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The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in Disks around Sun-like Stars
We have carried out a sensitive search for gas emission lines at IR andmillimeter wavelengths for a sample of 15 young Sun-like stars selectedfrom our dust disk survey with Spitzer. We have used mid-IR lines totrace the warm (300-100 K) gas in the inner disk and millimetertransitions of 12CO to probe the cold (~20 K) outer disk. Wereport no gas line detections from our sample. Line flux upper limitsare first converted to warm and cold gas mass limits using simpleapproximations allowing a direct comparison with values from theliterature. We also present results from more sophisticated modelsfollowing Gorti & Hollenbach that confirm and extend our simpleanalysis. These models show that the [S I] 25.23 μm line can setconstraining limits on the gas surface density at the disk inner radiusand traces disk regions up to a few AU. We find that none of the 15systems have more than 0.04MJ of gas within a few AU from thedisk inner radius for disk radii from 1 to ~40 AU. These gas mass upperlimits even in the eight systems younger than ~30 Myr suggest that mostof the gas is dispersed early. The gas mass upper limits in the 10-40 AUregion, which is mainly traced by our CO data, are <2M⊕. If these systems are analogs of the solar system,they either have already formed Uranus- and Neptune-like planets or willnot form them beyond 100 Myr. Finally, the gas surface density upperlimits at 1 AU are smaller than 0.01% of the minimum mass solar nebulafor most of the sources. If terrestrial planets form frequently andtheir orbits are circularized by gas, then circularization occurs early.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| An Infrared Coronagraphic Survey for Substellar Companions
We have used the F160W filter (1.4-1.8 μm) and the coronagraph on theNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) on theHubble Space Telescope to survey 45 single stars with a median age of0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7mag. For the median age we were capable of detecting a 30MJcompanion at separations between 15 and 200 AU. A 5MJ objectcould have been detected at 30 AU around 36% of our primaries. Forseveral of our targets that were less than 30 Myr old, the lower masslimit was as low as 1MJ, well into the high mass planetregion. Results of the entire survey include the proper-motionverification of five low-mass stellar companions, two brown dwarfs(HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| Multiplicity among Widely Separated Brown Dwarf Companions to Nearby Stars: Gliese 337CD
We present Lick Natural Guide Star Adaptive Optics observations of theL8 brown dwarf Gl 337C, which is resolved for the first time into twoclosely separated (0.53"+/-0.03"), nearly equal magnitude componentswith a Ks flux ratio of 0.93+/-0.10. Companionship isinferred from the absence of a 3.6" offset source in Two Micron All SkySurvey or photographic plate images, implying that the observedsecondary component is a comoving late-type dwarf. With a projectedseparation of 11 AU and nearly equal magnitude components, Gl 337CD hasproperties similar to those of other known companion and fieldsubstellar binaries. Its long orbital period (estimated to be ~140-180yr) inhibits short-term astrometric mass measurements, but the Gl 337CDsystem is ideal for studying the L-T transition at a fixed age andmetallicity. From a compilation of all known widely separated (>~100AU) stellar-brown dwarf multiple systems, we find evidence that thebinary fraction of brown dwarfs in these systems is notably higher thanthat of field brown dwarfs, 45+15-13% versus18+7-4% for analogous samples. We speculate onpossible reasons for this difference, including the possibility thatdynamic (ejection) interactions that may form such wide pairspreferentially retain binary secondaries because of their greatercombined mass and/or ability to absorb angular momentum.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Evolution of Cold Circumstellar Dust around Solar-type Stars
We present submillimeter (Caltech Submillimeter Observatory 350 μm)and millimeter (Swedish-ESO Submillimetre Telescope [SEST] 1.2 mm, OwensValley Radio Observatory [OVRO] 3 mm) photometry for 127 solar-typestars from the Formation and Evolution of Planetary Systems SpitzerLegacy program that have masses between ~0.5 and 2.0 Msolarand ages from ~3 Myr to 3 Gyr. Continuum emission was detected towardfour stars with a signal-to-noise ratio>=3: the classical T Tauristars RX J1842.9-3532, RX J1852.3-3700, and PDS 66 with SEST, and thedebris-disk system HD 107146 with OVRO. RX J1842.9-3532 and RXJ1852.3-3700 are located in projection near the R CrA molecular cloud,with estimated ages of ~10 Myr (Neuhäuser et al.), whereas PDS 66is a probable member of the ~20 Myr old Lower Centaurus-Crux subgroup ofthe Scorpius-Centaurus OB association (Mamajek et al.). The continuumemission toward these three sources is unresolved at the 24" SESTresolution and likely originates from circumstellar accretion disks,each with estimated dust masses of ~5×10-5Msolar. Analysis of the visibility data toward HD 107146(age~80-200 Myr) indicates that the 3 mm continuum emission is centeredon the star within the astrometric uncertainties and resolved with aGaussian-fit FWHM size of (6.5"+/-1.4")×(4.2"+/-1.3"), or185AU×120 AU. The results from our continuum survey are combinedwith published observations to quantify the evolution of dust mass withtime by comparing the mass distributions for samples with differentstellar ages. The frequency distribution of circumstellar dust massesaround solar-type stars in the Taurus molecular cloud (age~2 Myr) isdistinguished from that around 3-10 Myr and 10-30 Myr old stars at asignificance level of ~1.5 and ~3 σ, respectively. These resultssuggest a decrease in the mass of dust contained in small dust grainsand/or changes in the grain properties by stellar ages of 10-30 Myr,consistent with previous conclusions. Further observations are needed todetermine if the evolution in the amount of cold dust occurs on evenshorter timescales.
| Initial Results from the Palomar Adaptive Optics Survey of Young Solar-Type Stars: A Brown Dwarf and Three Stellar Companions
We present first results from the Palomar Adaptive Optics Survey ofYoung Stars conducted at the Hale 5 m telescope. Through direct imagingwe have discovered a brown dwarf and two low-mass stellar companions tothe young solar-type stars HD 49197, HD 129333 (EK Dra), and V522 Perand confirmed a previously suspected companion to RX J0329.1+0118(Sterzik et al.), at respective separations of 0.95" (43 AU), 0.74" (25AU), 2.09" (400 AU), and 3.78" (380 AU). Physical association of eachbinary system is established through common proper motion and/orlow-resolution infrared spectroscopy. Based on the companion spectraltypes, we estimate their masses at 0.06, 0.20, 0.13, and 0.20Msolar, respectively. From analysis of our imaging datacombined with archival radial velocity data, we find that the spatiallyresolved companion to HD 129333 is potentially identical to thepreviously identified spectroscopic companion to this star (Duquennoy& Mayor). However, a discrepancy with the absolute magnitudesuggests that the two companions could also be distinct, with theresolved one being the outermost component of a triple system. The browndwarf HD 49197B is a new member of a growing list of directly imagedsubstellar companions at 10-1000 AU separations from main-sequencestars, indicating that such brown dwarfs may be more common thaninitially speculated.
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Brown Dwarf Desert at 75-1200 AU
We present results of a comprehensive infrared coronagraphic search forsubstellar companions to nearby stars. The research consisted of (1) a178-star survey at Steward and Lick observatories, with opticalfollow-up from Keck Observatory, capable of detecting companions withmasses greater than 30 MJ, and semimajor axes between about140 to 1200 AU; (2) a 102-star survey using the Keck Telescope, capableof detecting extrasolar brown dwarfs and planets typically more massivethan 10 MJ, with semimajor axes between about 75 and 300 AU.Only one brown dwarf companion was detected, and no planets. Thefrequency of brown dwarf companions to G, K, and M stars orbitingbetween 75 and 300 AU is measured to be 1%+/-1%, the most precisemeasurement of this quantity to date. The frequency of massive (greaterthan 30 MJ) brown dwarf companions at 120-1200 AU is found tobe f=0.7%+/-0.7%. The frequency of giant planet companions with massesbetween 5 and 10 MJ orbiting between 75 and 300 AU ismeasured here for the first time to be no more than ~3%. Together withother surveys that encompass a wide range of orbital separations, theseresults imply that substellar objects with masses between 12 and 75MJ form only rarely as companions to stars. Theories of starformation that could explain these data are only now beginning toemerge.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are 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/418/989
| HD 77407 and GJ 577: Two new young stellar binaries. Detected with the Calar Alto Adaptive Optics system ALFA
We present the first results from our search for close stellar andsub-stellar companions to young nearby stars on the northern sky. Ourinfrared imaging observations are obtained with the 3.5 m Calar Altotelescope and the AO system ALFA. With two epoch observations which wereseparated by about one year, we found two co-moving companioncandidates, one close to HD 77407 and one close to GJ 577. For thecompanion candidate near GJ 577, we obtained an optical spectrum showingspectral type M 4.5; this candidate is a bound low-mass stellarcompanion confirmed by both proper motion and spectroscopy. We estimatethe masses for HD 77407 B and GJ 577 B to be 0.3 to 0.5Mȯ and 0.16 to 0.2 Mȯ,respectively. Compared to Siess et al. (\cite{Siess2000}) models, eachof the two pairs appears co-eval with HD 77407 A, B being 10 to 40 Myrsand GJ 577 A, B being ≥100 Myrs old. We also took multi-epochhigh-resolution spectra of HD 77407 to search for sub-stellarcompanions, but did not find any with 3 MJup as upper mass (msin i) limit (for up to 4 year orbits); however, we detected a long-termradial velocity trend in HD 77407 A, consistent with a 0.3Mȯ companion at 50 AU separation, i.e. the onedetected by the imaging. Hence, HD 77407 B is confirmed to be a boundcompanion to HD 77407 A. We also present limits for undetected, butdetectable companions using a deep image of HD 77407 A and B, alsoobserved with the Keck NIRC2 AO system; any brown dwarfs were detectableoutside of 0.5 arcsec (17 AU at HD 77407), giant planets with massesfrom 6.5 to 12 MJup were detectable at ≥1.5 arcsec.
| Age Dependence of the Vega Phenomenon: Observations
We study the time dependency of Vega-like excesses using infraredstudies obtained with the imaging photopolarimeter ISOPHOT on board theInfrared Space Observatory. We review the different studies published onthis issue and critically check and revise ages and fractionalluminosities in the different samples. The conclusions of our studydiffer significantly from those obtained by other authors (e.g., Hollandand coworkers; Spangler and coworkers), who suggested that there is aglobal power law governing the amount of dust seen in debris disks as afunction of time. Our investigations lead us to conclude that (1) forstars at most ages, a large spread in fractional luminosity occurs, but(2) there are few very young stars with intermediate or small excesses;(3) the maximum excess seen in stars of a given age is aboutfd~10-3, independent of time; and (4) Vega-likeexcess is more common in young stars than in old stars.
| Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars
The maximum amplitude (Amax) of spot-induced brightnessvariations from long-term V-band photometry and the ratioLX/Lbol between X-ray and bolometric luminositiesare suitable indicators of the level of magnetic activity in thephotosphere and in the corona of late-type stars, respectively. By usingthese activity indicators we investigate the dependence of coronal X-rayemission on the level of photospheric starspot activity in a homogeneoussample of low mass main sequence field and cluster stars of differentages (IC 2602, IC 4665,IC 2391, alpha Persei,Pleiades and Hyades). First, theactivity-rotation connection at the photospheric level is re-analysed,as well as its dependence on spectral type and age. The upper envelopeof Amax increases monotonically with decreasing rotationalperiod (P) and Rossby number (R0) showing a break around 1.1d that separates two rotation regimes where the starspot activity showsdifferent behaviours. The Amax-P andAmax-R0 relations are fitted with linear,exponential and power laws to look for the function which bestrepresents the trend of the data. The highest values of Amaxare found among K-type stars and at the ages of alphaPersei and Pleiades. We also analyse theactivity-rotation connection at the coronal level as well as itsdependence on spectral type. The level of X-ray emission increases withincreasing rotation rate up to a saturation level. The rotational periodat which saturation occurs is colour-dependent and increases withadvancing spectral type. Also the LX/Lbol-P andLX/Lbol-R0 relations are fitted withlinear, exponential and power laws to look for the best fittingfunction. Among the fastest rotating stars (P<=0.3 d) there isevidence of super-saturation. Also the highest values ofLXLbol are found among K-type stars. Finally, thephotospheric-coronal activity connection is investigated by using forthe first time the largest ever sample of light curve amplitudes asindicators of the magnetic filling factor. The activity parametersLX/Lbol and Amax are found to becorrelated with each other, thus confirming the dependence of coronalactivity on photospheric magnetic fields. More precisely, theLX/Lbol-Amax distribution shows thepresence of an upper envelope, which is constant at theLX/Lbol =~ -3.0 saturation level, and of a lowerenvelope. The best fit to the lower envelope is given by a power lawwith steepness decreasing from F-G to M spectral types. However, it isconsidered a tentative result, since the fit reduced chi-squares arelarge. Such spectral-type dependence may be related to a colourdependence of Amax on the total starspot filling factor, aswell as to the coronal emission being possibly more sensitive tostarspot activity variations in F- and G-type than in M-type stars. TheLX/Lbol-Amax mean values for eachcluster in our sample decrease monotonically with increasing age,showing that the levels of photospheric and coronal activity evolve intime according to a single power law till the Sun's age.Tables of the photometric and X-ray data sets are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/671
| Companions to Young Stars
Brown dwarfs occupy the important region in the mass range between starsand planets. Their existence, ambiguous until only recently, and theirproperties give insight into stellar and planetary formation. We presentstatistical results of an infrared, coronagraphic survey of young,nearby stars that includes probable companions to three young G-typestars, Gl 503.2 (G2V), HD 102982 (G3V), and Gl 577 (G5V). The companionto Gl 577 is a possible binary brown dwarf, according to evolutionarymodels. A dynamical determination of the components' masses will beachievable in the near future and will provide an excellent test of thepredictive ability of the evolutionary models.
| 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.
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| Chromospheric activity, lithium and radial velocities of single late-type stars possible members of young moving groups
We present here high resolution echelle spectra taken during threeobserving runs of 14 single late-type stars identified in our previousstudies (Montes et al. \cite{M_etal01b}, hereafter Paper I) as possiblemembers of different young stellar kinematic groups (Local Association(20-150 Myr), Ursa Major group (300 Myr), Hyades supercluster (600 Myr),and IC 2391 supercluster (35 Myr)). Radial velocities have beendetermined by cross correlation with radial velocity standard stars andused together with precise measurements of proper motions and parallaxestaken from Hipparcos and Tycho-2 Catalogues, to calculate Galactic spacemotions (U, V, W) and to apply Eggen's kinematic criteria. Thechromospheric activity level of these stars have been analysed using theinformation provided for several optical spectroscopic features (fromthe Ca II H & K to Ca II IRT lines) that are formed at differentheights in the chromosphere. The Li I lambda 6707.8 Å lineequivalent width (EW) has been determined and compared in the EW(Li I)versus spectral type diagram with the EW(Li I) of stars members ofwell-known young open clusters of different ages, in order to obtain anage estimation. All these data allow us to analyse in more detail themembership of these stars in the different young stellar kinematicgroups. Using both, kinematic and spectroscopic criteria we haveconfirmed PW And, V368 Cep, V383 Lac, EP Eri, DX Leo, HD 77407, and EKDra as members of the Local Association and V834 Tau, pi 1UMa, and GJ 503.2 as members of the Ursa Major group. A clearrotation-activity dependence has been found in these stars. Based onobservations made with the 2.2 m telescope of the German-SpanishAstronomical Centre, Calar Alto (Almería, Spain), operated by theMax-Planck-Institute for Astronomy, Heidelberg, jointly with the SpanishNational Commission for Astronomy, with the Nordic Optical Telescope(NOT), operated on the island of La Palma jointly by Denmark, Finland,Iceland, Norway and Sweden, in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias, and withthe Isaac Newton Telescope (INT) operated on the island of La Palma bythe Isaac Newton Group in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias. Tables 1,4, and 5 are only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/379/976
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| Dusty Debris around Solar-Type Stars: Temporal Disk Evolution
Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to searchfor evidence of emission from dust orbiting young main-sequence stars,both in clusters and isolated systems. Over half of the detections arenew examples of dusty stellar systems and demonstrate that such dust canbe detected around numerous stars older than a few times 106yr. Fluxes at 60 and either 90 or 100 μm for the new excess sourcestogether with improved fluxes for a number of IRAS-identified sourcesare presented. Analysis of the excess luminosity relative to the stellarphotosphere shows a systematic decline of this excess with stellar ageconsistent with a power-law index of -2.
| Discovery of 14 Nearby Double Stars
We report the discovery of 14 double-star systems with main-sequenceprimaries, most of which are M type. Infrared imaging indicates that thesecondaries are most likely late-type dwarfs. The fact that most of thestars are within 25 pc underscores the incompleteness of our presentcensus of our nearest neighbors.
| A Search for the Damped Lyα Absorber at z=1.86 toward QSO 1244+3443 with NICMOS
We have carried out a high-resolution imaging search for the galaxyassociated with the damped Lyα absorber (DLA) at z=1.859 towardthe zem=2.48 quasar QSO 1244+3443, using the Hubble SpaceTelescope (HST) and the near-infrared camera and multiobjectspectrometer (NICMOS). Images were obtained in the broad filter F160Wand the narrow filter F187N with camera 2 on NICMOS with the goal ofdetecting the rest-frame optical continuum and the Hα lineemission from the DLA. After point-spread function (PSF) subtraction,two weak features are seen at projected separations of 0.16"-0.24" fromthe quasar. Parts of these features may be associated with the DLA,although we cannot completely rule out that they could be artifacts ofthe PSF. If associated with the DLA, the objects would be ~1-2h-170 kpc in size with integrated flux densitiesof 2.5 and 3.3 μJy in the F160W filter, implying luminosities atλcentral=5600 Å in the DLA rest frame of4.4-5.9×109 h-270Lsolar at z=1.86, for q0=0.5. However, nosignificant Hα line emission is seen from these objects,suggesting low star formation rates (SFRs). Our 3 σ upper limit onthe SFR in the DLA is 1.3 h-270 Msolar yr-1 for q0=0.5 (2.4 h-270 Msolaryr-1 for q0=0.1). This together with our earlier result for LBQS1210+1731 mark a significant improvement over previous constraints onthe star formation rates of DLAs. Dust within the DLA could extinguishHα emission, but this would require the dust content in the DLA tobe much higher than that inferred from previous DLA observations. Acombination of low star formation rate and some dust extinction islikely to be responsible for the lack of Hα emission.Alternatively, the objects, if real, may be associated with the hostgalaxy of the quasar rather than with the DLA. In any case, ourobservations suggest that the DLA is not a large bright protodisk, but acompact object or a low surface brightness galaxy. If the two featuresare PSF artifacts or associated with the quasar host, then theconstraints on the size and luminosity of the DLA are even more severe.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable 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/qcar?J/A+A/367/521
| The ``rotation-activity connection'': Its extension to photospheric activity diagnostics
In this paper we present the results of a different approach in thestudy of the so-called rotation-activity connection, which is a wellestablished correlation between rotation and magnetic activity atchromospheric and outer atmospheric levels. The present study concernsthe photospheric level and was carried out by using V-band photometriclight curve amplitudes as indicators of starspot coverage and ofmagnetic activity. A high degree of correlation between the envelope ofmaximum V-band light curve amplitudes and the rotation period is foundfor the active star members of young open clusters (IC 2602, IC 2391,Alpha Persei, Pleiades and Hyades), as well as for active field stars.This correlation shows a different behaviour in two different rotationperiod ranges. Moreover, some evidence of a possible activity``saturation'' is found among the most rapidly rotating stars of thesample. Additional correlations between photospheric and other magneticactivity indicators in the chromosphere, transition region and coronaare also investigated. The results presented here can be considered asan extension of the well established rotation-activity connection validfrom the corona, transition region and chromosphere, down to thephotosphere.
| Resolving Low-Mass Companions to G-type stars - Two Stars and a Binary Brown Dwarf
Several candidate companions were discovered in our coronagraphic surveyof nearby, young stars with the Near-Infrared Camera and Multi-ObjectSpectrometer (NICMOS) on the Hubble Space Telescope (HST). Discovery andcharacterization of low mass stellar and sub-stellar objects isfundamental to observational astrophysics, especially in related areassuch as stellar evolution, the stellar mass function, and the origins ofstellar and planetary systems. Substellar objects are predicted to bemore luminous when younger, and therefore easier to detect. We haverecently acquired follow-up spectroscopic observations with the SpaceTelescope Imaging Spectrograph (STIS) and astrometric observations usingadaptive optics on the Canada-French-Hawaii and Hale 200-inch Telescopeswhich provide supporting evidence that three of our candidates areindeed bona-fide companions. Based on evolutionary models, the propermotion companions to the G-type stars Gl 503.2 and HD 102982 are verylow-mass stars, and the companions to Gl 577 likely form a brown dwarfbinary. A dynamical determination of the mass of the close (sep ~ fewAU) binary components of the brown dwarf secondary (Gl 577 B/C) will bepossible within only the next few years. This work is supported in partby NASA grants NAG 5-4688 to UCLA and NAG 5-3042 to the University ofArizona Instrument Definition Team. This talk is based on observationsunder grant GO-8176.01-97A with the NASA/ESA Hubble Space Telescope atthe Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc. under NASAcontract NAS 5-26555.
| The 75th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 916 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| 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.
| Photometry of Late Dwarf Stars
Multicolor broadband photometry is presented for 146 stars. For many ofthe stars, a comparison of photometric and trigonometric parallaxes isdiscussed.
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| Takýmyýldýz: | Ejderha |
| Sað Açýklýk: | 15h05m49.90s |
| Yükselim: | +64°02'49.9" |
| Görünürdeki Parlaklýk: | 8.417 |
| Uzaklýk: | 44.267 parsek |
| özdevim Sað Açýklýk: | -123.3 |
| özdevim Yükselim: | 110.1 |
| B-T magnitude: | 9.215 |
| V-T magnitude: | 8.483 |
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