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Search for brown-dwarf companions of stars
Context. The frequency of brown-dwarf companions in close orbit aroundSun-like stars is low compared to the frequency of planetary and stellarcompanions. There is presently no comprehensive explanation of this lackof brown-dwarf companions. Aims: By combining the orbitalsolutions obtained from stellar radial-velocity curves and Hipparcosastrometric measurements, we attempt to determine the orbit inclinationsand therefore the masses of the orbiting companions. By determining themasses of potential brown-dwarf companions, we improve our knowledge ofthe companion mass-function. Methods: The radial-velocitysolutions revealing potential brown-dwarf companions are obtained forstars from the CORALIE and HARPS planet-search surveys or from theliterature. The best Keplerian fit to our radial-velocity measurementsis found using the Levenberg-Marquardt method. The spectroscopicelements of the radial-velocity solution constrain the fit to theintermediate astrometric data of the new Hipparcos reduction. Theastrometric solution and the orbit inclination are found usingnon-linear ?2-minimisation on a two-parameter searchgrid. The statistical confidence of the adopted orbital solution isevaluated based on the distribution-free permutation test. Results: The discovery of nine new brown-dwarf candidates orbiting starsin the CORALIE and HARPS radial-velocity surveys is reported. NewCORALIE radial velocities yielding accurate orbits of sixpreviously-known hosts of potential brown-dwarf companions arepresented. Including the literature targets, 33 hosts of potentialbrown-dwarf companions are examined. Employing innovative methods, weuse the new reduction of the Hipparcos data to fully characterise theastrometric orbits of six objects, revealing M-dwarf companions ofmasses between 90 MJ and 0.52 M_&sun;. In addition, themasses of two companions can be restricted to the stellar domain. Thecompanion to HD 137510 is found to be a brown dwarf. At 95% confidence,the companion of HD 190228 is also a brown dwarf. Twenty-threecompanions remain brown-dwarf candidates. On the basis of the CORALIEplanet-search sample, we obtain an upper limit of 0.6% for the frequencyof brown-dwarf companions around Sun-like stars. We find that thecompanion-mass distribution function increases toward the lower end ofthe brown-dwarf mass range, suggesting that we detect the high-mass tailof the planetary distribution. Conclusions: Our findings agreewith the results of previous similar studies and confirm the pronouncedpaucity of brown-dwarf companions around Sun-like stars. They areaffected by the Hipparcos astrometric precision and mission duration,which limits the minimum detectable companion mass, and some of theremaining candidates are probably brown-dwarf companions.Based on observations made with the CORALIE spectrograph on the Swisstelescope at the European Southern Observatory in La Silla, Chile, withthe ESA Hipparcos astrometry satellite, and with the HARPS instrument onthe ESO 3.6-m telescope (GTO programme 072.C-0488).The CORALIE and HARPSradial-velocity measurements discussed in this paper are only availablein electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/vol/pge
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
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