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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
| A spectroscopy study of nearby late-type stars, possible members of stellar kinematic groups
Context. Nearby late-type stars are excellent targets for seeking youngobjects in stellar associations and moving groups. The origin of thesestructures is still misunderstood, and lists of moving group membersoften change with time and also from author to author. Most members ofthese groups have been identified by means of kinematic criteria,leading to an important contamination of previous lists by old fieldstars. Aims: We attempt to identify unambiguous moving groupmembers among a sample of nearby-late type stars by studying theirkinematics, lithium abundance, chromospheric activity, and otherage-related properties. Methods: High-resolution echelle spectra(R ~ 57 000) of a sample of nearby late-type stars are used to deriveaccurate radial velocities that are combined with the precise Hipparcosparallaxes and proper motions to compute galactic-spatial velocitycomponents. Stars are classified as possible members of the classicalmoving groups according to their kinematics. The spectra are also usedto study several age-related properties for young late-type stars, i.e.,the equivalent width of the lithium Li i 6707.8 Å line or theR'HK index. Additional information like X-ray fluxes from theROSAT All-Sky Survey or the presence of debris discs is also taken intoaccount. The different age estimators are compared and the moving groupmembership of the kinematically selected candidates are discussed. Results: From a total list of 405 nearby stars, 102 have beenclassified as moving group candidates according to their kinematics.i.e., only ~25.2% of the sample. The number reduces when age estimatesare considered, and only 26 moving group candidates (25.5% of the 102candidates) have ages in agreement with the star having the same age asan MG member.Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-PlanckInstitut für Astronomie and the Instituto de Astrofísica deAndalucía (CSIC) and observations made with the ItalianTelescopio Nazionale Galileo (TNG) operated on the island of La Palma bythe Fundación Galileo Galilei of the INAF (Istituto Nazionale diAstrofisica) at the Spanish Observatorio del Roque de los Muchachos ofthe Instituto de Astrofísica de Canarias.Appendices and Tables 1,5-15 are available in electronic form at http://www.aanda.orgTable 1 is alsoavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/521/A12
| Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter
Context. Chromospheric activity produces both photometric andspectroscopic variations that can be mistaken as planets. Large spotscrossing the stellar disc can produce planet-like periodic variations inthe light curve of a star. These spots clearly affect the spectral lineprofiles, and their perturbations alter the line centroids creating aradial velocity jitter that might “contaminate” thevariations induced by a planet. Precise chromospheric activitymeasurements are needed to estimate the activity-induced noise thatshould be expected for a given star. Aims: We obtain precisechromospheric activity measurements and projected rotational velocitiesfor nearby (d ? 25 pc) cool (spectral types F to K) stars, toestimate their expected activity-related jitter. As a complementaryobjective, we attempt to obtain relationships between fluxes indifferent activity indicator lines, that permit a transformation oftraditional activity indicators, i.e., Ca ii H & K lines, to othersthat hold noteworthy advantages. Methods: We used high resolution(~50 000) echelle optical spectra. Standard data reduction was performedusing the IRAF echelle package. To determine the chromospheric emissionof the stars in the sample, we used the spectral subtraction technique.We measured the equivalent widths of the chromospheric emission lines inthe subtracted spectrum and transformed them into fluxes by applyingempirical equivalent width and flux relationships. Rotational velocitieswere determined using the cross-correlation technique. To inferactivity-related radial velocity (RV) jitter, we used empiricalrelationships between this jitter and the R'_HK index. Results:We measured chromospheric activity, as given by different indicatorsthroughout the optical spectra, and projected rotational velocities for371 nearby cool stars. We have built empirical relationships among themost important chromospheric emission lines. Finally, we used themeasured chromospheric activity to estimate the expected RV jitter forthe active stars in the sample.Based on observations made with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto (Spain)and the Telescopio Nazionale Galileo (TNG) operated on the island of LaPalma by the Istituto Nazionale de Astrofisica Italiano (INAF), in theSpanish Observatorio del Roque de los Muchachos. This research has beensupported by the Programa de Acceso a InfraestructurasCientíficas y Tecnológicas Singulares (ICTS).Tables A1 toA4 are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/520/A79
| The PASTEL catalogue of stellar parameters
Aims: The PASTEL catalogue is an update of the [Fe/H] catalogue,published in 1997 and 2001. It is a bibliographical compilation ofstellar atmospheric parameters providing (T_eff, log g, [Fe/H])determinations obtained from the analysis of high resolution, highsignal-to-noise spectra, carried out with model atmospheres. PASTEL alsoprovides determinations of the one parameter T_eff based on variousmethods. It is aimed in the future to provide also homogenizedatmospheric parameters and elemental abundances, radial and rotationalvelocities. A web interface has been created to query the catalogue onelaborated criteria. PASTEL is also distributed through the CDS databaseand VizieR. Methods: To make it as complete as possible, the mainjournals have been surveyed, as well as the CDS database, to findrelevant publications. The catalogue is regularly updated with newdeterminations found in the literature. Results: As of Febuary2010, PASTEL includes 30151 determinations of either T_eff or (T_eff,log g, [Fe/H]) for 16 649 different stars corresponding to 865bibliographical references. Nearly 6000 stars have a determination ofthe three parameters (T_eff, log g, [Fe/H]) with a high qualityspectroscopic metallicity.The catalogue can be queried through a dedicated web interface at http://pastel.obs.u-bordeaux1.fr/.It is also available in electronic form at the Centre de DonnéesStellaires in Strasbourg (http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=B/pastel),at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/515/A111
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| Hipparcos preliminary astrometric masses for the two close-in companions to HD 131664 and HD 43848. A brown dwarf and a low-mass star
Context. Several mechanisms for forming brown dwarfs have been proposed,which today are not believed to be mutually exclusive. Among thefundamental characteristics of brown dwarfs that are intrinsically tiedto their origins, multiplicity is particularly relevant. Any successfuldetermination of the actual mass for such objects in any systems is thusworthwhile, as it allows one to improve on the characterization of themultiplicity properties (e.g., frequency, separation, mass-ratiodistribution) of sub-stellar companions. Aims: We attempt toplace better constraints on the masses of two Doppler-detectedsubstellar companions to the nearby G dwarfs HD 131664 and HD 43848. Methods: We carried out orbital fits to the Hipparcos IntermediateAstrometric Data (IAD) for the two stars, taking advantage of theknowledge of the spectroscopic orbits, and solving for the inclinationangle i and the longitude of the ascending node ?, the two orbitalelements that can be determined in principle solely by astrometry, Anumber of checks were carried out to assess the reliability of theorbital solutions thus obtained. Results: The best-fit solutionfor HD 131664 yields i=55±33 deg and ?=22±28 deg.The resulting inferred true companion mass is then Mc =23-5+26 M_J. For HD 43848, we find i=12±7deg and ?=288±22 deg, and a corresponding Mc =120-43+167 M_J. Based on the statistical evidencefrom an F-test, the study of the joint confidence intervals of variationin i and ? and the comparison of the derived orbital semi-majoraxes with a distribution of false astrometric orbits obtained for singlestars observed by Hipparcos, the astrometric signal of the twocompanions to HD 131664 and HD 43848 is then considered detected in theHipparcos IAD, with a level of statistical confidence not exceeding 95%. Conclusions: We constrain the true mass of HD 131664b to that ofa brown dwarf to within a somewhat statistically significant degree ofconfidence (~2-?). For HD 43848b, a true mass in the brown dwarfregime is ruled out at the 1-? confidence level. The results arediscussed in the context of the properties of the (few) close substellarand massive planetary companions to nearby solar-type stars and theirimplications for proposed models of formation and structure of massiveplanets and brown dwarfs.
| The main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filtersystem well defined stellar samples are needed. The nearby stars providea complete stellar sample representative for the thin disc population.We compare the filter transformations of different authors applied tothe main sequence stars from F to K dwarfs to SDSS filter system anddiscuss the properties of the main sequence. The location of the meanmain sequence in colour-magnitude diagrams is very sensitive tosystematic differences in the filter transformation. A comparison withfiducial sequences of star clusters observed in g', r', and i' show goodagreement. Theoretical isochrones from Padua and from Dartmouth havestill some problems, especially in the (r-i) colours.
| Automated classification of ELODIE stellar spectral library using probabilistic artificial neural networks
A Probabilistic Neural Network model has been used for automatedclassification of ELODIE stellar spectral library consisting of about2000 spectra into 158 known spectro-luminosity classes. The full spectrawith 561 flux bins and a PCA reduced set of 57, 26 and 16 componentshave been used for the training and test sessions. The results show aspectral type classification accuracy of 3.2 sub-spectral type andluminosity class accuracy of 2.7 for the full spectra and an accuracy of3.1 and 2.6 respectively with the PCA set. This technique will be usefulfor future upcoming large databases and their rapid classification.
| Astrometric Orbits for Hipparcos Stochastic Binaries
Taking advantage of an improved genetic optimization algorithm forfitting unconstrained Kepler orbits to the Hipparcos intermediateastrometric data, we obtain additional orbital solutions for 81Hipparcos stars with previous stochastic (failed) solutions. The sampleincludes astrophysically interesting objects, including the old diskwide binary HIP 754, the nearby AGB star HIP 34922 (L2 Pup),and the nearby M2 dwarf HIP 5496 (GJ 54, at 8 pc from the Sun), whichhas a resolved M dwarf companion.
| The eccentricity-mass distribution of exoplanets: signatures of different formation mechanisms?
We examine the distributions of eccentricity and host star metallicityof exoplanets as a function of their mass. Planets with M sin i ⪆ 4MJ have an eccentricity distribution consistent with that ofbinary stars, while planets with M sin i ⪉ 4 MJ are lesseccentric than binary stars and more massive planets. In addition, hoststar metallicities decrease with planet mass. The statisticalsignificance of both of these trends is only marginal with the presentsample of exoplanets. To account for these trends, we hypothesize thatthere are two populations of gaseous planets: the low-mass populationforms by gas accretion onto a rock-ice core in a circumstellar disk andis more abundant at high metallicities, and the high-mass populationforms directly by fragmentation of a pre-stellar cloud. Planets of thefirst population form in initially circular orbits and grow theireccentricities later, and may have a mass upper limit from the totalmass of the disk that can be accreted by the core. The second populationmay have a mass lower limit resulting from opacity-limitedfragmentation. This would roughly divide the two populations in mass,although they would likely overlap over some mass range. If most objectsin the second population form before the pre-stellar cloud becomeshighly opaque, they would have to be initially located in orbits largerthan ~30 AU, and would need to migrate to the much smaller orbits inwhich they are observed. The higher mean orbital eccentricity of thesecond population might be caused by the larger required intervals ofradial migration, and the brown dwarf desert might be due to theinability of high-mass brown dwarfs to migrate inwards sufficiently inradius.
| Unconstrained Astrometric Orbits for Hipparcos Stars with Stochastic Solutions
A considerable number of astrometric binaries whose positions on the skydo not obey the standard model of mean position, parallax, and linearproper motion were observed by the Hipparcos satellite. Some of themremain undiscovered, and their observational data have not been properlyprocessed with the more adequate astrometric model that includesnonlinear orbital motion. We develop an automated algorithm, based on``genetic optimization,'' to solve the orbital fitting problem in themost difficult setup, when no prior information about the orbitalelements is available (from, e.g., spectroscopic data or radial velocitymonitoring). We also offer a technique to accurately compute theprobability that an orbital fit is bogus, that is, that an orbitalsolution is obtained for a single star, and to estimate the probabilitydistributions for the fitting orbital parameters. We test this method onHipparcos stars with known orbital solutions in the catalog and furtherapply it to 1561 stars with stochastic solutions, which may beunresolved binaries. At a confidence level of 99%, orbital fits areobtained for 65 stars, most of which have not been known as binary. Itis found that reliable astrometric fits can be obtained even if theperiod is somewhat longer than the time span of the Hipparcos mission,that is, if the orbit is not closed. A few of the new probable binarieswith A-type primaries with periods 444-2015 days are chemically peculiarstars, including Ap and λ Bootis types. The anomalous spectra ofthese stars are explained by admixtures of light from the unresolved,sufficiently bright and massive companions. We estimate the apparentorbits of four stars that have been identified as members of the ~300Myr old Ursa Major kinematic group. Another four new nearby binaries mayinclude low-mass M-type or brown dwarf companions. Follow-upspectroscopic observations in conjunction with more accurate inclinationestimates will lead to better estimates of the secondary mass. Similarastrometric models and algorithms can be used for binary stars andplanet hosts observed by SIM and Gaia.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| Abundance Analysis of Planetary Host Stars. I. Differential Iron Abundances
We present atmospheric parameters and iron abundances derived fromhigh-resolution spectra for three samples of dwarf stars: stars that areknown to host close-in giant planets (CGP), stars for which radialvelocity data exclude the presence of a close-in giant planetarycompanion (no-CGP), as well as a random sample of dwarfs with a spectraltype and magnitude distribution similar to that of the planetary hoststars (control). All stars have been observed with the same instrumentand have been analyzed using the same model atmospheres, atomic data,and equivalent width modeling program. Abundances have been deriveddifferentially to the Sun, using a solar spectrum obtained with Callistoas the reflector with the same instrumentation. We find that the ironabundances of CGP dwarfs are on average 0.22 dex greater than that ofno-CGP dwarfs. The iron abundance distributions of both the CGP andno-CGP dwarfs are different than that of the control dwarfs, while thecombined iron abundances have a distribution that is very similar tothat of the control dwarfs. All four samples (CGP, no-CGP, combined, andcontrol) have different effective temperature distributions. We showthat metal enrichment occurs only for CGP dwarfs with temperatures justbelow solar and ~300 K higher than solar, whereas the abundancedifference is insignificant at Teff around 6000 K.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| Analysis of the Hipparcos Observations of the Extrasolar Planets and the Brown Dwarf Candidates
We analyzed the Hipparcos astrometric observations of 47 stars that werediscovered to harbor giant planets and 14 stars with brown dwarfsecondary candidates. The Hipparcos measurements were used together withthe corresponding stellar radial velocity data to derive an astrometricorbit for each system. To find out the significance of the derivedastrometric orbits, we applied a ``permutation'' technique by which weanalyzed the permuted Hipparcos data to get false orbits. The sizedistribution of these false orbits indicated the range of possiblyrandom, false orbits that could be derived from the true data. Thesetests could not find any astrometric orbit of the planet candidates withsignificance higher than 99%, suggesting that most if not all orbits arenot real. Instead, we used the Hipparcos data to set upper limits on themasses of the planet candidates. The lowest derived upper limit is thatof 47 UMa-0.014 Msolar, which confirms the planetary natureof its unseen companion. For 13 other planet candidates, the upperlimits exclude the stellar nature of their companions, although browndwarf secondaries are still an option. These negate the idea that all ormost of the extrasolar planets are disguised stellar secondaries. Of the14 brown dwarf candidates, our analysis reproduced the results ofHalbwachs et al., who derived significant astrometric orbits for sixsystems that imply secondaries with stellar masses. We show that anotherstar, HD 164427, which was discovered only very recently, also has asecondary with stellar mass. Our findings support Halbwachs et al.'sconclusion about the possible existence of the ``brown dwarf desert''that separates the planets and the stellar secondaries.
| A Spectrophotometric Technique for Detecting Companions of Low-Mass M Dwarfs
The most common stars in the Galaxy are the main-sequence M dwarfs, yetcurrent techniques are not optimized for detecting companions around thelowest mass M dwarfs, those with spectral designations ranging from M6to M10. Described in this paper is a search for companions around suchstars using a newly designed differential spectrophotometric technique.This novel method combines the strengths of the photometric andspectroscopic techniques, while it minimizes their inherent limitations.The scientific goal of this project is to search for short-periodsystems containing brown dwarfs and giant planets. The detection of orplacing limits on such systems will help discriminate among competingtheories of planetary formation.
| Screening the Hipparcos-based astrometric orbits of sub-stellar objects
The combination of Hipparcos astrometric data with the spectroscopicdata of putative extrasolar planets seems to indicate that a significantfraction of these low-mass companions could be brown or M dwarfs (Han etal. \cite{Han-2001:a}). We show that this is due to the adoptedreduction procedure, and consequently that the Hipparcos data do notreject the planetary mass hypothesis in all but one cases. Additionalcompanions, undetected so far, might also explain the large astrometricresiduals of some of these stars. Based on observations from theHipparcos astrometric satellite operated by the European Space Agency(ESA 1997).
| Exploring the brown dwarf desert with Hipparcos
The orbital elements of 11 spectroscopic binaries with brown dwarfcandidates (M2 sin i between 0.01 and 0.08 Msun)are combined with the Hipparcos observations in order to deriveastrometric orbits. Estimations of the masses of the secondarycomponents are thus calculated. It appears that 5 secondary masses aremore than 2 sigmaM2 above the limit of 0.08Msun, and are therefore not brown dwarfs. 2 other stars arestill discarded at the 1 sigmaM2 level, 1 browndwarf is accepted with a low confidence, and we are finally left with 3viable candidates which must be studied by other means. A statisticalapproach is developed, based on the relation between the semi-major axesof the photocentric orbit, a_0, their errors, sigma a_0, andthe frequency distribution of the mass ratios, q. It is investigatedwhether the set of values of a_0 and sigma a_0 obtained forthe sample is compatible with different frequency distributions of q. Itis concluded that a minimum actually exists for M2 betweenabout 0.01 and 0.1 Msun for companions of solar-type stars.This feature could correspond to the transition between giant planetsand stellar companions. Due to the relatively large frequency of singlebrown dwarfs found recently in open clusters, it is concluded that thedistribution of the masses of the secondary components in binary systemsdoes not correspond to the IMF, at least for masses below thehydrogen-ignition limit. Based on photoelectric radial-velocitymeasurements collected at Haute-Provence observatory and on observationsmade with the ESA Hipparcos astrometry satellite.
| On the Nature of Low-Mass Companions to Solar-like Stars
Low-mass companions (mass <70 Jupiter masses) to solar-like stars arecompared statistically to stellar-mass secondaries in binaries ofsimilar primary spectral types and orbital scales, based largely on thesurvey of Duquennoy & Mayor. To within the limits imposed byobservational constraints, the orbital properties of these low-masscompanions (LMCs) are statistically indistinguishable from those ofbinary systems. In both populations, orbital periods (P), semimajor axes(a), angular momenta (L), and binding energies (U) are all distributedapproximately as f(x)~x-1 for x=P,a,L,U. In both populations,eccentricities are broadly distributed approximately asf(e)~e-0.5, with no significant correlation with otherorbital elements, apart from a marked circularization of close orbits.The distribution of LMC masses is approximately a power law with indexbetween -1 and -2 there is ambiguous evidence in the data for a massspectrum bimodality about approximately 10 Jupiter masses. In bothpopulations the joint distributions of mass with all orbital propertiesare largely scattergrams, with no statistically significantcorrelations. The overall statistical properties of LMCs are suggestiveof a common formation mechanism with binary star systems. The similarform of the distributions of all orbital dynamic quantities in bothpopulations may result from postformation dissipative orbital decay.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| The mass distribution of extrasolar planet candidates and low-mass secondaries.
Not Available
| Habitable Moons
Not Available
| Possible Observational Criteria for Distinguishing Brown Dwarfs from Planets
The difference in formation process between binary stars and planetarysystems is reflected in their composition, as well as orbitalarchitecture, particularly in their orbital eccentricity as a functionof orbital period. It is suggested here that this difference can be usedas an observational criterion to distinguish between brown dwarfs andplanets. Application of the orbital criterion suggests that, with threepossible exceptions, all of the recently discovered substellarcompanions may be brown dwarfs and not planets. These criterion may beused as a guide for interpretation of the nature of substellar-masscompanions to stars in the future.
| The Discovery of a Planetary Companion to 16 Cygni B
High-precision radial-velocity observations of the solar-type star 16Cygni B (HR 7504, HD 186427), taken at McDonald Observatory and at LickObservatory, have each independently discovered periodic radial-velocityvariations indicating the presence of a Jovian-mass companion to thisstar. The orbital fit to the combined McDonald and Lick data gives aperiod of 800.8 days, a velocity amplitude (K) of 43.9 m s-1, and aneccentricity of 0.63. This is the largest eccentricity of any planetarysystem discovered so far. Assuming that 16 Cygni B has a mass of 1.0M&sun;, the mass function then implies a mass for the companion of1.5/sin i Jupiter masses. While the mass of this object is well withinthe range expected for planets, the large orbital eccentricity cannot beexplained simply by the standard model of growth of planets in aprotostellar disk. It is possible that this object was formed in thenormal manner with a low-eccentricity orbit and has undergonepostformational orbital evolution, either through the same process thathas been proposed to have formed the "massive eccentric" planets around70 Virginis and HD 114762, or by gravitational interactions with thecompanion star 16 Cygni A. It is also possible that the object is anextremely low mass brown dwarf formed through fragmentation of thecollapsing protostar. We explore a possible connection between stellarphotospheric Li depletion, pre--mainsequence stellar rotation, thepresence of a massive protoplanetary disk, and the formation of aplanetary companion.
| Auf der Suche nach Planeten um andere Sterne. Teil 2: Welche man schon kennt, und wie es weitergeht.
Not Available
| Some Cross-Reference Lists for the Catalog of Possible Nearby Stars
Not Available
| Possible nearby stars brighter than tenth magnitude
Basic data are compiled for 447 stars brighter than 10th visualmagnitude which may be within 25 pc of the sun and are missing from boththe Gliese (1969) and the Woolley et al. (1970) catalogs of nearbystars. The list includes 245 stars with photometric parallaxes, 17 starswith trigonometric parallaxes, and nine stars with dynamical parallaxes,all of which parallaxes are at least 0.040 arcsec, as well as 176 likelycandidates. The stars are grouped into six categories according to thereliability of absolute-magnitude estimates and ranked within each groupon the basis of calculated distance. The distance estimates incorporatea kinematic correction to the photometric parallaxes which is based onthe size of a star's proper motion. A list of stars brighter than 10thmag which appear in the Gliese but not in the Woolley et al. catalog isalso provided to facilitate cross-reference with existing catalogs ofnearby stars.
| Nearby Star Data Published 1969-1978
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979A&AS...38..423G&db_key=AST
| Dwarf K and M stars in the southern hemisphere.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972AJ.....77..486U&db_key=AST
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| Yükselim: | -04°25'05.9" |
| Görünürdeki Parlaklýk: | 9.415 |
| Uzaklýk: | 20.429 parsek |
| özdevim Sað Açýklýk: | 88.1 |
| özdevim Yükselim: | -91.1 |
| B-T magnitude: | 10.941 |
| V-T magnitude: | 9.541 |
Kataloglar ve belirtme:
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