from visible spectrophotometry. The results, whichrefer to 302 stars included in the Breger Catalogue, are derived fromthe comparison between the observed flux distributions and thepredictions of Kurucz's models (1979). The uncertainties to be expectedin individual results are discussed; their sizes are of the order of 5percent in effective temperature and 10 percent in angular diameter.
| Four-colour photometry of B stars north of B = + 45 deg and comparison with the south
Four-color photometry of 33 Henry Draper B stars north of b = + 45 degis presented. Most are little-reddened B or intermediate-A stars. A newAm star is discovered. The new measures essentially complete uvbyphotometry of all HD B stars within 45 deg of either galactic pole. Thenorthern and southern cones of HD B stars are compared, and selectioneffects deduced. Far from the galactic plane, it appears that B starsmay be equally distributed north and south of the plane; closer to thesun, an asymmetry associated with Gould's Belt is evident.
| Equivalent widths of spectral lines in B stars
Previously published equivalent widths of 26 strong spectral linesmeasured at various reciprocal dispersions, ranging from 1 to 75 A/mm,in 172 B stars and 14 A stars are collected and analyzed. The linesconsidered include H-alpha and H-beta; six of He I and one of He II; oneof C III; two of N II and one of N III; two of O II; one of Mg II; twoeach of Si II, Si III, and Si IV; one of Ca II; and three of Fe II. Acomparison of the measurements made at the different reciprocaldispersions reveals that the equivalent width of a given line increasessystematically with decreasing reciprocal dispersion. Only measurementsmade at a reciprocal dispersion of not more than 40 A/mm are used toplot average curves showing equivalent width as a function of spectraltype and luminosity class for all 26 lines. In all but two cases (He Iat 4121 A and Ca II at 4267 A), the behavior of the lines as a functionof temperature and gravity is found to be in accordance withmodel-atmosphere theory.
| Uvby-beta photometry of visual double stars - Absolute magnitudes of intrinsically bright stars
Individual absolute visual magnitudes M(v) are derived for intrinsicallybright stars and evolved stars. The results are collected for 106objects believed to be members of binary systems. uvby-beta photometrywas empirically calibrated in terms of M(v) for main sequence stars andphotoelectrically determined apparent magnitudes. The derived M(v)values are not significantly different from those stated in the Wilsoncatalogue (1976). Binary systems with main sequence primaries andsecondary components off the main sequence were also investigated.Several systems in which at least one component may be in the pre-mainsequence contraction stage are pointed out. A wide variety of commentsand derived data are given individually for 136 double stars, includingmetallicities, distance moduli, and masses.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Tukán |
| Rektaszcenzió: | 00h31m32.70s |
| Deklináció: | -62°57'29.0" |
| Vizuális fényesség: | 4.37 |
| Távolság: | 42.827 parszek |
| RA sajátmozgás: | 83.9 |
| Dec sajátmozgás: | -56.4 |
| B-T magnitude: | 4.242 |
| V-T magnitude: | 4.282 |
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