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Precise radial velocity measurements of G and K giants. Multiple systems and variability trend along the Red Giant Branch
We present the results of our radial velocity (RV) measurements of G andK giants, concentrating on the presence of multiple systems in oursample. Eighty-three giants have been observed for 2.5 years with thefiber-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in LaSilla, Chile. Seventy-seven stars (93%) of the targets have beenanalyzed for RV variability using simultaneous Th-Ar calibration and across-correlation technique. We estimate the long-term precision of ourmeasurement as better than 25 m s-1. Projected rotationalvelocities have been measured for most stars of the sample. Within ourtime-base only 21 stars (or 27%) show variability below 2\sigma, whilethe others show RV variability with amplitudes up to several kms-1. The large amplitude (several km s-1) andshape (high eccentricity) of the RV variations for 11 of the programstars are consistent with stellar companions, and possibly brown dwarfcompanions for two of the program stars. In those systems for which afull orbit could be derived, the companions have minimum masses from0.6 M\sun down to 0.1 M\sun. To thesemultiple systems we add the two candidates of giant planets alreadydiscovered in the sample. This analysis shows that multiple systemscontribute substantially to the long-term RV variability of giant stars,with about 20% of the sample being composed of multiple systems despitescreening our sample for known binary stars. After removing binaries,the range of RV variability in the whole sample clearly decreases, butthe remaining stars retain a statistical trend of RV variability withluminosity: luminous cool giants with B-V≥1.2 show RV variationswith \sigma_{/lineRV} > 60 m s-1, while giants with B-V< 1.2 including those in the clump region exhibit less variability orthey are constant within our accuracy. The same trend is observed withrespect to absolute visual magnitudes: brighter stars show a largerdegree of variability and, when plotted in the RV variability vs.magnitude diagram a trend of increasing RV scatter with luminosity isseen. The amplitude of RV variability does not increase dramatically, aspredicted, for instance, by simple scaling laws. At least two luminousand cooler stars of the sample show a correlation between RV andchromospheric activity and bisector asymmetry, indicating that in thesetwo objects RV variability is likely induced by the presence of(chromospheric) surface structures.Based on observations collected at the 1.52 m-ESO telescope at the LaSilla Observatory from Oct 1999 to Feb. 2002 under ESO programs and theESO-Observatório Nacional, Brazil, agreement and in part onobservations collected on the Alfred Jensch 2 m telescope of theThüringer Landessternwarte Tautenburg.
| VLT spectroscopy of XTE J2123-058 during quiescence: the masses of the two components
We present Very Large Telescope (VLT) low-resolution spectroscopy of theneutron star X-ray transient XTE J2123-058 during its quiescent state.Our data reveal the presence of a K7V companion which contributes 77 percent to the total flux at λ6300 and orbits the neutron star atK2=287+/-12kms-1. Contrary to other soft X-raytransients (SXTs), the Hα emission is almost exactly inantiphase with the velocity curve of the optical companion. Using thelight-centre technique we obtain K1=140+/-27kms-1and henceq=K1/K2=M2/M1=0.49+/-0.10.This, combined with a previous determination of the inclination angle(i=73°+/-4°) yields M1=1.55+/-0.31Msolarand M2=0.76+/-0.22Msolar. M2 agreeswell with the observed spectral type. Doppler tomography of the Hαemission shows a non-symmetric accretion disc distribution mimickingthat seen in SW Sex stars. Although we find a large systemic velocity of- 110+/-8kms-1 this value is consistent with the galacticrotation velocity at the position of J2123-058, and hence a halo origin.The formation scenario of J2123-058 is still unresolved.
| The Halo Black Hole X-Ray Transient XTE J1118+480
Optical spectra were obtained of the optical counterpart of thehigh-latitude (b~=62deg) soft X-ray transient XTE J1118+480near its quiescent state (R~=18.3) with the new 6.5 m Multiple MirrorTelescope and the 4.2 m William Herschel Telescope. The spectrumexhibits broad, double-peaked emission lines of hydrogen (FWHM~=2400 kms-1) arising from an accretion disk superposed withabsorption lines of a late-type secondary star. Cross-correlation of the27 individual spectra with late-type stellar template spectra reveals asinusoidal variation in radial velocity with amplitude K=701+/-10 kms-1 and orbital period P=0.169930+/-0.000004 days. The massfunction, 6.1+/-0.3 Msolar, is a firm lower limit on the massof the compact object and strongly implies that it is a black hole. Weestimate the spectral type of the secondary to be K7 V-M0 V, and that itcontributes 28%+/-2% of the light in the 5800-6400 Å region on2000 November 20, increasing to 36%+/-2% by 2001 January 4 as the diskfaded. Photometric observations (R-band) with the Instituto deAstrofísica de Canarias 0.8 m telescope reveal ellipsoidal lightvariations of full amplitude 0.2 mag. Modeling of the light curve givesa large mass ratio (M1/M2~20) and a high orbitalinclination (i=81deg+/-2deg). Our combined fitsyield a mass of the black hole in the range M1=6.0-7.7Msolar (90% confidence) for plausible secondary star massesof M2=0.09-0.5 Msolar. The photometric periodmeasured during the outburst is 0.5% longer than our orbital period andprobably reflects superhump modulations, as observed in some other softX-ray transients. The estimated distance is d=1.9+/-0.4 kpc,corresponding to a height of 1.7+/-0.4 kpc above the Galactic plane. Thespectroscopic, photometric, and dynamical results indicate that XTEJ1118+480 is the first firmly identified black hole X-ray system in theGalactic halo. Based in part on observations obtained at the MMTObservatory, a joint facility of the University of Arizona and theSmithsonian Institution.
| VLT optical observations of V821 Ara(=GX339-4) in an extended ``off'' state
We report on low-resolution spectroscopy of GX339-4 during its current,extended X-ray ``off'' state in May 2000 (r=20.1) obtained with the VLTFocal Reducer/low dispersion Spectrograph (FORS1). Although we do notpositively detect the secondary star in GX339-4 we place an upper limitof 30 percent on the contribution of a ``normal'' K-type secondary starspectrum to the observed flux. Using this limit for the observedmagnitude of the secondary star, we find a lower limit for the distanceof GX339-4 to be 5.6 kpc. Based on observations performed at theEuropean Southern Observatory, Paranal, Chile.
| 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.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
| 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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Osservazione e dati astrometrici
| Costellazione: | Idra |
| Ascensione retta: | 14h02m22.80s |
| Declinazione: | -27°25'48.0" |
| Magnitudine apparente: | 5.48 |
| Distanza: | 133.156 parsec |
| Moto proprio RA: | -31.7 |
| Moto proprio Dec: | -3.8 |
| B-T magnitude: | 7.203 |
| V-T magnitude: | 5.621 |
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INHABITED SKY
