Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
The lithium-rotation correlation for WTTS in Taurus–Auriga
Surface lithium abundance and rotation velocity can serve as powerfuland mutually complementary diagnostics of interior structure of stars.So far, the processes responsible for the lithium depletion duringpre-main sequence evolution are still poorly understood. We investigatewhether a correlation exists between equivalent widths of Li (EW(Li))and rotation period (Prot) for weak-line T Tauri stars(WTTSs). We find that rapidly rotating stars have lower EW(Li) and thefast burning of Li begins at the phase when star’s Protevolves towards 3 days among 0.9Mȯ to1.4Mȯ WTTSs in Taurus–Auriga. Our results supportthe conclusion by Piau and Turch-Chiéze about a model for lithiumdepletion with age of the star and by Bouvier et al. in relation torotation evolution. The turn over of the curve for the correlationbetween EW(Li) and Prot is at the phase of zero-age mainsequence (ZAMS). The EW(Li) decreases with decreasing Protbefore the star reaches the ZAMS, while it decreases with increasingProt (decreasing rotation velocity) for young low-mass mainsequence stars. This result could be explained as an age effect of Lidepletion and the rapid rotation does not inhibit Li destruction amonglow-mass PMS stars.
| 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.
| Metallicity and absolute magnitude calibrations for UBV photometry
Calibrations are presented here for metallicity ([Fe/H]) in terms of theultraviolet excess, [δ(U - B) at B - V = 0.6, hereafterδ0.6], and also for the absolute visual magnitude(MV) and its difference with respect to the Hyades(ΔMHV) in terms of δ0.6 and(B - V), making use of high-resolution spectroscopic abundances from theliterature and Hipparcos parallaxes. The relation[Fe/H]-δ0.6 has been derived for dwarf plus turn-offstars, and also for dwarf, turn-off, plus subgiant stars classifiedusing the MV-(B - V)0 plane of Fig. 11, which iscalibrated with isochrones from Bergbusch & VandenBerg (and alsoVandenBerg & Clem). The [Fe/H]-δ0.6 relations inour equations (5) and (6) agree well with those of Carney, as can beseen from Fig. 5(a). Within the uncertainties, the zero-points,+0.13(+/-0.05) of equation (5) and +0.13(+/-0.04) of equation (6), arein good agreement with the photometric ones of Cameron and of Carney,and close to the spectroscopic ones of Cayrel et al. and of Boesgaard& Friel for the Hyades open cluster. Good quantitative agreementbetween our estimated [Fe/H] abundances with those from uvby-βphotometry and spectroscopic [Fe/H]spec values demonstratesthat our equation (6) can be used in deriving quality photometric metalabundances for field stars and clusters using UBV data from variousphotometric surveys.For dwarf and turn-off stars, a new hybrid MV calibration ispresented, based on Hipparcos parallaxes withσπ/π <= 0.1 and with a dispersion of +/-0.24in MV. This hybrid MV calibration containsδ0.6 and (B - V) terms, plus higher order cross-termsof these, and is valid for the ranges of +0.37 <= (B - V)0<= +0.88,- 0.10 <= δ0.6 <= +0.29 and 3.44<= MV <= 7.23. For dwarf and turn-off stars, therelation for ΔMHV is revised and updated interms of (B - V) and δ0.6, for the ranges of -0.10<= δ0.6 <= +0.29, and +0.49 <= (B -V)0 <= +0.89, again making use of Hipparcos parallaxeswith σπ/π <= 0.1. These parallaxes formetal-poor dwarf and turn-off stars in our sample reveal that thedifference of ΔMHV(B - V) relative to Hyadesat (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given byLaird et al. In general, Hipparcos parallaxes are larger thanground-based ones, causing a divergence of ourΔMHV(B - V,δ0.6) relation(the solid line in Fig. 15b), from the one of Laird et al. (the dashedline) for the range +0.10 <= δ0.6 <= +0.29 ourabsolute magnitudes are fainter, as has been confirmed for localsubdwarfs by Reid. Our final calibrations forΔMHV(B - V, δ0.6),equations (16) and (17), are third-order polynomials inδ0.6, pass through the origin, and provide photometricdistances in reasonable agreement with those obtained directly fromHipparcos parallaxes (Fig. 18).
| Nuclear Spectra of Comet 162P/Siding Spring (2004 TU12)
We present visible and near-IR spectra of the nucleus of comet162P/Siding Spring (also known as 2004 TU12) obtained in 2004 December,while it had no detectable coma. This is the third object observed tohave intermittent cometary activity even when relatively close to theSun. The spectra show no strong features in this wavelength range. Thispaucity of deep absorptions is common among low-albedo asteroids and thefew comet nuclei observed in this spectral region. Marginal spectralstructure is observed in the visible spectrum, and beyond 2 μm theflux from the nucleus is dominated by thermal emission. We compare thespectrum of 162P with published spectra of other comet nuclei, primitiveasteroids, and meteorites. Comet nuclei display a range of spectralshapes and slopes not unlike those observed among outer main-beltasteroids but closest to Trojan asteroids. No suitable spectral matchesto comet 162P were found among primitive (chondritic) meteorites. Wemodeled our visible and near-IR spectra using the scattering theorydescribed by Shkuratov et al. (1999), and our approach is similar tothat used by Emery and Brown for modeling Trojan asteroids. Our bestfits to the spectral shape and albedo include mixtures containingamorphous carbons, organics, and silicates. The absence of strongspectral features prevents the identification of specific minerals, andthe resulting model compositions are not unique. The observations beyond2 μm are interpreted in a companion publication by Fernándezand coworkers.
| The Asiago Database of Spectroscopic Databases (ADSD)
Databases of observed stellar spectra are continuously being publishedand made publicly available, and the average number of stars perdatabase is increasing. This paper reviews the current status. TheAsiago Database of Spectroscopic Databases (ADSD) aims to provide acensus of publicly available libraries of observed stellar spectra, todocument their content and to homogenize their parameters for easierconsultation and access. Refereed journals, conference proceedings andpersonal web pages have been searched for libraries of a given minimumsize, properly documented and with data made publicly and directlyaccessible. A total of 294 databases (54 ultraviolet, 183 optical, 50infrared and 7 combined) have been found to match the selection criteriaand have been included in ADSD. They provide spectra of 16046 differentstars in electronic or printed formats. A card for each librarydescribes in a homogeneous way its aims, content, type of data, caveats,data download links, source paper, properties of included stars andmore. A dedicated web page allows direct access to ADSD, plans futureupdates, and provides interrogation tools to search all the librariesmatching given characteristics or including any given star.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| Oxygen from the λ7774 High-Excitation Triplet in Open Cluster Dwarfs: Hyades
Oxygen abundances have been derived from the near-IR, high-excitation OI λ7774 triplet in high-resolution, high signal-to-noise ratiospectra of 45 Hyades dwarfs using standard one-dimensional,plane-parallel LTE models. Effective temperatures of the stellar samplerange from 4319 to 6301 K, and the derived relative O abundances as afunction of Teff evince a trichotomous morphology. AtTeff>6100 K, there is evidence of an increase in the Oabundances with increasing Teff, consistent with non-LTE(NLTE) predictions. At intermediate Teff (5450K<=Teff<=6100 K), the O abundances are flat, andstar-to-star values are in good agreement, having a mean value of[O/H]=+0.25+/-0.02 however, systematic errors at the <~0.10 dex levelmight exist. The O abundances for stars with Teff<=5450 Kshow a striking increase with decreasing Teff, in starkcontrast to expectations and canonical NLTE calculations. The coolHyades triplet results are compared to those recently reported fordwarfs in the Pleiades cluster and the UMa moving group; qualitativedifferences between the trends observed in these stellar aggregatespoint to a possible age-related diminution of triplet abundance trendsin cool open cluster dwarfs. Correlations with age-related phenomena,i.e., chromospheric activity and photospheric spots, faculae, and/orplages, are investigated. No correlation with Ca II H+K chromosphericactivity indicators is observed. Multicomponent LTE ``toy'' models havebeen constructed in order to simulate photospheric temperatureinhomogeneities that could arise from the presence of starspots, and wedemonstrate that photospheric spots are a plausible source of thetriplet trends among the cool dwarfs.Based on observations obtained with the Mayall 4 m telescope at KittPeak National Observatory, a division of the National Optical AstronomyObservatory, which is operated by the Association of Universities forResearch in Astronomy, Inc., under cooperative agreement with theNational Science Foundation.This paper includes data taken with the Harlan J. Smith 2.7 m telescopeat the McDonald Observatory of the University of Texas at Austin.
| Chemical Homogeneity in the Hyades
We present an abundance analysis of the heavy elements Zr, Ba, La, Ce,and Nd for Hyades F-K dwarfs based on high-resolution, highsignal-to-noise ratio spectra from Keck HIRES. The derived abundancesshow the stellar members to be highly uniform, although some elementsshow a small residual trend with temperature. The rms scatter for eachelement for the cluster members is as follows: Zr=0.055, Ba=0.049,Ce=0.025, La=0.025, and Nd=0.032 dex. This is consistent with themeasurement errors and implies that there is little or no intrinsicscatter among the Hyades members. Several stars thought to be nonmembersof the cluster based on their kinematics are found to deviate from thecluster mean abundances by about 2 σ. Establishing chemicalhomogeneity in open clusters is the primary requirement for theviability of chemically tagging Galactic disk stars to common formationsites in order to unravel the dissipative history of early diskformation.
| Zero-Age Main Sequence in the HR Diagram of the Vilnius Photometric System
The zero-age main sequence for solar metallicity stars in the absolutemagnitude vs. color diagram of the Vilnius seven-color photometricsystem is determined. The calibration is based on the results ofphotoelectric photometry of stars in the Hyades, Pleiades and Praesepeopen clusters and the Ori OB1 association. A theoretical Victoria-Reginaisochrone, corresponding to an age of 10 million years, coincides wellwith the lower envelope of the unevolved main sequence.
| Search for surface variations on TNO 47171 and Centaur 32532
We present photometric and spectroscopic observations of oneTrans-Neptunian Object (TNO 47171 1999 TC{36}) and one Centaur (Thereusalso named 32532 2001 PT{13}). Near-infrared data were acquired with theISAAC instrument at one of the 8 m telecopes of the Very Large Telescope(VLT, ESO-Cerro Paranal, Chile), while visible data were obtained withthe EFOSC2 instrument on the 3.6 m telescope of La Silla (ESO, Chile).These observations were performed to search for rotational variationsfor both targets. Water ice has been confirmed on both objects. Thesurface composition models of the targets are presented and discussed,and are also compared to previous observations available in theliterature.
| Low-mass companions to Hyades stars
It is now well established that a large fraction of the low-mass starsare binaries or higher order multiples. Similarly a sizable fractionhave giant planets. In contrast to these, the situation for brown dwarfcompanions is complicated: While close systems seem to be extremelyrare, wide systems are possibly more common. In this paper, we presentnew results on a survey for low-mass companions in the Hyades. Aftermeasuring precisely the radial velocity of 98 Hyades dwarf stars for 5years, we have selected all stars that show low-amplitude long-periodtrends. With AO-observations of these 14 stars we found companioncandidates around nine of them, where one star has two companions. Thetwo companions of HIP 16548 have masses between 0.07 to 0.08Mȯ, and are thus either brown dwarfs or very low massstars. In the case of HAN 172 we found a companion with a mass between0.08 to 0.10 Mȯ, which is again between a star and abrown dwarf. The other seven stars all have stellar companions. In twoadditional cases, the RV-variations are presumably caused by stellaractivity, and in another case the companion could be a short-periodbinary. The images of the remaining two stars are slightly elongated,which might imply that even these are binaries. Because at least 12 ofthe 14 stars showing low-amplitude RV trends turn out to have companionswith a mass ≥ 70 M_Jupiter, or are just active, we finally estimatethe number of companions with masses between 10 M_Jupiter and 70M_Jupiter within 8 AU of the host stars in the Hyades as ≤2%.
| Is Sedna another Triton?
90377 Sedna is, so far, the largest and most distant trans-neptunianobject. It was observed at visible and near-infrared wavelengths usingsimultaneously two 8.2 m telescopes at the Very Large Telescope of theEuropean Southern Observatory. The spectrum of Sedna suggests thepresence on its surface of different ices (total abundance >50%). Itssurface composition is different from that determined for othertrans-neptunian objects, and apparently resembles that of Triton,particularly in terms of the possible presence of nitrogen and methaneices.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| Spectroscopic investigation of near-Earth objects at Telescopio Nazionale Galileo
In this paper we present new results obtained from our spectroscopicsurvey of near-Earth objects (called SINEO). We show a set of 36 visibleand near-infrared spectra, recorded with the 3.5-m Italian TelescopioNazionale Galileo at La Palma (Canary Island). We discuss theirtaxonomic classification (resulting in 25 objects belonging to theS-complex, five to the C-complex and six to the X-complex), and theiroverall compositional linkage with the principal source of near-Earthobjects, namely the Main Belt. Moreover, for some near-Earth objects wefound good spectral fit among meteorites. In particular, we achieved anexcellent fit for chondrites of different clans. Finally, we discuss theinfluences of space weathering among small S-type near-Earth objects.
| Analysis of near-IR spectra of 1 Ceres and 4 Vesta, targets of the Dawn mission
We obtained high signal to noise spectra of the two targets of the Dawnmission, 4 Vesta and 1 Ceres from observations carried out in remotecontrol between the Observatoire de Paris-Meudon and the NASA InfraredTelescope Facility on Mauna Kea. 4 Vesta was observed in the 0.7-2.5μm spectral region at three different rotational phases in order toi) determine the mineral composition; ii) understand the spectralvariations across the surface. Vesta was also observed in the 2.0-3.8μm range. The 3 μm absorption feature was not detected, implyingthe absence of OH and/or H2O-bearing minerals on the asteroid surface atthe latitude of our observations. The spectrum of 1 Ceres was obtainedin the 2.0-4.1 μm range and the presence of the 3.06 μm absorptionfeature confirmed. Laboratory measurement of ion-irradiated organics andices suggest that the 3.06 μm feature can be reproduced with a linearmixture of crystalyne ice and residues of ion-irradiated asphaltite.
| Fine structure of the chromospheric activity in Solar-type stars - The Hα line
A calibration of Hα as both a chromospheric diagnostic and an ageindicator is presented, complementing the works previously done on thissubject (\cite{herbig}; \cite{luca1}). The chromospheric diagnostic wasbuilt with a statistically significant sample, covering nine years ofobservations, and including 175 solar neighborhood stars. Regarding theage indicator, the presence of stars for which very accurate ages aredetermined, such as those belonging to clusters and kinematic groups,lends confidence to our analysis. We also investigate the possibilitythat stars of the same age might have gone through different tracks ofchromospheric decay, identifying - within the same age range - effectsof metallicity and mass. These parameters, however, as well as age, seemto be significant only for dwarf stars, losing their meaning when weanalyze stars in the subgiant branch. This result suggests that, inthese evolved stars, the emission mechanism cannot bemagnetohydrodynamical in nature, in agreement with recent models (Fawzyet al. 2002c, and references therein). The Sun is found to be a typicalstar in its Hα chromospheric flux, for its age, mass andmetallicity. As a byproduct of this work, we developed an automaticmethod to determine temperatures from the wings of Hα, which meansthe suppression of the error inherent to the visual procedure used inthe literature.Based on observations collected at Observatório do Pico dos Dias,operated by the Laboratório Nacional de Astrofísica, CNPq,Brazil.Table 5 is only available in electronic form at thehttp://www.edpsciences.org
| A new Böhm-Vitense gap in the temperature range 5560 to 5610 K in the main sequence hm-Vitense gap in the main sequence
Highly precise temperatures (σ = 10-15 K) have been determinedfrom line depth ratios for a set of 248 F-K field dwarfs of about solarmetallicity (-0.5 < [Fe/H] < +0.4), based on high resolution (R=42000), high S/N echelle spectra. A new gap has been discovered in thedistribution of stars on the Main Sequence in the temperature range 5560to 5610 K. This gap coincides with a jump in the microturbulent velocityVt and the well-known Li depression near 5600 K in fielddwarfs and open clusters. As the principal cause of the observeddiscontinuities in stellar properties we propose the penetration of theconvective zone into the inner layers of stars slightly less massivethan the Sun and related to it, a change in the temperature gradient.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute-Provence (France).Full Table 1 is only available in electronic form athttp://www.edpsciences.org
| Observed spectral properties of near-Earth objects: results for population distribution, source regions, and space weathering processes
We present new visible and near-infrared spectroscopic measurements for252 near-Earth (NEO) and Mars-crossing (MC) objects observed from 1994through 2002 as a complement to the Small Main-Belt AsteroidSpectroscopic Survey (SMASS, http://smass.mit.edu/). Combined withpreviously published SMASS results, we have an internally consistentdata set of more than 400 of these objects for investigating trendsrelated to size, orbits, and dynamical history. These data also providethe basis for producing a bias-corrected estimate for the total NEOpopulation (Stuart and Binzel, 2004, Icarus 170, 295-311). We find 25 ofthe 26 Bus (1999, PhD thesis) taxonomic types are represented, withnearly 90% of the objects falling within the broad S-, Q-, X-, andC-complexes. Rare A- and E-types are more common in the MC than NEOpopulation (about 5% compared to <1%) and may be direct evidence ofslow diffusion into MC orbits from the Flora and Hungaria regions,respectively. A possible family of MC objects (C-types) may reside atthe edge of the 5:2 jovian resonance. Distinct signatures are revealedfor the relative contributions of different taxonomic types to the NEOpopulation through different source regions. E-types show an originsignature from the inner belt, C-types from the mid to outer belt, andP-types from the outer belt. S- and Q-types have effectively identicalmain-belt source region profiles, as would be expected if they haverelated origins. A lack of V-types among Mars-crossers suggests entryinto NEO space via rapid transport through the ν6 and 3:1 resonancesfrom low eccentricity main-belt orbits, consistent with a Vesta origin.D-types show the strongest signature from Jupiter family comets (JFC),with a strong JFC component also seen among the X-types. A distincttaxonomic difference is found with respect to the jovian Tisserandparameter T, where C-, D-, and X-type (most likely low albedo P-class)objects predominate for T<=3. These objects, which may be extinctcomets, comprise 4% of our observed sample, but their low albedos makesthis magnitude limited fraction under-representative of the true value.With our taxonomy statistics providing a strong component to thediameter limited bias correction analysis of Stuart (2003, PhD thesis),we estimate 10-18% of the NEO population above any given diameter may beextinct comets, taking into account asteroids scattered into T<3orbits and comets scattered into T>3 orbits. In terms of possible spaceweathering effects, we see a size-dependent transition from ordinarychondrite-like (Q-type) objects to S-type asteroids over the size rangeof 0.1 to 5 km, where the transition is effectively complete at 5 km. Amatch between the average surface age of 5 km asteroids and the rate ofspace weathering could constrain models for both processes. However,space weathering may proceed at a very rapid rate compared withcollisional timescales. In this case, the presence or absence of aregolith may be the determining factor for whether or not an objectappears ``space weathered.'' Thus 0.1 to 5 km appears to be a criticalsize range for understanding the processes, timescales, and conditionsunder which a regolith conducive to space weathering is generated,retained, and refreshed.
| Near-IR spectroscopy of asteroids 21 Lutetia, 89 Julia, 140 Siwa, 2181 Fogelin and 5480 (1989YK8), potential targets for the Rosetta mission; remote observations campaign on IRTF
In the frame of the international campaign to observe potential targetasteroids for the Rosetta mission, remote observations have been carriedout between Observatoire de Paris, in Meudon-France and the NASAInfrared Telescope Facility on Mauna Kea. The SpeX instrument was usedin the 0.8-2.5 μm spectral region, for two observing runs in Marchand June 2003.This paper presents near-IR spectra of the asteroids 21 Lutetia, 89Julia, 140 Siwa, 2181 Fogelin and 5480 (1989YK8).Near-IR spectra of the asteroids 21 Lutetia and 140 Siwa are flat andfeatureless. The spectrum of 89 Julia reveals absorption bands around 1and 2 μm, which may indicate the presence of olivine andolivine-pyroxene mixtures and confirm the S-type designation.The small main-belt asteroids 2181 Fogelin and 5480 (1989YK8) areinvestigated spectroscopically for the first time. Near-IR spectra ofthese asteroids also show an absorption feature around 1 μm, whichcould be and indicator of igneous/metamorphic surface of the objects;new observations in visible as well as thermal albedo data are necessaryto draw a reliable conclusion on the surface mineralogy of bothasteroids.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| The Top Ten solar analogs in the ELODIE library
Several solar analogs have been identified in the library of highresolution stellar spectra taken with the echelle spectrograph ELODIE. Apurely differential method has been used, based on the χ2comparison of a large number of G dwarf spectra to 8 spectra of the Sun,taken on the Moon and Ceres. HD 146233 keeps its status of closest eversolar twin (Porto de Mello & da Silva \cite{PMDS97}). Some otherspectroscopic analogs have never been studied before, while the twoplanet-host stars HD 095128 and HD 186427 are also part of theselection. The fundamental parameters found in the literature for thesestars show a surprising dispersion, partly due to the uncertaintieswhich affect them. We discuss the advantages and drawbacks ofphotometric and spectroscopic methods to search for solar analogs andconclude that they have to be used jointly to find real solar twins.Based on observations made at the Observatoire de Haute-Provence(France).
| 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
| Properties of the Hermean regolith: V. New optical reflectance spectra, comparison with lunar anorthosites, and mineralogical modelling
We present new optical (0.4-0.65 μm) spectra of Mercury and lunarpure anorthosite locations, obtained quasi-simultaneously with theNordic Optical Telescope (NOT) in 2002. A comparative study is performedwith the model of Lucey et al. (2000, J. Geophys. Res. 105, 20297-20305,and references therein) between iron-poor, mature, pure anorthosite(>90% plagioclase feldspar) Clementine spectra from the lunar farsideand a combined 0.4-1.0 μm mercurian spectrum, obtained with the NOT,calculated for standard photometric geometry. Mercury is located at moreextreme locations in the Lucey ratio-reflectance diagrams than any knownlunar soil, specifically with respect to the extremely iron-poor matureanorthosites. Though quantitative prediction of FeO and TiO2abundances cannot be made without a more generally applicable model, wefind qualitatively that the abundances of both these oxides must be nearzero for Mercury. We utilize the theory of Hapke (2002, Icarus 157,523-534, and references therein), with realistic photometric parameters,to model laboratory spectra of matured mineral powders and lunar soils,and remotely sensed spectra of lunar anorthosites and Mercury. Animportant difference between fabricated and natural powders is the highvalue for the internal scattering parameter necessary to interpret thespectra for the former, and the requirement of rough andnon-isotropically scattering surfaces in the modelling of the latter.The mature lunar anorthosite spectra were well modelled with binarymixtures of calcic feldspars and olivines, grain sizes of 25-30 μmand a concentration of submicroscopic metallic iron (SMFe) of 0.12-0.15%in grain coatings. The mercurian spectrum is not possible to interpretfrom terrestrial mineral powder spectra without introducing an averageparticle scattering function for the bulk soil that increases inbackscattering efficiency with wavelength. The observed spectrum issomewhat better predicted with binary mixture models of feldspars andpyroxenes, than with single-component regoliths consisting of eitheralbite or diopside. Correct spectral reflectance values were predictedwith a concentration of 0.1 wt% SMFe in coatings of 15-30 μm sizedgrains. Since reasonable cosmogonical formation scenarios for Mercury,or meteoritic infall, predict iron concentrations at least this high, wedraw the conclusion that the average grain size of Mercury is about afactor of two smaller than for average returned lunar soil samples. The0.6-2.5 μm spectrum of McCord and Clark (1979, J. Geophys. Res. 178,745-747) is used to further limit the possible range of mineralogicalcomposition of Mercury. It is found that an intimately mixed and matured3:1 labradorite-to-enstatite regolith composition best matches both theoptical and near-infrared spectra, yielding an abundance of ~1.2 wt% FeOand ~0 wt% TiO2.
| Beryllium Abundances in F and G Dwarfs in Praesepe and Other Young Clusters from Keck HIRES Observations
The study of both Be and Li gives useful clues about stellar internalstructure. Of particular interest is the study of these light elementsin open clusters, which have a known age and metallicity. In this paperwe present a study of Be abundances in 10 F-type stars in Praesepe and acomprehensive discussion about Be abundances in other open clusters:Hyades, Pleiades, α Per, Coma, and UMa. We have made observationsof the doublet of Be II around 3130 Å in Praesepe stars, using theKeck I telescope and the High Resolution Echelle Spectrometer (HIRES).Beryllium abundances were derived from the spectra using the spectrumsynthesis method. We find four stars with definite Be depletion in thetemperature range of the Li dip like we found in our previous clusterstudies, notably for the Hyades and Coma clusters. Putting all theclusters together, we confirm the existence of a Be dip in a narrowtemperature range for F stars. Beryllium depletion in this dip is lesspronounced than Li depletion. For the cooler stars there is little or noBe depletion, even though there are large depletions of Li. For starsthat have little or no Li depletion, A(Li)>=3.0, the ratio Li/Be is75+/-4.6, compared to the meteoritic ratio of 77.6. For stars coolerthan ~5900 K there appears to be little or no Be depletion, and the meanA(Be) is 1.30+/-0.02. For these cooler stars within a given clusterthere is no evidence for intrinsic star-to-star differences in A(Be),with the possible exception of the cool Pleiades stars. In thetemperature range of the Li-Be dip, a strong correlation exists betweenLi and Be, consistent with the theory of rotationally induced mixing.Moreover, the slopes of the Li versus Be correlations are differentdepending on the temperature range. For the full sample of 42 starsbetween 5900 and 6650 K the slope is 0.43+/-0.05 [where A(Li) is theabscissa]. The slope is 0.48+/-0.08 for 6300K
| Magnesium Isotope Ratios in Hyades Stars
Using classical model atmospheres and an LTE analysis, Mg isotope ratios24Mg:25Mg:26Mg are measured in 32Hyades dwarfs covering effective temperatures4000K<=Teff<=5000K. We find no significant trend in anyisotope ratio versus Teff, and the mean isotope ratio is inexcellent agreement with the solar value. We determine stellarparameters and Fe abundances for 56 Hyades dwarfs covering4000K<=Teff<=6200K. For stars warmer than 4700 K, wederive a cluster mean value of [Fe/H]=0.16+/-0.02 (σ=0.1), in goodagreement with previous studies. For stars cooler than 4700 K, we findthat the abundance of Fe from ionized lines exceeds the abundance of Fefrom neutral lines. At 4700 K,[Fe/H]II-[Fe/H]I~=0.3dex, while at 4000 K[Fe/H]II-[Fe/H]I~=1.2dex. This discrepancy betweenthe Fe abundance from neutral and ionized lines likely reflectsinadequacies in the model atmospheres and the presence of non-LTE orother effects. Despite the inability of the models to reproduce theionization equilibrium for Fe, the Mg isotope ratios appear immune tothese problems and remain a powerful tool for studying Galactic chemicalevolution.Data presented here were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| The Distances to Open Clusters as Derived from Main-Sequence Fitting. II. Construction of Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances by comparingmulticolor photometry of single stars in the Hyades with theoreticalisochrones constructed with various color-temperature relations. Afterverifying that the isochrone effective temperatures agree well withspectroscopically determined values, we argue that mismatches betweenthe photometry and the theoretical colors likely arise from systematicerrors in the color-temperature relations. We then describe a method forempirically correcting the isochrones to match the photometry anddiscuss the dependence of the isochrone luminosity on metallicity.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| 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
| Searching for Planets in the Hyades. IV. Differential Abundance Analysis of Hyades Dwarfs
We present a differential abundance analysis of Hyades F-K dwarfs insearch for evidence of stellar enrichment from accretedhydrogen-deficient disk material. Metallicities and relative abundanceratios of several species have been determined. We derive a cluster mean[Fe/H]=0.13+/-0.01. Two stars show abundances ~0.2 dex larger than thecluster mean. In addition, one star, which was added by a recent studyas a cluster member, shows significantly lower abundances than thecluster mean. These three stars have questionable membershipcharacteristics. The remaining stars in the survey have an rms of 0.04dex in the differential [Fe/H] values. The Hyades cluster members haveapparently not been significantly chemically enriched. The abundanceratios of Si, Ti, Na, Mg, Ca, and Zn with respect to Fe are in theirsolar proportions.Some data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| A grid of synthetic spectra and indices Fe5270, Fe5335, Mgb and Mg2 as a function of stellar parameters and [alpha/Fe]
We have computed a grid of synthetic spectra in the wavelength rangelambda lambda 4600-5600 Å using revised model atmospheres, for arange of atmospheric parameters and values of [alpha -elements/Fe] = 0.0and +0.4. The Lick indices Fe5270, Fe5335, Mgb and Mg2 aremeasured on the grid spectra for FWHM = 2 to 8.3 Å. Relationsbetween the indices Fe5270, Fe5335 and Mg2 and the stellarparameters effective temperature Teff, log ; g, [Fe/H] and[alpha /Fe], valid in the range 4000 <= Teff <= 7000 K,are presented. These fitting functions are given for FWHM = 3.5 and 8.3Å. The indices were also measured for a list of 97 reference starswith well-known stellar parameters observed at ESO and OHP, and theseare compared to the computed indices. Finally, a comparison of theindices measured on the observed spectra and those derived from thefitting functions based on synthetic spectra is presented.Observations collected at the European Southern Observatory (ESO), LaSilla, Chile and at the Observatoire de Haute Provence (OHP), St-Michel,France.All Tables of Appendices A and B are only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/404/661
| Evidence for the Exposure of Water Ice on Titan's Surface
The smoggy stratosphere of Saturn's largest moon, Titan, veils itssurface from view, except at narrow wavelengths centered at 0.83, 0.94,1.07, 1.28, 1.58, 2.0, 2.9, and 5.0 micrometers. We derived a spectrumof Titan's surface within these ``windows'' and detected featurescharacteristic of water ice. Therefore, despite the hundreds of metersof organic liquids and solids hypothesized to exist on Titan's surface,its icy bedrock lies extensively exposed.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Бик |
| Ректацензија: | 04h26m40.12s |
| Deклинација: | +16°44'48.8" |
| Apparent магнитуда: | 8.1 |
| Даљина: | 46.685 parsecs |
| Proper motion RA: | 107 |
| Proper motion Dec: | -26.8 |
| B-T magnitude: | 8.947 |
| V-T magnitude: | 8.17 |
Каталог и designations:
|
