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Chemical composition and evolutionary status of nine UV-bright stars in five globular clusters from VLT/UVES spectra
We have derived the chemical composition of nine UV-bright starsbelonging to five Galactic globular clusters of various metallicities([Fe/H] from -1.0 to -2.4 dex). The analyses are based on highresolution spectra obtained with the UV-Visual Echelle Spectrograph(UVES) at VLT-UT2. The evolutionary status of the stars is assessed fromthe chemical analysis and location in the H-R diagram. The star ID7 inNGC 5986 is confirmed as a bona fide post-asymptotic giant branch star(post-AGB) whereas the high-luminosity star ID6 has probably left theAGB before the third dredge-up. ZNG 1 in NGC 6712 shows an overabundanceof sodium, oxygen, and silicon similar to overabundances we find in theUV-bright star ID6 in NGC 5986; both stars could be in a post-early-AGB(PEAGB) phase of evolution. The UV-bright star ZNG 7 in NGC 6218 seemsto be an AGB star. The stars V-4 and ZNG 5 in NGC 6656 are in apost-horizontal-branch phase of evolution, with V-4 being significantlyoverabundant in heavy elements. The origin of these overabundances isdiscussed in the context of the evolutionary versus primordial scenario.The three UV-bright stars K 260, K 996 and K 1082 observed in the verymetal-deficient globular cluster NGC 7078 are post-horizontal-branchstars, one of them being slightly enriched in s-elements but with aluminosity too low for third dredge-up to have occured. The abundancepatterns of K 1082 in NGC 7078 seem to indicate the presence of milddiffusion and a radiative levitation process, already reported in theblue HB stars of M 13 (Behr et al. \cite{Behr1999}, ApJ, 517, L135) andNGC 6752 (Moehler et al. \cite{Moehler1999}, A&A, 339, 537).Based on data collected at Paranal Observatory (ESO, Chile), programidentifier ID 69.D-0081.Full Table \ref{abund} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/353Appendix is only available in electronic form athttp://www.edpsciences.org
| A proper motion study of the globular cluster ω Centauri
A proper motion study is presented of the globular cluster ωCentauri. Based on 100 plates obtained with the Yale-Columbia 66 cm (26inch) refractor, and ranging in epoch from 1931 till 1935 and 1978 till1983, differential proper motions are obtained for 9847 stars. Thelimiting magnitude (photographic) is 16.0 for the centre of the cluster,16.5 for the outer parts. The optical system of the Yale-Columbia 26inch refractor (before and after its relocation from South Africa toAustralia) is reconstructed. The reconstruction model is analyzed forpossible systematic effects that changes in the objective may haveintroduced in the proper motions. The precisions of the proper motionsrange from an average of 0.1 milli arcsecond (mas) per year for thebrightest to an average of 0.65 mas yr-1 for the fainteststars. These precisions are high enough to enable the study of internalkinematics of the cluster. 9256 stars, situated within 29.'5 (0.6rt) from the cluster centre and with colour information, areused in cluster- membership determinations, identifying 7853 probablecluster members. Of the remaining 338 stars within 29.'5, those withoutcolour information, an estimated 243 are cluster members. The surfacedensity profile as obtained from integrated membership probabilities,and corrected for losses due to crowding, shows an increase continuingtill much closer to the cluster centre than shown in earlier studies. Aninternal proper motion dispersion of 1.0 to 1.2 mas yr-1(equivalent to 25 to 29 km s-1 for a distance of 5.1 kpc) isfound for members near the cluster centre, decreasing to 0.3 masyr-1 (7.5 km s-1 at 5.1 kpc) in the outer regions.There is an indication of anisotropy in the proper motion dispersionsover the intermediate part of the cluster halo, where the dispersions inthe radial direction are systematically higher than in the transversedirection. The proper motions also show a pronounced differentialrotation, which is not reflected in tests performed on the reconstructedtelescope model, and which looks similar to the rotation noticed inradial velocity studies of the cluster. A comparison between velocitydispersions obtained in radial velocity studies with the present resultsindicates a distance for the cluster of 4.5 kpc, compared to theexpected distance of 5.1 kpc. This result should, however, beinterpreted with caution, given a range of possible complications thatmay have affected the dispersion determinations for both the propermotions and the radial velocities, and the possibility that the velocitydistribution is intrinsically anisotropic. The absolute proper motion ofthe cluster is derived through linking the differential proper motionsand positions with the ICRF as defined by the Hipparcos catalogue andextended to a denser grid of stars by the Tycho-2 catalogue. Assuming acluster distance of 5.1 kpc, a tangential velocity of 142 ± 19 kms-1 is derived from the observed cluster proper motion. Aphotometric analysis of the measurements used to obtain the astrometricparameters provides new periods, zero points and mean magnitudes for 153variable stars in the cluster. A compilation of B and V photometry(required for the astrometric reductions) is used in an analysis ofabundance variations among the cluster stars as well. Complete Tables 2,3, 5 and 9 are available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:://cdsweb.u-strasbg.fr/Abstract.html
| 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
| Observations of Nearby Interstellar Gas and Dust in the Direction toward the Globular Cluster omega Centauri
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...417..572W&db_key=AST
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| Linear polarization of stars in seven metal-poor globular clusters
Polarization measurements of more than 250 stars, most of them redgiants, in the fields of seven metal-poor globular clusers arepresented. Strong spatial variations of the polarization in the fieldsof NGC 4372, NGC 4833, M4, and NGC 6266 are found, and it is concludedthat differential reddening is important in these clusters. Mildextinction variations are found across Omega Cen and M22, while theinterstellar medium toward NGC 6397 appears fairly uniform. After takingthe foreground polarization component into account, no evidence is foundfor significant intrinsic polarization in any of the stars observed. Itis argued that an extreme Population II environment is very deficient indust. This result is discussed in terms of different mechanisms for massloss proposed for Population II red giants.
| X-ray and UV observations of Omega Centauri with EXOSAT
The diffuse emission as well as the point sources discovered in theX-ray range with the Einstein satellite are studied. A diffuse UVemission due to the unresolved hot horizontal branch stars of Omega Cenis observed. The diffuse X-ray source discovered by Hartwick et al.(1982) is not detected, but the sensitivity threshold gives an upperlimit of the flux which is not inconsistent with the Einstein result.Four UV and three X-ray point sources are detected. One X-ray sourceshows variablity of about a factor 4.5 in flux between the two Exosatobservations. Flux variations for the X-ray sources between the twoExosat observations or with respect to the Einstein one are discussedusing spectral parameters derived from the recalibrated Einstein dataarchives.
| A soft X-ray observation of Omega Centauri with EXOSAT
In an observation with the LE1 CMA detector of the EXOSAT observatory,two of the X-ray sources previously discovered in the globular clusterOmega Cen with the IPC detector on board of the Einstein satellite havebeen detected. Improved positions for these two sources are given. Onthe basis of comparison of the EXOSAT and Einstein count rates, it isconcluded that these two sources in Omega Cen are either very soft (kTof about 40 eV) or highly variable. If they are very soft, previousestimates of their X-ray luminosity can be too high by an order ofmagnitude.
| Photometry of southern globular clusters I. Bright stars in omega Centauri
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973MNRAS.162..207C&db_key=AST
| Narrow-and broad-band photometry of red stars. VII. Luminosities and temperatures for halo-population red stars of high luminosity
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972ApJ...172..639E&db_key=AST
| Magnitudes and colors in omega Cen.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1959AJ.....64...58B&db_key=AST
| Southern hemisphere photometry II Photoelectric measures of bright stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958AJ.....63..118A&db_key=AST
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Datos observacionales y astrométricos
| Constelación: | Centauro |
| Ascensión Recta: | 13h25m49.00s |
| Declinación: | -47°31'30.3" |
| Magnitud Aparente: | 8.786 |
| Distancia: | 1020.408 parsecs |
| Movimiento Propio en Ascensión Recta: | -17.2 |
| Movimiento Propio en Declinación: | -2.3 |
| B-T magnitude: | 8.815 |
| V-T magnitude: | 8.789 |
Catálogos y designaciones:
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