Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The NASA-UC Eta-Earth Program. I. A Super-Earth Orbiting HD 7924
We report the discovery of the first low-mass planet to emerge from theNASA-UC Eta-Earth Program, a super-Earth orbiting the K0 dwarfHD 7924. Keplerian modeling of precise Doppler radial velocitiesreveals a planet with minimum mass MP sin i = 9.26 M⊕ in a P = 5.398 d orbit. Based on Keck-HIRESmeasurements from 2001 to 2008, the planet is robustly detected with anestimated false alarm probability of less than 0.001. Photometricobservations using the Automated Photometric Telescopes at FairbornObservatory show that HD 7924 is photometrically constant over theradial velocity period to 0.19 mmag, supporting the existence of theplanetary companion. No transits were detected down to a photometriclimit of ~0.5 mmag, eliminating transiting planets with a variety ofcompositions. HD 7924b is one of only eight planets detected by theradial velocity technique with MP sin i < 10 M⊕ and as such is a member of an emerging family oflow-mass planets that together constrain theories of planet formation.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Spectroscopic investigation of stars on the lower main sequence
Aims. The aim of this paper is to provide fundamental parameters andabundances with a high accuracy for a large sample of cool main sequencestars. This study is part of wider project, in which the metallicitydistribution of the local thin disc is investigated from a completesample of G and K dwarfs within 25 pc. Methods: The stars were observedat high resolution and a high signal-to-noise ratio with the ELODIEechelle spectrograph. The V sin i were obtained with a calibration ofthe cross-correlation function. Effective temperatures were estimated bythe line depth ratio method. Surface gravities (log g) were determinedby two methods: parallaxes and ionization balance of iron. The Mg and Naabundances were derived using a non-LTE approximation. Abundances ofother elements were obtained by measuring equivalent widths. Results:Rotational velocities, atmospheric parameters (T_eff, log g, [Fe/H],V_t), and Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, and Znabundances are provided for 131 stars. Among them, more than 30 starsare active stars with a fraction of BY Dra and RS CVn type stars forwhich spectral peculiarities were investigated. We find the meanabundances of the majority of elements in active and nonactive stars tobe similar, except for Li, and possibly for Zn and Co. The lithium isreliably detected in 54% of active stars but only in 20% of nonactivestars. No correlation is found between Li abundances and rotationalvelocities. A possible anticorrelation of log A(Li) with the index ofchromospheric activity GrandS is observed. Conclusions: Active andnonactive cool dwarfs show similar dependencies of most elemental ratiosvs. [Fe/H]. This allows us to use such abundance ratios to study thechemical and dynamical evolution of the Galaxy. Among active stars, noclear correlation has been found between different indicators ofactivity for our sample stars.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute Provence (France). Tables A.1-A3are only available 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/qcat?J/A+A/489/923
| Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants
We present the parameters of 891 stars, mostly clump giants, includingatmospheric parameters, distances, absolute magnitudes, spatialvelocities, galactic orbits and ages. One part of this sample consistsof local giants, within 100 pc, with atmospheric parameters eitherestimated from our spectroscopic observations at high resolution andhigh signal-to-noise ratio, or retrieved from the literature. The otherpart of the sample includes 523 distant stars, spanning distances up to1 kpc in the direction of the North Galactic Pole, for which we haveestimated atmospheric parameters from high resolution but lowsignal-to-noise Echelle spectra. This new sample is kinematicallyunbiased, with well-defined boundaries in magnitude and colours. Werevisit the basic properties of the Galactic thin disk as traced byclump giants. We find the metallicity distribution to be different fromthat of dwarfs, with fewer metal-rich stars. We find evidence for avertical metallicity gradient of -0.31 dex kpc-1 and for atransition at ~4-5 Gyr in both the metallicity and velocities. Theage-metallicity relation (AMR), which exhibits a very low dispersion,increases smoothly from 10 to 4 Gyr, with a steeper increase for youngerstars. The age-velocity relation (AVR) is characterized by thesaturation of the V and W dispersions at 5 Gyr, and continuous heatingin U.
| Neutron-capture elements in halo, thick-disk, and thin-disk stars. Strontium, yttrium, zirconium, cerium
We derived Sr, Y, Zr, and Ce abundances for a sample of 74 cool dwarfsand subgiants with iron abundances, [Fe/H], between 0.25 and‑2.43. These estimates were obtained using synthetic spectra,assuming local thermodynamic equilibrium (LTE) for Y, Zr, and Ce,allowing for non-LTE conditions for Sr. We used high-resolution(λ/Δλ≅40 000 and 60 000) spectra withsignal-to-noise ratios between 50 and 200. We find that the Zr/Y, Sr/Y,and Sr/Zr ratios for the halo stars are the same in a wide metallicityrange (‑2.43 ≤ [Fe/H] ≤ ‑0.90), within the errors,indicating a common origin for these elements at the epoch of haloformation. The Zr/Y ratios for thick-disk stars quickly decrease withincreasing Ba abundance, indicating a lower rate of production of Zrcompared to Y during active thick-disk formation. The thick-disk andhalo stars display an increase in the [Zr/Ba] ratio with decreasing Baabundance and a correlation of the Zr and Eu overabundances relative toBa. The evolutionary behavior of the abundance ratios found for thethick-disk and halo stars does not agree with current models for theGalaxy’s chemical evolution. The abundance ratios of Y and Zr toFe and Ba for thin-disk stars, as well as the abundance ratios withineach group, are, on average, solar, though we note a slight decrease ofZr/Ba and Zr/Y with increasing Ba abundance. These results provideevidence for a dominance of asymptotic-giant-branch stars in theenrichment of the interstellar medium in heavy elements during thethin-disk epoch, in agreement with the predictions of thenucleosynthesis theory for the main s-process component.
| Barium even-to-odd isotope abundance ratios in thick disk and thin disk stars
We present the Ba even-to-odd isotope abundance ratios in 25 cool dwarfstars with the metallicity [Fe/H] ranging between 0.25 and -1.35. Ourmethod takes advantage of the hyperfine structure (HFS) affecting the BaII resonance line of the odd isotopes. The fractional abundance of theodd isotopes of Ba is derived from a requirement that Ba abundances fromthe resonance line λ 4554 and subordinate lines λ 5853 andλ 6496 must be equal. The results are based on NLTE lineformation and analysis of high resolution (R 60 000) highsignal-to-noise ({S/N} ≥ 200) observed spectra. We find that thefraction of the odd isotopes of Ba grows toward the lower Ba abundance(or metallicity) and the mean value in the thick disk stars equals 33± 4%. This indicates the higher contribution of the r-process tobarium in the thick disk stars compared to the solar system matter. Theobtained fraction increases with the [Eu/Ba] abundance ratio growth inagreement with expectations. A significant fraction of the even isotopesof Ba found in old Galactic stars (the thick disk stars), 67%, is incontrast to the prediction of the "classical" model of the s-process andfavors the value predicted by the "stellar" models of Arlandini et al.(1999) and Travaglio et al. (1999).
| The N2K Consortium. VI. Doppler Shifts without Templates and Three New Short-Period Planets
We present a modification to the iodine cell Doppler technique thateliminates the need for an observed stellar template spectrum. For agiven target star, we iterate toward a synthetic template spectrumbeginning with an existing template of a similar star. We then perturbthe shape of this first-guess template to match the program observationof the target star taken through an iodine cell. The elimination of aseparate template observation saves valuable telescope time, a featurethat is ideally suited for the quick-look strategy employed by the``Next 2000 Stars'' (N2K) planet search program. Tests using Keck HIRES(High Resolution Echelle Spectrometer) spectra indicate that synthetictemplates yield a short-term precision of 3 m s-1 and along-term, run-to-run precision of 5 m s-1. We used this newDoppler technique to discover three new planets: a 1.50MJplanet in a 2.1375 day orbit around HD 86081; a 0.71MJ planetin circular, 26.73 day orbit around HD 224693; and a Saturn-mass planetin an 18.179 day orbit around HD 33283. The remarkably short period ofHD 86081b bridges the gap between the extremely short period planetsdetected in the Optical Gravitational Lensing Experiment (OGLE) surveyand the 16 Doppler-detected hot Jupiters (P < 15 days), which have anorbital period distribution that piles up at about 3 days. We haveacquired photometric observations of two of the planetary host starswith the automated photometric telescopes at Fairborn Observatory. HD86081 and HD 224693 both lack detectable brightness variability on theirradial velocity periods, supporting planetary-reflex motion as the causeof the radial velocity variability. HD 86081 shows no evidence ofplanetary transits in spite of a 17.6% transit probability. We have toofew photometric observations to detect or rule out transits for HD224693.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology.
| 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.
| How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars
Sun-like stars have stellar, brown dwarf, and planetary companions. Tohelp constrain their formation and migration scenarios, we analyze theclose companions (orbital period <5 yr) of nearby Sun-like stars. Byusing the same sample to extract the relative numbers of stellar, browndwarf, and planetary companions, we verify the existence of a very drybrown dwarf desert and describe it quantitatively. With decreasing mass,the companion mass function drops by almost 2 orders of magnitude from 1Msolar stellar companions to the brown dwarf desert and thenrises by more than an order of magnitude from brown dwarfs toJupiter-mass planets. The slopes of the planetary and stellar companionmass functions are of opposite sign and are incompatible at the 3σ level, thus yielding a brown dwarf desert. The minimum number ofcompanions per unit interval in log mass (the driest part of the desert)is at M=31+25-18MJ. Approximately 16%of Sun-like stars have close (P<5 yr) companions more massive thanJupiter: 11%+/-3% are stellar, <1% are brown dwarf, and 5%+/-2% aregiant planets. The steep decline in the number of companions in thebrown dwarf regime, compared to the initial mass function of individualstars and free-floating brown dwarfs, suggests either a differentspectrum of gravitational fragmentation in the formation environment orpost-formation migratory processes disinclined to leave brown dwarfs inclose orbits.
| An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei
CX Cep (WR 151) is the WR+O binary (WN5+O5 V) with the second shortestperiod known in our Galaxy. To examine the circumstellar matterdistribution and to better constraint the orbital parameters andmass-loss rate of the W-R star, we obtained broadband and multiband(i.e., UBVRI) linear polarization observations of the system. Ouranalysis of the phase-locked polarimetric modulation confirms the highorbital inclination of the system (i.e., i=65deg). Using theorbital solution of Lewis et al. (1993), we obtain masses of 33.9 and23.9 Msolar for the O and W-R stars, respectively, whichagree with their spectral types. A simple polarimetric model accountingfor finite stellar size effects allowed us to derive a mass-loss ratefor the W-R star of (0.3-0.5)×10-5 Msolaryr-1. This result was remarkably independent of the model'sinput parameters and favors an earlier spectral type for the W-Rcomponent (i.e., WN4). Finally, using our multiband observations, wefitted and subtracted from our data the interstellar polarization. Theresulting constant intrinsic polarization of 3%-4% is misaligned inrelation to the orbital plane (i.e.,θCIP=26deg vs. Ω=75deg)and is the highest intrinsic polarization ever observed for a W-R star.This misalignment points toward a rotational (or magnetic) origin forthe asymmetry and contradicts the most recent evolutionary models formassive stars (Meynet & Maeder 2003) that predict sphericallysymmetric winds during the W-R phase (i.e., CIP=0%).
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| First Ever Polarimetric Detection of a Wind-Wind Interaction Region and a Misaligned Flattening of the Wind in the Wolf-Rayet Binary CQ Cephei
In this paper we present unfiltered and multiband (i.e., UBVRI)polarimetric observations of the short-period Wolf-Rayet binary CQ Cep.Using the basic assumptions of an optically thin, corotating envelopeand pointlike sources (i.e., BME78 assumptions), we determined theorbital parameters of the system (i.e.,i=99deg+/-1deg andΩ=76deg+/-2deg at the 2 σ level) withan accuracy many times better than any previous work. Residual non-BME78variability around phase 0.0 was present in our data, which we associatewith the polarimetric eclipse of the dense central parts of theWolf-Rayet (W-R) wind by the orbiting O star. We attribute the observedphase lag of -0.15 between our residuals and those expected for astandard polarimetric eclipse to a wind-wind interaction (WWI) regiondistorted by Coriolis forces using the model presented by Marchenko etal. This model was also able to explain the strong wavelength dependenceof the polarimetric amplitudes in our multiband observations. Ouranalysis also reveals important epoch-dependent departures of the matterdistribution from spherical symmetry that were not related to theorbital plane and therefore cannot be the result of tidal interaction.We conclude that binarity is not playing an important role in drivingthe wind of the W-R star in CQ Cep and contributing to the observednonspherical matter distribution. On the other hand, this asymmetrycould be explained by a rotationally induced disk misaligned with theorbital plane.
| 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.
| A Catalog of Northern Stars with Annual Proper Motions Larger than 0.15" (LSPM-NORTH Catalog)
The LSPM catalog is a comprehensive list of 61,977 stars north of theJ2000 celestial equator that have proper motions larger than 0.15"yr-1 (local-background-stars frame). The catalog has beengenerated primarily as a result of our systematic search for high propermotion stars in the Digitized Sky Surveys using our SUPERBLINK software.At brighter magnitudes, the catalog incorporates stars and data from theTycho-2 Catalogue and also, to a lesser extent, from the All-SkyCompiled Catalogue of 2.5 million stars. The LSPM catalog considerablyexpands over the old Luyten (Luyten Half-Second [LHS] and New LuytenTwo-Tenths [NLTT]) catalogs, superseding them for northern declinations.Positions are given with an accuracy of <~100 mas at the 2000.0epoch, and absolute proper motions are given with an accuracy of ~8 masyr-1. Corrections to the local-background-stars propermotions have been calculated, and absolute proper motions in theextragalactic frame are given. Whenever available, we also give opticalBT and VT magnitudes (from Tycho-2, ASCC-2.5),photographic BJ, RF, and IN magnitudes(from USNO-B1 catalog), and infrared J, H, and Ks magnitudes(from 2MASS). We also provide an estimated V magnitude and V-J color fornearly all catalog entries, useful for initial classification of thestars. The catalog is estimated to be over 99% complete at high Galacticlatitudes (|b|>15deg) and over 90% complete at lowGalactic latitudes (|b|>15deg), down to a magnitudeV=19.0, and has a limiting magnitude V=21.0. All the northern starslisted in the LHS and NLTT catalogs have been reidentified, and theirpositions, proper motions, and magnitudes reevaluated. The catalog alsolists a large number of completely new objects, which promise to expandvery significantly the census of red dwarfs, subdwarfs, and white dwarfsin the vicinity of the Sun.Based on data mining of the Digitized Sky Surveys (DSSs), developed andoperated by the Catalogs and Surveys Branch of the Space TelescopeScience Institute (STScI), Baltimore.Developed with support from the National Science Foundation (NSF), aspart of the NASA/NSF NStars program.
| Sodium abundances in nearby disk stars
We present sodium abundances for a sample of nearby stars. All resultshave been derived from NLTE statistical equilibrium calculations. Theinfluence of collisional interactions with electrons and hydrogen atomsis evaluated by comparison of the solar spectrum with very precise fitsto the Na I line cores. The NLTE effects are more pronounced inmetal-poor stars since the statistical equilibrium is dominated bycollisions of which at least the electronic component is substantiallyreduced. The resulting influence on the determination of sodiumabundances is in a direction opposite to that found previously for Mgand Al. The NLTE corrections are about -0.1 in thick-disk stars with[Fe/H] -0.6. Our [Na/Fe] abundance ratios are about solar forthick- and thin-disk stars. The increase in [Na/Fe] as a function of[Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93}) isconfirmed. Our results suggest that sodium yields increase with themetallicity, and quite large amounts of sodium may be produced by AGBstars. We find that [Na/Fe] ratios, together with either [Mg/Fe] ratio,kinematic data or stellar evolutionary ages, make possible theindividual discrimination between thin- and thick-disk membership.Based on observations collected at the Germany-Spanish AstronomicalCenter, Calar Alto, Spain.Tables \ref{table2} and \ref{table3} are only available in electronicform at http://www.edpsciences.org
| The Atmospheric Lithium Abundances of Solar Analogues
We have analyzed the lithium abundance in the atmospheres of 20 starsthat are solar analogues based on high-resolution echelle spectra usingmodel atmospheres in a non-LTE approach. In terms of their lithiumabundances, the stars (which are located in a narrow range oftemperatures of 5650 5900 K) can be divided into two groups: one withlow lithium abundances, as in the solar atmosphere, and one with lithiumabundances that are higher than the solar value by about 1 dex (with theaccuracy of the lithium abundances being 0.15 dex).
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| Stars of Extragalactic Origin in the Solar Neighborhood
For 77 main-sequence F-G stars in the solar neighborhood with publishediron, magnesium, and europium abundances determined from high-dispersionspectra and with the ages estimated from theoretical isochrones, wecalculated the spatial velocities using Hipparcos data and the Galacticorbital elements. A comparison with the orbital elements of the globularclusters that are known to have been accreted by our Galaxy in the pastreveals stars of extragalactic origin. We show that the abundance ratiosof r- and alpha-elements in all the accreted stars differ sharply fromthose in the stars that are genetically associated with the Galaxy.According to current theoretical models, europium is produced mainly inlow-mass type-II supernovae (SNe II), while magnesium is synthesized inlarge quantities in high-mass SN II progenitors. Since all the oldaccreted stars of our sample exhibit a significant Eu overabundancerelative to Mg, we conclude that the maximum masses of the SN IIprogenitors outside the Galaxy were much lower than those inside it. Onthe other hand, only a small number of young accreted stars exhibit lownegative ratios [Eu/Mg] < 0. This can be explained by the delay ofprimordial star formation and the explosions of high-mass SNe II in arelatively small part of extragalactic space. We provide evidence thatthe interstellar medium was weakly mixed at the early evolutionarystages of the Galaxy formed from a single protogalactic cloud, and thatthe maximum mass of the SN II progenitors increased in it with timesimultaneously with the increase in mean metallicity.
| Neutron-Capture Elements in Halo, Thick-Disk, and Thin-Disk Stars: Neodymium
We have derived the LTE neodymium abundances in 60 cool stars withmetallicities [Fe/H] from 0.25 to -1.71 by applying a synthetic-spectrumanalysis to spectroscopic observations of NdII lines with a resolutionof λ/Δλ⋍60 000 and signal-to-noise ratios of100 200. We have improved the atomic parameters of NdII and blendinglines by analyzing the corresponding line pro files in the solarspectrum. Neodymium is overabundant with respect to iron in halo stars,[Nd/Fe]=0.33±0.09, with the [Nd/Fe] ratio decreasingsystematically with metallicity when [Fe/H]>-1. This reflects anonset of efficient iron production in type I supernovae during theformation of the thick disk. The [Nd/Ba] and [Nd/Eu] abundance ratiosbehave differently in halo, thick-disk, and thin-disk stars. Theobserved abundance ratios in halo stars, [Nd/Ba]=0.34±0.08 and[Nd/Eu]=-0.27±0.05, agree within the errors with the ratios ofthe elemental yields for the r-process. These results support theconclusion of other authors based on analyses of other elements that ther-process played the dominant role in the synthesis of heavy elementsduring the formation of the halo. The [Nd/Ba] and [Nd/Eu] ratios forthick-disk stars are almost independent of metallicity([Nd/Ba]=0.28(±0.03)-0.01(±0.04) [Fe/H] and[Nd/Eu]=-0.13(±0.03)+0.05(±0.04) [Fe/H]) but are smallerin absolute value than the corresponding ratios for halo stars,suggesting that the synthesis of s-process nuclei started during theformation of the thick disk. The s-process is estimated to havecontributed ⋍30% of the neodymium produced during this stage ofthe evolution of the Galaxy. The [Nd/Ba] ratio decreases abruptly by0.17 dex in the transition from the thick to the thin disk. Thesystematic decrease of [Nd/Ba] and increase of [Nd/Eu] with increasingmetallicity of thin-disk stars point toward a dominant role of thes-process in the synthesis of heavy elements during this epoch.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| Target Selection for SETI. II. Tycho-2 Dwarfs, Old Open Clusters, and the Nearest 100 Stars
We present the full target list and prioritization algorithm developedfor use by the microwave search for technological signals at the SETIInstitute. We have included the Catalog of Nearby Habitable StellarSystems (HabCat, described in Paper I), all of the nearest 100 stars and14 old open clusters. This is further augmented by a subset of theTycho-2 catalog based on reduced proper motions, and this larger catalogshould routinely provide at least three target stars within the largeprimary field of view of the Allen Telescope Array. The algorithm forprioritizing objects in the full target list includes scoring based onthe subset category of each target (i.e., HabCat, cluster, Tycho-2, ornearest 100), its distance (if known), and its proximity to the Sun onthe color-magnitude diagram.
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| 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.
| Multiplicity among solar-type stars. III. Statistical properties of the F7-K binaries with periods up to 10 years
Two CORAVEL radial velocity surveys - one among stars in the solarneighbourhood, the other in the Pleiades and in Praesepe - are merged toderive the statistical properties of main-sequence binaries withspectral types F7 to K and with periods up to 10 years. A sample of 89spectroscopic orbits was finally obtained. Among them, 52 relate to afree-of-bias selection of 405 stars (240 field stars and 165 clusterstars). The statistics corrected for selection effects yield thefollowing results: (1) No discrepancy is found between the binariesamong field stars and the binaries in open cluster. The distributions ofmass ratios, of periods, the period-eccentricity diagram and the binaryfrequencies are all within the same error intervals. (2) Thedistribution of mass ratios presents two maxima: a broad peak from q ~0.2 to q ~ 0.7, and a sharp peak for q > 0.8 (twins). Both arepresent among the early-type as well as among the late-type part of thesample, indicating a scale-free formation process. The peak for q >0.8 gradually decreases when long-period binaries are considered.Whatever their periods, the twins have eccentricities significantlylower than the other binaries, confirming a difference in the formationprocesses. Twins could be generated by in situ formation followed byaccretion from a gaseous envelope, whereas binaries with intermediatemass ratios could be formed at wide separations, but they are madecloser by migration led by interactions with a circumbinary disk. (3)The frequency of binaries with P<10 years is about 14%. (4) About0.3% of binaries are expected to appear as false positives in a planetsearch. Therefore, the frequency of planetary systems among stars ispresently 7+4-2%. The extension of thedistribution of mass ratios in the planetary range would result in avery sharp and very high peak, well separated from the binary stars withlow mass ratios. Based on photoelectric radial-velocity measurementscollected at Haute-Provence observatory and on observations made withthe ESA Hipparcos astrometry satellite.
| 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
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries
We report 25,563 radial velocity measurements for 1359 single-linedstars in the Carney-Latham sample of 1464 stars selected for high propermotion. For 171 of these, we present spectroscopic orbital solutions. Wefind no obvious difference between the binary characteristics in thehalo and the disk populations. The observed frequency is the same, andthe period distributions are consistent with the hypothesis that the twosets of binaries were drawn from the same parent population. Thissuggests that metallicity in general, and radiative opacities inparticular, have little influence over the fragmentation process thatleads to short-period binaries. All the binaries with periods shorterthan 10 days have nearly circular orbits, while the binaries withperiods longer than 20 days exhibit a wide range of eccentricities and amedian value of 0.37. For the metal-poor high-velocity halo binaries inour sample, the transition from circular to eccentric orbits appears tooccur at about 20 days, supporting the conclusion that tidalcircularization on the main sequence is important for the oldestbinaries in the Galaxy. Some of the results presented here usedobservations made with the Multiple Mirror Telescope, a joint facilityof the Smithsonian Institution and the University of Arizona.
| A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solarneighbourhood. This distribution is centred on solar metallicity. Weshow that previous metallicity distributions, selected on the basis ofspectral type, are biased against stars with solar metallicity orhigher. A selection of G-dwarf stars is inherently biased againstmetal-rich stars and is not representative of the solar neighbourhoodmetallicity distribution. Using a sample selected on colour, we obtain adistribution where approximately half the stars in the solarneighbourhood have metallicities higher than [Fe/H]=0. The percentage ofmid-metal-poor stars ([Fe/H]<-0.5) is approximately 4 per cent, inagreement with present estimates of the thick disc. In order to have ametallicity distribution comparable to chemical evolution modelpredictions, we convert the star fraction to mass fraction, and showthat another bias against metal-rich stars affects dwarf metallicitydistributions, due to the colour (or spectral type) limits of thesamples. Reconsidering the corrections resulting from the increasingthickness of the stellar disc with age, we show that the simpleclosed-box model with no instantaneous recycling approximation gives areasonable fit to the observed distribution. Comparisons with theage-metallicity relation and abundance ratios suggest that the simpleclosed-box model may be a viable model of the chemical evolution of theGalaxy at solar radius.
| Barium and europium abundances in cool dwarf stars and nucleosynthesis of heavy elements
We revise barium abundances in 29 cool stars with metallicities [Fe/H]ranging from -2.20 to 0.07 and europium abundances in 15 stars with[Fe/H] from -1.52 to 0.07. The sample has been extracted from Fuhrmann'slists (\cite{Fuhr3, Fuhr4}) and confined to main-sequence and turnoffstars with only one subgiant added. The results are based ondifferential NLTE model atmosphere analyses of spectra that have atypical S/N of 200 and a resolution of 40000 or 60000. The statisticalequilibrium of Eu Ii is first investigated with a model atom containing32 levels of Eu Ii plus the ground state of Eu Iii. NLTE effectsdecrease the equivalent widths of the Eu Ii lines compared with LTEresulting in positive NLTE abundance corrections which are below 0.08dex for all the stars investigated. The solar barium abundance eps{Ba,sun} = 2.21 and the europium abundance eps {Eu,sun} = 0.53 are foundfrom the Ba Ii and Eu Ii solar flux line profile fitting, and theycoincide within error bars with meteoritic abundances of Grevesse et al.(\cite{met96}). Here the usual scale with eps {H} = 12 is used. Theisotopic ratio \iso{151}{Eu}: \iso{153}{Eu} = 55: 45 is obtained fromsolar disk center intensity profile fitting of the Eu Ii lambda 4129Åline. We report here for the first time that the elemental ratios[Ba/Fe], [Eu/Fe] and [Eu/Ba] show a different behaviour for stars ofdifferent Galactic populations. For the halo stars the [Ba/Fe] ratiosare approximately solar, europium is overabundant relative to iron andbarium with the mean values [Eu/Fe] = 0.62 and [Eu/Ba] = 0.64. For thickdisk stars it is found that a) barium is slightly underabundant relativeto iron by about 0.1 dex; b) europium is overabundant relative to ironwith the [Eu/Fe] ratios between 0.30 and 0.44; and c) europium isoverabundant relative to barium with a mean value of [Eu/Ba] = 0.49 +/-0.03. A step-like change in the [Eu/Ba] and [Ba/Fe] ratios occurs at thethick to thin disk transition; so, nearly solar elemental ratios[Ba/Fe], [Eu/Fe] and [Eu/Ba] are found for the thin disk stars. Thesedata suggest that a) the halo and thick disk stellar population formedquickly during an interval comparable with the evolution time of an AGBprogenitor of 3 to 4 M_sun, and the r-process dominated heavy elementproduction at that epoch; b) there was a hiatus in star formation beforethe early stage of the thin disk developed. The even-to-odd Ba isotoperatios estimated from hyperfine structure (HFS) affecting the Ba Iiresonance line in the halo and thick disk stars favour a significantcontribution of \iso{138}{Ba} to barium for a pure r-process, and thisis supported by the recent data of Arlandini et al. (\cite{rs99}). Basedon observations at the German Spanish Astronomical Center, Calar Alto,Spain
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cassiopea |
| Ascensione retta: | 01h34m33.27s |
| Declinazione: | +68°56'53.3" |
| Magnitudine apparente: | 6.525 |
| Distanza: | 20.986 parsec |
| Moto proprio RA: | -376.8 |
| Moto proprio Dec: | 114.6 |
| B-T magnitude: | 7.364 |
| V-T magnitude: | 6.595 |
Cataloghi e designazioni:
|
