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Keplerian discs around post-AGB stars: a common phenomenon?
Aims.We aim at showing that the broad-band SED characteristics of oursample of post-AGB stars are best interpreted, assuming thecircumstellar dust is stored in Keplerian rotating passivediscs.Methods.We present a homogeneous and systematic study of theSpectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects.The selection criteria to define the whole sample were tuned to coverthe broad-band characteristics of known binary post-AGB stars. The wholesample includes 20 dusty RV Tauri stars from the General Catalogue ofVariable Stars (GCVS). We supplemented our own Geneva optical photometrywith literature data to cover a broad range of fluxes from the UV to thefar-IR.Results.All the SEDs display very similar characteristics: alarge IR excess with a dust excess starting near the sublimationtemperature, irrespective of the effective temperature of the centralstar. Moreover, when available, the long wavelength fluxes show ablack-body slope indicative of the presence of a component of large mmsized grains.Conclusions.We argue that in all systems, gravitationallybound dusty discs are present. The discs must be puffed-up to cover alarge opening angle for the central star and we argue that the discshave some similarity with the passive discs detected around youngstellar objects. We interpret the presence of a disc to be a signaturefor binarity of the central object, but this will need confirmation bylong-term monitoring of the radial velocities. We argue that dusty RVTauri stars are those binaries which happen to be in the Population IIinstability strip.
| The Open Cluster Ruprecht 91 and Its Cepheids
Photoelectric UBV photometry and star counts are presented for stars inthe previously unstudied open cluster Ruprecht 91, supplemented byobservations for stars in adjacent regions surrounding the Cepheids SXCar and VY Car, including new observations for the latter. Ruprecht 91is typical of groups associated with Cepheids, with an evolutionary ageof ~8×107 yr, but it is only 980+/-8 pc distant, muchcloser than the Cepheids. Both Cepheids are unlikely to be members ofRuprecht 91: VY Car on the basis of location beyond the cluster tidalradius, age, and implied distance, and SX Car on the basis of implieddistance. The brightest star in Ruprecht 91 is the M1 II supergiant HD93662, a likely member. The suspected variability of the star isconfirmed here with the aid of archival data, and its likely cyclelength is found to be 5756 days (15.8 yr). Period changes in VY Car andSX Car are also studied with the aid of archival data. The period of VYCar is decreasing, its rate of -75.92+/-0.37 s yr-1 beingconsistent with a second crossing of the instability strip. That for SXCar is increasing, its rate of +0.07+/-0.02 s yr-1 beingconsistent with a third crossing. VY Car and SX Car are established tohave space reddenings of EB-V=0.18+/-0.01 and 0.17+/-0.01,respectively.
| Dynamical mass estimates for two luminous young stellar clusters in Messier 83
Using new data from the UVES spectrograph on the ESO Very LargeTelescope and archive images from the Hubble Space Telescope, we havemeasured projected velocity dispersions and structural parameters fortwo bright young star clusters in the nearby spiral galaxy NGC5236. One cluster is located near the nuclear starburst of NGC5236, at a projected distance of 440 pc from the centre, while the otheris located in the disk of the galaxy at a projected galactocentricdistance of 2.3 kpc. We estimate virial masses for the two clusters of(4.2±0.7)×105 Mȯ and(5.2±0.8)×105 Mȯ and ages (frombroad-band photometry) of 107.1±0.2 years and108.0±0.1 years, respectively. Comparing the observedmass-to-light (M/L) ratios with simple stellar population models, wefind that the data for both clusters are consistent with a Kroupa-typestellar mass function (MF). In particular, we rule out any MF with asignificantly lower M/L ratio than the Kroupa MF, such as aSalpeter-like MF truncated at a mass of 1 Mȯ or higher.These clusters provide a good illustration of the fact that massive,globular cluster-like objects (``super star clusters'') can form at thepresent epoch even in the disks of seemingly normal, undisturbed spiralgalaxies.Based on observations collected at the European Southern Observatory,Chile under programme 71.B-0303A, and on observations obtained with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable 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/qcat?J/A+A/397/997
| Infrared carbon stars: new identifications and their space distribution in the Galaxy
We present JHKL photometry of a sample of 150 IRAS stars in the thirdand fourth galactic quadrant with | b | <2(deg) selected according totheir IRAS colour (0.160<=[12-25]<=1.156). We identify 27 carbonstar candidates using the [12-25] vs. K-L two-colour diagram method.Among them, 10 are listed in the Stephenson's catalogue of carbon starsand 17 are new infrared carbon stars (IRCS) candidates. Their distancesare found to be larger than 3 kpc. These data are combined with previousdata to study the space distribution of IRCS. The number density seemsto be independent of the galactocentric distance (R) toward the GalacticCenter and to decrease exponentially toward the anticenter. At thepresent stage, it is not possible to disentangle effects such asincompleteness of the sample, different space distribution law anddependence of the luminosity on metallicity. In particular, we suggestthat the apparent scarcity of IRCS in the central direction of theGalaxy (R<5 kpc) might be due to a selection effect. From theavailable data, we have found no IRCS within 1 kpc of the GalacticCenter. Based on observations obtained at the European SouthernObservatory, Chile
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| A multifrequency study of circumstellar envelopes of cool giants and supergiants
A multifrequency study of all parts of circumstellar envelopes of coolgiants and supergiants is presented. In order to investigate theinfluence of stellar parameters on spectral features of these envelopes,a sample of 77 cool giants and supergiants that occupies a horizontalstrip in the Hertzsprung-Russell diagram was observed. Spectroscopic andphotometric observations at optical, infrared and radio wavelengths ledto the following results: (1) for giants there exists a strongcorrelation between H-alpha emission and SiO masers, which led to thesuggestion that SiO masers are triggered by shock-waves and can bepumped by collisions, (2) the SiO expansion velocity was found to besystematically lower by 2 km/s compared to the CO expansion velocity,and (3) a relation between the asymmetry of the light curve and theintensity of the dust emission at 9.7 microns has been confirmed for awide range of periods. Rather than luminosity alone, pulsationalproperties of the variable play a dominant role for the structure ofcircumstellar envelopes of cool giants and supergiants.
| On the contribution of interstellar extinction to the 10 micron dust feature in OH/IR stars
The IRAS Low Resolution Spectra of 467 sources with the 10 micron dustfeature are analyzed. The strengths of the dust feature are determinedby the ratio of the flux at 9.7 micron to the fitted continuum level.Color temperatures are derived from the fluxes of the four IRASphotometric bands after correcting for the effect of the 10 micronfeature on the fluxes of the 12 micron band. A definite correlationbetween the strength of the feature and the color temperature of thecontinuum is found, implying that the 10 micron dust feature is largelycircumstellar in origin. A reexamination of the strength of the silicatefeature for seven of the OH/IR star used by Gehrs and colleagues in 1985has failed to reproduce the optical depth-distance relationship found bythese authors. It is concluded that interstellar extinction does notplay a major role in the formation of the 10 micron absorption feature.
| IRAS catalogues and atlases - Atlas of low-resolution spectra
Plots of all 5425 spectra in the IRAS catalogue of low-resolutionspectra are presented. The catalogue contains the average spectra ofmost IRAS poiont sources with 12 micron flux densities above 10 Jy.
| A clump of M stars in Velorum-Carina
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974PASP...86..429E&db_key=AST
| Spectroscopic and Photometric Observations of M Supergiants in Carina.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972ApJ...172...75H&db_key=AST
| Narrow-Band and Broad-Band Photometry of Red Stars. III. Southern Giants
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970ApJ...161..199E&db_key=AST
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| Takýmyýldýz: | Karina |
| Sað Açýklýk: | 10h47m38.70s |
| Yükselim: | -57°28'04.0" |
| Görünürdeki Parlaklýk: | 6.36 |
| Uzaklýk: | 847.458 parsek |
| özdevim Sað Açýklýk: | -11.8 |
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| B-T magnitude: | 8.524 |
| V-T magnitude: | 6.463 |
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