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Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| High-Resolution Spectroscopy of some Active Southern Stars
High-resolution échelle spectra of 42 nearby southern solar-typestars have been obtained, in a search for young, single, active, andrapidly rotating sun-like stars suitable for Doppler imaging and ZeemanDoppler imaging studies. As a result of this survey, 13 stars weredetermined to be youthful with ages less than 600Myr (Hyades age) andeight of these were found to have projected rotational velocities inexcess of 15kms-1. In addition, five spectroscopic binarysystems were identified. Of those stars observed for this survey, HD106506 is the most outstanding target for mapping active regions. It isan apparently young and single star with rapid rotation (v sin i~80kms-1), strong Hα chromospheric activity (logR'Hα~-4.2), and deformation of the spectral lineprofiles indicating the presence of large starspots.
| 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.
| Formation and Evolution of Planetary Systems: Upper Limits to the Gas Mass in HD 105
We report infrared spectroscopic observations of HD 105, a nearby (~40pc) and relatively young (~30 Myr) G0 star with excess infraredcontinuum emission, which has been modeled as arising from an opticallythin circumstellar dust disk with an inner hole of size >~13 AU. Wehave used the high spectral resolution mode of the Infrared Spectrometer(IRS) on the Spitzer Space Telescope to search for gas emission linesfrom the disk. The observations reported here provide upper limits tothe fluxes of H2 S(0) 28 μm, H2 S(1) 17 μm,H2 S(2) 12 μm, [Fe II] 26 μm, [Si II] 35 μm, and [SI] 25 μm infrared emission lines. The H2 line upper limitsplace direct constraints on the mass of warm molecular gas in the disk:M(H2)<4.6, 3.8×10-2, and3.0×10-3 MJ at T=50, 100, and 200 K,respectively. We also compare the line flux upper limits to predictionsfrom detailed thermal/chemical models of various gas distributions inthe disk. These comparisons indicate that if the gas distribution has aninner hole with radius ri,gas, the surface density at thatinner radius is limited to values ranging from <~3 g cm-2at ri,gas=0.5 AU to 0.1 g cm-2 atri,gas=5-20 AU. These values are considerably below the valuefor a minimum mass solar nebula, and suggest that less than 1 Jupitermass (MJ) of gas (at any temperature) exists in the 1-40 AUplanet-forming region. Therefore, it is unlikely that there issufficient gas for gas giant planet formation to occur in HD 105 at thistime.
| 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).
| 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.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| The Formation and Evolution of Planetary Systems: First Results from a Spitzer Legacy Science Program
We present 3-160 μm photometry obtained with the Infrared ArrayCamera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS)instruments for the first five targets from the Spitzer Space TelescopeLegacy Science Program ``Formation and Evolution of Planetary Systems''and 4-35 μm spectrophotometry obtained with the Infrared Spectrograph(IRS) for two sources. We discuss in detail our observations of thedebris disks surrounding HD 105 (G0 V, 30+/-10 Myr) and HD 150706 (G3 V,~700+/-300 Myr). For HD 105, possible interpretations include largebodies clearing the dust inside of 45 AU or a reservoir of gas capableof sculpting the dust distribution. The disk surrounding HD 150706 alsoexhibits evidence of a large inner hole in its dust distribution. Of thefour survey targets without previously detected IR excess, spanning ages30 Myr to 3 Gyr, the new detection of excess in just one system ofintermediate age suggests a variety of initial conditions or divergentevolutionary paths for debris disk systems orbiting solar-type stars.
| MARCS: Model Stellar Atmospheres and Their Application to the Photometric Calibration of the Spitzer Space Telescope Infrared Spectrograph (IRS)
We describe state-of-the-art MARCS-code model atmospheres generated fora group of A dwarf, G dwarf, and late-G to mid-K giant standard stars,selected to photometrically calibrate the Spitzer Space TelescopeInfrared Spectrograph (IRS) and compare the synthetic spectra toobservations of HR 6688, HR 6705, and HR 7891. The general calibrationprocesses and uncertainties are briefly described, and the differencesbetween various templated composite spectra of the standards areaddressed. In particular, a contrast between up-to-date modelatmospheres and previously published composite and synthetic spectra isillustrated for wavelength ranges around 8 μm (where the SiOΔv=1 band occurs for the cooler standards) and λ>=20μm, where the use of the Engelke function will lead to increasinglylarge discrepancies as a result of the neglect of gravity in cool stars.At this point, radiometric requirements are being met, absolute fluxcalibration uncertainties (1 σ) are ~20% in the Short-High andLong-High data and ~15% in the Short-Low and Long Low data, andorder-to-order flux uncertainties are ~10% or less. Iteration betweenthe MARCS model atmosphere inputs and the data processing will improvethe S/N ratios and calibration accuracies.
| Fire and Ice: Spitzer Infrared Spectrograph (IRS) Mid-Infrared Spectroscopy of IRAS F00183-7111
We report the detection of strong absorption and weak emission featuresin the 4-27 μm Spitzer Infrared Spectrograph (IRS) spectrum of thedistant ultraluminous infrared galaxy IRAS F00183-7111 (z=0.327). Theabsorption features of CO2 and CO gas, water ice,hydrocarbons, and silicates are indicative of a strongly obscured(A9.6>=5.4 AV>=90) and complex line of sightthrough both the hot diffuse interstellar medium and shielded coldmolecular clouds toward the nuclear power source. From the profile ofthe 4.67 μm CO fundamental vibration mode, we deduce that theabsorbing gas is dense (n~106 cm-3) and warm (720K) and has a CO column density of ~1019.5 cm-2,equivalent to NH~1023.5 cm-2. The hightemperature and density, as well as the small inferred size (<0.03pc), locates this absorbing gas close to the power source of thisregion. Weak emission features of molecular hydrogen, polycyclicaromatic hydrocarbons (PAHs), and Ne+, likely associated withstar formation, are detected against the 9.7 μm silicate feature,indicating an origin away from the absorbing region. Based on the 11.2μm PAH flux, we estimate the star formation component to beresponsible for up to 30% of the IR luminosity of the system. While ourmid-infrared spectrum shows no telltale signs of active galactic nucleus(AGN) activity, the similarities to the mid-infrared spectra of deeplyobscured sources (e.g., NGC 4418) and AGN hot dust (e.g., NGC 1068), aswell as evidence from other wavelength regions, suggest that the powersource hiding behind the optically thick dust screen may well be aburied AGN.
| 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.
| On Ca II Emission as an Indicator of the Age of Young Stars
Chromospheric emission in the Ca II H and K lines has often been used asan age diagnostic for solar mass stars. For 20 such stars with ages lessthan a few hundred megayears, we compare Ca II ages derived by Wright etal. with ages we derive based on a combination of lithium abundance,X-ray activity, and Galactic space motion. Typically, the Ca II ages arenoticeably older than the lithium/X-ray ages, suggesting that arecalibration of the Ca II ages may be necessary.
| Constraining the Lifetime of Circumstellar Disks in the Terrestrial Planet Zone: A Mid-Infrared Survey of the 30 Myr old Tucana-Horologium Association
We have conducted an N-band survey of 14 young stars in the ~30 Myr oldTucana-Horologium association to search for evidence of warm,circumstellar dust disks. Using the MIRAC-BLINC camera on the Magellan I(Baade) 6.5 m telescope, we find that none of the stars have astatistically significant N-band excess compared to the predictedstellar photospheric flux. Using three different sets of assumptions,this null result rules out the existence of the following around thesepost-T Tauri stars: (1) optically thick disks with inner hole radii of<~0.1 AU, (2) optically thin disks with masses of less than10-6 M⊕ (in ~1 μm sized grains) within<~10 AU of these stars, and (3) scaled-up analogs of the solar systemzodiacal dust cloud with more than 4000 times the emitting area. Oursurvey was sensitive to dust disks in the terrestrial planet zone withfractional luminosity oflog(Ldust/L*)~10-2.9, yet none werefound. Combined with results from previous surveys, these data suggestthat circumstellar dust disks become so optically thin as to beundetectable at N band before age ~20 Myr. We also present N-bandphotometry for several members of other young associations and asubsample of targets that will be observed with the Spitzer SpaceTelescope by the Formation and Evolution of Planetary Systems LegacyScience Program. Finally, we present an absolute calibration ofMIRAC-BLINC for four filters (L, N, 11.6, and Qs) on theCohen-Walker-Witteborn system.
| Dusty Debris Disks as Signposts of Planets: Implications for Spitzer Space Telescope
Submillimeter and near-infrared images of cool dusty debris disks andrings suggest the existence of unseen planets. At dusty but nonimagedstars, semimajor axes of associated planets can be estimated from thedust temperature. For some young stars these semimajor axes are greaterthan 1" as seen from Earth. Such stars are excellent targets forsensitive near-infrared imaging searches for warm planets. To probe thefull extent of the dust and hence of potential planetary orbits, Spitzerobservations should include measurements with the 160 μm filter.
| 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
| 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.
| Nearby young stars
We present the results of an extensive all-sky survey of nearby stars ofspectral type F8 or later in a systematic search of young (zero-age mainsequence) objects. Our sample has been derived by cross-correlating theROSAT All-Sky Survey and the TYCHO catalogue, yielding a total of 754candidates distributed more or less randomly over the sky. Follow-upspectroscopy of these candidate objects has been performed on 748 ofthem. We have discovered a tight kinematic group of ten stars withextremely high lithium equivalent widths that are presumably youngerthan the Pleiades, but again distributed rather uniformly over the sky.Furthermore, about 43 per cent of our candidates have detectable levelsof lithium, thus indicating that these are relatively young objects withages not significantly above the Pleiades age.Based on observations collected at the European Southern Observatory,Chile (ESO No. 62.I-0650, 66.D-0159(A), 67.D-0236(A)).
| Fast-rotating nearby solar-type stars sin i and X-ray luminosities relationships. II. Li abundances, v sin i and X-ray luminosities relationships
We present an analysis of our high-resolution spectroscopic andhigh-precision UBV(RI)_c photometric observations of a sample of 110nearby late-F and G-type stars selected for their large rotationalvelocity. The relationships between Li abundance, X-ray luminosity, andvsin i are investigated. We find that, as expected, the stars in oursample show statistically higher Li abundance and activity level thanfield star samples with similar characteristics, but slower rotation.Surprisingly, however, we also find four rapidly-rotating singlemain-sequence stars with very low Li abundance. For both single andbinary stars we find a large spread of Li abundance for stars withrotation lower than about 18 km s-1. The well-establishedcorrelation between X-ray luminosity and rotation rate is clearlyobserved. All single unevolved solar type stars with vsin i larger than18 km s-1 are strong X-ray emitters and have high Liabundance. Finally, we find also five evolved stars with very low Liabundance that are still rather fast rotators. The results from oursample confirm the presence of young very active stars close to the Sun,in agreement with recent findings from EUV and X-ray surveys, althoughour sample does not show such extreme characteristics as those selectedfrom EUV and X-ray surveys at the current flux limits.Based on data collected at the European Southern Observatory, La Silla,Chile.Tables, Figures and the complete data set are 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/397/987
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
| 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.
| The Dispersal of Young Stars and the Greater Sco-Cen Association
We review topics related to the dispersal of young stars from theirbirth-sites, and focus in particular on the entourage of young starsrelated to the ongoing star-formation event in the Sco-Cen OBassociation. We conduct a follow-up kinematic study to that presented inMamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby,isolated, young stars. In addition to the eta Cha and TW Hya groups, wefind several more intriguing Sco-Cen outlier candidates: most notablyβ Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connectionbetween Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds,and in particular, Corona Australis. The kinematic evidence suggeststhat many of the nearby, isolated ~10 Myr-old stars were born nearSco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesizethat these stars inherited 5-10 km/s velocities moving away fromSco-Cen, either through molecular cloud turbulence, or through formationin molecular clouds associated with the expanding Sco-Cen superbubbles(e.g. Loop I).
| Detection of moving clusters by a method of cinematic pairs.
Not Available
| XID: Cross-Association of ROSAT/Bright Source Catalog X-Ray Sources with USNO A-2 Optical Point Sources
We quantitatively cross-associate the 18,811 ROSAT Bright Source Catalog(RASS/BSC) X-ray sources with optical sources in the USNO A-2 catalog,calculating the probability of unique association (Pid)between each candidate within 75" of the X-ray source position, on thebasis of optical magnitude and proximity. We present catalogs ofRASS/BSC sources for which Pid>98%, Pid>90%,and Pid>50%, which contain 2705, 5492, and 11,301 uniqueUSNO A-2 optical counterparts respectively down to the stated level ofsignificance. Together with identifications of objects not cataloged inUSNO A-2 due to their high surface brightness (M31, M32, ...) andoptical pairs, we produced a total of 11,803 associations to aprobability of Pid>50%. We include in this catalog a listof objects in the SIMBAD database within 10" of the USNO A-2 position,as an aid to identification and source classification. This is the firstRASS/BSC counterpart catalog which provides a probability of associationbetween each X-ray source and counterpart, quantifying the certainty ofeach individual association. The catalog is more useful than previouscatalogs which either rely on plausibility arguments for association ordo not aid in selecting a counterpart between multiple off-band sourcesin the field. Sources of high probability of association can beseparated out, to produce high-quality lists of classes (Seyfert 1/2s,QSOs, RS CVns) desired for targeted study, or for discovering newexamples of known classes (or new classes altogether) through thespectroscopic classification of securely identified but unclassifiedUSNO A-2 counterparts. Low Pid associations can be used forstatistical studies and follow-on investigation-for example, performingfollow-up spectroscopy of the many low-mass stars to search forsignatures of coronal emission, or to investigate the relationshipbetween X-ray emission and classes of sources not previouslywell-studied for their X-ray emissions (such as pulsating variablestars). We find that a fraction ~65.8% of RASS/BSC sources have anidentifiable optical counterpart, down to the magnitude limit of theUSNO A-2 catalog which could be identified by their spatial proximityand high optical brightness.
| Metallicity effects on the chromospheric activity-age relation for late-type dwarfs
We show that there is a relationship between the age excess, defined asthe difference between the stellar isochrone and chromospheric ages, andthe metallicity as measured by the index [Fe/H] for late-type dwarfs.The chromospheric age tends to be lower than the isochrone age formetal-poor stars, and the opposite occurs for metal-rich objects. Wesuggest that this could be an effect of neglecting the metallicitydependence of the calibrated chromospheric emission-age relation. Wepropose a correction to account for this dependence. We also investigatethe metallicity distributions of these stars, and show that there aredistinct trends according to the chromospheric activity level. Inactivestars have a metallicity distribution which resembles the metallicitydistribution of solar neighbourhood stars, while active stars appear tobe concentrated in an activity strip on the logR'_HKx[Fe/H] diagram. Weprovide some explanations for these trends, and show that thechromospheric emission-age relation probably has different slopes on thetwo sides of the Vaughan-Preston gap.
| The Problem of HIPPARCOS Distances to Open Clusters. II. Constraints from Nearby Field Stars
This paper examines the discrepancy between distances to nearby openclusters as determined by parallaxes from Hipparcos compared totraditional main-sequence fitting. The biggest difference is seen forthe Pleiades, and our hypothesis is that if the Hipparcos distance tothe Pleiades is correct, then similar subluminous zero-age main-sequence(ZAMS) stars should exist elsewhere, including in the immediate solarneighborhood. We examine a color-magnitude diagram of very young andnearby solar-type stars and show that none of them lie below thetraditional ZAMS, despite the fact that the Hipparcos Pleiades parallaxwould place its members 0.3 mag below that ZAMS. We also presentanalyses and observations of solar-type stars that do lie below theZAMS, and we show that they are subluminous because of low metallicityand that they have the kinematics of old stars.
| Evolutionary Oddities in Old Disk Population Clusters
With a luminosity zero point fixed by the kinematics of old disksuperclusters (HR 1614, t = 6 Gyr, [Fe/H] = +0.1 dex) and groups(Arcturus, t = 14 Gyr, [Fe/H] = -0.65 dex), the luminosities and colorsof evolved old disk stars, especially red horizontal branch (RHB), earlyasymptotic branch [AGB(1)], thermally pulsing asymptotic giant branch[AGB(2)], and sdOB stars in old disk clusters (NGC 6791, 47 Tuc, M71,M67, Mel 66, NGC 2420, NGC 2204, and NGC 2443) are discussed. (1) TheRHB stars in the old disk all have M_V = +0.7 +/- 0.1 (M_K = -1.3 +/-0.1) mag. (2) Large-amplitude red variables (LARVs) with quasi-stableperiods and light curves are old disk stars on AGB(2). (3) AGB(1)objects include CH stars and semiregular (SRa) variables. (4) Thepopulous and overabundant cluster NGC 6791 may be the only disk clusterwith sdOB stars, populating the lower portion of the bifurcated extendedhorizontal branch that is usual in most ``blue tailed'' and high-densityhalo clusters. (5) Post-red giant branch (RGB) stars in old diskclusters show a B - V (b - y) defect when compared with RGB stars,possibly because of a change in the character of the atmospheres. (6) Ifthe bulk of the LARVs are pulsating in the fundamental mode, R Vir (P =145 days) is possibly a first-overtone pulsator. (7) The overabundantold disk clusters are within the solar circle, with Liller 1 being atthe Galactic center. (8) Several probable RHB stars at the southGalactic pole are identified. (9) The period-age relation, combined withthe known spatial distribution of Galactic LARVs, leads to a relationbetween age and scale height of distribution that monotonicallyincreases with age, leaving no obvious reason for a bifurcation of thepopulation.
| F, G and K stars in the ROSAT all-sky survey. I. Photometry
We present accurate BV(RI)_c photometry for a sample of F, G and K starsdetected in selected areas of the ROSAT all-sky survey (RASS). We haveused the photometry, in addition to low-resolution spectroscopy, toestimate spectral classifications, distances and X-ray luminosities. Thelog (L_X / L_V) in the sample lies below -2. Although the samplecontains also nearby, inactive stars, it is dominated by active objects.The median X-ray luminosity in our sample is < L_X > = 29.88 andthe mean value of the hardness ratios = 0.13 +/- 0.35. Wecompare the derived X-ray luminosity function with similar functionsobtained from the serendipitous samples of the Einstein Observatorymedium sensitivity survey (EMSS) and EXOSAT. Our sample is completelyconsistent with the EMSS sample of solar type stars, indicating thatboth our sources and the EMSS sources are representative of the highgalactic latitude X-ray stellar population. We do not find extremelyactive stars (log (L_X) >= 32), as are found in the EMSS sample, andwe argue that these objects are rare. Table 3 is also available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| Consistency of the metallicity distributions of nearby F, G and K dwarfs
The consistency of the metallicity distributions of F, G and K dwarfs isstudied. We present a new metallicity distribution for K dwarfs usingmetallicities determined from uvby photometry. There is a remarkablepaucity of metal-poor K dwarfs in analogy with the G dwarf problem. Weshow that late-type dwarfs have consistent metallicity distributions. Wealso propose preliminary corrections to these distributions to take intoaccount the contamination of the uvby indices due to the chromosphericactivity in these stars, since around 30% of the nearby late-type dwarfshave active chromospheres. We consider the possibility that themetallicity distribution of cooler stars may be different from that ofthe hotter stars due to (i) metal-enhanced star formation and (ii) ametallicity bias in the catalogue of nearby stars. We conclude thatthese hypotheses are unlikely to produce important differences in themetallicity distributions of late-type dwarfs.
| The evolutionary status of activity-selected solar-type stars and of T Tauri stars as derived from HIPPARCOS parallaxes: evidence for long-lived T Tauri disks?
We have used the Hipparcos parallaxes to study the evolutionary statusof a sample of stars with spectral types from late F to M0 (hereafter``solar-type stars''), selected on the basis of their activity, mainlyfrom Einstein-based surveys. The parallaxes have been used to place theobjects in the H-R diagram, determining their age by comparison withtheoretical evolutionary tracks and observational main sequences. Thisage is compared with age estimates derived from the lithium abundance,the activity level and the presence of circumstellar disks. Tocomplement our sample at the young end we have also studied theHipparcos-determined distances of a sample of optically-selectedpre-main sequence stars, mostly classical T Tauri stars (CTTS). SomeCTTS appear to be much nearer to us than previously determined, and faraway from their putative parent cloud. This implies a significantlylarger age providing observational evidence for the existence oflong-lived T Tauri disks which could produce slow rotators on theZero-Age Main Sequence (ZAMS). None of the above-mentioned age proxiesappears to reliably and unambiguously select very young stars in therange of spectral types considered here, with some apparently very youngobjects effectively lying onto or very close to the main sequence. Theattribution of ages to young solar-type stars on the basis of any of thestandard proxies may thus significantly under- or over-estimate theevolutionary age of the object. Caution must therefore be exercised whenattributing ages to individual stars, and claims about the large numberof PMS stars found in X-ray based surveys may need to be at least inpart reconsidered in this light. Based on data from the ESA Hipparcossatellite.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | עוף החול |
| התרוממות ימנית: | 00h05m52.55s |
| סירוב: | -41°45'11.0" |
| גודל גלוי: | 7.518 |
| מרחק: | 40.241 פארסק |
| תנועה נכונה: | 98.5 |
| תנועה נכונה: | -75.8 |
| B-T magnitude: | 8.226 |
| V-T magnitude: | 7.577 |
קטלוגים וכינוים:
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