目录
图像
上传图像
DSS Images Other Images
相关文章
A Study on the Abundance Distribution of Heavy Elements in Metal-poor Stars by the Parameterization Method
Based on a large amount of observed data of element abundances inmetal-poor stars, taking the abundance distribution of heavy elements inthe solar system as a standard, and selecting Sr, Ba and Eu as thetypical elements of the three nucleosynthetic processes in metal-poorstars, namely the weak sprocess, main s-process and r-process, we havestudied the contributions of the three kinds of neutron-captureprocesses to the abundance distribution of heavy elements in metal-poorstars, with the parameterization method. It is found that the higher themetal abundance, the greater the contributions of the weak s-process andthe chief s-process to the abundances of lighter neutron-captureelements. The heavier neutron-capture elements are mainly produced bythe r-process and the chief s-process; and that at low metallicity, theabundances of heavy neutron-capture elements are mainly produced by ther-process. In the early Galaxy, the weak s-process has almost nocontribution to the element abundance.
| The origin of palladium and silver
Context. In recent years, progress has been made in understanding therapid neutron-capture process (r-process) and current studies suggestthe need for a second, so-called weak r-process. Observational proof ofthis possible second branch of the r-process may come from a detailedknowledge of the formation and evolution of silver (Ag) and to someextent palladium (Pd) abundances in halo stars. Aims: We studythe silver and palladium abundances of a large sample of stars (only afew measurements have been made so far) to gain insight into theformation process of lighter neutron-capture elements. Methods:We analysed a sample of stars (including both dwarfs and giants), forwhich we determined in a consistent manner stellar parameters andabundances. The abundances were derived via spectrum synthesis, makinguse of the MOOG synthetic spectrum code (1D LTE) and MARCS stellar modelatmospheres (1D LTE). Results: We derived the Ag and Pdabundances of 56 stars, the largest sample to date for which both Pd andAg have been studied. The stars span a metallicity range of 2.5 dex,from the metal-poor giants at [Fe/H] ~ -3.2 to the more metal-richdwarfs at [Fe/H] ~ -0.6. Both elements display rather flat trends withmetallicity, with some dispersion being present at the lowestmetallicities. Conclusions: The similarity detected in theevolutionary trends of both Ag and Pd is indicative of a common origin.Qualitative comparisons with some theoretical calculations seem toconfirm the need for a second r-process, responsible for theirformation. Further abundance studies (exploring more light n-captureelements) and comparisons with a wider variety of theoretical models areneeded to fully characterise the details and site of the formationmechanism(s).Appendix is only available in electronic form at http://www.aanda.org
| Forming the first planetary systems: debris around Galactic thick disc stars
The thick disc contains stars formed within the first Gyr of Galactichistory, and little is known about their planetary systems. The SpitzerMIPS instrument was used to search 11 of the closest of these oldlow-metal stars for circumstellar debris, as a signpost that bodies atleast as large as planetesimals were formed. A total of 22 thick discstars has now been observed, after including archival data, but dust isnot found in any of the systems. The data rule out a high incidence ofdebris among star systems from early in the Galaxy's formation. However,some stars of this very old population do host giant planets, atpossibly more than the general incidence among low-metal Sun-like stars.As the Solar system contains gas giants but little cometary dust, thethick disc could host analogue systems that formed many Gyr before theSun.
| A Survey of Stellar Families: Multiplicity of Solar-type Stars
We present the results of a comprehensive assessment of companions tosolar-type stars. A sample of 454 stars, including the Sun, was selectedfrom the Hipparcos catalog with ?>40 mas,??/? < 0.05, 0.5 <= B - V <= 1.0(~F6-K3), and constrained by absolute magnitude and color to excludeevolved stars. These criteria are equivalent to selecting all dwarf andsubdwarf stars within 25 pc with V-band flux between 0.1 and 10 timesthat of the Sun, giving us a physical basis for the term "solar-type."New observational aspects of this work include surveys for (1) veryclose companions with long-baseline interferometry at the Center forHigh Angular Resolution Astronomy Array, (2) close companions withspeckle interferometry, and (3) wide proper-motion companions identifiedby blinking multi-epoch archival images. In addition, we include theresults from extensive radial-velocity monitoring programs and evaluatecompanion information from various catalogs covering many differenttechniques. The results presented here include four new commonproper-motion companions discovered by blinking archival images.Additionally, the spectroscopic data searched reveal five new stellarcompanions. Our synthesis of results from many methods and sourcesresults in a thorough evaluation of stellar and brown dwarf companionsto nearby Sun-like stars. The overall observed fractions of single,double, triple, and higher-order systems are 56% ± 2%, 33%± 2%, 8% ± 1%, and 3% ± 1%, respectively, countingall confirmed stellar and brown dwarf companions. If all candidate,i.e., unconfirmed, companions identified are found to be real, thepercentages would change to 54% ± 2%, 34% ± 2%, 9%± 2%, and 3% ± 1%, respectively. Our completeness analysisindicates that only a few undiscovered companions remain in thiswell-studied sample, implying that the majority (54% ± 2%) ofsolar-type stars are single, in contrast to the results of priormultiplicity studies. Our sample is large enough to enable a check ofthe multiplicity dependence on various physical parameters by analyzingappropriate subsamples. Bluer, more massive stars are seen as morelikely to have companions than redder, less massive ones, consistentwith the trend seen over the entire spectral range. Systems with largerinteraction cross sections, i.e., those with more than two components orlong orbital periods, are preferentially younger, suggesting thatcompanions may be stripped over time by dynamical interactions. Weconfirm the planet-metallicity correlation (i.e., higher metallicitystars are more likely to host planets), but are unable to check it forbrown dwarfs due to the paucity of such companions, implying that thebrown dwarf desert extends over all separation regimes. We find nocorrelation between stellar companions and metallicity for B - V< 0.625, but among the redder subset, metal-poor stars ([Fe/H]<-0.3) are more likely to have companions with a 2.4?significance. The orbital-period distribution of companions is unimodaland roughly log normal with a peak and median of about 300 years. Theperiod-eccentricity relation shows the expected circularization forperiods below 12 days, caused by tidal forces over the age of theGalaxy, followed by a roughly flat distribution. The mass-ratiodistribution shows a preference for like-mass pairs, which occur morefrequently in relatively close pairs. The fraction of planet hosts amongsingle, binary, and multiple systems are statisticallyindistinguishable, suggesting that planets are as likely to form aroundsingle stars as they are around components of binary or multiple systemswith sufficiently wide separations. This, along with the preference oflong orbital periods among stellar systems, increases the space aroundstars conducive for planet formation, and perhaps life.
| Accurate fundamental parameters for 23 bright solar-type stars
We combine results from interferometry, asteroseismology andspectroscopy to determine accurate fundamental parameters of 23 brightsolar-type stars, from spectral type F5 to K2 and luminosity classesIII-V. For some stars we can use direct techniques to determine themass, radius, luminosity and effective temperature, and we compare withindirect methods that rely on photometric calibrations or spectroscopicanalyses. We use the asteroseismic information available in theliterature to infer an indirect mass with an accuracy of 4-15 per cent.From indirect methods we determine luminosity and radius to 3 per cent.We find evidence that the luminosity from the indirect method isslightly overestimated (~ 5 per cent) for the coolest stars, indicatingthat their bolometric corrections (BCs) are too negative. ForTeff we find a slight offset of -40 +/- 20K between thespectroscopic method and the direct method, meaning the spectroscopictemperatures are too high. From the spectroscopic analysis we determinethe detailed chemical composition for 13 elements, including Li, C andO. The metallicity ranges from [Fe/H] = -1.7 to +0.4, and there is clearevidence for ?-element enhancement in the metal-poor stars. Wefind no significant offset between the spectroscopic surface gravity andthe value from combining asteroseismology with radius estimates. Fromthe spectroscopy we also determine v sin i and we present a newcalibration of macroturbulence and microturbulence. From the comparisonbetween the results from the direct and spectroscopic methods we claimthat we can determine Teff, log g and [Fe/H] with absoluteaccuracies of 80K, 0.08 and 0.07dex. Photometric calibrations ofStrömgren indices provide accurate results for Teff and[Fe/H] but will be more uncertain for distant stars when interstellarreddening becomes important. The indirect methods are important toobtain reliable estimates of the fundamental parameters of relativelyfaint stars when interferometry cannot be used. This paper is the firstto compare direct and indirect methods for a large sample of stars, andwe conclude that indirect methods are valid, although slight correctionsmay be needed.
| Stellar atmosphere parameters with MA?, a MAssive compression of ?2 for spectral fitting
MA? is a new tool to estimate parameters from stellar spectra. It isbased on the maximum likelihood method, with the likelihood compressedin a way that the information stored in the spectral fluxes isconserved. The compressed data are given by the size of the number ofparameters, rather than by the number of flux points. The optimumspeed-up reached by the compression is the ratio of the data set to thenumber of parameters. The method has been tested on a sample oflow-resolution spectra from the Sloan Extension for GalacticUnderstanding and Exploration (SEGUE) survey for the estimate ofmetallicity, effective temperature and surface gravity, with accuraciesof 0.24 dex, 130 K and 0.5 dex, respectively. Our stellar parameters andthose recovered by the SEGUE Stellar Parameter Pipeline agree reasonablywell. A small sample of high-resolution VLT-UVES spectra is also used totest the method and the results were compared to a more classicalapproach. The speed and multi-resolution capability of MA? combinedwith its performance compared with other methods indicates that it willbe a useful tool for the analysis of upcoming spectral surveys.
| An Overview of the Rotational Behavior of Metal-poor Stars
This paper describes the behavior of the rotational velocity inmetal-poor stars ([Fe/H] <= -0.5 dex) in different evolutionarystages, based on vsin i values from the literature. Our sample iscomprised of stars in the field and some Galactic globular clusters,including stars on the main sequence, the red giant branch (RGB), andthe horizontal branch (HB). The metal-poor stars are, mainly, slowrotators, and their vsin i distribution along the HR diagram is quitehomogeneous. Nevertheless, a few moderate to high values of vsin i arefound in stars located on the main sequence and the HB. We show that theoverall distribution of vsin i values is basically independent ofmetallicity for the stars in our sample. In particular, thefast-rotating main sequence stars in our sample present rotation ratessimilar to their metal-rich counterparts, suggesting that some of themmay actually be fairly young, in spite of their low metallicity, or elsethat at least some of them would be better classified as blue stragglerstars. We do not find significant evidence of evolution in vsin i valuesas a function of position on the RGB; in particular, we do not confirmprevious suggestions that stars close to the RGB tip rotate faster thantheir less-evolved counterparts. While the presence of fast rotatorsamong moderately cool blue HB stars has been suggested to be due toangular momentum transport from a stellar core that has retainedsignificant angular momentum during its prior evolution, we find thatany such transport mechanisms most likely operate very fast as the stararrives on the zero-age HB (ZAHB), since we do not find a link betweenevolution off the ZAHB and vsin i values. We present an extensivetabulation of all quantities discussed in this paper, including rotationvelocities, temperatures, gravities, and metallicities [Fe/H], as wellas broadband magnitudes and colors.
| On the Relationship Between Debris Disks and Planets
Dust in debris disks is generated by collisions among planetesimals. Theexistence of these planetesimals is a consequence of the planetformation process, but the relationship between debris disks and planetshas not been clearly established. Here we analyze Spitzer/MIPS 24 and 70μm data for 150 planet-bearing stars, and compare the incidence ofdebris disks around these stars with a sample of 118 stars around whichplanets have been searched for, but not found. Together they comprisethe largest sample ever assembled to deal with this question. The use ofsurvival analysis techniques allows us to account for the large numberof nondetections at 70 μm. We discovered 10 new debris disks aroundstars with planets and one around a star without known planets. We foundthat the incidence of debris disks is marginally higher among stars withplanets, than among those without, and that the brightness of theaverage debris disk is not significantly different in the two samples.We conclude that the presence of a planet that has been detected viacurrent radial velocity techniques is not a good predictor of thepresence of a debris disk detected at infrared wavelengths.
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| Beryllium abundances and star formation in the halo and in the thick disk
Context: Beryllium is a pure product of cosmic ray spallation. Thisimplies a relatively simple evolution in time of the beryllium abundanceand suggests its use as a time-like observable. Aims: Our goal is toderive abundances of Be in a sample of 90 stars, the largest sample ofhalo and thick disk stars analyzed to date. We study the evolution of Bein the early Galaxy and its dependence on kinematic and orbitalparameters, and investigate its use as a cosmochronometer. Abundances ofBe, Fe, and α-elements of 73 stars are employed to study theformation of the halo and the thick disk of the Galaxy. Methods:Beryllium abundances are determined from high-resolution, highsignal-to-noise UVES spectra with spectrum synthesis. Atmosphericparameters and abundances of α-elements are adopted from theliterature. Lithium abundances are used to eliminate mixed stars fromthe sample. The properties of halo and thick disk stars are investigatedin diagrams of log(Be/H) vs. [ α/H] , log(Be/H) vs. [Fe/H], and [α/Fe] vs. log(Be/H) and with orbital and kinematic parameters. Results: We present our observational results in various diagrams. (i)In a log(Be/H) vs. [Fe/H] diagram we find a marginal statisticaldetection of a real scatter, above what is expected from measurementerrors, with a larger scatter among halo stars. The detection of thescatter is further supported by the existence of pairs of stars withidentical atmospheric parameters and different Be abundances; (ii) in alog(Be/H) vs. [ α/Fe] diagram, the halo stars separate into twocomponents; one is consistent with predictions of evolutionary models,while the other has too high α and Be abundances and is chemicallyindistinguishable from thick disk stars. This suggests that the halo isnot a single uniform population where a clear age-metallicity relationcan be defined; (iii) In diagrams of R_min vs. [ α/Fe] andlog(Be/H), the thick disk stars show a possible decrease in [α/Fe] with R_min, whereas no dependence of Be with R_min is seen.This anticorrelation suggests that the star formation rate was lower inthe outer regions of the thick disk, pointing towards an inside-outformation. The lack of correlation for Be indicates that it isinsensitive to the local conditions of star formation.Based on observations made with ESO VLT, at Paranal Observatory, underprograms 076.B-0133 and 077.B-0507, and on data obtained from theESO/ST-ECF Science Archive Facility and the UVES Paranal ObservatoryProject 266.D-5655.Tables 1-3, 6 and Appendices A-C are only available in electronic format http://www.aanda.org
| Calibration of Strömgren uvby-H? photometry for late-type stars - a model atmosphere approach
Context: The use of model atmospheres for deriving stellar fundamentalparameters, such as T_eff, log g, and [Fe/H], will increase as we findand explore extreme stellar populations where empirical calibrations arenot yet available. Moreover, calibrations for upcoming large satellitemissions of new spectrophotometric indices, similar to the uvby-H?system, will be needed. Aims: We aim to test the power oftheoretical calibrations based on a new generation of MARCS models bycomparisons with observational photomteric data. Methods: Wecalculated synthetic uvby-H? colour indices from synthetic spectra.A sample of 367 field stars, as well as stars in globular clusters, isused for a direct comparison of the synthetic indices versus empiricaldata and for scrutinizing the possibilities of theoretical calibrationsfor temperature, metallicity, and gravity. Results: We show thatthe temperature sensitivity of the synthetic (b-y) colour is very closeto its empirical counterpart, whereas the temperature scale based uponH? shows a slight offset. The theoretical metallicity sensitivityof the m1 index (and for G-type stars its combination withc_1) is somewhat higher than the empirical one, based upon spectroscopicdeterminations. The gravity sensitivity of the synthetic c1index shows satisfactory behaviour when compared to obervations of Fstars. For stars cooler than the sun, a deviation is significant in thec1-(b-y) diagram. The theoretical calibrations of (b-y),(v-y), and c1 seem to work well for Pop II stars and lead toeffective temperatures for globular cluster stars supporting recentclaims that atomic diffusion occurs in stars near the turnoff point ofNGC 6397. Conclusions: Synthetic colours of stellar atmospherescan indeed be used, in many cases, to derive reliable fundamentalstellar parameters. The deviations seen when compared to observationaldata could be due to incomplete linelists but are possibly also due tothe effects of assuming plane-parallell or spherical geometry and LTE.Model colours 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/498/527
| The main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filtersystem well defined stellar samples are needed. The nearby stars providea complete stellar sample representative for the thin disc population.We compare the filter transformations of different authors applied tothe main sequence stars from F to K dwarfs to SDSS filter system anddiscuss the properties of the main sequence. The location of the meanmain sequence in colour-magnitude diagrams is very sensitive tosystematic differences in the filter transformation. A comparison withfiducial sequences of star clusters observed in g', r', and i' show goodagreement. Theoretical isochrones from Padua and from Dartmouth havestill some problems, especially in the (r-i) colours.
| ADONIS high contrast infrared imaging of Sirius-B
Context: Sirius is the brightest star in the sky and a strong source ofdiffuse light for modern telescopes so that the immediate surroundingsof the star are still poorly known. Aims: We study the closesurroundings of the star (2 to 25´´) by means of adaptiveoptics and coronographic device in the near-infrared, using theESO/ADONIS system. Methods: The resulting high contrast imagesin the JHKs bands have a resolution of ~0.2´´ and limitingapparent magnitude ranging from m_K=9.5 at 3´´ from Sirius-Ato m_K=13.1 at 10´´. These are the first and deepest imagesof the Sirius system in this infrared range. Results: From theseobservations, accurate infrared photometry of the Sirius-B white dwarfcompanion is obtained. The JH magnitudes of Sirius-B are found to agreewith expectations for a DA white dwarf of temperature (T=25 000 K) andgravity (log g = 8.5), consistent with the characteristics determinedfrom optical observations. However, a small, significant excess ismeasurable for the K band, similar to that detected for“dusty” isolated white dwarfs harbouring suspected planetarydebris. The possible existence of such circumstellar material aroundSirius-B has still to be confirmed by further observations. Conclusions: These deep images allow us to search for small but yetundetected companions to Sirius. Apart from Sirius-B, no other source isdetected within the total 25´´ field. A comparison of theflux expected from the faintest known brown dwarfs at the distance ofSirius demonstrates that the above limiting magnitudes correspond to astar of spectral type later than T5 at 5´´ and T7 at10´´. Using theoretical spectra of brown dwarfs andplanet-size objects, we also show that the end of the brown dwarfsequence is reached in the outer part of the image. The minimumdetectable mass is around 10 M_Jup inside the planetary limit,indicating that an extrasolar planet at a projected distance of ~25 AUfrom Sirius would have been detected.
| A catalogue of multiplicity among bright stellar systems
We consider the multiplicity of stellar systems with (combined)magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4559such bright systems (including the Sun), and the frequencies ofmultiplicities 1, 2,..., 7 are found to be 2718, 1437, 285, 86, 20, 11and 2. We discuss the uncertainties, which are substantial. We alsoconsider the distributions of periods of orbits and suborbits. We notethat for even more restricted set of 478 systems with VH<= 4.00, the proportions of higher multiples up to sextuple areprogressively larger (213, 179, 54, 19, 8, 5), suggesting substantialincompleteness in even the reasonably well studied larger sample.This sample can be seen as relatively thoroughly studied formultiplicity, and reasonably representative of stars more massive thanthe Sun. But the restriction to VH <= 6 means that oursample contains hardly any systems where all components are low-massmain-sequence stars (K or M).Data on multiplicity are important as a constraint on (i) the starformation problem, (ii) the problem of the evolution of the Galacticstellar population and (iii) the interaction of dynamics and evolutionthrough the effect of Kozai cycles. We discuss these topics briefly.
| a New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of whitedwarfs lying within 20 pc of the Sun. A new revised catalog of localwhite dwarfs containing 122 entries (126 individual degenerate stars) ispresented. This list contains 27 white dwarfs not included in a previouslist from 2002, as well as new and recently published trigonometricparallaxes. In several cases new members of the local white dwarfpopulation have come to light through accurate photometric distanceestimates. In addition, a suspected new double degenerate system (WD0423+120) has been identified. The 20 pc sample is currently estimatedto be 80% complete. Using a variety of recent spectroscopic,photometric, and trigonometric distance determinations, we re-compute aspace density of 4.8 ± 0.5 × 10 3 pc3 corresponding to a mass density of 3.2 ± 0.3 ×10 3 M sun pc 3 from the completeportion of the sample within 13 pc. We find an overall mean mass for thelocal white dwarfs of 0.665 M sun, a value larger than mostother non-volume-limited estimates. Although the sample is small, wefind no evidence of a correlation between mass and temperature in whichwhite dwarfs below 13,000 K are systematically more massive than thoseabove this temperature. Within 20 pc 25% of the white dwarfs are inbinary systems (including double degenerate systems). Approximately 6%are double degenerates and 6.5% are Sirius-like systems. The fraction ofmagnetic white dwarfs in the local population is found to be 13%.
| Oxygen abundances in nearby stars. Clues to the formation and evolution of the Galactic disk
The abundances of iron and oxygen are homogeneously determined in asample of 523 nearby (d<150 pc) FGK disk and halo stars withmetallicities in the range -1.5<[Fe/H]<0.5. Iron abundances wereobtained from an LTE analysis of a large set of Fe I and Fe II lineswith reliable atomic data. Oxygen abundances were inferred from arestricted non-LTE analysis of the 777 nm O I triplet. We adopted theinfrared flux method temperature scale and surface gravities based onHipparcos trigonometric parallaxes. Within this framework, theionization balance of iron lines is not satisfied: the mean abundancesfrom the Fe I lines are systematically lower by 0.06 dex than those fromthe Fe II lines for dwarf stars of Teff>5500 K and[Fe/H]<0.0, and giant stars of all temperatures and metallicitiescovered by our sample. The discrepancy worsens for cooler and metal-richmain-sequence stars. We use the stellar kinematics to compute theprobabilities of our sample stars to be members of the thin disk, thickdisk, or halo of the Galaxy. We find that the majority of thekinematically-selected thick-disk stars show larger [O/Fe] ratioscompared to thin-disk stars while the rest show thin-disk abundances,which suggests that the latter are thin-disk members with unusual(hotter) kinematics. A close examination of this pattern for disk starswith ambiguous probabilities shows that an intermediate population withproperties between those of the thin and thick disks does not exist, atleast in the solar neighborhood. Excluding the stars with unusualkinematics, we find that thick-disk stars show slowly decreasing [O/Fe]ratios from about 0.5 to 0.4 in the -0.8<[Fe/H]<-0.3 range. Usinga simple model for the chemical evolution of the thick disk we show thatthis trend results directly from the metallicity dependence of the TypeII supernova yields. At [Fe/H]>-0.3, we find no obvious indication ofa sudden decrease (i.e., a "knee") in the [O/Fe] vs. [Fe/H] pattern ofthick-disk stars that would connect the thick and thin disk trends at ahigh metallicity. We conclude that Type Ia supernovae (SN Ia) did notcontribute significantly to the chemical enrichment of the thick disk.In the -0.8<[Fe/H]<+0.3 range, thin-disk stars show decreasing[O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution.The implications of these results for studies of the formation andevolution of the Galactic disk are discussed.Tables 4-6 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/465/271 Partially based onobservations obtained with the Hobby-Eberly Telescope, which is a jointproject of the University of Texas at Austin, the Pennsylvania StateUniversity, Stanford University, Ludwig-Maximilians-UniversitätMünchen, and Georg-August-Universität Göttingen; and datafrom the UVES Paranal Observatory Project (ESO DDT Program ID266.D-5655).
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is 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/464/377
| New Debris Disks around Nearby Main-Sequence Stars: Impact on the Direct Detection of Planets
Using the MIPS instrument on Spitzer, we have searched for infraredexcesses around a sample of 82 stars, mostly F, G, and K main-sequencefield stars, along with a small number of nearby M stars. These starswere selected for their suitability for future observations by a varietyof planet-finding techniques. These observations provide information onthe asteroidal and cometary material orbiting these stars, data that canbe correlated with any planets that may eventually be found. We havefound significant excess 70 μm emission toward 12 stars. Combinedwith an earlier study, we find an overall 70 μm excess detection rateof 13%+/-3% for mature cool stars. Unlike the trend for planets to befound preferentially toward stars with high metallicity, the incidenceof debris disks is uncorrelated with metallicity. By newly identifyingfour of these stars as having weak 24 μm excesses (fluxes ~10% abovethe stellar photosphere), we confirm a trend found in earlier studieswherein a weak 24 μm excess is associated with a strong 70 μmexcess. Interestingly, we find no evidence for debris disks around 23stars cooler than K1, a result that is bolstered by a lack of excessaround any of the 38 K1-M6 stars in two companion surveys. Onemotivation for this study is the fact that strong zodiacal emission canmake it hard or impossible to detect planets directly with futureobservatories such as the Terrestrial Planet Finder (TPF). Theobservations reported here exclude a few stars with very high levels ofemission, >1000 times the emission of our zodiacal cloud, from directplanet searches. For the remainder of the sample, we set relatively highlimits on dust emission from asteroid belt counterparts.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| A catalog of rotational and radial velocities for evolved stars. IV. Metal-poor stars^
Aims.The present paper describes the first results of an observationalprogram intended to refine and extend the existing v sin i measurementsof metal-poor stars, with an emphasis on field evolved stars.Methods: .The survey was carried out with the FEROS and CORALIEspectrometers. For the v sin i measurements, obtained from spectralsynthesis, we estimate an uncertainty of about 2.0 km s-1. Results: .Precise rotational velocities v sin i are presented for alarge sample of 100 metal-poor stars, most of them evolving off themain-sequence. For the large majority of the stars composing the presentsample, rotational velocities have been measured for the first time.
| Metallicity and absolute magnitude calibrations for UBV photometry
Calibrations are presented here for metallicity ([Fe/H]) in terms of theultraviolet excess, [δ(U - B) at B - V = 0.6, hereafterδ0.6], and also for the absolute visual magnitude(MV) and its difference with respect to the Hyades(ΔMHV) in terms of δ0.6 and(B - V), making use of high-resolution spectroscopic abundances from theliterature and Hipparcos parallaxes. The relation[Fe/H]-δ0.6 has been derived for dwarf plus turn-offstars, and also for dwarf, turn-off, plus subgiant stars classifiedusing the MV-(B - V)0 plane of Fig. 11, which iscalibrated with isochrones from Bergbusch & VandenBerg (and alsoVandenBerg & Clem). The [Fe/H]-δ0.6 relations inour equations (5) and (6) agree well with those of Carney, as can beseen from Fig. 5(a). Within the uncertainties, the zero-points,+0.13(+/-0.05) of equation (5) and +0.13(+/-0.04) of equation (6), arein good agreement with the photometric ones of Cameron and of Carney,and close to the spectroscopic ones of Cayrel et al. and of Boesgaard& Friel for the Hyades open cluster. Good quantitative agreementbetween our estimated [Fe/H] abundances with those from uvby-βphotometry and spectroscopic [Fe/H]spec values demonstratesthat our equation (6) can be used in deriving quality photometric metalabundances for field stars and clusters using UBV data from variousphotometric surveys.For dwarf and turn-off stars, a new hybrid MV calibration ispresented, based on Hipparcos parallaxes withσπ/π <= 0.1 and with a dispersion of +/-0.24in MV. This hybrid MV calibration containsδ0.6 and (B - V) terms, plus higher order cross-termsof these, and is valid for the ranges of +0.37 <= (B - V)0<= +0.88,- 0.10 <= δ0.6 <= +0.29 and 3.44<= MV <= 7.23. For dwarf and turn-off stars, therelation for ΔMHV is revised and updated interms of (B - V) and δ0.6, for the ranges of -0.10<= δ0.6 <= +0.29, and +0.49 <= (B -V)0 <= +0.89, again making use of Hipparcos parallaxeswith σπ/π <= 0.1. These parallaxes formetal-poor dwarf and turn-off stars in our sample reveal that thedifference of ΔMHV(B - V) relative to Hyadesat (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given byLaird et al. In general, Hipparcos parallaxes are larger thanground-based ones, causing a divergence of ourΔMHV(B - V,δ0.6) relation(the solid line in Fig. 15b), from the one of Laird et al. (the dashedline) for the range +0.10 <= δ0.6 <= +0.29 ourabsolute magnitudes are fainter, as has been confirmed for localsubdwarfs by Reid. Our final calibrations forΔMHV(B - V, δ0.6),equations (16) and (17), are third-order polynomials inδ0.6, pass through the origin, and provide photometricdistances in reasonable agreement with those obtained directly fromHipparcos parallaxes (Fig. 18).
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 pc-The Southern Sample
We are obtaining spectra, spectral types, and basic physical parametersfor the nearly 3600 dwarf and giant stars earlier than M0 in theHipparcos catalog within 40 pc of the Sun. Here we report on resultsfor 1676 stars in the southern hemisphere observed at Cerro TololoInter-American Observatory and Steward Observatory. These resultsinclude new, precise, homogeneous spectral types, basic physicalparameters (including the effective temperature, surface gravity, andmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. We include notes on astrophysically interesting stars inthis sample, the metallicity distribution of the solar neighborhood, anda table of solar analogs. We also demonstrate that the bimodal nature ofthe distribution of the chromospheric activity parameterlogR'HK depends strongly on the metallicity, andwe explore the nature of the ``low-metallicity'' chromosphericallyactive K-type dwarfs.
| 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.
| 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.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Chemical Composition in the Globular Cluster M71 from Keck HIRES Spectra of Turnoff Stars
We have made observations with the Keck I telescope and HIRES at aresolution of ~45,000 of five nearly identical stars at the turnoff ofthe metal-rich globular cluster M71. We derive stellar parameters andabundances of several elements. Our mean Fe abundance,[Fe/H]=-0.80+/-0.02, is in excellent agreement with previous clusterdeterminations from both giants and near-turnoff stars. There is noclear evidence for any star-to-star abundance differences orcorrelations in our sample. Abundance ratios of the Fe peak elements(Cr, Ni) are similar to Fe. The turnoff stars in M71 have remarkablyconsistent enhancements of 0.2-0.3 dex in [Si/Fe], [Ca/Fe], and [Ti/Fe],like the red giants. Our [Mg/Fe] ratio is somewhat lower than thatsuggested by other studies. We compare our mean abundances for the fiveM71 stars with field stars of similar metallicity [Fe/H]: eight withhalo kinematics and 17 with disk kinematics. The abundances of theα-fusion products (Mg, Si, Ca, Ti) agree with both samples butseem a closer match to the disk stars. The Mg abundance in M71 is at thelower edge of the disk and halo samples. The neutron-capture elements, Yand Ba, are enhanced relative to solar in the M71 turnoff stars. Ourratio [Ba/Fe] is similar to that of the halo field stars but a factor of2 above that for the disk field stars. The important [Ba/Y] ratio issignificantly lower than M71 giant values; the precluster material mayhave been exposed to a higher neutron flux than the disk stars orself-enrichment has occurred subsequent to cluster star formation. TheNa content of the M71 turnoff stars is remarkably similar to that in thedisk field stars but more than a factor of 2 higher than the halo fieldstar sample. We find [Na/Fe]=+0.14+/-0.04 with a spread less than halfof that found in the red giants in M71. Excluding Mg, the lack ofintracluster α-element variations (turnoff vis-à-visgiants) suggests that the polluting material needed to explain theabundance patterns in M71 did not arise from explosive nucleosynthesisbut in a more traditional s-process environment such as AGB stars. Thedetermination of light s-peak abundances should reveal whether thispollution occurred before or after cluster formation.
| Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood
A new metallicity distribution and an age-metallicity relation arepresented for 437 nearby F and G turn-off and sub-giant stars selectedfrom radial velocity data of Nidever et al. Photometric metallicitiesare derived from uvby- Hβ photometry, and the stellar ages from theisochrones of Bergbusch & VandenBerg as transformed to uvbyphotometry using the methods of Clem et al.The X (stellar population) criterion of Schuster et al., which combinesboth kinematic and metallicity information, provides 22 thick-discstars. σW= 32 +/- 5 km s-1,= 154 +/- 6 km s-1 and<[M/H]>=-0.55 +/- 0.03 dex for these thick-disc stars, which is inagreement with values from previous studies of the thick disc.α-element abundances which are available for some of thesethick-disc stars show the typical α-element signatures of thethick disc, supporting the classification procedure based on the Xcriterion.Both the scatter in metallicity at a given age and the presence of old,metal-rich stars in the age-metallicity relation make it difficult todecide whether or not an age-metallicity relation exists for the olderthin-disc stars. For ages greater than 3 Gyr, our results agree with theother recent studies that there is almost no correlation between age andmetallicity, Δ([M/Fe])/Δ(age) =-0.01 +/- 0.005 dexGyr-1. For the 22 thick-disc stars there is a range in agesof 7-8 Gyr, but again almost no correlation between age and metallicity.For the subset of main-sequence stars with extra-solar planets, theage-metallicity relation is very similar to that of the total sample,very flat, the main difference being that these stars are mostlymetal-rich, [M/H]>~-0.2 dex. However, two of these stars have[M/H]~-0.6 dex and have been classified as thick-disc stars. As for thetotal sample, the range in ages for these stars with extra-solarplanetary systems is considerable with a nearly uniform distributionover 3 <~ age <~ 13 Gyr.
| 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.
| Doppler imaging and surface differential rotation of young open cluster stars - I. HD 307938 (R58) in IC 2602
In this paper we present Doppler images of a young active G dwarf (HD307938) in the southern open cluster IC 2602. Spectroscopic data wereobtained over a four-night period in 2000 January at the 3.9-mAnglo-Australian Telescope using the University College London EchelleSpectrograph. Simultaneous photometric observations (in the V and Rbands) were obtained at the 1.0-m Australian National Universitytelescope. By applying least-squares deconvolution (LSD) to the 2500+photospheric lines in each echelle spectrum a single highsignal-to-noise ratio LSD profile was produced for each phase of thespectroscopic observations. Maximum-entropy image reconstruction,incorporating both the LSD profiles and the photometric data, was usedto produce maps of the surface features of the star, with the inclusionof the photometric data producing an increase (compared with the use ofspectroscopic data alone) in the spot occupancy in both low- andmid-latitude regions of the star. The maps show that HD 307938 possessesa large, broken polar spot extending down to ~60° latitude, as wellas lower-latitude spots similar to other rapidly rotating G dwarfs. Byincorporating a solar-like differential rotation law into the imagingprocess the surface differential rotation of HD 307938 was determined.This gave a surface shear of dΩ= 0.025 +/- 0.015 radd-1 (for an inclination angle of 60°). Thus the equatorof HD 307938 laps the poles every ~250 d and has a photospheric sheararound half that of the Sun.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
