Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Metallicity, debris discs and planets
We investigate the populations of main-sequence stars within 25 pc thathave debris discs and/or giant planets detected by Doppler shift. Themetallicity distribution of the debris sample is a very close match tothat of stars in general, but differs with >99 per cent confidencefrom the giant planet sample, which favours stars of above averagemetallicity. This result is not due to differences in age of the twosamples. The formation of debris-generating planetesimals at tens of authus appears independent of the metal fraction of the primordial disc,in contrast to the growth and migration history of giant planets withina few au. The data generally fit a core accumulation model, with outerplanetesimals forming eventually even from a disc low in solids, whileinner planets require fast core growth for gas to still be present tomake an atmosphere.
| IRS Spectra of Solar-Type Stars: A Search for Asteroid Belt Analogs
We report the results of a spectroscopic search for debris diskssurrounding 41 nearby solar-type stars, including eight planet-bearingstars, using the Infrared Spectrometer (IRS) on the Spitzer SpaceTelescope. With the accurate relative photometry of the IRS between 7and 34 μm we are able to look for excesses as small as ~2% ofphotospheric levels, with particular sensitivity to weak spectralfeatures. For stars with no excess, the 3 σ upper limit in a bandat 30-34 μm corresponds to ~75 times the brightness of our zodiacaldust cloud. Comparable limits at 8.5-13 μm correspond to ~1400 timesthe brightness of our zodiacal dust cloud. These limits correspond tomaterial located within the <1 to ~5 AU region that, in our solarsystem, originates predominantly from debris associated with theasteroid belt. We find excess emission longward of ~25 μm from fivestars, of which four also show excess emission at 70 μm. Thisemitting dust must be located in a region starting around 5-10 AU. Onestar has 70 μm emission but no IRS excess. In this case, the emittingregion must begin outside 10 AU; this star has a known radial velocityplanet. Only two stars of the five show emission shortward of 25 μm,where spectral features reveal the presence of a population of small,hot dust grains emitting in the 7-20 μm band. One of these stars, HD72905, is quite young (300 Myr), while the other, HD 69830, is olderthan 2 Gyr. The data presented here strengthen the results of previousstudies to show that excesses at 25 μm and shorter are rare: only 1out of 40 stars older than 1 Gyr or ~2.5% shows an excess. Asteroidbelts 10-30 times more massive than our own appear are rare amongmature, solar-type stars.
| Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer MIPS Survey
We have searched for infrared excesses around a well-defined sample of69 FGK main-sequence field stars. These stars were selected withoutregard to their age, metallicity, or any previous detection of IRexcess; they have a median age of ~4 Gyr. We have detected 70 μmexcesses around seven stars at the 3 σ confidence level. Thisextra emission is produced by cool material (<100 K) located beyond10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Only one star,HD 69830, shows excess emission at 24 μm, corresponding to dust withtemperatures >~300 K located inside of 1 AU. While debris disks withLdust/L*>=10-3 are rare around oldFGK stars, we find that the disk frequency increases from 2%+/-2% forLdust/L*>=10-4 to 12%+/-5% forLdust/L*>=10-5. This trend in thedisk luminosity distribution is consistent with the estimated dust inour solar system being within an order of magnitude greater or less thanthe typical level around similar nearby stars. Although there is nocorrelation of IR excess with metallicity or spectral type, there is aweak correlation with stellar age, with stars younger than a gigayearmore likely to have excess emission.
| 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.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo not show dilution of metallicity, (3) no abundance variations for Na,Si, Ti, or Ni are found as a function of condensation temperature, and(4) no correlations between metallicity and orbital period oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| Abundances of Mn, Co and Eu in a sample of 20 F-G disk stars: the influence of hyperfine structure splitting
We present Mn, Co and Eu abundances for a sample of 20 disk F and Gdwarfs and subgiants with metallicities in the range-0.8≤[Fe/H]≤+0.3. We investigate the influence of hyperfinestructure (HFS) on the derived abundances of Mn and Co by using HFS datafrom different sources in the literature, as well as calculated HFS frominteraction factors A and B. Eu abundances were obtained from spectralsynthesis of one Eu II line that takes into account HFS from a series ofrecent laboratory measurements. For the lines analysed in this study, wefind that for manganese, the differences between abundances obtainedwith different HFSs are no greater than 0.10 dex. Our cobalt abundancesare even less sensitive to the choice of HFS than Mn, presenting a 0.07dex maximum difference between determinations with different HFSs.However, the cobalt HFS data from different sources are significantlydifferent. Our abundance results for Mn offer an independentconfirmation of literature results, favouring type Ia supernovae as themain nucleosynthesis site of Mn production, in contrast to trends of Mnversus metallicity previously reported in the literature. For Co, weobtain [Co/Fe]0.0 in the range -0.3<[Fe/H]<+0.3 and [Co/Fe]rising to a level of +0.2 when [Fe/H] decreases from -0.3 to -0.8, indisagreement with recent results in the literature. The observeddiscrepancies may be attributed to the lack of HFS in the works we usedfor comparison. Our results for Eu are in accordance with low-mass typeII supernovae being the main site of the r-process nucleosynthesis.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| The age of the Galactic thin disk from Th/Eu nucleocosmochronology. I. Determination of [Th/Eu] abundance ratios
The purpose of this work is to resume investigation of Galactic thindisk dating using nucleocosmochronology with Th/Eu stellar abundanceratios, a theme absent from the literature since 1990. A stellar sampleof 20 disk dwarfs/subgiants of F5 to G8 spectral types with-0.8≤[Fe/H]≤+0.3 was selected. In stars with such spectral typesand luminosity classes, spectral synthesis techniques must be employedif we wish to achieve acceptably accurate results. An homogeneous,self-consistent set of atmospheric parameters was determined. Effectivetemperatures were determined from photometric calibrations and Hαprofile fitting; surface gravities were obtained from Teff,stellar masses and luminosities; microturbulence velocities andmetallicities were obtained from detailed, differential spectroscopicanalysis, relative to the Sun, using equivalent widths of Fe I and Fe IIlines. Chemical abundances of the elements that contaminate the Th andEu spectral regions (Ti, V, Cr, Mn, Co, Ni, Ce, Nd, and Sm) weredetermined through spectroscopic analysis. Abundance uncertainties werethoroughly scrutinised, their average value (0.10±0.02) dex being found to be satisfactorily low. Eu and Th abundances weredetermined by spectral synthesis of one Eu II line (4129.72 Å) andone Th II line (4019.13 Å), taking into account the detailedhyperfine structures of contaminating Co lines, as well as the hyperfinestructure and isotope shift of the Eu line. Comparison of our abundanceswith literature data shows that our results exhibit a similar behaviour,but a considerably lower scatter (36% lower for Eu, and 61% lower forTh). The [Th/Eu] abundance ratios thus obtained were used, in the secondpaper of this series, to determine the age of the Galactic disk.
| 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 search for debris discs around stars with giant planets
Eight nearby stars with known giant planets have been searched forthermal emission in the submillimetre arising from dust debris. The nullresults imply quantities of dust typically less than 0.02 Earth massesper star. Conversely, literature data for 20 Sun-like stars with debrisdiscs show that <= 5 per cent have gas giants inside a fewastronomical units - but the dust distribution suggests that nearly allhave more distant planets. The lack of overlap in these systems - i.e.few stars possess both inner planets and a disc - indicates that thesephenomena either are not connected or are mutually exclusive. Comparisonwith an evolutionary model shows that debris masses are predicted to below by the stellar ages of 2-8 Gyr (unless the colliding parent bodiesare quite distant, located beyond 100-200 au), but it remains to beexplained why stars that do have debris should preferentially only havedistant planets. A simple idea is proposed that could produce theselargely different systems, invoking a difference in the primordial discmass. Large masses promote fast gas giant growth and inwards migration,whereas small masses imply slow evolution, low-mass gas giants andoutwards migration that increases the collision rate of Kuiper Belt-likeobjects. This explanation neglects other sources of diversity betweendiscs (such as density and planetesimal composition and orbits), but itdoes have the merit of matching the observational results.
| Stellar Chemical Signatures and Hierarchical Galaxy Formation
To compare the chemistries of stars in the Milky Way dwarf spheroidal(dSph) satellite galaxies with stars in the Galaxy, we have compiled alarge sample of Galactic stellar abundances from the literature. Whenkinematic information is available, we have assigned the stars tostandard Galactic components through Bayesian classification based onGaussian velocity ellipsoids. As found in previous studies, the[α/Fe] ratios of most stars in the dSph galaxies are generallylower than similar metallicity Galactic stars in this extended sample.Our kinematically selected stars confirm this for the Galactic halo,thin-disk, and thick-disk components. There is marginal overlap in thelow [α/Fe] ratios between dSph stars and Galactic halo stars onextreme retrograde orbits (V<-420 km s-1), but this is notsupported by other element ratios. Other element ratios compared in thispaper include r- and s-process abundances, where we find a significantoffset in the [Y/Fe] ratios, which results in a large overabundance in[Ba/Y] in most dSph stars compared with Galactic stars. Thus, thechemical signatures of most of the dSph stars are distinct from thestars in each of the kinematic components of the Galaxy. This resultrules out continuous merging of low-mass galaxies similar to these dSphsatellites during the formation of the Galaxy. However, we do not ruleout very early merging of low-mass dwarf galaxies, since up to one-halfof the most metal-poor stars ([Fe/H]<=-1.8) have chemistries that arein fair agreement with Galactic halo stars. We also do not rule outmerging with higher mass galaxies, although we note that the LMC and theremnants of the Sgr dwarf galaxy are also chemically distinct from themajority of the Galactic halo stars. Formation of the Galaxy's thickdisk by heating of an old thin disk during a merger is also not ruledout; however, the Galaxy's thick disk itself cannot be comprised of theremnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarfgalaxy like the LMC or Sgr, because of differences in chemistry.The new and independent environments offered by the dSph galaxies alsoallow us to examine fundamental assumptions related to thenucleosynthesis of the elements. The metal-poor stars ([Fe/H]<=-1.8)in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than[Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy.Predictions from the α-process (α-rich freeze-out) would beconsistent with this result if there have been a lack of hypernovae indSph galaxies. The α-process could also be responsible for thevery low Y abundances in the metal-poor stars in dSph's; since [La/Eu](and possibly [Ba/Eu]) are consistent with pure r-process results, thelow [Y/Eu] suggests a separate r-process site for this light(first-peak) r-process element. We also discuss SNe II rates and yieldsas other alternatives, however. In stars with higher metallicities([Fe/H]>=-1.8), contributions from the s-process are expected; [(Y,La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still muchhigher in the dSph stars than similar metallicity Galactic stars. Thisresult is consistent with s-process contributions from lower metallicityAGB stars in dSph galaxies, and is in good agreement with the slowerchemical evolution expected in the low-mass dSph galaxies relative tothe Galaxy, such that the build-up of metals occurs over much longertimescales. Future investigations of nucleosynthetic constraints (aswell as galaxy formation and evolution) will require an examination ofmany stars within individual dwarf galaxies.Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster isconfirmed in Galactic halo stars, but we discuss this in terms of thegeneral nucleosynthesis of neutron-rich elements. We do not confirm thatthe Na-Ni trend is related to the accretion of dSph galaxies in theGalactic halo.
| 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
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Oxygen line formation in late-F through early-K disk/halo stars. Infrared O I triplet and [O I] lines
In order to investigate the formation of O I 7771-5 and [O I] 6300/6363lines, extensive non-LTE calculations for neutral atomic oxygen werecarried out for wide ranges of model atmosphere parameters, which areapplicable to early-K through late-F halo/disk stars of variousevolutionary stages.The formation of the triplet O I lines was found to be well described bythe classical two-level-atom scattering model, and the non-LTEcorrection is practically determined by the parameters of theline-transition itself without any significant relevance to the detailsof the oxygen atomic model. This simplifies the problem in the sensethat the non-LTE abundance correction is essentially determined only bythe line-strength (Wlambda ), if the atmospheric parametersof Teff, log g, and xi are given, without any explicitdependence of the metallicity; thus allowing a useful analytical formulawith tabulated numerical coefficients. On the other hand, ourcalculations lead to the robust conclusion that LTE is totally valid forthe forbidden [O I] lines.An extensive reanalysis of published equivalent-width data of O I 7771-5and [O I] 6300/6363 taken from various literature resulted in theconclusion that, while a reasonable consistency of O I and [O I]abundances was observed for disk stars (-1 <~ [Fe/H] <~ 0), theexistence of a systematic abundance discrepancy was confirmed between OI and [O I] lines in conspicuously metal-poor halo stars (-3 <~[Fe/H] <~ -1) without being removed by our non-LTE corrections, i.e.,the former being larger by ~ 0.3 dex at -3 <~ [Fe/H] <~ -2.An inspection of the parameter-dependence of this discordance indicatesthat the extent of the discrepancy tends to be comparatively lessenedfor higher Teff/log g stars, suggesting the preference ofdwarf (or subgiant) stars for studying the oxygen abundances ofmetal-poor stars.Tables 2, 5, and 7 are 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/402/343 and Table\ref{tab3} is only available in electronic form athttp://www.edpsciences.org
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is 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/398/721
| Brightness variations of an extrasolar giant planet rotating around its central star
Extrasolar giant planets (EGPs) were discovered around 65 stars (visualmagnitude 4m-8m) in the last seven years. Almostall information on them was derived from precise measurements of radialvelocities. The mass-radius ratio for EGP orbiting the star HD 209458 isknown to be R/M = 1.55/0.69(RJ/MJ) =2.25(RJ/MJ). This ratio enabled us to estimatemagnitudes of EGPs around 65 stars. These magnitudes lie between25.1m and 12.8m. The planet's contribution is0.003-0.047% of the star radiation for 21 stars. Therefore, the systemsof this kind can be accessible for photometric observations. The planetsorbiting the stars τ Boo, HD 121504, HD 217107, HD 187123, HD209458, HD 83443, HD 75289, 51 Peg, and HD 46375 are most suitable forthese observations.
| Extrasolar planets around HD 196050, HD 216437 and HD 160691
We report precise Doppler measurements of the stars HD 216437, HD 196050and HD 160691 obtained with the Anglo-Australian Telescope using theUCLES spectrometer together with an iodine cell as part of theAnglo-Australian Planet Search. Our measurements reveal periodicKeplerian velocity variations that we interpret as evidence for planetsin orbit around these solar type stars. HD 216437 has a period of 1294+/- 250 d, a semi-amplitude of 38 +/- 3 m s-1 and aneccentricity of 0.33 +/- 0.09. The minimum (M sin i) mass of thecompanion is 2.1 +/- 0.3 MJUP and the semi-major axis is 2.4+/- 0.5 au. HD 196050 has a period of 1300 +/- 230 d, a semi-amplitudeof 49 +/- 8 m s-1 and an eccentricity of 0.19 +/- 0.09. Theminimum mass of the companion is 2.8 +/- 0.5 MJUP and thesemi-major axis is 2.4 +/- 0.5 au. We also report further observationsof the metal-rich planet bearing star HD 160691. Our new solutionconfirms the previously reported planet and shows a trend indicating asecond, longer-period companion. These discoveries add to the growingnumbers of mildly eccentric, long-period extrasolar planets aroundmetal-rich Sun-like stars.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| The planet search program at the ESO Coudé Echelle spectrometer. III. The complete Long Camera survey results
We present the complete results of the planet search program carried outat the ESO Coudé Echelle Spectrometer (CES) on La Silla, usingthe Long Camera from Nov. 1992 to April 1998. The CES survey hasmonitored 37 late-type (F8V - M5V) stars in the southern hemisphere forvariations in their differential radial velocities (RV) in order todetect Doppler reflex motions caused by planetary companions. This ledto the discovery of the first extrasolar planet in an Earth-like orbitaround the young (ZAMS) and active G0V star iota Horologii (Kürsteret al. \cite{martin00}). Here we present the RV results for all surveystars and perform a statistical examination of the whole data-set. Eachstar is tested for RV variability, RV trends (linear and non-linear) andsignificant periodic signals. beta Hyi and epsilon Ind are identified aslong-term, low-amplitude RV variables. Furthermore, for 30 CES surveystars we determine quantitative upper mass-limits for giant planetsbased on our long-term RV results. We find that the CES Long Camerasurvey would have detected short-period (``51 Peg-type'') planets aroundall 30 stars but no planets with msin i < 1 {M}_Jup at orbitalseparations larger than 2 AU. Finally, we demonstrate that the CESplanet search can be continued without applying velocity corrections tothe RV results coming from the currently installed Very Long Camera atthe CES. Based on observations collected at the European SouthernObservatory, La Silla. Appendices A and B are only available inelectronic form at http://www.edpsciences.org
| Europium abundances in F and G disk dwarfs
Europium abundances for 74 F and G dwarf stars of the galactic disk havebeen determined from the 4129.7 Å Eu II line. The stars wereselected from the sample of Edvardsson et al. (1993) and [Eu/Fe] shows asmaller scatter and a slightly weaker trend with [Fe/H] than found byWoolf et al. (1995). The data of the two analyses are homogenized andmerged. We also discuss the adopted effective temperature scale. Basedon observations carried out at the European Southern Observatory, LaSilla, Chile. Tables 2 and 6 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcal?J/A+A/381/500
| The theory of brown dwarfs and extrasolar giant planets
Straddling the traditional realms of the planets and the stars, objectsbelow the edge of the main sequence have such unique properties, and arebeing discovered in such quantities, that one can rightly claim that anew field at the interface of planetary science and astronomy is beingborn. This article extends the previous review of Burrows and Liebert(1993) and describes the essential elements of the theory of browndwarfs and giant planets. It discusses their evolution, atmosphericcomposition, and spectra, including the new spectroscopic classes L andT. Particular topics which are important for an understanding of thespectral properties include the effects of condensates, clouds,molecular abundances, and atomic opacities. Moreover, it discusses thedistinctive features of these extrasolar giant planets that areirradiated by a central primary, in particular, their reflectionspectra, albedos, and transits. Overall, the theory explains the basicsystematics of substellar-mass objects over three orders of magnitude inmass and age, and a factor of 30 in temperature.
| Two New Planets from the Anglo-Australian Planet Search
Precise Doppler measurements from the Anglo-Australian Telescope (AAT)UCLES spectrometer reveal periodic Keplerian velocity variations in thestars HD 160691 and HD 27442. HD 160691 has a period of 743 days, asemiamplitude of 54 m s-1, and a high eccentricity, e=0.62,typical of extrasolar planets orbiting beyond 0.2 AU. The minimum(Msini) mass of the companion is 1.97 MJ, and the semimajoraxis is 1.65 AU. HD 27442 has a 415 day period, a semiamplitude of 32 ms-1, and an eccentricity of 0.058. The minimum mass is 1.43MJ, and the semimajor axis is 1.18 AU. This is the firstextrasolar planet orbiting beyond 0.2 AU that is in a circular orbitsimilar to solar system planets. The photon-limited precision ofAAT/UCLES measurements is 3 m s-1 as demonstrated by stablestars and Keplerian fits to planet-bearing stars. In addition, wepresent confirmation of four previously announced planets. Based onobservations obtained at the Anglo-Australian Telescope, Siding Spring,Australia.
| 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
| Albedo and Reflection Spectra of Extrasolar Giant Planets
We generate theoretical albedo and reflection spectra for a full rangeof extrasolar giant planet (EGP) models, from Jovian to 51 Pegasi classobjects. Our albedo modeling utilizes the latest atomic and molecularcross sections, Mie theory treatment of scattering and absorption bycondensates, a variety of particle size distributions, and an extensionof the Feautrier technique, which allows for a general treatment of thescattering phase function. We find that, because of qualitativesimilarities in the compositions and spectra of objects within each offive broad effective temperature ranges, it is natural to establish fiverepresentative EGP albedo classes. At low effective temperatures(Teff<~150 K) is a class of ``Jovian'' objects (class I)with tropospheric ammonia clouds. Somewhat warmer class II, or ``watercloud,'' EGPs are primarily affected by condensed H2O.Gaseous methane absorption features are prevalent in both classes. Inthe absence of nonequilibrium condensates in the upper atmosphere, andwith sufficient H2O condensation, class II objects areexpected to have the highest visible albedos of any class. When theupper atmosphere of an EGP is too hot for H2O to condense,radiation generally penetrates more deeply. In these objects, designatedclass III or ``clear'' because of a lack of condensation in the upperatmosphere, absorption lines of the alkali metals, sodium and potassium,lower the albedo significantly throughout the visible. Furthermore, thenear-infrared albedo is negligible, primarily because of strongCH4 and H2O molecular absorption andcollision-induced absorption (CIA) by H2 molecules. In thoseEGPs with exceedingly small orbital distance (``roasters'') and 900K<~Teff<~1500 K (class IV), a tropospheric silicatelayer is expected to exist. In all but the hottest(Teff>~1500 K) or lowest gravity roasters, the effect ofthis silicate layer is likely to be insignificant because of the verystrong absorption by sodium and potassium atoms above the layer. Theresonance lines of sodium and potassium are expected to be salientfeatures in the reflection spectra of these EGPs. In the absence ofnonequilibrium condensates, we find, in contrast to previous studies,that these class IV roasters likely have the lowest visible and Bondalbedos of any class, rivaling the lowest albedos of our solar system.For the small fraction of roasters with Teff>~1500 Kand/or low surface gravity (<~103 cm s-2 classV), the silicate layer is located very high in the atmosphere,reflecting much of the incident radiation before it can reach theabsorbing alkali metals and molecular species. Hence, the class Vroasters have much higher albedos than those of class IV. In addition,for class V objects, UV irradiation may result in significant alkalimetal ionization, thereby further weakening the alkali metal absorptionlines. We derive Bond albedos (AB) andTeff estimates for the full set of known EGPs. A broad rangein both values is found, with Teff ranging from ~150 tonearly 1600 K, and AB from ~0.02 to 0.8. We findthat variations in particle size distributions and condensation fractioncan have large quantitative, or even qualitative, effects on albedospectra. In general, less condensation, larger particle sizes, and widersize distributions result in lower albedos. We explore the effects ofnonequilibrium condensed products of photolysis above or withinprincipal cloud decks. As in Jupiter, such species can lower the UV/bluealbedo substantially, even if present in relatively small mixing ratios.
| Scale-relativity and quantization of exoplanet orbital semi-major axes
In a recent study (Nottale \cite{xtrasol}), it was found that thedistribution of the semi-major axes of the firstly discovered exoplanetswas clustered around quantized values according to the lawa/GM=(n/w02, in the same manner and in terms ofthe same constant w0=144 km/s as in our own inner SolarSystem. The ratio alpha g=w0/c actually stands outas a gravitational coupling constant. The number of exoplanets has nowincreased fivefold since this first study, including a full system ofthree planets around Ups And. In the present paper, we apply the sameanalysis to the new exoplanets and we find that their distributionagrees with this structuration law in a statistically significant way(probability ~ 10-4). Such a n2 law is predictedby the scale-relativity approach to planetary system formation, in whichthe evolution of planetesimals is described in terms of a generalizedSchrödinger equation. In particular, one was able to predict fromthis model (Nottale \cite{liwos}) the occurrence of preferentialdistances of planets at ~ 0.043 AU/Msun and ~ 0.17AU/Msun from their parent stars. The observational datasupports this theoretical prediction, since the semimajor axes of ~ 50%of the presently known exoplanets cluster around these values (51Peg-type planets).
| zeta 1 and zeta 2 Reticuli and the existence of the zeta Herculis group
We report the detailed analysis of the solar type stars zeta1 and zeta 2 Reticuli. We obtained accurateeffective temperatures (T_eff = 5746 +/- 27 K and 5859 +/- 27 Krespectively) and surface gravities (log g = 4.54 +/- 0.02 and 4.46 +/-0.01 respectively). Both stars are slightly metal deficient ([Fe/H] =-0.22 +/- 0.05) and their element abundance patterns are compatible withone another and with the Sun. The hypothesis, suggested by previousdetailed analyses, that these stars could be helium rich relative to theSun, was investigated. The stars were found to have a normal, solarhelium abundance. We analysed the stars' membership of the zetaHerculis stellar kinematic group (SKG). Some probable membershave nearly the same galactic orbital parameters, chemical compositionand evolutionary states, which confirm the existence of a metaldeficient SKG. Since we determined that zeta Herculis does not belong tothis group, we propose it be renamed zeta Reticuli SKG. Based onobservations collected at the European Southern Observatory, La Silla,Chile, and at the Observatório do Pico dos Dias, operated by theLaboratório Nacional de Astrofísica, CNPq, Brazil.
| The Vega phenomenon around G dwarfs
In this paper, we present and discuss photometric infrared measurementstaken with ISOPHOT at 60 mu m for a sample of 30 G dwarfs, using C1003X3 minimaps. This sample was selected from the set of more than 1600 Gdwarfs which will be scrutinized for radial-velocity variations by theGeneva group, aiming at the detection of extra-solar planets. In oursample, 5 stars display an infrared excess at 60 mu m. We have comparedour results with those of Habing et al. (1999, in prep.), who haveinvestigated the incidence and the survival of remnant disks aroundmain-sequence stars. They concluded that most stars which arrive on themain-sequence still possess a disk, and that this disk then decays inthe next 400 Myr. The 5 stars in our sample which have an infraredexcess, are probably older than 3 Gyr, however, from which we suggestthat the disks around cool stars may survive longer than those amongearlier-type objects. For two stars in our sample, the fractionalluminosity of the disk is significantly higher than for typicalVega-type stars, and approaches the exceptional value observed for betaPic. To investigate the correlation between an infrared excess andplanets/companion stars, we combine our results with the first resultsof the CORALIE survey. None of the until now detected radial-velocityvariables display an infrared excess. That the stars for which aninfrared excess is found lack a companion, can however not be concludedat the present stage. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the United Kingdom) andwith the participation of ISAS and NASA
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Pavone |
| Ascensione retta: | 20h40m02.60s |
| Declinazione: | -60°32'56.0" |
| Magnitudine apparente: | 5.12 |
| Distanza: | 24.195 parsec |
| Moto proprio RA: | 312.8 |
| Moto proprio Dec: | -568.8 |
| B-T magnitude: | 5.74 |
| V-T magnitude: | 5.181 |
Cataloghi e designazioni:
|
